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Preparation and properties of reactively sputtered copper oxide films Drobny, Vladimir F. 1978

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PREPARATION AND PROPERTIES OF REACTIVELY SPUTTERED COPPER OXIDE FILMS by V l a d i m i r F. Drobny D i p l . Ing. 1969 Slovak T e c h n i c a l U n i v e r s i t y A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENT FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n THE FACULTY OF GRADUATE STUDIES (Department of E l e c t r i c a l Engineering) We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA August, 1978 0 V l a d i m i r F. Drobny, 1978 In presenting t h i s t h e s i s in p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree that permission for extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . It i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission. V l a d i m i r F. Drobny Department of E l e c t r i c a l Engineering The U n i v e r s i t y of B r i t i s h Columbia 2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5 Date October 16, 1978 ABSTRACT The d e p o s i t i o n of t h i n copper oxide f i l m s by r e a c t i v e s p u t t e r i n g has been i n v e s t i g a t e d from both experimental and t h e o r e t i c a l s t a n d p o i n t s . Experimental work u t i l i z e d both r . f . and d.c. s p u t t e r i n g i n argon/oxygen mixtures. The f i l m p r o p e r t i e s of [0]/[Cu] compositional r a t i o , o p t i c a l constants and r e s i s t i v i t y were e x p e r i m e n t a l l y determined. The composition of the f i l m s was found to change w i t h i n c r e a s i n g oxygen p a r t i a l pressure i n the f o l l o w i n g manner Cu Cu + Cu 20 -> Cu 20 -> Cu 20 + CuO -+ CuO The above trend could a l s o be observed a t a given oxygen p a r t i a l pressure by v a r y i n g the metal d e p o s i t i o n rate. This o b s e r v a t i o n l e d to the p o s t u l a t e that the f i l m c ompositional r a t i o was determined by the r e l a t i v e magnitudes of the f l u x e s o f copper and oxygen atoms i n c i d e n t on the s u b s t r a t e plane. Based on t h i s p o s t u l a t e a comprehensive theory d e s c r i b i n g f i l m d e p o s i t i o n by r e a c t i v e s p u t t e r i n g was developed. The theory i s s u f f i c i e n t l y general to apply to any metal - elemental r e a c t i v e gas system. Good agreement was ob-tai n e d between the p r e d i c t i o n s of the theory and the phase composition of copper oxide f i l m s deposited over a wide range of experimental c o n d i t i o n s . C o r r e l a t i o n of t h e o r e t i c a l and p r a c t i c a l data a l s o allowed determination of the s t i c k i n g c o e f f i c i e n t f o r oxygen on copper-coated s u r f a c e s . The theory a l s o served to guide experiments i n producing, i n a very r e p r o d u c i b l e manner, copper oxide f i l m s w i t h a wide range of o p t i c a l and e l e c t r i c a l p r o p e r t i e s . The o p t i c a l constants of complex r e f r a c t i v e index and a b s o r p t i o n c o e f f i c i e n t were determined f o r the f i r s t time f o r r e a c t i v e l y - s p u t t e r e d r f i l m s of Cu + Cu 20, Cu 20, Cu 20 + CuO and CuO. The presence of Cu 20 was found to be c h a r a c t e r i z e d by a peak i n the r e a l p a r t of the r e f r a c t i v e index i i (n = 3.36) o c c u r r i n g at a wavelength of 0.48ym. Evidence of f r e e c a r r i e r a b s o r p t i o n and acceptor l e v e l - conduction band t r a n s i t i o n s was obtained f o r (X^O f i l m s of v a r i o u s stochiometry. The r e s i s t i v i t y of the C^O f i l m s depended s t r o n g l y on the stochiometry and values as low as 30 ohm cm could be e a s i l y obtained. Such low values of r e s i s t i v i t y are not r e a d i l y ob-t a i n a b l e by other p r e p a r a t i v e techniques and a l l o w Ci^O t o be considered as a semiconductor m a t e r i a l f o r a v a r i e t y of a p p l i c a t i o n s . One'such a p p l i c a t i o n , namely C^O/Si h e t e r o j u n c t i o n s o l a r c e l l s , was i n v e s t i g a t e d i n the present work. -i i i TABLE OF CONTENTS Page ABSTRACT . . i i TABLE OF CONTENTS . . . i v LIST OF TABLES . v i LIST OF ILLUSTRATIONS . . . . . . . . ' . . . . . v i i LIST OF SYMBOLS x i ACKNOWLEDGEMENT x v i 1. INTRODUCTION 1 2. THEORY OF REACTIVE SPUTTERING • 2.1 I n t r o d u c t i o n 5 2.2 Proposed Theory of R e a c t i v e S p u t t e r i n g . . 9 2.3 A p p l i c a t i o n of the Theory to the R e a c t i v e S p u t t e r i n g of Copper i n Oxygen . . . . . . . . . . . . . . . . . 21 3. COPPER OXIDE FILM PREPARATION 3.1 General Considerations 25 3.2, The S p u t t e r i n g System . . . . .... . . . . . . . . . . 27 3.3 Substrate P r e p a r a t i o n 29 3.4 D e p o s i t i o n Conditions 32 4. PROPERTIES OF COPPER OXIDE FILMS 4.1 F i l m Thickness Measurement 40 4.2 Phase Composition . . . . . . . . . . . . 41 4.3 O p t i c a l Transmittance . . . . . . . 47 4.4 R e s i s t i v i t y 51 4.5 Thermal A c t i v a t i o n Energies f o r C o n d u c t i v i t y 58 4.6 Comparison of Exp e r i m e n t a l and T h e o r e t i c a l D a t a . . . 66 i v 5. OPTICAL CONSTANTS OF COPPER OXIDE THIN FILMS 5.1 P o s s i b l e Measurement Methods . . . 80 5.2 Technique used f o r O p t i c a l Constants Determination 89 5.3 Re s u l t s 92 6. COPPER OXIDE/SILICON HETEROJUNCTION DIODES 6.1 I n t r o d u c t i o n 103 6.2 Wafer P r e p a r a t i o n . 108 6.3 C^O/Si H e t e r o j u n c t i o n P r e p a r a t i o n . 112 6.4 I-V C h a r a c t e r i s t i c s 117 1 7. CONCLUSIONS . . . 127 APPENDIX A 129 APPENDIX B 138 REFERENCES . . . . . . . . . . . . . . . 144 v LIST OF TABLES Table Page 4.1 I d e n t i f i c a t i o n of the D i f f r a c t i o n Rings i n F i g . 4.1 . . . . 45 4.2 Oxygen P a r t i a l Pressures Ranges Required f o r Various Compositional S t r u c t u r e s 46 4.3 Thermal A c t i v a t i o n Energies f o r C o n d u c t i v i t y 65 v i LIST OF ILLUSTRATIONS Figure Page 3.1 Schematic diagram of reactive sputtering system used for copper oxide film deposition 28 3.2 3 mm diameter 150. mesh EM copper grid . . . . . . . . . . . 30 3.3a Substrate with contact areas used for preparation of samples for determination of thermal activation energies for conductivity . . . . . . . 30 3.3b Sample for measurements of thermal activation energies for conductivity 30 3.4a Custom-made substrate table enabling e f f i c i e n t cooling of samples during their deposition . . . 36 3.4b Substrate table sample holder as used for deposition of samples for determination of thermal activation energies for conductivity . . . . . . . 37 4.1 Electron diffraction patterns for copper oxide films . . . 44 4.1a Electron diffraction pattern taken from gold film . . . . 46 4.2 Optical transmittance data for copper oxide films deposited by r . f . sputtering at 200W of forward power and total pressure of 25 mTorr 49 4.3 Variation of optical transmittance with radial distance along copper oxide film . . . . . . 50 4.4 Circuit diagram of the four-point probe set-up used for r e s i s t i v i t y measurements . 52 4.5 Resistivity data for films deposited by r . f . sputtering at 200W and total pressure of 25 mTorr . . . . . . . . . . 54 4.6 Resistivity data for films deposited by d.c. sputtering at 70 mA and total pressure of 75 mTorr . . . . . . . . . 55 v i i 4.7 Me t a l mask f o r d e p o s i t i o n of samples used f o r . . measurement of thermal a c t i v a t i o n energies f o r c o n d u c t i v i t y . . 60 4.8 Cryostat used f o r measurement of thermal a c t i v a t i o n energies f o r c o n d u c t i v i t y 60 4.9 Sample ho l d e r 61 4.10 Schematic diagram of the set-up f o r measurement of thermal a c t i v a t i o n energies f o r c o n d u c t i v i t y 63 4.11 D e p o s i t i o n r a t e of pure copper f o r v a r i o u s r . f . power l e v e l s at the t o t a l pressure of 25 mTorr . . . . . . . . . . . . 68 4.12 Comparison of t h e o r e t i c a l and experimental data regarding the oxygen p a r t i a l pressure r e q u i r e d t o form p a r t i c u l a r copper oxide f i l m s at various r . f . power l e v e l s . . 69 4.13 V a r i a t i o n of r e s i s t i v i t y w i t h r a d i a l d i s t a n c e along copper oxide f i l m s . . 72 4.14 Dep o s i t i o n r a t e of pure copper r . f . s p u t t e r e d i n 100% Ar, normalized w i t h respect to the d e p o s i t i o n r a t e at the centre of the s u b s t r a t e t a b l e . . . . . . . . . . 74 o 5.1 Transmittance curves f o r two samples w i t h t h i c k n e s s 1080A o o and 540A, n = 1.5 and X = 4000A 84 s 5.2 Transmittance curves f o r two samples w i t h thicknesses of 1080A and 360A, n = 1.5 and X = 6000A . 86 • s . 5.3 Transmittance curves f o r two samples w i t h t h i c k n e s s 1080A and 360A, n = 1.5 and X = 6000A 87 • s • 5.4 Transmittance and r e f l e c t a n c e curves f o r f i l m w i t h 0 thi c k n e s s of 1080A 88 5.5 Transmittance and r e f l e c t a n c e curves f o r f i l m w i t h o o t h i c k n e s s of 1000A, n = 1.5 and X = 5000A . 91 v i i i 5.6 O p t i c a l p r o p e r t i e s of Cu.O + Cu 1030A t h i c k f i l m deposited on S u p r a s i l 2 su b s t r a t e at 200W of r . f . power and 0.32 mTorr of oxygen 95 o 5.7 O p t i c a l p r o p e r t i e s of Cu^O 1010A t h i c k f i l m deposited on S u p r a s i l 2 s u b s t r a t e at 200W of r . f . power and 0.36 mTorr of oxygen. The f i l m r e s i s t i v i t y = 91 ohm cm 96 o 5.8 O p t i c a l p r o p e r t i e s of the Cu 20 1050A t h i c k f i l m deposited on the S u p r a s i l 2 s u b s t r a t e a t 200W of r . f . power and 0.4 mTorr of oxygen. The f i l m r e s i s t i v i t y = 48 ohm cm . . . . 97 o 5.9 O p t i c a l p r o p e r t i e s of the Cu 20 1050A t h i c k f i l m deposited on the S u p r a s i l 2 su b s t r a t e a t 200W of r . f . power and 0.35 mTorr of oxygen. The f i l m r e s i s t i v i t y = 892 ohm cm . . . 98 o 5.10 O p t i c a l p r o p e r t i e s of Cu 20 + CuO 1050A t h i c k f i l m deposited on the S u p r a s i l 2 sub s t r a t e at 200W of r . f . power and 0.7 mTorr of oxygen 99 o 5.11 O p t i c a l p r o p e r t i e s of CuO 1200A t h i c k f i l m deposited at 200W of r . f . power and 6 mTorr of oxygen 100 5.12 Dependence of the absor p t i o n c o e f f i c i e n t on the i n c i d e n t photon energy f o r v a r i o u s copper oxide f i l m s 101 5.13 Dependence of the absorption c o e f f i c i e n t on the photon energy of i n c i d e n t l i g h t f o r cuprous oxide f i l m s of d i f f e r e n t r e s i s t i v i t e s . 102 6.1 Substrate t a b l e sample h o l d e r 113 6.2 Mask mounting f o r Cu 20/Si h e t e r o j u n c t i o n formation 114 6.3 Metal mask f o r Cu 20/Si h e t e r o j u n c t i o n formation 115 6.4 Met a l mask f o r s o l a r c e l l top m e t a l l i z a t i o n 115 6.5 Mask f o r S i 0 2 windows formation . 115 6.6 He t e r o j u n c t i o n Cu 90/Si s o l a r c e l l c o n f i g u r a t i o n 116 i x 6.7 Dark and illuminated (100 mW cm i) I-V characteristics for Cu20/n-Si diodes fabricated at different sputtering power levels 118 6.8 Illuminated (100 mW cm ) I-V characteristics for Cu^O/n-Si heterojunction diodes fabricated at different sputtering power levels . . 119 6.9 Semilog plot of the forward I-V characteristics for Cu20/n-Si diodes 120 6.10 Schematic energy diagram of a reverse biased sputter-damaged Schottky barrier 121 6.11 Dark reverse I-V characteristics for C^O/n-Si diodes fabricated at different sputtering power levels . . . 123 -2 6.12 Illuminated (100 mW cm ) I-V characteristics for • C^O/p-Si heterojunction diodes fabricated at ' different r . f . power levels . . . . . 124 6.13 Semilog plot of the forward I-V characteristics of Cu20/p-Si diodes . . ; 125 A.l Vertical furnace for thermal oxidation and single crystal growth of Cu^O . 130 A.2 Temperature profile of the oxidation furnace at 1.48 1/min of air flow 131 A.3 SEM print of Cu^ O shortly after the completion of oxidation process . . . . . 133 A.4 Cross section of partially oxidized Cu plate . . . . 133 A.5 SEM cross section print of completely oxidized Cu f o i l . . . . 134 A.6 Photograph of cuprous oxide plate prepared from 99.999% Cu f o i l 20 mil thick and annealed for 14 hours . . . . . . . . . 134 A.7 Photographs of the C^O samples prepared from 99.99% Cu with 63 ppm of Ni content 135 A.8 Photograph of the Cu20 sample prepared from 99.9% Cu 5 mil thick, annealed for 7 hours 135 A.9 SEM cross section print of completely annealed Cu20 sample . . 137 x LIST OF SYMBOLS r a t i o of the i n i t i a l copper t h i c k n e s s to the thickness of r e s u l t i n g Cu^O surface area of the regions of the evacuated enclosure coated by spu t t e r e d deposits area coated by metal area coated by metal oxide species s u b s t r a t e t a b l e area t a r g e t area constant, see d e f i n i t i o n i n chapter 2.2 re-emission r a t e o f r e a c t i v e gas atoms compositional r a t i o compositional r a t i o c o n t r i b u t e d from r e a c t i v e gas i n atomic form compositional r a t i o c o n t r i b u t e d from r e a c t i v e gas i n molecul. form f i l m t h i c k n e s s s u b s t r a t e t h i c k n e s s i n t e r p l a n a r spacing d e p o s i t i o n r a t e Fermi l e v e l i n the metal q u a s i - f e r m i l e v e l f o r e l e c t r o n s i n the semiconductor c h a r a c t e r i s t i c energy normalized s p u t t e r i n g r a t e constant, see d e f i n i t i o n i n chapter 2.2 f i l l f a c t o r i n c i d e n t f l u x of r e a c t i v e gas atoms at p a r t i a l pressure P x i G(P ) - i n c i d e n t f l u x of r e a c t i v e gas atoms at p a r t i a l pressure P o o h - conduction h o l e I - e l e c t r i c c u r r e n t I - i n c i d e n t l i g h t i n t e n s i t y I - s a t u r a t i o n current (chapter 6) I - reference beam i n t e n s i t y I ^ - r e f l e c t e d l i g h t i n t e n s i t y ERQ - i n t e n s i t y of l i g h t r e f l e c t e d from a reference aluminum m i r r o r I^, - t r a n s m i t t e d l i g h t i n t e n s i t y J g ^ - short c i r c u i t c u r r e n t d e n s i t y k - Boltzmann's constant k - e x t i n c t i o n c o e f f i c i e n t (chapter 5) K - i s o b a r i c thermal constant L - v a r i a b l e oxygen leak r a t e L - e f f e c t i v e specimen - p l a t e d i s t a n c e (Chaper 4) L - c h a r a c t e r i s t i c l ength (chapter 6) M - molecular weight n - number of oxygen atoms i n a volume V (chapter 2.2) n - order of the r e a c t i o n (chapter 2.1) n - r e f r a c t i v e index (chapter 5) n - complex r e f r a c t i v e index n - r e f r a c t i v e index of the s u b s t r a t e s N - number of s i t e s at the t a r g e t occupied by r e a c t i v e gas s p e c i e s - Avogadro's number N g — number of empty s i t e s at the s u b s t r a t e N., - number of metal atoms i n one molecule of compound M N - t o t a l number of s i t e s at the s u b s t r a t e at which r e a c t i v e gas s species might bond x i i N f c(x) - trap d e n s i t y N - trap d e n s i t y at the surface of s i l i c o n N^ , - t o t a l number of s i t e s at the target at which r e a c t i v e gas species might bond O.D. - o p t i c a l d e n s i t y P - p a r t i a l pressure of r e a c t i v e gas d u r i n g s p u t t e r i n g ^ e££ ~ e f f e c t i v e p a r t i a l pressure of r e a c t i v e gas P^ - constant r e l a t e d to l i m i t i n g pressure P Q - i n i t i a l p a r t i a l pressure of r e a c t i v e gas q - e l e c t r o n i c charge . - gas c o n t r i b u t e d from outgassing processes Q^k ~ g a s c o n t r i b u t e d from a i r leakage, s t e a d y - s t a t e permeation and other constant vapor emissions r(T) - s p e c i f i c r a t e of r e a c t i v e species a r r i v a l from the gas phase ( h ^ k j l ^ ) - r a d i u s on the d i f f r a c t i o n p a t t e r n of the r i n g r e p r e s e n t i n g a p a r t i c u l a r c r y s t a l o g r a p h i c plane (h^k^l ^) R - r e f l e c t a n c e from f i l m s i d e R ! - r e f l e c t a n c e from s u b s t r a t e s i d e R ( P Q ) - s p u t t e r i n g r a t e at i n i t i a l p a r t i a l pressure P Q R^ - s p u t t e r i n g r a t e of metal from clean m e t a l l i c t a r g e t R Q X - s p u t t e r i n g r a t e of metal from t a r g e t f u l l y covered by r e a c t i v e gas R^,g - s p u t t e r i n g r a t e from the t a r g e t of atomic oxygen RTMo - f r a c t i o n of oxygen spu t t e r e d from the target i n the form of molecular species RO(A) - r e f l e c t a n c e of the reference aluminium m i r r o r R.O.D. - o p t i c a l d e n s i t y f o r r e f l e c t a n c e s - s p e c i f i c d e n s i t y x i i i S - pumping speed t - time T - absolute temperature T - o p t i c a l transmittance (chapter 4 and 5) T g - r a t e of s o r p t i o n of r e a c t i v e gas by s p u t t e r e d metal d e p o s i t s T,j, - r a t e of s o r p t i o n of r e a c t i v e gas by the t a r g e t T.O.D. - o p t i c a l d e n s i t y f o r transmittance V - voltage V Q C - open c i r c u i t v o l t a g e [V 3 - copper i o n vacancy W - f i l m t hickness x - d i s t a n c e from the surface a - s t i c k i n g c o e f f i c i e n t a - absorption c o e f f i c i e n t f o r l i g h t (chapter 5) T a Q - s t i c k i n g p r o b a b i l i t y f o r r e a c t i v e gas at the t a r g e t a o Mo * s t i c k i n g c o e f f i c i e n t of oxygen at the s u b s t r a t e a - s t i c k i n g c o e f f i c i e n t of metal a t the s u b s t r a t e M - s t i c k i n g c o e f f i c i e n t of metal oxide species at the s u b s t r a t e ot^ - s t i c k i n g c o e f f i c i e n t of metal species at the s u b s t r a t e $ - constant, see d e f i n i t i o n i n chapter 2.2 g' - constant, see d e f i n i t i o n i n chapter 2.2 - constant, see d e f i n i t i o n i n chapter 2.2 ^ - maximum composition r a t i o a v a i l a b l e f o r p a r t i c u l a r metal-oxygen system A^ - r e d i f f u s i o n c o e f f i c i e n t f o r metal species A.. - e f f e c t i v e r e d i f f u s i o n c o e f f i c i e n t f o r metal species M A w - r e d i f f u s i o n c o e f f i c i e n t f o r molecular species Mo x i v e - constant, see d e f i n i t i o n i n chapter 2.2 e - thermal a c t i v a t i o n energy f o r c o n d u c t i v i t y e' - constant, see d e f i n i t i o n i n chapter 2.2 V, - r a t i o of the s p u t t e r i n g r a t e of metal i n molecular form to the t o t a l s p u t t e r i n g r a t e of metal from compound target n - e f f i c i e n c y (chapter 6.1) n - diode q u a l i t y f a c t o r (chapter 6.4) 6 - target surface coverage by r e a c t i v e gas atoms £ - . p r o p o r t i o n a l i t y constant r e l a t i n g the e f f e c t i v e g e t t e r i n g r a t e of s p u t t e r e d deposits to the s p u t t e r i n g r a t e of metal X - wavelength of e l e c t r o n s (chapter 4) A - wavelength of l i g h t p - e l e c t r i c a l r e s i s t i v i t y - e l e c t r i c a l c o n d u c t i v i t y a <j>go - b a r r i e r height - r a t i o of p a r t i a l pressure of oxygen to the s p u t t e r i n g r a t e of copper - constant, see d e f i n i t i o n i n chapter 2.2 xv ACKNOWLEDGEMENT I thank my research s u p e r v i s o r , Dr. David L. P u l f r e y , f o r h i s encouragement, support and c a r e f u l guidance throughout the course o f t h i s research. S p e c i a l thanks are due to Dr. L. Young, Dr. D. Tromans, Mr. H. Hogenboom and Mr. A. L a c i s f o r numerous h e l p f u l d i s c u s s i o n s and to Mr. J . Stuber and Mr. D. F l e t c h e r f o r t e c h n i c a l a s s i s t a n c e . The f i n a n c i a l support of the N a t i o n a l Research C o u n c i l of Canada i s g r a t e f u l l y acknowledged. F i n a l l y , I would l i k e to thank my f e l l o w graduate students i n the S o l i d - S t a t e group f o r h e l p f u l d i s c u s s i o n s and a l s o Miss L i s a O h r l i n g f o r typ i n g t h i s t h e s i s . x v i 1. INTRODUCTION I t i s becoming i n c r e a s i n g l y c l e a r t h a t the methods p r e s e n t l y used f o r the l a r g e s c a l e generation of e l e c t r i c i t y w i l l need to be augmented by other, s o - c a l l e d non-conventional methods, i f f u t u r e , major c r i s e s i n the supply of e l e c t r i c i t y are to be avoided. One of the p o s s i b l e methods that i s r e c e i v i n g much a t t e n t i o n i s p h o t o v o l t a i c s . The goal f o r c o s t - e f f e c t i v e e l e c t r i c i t y generation by t h i s method i s a s o l a r array cost of $500 f o r each r a t e d peak k i l o w a t t of p h o t o v o l t a i c a l l y - p r o d u c e d e l e c t r i c i t y [ 1 ] . The cost of s o l a r c e l l arrays has f a l l e n from $30,000/kW p k to 6 ,000/kWpk i n the l a s t four years and i n t e n s i v e research and development e f f o r t s around the world are seeking to f u r t h e r reduce t h i s f i g u r e . One approach i s to u t i l i z e l a r g e area, f l a t p l a t e s o l a r c e l l s f a b r i c a t e d from t h i n f i l m semi-conductors. Various candidate m a t e r i a l s being i n v e s t i g a t e d are Cu^S, CdS, S i , GaAs, CdZnS, CdTe, Z n . ^ * ItiP, CuInSe2 and s e v e r a l organics [ 2 ] . Another m a t e r i a l that has only r e c e i v e d l i t t l e a t t e n t i o n i s C^O [3-6]. I t i s perhaps s u r p r i s i n g that more work has not been c a r r i e d out on t h i s m a t e r i a l as i t can be formed by the simple thermal o x i d a t i o n of an abundant and inexpensive metal. Also the bandgap of Cu^O i s 2.04 eV and i s d i r e c t , thus suggesting use of C^O e i t h e r as a window m a t e r i a l i n a h e t e r o j u n c t i o n s t r u c t u r e w i t h lower bandgap semiconductor p a r t n e r s , or as.a short wavelength absorbing component i n a tandem c e l l arrangement [ 2 ] , The t r a d i t i o n a l method of thermal o x i d a t i o n of Cu to Cu^O may be s u i t e d t o c e l l s of the l a t t e r type, provided s t a b l e m a t e r i a l of r e s i s t i v i t y c o n s i d e r a b l y lower than p r e s e n t l y a t t a i n a b l e ('v 2500 ohm-cm) can be achieved. However t h i s does not appear too l i k e l y i n view of the d i f f i c u l t i e s i n v o l v e d i n doping thermally grown Cu^O [ 7 ] . The e f f e c t of doping by s u b s t i t u t i o n a l i m p u r i t i e s on Cu^O c o n d u c t i v i t y i s p r e s e n t l y not known and thus only a d e f e c t doping e f f e c t r e s u l t i n g from a nonstochiometry has so f a r been u t i l i z e d i n lowering the r e s i s t i v i t y of Cu^O m a t e r i a l used i n s o l a r c e l l experiments. The usual doping procedure f o r the r m a l l y grown C^O i n v o l v e s the s a t u r a t i o n of Cu^O by oxygen at high temperature (-v 1100°C) fo l l o w e d by an abrupt c o o l i n g of the sample to ^  25°C r e s u l t i n g i n a f r e e z i n g - i n of copper i o n vacancies. Various quenching procedures have been t r i e d [8,9] but a l l have so f a r f a i l e d to produce Cu^O m a t e r i a l w i t h r e s i s t i v i t y low enough to make i t p r a c t i c a l f o r s o l a r c e l l use. The l i m i t a t i o n s would appear to be the nature of the annealing and quenching processes i n as much as they r e s t r i c t the amount of excess oxygen t h a t can be in c o r p o r a t e d i n the sample and the c r y s t a l l i n e p e r f e c t i o n r e s p e c t i v e l y . Thus, to r e a l i z e the p h o t o v o l t a i c p o t e n t i a l of Cu^O, formation technologies other than thermal o x i d a t i o n need to be developed. In p a r t i c u l a r to f u l l y r e a l i z e the t h i n f i l m c a p a b i l i t y of Cu^O a method of d e p o s i t i n g the m a t e r i a l ou a d e s i r a b l e s u b s t r a t e would be p r e f e r a b l e to a method l i m i t e d to "growing" the f i l m on a copper s u b s t r a t e . D e p o s i t i o n of copper oxide f i l m s by r e a c t i v e s p u t t e r i n g i s one p o s s i b i l i t y and t h i s method has recei v e d some a t t e n t i o n i n the past i n stu d i e s on the fundamental processes i n v o l v e d i n both r . f . [24] and d.c. [28] r e a c t i v e s p u t t e r i n g . As would be expected, the composition of the deposited oxide i s a f u n c t i o n of the oxygen p a r t i a l pressure i n the s p u t t e r i n g discharge, although the previous works are not i n agreement as to the r e l a t i o n s h i p between these two p r o p e r t i e s . I t was p o s t u l a t e d by the present author that t h i s c o n t r o l over the m a t e r i a l composition r a t i o may lead to a convenient method of e f f e c t i v e l y doping cuprous oxide and thus a c h i e v i n g the d e s i r e d low values of r e s i s t i v i t y . A c c o r d i n g l y the r e a c t i v e s p u t t e r d e p o s i t i o n of t h i n f i l m s of copper oxide was i n v e s t i g a t e d i n d e t a i l i n t h i s t h e s i s . The r e s u l t s obtained l e d to p o s t u l a t i o n of a comprehensive theory of f i l m d e p o s i t i o n by r e a c t i v e s p u t t e r i n g based on the premise that the deposited f i l m compositional r a t i o i s r e l a t e d to the magnitudes of the f l u x e s of r e a c t i v e gas species and sputtered metal species at the s u b s t r a t e plane. This theory i s described i n chapter 2, both from a general standpoint and f o r the p a r t i c u l a r case of the copper-oxygen system. Chapter 3 describes s u b s t r a t e p r e p a r a t i o n techniques and the s p u t t e r i n g system used i n the experiments. General c o n s i d e r a t i o n s r e l a t e d to f i l m p r e p a r a t i o n techniques l e a d i n g to a high r e p r o d u c i b i l i t y of e x p e r i -mental data are a l s o discussed i n t h i s chapter. The u l t i m a t e aim of t h i s program on copper oxide t h i n f i l m s i s to produce acceptably e f f i c i e n t (5 - 10%) s o l a r c e l l s , but before t h i s can be achieved a number of r e l e v a n t m a t e r i a l p r o p e r t i e s of the deposited f i l m s have to be f i r m l y e s t a b l i s h e d . This task formed the main p a r t of the experimental work described i n t h i s t h e s i s and i s recorded i n chapters 4 and 5. In chapter 4 measurements of phase composition, o p t i c a l t r a nsmittance, e l e c t r i c a l r e s i s t i v i t y and thermal a c t i v a t i o n energies f o r c o n d u c t i v i t y of copper oxide f i l m s are presented. Experiments designed to d i r e c t l y t e s t the proposed theory on r e a c t i v e s p u t t e r i n g are a l s o d e s c r i b e d , and the agreement obtained i s very good. The t h e o r e t i c a l treatment i s then used i n c o n j u n c t i o n w i t h the experimental data to determine the s t i c k i n g c o e f f i c i e n t of r e a c t i v e gas at the s u b s t r a t e , so i l l u s t r a t i n g a novel way of determining t h i s parameter. Chapter 5 i s devoted to d e s c r i b i n g the method used and the r e s u l t s obtained i n determining the o p t i c a l constants of the t h i n copper oxide f i l m s . Knowledge of the o p t i c a l constants i s of importance not only f o r a s s e s s i n g the p h o t o v o l t a i c p o t e n t i a l of copper oxide f i l m s but a l s o f o r e s t a b l i s h i n g a fundamental property of these f i l m s . The measured e l e c t r i c a l and o p t i c a l p r o p e r t i e s of the copper oxide f i l m s suggest t h a t , as f a r as p h o t o v o l t a i c a p p l i c a t i o n s are concerned, a h e t e r o j u n c t i o n diode s o l a r c e l l s t r u c t u r e i n v o l v i n g Cu^O and a lower bandgap semiconductor i s worthy of study. A c c o r d i n g l y the technology o f pr e p a r a t i o n of (X^O/Si h e t e r o j u n c t i o n diodes i s d e s c r i b e d i n chapter 6. F i r s t r e s u l t s of I-V measurements both i n the dark and under simulated s u n l i g h t are presented. The conclusions to be drawn from t h i s work are presented i n chapter 7 and suggestions f o r f u r t h e r work are made. 2. THEORY OF REACTIVE SPUTTERING 2.1 INTRODUCTION There has r e c e n t l y been much d i s c u s s i o n on the fundamental aspects of r e a c t i v e s p u t t e r i n g , e s p e c i a l l y concerning the mechanism, of f i l m formation and the p a r t i c u l a r l o c a t i o n at which the f i l m - b u i l d i n g species are formed [10-16]. A l l three p o s s i b l e l o c a t i o n s have been considered; namely the ta r g e t s u r f a c e , the vapor phase and the s u b s t r a t e . At pressures i n the range t y p i c a l l y used f o r r . f . and d.c. diode s p u t t e r i n g the mean f r e e path of sputtered atoms i s l a r g e 1 cm). Therefore, when a low p a r t i a l pressure of r e a c t i v e gas i s used ('v 1% of t o t a l pressure) , the number of c o l l i s i o n s between sputtered metal species (M) and the r e a c t i v e gas species (0) w i l l be s m a l l . Thus the r a t e of c r e a t i o n of M-0 species i n the vapor phase i s l i k e l y to be much l e s s than the r a t e of a r r i v a l of e i t h e r r e a c t i v e species at the t a r g e t or r e a c t i v e and metal species at the s u b s t r a t e . Obviously the p r o b a b i l i t y of formation of more complicated s p e c i e s , such as ^ 2 ' ^2^ e t c ' ' -*-s e v e n lower. Also the p r o b a b i l i t y of a s y n t h e s i s r e a c t i o n i n the gas phase i s very low because some means f o r the r e l e a s e of the energy of r e a c t i o n as w e l l as the k i n e t i c energy of the atoms i s necessary to prevent a spontaneous decomposition [10]. Thus at the low t o t a l pressures and r e a c t i v e gas concentrations used i n r e a c t i v e s p u t t e r i n g the p o s s i b i l i t y of forming the f i l m - b u i l d i n g species i n the gas phase would appear to be very u n l i k e l y . I t i s known that when s p u t t e r i n g m e t a l l i c t a r g e t s i n a r e a c t i v e gas mixture, an abrupt decrease i n the d e p o s i t i o n r a t e takes p l a c e above a c e r t a i n value of r e a c t i v e gas p a r t i a l pressure [17-21]. This has been 6. a t t r i b u t e d to a r e a c t i o n t a k i n g place at the ta r g e t s u r f a c e . I t has been assumed t h a t , above a c e r t a i n c r i t i c a l pressure of r e a c t i v e gas, the target becomes p a r t i a l l y or f u l l y , depending on the pressure, covered by the compound formed by r e a c t i o n between the r e a c t i v e gas species and the t a r g e t . U s u a l l y , e s p e c i a l l y i n the case of oxides, these compounds have much lower s p u t t e r i n g y i e l d s than t h e i r parent metals, r e s u l t i n g i n a d e p o s i t i o n r a t e which decreases i n p r o p o r t i o n to the area of ta r g e t covered by the compound. During the l a s t decade s e v e r a l models of r e a c t i v e s p u t t e r i n g have been published which seek to e x p l a i n the processes o c c u r r i n g at the t a r g e t ' s surface [17-21], w i t h the aim of modelling and p r e d i c t i n g the changes of s p u t t e r i n g r a t e w i t h p a r t i a l pressure of r e a c t i v e gas. H e l l e r [21] f i r s t presented a model, i n which the r a p i d decrease of the s p u t t e r i n g r a t e above a c e r t a i n pressure of r e a c t i v e gas was ex p l a i n e d i n terms of the d i f f e r e n c e between the s p u t t e r i n g and o x i d a t i o n r a t e at the target surface as a f u n c t i o n of the p a r t i a l pressure of r e a c t i v e gas. La t e r Abe and Yamashina [18] f u r t h e r developed t h i s model and were able to c a l c u l a t e the d e p o s i t i o n r a t e at any p a r t i a l pressure of r e a c t i v e gas f o r a p a r t i c u l a r m e t a l - r e a c t i v e gas system, provided some constants were determined ex p e r i m e n t a l l y . They expressed the normalized s p u t t e r i n g r a t e -2 -1 i n terms of the i n c i d e n t f l u x of r e a c t i v e gas atoms G (P ) [ ™ sec ], the -2 -1 re-emission r a t e of r e a c t i v e gas atoms B [m sec ], the s t i c k i n g c o e f f i c -i e n t a, the i n i t i a l ( i . e . before e n e r g i z i n g the plasma) p a r t i a l pressure of r e a c t i v e gas P [Pa], and the constant P T [Pa] r e l a t e d to some l i m i t i n g pressure, by the f o l l o w i n g r e l a t i o n s : R0?o) " Rox ( a / B ) ( P Q / P L ) n G(P Q) f ( P 0 ) = = 1 - • (2.D R M - R D x 1 + ( a / B ) ( P o / P L ) n G ( P 0 ) where R ( P D ) i s the s p u t t e r i n g r a t e at a given i n i t i a l p a r t i a l pressure of r e a c t i v e gas P Q R ^ t h e s p u t t e r i n g r a t e from a cle a n m e t a l l i c t a r g e t , and R Q X the s p u t t e r i n g r a t e of the metal from the ta r g e t f u l l y covered by a r e a c t i v e gas s p e c i e s . The values f o r a/B, P L and n (the s o - c a l l e d order of the r e a c t i o n [18]) can be determined from experimental data f o r R ( P D ) , Rj4 and R O X r e s p e c t i v e l y . The l i m i t i n g pressure P L r e f e r s to the value of p a r t i a l pressure of r e a c t i v e gas at which the r e d u c t i o n i n the s p u t t e r i n g r a t e , due to compound formation at the t a r g e t , s t a r t s to be s i g n i f i c a n t . ? L i s c e r t a i n l y a f u n c t i o n of the metal s p u t t e r i n g r a t e but no attempt was made to r e l a t e these two p r o p e r t i e s . The model a l s o d i d not account f o r the re d u c t i o n i n the pressure of r e a c t i v e gas during s p u t t e r i n g due to i t s absorption by deposits and the t a r g e t . A s i m i l a r model published i n the same year by S h i n o k i and I t o h took i n t o c o n s i d e r a t i o n the abso r p t i o n of r e a c t i v e gas by sputtered metal deposits at the s u b s t r a t e , and the e f f e c t i v e p a r t i a l pressure of reactive-gas during s p u t t e r i n g was expressed by the f o l l o w i n g equation [19] : K N Pe f f = P 0 " - % A T ( 1 - ¥ t ) (2-2) where P Q and R M are as defined i n equation (2.1), A^ i s the target area, S i s the pumping speed, N^is the t o t a l number of s i t e s on the tar g e t at which r e a c t i v e gas species might bond, N i s the number of s i t e s on the ta r g e t a c t u a l l y occupied by a r e a c t i v e gas species and E, i s a p r o p o r t i o n a l i t y constant which r e l a t e s the e f f e c t i v e g e t t e r i n g r a t e of sputtered deposits to the s p u t t e r i n g r a t e of the metal. However, i n the above model no mathematical expression was given f o r the determination of £, n e i t h e r was t h i s f a c t o r c o r r e l a t e d i n a p h y s i c a l sense w i t h other v a r i a b l e s . This model a l s o d i d not account f o r the r e d u c t i o n of r e a c t i v e 8. gas pressure stemming from abs o r p t i o n by the t a r g e t . Target abs o r p t i o n becomes important when a l a r g e number of r e a c t i v e gas species are sputtered from the target i n molecular form (MO, K^O etc.) In a l l the above models i t has been assumed that the e f f e c t i v e metal s p u t t e r i n g r a t e i s a f u n c t i o n of the t a r g e t ' s s u r f a c e coverage by a r e a c t i v e gas sp e c i e s . To express t h i s mathematically l e t 8 be the t a r g e t surface coverage by r e a c t i v e gas atoms (0 = N/N^ ,) , the e f f e c t i v e s p u t t e r i n g r a t e of metal at a t a r g e t surface w i t h coverage 6, and R ox the s p u t t e r i n g r a t e s when N/N^ = 0 (cl e a n m e t a l l i c t a r g e t surface) and N/N^ j, = 1 (the target surface f u l l y covered b y r e a c t i v e species) r e s p e c t i v e l y . The e f f e c t i v e s p u t t e r i n g r a t e of metal i s then given by a l i n e a r combination of R^ and R Q X> namely \ = *M " ( RM ~ O 6 [ m " 2 s e C ~ 1 ] ( 2 ' 3 ) Another expression i n v o l v i n g 0 stems from c o n s i d e r a t i o n of the r a t e of change of the number of s i t e s at the ta r g e t occupied by r e a c t i v e gas s p e c i e s , i . e . H = C£G(P) ( 1 - 6 ) - B 0 ( 2 . 4 ) T where i s the s t i c k i n g p r o b a b i l i t y f o r r e a c t i v e gas at the ta r g e t and B i s the reemission r a t e of r e a c t i v e gas atoms when the t a r g e t i s f u l l y - 2 - 1 covered by r e a c t i v e species [m sec ]. The f i r s t term on the r i g h t hand si d e of equation ( 2 . 4 ) expresses the r a t e of r e a c t i v e gas tra p p i n g on the tar g e t surface and the second term the r a t e of r e a c t i v e gas removal from the ta r g e t s u r f a c e . For steady s t a t e c o n d i t i o n s at the t a r g e t , dN/dt = 0 and from Equation ( 2 . 4 ) . <VG(P) e = — (2.5) B + a T G(P) o thus t h i s value of 6 can be s u b s t i t u t e d i n t o Eq. (2.3) and c a l c u -l a t e d as R^ and R can be e a s i l y determined e x p e r i m e n t a l l y . The above models are u s e f u l i n d e s c r i b i n g c o n d i t i o n s a t the ta r g e t during r e a c t i v e s p u t t e r i n g and as such form a b a s i s f o r developing a more complete model and theory of r e a c t i v e s p u t t e r i n g that would r e l a t e s p u t t e r -i n g c o n d i t i o n s to the p r o p e r t i e s of sp u t t e r - d e p o s i t e d f i l m s . Such a theory i s proposed and described i n the next s e c t i o n . F o l l o w i n g t h i s the theory i s a p p l i e d to a p a r t i c u l a r system (copper-oxygen) thus p r o v i d i n g t h e o r e t i -c a l r e s u l t s f o r the composition of deposited copper oxides which can then be used to guide experiments and to compare w i t h experimental data (chapter 4 ) . 2.2 PROPOSED THEORY OF REACTIVE SPUTTERING In the f o l l o w i n g treatment r e a c t i v e s p u t t e r i n g i n a diode arrange-ment i s considered. Discharge.pressure i s taken to be c o n t r o l l e d on a dynamic b a s i s and the mode of e n e r g i z i n g the plasma i s not s p e c i f i e d ( i . e . i t i s not considered to be s i g n i f i c a n t whether a d.c.'or an r . f . e n e r g i z i n g source i s used). The ta r g e t i s taken to be metal and the r e a c t i v e gas species i s taken, f o r d e f i n i t e n e s s , to be oxygen but the theory so developed i s r e l e v a n t to other elemental r e a c t i v e gases, e.g. N^, H^ e t c . I t i s f i r s t proposed that the r e a c t i o n s i t e f o r the f i l m - b u i l d i n g species i s the s u b s t r a t e and that the deposited f i l m composition i s determined by the r e l a t i v e magnitudes of the f l u x e s of metal and oxygen species sorbed th e r e a t . The a r r i v a l of metal species at the s u b s t r a t e c o n t i n u a l l y creates s i t e s at which oxygen may be sorbed. Thus the f i l m b u i l d s up and under these circumstances the f i l m composition r a t i o can be expressed as 10. [0] oxygen flow sorbed by the f i l m ^ [M] metal flow sorbed by the f i l m a G(P) (N /N ) + A a M C0R ,„ o e s Mo Mo ox (2.6) A A A AM aM \ A where a , a , and a\. are the s t i c k i n g c o e f f i c i e n t s at the s u b s t r a t e of o Mo M oxygen, metal oxide and metal species r e s p e c t i v e l y , N g i s the t o t a l number of a c t i v e s i t e s on the s u b s t r a t e created per u n i t time which can be occupied -2 -1 by r e a c t i v e atoms [m sec ], and N i s the number of these s i t e s which -2 -1 remains empty per u n i t time [m sec ], G(P) i s the f l u x of oxygen sp e c i e s to -2 -1 A the s u b s t r a t e when the discharge i s on [m sec ], i s the e f f e c t i v e r e d i f f u s i o n . c o e f f i c i e n t f o r metal sputtered species and A w i s the r e d i f f u s i o n Mo c o e f f i c i e n t f o r molecular s p u t t e r e d s p e c i e s , 6 (as i n eqn.2.5) i s the f r a c -t i o n of the target covered by oxygen atoms,£ i s the r a t i o of the s p u t t e r i n g r a t e of molecular species to the t o t a l s p u t t e r i n g r a t e ( i . e . of atomic and molecular s p e c i e s ) . A measure of £ has been obtained by Coburn, Taglauer and Kay [23] f o r v a r i o u s metal oxide t a r g e t s u s i n g glow discharge spectrometry (GDS) . I t was found that C, increased w i t h the b i n a r y M-0 bond energy. Because more complex species such as MO^ or were only o c c a s i o n a l l y observed and at very low i n t e n s i t i e s [23], these s p e c i e s , or more s p e c i f i c a l l y the n e u t r a l s MO2, M2^> a r e neglected i n the present treatment and only MO A formation i s considered. The c o e f f i c e n t s A., and A., which appear i n Eq. (2.6) M Mo express the p r o p o r t i o n s of the t o t a l s p u t t e r e d metal or metal oxide species r e s p e c t i v e l y that impinge on the s u b s t r a t e t a b l e , under the c o n d i t i o n of A equal areas of the t a r g e t and the s u b s t r a t e t a b l e . The c o e f f i c i e n t s A and M 11. ^Mo ar:i'-se ^ r o m the. d i f f u s i v e nature of s p u t t e r e d s p e c i e s as they are t r a n s -ported towards the s u b s t r a t e at pressures t y p i c a l l y used i n r . f . and d.c. diode s p u t t e r i n g [26]. However i n most cases, when the t a r g e t - s u b s t r a t e * d i s t a n c e i s much s m a l l e r than the target diameter, A„, A„ can both be taken M, Mo as approximately equal to u n i t y , e s p e c i a l l y at the centre p o r t i o n of the sub-s t r a t e t a b l e . The f i r s t term i n the numerator of Eq.(2.6) represents the amount of oxygen sorbed by the s u b s t r a t e from the gas phase, w h i l e the second term (A„ a.,, C 6 R ) a r i s e s because the t a r g e t can be expected to become p a r t i a l l y Mo Mo ox • • • r r J covered w i t h oxide [18-21], and s p u t t e r i n g from t h i s oxide w i l l y i e l d molecular Species as w e l l as atomic oxygen and metal s p e c i e s . The c o n t r i b u t i o n of the l a t t e r two components to t h e i r r e s p e c t i v e p a r t i c l e f l u x e s are i n c l u d e d i n Eq.(2.6) v i a G(P) and R^ r e s p e c t i v e l y . The e f f e c t i v e s p u t t e r i n g r a t e of metal i s defined as the t o t a l number of metal species l e a v i n g a u n i t area of t a r g e t i n u n i t time, and thus the metal d e p o s i t i o n r a t e at the s u b s t r a t e can be described by the f o l l o w i n g r e l a t i o n : = V M ( 1 - e ) RM + V M ^ - ^ 8 V W 9 R O X • '• • • - 2 - 1 . . . . [m sec ] (2.7) where A w i s the r e d i f f u s i o n c o e f f i c i e n t f o r s p u t t e r e d atomic metal s p e c i e s . M The f i r s t two terms on the r i g h t hand s i d e of equation (2.7) represent metal species sorbed upon the s u b s t r a t e i n t h e i r elemental form. The l a s t term denotes a c o n t r i b u t i o n from the metal species a r r i v i n g i n a bonded metal-oxygen form. Note that G(P) represents the f l u x of elemental oxygen spe c i e s to the s u b s t r a t e under d e p o s i t i o n c o n d i t i o n s , i . e . when the a c t u a l p a r t i a l pressure of oxygen P [Pa] w i l l be somewhat d i f f e r e n t from that pressure P Q [Pa] e s t a b l i s h e d p r i o r to i n i t i a t i o n of the s p u t t e r i n g discharge. Thus, 12. from the k i n e t i c theory of gases we can w r i t e 24 5.25 x 10 P G ( P ) = = r ( T ) P [atoms m s e c ] (2.8) 1/2 (MT) where M i s the molecular weight of oxygen (31.9988), T the gas temperature [K] and r(T) the s p e c i f i c r a t e of r e a c t i v e species a r r i v a l from the gas - 2 - 1 - 1 phase [m sec Pa ] [22]. To develop the theory f u r t h e r i t i s necessary to express the a c t u a l pressure P i n terms of the measured i n i t i a l p a r t i a l pressure P q . This can be achieved by c o n s i d e r i n g the system equation p r i o r to s p u t t e r i n g , i . e . dP, V — - = - P S + L (2.9) dt o . ' 3 3 - 1 where V i s the system volume [m ] , S the pumping r a t e [m sec ] and L the 3 -1 v a r i a b l e oxygen lea k r a t e [m Pa sec ] . In the steady s t a t e dP Q/dt = 0 and thus P = ^ o S (2.10) Now, on a l l o w i n g s p u t t e r i n g to commence the surfaces w i t h i n the b e l l j a r w i l l become coated w i t h metal atoms, l e a d i n g to the p o s s i b i l i t y of oxygen s o r p t i o n and hence a r e d u c t i o n i n the e f f e c t i v e oxygen p a r t i a l pressure. A l s o some oxygen i s sorbed by the t a r g e t . Thus the new system equation becomes it = ~ V S + H " V TT " R T g } ~ A T s . (2.11) where A^ ,, A are the sur f a c e areas of the t a r g e t and of the regions of the evac-2 uated enclosure coated by sput t e r e d deposits r e s p e c t i v e l y [m ] ; T T are i , s the r a t e s of s o r p t i o n of oxygen by the t a r g e t and by sput t e r e d metal on the -2 -1 su b s t r a t e r e s p e c t i v e l y [m sec ] ; R^ , i s the s p u t t e r i n g r a t e from the t a r g e t - 2 - 1 -1 of atomic oxygen [m sec ] ; k i s .Boltzmann's constant [ j o u l e s K ] 13. and n the number of oxygen atoms i n a volume V corresponding to the pressure P assuming i d e a l gas behavior, i . e . PV = nkT (2.12) When steady s t a t e c o n d i t i o n s p r e v a i l at the t a r g e t the r a t e of oxygen reemission from the p a r t i a l l y o x i d i z e d target w i l l equal the rat e of s o r p t i o n of oxygen at the target (i.e..6..reaches•a constant v a l u e ) . I t i s obvious that a l l oxygen sorbed at the t a r g e t s u r f a c e a r r i v e s from the gas phase. On the other hand the oxygen species reemitted from the t a r g e t are i n both atomic and molecular (MO) form and the r a t i o of t h e i r r e l a t i v e f l u x e s depends on an M-0 bond energy as discussed above. Thus the steady s t a t e oxygen balance at the target can be described by the f o l l o w i n g r e l a t i o n : TT " ( R T g + *W = °' or R_._ = T - R r -2 -1, / 0 n O N TMo x Tg [m sec ] (2.13) where R r j ^ i s t n e f r a c t i o n of oxygen sputtered from the t a r g e t i n the form of molecular (MO) sp e c i e s . I t i s obvious that t h i s f r a c t i o n of oxygen does not r e t u r n to the gas phase and thus causes a r e d u c t i o n i n oxygen p a r t i a l pressure, The molecular f r a c t i o n R m w can be r e l a t e d to R , 9 and C by the f o l l o w i n g TMo ox J r e l a t i o n s h i p : \*o = 6 ^ R o x (2.14) I n s e r t i n g Eq. (2.13) i n t o Eq. (2.11) and assuming that the whole s p u t t e r i process i s i n a steady s t a t e (dn/dt = 0) one can w r i t e that 14. V" S + kT AT RTMo A T s (2.15) The r a t e at which oxygen i n the gas phase i s sorbed by deposits i s given by N T =- a G(P) (-£) (2.16) s o N s Combining equations (2.6 and 2.16) the expre s s i o n f o r T g emerges as T = A*Ca*R* - A a M ?6R s M M M Mo Mo ox (2.17) Let us assume that no metal and MO species leave the s p u t t e r i n g chamber during s p u t t e r i n g and a l s o that the deposited m a t e r i a l , r e g a r d l e s s of i t s l o c a t i o n , has a uniform composition. Then i n terms of and , the areas coated by 'metal and metal oxide species r e s p e c t i v e l y , u s i n g Eqns. (2.17) and (2.14) i t f o l l o w s that A T s = V M ^ A * " V W 9 R o x (2.18) because a l l deposits w i t h i n the vacuum chamber are a c t i v e i n the g e t t e r i n g process. I n s e r t i n g equations (2.14 and 2.18) i n t o equation (2.15), then using the r e l a t i o n s given by Eqns. (2.10 and "2.12), and then s o l v i n g f o r the p a r t i a l pressure of oxygen p the f o l l o w i n g r e l a t i o n can be obtained: A A A AJcTCOR A kTCot^R^A A k l a , ?eR A„ / 9 1 Q . P T ox _ M M M M T-lo Mo ox Mo (2.19) o S S S 15. I f no metal and metal-oxide species leave the s p u t t e r i n g chamber, i t i s obvious that * A * * V h = A M V M R M (2.20a) and A_,C6R = A^ . A r,6oi R (? 20b) T ox Mo Mo Mo ox \^.,^UDJ Then, when the target area i s taken to equal the s u b s t r a t e area (A = A ) T s A s AM = ~*~* M M (2.21a) and A . V = A ^ (2.21b) Mo Mo S u b s t i t u t i n g the r e l a t i o n s f o r and A^ given by equations (2.21a and 2.21b) i n t o Eq.(2.19) and again t a k i n g A^ , = A g the f o l l o w i n g r e l a t i o n fc the reduced p a r t i a l pressure of oxygen can be obtained: 6r „ , ' (2.22) A kTCR. P _ P o S [ pa] By s u b s t i t u t i n g Eq.(2.22) i n t o Eq.(2.6) us i n g Eq. (2.8) and s o l v i n g f o r the composition r a t i o C we have: 1 6 . P A a CQ R N a o r(T) P Q V rN s A a M N 'a r(T) e o A kT + _s (2.23) A p a r t i c u l a r metal oxide f i l m w i l l be c h a r a c t e r i s e d by a p a r t i c u l a r value of (N^/N^). Thus f o r a given composition r a t i o , temperature, pumping speed and f o r s t i c k i n g c o e f f i c i e n t s independent of oxygen p a r t i a l pressure and metal s p u t t e r i n g r a t e , i t f o l l o w s that the denominator i n the square bracketed term i n Eq.(2.23) i s a constant. Assuming the same s p u t t e r i n g power dependence f o r both R q x and R^ the numerator i n the square bracketed term w i l l a l s o remain constant f o r a given r a t i o of P /R . Note that i f the r a t i o ^ j / ^ o x ^ s independent of s p u t t e r i n g power then the r a t i o R^/R^ a u t o m a t i c a l l y remains constant, see Eq.(2.3). Hence Eq.(2.23) p r e d i c t s that f o r p r e p a r a t i o n of f i l m s of a given composition r a t i o the discharge c o n d i t i o n s must be such that the r a t i o P ^ / R ^ i s kept constant. This s i t u a t i o n of keeping a constant value of PQ/R^ f° r a range of P q and R^ i s d i f f i c u l t to r e a l i z e i n p r a c t i c e as, f o r example, changes i n P q are not l i k e l y to leave the s t i c k i n g c o e f f i c i e n t s u n a l t e r e d and changing R^ w i l l * a f f e c t the gas and s u b s t r a t e temperatures (and hence a , OL, OL, a l s o ) . How-• o M, Mo ever the appropriateness of equation (2.23) to d e s c r i b i n g the r e a c t i v e s p u t t e r i n g of metal oxides can be appreciated by assuming that R^ i s given by the s p u t t e r i n g r a t e R^ from a c l e a n metal t a r g e t i n pure argon, and that changes i n R^ (caused by a d j u s t i n g the r . f . power l e v e l f o r instance) r e q u i r e corresponding changes i n i n i t i a l oxygen p a r t i a l pressure P q to produce f i l m s of a given composition r a t i o , (see s e c t i o n 4 . 6 ) . 17. To f i n d an absolute r e l a t i o n s h i p between the oxygen p a r t i a l pressure, metal s p u t t e r i n g r a t e and f i l m composition r a t i o , N^/N^ has to be computed. The number of empty s i t e s w i t h i n a f i l m i s given by the d i f f e r e n c e between the t o t a l number of s i t e s able to bond oxygen and the number of oxygen species absorbed by f i l m . Thus during a growth p e r i o d t j - ^ t ^ we have t2 t2 fc2 N / N dt =/ N dt - a G ( P ) / S- dt (2.24) e s O N C l fcl V s The t o t a l number of s i t e s N a v a i l a b l e f o r bonding oxygen can be described s as the d i f f e r e n c e betx^een the number of metal species sorbed by the f i l m during i t s growth, m u l t i p l i e d by y (the maximum number of a v a i l a b l e bonds per metal atom, i . e . y i s e q u i v a l e n t to the maximum composition r a t i o a v a i l a b l e f o r a p a r t i c u l a r metal-oxygen system ), and the number of s i t e s f i l l e d d uring the growth by oxygen a r r i v i n g i n molecular (MO) form. Thus N =YvV - A a r e R (2.25) s ' M M M Mo Mo ox Assuming that the e f f e c t i v e d e p o s i t i o n r a t e of metal R^, the pressure P and the N /N r a t i o remain constant during a d e p o s i t i o n process ( i . e . the e S o r co n d i t i o n s necessary f o r o b t a i n i n g a uniform composition through the f i l m t h i c k n e s s ) , then by combining equations (2.24 and 2.25) and s o l v i n g f o r N /N one can o b t a i n s e ! j L = V ( T ) P (2.26) ' M M M Mo Mo ox 18. S u b s t i t u t i n g Eq.(2.22) f o r P i n Eq.(2.26) and then by i n s e r t i n g Eq.(2.26) i n t o Eq.(2.23) and s o l v i n g f o r the composition r a t i o C the f o l l o w i n g equation can be obtained P R O I „ ox, l * * 2 P P * (y + Ywe + a) — - we £0 —;- ) where P R / . ° , • r> OX 4o)(Y T + Y^ ? e~*~ e ' g " c 2 e 2 R ° x *2^ V (2.27) a tx A a _ S o_ , _ o ,, _ Mo Mo M M Mo Mo M M 1 1 c = . e . = _ . Br(T) B'r(T) Obviously i f Eq.(2.27) i s to be used to guide experiments i n preparing f i l m s of a re q u i r e d C a l l parameters i n the equation have to be known or e x p e r i -mentally c o n t r o l l a b l e . Of these parameters the two l i k e l y to give most d i f f i c u l t y i n determination are 6 and R^. Determination of these i s addressed below. Regarding 6, Q = R M - V RM - R o x (2.28) In many cases 6 can be approximated, namely; at high p a r t i a l pressures of r e a c t i v e gas, when R,^  ~- R then 8 - 1; at very low p a r t i a l pressures, when 19 = R^ then 6 - 0 . However at medium p a r t i a l pressures, when n e i t h e r of these c o n d i t i o n s i s f u l f i l l e d a simple approximation does not work and has to be determined ex p e r i m e n t a l l y or c a l c u l a t e d t h e o r e t i c a l l y , see below. As.mentioned-in s e c t i o n 2.1, when steady s t a t e c o n d i t i o n s p r e v a i l at the t a r g e t , the r a t e of oxygen s o r p t i o n by the t a r g e t i s equal to the ra t e of oxygen removal from the t a r g e t , i . e . from Eq.(2.4) r ( T ) P ( l - 8) = B6 (2.29) B was o r i g i n a l l y defined as the reemission r a t e of r e a c t i v e gas atoms from the t a r g e t when i t was f u l l y covered by the r e a c t i v e gas s p e c i e s . However the reemission r a t e B can a l s o be described as the s p u t t e r i n g r a t e of oxygen atoms from the target f u l l y covered by oxygen atoms. Thus, assuming the same sput-t e r i n g power dependence (or discharge cu r r e n t dependence i n the d.c. s p u t t e r i n g case) f o r both, R and B, one can simply r e l a t e B to R by some constant b ox J ox B = bR °x (2-30) I n s e r t i n g Eq.(2.30) i n t o Eq.(2.29) and s o l v i n g f o r 8 the f o l l o w i n g r e l a t i o n -ship can be obtained: F r ( T ) P = R o x (2.31) R +.Fr(T)P 1 R + Fr(T)P ox ox where T 01 F ° b The r e l a t i o n f o r the e f f e c t i v e s p u t t e r i n g r a t e of the metal at a p a r t i c u l a r p a r t i a l pressure can then be obtained by combining Eqns.(2.3,.2.22 and 2.31), 20. i . e . n ox \ t ~ R 2 ^ rl OX R + F r ( T ) ( P - - R*) ox o w M (2.32) The e f f e c t i v e s p u t t e r i n g r a t e of metal R^ can be c a l c u l a t e d d i r e c t l y from Eq.(2.32), i f C and F are known. In most cases, however, C i s unknown. I t i s then necessary to s u b s t i t u t e Eq.(2.28) f o r 6 i n t o Eq.(2.27) and Eq.(2.27) f o r C i n t o Eq.(2.32) and then s o l v e the complete set of equations i t e r a t i v e l y . In the cases when F i s unknown, i t can be determined e x p e r i -mentally f o r a given metal-oxygen system by d e p o s i t i n g a f i l m of known comp-o s i t i o n and t h i c k n e s s . Then the number of metal atoms w i t h i n a f i l m per u n i t area can be c a l c u l a t e d . D i v i d i n g t h i s value by a d e p o s i t i o n time i n seconds the e f f e c t i v e s p u t t e r i n g r a t e of metal can be obtained. By sub-it s t i t u t i o n of R^ and C values i n t o Eq.(2.32) the value of F can be c a l c u l a t e d . For some metal-oxygen systems having an M-0 b i n a r y bond energy so s m a l l that the c o n t r i b u t i o n of MO species to f i l m formation can be n e g l e c t e d ( i . e . the number of MO species i s only a very s m a l l f r a c t i o n of the t o t a l number of sputtered metal s p e c i e s ) , Eqns(2.23 and 2.27) can be s i m p l i f i e d by o m i t t i n g a l l terms c o n t a i n i n g £. Such a system, which can be c l a s s i f i e d as the low M-0 bond energy system, w i l l be discussed i n the next s e c t i o n . Note that Eqns(2.23 and 2.27) can a l s o be used f o r c a l c u l a t i o n (or estimation) of the concentration i n a deposited f i l m of i m p u r i t i e s from i the gas phase. In t h i s case P q can be replaced by P q , the i n i t i a l p a r t i a l pressure of i m p u r i t y gas. A l s o y i n t h i s case would now represent e i t h e r a maximum composition r a t i o or s o l u b i l i t y f o r a p a r t i c u l a r i m p u r i t y gas-deposited metal (or metal oxide) system. 21. 2.3 APPLICATION OF THE THEORY TO THE REACTIVE SPUTTERING OF COPPER IN OXYGEN To complement the experimental work on the p r e p a r a t i o n of r e a c t i v e l y sputtered t h i n copper oxide f i l m s as described elsewhere i n t h i s t h e s i s the above general theory was a p p l i e d to the p a r t i c u l a r case of the Cu-0 system. I t has already been mentioned i n s e c t i o n 2.2 that the molecular to atomic s p u t t e r i n g r a t i o decreases as the value of b i n a r y M-0 bond energy decreases [23]. A l s o when the elemental p a r t n e r s i n the oxide have a h i g h mass r a t i o , the molecular to atomic s p u t t e r i n g r a t i o tends to have lower values than expected from the pure M-0 bond energy view p o i n t [23]. The molecular bond energy f o r the Cu-0 system i s low ('v 90 Kcal/mole [25]) and thus the species e j e c t e d from the t a r g e t w i l l more l i k e l y be atomic than molecular. From the data of Coburn et a l . [23] i t can be i n f e r r e d t h a t , f o r a completely o x i d i z e d copper t a r g e t t, - 0.02. This i s i n a good agreement w i t h r e s u l t s of Purdes, B o l k e r , B u c c i and Tisone [24] f o r the case of a copper t a r g e t r e a c t i v e l y sput-tered i n an oxygen-neon r . f . plasma. The l a t t e r workers s t u d i e d the nature of the species coming from the target using GDS techniques and found that the CuO s i g n a l was about two orders of magnitude lower than that of Cu . For cases where such a low value of £ a p p l i e s Eqns.(2.23 and 2.27)can be s i m p l i f i e d , as i s shown below. Let us r e w r i t e Eq. (2.6) i n the f o l l o w i n g form: a r(T)PN A„ a, ?6R r _ _£ e Mo Mo ox _ . ,~ . C - + — £ ~ C i + C 2 ( 2 * 3 3 ) KA,^S A M a M ^ MM For values of £<<1 the second term on the r i g h t hand s i d e of Eq. (2.33) can be neglected i n comparison with the f i r s t term provided A*<x*R* i s never much M M M l e s s than A M o a M o e R o x - To show that t h i s i n e q u a l i t y i s u n l i k e l y to be 22. V i o l a t e d i n the system under d i s c u s s i o n , consider the f o l l o w i n g two extreme cases. F i r s t l y , when the p a r t i a l pressure of r e a c t i v e gas i s so high that the target i s f u l l y covered by r e a c t i v e species (6 = 1) then = R Thus Mo Mo C = C. + ? 1 A* o* <2-34> M M I f £ <<1, most of the metal i s sputtered i n the form of atomic s p e c i e s . •k Thus obviously A^-A,,,, see Eq.(2.7). Both A., and A W depend on the mean fr e e M M M Mo c path of sput t e r e d metal atoms and spu t t e r e d metal oxide species r e s p e c t i v e l y . The mean fr e e path of sputtered species decreases w i t h . t h e i r i n c r e a s i n g mass and diameter [26]. Because the masses of Cu and CuO species do not d i f f e r s i g n i f i c a n t l y , and the CuO diameter i s l a r g e r than that of Cu, the mean free path of sputtered CuO species i s sm a l l e r than that of Cu spe c i e s . Thus energies of CuO species w i l l be reduced to the thermal energy of the s p u t t e r i n g gas at s h o r t e r distances from the t a r g e t than energies of Cu spec i e s [26], r-. * r e s u l t i n g i n A M Q < A . The values of a M and ^  at the su b s t r a t e temperature O used (<50 C) are approximately equal to u n i t y and thus A.. OL . A X. CL. Mo Mo Mo Mo c *~ < 1 \ l °M AM KM Thus at high p a r t i a l pressures of oxygen, where 0 ^ 1 and CuO i s being depos-i t e d , we have C = 1; i . e . from Eq.(2.34) C1>>C, and hence from Eq.(2.33) a r(T)PN A a„ C9R o e >> Mo Mo ox VA N s AM aM RM 23. or N N a r(T)P (r^-) > a r ( T ) P ( - ^ ) » A a M £6R o o N o N Mo Mo ox s s (2.35) From Eq. (2.35) one can see that the r i g h t hand s i d e term i n the numerator i n the square brackets of Eq.(2.23) i s much s m a l l e r than u n i t y and th e r e f o r e can be neglected. Eqns (2.23'and 2.27) can then be r e w r i t t e n r e s p e c t i v e l y i n the form and C = A N a s A A kT MM , s N a r(T) e o (2.36) 1 P o 1 P o C = -x (Y + ywe + W - T - ) - — [ ( y + ywe + to ) 2 *M • 2 *M 4ajy - T - ] (2.37) At low p a r t i a l pressures of oxygen, when Cu^O i s being formed f o r example, 6 « 1 , and thus then the term i n v o l v i n g 6 and £ i n Eq.(2.35) becomes even l e s s s i g n i f i c a n t than i n the high pressure case and can be neglected. I t i s encouraging that Eq.(2.36), as obtained by r e d u c t i o n from the general a n a l y s i s , i s i d e n t i c a l to that p r e v i o u s l y derived by Drobny and P u l f r e y [27] from an a n a l y s i s that used Cu-0 system parameters at the outset. I t f o l l o w s from Eq. (2.36) that a p a r t i c u l a r copper oxide f i l m w i l l be c h a r a c t e r i z e d by a p a r t i c u l a r value of N /N , thus f o r a given composition 6 S r a t i o , temperature, pumping speed and f o r s t i c k i n g c o e f f i c i e n t s independent of oxygen pressure and copper s p u t t e r i n g r a t e , i t f o l l o w s that the term i n the square b r a c k e t s i n Eq.(2.36) remains constant. Hence Eq.(2.36) p r e d i c t s that f o r p r e p a r a t i o n of copper oxide f i l m s of a given composition r a t i o the discharge 24. c o n d i t i o n s must be such that the r a t i o P /R^ i s constant. This dependence of composition r a t i o on P q and was u t i l i z e d e x p e r i m e n t a l l y to prepare, i n a very r e p r o d u c i b l e f a s h i o n , copper oxide f i l m s w i t h C ranging, as d e s i r e d , from 0 to 1 (see chapter 4.). Al s o p r e c i s e measurements on f i l m s of a p a r t i c u l a r composition r a t i o (0.5 = Cu^O) allowed determination of the s t i c k i n g c o e f f i c i e n t a and the constant F (see s e c t i o n 4.5). 25. 3. COPPER OXIDE FILM PREPARATION 3.1. GENERAL CONSIDERATIONS Because of the l a r g e amount of heat d i s s i p a t i o n and the chance of i o n i c bombardment of any plasma-exposed s u r f a c e s , there i s u s u a l l y c o n s i d e r -ably more outgassing i n s p u t t e r i n g systems than i n comparable evaporation systems. Thus s e r i o u s contamination of s p u t t e r e d deposits can r e s u l t . In order to avoid or reduce t h i s e f f e c t , a h i g h background vacuum i n the range of 10 ^ Torr or l e s s should be a t t a i n e d before a d m i t t i n g the s p u t t e r i n g gas. The i n i t i a l pressure of contaminant gases should be thus q u i t e low. I n i t i a l p r e s p u t t e r i n g , w i t h the s u b s t r a t e being p r o t e c t e d by a s h u t t e r should take care of a d d i t i o n a l gases f r e e d by i o n bombardment and thermal outgassing from surfaces adjacent to the plasma. P r e s p u t t e r i n g u t i l i z e s the f a c t that a deposited f i l m i s an a c t i v e g e t t e r f o r i m p u r i t i e s . I t has been found [12] that r e a c t i v e gases such as oxygen, n i t r o g e n and water vapor show a f a s t decrease i n c o n c e n t r a t i o n as soon as s p u t t e r i n g i s i n i t i a t e d due to the g e t t e r i n g a c t i o n of f r e s h metal d e p o s i t s . Note that outgassing can be minimized by c o o l i n g and grounding a l l plasma-exposed s u r f a c e s . There are b a s i c a l l y two methods which can be used to m a i n t a i n a required pressure during s p u t t e r i n g . One i s to use a s e a l e d (closed) system arrangement, where the s p u t t e r i n g chamber i s f i r s t pumped down to the d e s i r e d pressure (^  10 ^ Torr) i n the high vacuum range. Then the s p u t t e r i n g gas i s admitted through a metering v a l v e i n t o the chamber to b u i l d - u p the d e s i r e d pressure f o r s p u t t e r i n g . A c l o s e d system method r e q u i r e s the s p u t t e r i n g chamber to be p e r f e c t l y leak proof (a very u n l i k e l y event) i f one wants to avoid." a f u r t h e r pressure b u i l d - u p and contamination of s p u t t e r e d d e p o s i t s . 26. This method, however, i s not s u i t a b l e f o r r e a c t i v e s p u t t e r i n g because the r e a c t i v e gas consumed i n deposited f i l m s i s not r e p l e n i s h e d , r e s u l t i n g i n a continuously decreasing p a r t i a l pressure of r e a c t i v e gas. This i n turn w i l l cause a change i n the f l u x of r e a c t i v e species to the s u b s t r a t e , thus r e s u l t i n g i n compositional v a r i a t i o n through the f i l m t h i c k n e s s . Despite t h i s drawback the closed system has been used i n e a r l y work where t h r o t t l i n g of the d i f f u s i o n pump was presumably not a v a i l a b l e [28]. The other, more common, p r a c t i c e i s to have a continuously pumped system, where the de s i r e d pressure i s maintained by an ad j u s t a b l e l e a k of r e a c t i v e gas. Here, however, one should be aware that at the r e l a t i v e l y high pressures used d u r i n g r . f . and d.c. s p u t t e r i n g the d i f f u s i o n pump i s very i n e f f i c i e n t , unstable and a s i g n i f i c a n t backstreaming of d i f f u s i o n pump o i l w i l l most l i k e l y occur. This i s avoidable by t h r o t t l i n g the d i f f u s i o n pump e i t h e r by only p a r t i a l l y opening the h i g h vacuum v a l v e or by u t i l i z i n g an a d d i t i o n a l t h r o t t l e v a l v e . The pressure i n the vacuum chamber P, the v a r i a b l e leak L and a t h r o t t l e d pumping speed S are r e l a t e d by r L , Q 1 k , % ™ S S S ( 3 . 1 ) where Q ^ i s the gas c o n t r i b u t e d from a i r leakage, s t e a d y - s t a t e permeation or other constant vapor emissions and Q'^(t) i s the gas c o n t r i b u t e d from out-gassing processes. I t i s c l e a r that the c o n c e n t r a t i o n of i m p u r i t i e s w i t h i n the vacuum chamber decreases w i t h i n c r e a s i n g pumping r a t e S. However t h i s i s only u s e f u l i f the s p u t t e r i n g gas used i s of high p u r i t y . I f the concen-t r a t i o n of i m p u r i t i e s i n the s p u t t e r i n g gas i s • t h e dominant f a c t o r which determines the number of im p u r i t y species i n the s p u t t e r i n g atmosphere, then s p u t t e r i n g pressures as low as p o s s i b l e should be used. An a d d i t i o n a l e f f e c t 27. -of high system throughput i s an improved c o o l i n g of elements and f i x t u r e s exposed to heating by the plasma. Considering co n v e n t i o n a l d.c. or r . f . diode s p u t t e r i n g arrangements, the minimum value of discharge pressure i s given e i t h e r by a minimum value of pressure at which the discharge of a p a r t i c u l a r . e x c i t a t i o n type can be maintained, or by a minimum pressure at which a d e s i r e d d e p o s i t i o n r a t e can be obtained. Thus as f a r as the p u r i t y of deposited f i l m s i s concerned,films of b e t t e r q u a l i t y can be ob-tain e d by r . f . r a t h e r than by d.c. s p u t t e r i n g . 3.2 THE SPUTTERING SYSTEM The b a s i c s p u t t e r i n g system used i n a l l experiments reported i n t h i s work was a P e r k i n Elmer Randex type 3140 - 6J system. The evacuation u n i t comprised a 6 - i n c h 2500 1/sec d i f f u s i o n pump ( V a r i a n ) , b a l l a s t e d 500 1/ min mechanical pump (Sargent Welch), 1.5" I.D. f o r e l i n e and roughing tubing system and an a u t o m a t i c a l l y operated solenoid-pneumatic v a l v e system. The c r y o - b a f f l e i n t h i s arrangement was placed between the d i f f u s i o n pump and the baseplate throat of the s p u t t e r i n g u n i t . During a s p u t t e r i n g process the d i f f u s i o n pump was t h r o t t l e d by means of a t h r o t t l i n g v a l v e placed i n the baseplate t h r o a t , (see F i g . , 3.1). I t i s noteworthy that f o r an optimum s p u t t e r i n g system the c r y o - b a f f l e should be placed above the t h r o t t l i n g v a l v e i n order to have maximum pumping speed f o r water vapors l i b e r a t e d during the s p u t t e r i n g process. The system was equipped w i t h a semiautomatic gauge monitoring system w i t h d i g i t a l readout. I n the system as s u p p l i e d i t was p o s s i b l e to continuously monitor the reading of the thermocouple gauge A, which served a l s o as the sensor f o r the automatic v a l v e c o n t r o l system, and any one of e i t h e r the two thermocouple gauges B and C or the i o n i z a t i o n gauge D. The base pressure, which the system r e g u l a r l y achieved a f t e r 28. r.f. MATCHING NETWORK + MODE SELEC-TOR H> Q r.f. (GENERATOR IONIZATION GAUGE MONITOR d.c. POWER SUPPLY J X METERING VALVE ° N 0 H ' j _ VALVE METERING VALVE ON-OFF VALVE F i g . 3.1 Schematic diagram of r e a c t i v e s p u t t e r i n g system used f o r copper oxide f i l m d e p o s i t i o n 29. , >• —8 overnight pumping was i n the order of 4x10 Torr. The s p u t t e r i n g u n i t had a s i n g l e t a r g e t 6 inches i n diameter. Both the target and s u b s t r a t e t a b l e were watercooled. The system's mode s e l e c t o r switch enabled the e n e r g i z i n g r . f . v o l t a g e to be switched to the ta r g e t s o l e l y ( s p u t t e r - d e p o s i t o n mode), or to the s u b s t r a t e t a b l e s o l e l y ( s p u t t e r - e t c h mode) or to both the ta r g e t and the su b s t r a t e ( b i a s - s p u t t e r mode). In the present work only the s p u t t e r -d e p o s i t i o n mode was used. The as - s u p p l i e d r . f . network was used f o r r . f . s p u t t e r i n g . By removing the f i n a l connection to the t a r g e t the l a t t e r could a l s o then be energized from a separate high v o l t a g e d.c. source, thus per-m i t t i n g d.c. s p u t t e r i n g . In order to measure the n e c e s s a r i l y low pressure of r e a c t i v e gas (oxygerj) 5 d e l i b e r a t e l y admitted to the s p u t t e r i n g chamber, an a d d i t i o n a l i o n i z a t i o n gauge E was coupled d i r e c t l y to the s p u t t e r i n g chamber. The gas d i s t r i b u t i o n system, comprising metering v a l v e s , on-off v a l v e s , mixing chamber, and conduits was made from s t a i n l e s s s t e e l . The schematic drawing of the whole system can be seen on F i g . 3.1. 3.3. SUBSTRATE PREPARATION The b a s i c p r o p e r t i e s of the copper oxide f i l m s of i n t e r e s t i n the present work were composition, e l e c t r i c a l r e s i s t i v i t y and o p t i c a l p r o p e r t i e s (see chapters 4 and 5) . Transmission e l e c t r o n microscopy was used to provide i n f o r m a t i o n on f i l m phase composition and the su b s t r a t e s used i n t h i s a p p l i c a t i o n con-s i s t e d of a f i n e copper mesh ( F i g . 3.2) supporting an amorphous carbon f i l m . The carbon f i l m was prepared by evaporating carbon onto a microscope g l a s s s l i d e . The f i l m was then s t r i p p e d from the gl a s s s l i d e by immersing i t i n water. The f l o a t i n g carbon f i l m was then picked up by the copper mesh and allowed to dry. I i [ [ I I \ [ I \ r i r r'f i i . I N I I [._( i 1 MM r n ; L c r 1 i (TL-.rr:c;cr;c i • n ^ r r r r m ; , t i [ L r r r r f E rp -i t r r r x r r r r r i 11 f r e r r reiL. free r r i T r r r r r ' i r T [ - r r r r r r r r r r 30. F i g . 3.2 3:mr. diameter 150 mesli EM ropper g r i d . 25.4 1 (a) (b) F i g . 3.3a Substrate w i t h contact areas used f o r p r e p a r a t i o n of samples f o r d etermination of the thermal a c t i v a t i o n e n e r g i e s of c o n d u c t i v i t y . A l l dimensions are i n m i l l i m e t e r s . F i g . 3.3b Sample f o r thermal a c t i v a t i o n energy of c o n d u c t i v i t y measurements. / 31. Films intended f o r transmittance and r e s i s t i v i t y e v a l u a t i o n were deposited onto Corning 2947 3 " x l " glass s l i d e s . Glass s l i d e s were precleaned P at a temperature between 7 0 - 8 0 C i n a g l a s s c l e a n i n g s o l u t i o n of 2% (by weight) of chromium t r i o x i d e (Cr0 o) i n s u l f u r i c a c i d f o r 3 minutes, f o l l o w e d by r i n s i n g i n d e i o n i z e d water and blowing dry i n n i t r o g e n . In some cases, when some traces of i m p u r i t i e s remained on the s l i d e s , the procedure was repeated. The glass c l e a n i n g s o l u t i o n was prepared by d i s s o l v i n g chromic t r i o x i d e i n the minimum amount of deio n i z e d water necessary to d i s s o l v e the c r y s t a l s completely. Then the s u l f u r i c a c i d was added. The measurement of a c t i v a t i o n energies was done on H a l l e f f e c t shaped f i l m s deposited on e i t h e r glass or alumina s u b s t r a t e s ( F i g . 3.3), w i t h the sample contacts prepared beforehand. This c o n t a c t i n g method was found to be f a s t and r e l i a b l e because the sample was ready f o r measurement immediately f o l l o w i n g a copper oxide f i l m d e p o s i t i o n . I t a l s o avoided the contamination of the f i l m or i t s exposure to an undesired temperature treatment, that may have r e s u l t e d , i f the c o n t a c t i n g procedure had followed the f i l m d e p o s i t i o n O procedure. Substrates were prepared by s p u t t e r i n g f i r s t a ^ 200 A t h i c k o chromium f i l m onto the glass or alumina s l i d e f o l l o w e d by a ^ 2000 A t h i c k gold f i l m . The purpose of the chromium l a y e r was to assure a good adherence of the gold f i l m to the s u b s t r a t e . The s u b s t r a t e was then s c r i b e d i n t o i n d i v i d u a l samples of r e q u i r e d s i z e (1" x 9/16"). The contact area was defined by p h o t o l i t h o g r a p h y , using a p o s i t i v e Waycoat p h o t o r e s i s t . (The complete p h o t o r e s i s t process i s described i n chapter 6). The gold f i l m was then etched away i n a s o l u t i o n of KI + I i n d e i o n i z e d water. The s o l u t i o n was made from 400g K I , lOOg I ^ and 400 ml of d e i o n i z e d water. Subsequently, chromium was etched o f f i n a s o l u t i o n prepared from 330 g of e e r i e ammonium n i t r a t e ((NH,) Ce(N0_) ,) and 100 ml of p e r c h l o r i c a c i d (HC10.) made up to a volume of 2 l i t r e s w i t h d e i o n i z e d water. (Note, when r e s o l u t i o n b e t t e r than 50um i s r e q u i r e d , the s o l u t i o n should be f i l t e r e d ) . The measurement of r e f l e c t a n c e and transmittance of f i l m s f o r determination of o p t i c a l constants was done on S u p r a s i l 2 (fused q u a r t z -fused s i l i c a m a t e r i a l made by A m e r s i l Inc.) s u b s t r a t e s . The S u p r a s i l 2 sub-s t r a t e s were precleaned by using the same procedure as used f o r glass s l i d e s . Copper oxide f i l m s were a l s o deposited onto S i s u b s t r a t e s i n order to form h e t e r o j u n c t i o n diodes. The s u b s t r a t e p r e p a r a t i o n procedure f o r t h i s case i s d e s c r i b e d i n Chapter 6. 3.4 DEPOSITION CONDITIONS Most of the experimental work was performed us i n g r . f . r e a c t i v e sput-t e r i n g but, as described i n Chapter 4.,d.c. r e a c t i v e s p u t t e r i n g was a l s o i n v e s t i g a t e d . The general procedure f o r r e a c t i v e s p u t t e r i n g w i t h e i t h e r v o l -tage source was very s i m i l a r and i s described below. The t a r g e t used was made of 14 m i l t h i c k 99.999% copper f o i l fastened to the water cooled copper t a r g e t holder by four screws. The target-anode di s t a n c e was f i x e d at 5cm. I t f o l l o w s from the t h e o r e t i c a l r e s u l t s of Chapter 2.that the f i l m composition i s determined by the r e l a t i v e magnitudes of the f l u x e s of copper and oxygen species at the s u b s t r a t e . I t i s a l s o known that the s p u t t e r i n g y i e l d from a p a r t i c u l a r t a r g e t m a t e r i a l depends on the c o n d i t i o n of i t s s u r f a c e . I t was observed t h a t , f o r given d e p o s i t i o n c o n d i t i o n s , a new copper t a r g e t w i t h a smooth su r f a c e always showed a somewhat higher s p u t t e r i n g r a t e than a t a r g e t e x h i b i t i n g the s u r f a c e t e x t u r e developed by pro-longed s p u t t e r i n g . Therefore, whenever a new t a r g e t was used, i n t e n s i v e pre-s p u t t e r i n g had to be c a r r i e d out to.ensure attainment of c h a r a c t e r i s t i c s u r -face t e x t u r e and thus the s t a b i l i z a t i o n of the s p u t t e r i n g y i e l d from the t a r g e t 33. and the r e p r o d u c i b i l i t y of the deposited f i l m s . Another parameter which can s i g n i f i c a n t l y change the r a t i o of f l u x e s of copper and oxygen species im-pin g i n g on the su b s t r a t e from the value expected from the i n i t i a l p a r t i a l pressure of oxygen and the s p u t t e r i n g power l e v e l i s the o x i d a t i o n s t a t e of the target s u r f a c e . In a l l cases p r i o r to an a c t u a l d e p o s i t i o n of a copper oxide f i l m onto the s u b s t r a t e i t was necessary to ensure that the target was i n the ste a d y - s t a t e c o n d i t i o n , otherwise the f i l m composition was not repro-d u c i b l e . That i s , the o x i d a t i o n r a t e and oxygen removal r a t e at the target surface had to be arranged to be equal. This s t e a d y - s t a t e c o n d i t i o n at the target can be achieved by p r e s p u t t e r i n g of the ta r g e t at the same r . f . power (or c u r r e n t i n the d.c. s p u t t e r i n g case) l e v e l and oxygen pressure as i n -tended f o r use i n f i l m d e p o s i t i o n . The p r e c i s e procedure used f o r t a r g e t p r e s p u t t e r i n g depended on the nature of the previous d e p o s i t i o n . For ex-ample" i f i t was intended to perform a d e p o s i t i o n at an oxygen p a r t i a l pressure equal to or higher than the previous oxygen p a r t i a l pressure used and at an r . f . power (or the curr e n t i n the d.c. case) l e v e l equal to or lower than the r . f . power (or d.c. c u r r e n t ) l e v e l p r e v i o u s l y used, „ then the target could be simply e q u i l i b r a t e d by p r e s p u t t e r i n g at the r . f . power (or the current i n a d.c. s p u t t e r i n g case) and the oxygen pressure to be used during the a c t u a l d e p o s i t i o n , provided the s h u t t e r covered the subs t r a t e . The same pocedure could be a p p l i e d when the previous d e p o s i t i o n was done at a s l i g h t l y higher p a r t i a l pressure of oxygen and the same or s l i g h t l y lower r . f . power (or current i n d.c."case) l e v e l than the intended one. However, when the previous p a r t i a l pressure of oxygen was s i g n i f i c a n t l y higher or the s p u t t e r i n g power (or the curr e n t i n d.c. case) l e v e l much lower than those intended to be used, then the target oxide coverage had to be reduced by l a r g e amount. I t takes a much longer time to reduce the target oxide coverage than i t does to b u i l d i t up. This i s due to a much lower 34. s p u t t e r i n g r a t e of copper oxide than of the metal copper. The recommended procedure i n t h i s case i s to pr e s p u t t e r the ta r g e t i n pure argon u n t i l the oxide l a y e r completely disappears. Then one can u t i l i z e the f i r s t procedure, as described above. Let us summarize these c o n s i d e r a t i o n f o r s i m p l i c i t y . One can denote by the r a t i o of p a r t i a l pressure of oxygen to the s p u t t e r i n g r a t e of copper: P (3.2) Then the f i r s t procedure f o r target e q u i l i b r a t i o n can be a p p l i e d i f ^ 1 ^ 2 ' w ^ e r e AN<^ ^2 r e P r e s e n t s previous and intended d e p o s i t i o n c o n d i t i o n s r e s p e c t i v e l y . In a l l cases when ij;^ i s s i g n i f i c a n t l y higher than i / ^ j the combination of pure argon p r e s p u t t e r i n g f o l l o w e d by presput-t e r i n g at intended c o n d i t i o n i s recommended. I t i s noteworthy that i n a l l cases the p r e s p u t t e r i n g , except the pure argon t a r g e t p r e e t c h , has to be done at the same c o n d i t i o n s as an intended d e p o s i t i o n . The way to recognize whether the ta r g e t was i n a steady-state c o n d i t i o n was d i f f e r e n t f o r d.c. and r . f . s p u t t e r i n g modes but i n both cases the dependence of plasma imped-ance on the c o n d i t i o n of the ta r g e t surface was u t i l i z e d . . The discharge current (and thus a l s o the plasma impedance) i s a complex f u n c t i o n of ap p l i e d anode-cathode v o l t a g e , the t o t a l pressure i n the s p u t t e r i n g chamber, the volume i o n i z a t i o n c o e f f i c i e n t , which depends on the s p u t t e r i n g gas, and the i o n - e l e c t r o n emission c o e f f i c i e n t under discharge c o n d i t i o n s , which depends on the cathode m a t e r i a l and s p u t t e r i n g gas [29]. I f the ta r g e t i s not i n a steady s t a t e c o n d i t i o n , i t s coverage by oxide i s changing w i t h time. I f <ip , the ta r g e t area covered by oxide reduces and r e s u l t s i n an 35. increase i n p a r t i a l pressure of oxygen i n the s p u t t e r i n g chamber. I f ^2 >^1' t* i e t a r S e t o x i d e coverage b u i l d s up, r e s u l t i n g i n the trapping-of oxygen by the ta r g e t and i n a decrease of oxygen p a r t i a l pressure w i t h i n the chamber. Thus i n e i t h e r of the above cases both the volume i o n i z a t i o n c o e f f i c i e n t and the i o n - e l e c t r o n emission c o e f f i c i e n t are a f f e c t e d by non-e q u i l i b r i u m c o n d i t i o n at the ta r g e t . In the d.c. s p u t t e r i n g case the non e q u i l i b r i u m c o n d i t i o n at the target i s observed by discharge c u r r e n t i n s t a b i l i t i e s and i n the r . f . s p u t t e r i n g case by i n s t a b i l i t i e s of r e f l e c t e d power r e s u l t i n g from the r . f . network mismatch . In instances when the deposited f i l m s were intended f o r measurement of transmittance and room temperature r e s i s t i v i t y , the sub s t r a t e s were supported d i r e c t l y by the water cooled s u b s t r a t e t a b l e . The a c t u a l copper oxide f i l m d e p o s i t i o n times were i n t h i s case l i m i t e d to 2 minutes i n order to prevent a s i g n i f i c a n t r i s e i n su b s t r a t e temperature. (Note that the de-composition temperature f o r Cu^O at the p a r t i a l pressure of oxygen used f o r d e p o s i t i o n of cuprous oxide i s only a few hundred degrees Celsius ). When the s u b s t r a t e i s j u s t l o o s e l y supported by the s u b s t r a t e t a b l e the thermal conductance between the subs t r a t e and the su b s t r a t e t a b l e i s very l i m i t e d . Therefore s i g n i f i c a n t heating of the su b s t r a t e by the plasma i n t h i s case i s l i m i t e d only by the thermal c a p a c i t y of the s u b s t r a t e . In the cases when a longer d e p o s i t i o n time was r e q u i r e d , b e t t e r thermal conductance between the s u b s t r a t e t a b l e had to be ensured. The problem was solved by p l a c i n g a t h i n l a y e r of Corning h i g h vacuum s i l i c o n e grease between the s u b s t r a t e and the custom-made s u b s t r a t e t a b l e (see F i g . 3.4.). When usin g s i l i c o n wafers as sub s t r a t e s these were contacted to the s u b s t r a t e t a b l e using g a l l i u m as a bonding m a t e r i a l . 0 36. i 1 F i g . 3.4a Custom-made sub s t r a t e t a b l e e n a b l i n g e f f i c i e n t c o o l i n g of samples during t h e i r d e p o s i t i o n . A l l dimensions are i n inches. 37. SILICONE VACUUM GREASE F i g . 3.4b Substrate t a b l e sample holder as used f o r d e p o s i t i o n of samples f o r determination of thermal a c t i v a t i o n energies f o r c o n d u c t i v i t y . 38. The standard copper oxide f i l m d e p o s i t i o n procedure comprised the f o l l o w i n g steps: Pumping down the b e l l j a r to a base pressure of 5 x 10 ^ Torr or l e s s as i n d i c a t e d by the i o n i z a t i o n gauge E ( F i g . 3.1) coupled d i r e c t l y to the vacuum chamber. T h r o t t l i n g the d i f f u s i o n pump by t u r n i n g the t h r o t t l e v a l v e i n t o i t s minimum conductance p o s i t i o n , w h i l e watching the reading of the i o n -i z a t i o n gauge E. I f the reading went up more than 20 - 30%, more pumping time had to be allowed. When the pressure s t a b i l i z e d at a value equal to, or lower than, 6 x 10 Torr w i t h the t h r o t t l e valve i n i t s minimum conductance p o s i t i o n , the next step could be fo l l o w e d . Switching the oxygen on-off v a l v e i n i t s "on" p o s i t i o n and a d m i t t i n g the oxygen alone at such a r a t e that the d e s i r e d p a r t i a l pressure was reached. The leak r a t e was adjusted by means of the oxygen metering v a l v e . Oxy.Ren pressures below 2 mTorr were measured by the i o n i z a t i o n gauge E and higher pressures were measured by the thermocouple gauge A. Admitti n g argon at such a r a t e as to b u i l d up the t o t a l pressure to 25 mTorr i n the r . f . s p u t t e r i n g case, or 75 mTorr i n the d.c. sput-t e r i n g case. r . f . s p u t t e r i n g : Turning the r . f . plasma on and a d j u s t i n g the r . f . power l e v e l as d e s i r e d . Then p r e s p u t t e r i n g the target f o l l o w i n g the p r e s p u t t e r i n g procedure as p r e v i o u s l y described w i t h the s h u t t e r p r o t e c t i n g the sample. A f t e r s t a b i l i z a t i o n of the r e f l e c t e d power the a c t u a l d e p o s i t i o n was i n i t i a t e d by removing the s h u t t e r from i t s s u b s t r a t e p r o t e c t i n g p o s i t i o n and f i n e tuning the forward power to the d e s i r e d l e v e l and the r e f l e c t e d power to a minimum once again. 3 9 . d.c. s p u t t e r i n g : A d j u s t i n g the discharge c u r r e n t to 70 mA and f o l l o w i n g the p r e s p u t t e r i n g procedure as described above w i t h the s h u t t e r p r o t e c t i n g the sample u n t i l the discharge current s t a b i l i z e d . Then the s h u t t e r was removed, the discharge current was readjusted to the 70 mA l e v e l ( i . e . an anode-cathode v o l t a g e of from 2.65 - 3.0 kV, depending on the oxygen p a r t i a l pressure) and the d e p o s i t i o n was i n i t i a t e d . 4. PROPERTIES OF COPPER OXIDE FILMS 4.1 FILM THICKNESS MEASUREMENT The thickness of the copper oxide f i l m s was determined by the Fizeau f r i n g e s method on a Sloan M-100 Angstrometer. This method re q u i r e d the formation of an abrupt f i l m step on the sample. Because of the d i f f u s i v e nature of the t r a n s p o r t of sputtered species at the pressures used f o r copper oxide f i l m d e p o s i t i o n s (25 mTorr i n r . f . and 75 mTorr i n the d.c. s p u t t e r i n g case) a sharp enough step, e n a b l i n g an accurate thickness measurement, could not be formed by the simple method of using a metal mask during a f i l m d e p o s i t i o n process. Therefore the edge was formed by an e t c h i n g technique whereby a p o r t i o n of the sample was b r i e f l y dipped i n t o an e t c h i n g s o l u t i o n and etched completely away. (The p o s i t i o n of the f i step on the sample was determined by the depth of immersion i n the e t c h i n g s o l u t i o n ) . The e t c h i n g time was kept as short as necessary to j u s t completely remove the p o r t i o n of the f i l m immersed i n the e t c h i n g s o l u t i o n and t y p i c a l l y ranged from < 1 sec f o r Cu^O + Cu f i l m s to ^ 5 sees f o r CuO f i l m s . Thus the formation of a sharp f i l m step was ensured. E t c h i n g s o l u t i o n s used were HNO^ : H^ O i n volume r a t i o 1:1 f o r the Cu^O e t c h , and HN0 3 : HC1 : H 20 i n volume r a t i o 1 : 2 : 2 f o r the CuO and C^O + CuO etches. In a l l cases the e t c h i n g step was followed immediately by r i n s i n g the sample i n deionized water, subsequent r i n s i n g i n methanol and blowing dry i n a stream of n i t r o g e n . Then to ensure a good r e f l e c t i v i t y from both surfaces of f i l m and s u b s t r a t e r e s p e c t i v e l y the sample was coated on the f i l m s i d e by approximately 2500 A* of copper ( r . f . s p u t t e r e d ) . I t i s note-worthy that d.c. sputtered copper f i l m s were not found to be r e f l e c t i n g enough to ensure a good r e s o l u t i o n of f r i n g e s f o r the thickness measurements. 41. The thickness of t h i c k (.3 - 3 u ) copper f i l m s (deposited f o r the purpose of e s t i m a t i n g the copper s p u t t e r i n g rate) was determined from the mass d i f f e r e n c e between a glass s l i d e w i t h and without the copper f i l m . Assuming the same den s i t y f o r spu t t e r e d and bulk copper, and knowing the area of the f i l m , the thickness could be then c a l c u l a t e d . 4.2 PHASE COMPOSITION The f a c t that the r e a c t i v e l y sputtered copper oxide f i l m s possessed a p o l y c r y s t a l l i n e s t r u c t u r e enabled use of tr a n s m i s s i o n e l e c t r o n d i f f r a c t i o n as the research t o o l f o r i n v e s t i g a t i o n of t h e i r c o m p o s i t i o n a l c h a r a c t e r . The e l e c t r o n d i f f r a c t i o n (E.D.) measurements were performed on an H i t a c h i (110 A model) tr a n s m i s s i o n e l e c t r o n microscope. The e l e c t r o n microscope was used i n the tra n s m i s s i o n e l e c t r o n d i f f r a c t i o n mode on the 75 kV range. With three a c c e l e r a t i o n v o l t a g e ranges a v a i l a b l e , namely 50, 75 and 100 kV, the 75 keV e l e c t r o n s were s e l e c t e d on the b a s i s of the t r a d e - o f f between the t r a n s -parency of 200 - 1100 X t h i c k copper oxide f i l m s to high energy e l e c t r o n s and the p h y s i c a l s i z e of the E.D. p a t t e r n s . The t y p i c a l thicknesses of f i l m s , f o r example deposited by r . f . r e a c t i v e s p u t t e r i n g at 200 W of forward power o o f o r a 2 minute d e p o s i t i o n , were Cu 20 ^ 1000 X, Cu 20 + CuO ^ 800A.and Cu0^225A. The f i l m s were deposited on t h i n amorphous carbon f i l m s (transparent f o r e l e c -trons) supported by a f i n e copper mesh. For d e t a i l s of su b s t r a t e p r e p a r a t i o n see s e c t i o n 3.3. I t i s noteworthy that during a d e p o s i t i o n the su b s t r a t e s had to be placed c l o s e to the centre of the s u b s t r a t e t a b l e . This r e q u i r e -ment stems from the f a c t that the metal (copper) d e p o s i t i o n r a t e decreases towards the outer regions of the s u b s t r a t e t a b l e r e s u l t i n g i n a v a r i a t i o n of f i l m composition w i t h the d i s t a n c e from the s u b s t r a t e t a b l e c e n t r e . 42. Note that i n a l l cases the s u b s t r a t e t a b l e centre was used as the reference p o i n t , to which the measured p r o p e r t i e s of deposited f i l m s and the f i l m d e p o s i t i o n c o n d i t i o n s were c o r r e l a t e d . A l l e l e c t r o n d i f f r a c t i o n patterns were observed and t h e i r photo-graphs taken at as low e l e c t r o n beam i n t e n s i t y as p o s s i b l e , thereby e l i m i -n a t i n g or at l e a s t m i n i m i z i n g the chance of decomposing the f i l m s . I n t e r -planar spacings ("d - values") were obtained from the r e l a t i o n : d ( h . k . l ) = X L / r ( h . k . l . ) (4.D v x x 1 i i i where r,, , _ * i s the r a d i u s on the d i f f r a c t i o n p a t t e r n of the r i n g rep-(h.k.1.) 1 x x r e s e n t i n g a p a r t i c u l a r c r y s t a l l o g r a p h i c plane (h^k_^l_^), A the wavelength of e l e c t r o n s , and L the e f f e c t i v e specimen - p l a t e d i s t a n c e . The camera constant "AL" was determined by using the d i f f r a c t i o n p a t t e r n from a gold f i l m as the reference ( F i g . 4.1e). The procedure was to measure the d i a -meter of a p a r t i c u l a r d i f f r a c t i o n r i n g , m u l t i p l y t h i s value by a c o r r e -sponding i n t e r p l a n a r spacing value from standard tables [30] (comparison of the i n t e n s i t y d i s t r i b u t i o n of observed E.D. r i n g s w i t h the standard normalized i n t e n s i t i e s from ta b l e s helps to i d e n t i f y the plane correspond-i n g to a p a r t i c u l a r observed E.D. r i n g ) and so o b t a i n the. camera constant. B e t t e r accuracy was achieved by repeating the above procedure f o r the f i r s t 4 - 6 r i n g s of the gold e l e c t r o n d i f f r a c t i o n p a t t e r n and c a l c u l a t i n g the average from Eq.(4.2) The camera constant so determined was then taken as the reference i n a l l measurements, and i t s v a l i d i t y was ensured by keeping the current s e t t i n g s of the o b j e c t i v e , intermediate and p r o j e c t i o n lenses the same i n a l l measurements. The phase composition of the deposited copper oxide f i l m s was determined by f i r s t measuring the diameter of d i f f r a c t i o n r i n g s from a p a r t i c u l a r sample, then c a l c u l a t i n g the i n t e r p l a n a r spacings from Eq.(4.1), and comparing these numbers and the r e l a t i v e i n t e n s i t i e s of d i f f r a c t i o n r i n g s w i t h t a b u l a t e d l i s t s of "d"-values f o r known s t r u c t u r e s (Cu, CuO and Cu^O), u n t i l an i d e n t i f i c a t i o n was made. T y p i c a l e l e c t r o n d i f f r a c t i o n patterns f o r f i l m s corresponding to each of the four oxide phases observed are shown i n F i g . 4.1 a-d, and the r e s u l t s of t h e i r i d e n t i f i c a t i o n are summarized i n Table 4.1 a-d. In F i g . 4.1a the f o u r t h r i n g from the center r e v e a l s the presence of m e t a l l i c copper This i s the second most intense copper r i n g . The s t r o n g e s t copper r i n g (111) i s masked somewhat by the (200) cuprous oxide r i n g . In F i g . 4»lc the presence of CuO i s revealed p r i n c i p a l l y by the strong t h i r d d i f f r a c t i o n r i n g from the (111) and (200) CuO planes. The other s t r o n g c u p r i c oxide d i f f r a c t i o n r i n g s , (002) and (111) are masked by the (111) d i f f r a c t i o n r i n g from cuprous oxide. Cuprous oxide i s u n e q u i v o c a l l y i d e n t i f i e d i n F i g . 4.1c by the f i r s t d i f f r a c t i o n r i n g (110) (note i t s absence from F i g . 4. I d ) . Each of the compositions represented by F i g . 4.1 a-d could be obtained w i t h e i t h e r d.c. or r . f . plasma e x c i t a t i o n . In both instances the trend i n f i l m composition w i t h i n c r e a s i n g oxygen p a r t i a l pressure was found to be: Cu Cu + Cu o0 -> Cu„0 -y Cu o0 + CuO -> CuO 44. Table-4.1 Identification of the diffraction rings in Fig. 4.1 45. * Ring No. Cu20 CuO Cu * Ring No. Cu20 CuO 1 110 1 110 2 111 2 111 3 200 111 3 200 4 200 lb 4 211 5 211 5 220 6 220 6 311 7 311 7' 8 8 9 9 1 110 1 110 2 111 002 111) 2 002 111) 3 111 200 J 3 111 200 J 4 200 Id 4 112 5 202 5 202 6 202 6 202 7 220 113 7 113 8 9 311 8 9 311 310 * Starting at the innermost ring. Table 4.2 Oxygen part i a l pressure ranges required for various compositional structures. Phase observed by E.D. Oxygen p a r t i a l pr essure range [in Torr] R.F. Sputtering P = 200 W D.C. Sputtering I = 70 mA Cu 20 + Cu Cu 20 Cu20 + CuO CuO < .3 .34 - .52 .57 - .92 >1.0 < .26 .33 - .51 .62 - .89 >1.06 Fig. 4.1e Electron diffraction pattern taken from gold film. The same trend could be observed, at constant oxygen p a r t i a l p r essure, by decreasing the copper d e p o s i t i o n r a t e . The p a r t i a l pressures of oxygen req u i r e d to o b t a i n the va r i o u s compositions f o r given e x c i t a t i o n c o n d i t i o n s are summarized i n Table 4.2. 4.3 OPTICAL TRANSMITTANCE S'ubstrate p r e p a r a t i o n was as described i n s e c t i o n 3.3 and f i l m s intended f o r o p t i c a l transmittance determination were u s u a l l y deposited at the same time as f i l m s to be used f o r e l e c t r o n d i f f r a c t i o n a n a l y s i s . Thus a good c o r r e l a t i o n between the compositional character and o p t i c a l p r o p e r t i e s of f i l m was assured. The measurement of o p t i c a l d e n s i t y of f i l m s u t i l i z e d a Cary 14 double beam spectrophotometer. I n a l l cases the sample was placed i n the sample beam compartment and the reference beam remained unchanged (no n e u t r a l d e n s i t y f i l t e r s were used i n t h i s case). O p t i c a l d e n s i t y (O.D.) i s r e l a t e d to the i n t e n s i t y of l i g h t i n c i d e n t on the sample I and the i n t e n s i t y of l i g h t l e a v i n g the sample I T by The t r a n s m i s s i o n c o e f f i c i e n t I i s defined as the r a t i o of t r a n s m i t t e d to i n c i d e n t l i g h t i n t e n s i t y O.D. = (4.3) (4.4) Then from Eqns. (4.3 and 4.4) T = 10 -O.D. (4.5) 48. When using the double beam spectrophotometer I i s equal to the reference beam i n t e n s i t y i f the spectrophotometer i s zero-balanced w i t h the sample withdrawn from the sample beam. In a l l cases, when c a l c u l a t i o n of transmittances from o p t i c a l d e nsity data were r e q u i r e d , the o p t i c a l d e n s i t y curves were d i g i t i z e d on an I n s t r o n i c s Gradicon d i g i t i z e r . Transmittances were then c a l c u l a t e d on an IBM 370/168 computer and p l o t t e d as a f u n c t i o n of wavelength on a Calcomp p l o t t e r . The o p t i c a l transmittance data taken from f i l m s deposited at various oxygen pressures i n an r . f . discharge at 200 W are shown i n F i g . 4.2. Films deposited using a d.c. discharge at 70 mA of discharge c u r r e n t showed very s i m i l a r f e a t u r e s . The pure cuprous oxide phase (curve 3 on F i g . 4.2) i s c h a r a c t e r i z e d by a steep absorption edge, as would be expected from a d i r e c t band gap semiconductor m a t e r i a l [31]. The band gap of Cu^O i s 2.04 eV which corresponds to a wavelength of 607 hm. As the oxygen p a r t i a l pressure i s reduced below that r e q u i r e d to form pure C^O (curves 2-1) the f r e e copper content of the f i l m i n c r e a s e s , l e a d i n g to a decrease i n transmittance. On i n c r e a s i n g the oxygen p a r t i a l pressure above that r e q u i r e d f o r pure cuprous oxide formation the nature of the abso r p t i o n curve changes, and tends towards a shallow absorption c h a r a c t e r i s t i c (curve 5 ) , which i s presumably t y p i c a l of sputtered CuO. S i m i l a r l y shaped transmittance curves f o r 880^ t h i c k t her-mally grown CuO f i l m s have been observed by Weider and Czanderna [46] • O p t i c a l transmittance alone ( o p t i c a l constants were determined using both r e f l e c t a n c e and transmittance data, see chapter 5) was u t i l i z e d as a supple-mental research t o o l i n i n v e s t i g a t i n g the compositional character of deposited copper oxide f i l m s . Here the f a c t that Cu^O i s c h a r a c t e r i z e d by a steep absorption edge around 600 nm wavelength, CuO by a shallow absorption curve 49. i.o r 400.0 500.0 600.0 700.0 800.0 wavelength [nin] F i g . 4.2 O p t i c a l transmittance data f o r copper oxide f i l m s deposited by r . f . s p u t t e r i n g at 200W of forward power and t o t a l pressure of 25 mTorr. A l l data normalized to represent r e s u l t s f o r 1000 A t h i c k f i l m s . 50 DISTANCE FROM SUBSTRATE CENTRE [ i n ] 400.0 500.0 600.0 wavelength [nm] 700.0 F i g . 4.3 V a r i a t i o n of o p t i c a l transmittance w i t h r a d i a l d i s t a n c e along copp oxide f i l m (sample //234, see F i g . 4 . 5 ) . F i l m thickness = 950A. Compare data w i t h that of F i g . 4.2 and note the r e d u c t i o n i n Cu content on moving away from the centre of the s u b s t r a t e . 51. and that an increased f r e e copper content i n a copper oxide f i l m causes s i g n i -f i c a n t decrease i n transmittance between 500 - 600 nm was u t i l i z e d . As a p a r t i c u l a r example o p t i c a l transmittance d a t a was used i n seeking an explanation of the v a r i a t i o n of f i l m r e s i t i v i t y across a deposited copper oxide f i l m (see s e c t i o n 4.6). I t was found that due to the nonuniform metal d e p o s i t i o n r a t e w i t h distance from the s u b s t r a t e t a b l e centre ( d e p o s i t i o n ra t e decreasing w i t h distance from the centre of the s u b s t r a t e t a b l e )•, a nonuniform composition of f i l m across the sample r e s u l t e d , thus o b v i o u s l y a f f e c t i n g the f i l m r e s i s t i v i t y (see s e c t i o n 4.6 f o r more d e t a i l s ) . The theory on r e a c t i v e s p u t t e r i n g described i n chapter 2 thus p r e d i c t s an i n -crea s i n g composition r a t i o ([0]/[Cu] w i t h an i n c r e a s i n g distance from the s u b s t r a t e t a b l e centre. I t was suggested that the v a r i a t i o n i n r e s i s t i v i t y across the sample was due to the above e f f e c t and t h i s was confirmed by transmittance measurements. F i g . 4.3 represents t y p i c a l transmittance curves taken from a Cu^O f i l m that was n e a r l y s t o c h i o m e t r i c at the centre p o i n t of the s u b s t r a t e and s u b s t r a t e t a b l e . A decrease i n f r e e copper content i n the f i l m on moving away from the centre of the s u b s t r a t e t a b l e can be seen (compare F i g s . 4.2 and 4.3). Measurement of o p t i c a l transmittance was a l s o u t i l i z e d f o r d e t e r -mination of o p t i c a l constants of copper oxide f i l m s , and t h i s i s discussed i n chapter 5. 4.4 RESISTIVITY The r e s i s t i v i t y measurements were done on the same samples as used f o r transmittance measurements. Here great care was taken to have an obser-v a t i o n of phase composition (by TEM) and a measurement of r e s i s t i v i t y on samples prepared during the same d e p o s i t i o n run, i n order to have a d i r e c t c o r r e l a t i o n between the r e s i s t i v i t i e s and the phases observed. GND CM o w •J H M W CONSTANT CURRENT SCARCE' Q_ (HP-6186) r GND F i g . 4.4 C i r c u i t diagram of the f o u r - p o i n t probe set-up used f o r r e s i s t i v i t y measurements. The f o u r - p o i n t probe technique was chosen as the measurement method f o r r e s i s t i v i t y because of i t s s i m p l i c i t y and speed. For an i n f i n i t e f i l m sheet w i t h thickness much s m a l l e r than the probe spacing the r e s i s t i v i t y of a f i l m can be c a l c u l a t e d from the f o l l o w i n g c u r r e n t - v o l t a g e r e l a t i o n s h i p [49] 53. In2 I V W (4.6) where p i s the f i l m r e s i s t i v i t y , I i s the c u r r e n t f l o w i n g through the outer probes, V i s the v o l t a g e measured between two i n n e r probes and W i s the f i l m t h i c k n e s s . F i g . 4.4 shows the c i r c u i t diagram of the f o u r - p o i n t probe set-up, as used i n r e s i s t i v i t y measurements. The outer probe c u r r e n t was s u p p l i e d by the constant current source HP 6186. Both the v o l t a g e and the c u r r e n t were measured by electrometers ( K e i t h l e y model 602) because of t h e i r high i n -put r e s i s t a n c e and low current measurement c a p a b i l i t i e s . The f o u r - p o i n t probe used was a K u l i c k e & S o f f a instrument, model number 3007. The r e s i s t i v i t y was measured i n the c e n t r a l p o r t i o n 1") of each f i l m . The measurement of thickness u t i l i z e d a Sloan M-100 Angstrometer, as d e s c r i b e d i n s e c t i o n 4.1. Each f i l m composition e x h i b i t e d a c h a r a c t e r i s t i c range of r e s i s t -i v i t i e s as can be seen from F i g . 4.5 f o r r.f.-and F i g . 4.6 f o r d . c . - r e a c t i v e l y sputtered f i l m s . Note the s i m i l a r trends i n v a r i a t i o n of copper oxide f i l m r e s i s t i v i t y w i t h the oxygen p a r t i a l pressure used during d e p o s i t i o n f o r d.c. and r . f . sputtered f i l m s . This suggests that a common mechanism of copper oxide f i l m formation i n both an r . f . and a d.c. r e a c t i v e s p u t t e r i n g environment e x i s t s . However, on comparing the absolute values of r e s i s t -i v i t i e s of f i l m s w i t h the same composition i t can be seen that d.c. s p u t t e r e d copper oxide f i l m s possessed r e s i s t i v i t i e s higher than those of r . f . sputtered f i l m s . This can be a t t r i b u t e d to d i f f e r e n c e s i n both r a d i a t i o n damage (caused by h i g h energy e l e c t r o n bombardment of the f i l m and the sub-s t r a t e ) and i m p u r i t y effects.. For example, l e t us compare some aspects of r . f . and d.c. s p u t t e r i n g where the s p u t t e r i n g c o n d i t i o n s are such that i n both cases approximately equal s p u t t e r i n g r a t e s are obtained. Cases i n p o i n t would be:- f o r d.c. s p u t t e r i n g at 75 mTorr of t o t a l Ar + 0^ pressure 54. 10* 10 10' 7 i o 2 o 6 o 3 i o 1 > •H 4-1 w •H a) 10 -1 10 _2 io h // 234 # 270 X X X X # 2 6 6 — X X X ^ // 261 *~X .// 267 X Cu o0 Cu + Cu 20 ^ _ ] c u 2 0 | C u ; [ CuO 0.1 10 oxygen p a r t i a l pressure [mTorr] F i g . 4.5 R e s i s t i v i t y data f o r f i l m s deposited by r . f . s p u t t e r i n g at 200W and t o t a l pressure of 25 mTorr. The p a r t i a l pressure boundaries f o r the v a r i -ous f i l m compositions are from Table 4.2. The sample numbers shown i d e n t i f y the samples used to produce F i g s . 4.3 and 4.13. 10 10' s u B o •u •H > U w •rl CO CU V-i 10 10 J 10 1 10 -1 10 - 2 X Cu + Cu 20 0.1 x x x x x* X Cu 2o ICu o + CuO X .CuO 10 oxygen p a r t i a l pressure [mTorr] F i g . 4.6 R e s i s t i v i t y data f o r f i l m s deposited by d.c. s p u t t e r i n g at 70 mA and t o t a l pressure of .75 mTorr. The p a r t i a l pressure boundaries f o r the v a r i o u s f i l m compositions are from Table 4.2. 56. and 70 mA of discharge c u r r e n t : - f o r , r . f . s p u t t e r i n g at 25 mTorr of t o t a l pressure and 200 W of r . f . power. Then i f the d.c. case i s compared w i t h the r . f . case, i t fo l l o w s t h a t : - ( i ) . the anode-cathode voltage i s ^  900 V i n the r . f . case and ^ 3kV i n a d.c. case. Thus the d.c. s p u t t e r i n g w i l l r e s u l t i n more s e r i o u s r a d i a t i o n damage to the f i l m caused by high energy e l e c t r o n bom-bardment; ( i i ) . more en e r g e t i c e l e c t r o n s w i l l r e s u l t i n a higher r a t e of emission of i m p u r i t y gases from the b e l l j a r w a l l s and f i x t u r e s . Thus i t w i l l r e s u l t i n higher concentrations of i m p u r i t i e s i n the s p u t t e r i n g chamber; ( i i i ) . the higher t o t a l pressure at the same pumping r a t e r e q u i r e s a higher Ar leak r a t e . Thus,if the c o n c e n t r a t i o n of i m p u r i t i e s i n the argon gas makes a s i g n i f i c a n t c o n t r i b u t i o n to the im p u r i t y species c o n c e n t r a t i o n i n the s p u t t e r i n g chamber, then the d.c. case w i l l be c h a r a c t e r i z e d by a higher c o n c e n t r a t i o n of i m p u r i t i e s i n the s p u t t e r i n g chamber,; ( i v ) . the higher t o t a l pressure used i n the d.c. case a l s o r e s u l t s i n a higher thermal conductance of the s p u t t e r i n g gas and plasma, thus r e s u l t i n g i n a higher heat t r a n s f e r from the cathode to the b e l l : j a r w a l l s and i n s i d e f i x t u r e s . This again could r e s u l t i n greater emission of i m p u r i t y gases. The p a r t i a l pressure boundaries ( F i g s . 4.5 and 4.6) f o r v a r i o u s f i l m compositions were determined by a n a l y s i s of e l e c t r o n d i f f r a c t i o n data. The procedure was described i n s e c t i o n 4.2. At low oxygen p a r t i a l pressures the l a r g e f r e e copper content i n the f i l m s produces m e t a l l i c - l i k e conduction. I n c r e a s i n g the oxygen p a r t i a l pressure leads to a peak i n r e s i s t i v i t y which i s thought to be a s s o c i a t e d w i t h the formation of s t o c h i o m e t r i c cuprous oxide. This c o n c l u s i o n stems from the e x c e l l e n t correspondence between the occurrence of t h i s peak and the onset of the C^O phase. Further i n c r e a s e s i n oxygen pressure r e s u l t i n a lowering of the r e s i s t i v i t y by doping of the f i l m w i t h excess oxygen. I t i s w e l l known that cuprous oxide i s doped by excess oxygen r e s u l t i n g i n the formation of copper i o n vacancies [48]. The doping process i s described by the f o l l o w i n g equation: 1/2 0 2 + 2 Cu 20?=£3 Cu 20 + 2 [ V C u ] + 2h (4.7) where [V^, ] denotes a copper i o n vacancy and "h" i s a conduction h o l e . Doping of t h i s type t h e r e f o r e r e s u l t s i n an enhanced p-type conduction. Attainment of r e s i s t i v i t y values of 20-100 ohm-cm. i n Cu 20 i s encouraging i n view of the f a c t that other p r e p a r a t i o n methods (e.g. thermal o x i d a t i o n of copper [3-6, 32], e l e c t r o d e p o s i t i o n [35]) have not been able to y i e l d s t a b l e cuprous oxide m a t e r i a l of r e s i s t i v i t y l e s s than s e v e r a l thousand ohm-cm. On f u r t h e r i n c r e a s i n g the oxygen p a r t i a l pressure, the co n c e n t r a t i o n of defects reaches a c e r t a i n c r i t i c a l value at which a change i n valency at some l o c a l d e fects occurs and thus the n u c l e a t i o n of c u p r i c oxide begins. The formation of these e l e c t r i c a l l y n e u t r a l d e f e c t s i n tu r n reduces the amount of e l e c t r i c a l l y a c t i v e copper i o n vacancies, thus reducing the conduc-t i v i t y of the semiconducting oxide. The cuprous o x i d e - c u p r i c oxide mixture produces a semiconducting m a t e r i a l of c h a r a c t e r i s t i c r e s i s t i v i t y around 500 ohm-cm. At even higher p a r t i a l pressures of oxygen the CuO phase i s de-p o s i t e d , w i t h r e s i t i v i t y f i r s t decreasing w i t h i n c r e a s i n g p a r t i a l pressure of oxygen and then, above a c e r t a i n value of oxygen pressure, s a t u r a t i n g i n the range 0.1 - 1.0 ohm-cm (see F i g s . 4.5 and 4.6). This r e s u l t i s s u r p r i s i n g , because CuO i s known to be an i n s u l a t o r when grown by the thermal o x i d a t i o n of copper. The f a c t that the CuO f i l m s were semiconducting p-type (deter-mined by t h e r m o e l e c t r i c probe measurement) suggests that excess oxygen i n CuO f i l m s could provide a doping e f f e c t . However, on the other hand, Purdes et a l . [24] have found that CuO f i l m s deposited by r e a c t i v e sput-t e r i n g are oxygen d e f i c i e n t and that the composition r a t i o of CuO sa t u r a t e s -4 at a value of 0.86 at oxygen pressures of 6.25x10 To r r . 58. 4.5 THERMAL ACTIVATION ENGERGIES FOR CONDUCTIVITY In semiconductors i n which donors and acceptors are not f u l l y i o n i z e d i n the temperature range explored, the bulk c o n d c t i v i t y depends, i n general, e x p o n e n t i a l l y on 1/T w i t h a unique a c t i v a t i o n energy as described i n a = o exp(-e /kT) (4.7) o a where a i s the sample c o n d u c t i v i t y , Q the constant of p r o p o r t i o n a l i t y , k Boltzmann's constant, T the sample temperature. The a c t i v a t i o n energy E A can be found by measuring the c o n d u c t i v i t y dependence on temperature and i t s value i s given by the slope of the l n a versus 1/T p l o t i . e . from In a 9 - In a -, (4.8) where a ^ and a 2 a r e t n e c o n d u c t i v i t i e s at temperatures T^ and T^ r e s p e c t i v e l y . In some semiconductors, of which cuprous oxide i s an example, m u l t i p l e a c t i v a t i o n energies can be found. Thus over some temperature ranges more than one a c t i v a t i o n energy a f f e c t s the thermal dependence of c o n d u c t i v i t y . Thus without proper care a f a l s e value of a c t i v a t i o n energy can be determined. A l s o the temperature dependence of hole m o b i l i t y can co n t r i b u t e to an e r r o r i n determining the a c t i v a t i o n energy. To avoid the i n f l u e n c e of both the above mentioned e f f e c t s , only l i n e a r p a r t s of lna versus 1/T p l o t s should be considered. A c t i v a t i o n energy measurements were made on H a l l e f fect-shaped, r . f . sputtered samples as described i n s e c t i o n 3.4 ( F i g . 3.3). Sample 59. shapes were defined by s p u t t e r d e p o s i t i o n of copper oxide f i l m s through a t h i n metal mask ( F i g . 4.7). The mask was made of s t a i n l e s s s t e e l ( 100 um t h i c k ) or brass (70 um t h i c k ) f o i l . The d e s i r e d p a t t e r n was formed by the combination of p h o t o l i t h o g r a p h i c masking techniques and subsequent spray e t c h i n g of the p a t t e r n i n 50 °C commercial f e r r i c c h l o r i d e (FeCl ) s o l u t i o n . In order to achieve a s u f f i c i e n t l y high current (minimum 0.2 ViA)for I-V measurements that could be c a r r i e d by the sample without applying a high b i a s voltage across the sample, the f i l m s used here were much t h i c k e r than those used f o r o p t i c a l and r e s i s t i v i t y measurements. The minimum cur r e n t used f o r a c t i v a t i o n energy measurements was determined by the s i g n a l to noise r a t i o and a l s o by the l i m i t e d a b i l i t y of the constant current source HP 6186 to c o n t r o l at very low c u r r e n t s . The maximum sample b i a s v o l t a g e (o,40 V) was l i m i t e d by the maximum v o l t a g e range of the K e i t h l e y 602 electrometer (10 V), which, was used to measure the v o l t a g e prop-o r t i o n a l to sample r e s i s t i v i t y . The K e i t h l e y 602 was used f o r v o l t a g e 14 measurement because i t o f f e r s a high input impedance c a p a b i l i t y (10 , ohms). No back-off v o l t a g e was used to extend the measured v o l t a g e range because i t introduced inconveniences i n f u r t h e r data p r o c e s s i n g . To o b t a i n samples w i t h the d e s i r e d f i l m thickness of 5,000 -10000 k\ d e p o s i t i o n times of 10-20 minutes (at 200 W of r . f . power) were re q u i r e d . Such long d e p o s i t i o n times r e q u i r e d good sample c o o l i n g during the f i l m d e p o s i t i o n process, otherwise a s i g n i f i c a n t r i s e i n the s u b s t r a t e temperature r e s u l t e d . A good thermal conductance between the s u b s t r a t e and the s u b s t r a t e t a b l e was ensured by employing s i l i c o n vacuum grease (see s e c t i o n 3.4, F i g . 3.4). An excessive shadowing at the edges of the metal mask, which i s t y p i c a l f o r r . f . diode s p u t t e r i n g at the pressure used (25 mTorr) 60. 28.5 F i g . 4.7 Metal mask f o r d e p o s i t i o n of samples used f o r measurement of thermal a c t i v a t i o n energies f o r c o n d u c t i v i t y . A l l dimensions are i n m i l l i m e t -res , SAMPLE HOLDER THERMAL INSULATION SAMPLE COPPER COLLAR CONNECTORS SAMPLE HEATER DEWAR LIQUID NITROGEN SAMPLE HOLDER PLATE THERMAL INSULATION COPPER TUBE EVAP. RESISTOR F i g . 4.8 Cryos t a t used f o r measurement of thermal a c t i v a t i o n energies f o r c o n d u c t i v i t y . CONNECTOR CERAMIC (ALUMINA) TUBING HOLDER BERGLASS IP THERMOCOUPLE F i g . 4.9 Sample holder 62. [26], was avoided by contact mounting the mask on the s u b s t r a t e u s i n g four s t e e l s p r i n g s , as shown i n F i g . 3.4. Measurement of the temperature dependence of c o n d u c t i v i t y over the range 90-400 K was performed i n the simple c r y o s t a t shown i n F i g . 4.8. Heat developed by the r e s i s t o r immersed i n l i q u i d n i t r o g e n caused evaporation of n i t r o g e n from the f l a s k through the 1" I.D., 1/8" t h i c k copper tubing a l s o immersed i n l i q u i d n i t r o g e n . The e n t i r e copper p i p e , being a good thermal conductor, was thus held at a temperature very c l o s e to 77 K. C o o l i n g of the sample holder and sample was provided by the a c t i o n of the n i t r o g e n gas passing through the copper tubing. The e f f e c t i v e n e s s of the c o o l i n g a c t i o n was d i r e c t l y p r o p o r t i o n a l to the evaporation r a t e of n i t r o g e n . This arrange-ment of a continuous flow of n i t r o g e n a l s o prevented water condensation on the sample s u r f a c e . When a sample temperature r i s e above room temperature was de-s i r e d , the evaporator r e s i s t o r power was switched-off and the heater c o i l ( part of the sample ho l d e r assembly) power was switched-on. The d e s i r e d r a t e of temp-erature r i s e was adjusted by the l e v e l of the power s u p p l i e d to the heater c o i l . Good thermal conductance between the sample and the s u b s t r a t e p l a t e was ensured by applying a t h i n l a y e r of thermal compound(Wakerfield 120-2) and by p r e s s i n g down the sample by the f i b e r - g l a s s c l i p , see F i g . 4.9. A l l e l e c t r i c a l con-nections to a sample were achieved by a standard PbSn s o l d e r . The sample temp-erature was measured using a copper-constantan thermocouple fastened between two mica p l a t e s by a copper c l i p to the s u b s t r a t e p l a t e . The reference j u n c t i o n was kept at 0°C i n a w a t e r - i c e mixture. Data was recorded on an X-Y p l o t t e r (model HP 7044). Sample temp-e r a t u r e , i n the form of the thermocouple output v o l t a g e , was d i s p l a y e d on the X channel and sample r e s i s t i v i t y , i n the form of the v o l t a g e V between sam-o p i e terminals a,b (see F i g . 4.10), was d i s p l a y e d on the Y-channel. 63. D.C. POWER SUPPLY °-0 - 40V Q SAMPLE HEATER (KEITHLEY 602 or FLUKE 8000) f f f i l uA KFJ%. .... U ft b V Cu-Constantan THERMOCOUPLE YREF n o ERENCE JUNCTION 0 °C CONSTANT CURRENT SOURCE i t (HP 6186) X X-Y PLOTTER (HP 7044) (KEITHLEY 602) F i g . 4.10 Schematic diagram of the setup f o r measurement of thermal a c t i v a t i o n energy f o r c o n d u c t i v i t y . 64. Because the input impedance of the X-Y p l o t t e r was so low 10 Mohm) and might cause s i g n i f i c a n t l o a d i n g of the sample v o l t a g e V , e s p e c i a l l y at a lower temperatures and f o r the higher r e s i s t i v i t y samples, the input of the Y-channel was i n t e r f a c e d w i t h the sample through a K e i t h l e y 602 electrometer. The sample current was measured e i t h e r by a K e i t h l e y 602 electrometer or by a Fluke 8000 multimeter. E v a l u a t i o n of r e s u l t s f o r e r e q u i r e d the p l o t t i n g of lna versus l / T , s e l e c t i o n of l i n e a r p a r t s of t h i s curve and then c a l c u l a t i o n of a c t i v a t i o n energies by using Eq. (4.8). The e n t i r e e v a l u a t i o n process f o r a c t i v a t i o n energy c a l c u l a t i o n was mechanized as f o l l o w s . A l l graphs from the X-Y p l o t t e r were f i r s t d i g i t i z e d on an I n s t r o n i c s Gradicon d i g i t i z e r and t r a n s f e r r e d to IBM cards f o r f u r t h e r processing on an IBM 370/168 computer. The computer program allowed f o r the c a l c u l a t i o n of temperature from thermocouple voltages by using a s p l i n e f u n c t i o n i n t e r p o l a t i o n . A thermocouple voltage-temperature o data s e t f o r 100 p o i n t s between.-200 t o + 250 C was s t o r e d on a f i l e , t o which the program had access. Data output was i n a p r i n t - o u t form f o r 1/T, a and l n a as w e l l as a g r a p h i c a l form, which y i e l d e d a p l o t of l n a versus 1/T. 'From these p l o t s one could then s e l e c t v i s u a l l y the l i n e a r p a r t s of the slope g i v i n g p a r t i c u l a r a c t i v a t i o n energy values. The data i n p r i n t - o u t form from the s e l e c t e d temperature ranges were then used i n Eq. (4.8) to c a l c u l a t e the values of a c t i v a t i o n energies. The c a l c u l a t e d values of a c t i v a t i o n energies f o r c o n d u c t i v i t y , cor-responding to a p a r t i c u l a r oxide phase and temperature range, are summarized i n Table 4.3. For samples deposited at the lowest p a r t i a l pressures of oxygen, v/here the Cu^O + Cu phase i s formed, the r e s i s t i v i t y was found to be almost •independent of temperature i n the range from 77-400 K. Thus no a c t i v a t i o n 65. TABLE 4.3 Thermal a c t i v a t i o n energies f o r c o n d u c t i v i t y of copper oxide f i l m s deposited by r e a c t i v e s p u t t e r i n g . Oxide Phase Temperature Range A c t i v a t i o n Energy [°c] [eV] Cu 20 -60 - -4 0.09 -30 - + 60 0.19 + 50 - + 116 0.23 Cu 20 + CuO -90 - - 56 0.14 •+ 2 - + 47 0.19 + 56 - + 76 0.23 CuO -150 - + 20 0.11 -150 - + 20 0.14 66. energy f o r t h i s c e r m e t - l i k e m a t e r i a l i n t h i s temperature region was determined. Pure r e a c t i v e l y sputtered cuprous oxide i s c h a r a c t e r i z e d by three thermal a c t i v a t i o n energies, namely 0.09, 0.19 and 0.23 eV. Therefore the e l e c t r i c a l conduction of r e a c t i v e l y sputtered cuprous oxide cannot be e x p l a i n e d i n terms of a simple energy l e v e l diagram. In a d d i t i o n the f a c t that f i l m s are poly-c r y s t a l l i n e makes i n t e r p r e t a t i o n of these r e s u l t s even more d i f f i c u l t . Comparison of the above r e s u l t s w i t h r e s u l t s obtained f o r thermally grown Cu^O i s not easy, because a c t i v a t i o n energies f o r thermally grown cuprous oxide depend s t r o n g l y on the thermal h i s t o r y of the measured samples and have been found to have values between 0.1 and 1 eV [47]. For samples of comp-o s i t i o n Cu^O +. CuO two a c t i v a t i o n energy l e v e l s w i t h the same values as f o r Cu^O f i l m s were found. This i n d i c a t e s a s i m i l a r i t y i n the mechanism r e s p o n s i -b l e f o r conduction processes i n both Cu^O and Cu^O + CuO f i l m s . However, the f a c t that a s i g n i f i c a n t p o r t i o n of e l e c t r i c a l l y n e u t r a l defects ( n u c l e a t i o n of CuO) replaces e l e c t r i c a l l y a c t i v e defects (copper i o n vacancies) on i n c r e a s i n g the oxygen p a r t i a l pressure r e s u l t s i n a lowering of the con-d u c t i v i t y and disappearing of the 0.09 eV l e v e l . The new l e v e l (0.14 eV) that appears i s presumably a s s o c i a t e d w i t h formation of CuO. 4.6 COMPARISON OF REACTIVE SPUTTERING THEORY WITH EXPERIMENTAL DATA. To i l l u s t r a t e the v a l i d i t y of the theory proposed i n chapter 2, the r a t i o of ^0'^~^ re q u i r e d f o r formation of n e a r - s t o c h i o m e t r i c cuprous oxide f i l m s was found ex p e r i m e n t a l l y f o r v a r i o u s r . f . power l e v e l s and then compared w i t h t h e o r e t i c a l values c a l c u l a t e d from Eq. (2.36). The d e p o s i t i o n r a t e R^ of copper sputtered from a pure copper t a r g e t i n pure argon was measured at various r . f . power l e v e l s . The standard procedure was to deposit copper f o r a p e r i o d of one hour at the d e s i r e d power l e v e l , and then to estimate the 67. thickness of the deposited f i l m by the weighing method described i n s e c t i o n 4.1. The r e s u l t s obtained are g r a p h i c a l l y presented i n F i g . 4.11 and as described i n s e c t i o n 2.2, can be taken as a measure of the dependence of on r . f . power. Then the r e q u i r e d values of P to produce n e a r - s t o c h i o m e t r i c o Cu^O f i l m s at each r . f . power l e v e l were determined. This n e c e s s i t a t e d many dep o s i t i o n s at each r . f . power l e v e l , each time s l i g h t l y v a r y i n g P and then studying the e l e c t r o n d i f f r a c t i o n data f o r each d e p o s i t i o n u n t i l the occurrence of pure Cu^O f i l m s was i n d i c a t e d . S t o c h i o m e t r i c , undoped cuprous oxide i s an i n s u l a t o r at room temperature and i t i s b e l i e v e d that the sharp peak i n the r e s i s t i v i t y data shown i n F i g s . 4.5 and 4.6 i s i n d i c a t i v e of formation of t h i s m a t e r i a l . C l e a r l y , exact attainment of the r e q u i r e d P^ at each power l e v e l ( t h e r e f o r e s p u t t e r i n g r a t e R^) would r e q u i r e a c o n s i d e r a b l e number of f i l m p r e p a r a t i o n s . The procedure adopted was to prepare 10-15 samples at each r . f / power l e v e l and record the highest value of P q at which the e l e c t r o n d i f f r a c t i o n p a t t e r n of Cu + Cu„0 was observed and the lowest value of P at . 2 o which Cu^O + CuO was detected. The r e s u l t s are p l o t t e d i n F i g . 4.12. Taking the data a t 200 W as references (see a l s o F i g . 4.5 f o r the p r e c i s e extreme values of P_ needed to produce Cu^O) the v a r i a t i o n i n R^ w i t h power l e v e l o ( F i g . 4.11) was used to p r e d i c t the extreme values of P q f o r formation of Cu 20 at r . f . power l e v e l s other than 200 W using Eq. (2.36). These r e s u l t s are shown as the s o l i d l i n e s on F i g . 4.12 w h i l e the e x p e r i m e n t a l l y determined P values are represented by the dots, o Note that i n the above experiments and c a l c u l a t i o n s R r a t h e r than R^ was used because i f was easy to determine R^ at any r . f . power l e v e l (using the above discussed method), w h i l e the determination of R^ re q u i r e d knowledge of the compositon, thickness and d e n s i t y of the deposited f i l m s . Obviously the composition and de n s i t y of the f i l m are p r o p e r t i e s which are 500 400 h 300 h 200 100 h O / 50 100 150 200 250 r . f . forward power [W] F i g . 4.11 Dep o s i t i o n r a t e of pure copper (measured at the center of substrate t a b l e ) f o r v a r i o u s r . f . power l e v e l s . a n d the t o t a l pressure of 25 mTorr. 69. u u o H 0 u 3 CO CO 2 0.3 0-. rH ctf •rH •U r-l ca a a cu to X o Cu20/CuO. 50 100 150 200 r . f . forward power [W] 250 Cu 20/Cu F i g . 4.12 Comparison of t h e o r e t i c a l and experimental data regarding the oxygen p a r t i a l pressure r e q u i r e d to form p a r t i c u l a r copper oxide f i l m s at v a r i o u s r . f . power l e v e l s . © = experimental data i n d i c a t i n g lowest observed p a r t i a l pressures f o r producing Cu20+CuO; O = experimental data i n d i c a t i n g highest observed p a r t i a l pressures f o r producing Cu 20+Cu; s o l i d l i n e s based on constant values of P^R^, w i t h R^ being taken from F i g . 4.11 and reference values of P at 200W taken from Table 4.2. o 70. much more d i f f i c u l t to measure than the thickness and mass of copper f i l m necessary f o r determination of R^. The v a l i d i t y of using i n s t e a d of R^ i n the above c a l c u l a t i o n s was discussed i n chapter 2. Let us now discuss some f a c t o r s i n the denominator of Eq. (2.36) which can i n f l u e n c e the above experiments. P C = •k s M M N a r(T) e o + A kT s (2.36) The value of N g/N e f o r a given composition r a t i o C remains constant, regardless of the d e p o s i t i o n c o n d i t i o n s used to o b t a i n t h i s composition. G e n e r a l l y , the temperature of the s p u t t e r i n g gas can be a f f e c t e d by the r . f . power l e v e l . However, i n a dynamic s p u t t e r i n g system ( c o n t i n u o u s l y pumped system w i t h a constant leak r a t e of the s p u t t e r i n g gas, see s e c t i o n 3.1) having a constant supply of a f r e s h s p u t t e r i n g gas, the i n f l u e n c e of the r . f . power l e v e l (over the range used here)on the s p u t t e r i n g gas temperature can be assumed to be n e g l i g i b l e . Thus the terms r (T) and A gkT/S remain constant provided, i n the l a t t e r case,that the t h r o t t l e d pumping speed S remains un-changed i n a l l experiments. Therefore n e i t h e r r ( T ) , A^kT/S nor N g/N e, f o r a given f i l m composition r a t i o , have an e f f e c t on the value of the square bracketed term of Eq. (2.36). The value of A^ may s l i g h t l y i n c r e a s e w i t h i n c r e a s i n g r . f . power l e v e l due to the f a c t that atoms e j e c t e d from the target w i t h higher energies are thermally e q u i l i b r a t e d w i t h the s p u t t e r i n g gas at a s h o r t e r d i s t a n c e from the s u b s t r a t e than those having lower energies However, i f the t a r g e t - s u b s t r a t e t a b l e d i s t a n c e i s much s m a l l e r than the targ e t diameter, then, p a r t i c u l a r l y near the centre of the s u b s t r a t e t a b l e , 71. t h i s e f f e c t can be neglected. The temperature of the t a r g e t and the sub-s t r a t e depends on the s p u t t e r i n g power l e v e l . This a f f e c t s the values of T cxo and a^, the s t i c k i n g c o e f f i c i e n t s f o r oxygen at the s u b s t r a t e and the T target r e s p e c t i v e l y . Values of and decrease w i t h i n c r e a s i n g temperature and, thus, a l s o w i t h the r . f . power l e v e l used f o r s p u t t e r i n g . Thus an i n -crease of r . f . power l e v e l can r e s u l t i n an increase i n the value of the denominator i n the square bracketed term of Eq. (2.36). At the target a T decrease i n w i l l r e s u l t i n a lowering of the t a r g e t coverage by oxygen (see Eq.(2.31)) and thus enhanced R^, see a l s o Eq. (2.32). The e f f e c t of T the temperature dependence of and a on the composition r a t i o thus depends on the experimental c o n d i t i o n s . In p a r t i c u l a r i f the e f f i c i e n c y of the c o o l i n g systems f o r the target and the su b s t r a t e i s poor then there w i l l be a tendency fo r the composition r a t i o at a given PQ/R^ r a t i o to decrease w i t h i n c r e a s i n g s p u t t e r i n g power l e v e l . Thus at a higher r . f . power l e v e l a higher P /Rjyj r a t i o may be r e q u i r e d than at a lower power l e v e l , to o b t a i n the same composition T r a t i o i n both cases. Therefore, c o n s i d e r i n g the e f f e c t of a and a on the o o composition r a t i o and that a l i m i t e d number of experiments were done, e s p e c i a l l y i n the Cu^O + CuO r e g i o n , the f i t of t h e o r e t i c a l l y p r e d i c t e d values and e x p e r i -mental data shown i n F i g . 4.12 i s very good. The proposed theory a l s o provides a b a s i s f o r e x p l a i n i n g the v a r i a t i o n s i n f i l m r e s i s t i v i t y that were observed on t a k i n g measurements at d i f f e r e n t p o s i t i o n s (with respect to the centre of the s u b s t r a t e table) on the deposited f i l m s . Data f o r f i l m s deposited at v a r i o u s oxygen p a r t i a l pressures are shown i n F i g . 4.13. This e f f e c t i s not due to a v a r i a t i o n i n f i l m t hickness as t h i s has been allowed f o r i n the data presented i n F i g . 4.13. The c o r r e c t i o n procedure was as f o l l o w s : the s p a t i a l dependence of the thickness of a pure copper f i l m (deposited i n 100% argon) was estimated from a thickness meas-urement of the c e n t r a l p a r t of the f i l m and r e s i s t a n c e data taken at 1/4" 72. F i g . 4.13 V a r i a t i o n of r e s i s t i v i t y w i t h r a d i a l d i s t a n c e along copper oxide f i l m s . The sample numbers r e f e r to those marked on F i g . 4.5. 73. i n t e r v a l s along the f i l m (a constant value of f i l m r e s i s t i v i t y was assumed), see F i g . 4.14. Resistance data were obtained by the f o u r - p o i n t probe technique. Each f i l m deposited at each p a r t i a l pressure of oxygen ( F i g . 4.13) was then assumed to e x h i b i t the same thickness v a r i a t i o n (on a per-centage b a s i s ) as the pure copper f i l m , and the reference thickness f o r each f i l m was determined by an Angstronmeter measurement on the c e n t r a l p o r t i o n of each f i l m . The s l i g h t asymmetry around the centre p o i n t of the sub-s t r a t e t a b l e e x h i b i t e d by the curves i n F i g s . 4.13 and 4.14 i s r e l a t e d to the plasma disturbance caused by the s h u t t e r f i x t u r e . The s i g n i f i c a n c e of the curves i n F i g . 4.13 i s that they suggest a v a r i a t i o n of f i l m composition across the s u b s t r a t e . With reference to F i g s . 4.5 and 4.13, l a r g e changes i n r e s i s t i v i t y w i l l r e s u l t across the s u b s t r a t e s l i d e at P values s u i t a b l e o f o r p r e p a r a t i o n of n e a r l y s t o c h i o m e t r i c cuprous oxide at the centre of the s l i d e (sample //270) ; a l s o the r e s i s t i v i t y of the outer regions of the f i l m w i l l become greater than that at the centre f o r f i l m s prepared at P q values approaching the value appropriate to Cu^O + CuO formation (sample # 2 6 7 ) . The observed nonuniformity (across the s u b s t r a t e t a b l e ) i n d e p o s i t i o n r a t e of copper i s caused by the d i f f u s i v e nature of the mechanism of tr a n s p o r t of copper species towards the s u b s t r a t e , r e s u l t i n g from the f a c t that A M ( X ) i s a f u n c t i o n of the di s t a n c e x from the centre of the s u b s t r a t e t a b l e . Thus the decrease of A ^ ( x ) towards the outer regions of the s u b s t r a t e t a b l e ( e q u i v a l e n t to a decrease i n the f l u x of copper species) r e s u l t s i n an i n -c r e a s i n g compositional r a t i o w i t h the di s t a n c e from the s u b s t r a t e t a b l e center. An i n c r e a s i n g composition r a t i o w i t h tha di s t a n c e from the s u b s t r a t e t a b l e center was observed by measurement of the s p a c i a l dependence of f i l m r e s i s t -i v i e s and was confirmed by spectrophotometer measurements taken at va r i o u s p o s i t i o n s along the f i l m , see F i g . 4.3 and compare w i t h F i g . 4.2. This i s 74. 0.5 h 0.4 \-0.3 0.2 -1.5 -1.0 -0.5 0.0 0.5 .1.0 1.5 d i s t a n c e from s u b s t r a t e centre [ i n ] F i g . 4.14 D e p o s i t i o n r a t e of pure copper r . f . sputtered i n 100% Ar, normalized w i t h respect to the d e p o s i t i o n r a t e at the centre of the su b s t r a t e t a b l e . ( T o t a l pressure = 25 mTorr). 75. f u r t h e r d i r e c t evidence of the v a l i d i t y of the above proposed theory embodied i n Eq. (2.36), namely that the f i l m c ompositional r a t i o i s d i r e c t l y prop-o r t i o n a l to the r e l a t i v e magnitudes of the f l u x e s of metal and oxygen species at the s u b s t r a t e . A f u r t h e r way to show the v a l i d i t y of the r e a c t i v e s p u t t e r i n g theory proposed i n chapter 2 i s to compare the r . f . and d.c. F /R^ r a t i o s r e q uired to o b t a i n n e a r - s t o c h i o m e t r i c Cu^O, where the composition i s p r e c i s e l y known (C = 0.5). From e l e c t r o n d i f f r a c t i o n data ( s e c t i o n 4.2) and r e s i s t i v i t y data ( s e c t i o n 4.4) the p a r t i a l pressures of oxygen r e q u i r e d f o r formation of near-s t o c h i o m e t r i c Cu^O f i l m s could be very p r e c i s e l y determined. Let us con-centrate on two cases where s i m i l a r s p u t t e r i n g r a t e s f o r r . f . and d.c. de-p o s i t i o n were obtained, namely r . f . s p u t t e r i n g at 200 W of forward power which r e q u i r e d a P of 0.0427 Pa (0.32 mTorr) f o r n e a r - s t o c h i o m e t r i c cuprous o oxide formation, and d.c. s p u t t e r i n g at 70 mA of discharge c u r r e n t , where a P q of 0.0347 Pa (0.26 mTorr) was r e q u i r e d to form the n e a r - s t o c h i o m e t r i c Cu^O phase. In both cases d e p o s i t i o n times of 2 minutes were used, which produced f i l m s of thickness 1080 X and 870 X f o r the cases of r . f . and a d.c. s p u t t e r i n g r e s p e c t i v e l y . From the known Cu^O d e p o s i t i o n r a t e D [cm -1 -3 sec ], C^O d e n s i t y ( s = 6.2 g cm ), Cu^O molecular weight (M = 143 g -1 23 -1 mole ) and Avogadrc's number (N^ = 6.023x10 molecules mole )one can calculate,assuming ±- 1 ( t h i s approximation can be allowed near the centre of the sub s t r a t e t a b l e ) , t h e e f f e c t i v e copper s p u t t e r i n g r a t e as D s N A _2 - l \ = ~ M N M [ A T - C M S 6 C 1 (4-9) where m u l t i p l i c a t i o n by N M(=2) a r i s e s from the f a c t that one molecule of Cu 20 has two copper atoms. Applying Eq. (4.9) to the above r . f . and d.c. 76. cases one can show that i n the r . f . s p u t t e r i n g case p A 19 _2 - i o -22 2 = 4.7 x 10 at m sec ; and — = 9.08 x 10 Pa m sec; and i n the d.c. s p u t t e r i n g case p * 19 - 2 - 1 o -22 2 R^ = 3.79 x 10 at m sec ; and — = 9.15 x 10 Pa m sec. Here, i f one takes i n t o c o n s i d e r a t i o n that two d i f f e r e n t plasma e n e r g i z i n g methods were used and that the t o t a l pressure and anode-cathode voltage were both about 3 times l a r g e r i n the d.c. case than i n the r . f . s p u t t e r i n g case T ( p o s s i b l y l e a d i n g to a d i f f e r e n t a Q and a ) the agreement between the d.c. and r . f . values of P^/R^" i s e x c e l l e n t . Eq. (2.37) i s u s e f u l i n a l l o w i n g comparison of theory arid experimental data, but i t s main f u n c t i o n i s as a s t a r t i n g p o i n t from which the experimental c o n d i t i o n s r e q u i r e d to form a f i l m of given composition r a t i o could be determined. To use Eq. (2.37) i n t h i s f a s h i o n r e q u i r e s a knowledge of the system constants, namely y, w and e,(defined i n chapter 2, Eq. (2.27)). Determination of w r e q u i r e d knowledge of the t h r o t t l e d pumping speed S. The f o l l o w i n g procedure was used to determine the pumping speed S. F i r s t the s p u t t e r i n g system was pumped —6 down to a vacuum b e t t e r than 10 Torr. Then the t h r o t t l i n g v a l v e was turned i n t o i t s minimum conductance ( s p u t t e r i n g ) p o s i t i o n and oxygen was -4 admitted to the system at such a rate that a pressure of 4 x 10 Torr was reached (the pressure reading was taken according to the i o n i z a t i o n gauge E, see F i g . 3.1). A f t e r the pressure s t a b l i l i z e d , the oxygen on-off v a l v e was clo s e d , the s e t t i n g of the oxygen metering v a l v e was unchanged and the system —6 was allowed to pump down to a base pressure of approximately 10 Torr again. Then the high vacuum va l v e was cl o s e d and the oxygen on-off v a l v e opened. Thus the pressure i n the b e l l j a r s t a r t e d to increase and the time r e q u i r e d to 77. increment the t o t a l pressure by 60 mTorr was measured. This procedure was repeated 5 times and the average time At was c a l c u l a t e d . The pressure reading was given by the thermocouple gauge B. In order to avoid any e r r o r caused by i n c o r r e c t z e r o i n g and n o n l i n e a r i t y of the gauge, the pressure i n c r e -ments were taken i n d i f f e r e n t pressure r e g i o n s , f o r example between 1-61 mTorr, 20-80 mTorr, e t c . The average time At r e q u i r e d to increment the pressure by AP = 60 m Torr was 67 seconds. Knowing the t o t a l volume of the enclosure -2 3 V(2.23x10 m ), the leak r a t e of oxygen could be c a l c u l a t e d from the r e l a t i o n L - - g - S (4.10) 60xl0~ 3x0.023 . -5 3 T -1 = 2.027x10 m Torr sec 67 The t h r o t t l e d pumping speed S can be then be c a l c u l a t e d from the r e l a t i o n c - L 2.027x10 5 _ n 7 1 n-2 3 -1 S = — = . = 5.07x10 m sec P —4 4x10 2 Then, knowing the t a r g e t area (0.0182 m ) and assuming T = 300 K 0.0507 0.0507 on _1 -1 co = - t — • Z 9 T 1 = 6 - 7 x K T U N m 0.0182 x 1.38x10 x 300 The constant 6 was determined from (Eq.2.37) and from the values of P q and R^ r e q u i r e d to o b t a i n n e a r - s t o c h i o m e t r i c C^O of which the compositional r a t i o i s kn (C=0.5). Using the data from r . f . d e p o s i t i o n at 200 W where the n e a r - s t o c h i o -* 1 9 - 2 m e t r i c cuprous oxide was formed, namely P q = 0.0427 Pa and R^ ?= 4.7x10 m sec \ 3 equal to 0.1148 was obtained. Assuming again and a l s o 78. a ^ l then B-ct Q = 0.1148, which appears to be a reasonable value f o r the s t i c k i n g c o e f f i c i e n t of oxygen on copper, e s p e c i a l l y t a k i n g i n t o c o nsidera-t i o n that the s u b s t r a t e i s exposed to bombardment by high energy e l e c t r o n s . As the f a c t o r s to, 3 (hence e) and S can be maintained constant over a wide range of s p u t t e r i n g c o n d i t i o n s , the numbers c a l c u l a t e d above can be used i n Eq. (2.37) to p r e d i c t the r e q u i r e d values of and neeueu to produce a copper oxide f i l m of given composition r a t i o . I t remains only to estimate A R^ from observable parameters. A C a l c u l a t i o n of from Eq. (2.32) r e q u i r e s R o x> a n d F to be known. The s p u t t e r i n g r a t e s R q ^ and R^ have to be determined e x p e r i m e n t a l l y f o r each d e p o s i t i o n power, but F, which i s independent of s p u t t e r i n g power, p r o v i d i n g good c o o l i n g of the t a r g e t i s ensured, can be determined once f o r a given t a r g e t - r e a c t i v e gas system. The c o e f f i c i e n t F was determined from Eq. (2.32) using data from s p u t t e r d e p o s i t i o n at 200 W and 7^ = 0.0427 Pa, where near s t o c h i o m e t r i c Cu^O (C = 0.5) was deposited. The s p u t t e r i n g rates R^ and R were determined.from the d e p o s i t i o n r a t e of copper i n 100% argon and the d e p o s i t i o n r a t e of CuO i n 100% oxygen atmosphere r e s p e c t i v e l y , A using the same c a l c u l a t i o n method as used p r e v i o u s l y to determine R (Eq. 4.9). Thus from: A 1 9 - 2 - 1 -2 R,, = 4.7x10 m sec P = 4.27x10 P M o a = 5 . 4 2 x l 0 1 9 m ~ 2 s e c _ 1 C = 0.5 18 —2 —1 R = 8.55x10 m sec ox -3 one obtains F = 2.73x10 A convenient check on the usefulness of the above values f o r the system constants i s to use them to compute the composition r a t i o i n a range of pressure and power values which ex p e r i m e n t a l l y are known to produce a w e l l - d e f i n e d phase, e.g. CuO f o r which C=l. This was done f o r oxygen p a r t i a l pressures of 1 mTorr and 10 mTorr and the r e s u l t i n g values of C, computed using the above values of constants i n the s i m p l i f i e d v e r s i o n of Eqn. (2.27) i . e . Eqn.(2.37), were 0.987 and 0.995 r e s p e c t i v e l y . 80. I 5. OPTICAL CONSTANTS OF COPPER OXIDE THIN FILMS 5.1 POSSIBLE MEASUREMENT METHODS The o p t i c a l constants of t h i n f i l m s can be determined e i t h e r from i n t e n s i t y measurements (photometric) o r from p o l a r i z a t i o n measurements ( p o l a r i m e t r i c ) . Photometric measurements can be performed e i t h e r at normal or non-normal i n c i d e n c e , w h i l e p o l a r o m e t r i c at non-normal only. Only photometric methods were considered f o r the determination of the o p t i c a l constants of the v a r i o u s copper oxide f i l m s prepared i n t h i s work, and a l l measurements were done at normal incidence of l i g h t . There are many ex-perim e n t a l advantages to be gained by making measurements at normal i n c i d e n c e . The angle of i n c i d e n c e does not need to be known a c c u r a t e l y , measurements are i n s e n s i t i v e to the convergence angle of l i g h t on the sample and p o l a r i z a t i o n e f f e c t s are unimportant [36], A l s o s u r f a c e n o n u n i f o r m i t i e s i n f l a t n e s s and contamination o f t e n do not g r e a t l y i n f l u e n c e the r e s u l t [37]. Because a l l the present experiments were performed on absorbing f i l m s , a determination of both the r e f r a c t i v e index n and the e x t i n c t i o n c o e f f i c i e n t k was necessary, where the t o t a l complex index o f r e f r a c t i o n i s n = n - i k . G e n e r a l l y , when a number N of o p t i c a l constants are to be determined, N independent measurements must be made. Then N independent equations for the measured q u a n t i t i e s w r i t t e n i n terms of the N unknown o p t i c a l parameters have to be s o l v e d . C o n s i d e r i n g photometric methods o n l y , r e f l e c t a n c e R of l i g h t from the f i l m s i d e , r e f l e c t a n c e R' from the s u b s t r a t e s i d e and transmittance T are the measured q u a n t i t i e s . The a n a l y t i c a l expressions f o r the dependencies of R, R' and T on the r e f r a c t i v e index and e x t i n c t i o n c o e f f i c i e n t , the wavelength o f the l i g h t X, the f i l m t h i c k n e s s d and the r e f r a c t i v e index and thi c k n e s s of the s u b s t r a t e n and d are very s s 81. complicated. I n v e r t i n g of f u n c t i o n s a n a l y t i c a l l y to o b t a i n "n" and "k" i n terms of the measured q u a n t i t i e s R and T i s v i r t u a l l y i m p o s s i b l e . The s o l u t i o n s used to be obtained i n the past through approximate formulas and/or g r a p h i c a l methods. Reviews of such methods have appeared elsewhere [38-40,45]. The method used i n the present work f o r determining the o p t i c a l constants of t h i n f i l m s takes advantage of powerful numerical i t e r a t i o n methods f o r s o l v i n g simultaneous n o n l i n e a r equations and the a v a i l a b i l i t y of f a s t powerful computers. This allows a t t e n t i o n to be concentrated on accuracy i n s t e a d of the former s i m p l i c i t y of e v a l u a t i o n . There are a number of independent measurement combinations. Considering only p e r p e n d i c u l a r i n c i d e n c e , n,k and d can be obtained from a combination of T-R-R'. Because i n our case d was determined independently by a Sloan M-100 Angstrometer, we can concentrate on determination of n and k only. In that case two independent measured q u a n t i t i e s which can be expressed i n terms of n and k (to be determined), have to be obtained. The most common choice i s t o use R and T measurements [36,37, 41-44]. U s u a l l y R' measurements are not used, because they o f t e n contain systematic e r r o r s [36]. In some cases, however, the r a t i o R/R' can be used, and thus the measurement of absolute r e f l e c t i v i t y can be avoided. In the cases when the measurement of r e f l e c t i v i t y i s not a v a i l a b l e , a combination of two transmittance measurements f o r s i m i l a r f i l m s of two d i f f e r e n t thickness can be used. Speaking of accuracy i n the o p t i c a l constants d e termination, one would l i k e to know how the e r r o r i n some of the measured q u a n t i t i e s a f f e c t s the accuracy of the c a l c u l a t i o n of o p t i c a l constants. This i s very d i f f i c u l t to g e n e r a l i z e because the accuracy depends on the s e l e c t i o n of measured q u a n t i t i e s , the wavelength and the range;of values of o p t i c a l constants to be determined. L e t us consider the cases of measurements i n v o l v i n g R-T and T ( d ^ ) - T ( d 2 ) . The expressions f o r the dependence of R and T on n, n g the r e f r a c t i v e indeces of the f i l m and the s u b s t r a t e r e s p e c t i v e l y , k the e x t i n c t i o n c o e f f i c i e n t of the f i l m , d, d the thicknesses of the f i l m and s the s u b s t r a t e r e s p e c t i v e l y and the l i g h t wavelength A have the f o l l o w i n g form [36]: a exp(4Trdk/A) + b cos(4iTdn/A) + c. sin(4Trnk/A) + f 1 exp(-4iTdk/A) a 2 exp(4Trdk/A) + b 2 c o s (4-rrdn/A) + c 2 sin(4irdn/A) + f 2 exp (-4iTdk/A) (5.1) a 2 exp(4Trdk/A) + b 2 cos (4Trdn/A) + sin(4iTdn/A) + exp(-4irdk/A) (5.2) where a, = [(n - l ) 2 + k 2 ] [ ( n 2 + 1 ) ( n 2 + k 2 + n 2 ) + 4nn 2 ] , 1 / J L N s s s -a. = [(n + l ) 2 + k 2 ] [ ( n 2 + 1) ( n 2 + k 2 + n 2 ) + 4nn 2 ] , I s s s 2 2 2 2 ? 2 2 2 b, = - 2 [ ( n + l ) ( n + k - 1)(n + k - n ) + 8k n ], 1 s s s 2 7 2 2 2 2 2 2 b„ = - 2[(n + l ) ( n + k - l ) ( n + k - n ) - 8k n ], I s s s c x = 4 k [ - ( n g 2 + l ) ( n 2 + k 2 - n g 2 ) + 2 n g 2 ( n 2 + k 2 - 1 ) ] , c 0 = 4k[(n 2 + l ) ( n 2 + k 2 - n 2 ) + 2n 2 ( n 2 - + k 2 - 1 ) ] , £ s s s f n = [(n + l ) 2 + k 2 ] [ ( n 2 + l ) ( n 2 + k 2 + n 2 ) - 4nn 2 ] , i s s s f„ = [(n - l ) 2 + k 2 ] [ ( n 2 + l ) ( n 2 + k 2 + n 2 ) - 4nn 2 ] , and z s s s 2 2 2 a = 32 n (n + k ). 83. To i l l u s t r a t e g r a p h i c a l l y the c o r r e l a t i o n between the e r r o r i n measurement of r e f l e c t a n c e and transmittance and accuracy of o p t i c a l constants determination, the contours of constant R and T were p l o t t e d i n the n-k plane f o r a region of n and k values where the o p t i c a l constants of Cu^O were expected t o l i e . The r e f r a c t i v e index n f o r thermally grown Cu^O as a f u n c t i o n of wavelength over the v i s i b l e region of l i g h t has been determined by Pastrnak [98] to have values near 2.7. From values of a b s o r p t i o n c o e f f i c i e n t a(A) f o r b u l k Cu^O determined by Pastrnyak [99] i t could be i n f e r r e d that the maximum value of e x t i n c t i o n c o e f f i c i e n t k should have a 0 0 value l e s s than u n i t y i n the wavelength region of i n t e r e s t (4000A - 8000A). M u l t i p l e s o l u t i o n s f o r n and k may e x i s t at given values of r e f l e c t a n c e and transmittance. In order to gain some i n f o r m a t i o n about the l o c a t i o n of s o l u t i o n s other than the c o r r e c t one and having r e f r a c t i v e i n d i c e s of lower values than expected f o r C^O f i l m s , the contours of constant R and T were p l o t t e d f o r the range of n-values from 0.5 to 3.0 and of k-values from 0.0 to 1.0. The p r o x i m i t y of the m u l t i p l e s o l u t i o n s and manner i n which the measured q u a n t i t i e s i n t e r s e c t i n the n-k plane were used to judge the s u i t a b i l i t y of a given method f o r determination of the o p t i c a l constants of copper oxide f i l m s at a given wavelength. L e t us consider the two transmittances method, where a measurement of absolute specular r e f l e c t a n c e can be avoided. As the f i r s t example the o o transmittance data f o r f i l m s w i t h thicknesses of 1080A and 540A were considered, see F i g . 5.1. Transmittances i n t h i s case i n t e r s e c t at very s m a l l angles l e a d i n g to a l a r g e inaccuracy i n determining the o p t i c a l constants, e s p e c i a l l y the r e f r a c t i v e index n. This i s e s p e c i a l l y c r i t i c a l i n the region o of n > 1.5 and k > 0.3, where Cu 20 o p t i c a l constants at 4000A are expected to f a l l . Moreover, n and k are m u l t i p l e - v a l u e d f u n c t i o n s of the transmittances 84. r e f r a c t i v e index n F i g . 5.1 Transmittance curves f o r cwo samples of d i f f e r e n t t h i c k n e s s as a f u n c t i o n of r e f r a c t i v e i n d i c e s of the f i l m . The width of the countours i n d i c a t e s an experimental e r r o r i n transmittance measurement of ±1% of the nominal transmittance v a l u e . The double l i n e represents a sample w i t h f i l m t h i c k n e s s = 540A (T') and the t r i p l e l i n e a sample with thickness = 1080A.(T). Curves are computed f o r n = 1.5 and A = 4000A. i n the r e g i o n of n > 1.5. Thus i f e r r o r s i n measurement of tran s m i t t a n c e s are l a r g e (> 1%) , the u n c e r t a i n ! t y i n n and k determination can be so .large t h a t t h i s method i s not able to o f f e r r e l i a b l e r e s u l t s . S i g n i f i c a n t improvement i n the r e l i a b i l i t y and accuracy o f the two transmittances method: can be achieved by i n c r e a s i n g the d i f f e r e n c e between the t h i c k n e s s e s o f the samples used. This i s shown i n F i g s . 5.2 and 5.3 where the tran s m i t t a n c e s o o from f i l m s of thicknesses 1080A and 360A are given. Note t h a t n and k are i n t h i s case s i n g l e - v a l u e d f u n c t i o n s of R and T over the p l o t t e d n - r e g i o n . However, one has to be aware when usin g t h i s method that i n some cases two f i l m s w i t h very d i f f e r e n t thicknesses may not have i d e n t i c a l p h y s i c a l p r o p e r t i e s and t h i s may y i e l d an a d d i t i o n a l inaccuracy i n the det e r m i n a t i o n of o p t i c a l constants. Turning now to the R-T method F i g . 5.4 d i s p l a y s transmittance and o r e f l e c t a n c e curves, i n the n-k plane, computed f o r a 1080A t h i c k f i l m . Here a l l R and T curves i n t e r s e c t at very l a r g e angles, except f o r f i l m s w i t h n ^ n g = 1.5 and thus a h i g h accuracy of n and k determination u s i n g t h i s method f o r f i l m s w i t h n 4 n can be achieved. The f a c t that n and k are s m u l t i p l e - v a l u e d f u n c t i o n s of R and T does not seem to be a drawback of t h i s method, because i n d i v i d u a l s o l u t i o n s are f a r apart and by c a r e f u l c o n s i d e r a t i o n an acceptable s o l u t i o n can be e a s i l y determined. I f one f o l l o w s the behaviour of n and k over the measured wavelength r e g i o n , and n-values f o r some wave-l e n g t h p o i n t s are known e i t h e r from the l i t e r a t u r e o r from independent measurements, i t i s not very l i k e l y t h a t an erroneous s o l u t i o n can be obtained. C o n s i d e r i n g the t h e o r e t i c a l accuracy of the R and T method t h i s method was s e l e c t e d f o r determination of the o p t i c a l constants of copper oxide f i l m s . The technique used i n t h e i r determination and the r e s u l t s o btained are discussed i n the next s e c t i o n of t h i s chapter. 86. r e f r a c t i v e index n F i g . 5.2 Transmittance curves f o r two samples of the same m a t e r i a l w i t h thicknesses of 1080A ( t r i p l e l i n e (T)) and 360& (double l i n e ( T ' ) ) . The width of the countours represents an experimental e r r o r i n transmittance measurements of ±1% of t h e i r nominal v a l u e s . Curves were computed f o r n c = 1.5 and A = 4000A. 87. o r e f r a c t i v e index n F i g . 5.3 Transmittance curves f o r a sample w i t h t h i c k n e s s of 1080A (T) and f o r a sample w i t h thickness of 360A ( T 1 ) . The width of the countours represents an experimental e r r o r i n transmittance measurements of ±1% of t h e i r nominal o v a l u e s . Curves were computed f o r n Q = 1.5 and A = 6000A. 88; r e f r a c t i v e index n F i g . 5.4 Transmittance and r e f l e c t a n c e curves f o r f i l m w i t h t h i c k n e s s of 1080A. The countours width i n d i c a t e s an e r r o r i n transmittance and r e f l e c t a n c e measurements ±1% of t h e i r nominal v a l u e s . Curves were computed f o r n g = 1.5 and A = 4000A. 89. 5.2 TECHNIQUE USED FOR OPTICAL CONSTANTS DETERMINATION The transmittance T and absolute s p e c u l a r r e f l e c t a n c e R from the f i l m s i d e of the copper oxide coated s u b s t r a t e s were measured at p e r p e n d i c u l a r i n c i d e n c e of l i g h t u s i n g a Cary 14R spectrophotometer and p l o t t e d as a f u n c t i o n of wavelength. The data output f o r r e f l e c t a n c e and transmittance were i n the form of o p t i c a l d e n s i t i e s , namely T.O.D. = l o g i r / I T ( 5 > 3 ) R.O.D. = l o g I R 0 / l R ( 5 > 4 ) where T.O.D. was the o p t i c a l d e n s i t y corresponding to transmittance, R.O.D. the o p t i c a l d ensity corresponding to the r e f l e c t a n c e , I the i n t e n s i t y of the l i g h t beam i n c i d e n t upon the sample during measurement of transmittance (equal to the i n t e n s i t y of the reference beam) , I the i n t e n s i t y of the tran s m i t t e d beam, I the i n t e n s i t y of l i g h t r e f l e c t e d from a reference aluminium m i r r o r and I the i n t e n s i t y of l i g h t r e f l e c t e d from the f i l m s i d e K of the sample. The transmittance was then c a l c u l a t e d from the f o l l o w i n g r e l a t i o n T = 10- T-°' D- (5.5) and s i m i l a r l y the r e f l e c t a n c e was computed from • R = R O ( A ) x l O " R , 0 - D ' (5.6) where RO(A) was the r e f l e c t a n c e of the reference aluminium m i r r o r d e r i v e d from data s u p p l i e d by the N a t i o n a l Bureau of Standards. For f u r t h e r data p r o c e s s i n g the curves obtained from the spectrophotometer measurements were d i g i t i z e d and s t o r e d on f i l e . Measure-ments of f i l m t hickness were performed us i n g the M-100 Sloan Angstrometer, as described i n chapter 4.1. Footnote: * These R and T measurements were performed at RCA Labs, P r i n c e t o n , N.J. by Dr. P. Zanzucchi. 90. The computation of the o p t i c a l constants n, k f o r given R,T,A values was based on the secant i t e r a t i o n method and u t i l i z e d the NDINVT r o u t i n e [103] a v a i l a b l e from the UBC computing centre. Eqns. (5.1) and (5.2) , which give the r e l a t i o n s h i p between n,k, and R,T were used i n i m p l i c i t form. Because n,k are m u l t i p l e - v a l u e d f u n c t i o n s of R,T,A, the p a r t i c u l a r s o l u t i o n obtained depended on a guess n,k v e c t o r requested by the i t e r a t i o n r o u t i n e . The r e f r a c t i v e index n i s expected to be a smooth f u n c t i o n of wavelength and thus an erroneous s o l u t i o n could o f t e n be e a s i l y recognized by any d i s c o n t i n u i t i e s i n n values near some wavelength p o i n t . A l l n,k computations were performed us i n g a c o n v e r s a t i o n a l t e r m i n a l , which enabled d i r e c t operator c o n t r o l over the s e l e c t i o n of a p a r t i c u l a r s o l u t i o n . The program allowed computation of n,k e i t h e r continuously f o r the whole range of wavelength (0.38 - 0.8um). where R,T was determined, or only f o r a s e l e c t e d r e g i o n of t h i s range which could be, when requested, a given s i n g l e wavelength p o i n t . In the case when continuous computation over the whole o r a p a r t i c u l a r r e gion of wavelength was requested, the i n i t i a l n,k guess v e c t o r s u p p l i e d by the operator was assumed f o r the f i r s t wavelength p o i n t only . For the next wavelength p o i n t the n,k s o l u t i o n determined f o r the pr e v i o u s wavelength p o i n t was assumed as the n,k guess v e c t o r . In the s i n g l e p o i n t run mode an n,k guess v e c t o r had to be s u p p l i e d f o r each p o i n t by the o p e r a t o r . The f o l l o w i n g procedure was adopted i n the c a l c u l a t i o n of n,k usin g the above mentioned program. F i r s t f o r a given n,k guess v e c t o r , the values of o p t i c a l constants were determined f o r the whole measured wavelength range. I f a d i s c o n t i n u i t y i n n occured at a c e r t a i n wavelength p o i n t , the c a l c u l a t i o n was repeated u s i n g a new n,k v e c t o r , u n t i l a l l d i s c o n t i n u i t i e s were removed. I t could a l s o happen th a t f o r some wavelength p o i n t no s o l u t i o n o r at l e a s t no p h y s i c a l l y acceptable s o l u t i o n could be obtained. This could be caused by 91. 0.5 0.75 1.0 1.25 1.5 1.75 2.0 2.25 2.5 2.75 3.0 r e f r a c t i v e index n F i g . 5.5 Transmittance and r e f l e c t a n c e curves f o r f i l m w i t h t h i c k n e s s of 1000A at X = 5000A and n g = 1.5. Note, that i n t h i s p a r t i c u l a r case f o u r d i f f e r e n t s o l u t i o n s f o r n,k can be obtained f o r example, f o r combination of R = 0.1 and T = 0.6 - 0.8. • 92. a s m a l l experimental e r r o r i n the R,T measurements f o r which the s o l u t i o n was most probably c l o s e to one of the branch p o i n t s , see f o r example p o i n t P(R =0.1, T = 0.2) i n F i g . 5.5. In such cases a small e r r o r i n R,T measurements may cause the R,T curves not to i n t e r s e c t and thus no s o l u t i o n f o r t h a t given R,T combination can be found. Any such p o i n t s were e i t h e r removed from the d a t a , or n,k were determined f o r two n e i g h b o r i n g wavelength r e g i o n s , o m i t t i n g the p o i n t i n q u e s t i o n . Note that the s i n g l e p o i n t run mode was used t o determine n,k f o r such c r i t i c a l p o i n t s i n order t o f i r m l y e s t a b l i s h whether a s o l u t i o n f o r such a p o i n t could be found. The program l i s t i n g i n c l u d i n g i t s d e s c r i p t i o n i s given i n Appendix B. 5.3 RESULTS The absolute R,T data c a l c u l a t e d from the measured o p t i c a l d e n s i t i e s and the o p t i c a l constants determined from t h i s data f o r v a r i o u s copper oxide f i l m s are reported i n F i g s . 5.6 - 5.13. The d e s c r i p t i o n s of the experimental c o n d i t i o n s at which the p a r t i c u l a r f i l m s were d e p o s i t e d , i n c l u d i n g some of t h e i r : r e l a t e d : p r o p e r t l e s are i n c l u d e d i n t h e i r r e s p e c t i v e c a p t i o n s . A l l curves were computer-generated and p l o t t e d by a Calcomp p l o t t e r . Values of a b s o r p t i o n c o e f f i c i e n t a were c a l c u l a t e d from the f o l l o w i n g r e l a t i o n : _ 4 i i k " ~ A (5.7) By comparing F i g s . 5.6 (a-b) - 5.11 (a-b), the v a r i a t i o n of o p t i c a l p r o p e r t i e s w i t h the f i l m c o m p o s i t i o n a l character can be c l e a r l y seen. The C h a r a c t e r i s t i c transmittances f o r each f i l m phase have been d i s c u s s e d e a r l i e r ( s e c t i o n 4.3). I t can now be a p p r e c i a t e d that the r e d u c t i o n i n transmittance on going from n e a r - s t o c h i o m e t r i c Cu^O f i l m s to Cu^O + Cu f i l m s (compare e.g. F i g s . 5.6a and 5.9a) i s not due to l a r g e changes i n r e f l e c t a n c e but more t o an i n c r e a s e i n absorptance. E x t i n c t i o n and absorption c o e f f i c i e n t values f o r 93. Cu 20 + Cu f i l m s are thus s i g n i f i c a n t l y l a r g e r than i n Cu 20 f i l m s , whereas the r e a l p a r t of the r e f r a c t i v e i n d i c e s are s i m i l a r ( F i g s . 5.6b, c and 5.9b, c) . There i s l i t t l e change i n absorptance as the stochiometry of Cu 20 on the s i d e of copper d e f i c i e n c y i s changed (compare F i g s . 5.7c, 5.8c and 5.9c) u n t i l the n u c l e a t i o n of CuO s t a r t s to occur, whereupon the absorptance i n c r e a s e s . The o p t i c a l p r o p e r t i e s f o r Cu 20 + CuO f i l m s are thus,as expected, intermediate between those of pure Cu 20 and pure CuO ( F i g s . 5.7, 5.10 and 5.11). The l a r g e increase i n CuO n-value w i t h i n c r e a s i n g wavelength ( F i g . 5.11b) has a l s o been reported i n thermally-prepared CuO f i l m s [46]. Values of n and k f o r the l a t t e r f i l m s and the r e a c t i v e l y - s p u t t e r e d CuO f i l m s prepared i n the present work are very s i m i l a r . P u b l i s h e d data on o p t i c a l p r o p e r t i e s of copper oxide f i l m s i s very meagre. Wieder and Czanderna [46] have made some measurements on CuO (quoted above) and on thermally-prepared CUOQ ^ which i s a gross defect s t r u c t u r e of Cu„0. Values of k reported f o r the Cu0_ f i l m s are s i m i l a r to those found 2. r 0.67 to o b t a i n i n the present r e a c t i v e l y - s p u t t e r e d Cu 20 + CuO f i l m s . As regards other work on copper oxides Pastrnak [98] has given n-values f o r b u l k Cu 20 m a t e r i a l out to a wavelength o f 12um, but the d e t a i l i n the range of i n t e r e s t i n the present work (0.4 - 0.8um) i s only s u f f i c i e n t to say th a t n i s about 2.7 f o r bulk Cu 20 m a t e r i a l [46]. This value i s approached i n the present f i l m s towards the higher wavelength end of the range covered. Around 0.48um a peak i n n-value (n % 3.36) was observed which appears to be c h a r a c t e r i s t i c of the presence of Cu 20 i r r e s p e c t i v e of whether Cu, CuO o r copper i o n vacancies are a l s o present ( F i g s . 5.7b, 5.8b and 5.9b). Values of a f o r bulk Cu 20 have been reported by Pastrnyak [99] and from these k values have been deduced [46]. The reported data bear l i t t l e resemblence to the present r e s u l t s , as i s perhaps to be expected [91] from the f a c t that one s e t of data i s f o r b u l k 94. m a t e r i a l and one f o r very t h i n f i l m s . Nevertheless the ab s o r p t i o n c o e f f i c i e n t data f o r the present f i l m s i n d i c a t e s a band gap value (see F i g . 5.13) which f a l l s w i t h i n the range of values of 1.94 - 2.14 g e n e r a l l y accepted f o r bu l k Cu^O [35, 74, 99]. The appearance of e x c i t o n peaks i n the absorption spectrum, which i s apparently common i n b u l k samples [99, 100, 101], was not observed w i t h the present f i l m samples at room temperature. For u n s p e c i f i e d "C^O f i l m s " Wieder and Czanderna [46] were a l s o not able to i d e n t i f y d e f i n i t e e x c i t o n peaks. There i s however some s t r u c t u r e at long wavelengths i n the values of abso r p t i o n c o e f f i c i e n t f o r the three C^O f i l m s shown i n F i g . 5.13. The correspondence between i n c r e a s e d absorption and c o n d u c t i v i t y suggests the presence of e i t h e r f r e e c a r r i e r a bsorption or s i g n i f i c a n t absorption by copper i o n vacancies. The l a t t e r has been p o s t u l a t e d [46] as r e s p o n s i b l e f o r the high k values i n the CuO n ^ m a t e r i a l discussed above. .In the present case i t would appear t h a t the broad d i s p e r s i o n shown by the high c o n d u c t i v i t y samples can be a t t r i b u t e d to the free c a r r i e r a b s o r p t i o n . However i n the case of the n e a r - s t o c h i o m e t r i c C^O sample of r e s i s t i v i t y 892 ohm cm ( f o r which the f r e e c a r r i e r a b s o r p t i o n would c e r t a i n l y be l e s s than i n the other two samples) there i s evidence ( F i g s . 5.9c and 5.13) of an a d d i t i o n a l a b s o r p t i o n "shoulder" c l o s e to the absorption edge. The t h r e s h o l d energy f o r t h i s shoulder i s lower than the bandgap by about 0.2 eV, suggesting the p o s s i b i l i t y of a t r a n s i t i o n between an i o n i z e d acceptor l e v e l (copper i o n vacancy) and the conduction band. T h i s value of i o n i z a t i o n energy c o r r e l a t e s w e l l w i t h the range of thermal a c t i v a t i o n energies f o r c o n d u c t i v i t y as reported i n s e c t i o n 4.5. (a) Wavelength dependence of the transmittance and r e f l e c t a n c e . o.s o u u o c a -u o.i 95. wavelength [ym] (b) Wavelength dependence of the r e f r a c t i v e index n and the e x t i n c t i o n c o e f f i c i e n t k. •a c •rl <D O a u (c) Depenedence of the ab s o r p t i o n c o e f f i c i e n t a on the i n c i d e n t photon energy. F i g . 5.6 O p t i c a l p r o p e r t i e s of Cu20+Cu 1030A t h i c k f i l m deposited on S u p r a s i l 2 s u b s t r a t e at 200W of r . f . power arid .32 mTorr of oxygen. 0.4 0.) 0.6 wavelength [pm] —r~ 1.0 f I.) I — J.O ~ i — 3.) I 4.0 photon enc-rgy [cV] (a) Wavelength dependence of the transmittance and r e f l e c t a n c e . 0.5 0.6 Q. wavelength [pra] (b) Wavelength dependence of the r e f r a c t i v e index n and the e x t i n c t i o n c o e f f i c i e n t k. c X 3.0 -V -o c •H 2.6 -> u 2.6 O a u *M D 2.4 • U(c) Dependence of the absorp-t i o n c o e f f i c i e n t a on the phofcon energy of i n c i d e n t l i g h t . Fig.5.7 O p t i c a l p r o p e r t i e s of the Cu 20 1010A t h i c k f i l m deposited on S u p r a s i l 2 subs-t r a t e at 200W of r . f . power and .36 mTorr of oxygen. The f i l m r e s i s t i v i t y p = 91 ohm cm. t>0 *-0 O 0.5 0.6 wavelength [pm] 2.0 ~ r — 2.V — r ~ j.a — I ~ I>lii>ton energy [eV] 97 (a) Wavelength dependence of the transmittance and r e f l e c t a n c e . H 0.2 H c Q 1-1 *• 0.3 u o si o.« o c •«. U o j o 111 >-f 01 0.2 0.1 0.«- 0.5 0.4 wavelength [pm] (b) Wavelength dependence of the r e f r a c t i v e index n and the e x t i n c t i o n c o e f f i c i e n t k. a 01 > (c) Dependence of the absorp-t i o n c o e f f i c i e n t a on the photon energy of i n c i d e n t l i g h t . Fig.5.8 O p t i c a l p r o p e r t i e s of the Cu 20 1050A t h i c k f i l m deposited on the S u p r a s i l 2 s u b s t r a t e at 200W of r . f . power and .4 mTorr of oxygen. The f i l m r e s i s t i v i t y p = 48 ohm cm. i 5 0 00 «.3 O 0.4 0.S 0.4 wavelength [pm] — T -2.3 —I— 3.0 !~ 3.3 photon energy [e-'] (a) Wavelength dependence of the transmittance and r e f -l e c t a n c e . 0.9 <u o c «l 0.8 6 o.t in a a u o.t u o o c to u o.« o o 98. 0.1. 0.1 o.t wavelength [ym] (b) Wavelength dependence of the r e f r a c t i v e index n and the e x t i n c t i o n c o e f f i c i e n t k. c • X 111 •o in 3.2 -> 3.0 -u U a u 2.8 • a 2.6 . w a v e l e n g t h ^-la] (c) Dependence of the absorp-t i o n c o e f f i c i e n t a on the photon energy of i n c i d e n t l i g h t . O p t i c a l p r o p e r t i e s o F i g . 5.9 of the Cu 20 1050i\ t h i c k f i l m deposited on the S u p r a s i l 2 su b s t r a t e at 200W of r . f . power and .35 mTorr of oxygen, The f i l m r e s i s t i v i t y p = 892 ohm cm. 00 4.5 - J — 2.0 2.3 photon energy [eV] (a) Wavelength dependence of the transmittance and r e f l e c t a n c e . (b) Wavelength dependence of the r e f r a c t i v e index n and the e x t i n c t i o n c o e f f i c i e n t k. 0.9 0) u c O.t a -g 0.7 IB c a n o.t ~ 0.1 HI o c •a o.t u u CI 0.2 0.1 J.t X r3 ^ 2.8 wavelength [pa] 99. O.t 0.3 0.6 wavelength (umj (c) Dependence of the ab s o r p t i o n c o e f f i c i e n t a on the photon energy of i n c i d e n t l i g h t . F i g . 5.10 O p t i c a l p r o p e r t i e s of Cu20+CuO 1050A t h i c k f i l m d eposited on the S u p r a s i l 2 su b s t r a t e at 200W of r . f . power and .7 mTorr of oxygen. 00 o —r~ 2.V ' I 3.9 —r~ 3.1 photon cn.irgy [eV] I t.o (a) Wavelength dependence of the transmittance and ref-l e c t a n c e . c u o 01 o.s O C a iS O.J 0-4 0.5 0. wavelength [yra] (b) Wavelength dependence of r e f r a c t i v e index n and e x t i n c t i o n • c o e f f i c i e n t k. (c) Dependence of the a b s o r p t i o n c o e f f i c i e n t a on the i n c i d e n t photon energy. F i g . 5.11 O p t i c a l prop-e r t i e s of CuO 1200A t h i c k f i l m deposited at 200W of r . f . power and 6. mTorr of oxygen. 4.0 3.4 •a 0) > J.S CI u to o 0.4 0.3 wavelength x.i i.o 2.5 3.0 photon energy [eV] F i g . 5.13 Dependence of the a b s o r p t i o n c o e f f i c i e n t a on the photon energy of i n c i d e n t l i g h t f o r cuprous oxide f i l m s o§ d i f f e r e n t r e s i s t i v i t i e s . 103. 6'. COPPER OXIDE/SILICON HETEROJUNCTION DIODES 6.1 ' INTRODUCTION As mentioned i n chapter 1, one of the a t t r a c t i o n s of copper oxide f i l m s i s t h e i r p o t e n t i a l s u i t a b i l i t y to l a r g e , t h i n f i l m p h o t o v o l t a i c converters. As a f i r s t i n v e s t i g a t i o n of t h i s p o s s i b i l i t y a f a b r i c a t i o n procedure f o r Cu^O/Si s o l a r c e l l s was developed and the p h o t o v o l t a i c prop-e r t i e s of these diodes were i n v e s t i g a t e d . This work i s described l a t e r i n t h i s chapter, f o l l o w i n g a b r i e f review of h e t e r o j u n c t i o n s o l a r c e l l s based on s i l i c o n . H e t e r o j u n c t i o n s o l a r c e l l s have very i n t e r e s t i n g o p t i c a l , e l e c t r i c a l and t e c h n o l o g i c a l p r o p e r t i e s , which make these c e l l s a t t r a c t i v e f o r p o s s i b l e use i n t e r r e s t r i a l p h o t o v o l t a i c power generation. In some cases hetero-j u n c t i o n c e l l s can be s u p e r i o r to c l a s s i c a l homojunction c e l l s . In a con-v e n t i o n a l homojunction s o l a r c e l l photons, w i t h an energy equal to or only s l i g h t l y higher than the bandgap energy of the s o l a r c e l l semiconductor, are mostly absorbed deeply w i t h i n the semiconductor, e s p e c i a l l y i f the semi-conductor has an i n d i r e c t bandgap. E l e c t r o n - h o l e p a i r s created deeply w i t h -i n the semiconductor ( t h e r e f o r e f a r from the c o l l e c t i n g j u n c t i o n ) have a l a r g e chance of recombining before they can be separated by a p-n j u n c t i o n . Photons w i t h energy much higher than the bandgap energy of the semi-conductor are absorbed very c l o s e to the su r f a c e of the s o l a r c e l l , e s p e c i a l l y i n the cases when the semiconductor has a d i r e c t bandgap nature. I f the surface recombination v e l o c i t y at the su r f a c e i s very h i g h , most of these 104. photogenerated e l e c t r o n - h o l e p a i r s w i l l recombine, r a t h e r than d i f f u s e towards the p-n j u n c t i o n , where they could be otherwise c o l l e c t e d . This r e s u l t s i n c o l l e c t i o n l o s s e s . By employment of a h e t e r o j u n c t i o n c e l l s t r u c t u r e , p r o v i d i n g the semiconductor f a c i n g the r a d i a t i o n has a l a r g e r bandgap than the base semiconductor and that the h e t e r o j u n c t i o n - i n t e r f a c e recombination v e l o c i t y i s s u f f i c i e n t l y low, t h i s problem can be e f f i c i e n t l y overcome. The higher energy bandgap semiconductor (the "window") c l e a r l y needs to have a lower absorption c o e f f i c i e n t than the base semiconductor. As a r e s u l t , i n the h e t e r o j u n c t i o n c e l l , high energy photons are absorbed at a l a r g e r d i s t a n c e from the surface of the c e l l than i n the comparable p-n homojunction c e l l . E l e c t r o n - h o l e p a i r s c o n t r i b u t e d from hi g h energy photons thus have, i n the h e t e r o j u n c t i o n c e l l , l e s s chance to recombine at the surface than i n a comparable p-n homojunction c e l l . This can r e s u l t i n an improved short wavelength response of a h e t e r o j u n c t i o n c e l l , compared to a homojunction c e l l [33,341-The cost of forming a homojunction by d i f f u s i o n or e p i t a x i a l processes forms a s u b s t a n t i a l p o r t i o n of the s o l a r c e l l f a b r i c a t i o n cost. On the other hand a h e t e r o j u n c t i o n can oft e n be formed by employing low cost p r o c e s s i n g , such as s p u t t e r i n g , evaporation, p y r o l i t i c s p r a y i n g , e t c . , and t h i s i s another m o t i v a t i o n f o r i n v e s t i g a t i n g h e t e r o j u n c t i o n s o l a r c e l l technology. There are a l s o cases where the formation of a h e t e r o j u n c t i o n i s e s s e n t i a l , because nowadays, when l o o k i n g f o r low cost s o l a r c e l l semiconductors, many of them are a v a i l a b l e i n only one type of c o n d u c t i v i t y , so making formation of a p-n homojunction i n t h i s case i m p o s s i b l e . There are four b a s i c groups of h e t e r o j u n c t i o n s o l a r c e l l s , namely 1. Abrupt h e t e r o j u n c t i o n c e l l s 2. Heteroface s o l a r c e l l s 105. 3. Graded bandgap s o l a r c e l l s 4. Metal-Semiconductor (MS) and Metal-Insulator-Semiconductor (MIS) s o l a r c e l l s In an abrupt h e t e r o j u n c t i o n s o l a r c e l l the r e c t i f y i n g j u n c t i o n i s formed between the window semiconductor and a base semiconductor o f } u s u a l l y , opposite c o n d u c t i v i t y type from the window semiconductor. The heteroface s o l a r c e l l [50] i s b a s i c a l l y a p-n homojunction c e l l having a l a y e r of a l a r g e r bandgap semiconductor on i t s top, and i t i s u s u a l l y arranged that the two top semiconductors have the same type of c o n d u c t i v i t y . The graded bandgap c e l l [51] i s s i m i l a r to the abrupt s o l a r c e l l , except that the window semiconductor has a band gap which i n c r e a s e s towards the surface of the s o l a r c e l l . Such a b u i l t - i n f i e l d helps i n the c o l l e c t i o n of c a r r i e r s photogenerated i n the l a r g e r band gap semiconductor. In an MS s o l a r c e l l [52] a s u i t a b l e semitransparent metal f i l m , u s u a l l y deposited on top of the semiconductor by vacuum evaporation,forms a b a r r i e r (so c a l l e d Schottky b a r r i e r ) w i t h r e c -t i f y i n g p r o p e r t i e s . The induced energy band bending at the surface of the semiconductor r e s u l t s i n a b u i l t - i n f i e l d which separates m a j o r i t y and m i n o r i t y c a r r i e r s . The MIS s t r u c t u r e d i f f e r s from the MS s t r u c t u r e i n that a t h i n i n t e r f a c i a l i n s u l a t i n g l a y e r i s grown, e i t h e r n a t u r a l l y or d e l i b e r a t e l y , between the metal and the semiconductor. This i n t e r f a c i a l l a y e r , i f o p t i -mally designed, can g r e a t l y improve performance of the c e l l [53]. Because i n h e t e r o j u n c t i o n ( H J ) s o l a r c e l l s most of the photons are absorbed i n the s m a l l e r bandgap m a t e r i a l , t h e . l a t t e r should have a bandgap i n the range from 1.1 - 1.9 e.V [54] to achieve a high e f f i c i e n c y s o l a r c e l l . The l a r g e r bandgap semiconductor should have an energy bandgap as l a r g e as p o s s i b l e , so that i t absorbs a minimum number of photons. The l a t t i c e 106. constants and the thermal expansion c o e f f i c i e n t s of both h e t e r o j u n c t i o n partners should be n e a r l y equal and the e l e c t r o n a f f i n i t i e s compatible, so that r e s u l t i n g d i s c o n t i n u i t i e s i n energy bands w i l l have a minimal e f f e c t on the c o l l e c t i o n e f f i c i e n c y of a c e l l . The window semiconductor should have a low r e s i s t i v i t y and an o p t i c a l r e f r a c t i v e index i n a s u i t a b l e range, so as to serve as an a n t i r e f l e c t i o n c o a t i n g f o r the lower bandgap semi-conductor. . I t has been found r e c e n t l y that c e r t a i n oxide semiconductors, namely Sn0 2 [55], 1 ^ 0 ^ 5 6 ] , t h e i r mixture ITO [57-59] and Cd^SnO^ [60] show promise as s u i t a b l e window semiconductor m a t e r i a l s f o r h e t e r o j u n c t i o n or heteroface s o l a r . c e l l s u s i n g S i as the base m a t e r i a l . I n t e r e s t i n these m a t e r i a l s arose because of t h e i r combination of low r e s i s t i v i t y , high AMI s o l a r t r a n s m i t t i v i t y and high thermal i n f r a r e d r e f l e c t a n c e [61]. The r e -f r a c t i v e i n d i c e s of these oxides are i n the range of 1.9 - 2.0 [11], and thus are able to perform as an a n t i r e f l e c t i o n c o a t i n g on S i [57,58,62], or other semiconductors. The h i g h e s t oxide semiconductor-semiconductor (0S0). s o l a r c e l l e f f i c i e n c i e s that have been reported to the present, date are 9.9% f o r Sn0 2/nSi c e l l s [55], where the SnO^ was deposited on S i by e l e c t r o n beam eva p o r a t i o n , 12% from an .; ITO/pSi c e l l [57], where the ITO was deposited on S i by n e u t r a l beam s p u t t e r i n g , and 10% f o r ITO/nSi.[58], where ITO was deposited on S i by a p y r o l i t i c s p r a y i n g method. A l l h i g h l y conductive Sn0 2, 1 ^ 0 ^ and ITO m a t e r i a l s are degenerate semiconductors and thus 0S0 s o l a r c e l l s employing any of the above oxide semiconductors cannot be u n e q u i v o c a l l y c l a s s i f i e d i n any of the f o u r groups as d e f i n e d at the beginning of t h i s chapter. R e c e n t l y , Schewchun, DuBow, Myszkowski and Singh[102] proposed to d e s c r i b e the p r o p e r t i e s of the above-mentioned h e t e r o j u n c t i o n s t r u c t u r e s i n terms of a degenerate semiconductor-insulator-semiconductor s t r u c t u r e ( S I S ) , 107. which, i n p r i n c i p l e , i s very s i m i l a r to the MIS c o n f i g u r a t i o n . They have shown that the maximum e f f i c i e n c y of ITO/pSi c a l l s could reach 19.9% under AMI i l l u m i n a t i o n . Turning now to the relevance of Cu^O to s o l a r c e l l s i t i s noted that the bandgap of Cu^O i s 2.04 eV and i s d i r e c t , thus suggesting use of C^O e i t h e r as a window m a t e r i a l i n a h e t e r o j u n c t i o n s t r u c t u r e w i t h lower bandgap semiconductor p a r t n e r s , or as an absorbing semiconductor component f o r t h i n f i l m s o l a r c e l l s . Even i n the l a t t e r case the formation of a h e t e r o j u n c t i o n s t r u c t u r e i s necessary, because Cu^O at the present time i s a v a i l a b l e only w i t h p-type c o n d u c t i v i t y . C e l l s based on Cu^O as the absorbing component have been f a b r i c a t e d from thermally-grown Cu^O and have u t i l i z e d the r e c t i f y i n g nature of the CU/CU2O contact to o b t a i n weak p h o t o v o l t a i c performance (n = 1% [3,4,6,9 ] ) . However i t seems c l e a r that to f u l l y implement a s u i t a b l e h e t e r o j u n c t i o n technology a d e p o s i t i o n method, r a t h e r than a "grown-method" i s r e q u i r e d . A d e p o s i t i o n technique able to produce CU2O w i t h p r o p e r t i e s p o t e n t i a l l y acceptable from the standpoint of photo-v o l t a i c a p p l i c a t i o n [27,90] has been developed based on the r e a c t i v e s p u t t e r i n g procedures and theory described elsewhere i n t h i s t h e s i s . Cu^O produced by t h i s technique e x h i b i t s d e s i r a b l y low r e s i s t i v i t i e s (20-60 ohm-cm) and p o t e n t i a l l y s u i t a b l e o p t i c a l a b s o r p t i o n p r o p e r t i e s (chapter 5 ) . There i s however l i t t l e known about other C^O p r o p e r t i e s which can a f f e c t hetero-j u n c t i o n performance, namely e l e c t r o n a f f i n i t y , thermal expansion c o e f f i c i e n t and l a t t i c e match w i t h other h e t e r o j u n c t i o n p a r t n e r s . In the present work S i was used as the partner semiconductor w i t h C^O, mainly f o r the reason that S i i s a w e l l defined ,abundant m a t e r i a l , w i t h d e s i r a b l e low bandgap and w e l l - e s t a b l i s h e d p r o cessing technology. In the next s e c t i o n experiments and r e s u l t s obtained w i t h the Cu o0/Si s t r u c t u r e are presented. 108. 6.2 WAFER PREPARATION The s i l i c o n s u b s t r a t e s used were p o l i s h e d on the f r o n t s i d e and were e i t h e r n-type ([111], 2 ohm-cm) or p-type ([100], 2-8 ohm-cm). In a l l cases the p r e p a r a t i o n process was as f o l l o w s : i . PREFURNACE CLEANING 1. Immerse the sample f o r 10 minutes i n a f r e s h l y prepared s o l u t i o n of H o0 : H o0 o : NH.OH i n the r a t i o of 5: 1: 1 : by volume h e l d at 75-85°C. 2 2 2 4 The s o l u t i o n was prepared by adding NH^OH and H^O^ to b o i l i n g d e i o n i z e d water. 2. Rinse i n a deioni z e d water cascade,2 minutes ( f i r s t bath) and 8 minutes (second bath) 3. Dip i n s o l u t i o n of D.I. R^O : HF i n the r a t i o of 9: 1 by volume f o r 30 sec. 4. Repeat step 2 r i n s e s . 5. Immerse f o r 10 minutes i n a f r e s h l y prepared s o l u t i o n of D.I. R^O : ^2°2 : H C 1 i n t h e r a t i o s of 6 : 1 : 1 by volume h e l d at 75-85°C. The s o l u t i o n was prepared by adding HC1 and ^ 0 2 i n t o b o i l i n g D.I. water. 6. Repeat step 2. 7. Dip i n i s o p r o p y l or methyl a l c o h o l f o r 5 minutes. 8. Dry i n freon vapor f o r 2-3 minutes. i i . WAFER OXIDATION o A f t e r cleaning,the S i wafer was o x i d i z e d at 1100 C using a wet o x i d a t i o n process to o b t a i n a l a y e r of S i 0 o of thickness approximately 6000 X. 109. The f o l l o w i n g o x i d a t i o n c y c l e was used: 1. 5 minutes i n 0^ (1.0 l/min) 2. 2 hours i n (1.6 l/min) + 0 2 (1.0 l/min) 3. 30 minutes i n (1.0 l/min) i i i : ; BACK SIDE OXIDE STRIPPING The purpose of t h i s step was to prepare the wafer f o r the back contact doping procedure and i t i n c l u d e d : 1. A p p l i c a t i o n of Waycoat P o s i t i v e LSI p h o t o r e s i s t at 4500 rpm on the f r o n t s i d e of the wafer, followed by a 1 minute dip i n Waycoat P o s i t i v e LSI developer, a 60 sec. r i n s e i n D.I. water and then blowing dry i n n i t r o g e n . 2. Baking the sample i n a convection oven f o r 30 minutes at 120°C. 3. Removal of the back s i d e oxide i n a 6:1 s o l u t i o n by volume of NH.F stock 4 and HF. The NH^F stock was a 40% by weight s o l u t i o n of NH^F i n D.I. water. 4. Removal of the p h o t o r e s i s t i n b o i l i n g acetone. This procedure was r e -peated twice, always w i t h new acetone. i v . PREFURNACE CLEANING (Same procedure as described i n step i ) . v. PREDEPOSITION Here i t i s necessary to d i s t i n g u i s h between the processes used f o r p-type and n-type wafers. N_+ p r e d e p o s i t i o n f o r back s i d e ohmic contact to n-type wafers (Furnace temperature = 965°C) 1. Predoping of the p r e d e p o s i t i o n tube and the p r e d o p o s i t i o n boat f o r 20 minutes i n N 2 (2.0 l/min) + 0 2 (60 cc/min) + P0C1 3 (15°C) d r i v e n by N 2 (60 cc/min). 110. v. PREDEPOSITION (cont'd) 2. I n s e r t i n g the samples i n t o the p r e d e p o s i t i o n tube. 3. 10 minute c y c l e i n (2.0 1/min) + 0^ (60 cc. min) 4. 55 minutes exposure to the same gas flow r a t e s as i n step 1. 5. 5 minute exposure to the same gas flow r a t e s as i n step 3. P"*~ p r e d e p o s i t i o n f o r back s i d e ohmic contact to p-type wafers (Furnace temperature = 1090°C) 1. 10 minutes i n N 2 (2.0 1/min) 2. 5 minutes i n (2.0 1/min) + dopant ^ n methanol and some HC1 kept at 15°C) d r i v e n by (60 cc/min). 3. 15 minutes i n N 2 (2.0 1/min) v i . DEGLAZING J J + - 1 minute etch i n a 10 : 1 s o l u t i o n of D.I. H^ O and HF, fo l l o w e d by D.I. water r i n s i n g and blowing dry i n n i t r o g e n . P + - 20 sec. etch i n a 1: 1 s o l u t i o n of D. I H^ O and HF, fo l l o w e d by D.I. water r i n s i n g and blowing dry i n n i t r o g e n . v i i . DIFFUSION N + - Furnace temperature was 1090°C and the timing used was as f o l l o w s : 1. 5 minutes i n 0^ (1.5 1/min) 2. 50 minutes i n 0 2 (1.5 1/min) + H 2 (2.4 1/min) 3. 5 minutes i n 0 2 (1.5 1/min) P_+ - Furnace temperature was 1090°C and timing used was as f o l l o w s : 1. 5 minutes i n 0 2 (1.5 1/min). 2. 2 hours i n 0 '+ HC1 (60 cc/min) 111. DIFFUSION (cont'd) 3. 30 minutes i n 0 2 + HC1 + H 2 (2.4 l/min) 4. 5 minutes i n 0^ v i i i . OPENING THE DIODE WINDOWS ON THE WAFER FRONT JSIDE In t h i s step the oxide was s t r i p p e d from the back s i d e of the wafer and, simultaneously, windows of 4.7 mm diameter were opened photo-l i t h o g r a p h i c a l l y i n the oxide covering the f r o n t S i s u r f a c e . The f o l l o w i n g steps were necessary: 1. A p p l i c a t i o n of Waycoat P o s i t i v e LSI 295 p h o t o r e s i s t at 4500 rpm. 2. Prebaking the samples at 90°C f o r 30 minutes. 3. Exposure by an u l t r a v i o l e t l i g h t source f o r 2 minutes through the mask shown i n F i g . 6.5. 4. Developing i n Waycoat P o s i t i v e LSI Developer, d i l u t e d 1 : 1 w i t h D.I. water, f o r 90 seconds, followed by D.I. water r i n s e f o r 60 sec. and blowing dry i n n i t r o g e n . 5. Postbaking the samples at 120°C f o r 30 minutes 6. Removal of unmasked S i 2 0 i n s o l u t i o n of 6 :1 by volume NH^F stock and HF. 7. 2 and 8 minutes D.I. water cascade r i n s e s . 8. Blowing dry i n n i t r o g e n . 9. Removing the p h o t o r e s i s t i n b o i l i n g acetone. 10. D e p o s i t i o n of a V 2000 A* t h i c k gold f i l m on the back s i d e of the wafer by vacuum evaporation. 11. A l l o y i n g the contact at 500°C f o r 20 minutes i n a n i t r o g e n atmosphere. 12. S c r i b i n g and breaking the wafers i n t o q u a r t e r s . 112. The above procedure produced samples w i t h an ohmic back contact and a f r o n t s i d e comprising e i g h t 4.7 mm diameter c i r c l e s of exposed S i , i s o l a t e d from each other by ^ 6000 S of S i 0 2 . C^O d e p o s i t i o n and subsequent diode f a b r i c a t i o n i s described i n the next s e c t i o n . 6.3 Cu 20/Si HETEROJUNCTION PREPARATION. P r i o r - to;Cu^O d e p o s i t i o n each sample ( s i l i c o n wafer quarter) was immersed i n 10% HF f o r 10 seconds, r i n s e d thoroughly i n d e i o n i z e d water (2 minute and 8 minute D.I. water cascade r i n s e s ) , blown dry i n n i t r o g e n and then contacted to the s u b s t r a t e t a b l e sample holder (see F i g . 6.1) using g a l l i u m as a bonding m a t e r i a l . The sample bonding was a very s t r a i g h t f o r w a r d procedure. The s u b s t r a t e t a b l e sample holder was removed from the s u b s t r a t e t a b l e ( F i g . 3.4) and heated by a p l a t e heater to a temperature of approximately 50°C. Then the wafer was placed i n the i n l a y of the s u b s t r a t e t a b l e sample holder which was f i l l e d w i t h g a l l i u m . Areas f o r C^O d e p o s i t i o n (6.5 mm d i a -meter, centred on the oxide windows) were defined by a metal mask ( F i g . 6.3 ) which was pressed down on the sample by three s t e e l s p r i n g c l i p s , (see F i g . 6.2 ). Thus an i n t i m a t e contact of the mask and the sample was ensured, avoid-i n g excess shadowing at the mask edges. The s u b s t r a t e t a b l e sample holder was then i n s e r t e d i n t o the s u b s t r a t e t a b l e and the s p u t t e r i n g chamber was evacuated to a vacuum of b e t t e r than 5x10 ^ Torr. Then s p u t t e r i n g at the d e s i r e d r . f . power l e v e l was c a r r i e d out such as to produce Cu^O f i l m s ^ 1500 X t h i c k and of r e s i s t i v i t y ^ 50 ohm cm. This combination of Cu 20 and S i e x h i b i t e d a b l a c k appearance, suggesting that t h i s p a r t i c u l a r s t r u c t u r e had good a n t i -r e f l e c t i o n p r o p e r t i e s . F i n a l l y , i n another vacuum system, gold was evap-orated through another metal mask ( F i g . 6.4) to produce e i t h e r a 4 - f i n g e r , 113. F i g . 6.1 Substrate t a b l e sample holder ICON s i o 2 F i g . 6.2 Mask mounting f o r Cu O/Si h e t e r o j u n c t i o n formation 115, Metal mask f o r Cu 20/Si Fig.6.3 h e t e r o j u n c t i o n formation. V 1 H Fig.6.4 Metal mask f o r top s o l a r 11 #11 r-l c e l l m e t a l l i z a t i o n . • • X'4 Fig.6.5 Mask f o r SiG y windows formation. fl&3 K'.'i © © © © © © ' Q © © © O to 23/4' (a) S i l i c o n wafer prepared f o r Cu„0 d e p o s i t i o n . ( b ) F i n a l C u 2 0 / S i s o l a r c e l l c o n f i g u r a t i o n . Au TOP METALLIZATION -\-2000A 1500A / S i 5000A (c) Schematic view of Cu 20/Si s o l a r c e l l cross s e c t i o n . F i g . 6.6 H e t e r o j u n c t i o n C u 9 0 / S i s o l a r c e l l c o n f i g u r a t i o n comb-like contact g r i d p a t t e r n or a s o l i d c i r c u l a r p a t t e r n . Each qu a r t e r s l i c e y i e l d e d 3 diodes and 5 s o l a r c e l l d e v i c e s , each w i t h an a c t i v e area of 2 0.106 cm and mutually i s o l a t e d by the remaining S i 0 2 l a y e r (see F i g . 6.6). 6.A I-V CHARACTERISTICS I-V measurements were performed on the diodes both i n the dark and _2 under 100 mWcm of simulated AMI s u n l i g h t obtained from a 3200 K tungsten lamp, f i l t e r e d by a wide band hot m i r r o r . R e c t i f i c a t i o n was observed w i t h both p-type and n-type s i l i c o n s u b s t r a t e s , the d i r e c t i o n of forward b i a s being s i l i c o n p o s i t i v e f o r p-type s u b s t r a t e s and s i l i c o n negative f o r n-type s u b s t r a t e s . T y p i c a l c h a r a c t e r i s t i c s f o r C^O/nSi devices are shown i n F i g s . 6.7 and 6.8. The r e d u c t i o n i n magnitude of the reverse b i a s s a t u r a t i o n current and the i n c r e a s e i n forward "turn-on" voltage w i t h decreasing s p u t t e r i n g power ( F i g . 6.7) suggest that the device performance i s s t r o n g l y i n f l u e n c e d by the presence of bombardment-induced su r f a c e s t a t e s . This i s f u r t h e r evinced by the dark forward b i a s data p l o t t e d i n semilog f a s h i o n as shown i n F i g . 6.9. S i m i l a r d e v i a t i o n s from l i n e a r i t y of l n l - V p l o t s w i t h i n c r e a s i n g s p u t t e r i n g power have been observed i n s p u t t e r e d Mo-Si Schottky diodes [63]. M u l l i n s and Brunnschweiler [63] suggest that during s p u t t e r d e p o s i t i o n a h i g h d e n s i t y of " d o n o r - l i k e " traps i s created i n the surface region of s i l i c o n . The trap d e n s i t y was assumed to decrease e x p o n e n t i a l l y i n t o the bulk as N f c(x) = N T G exp (" L) (6.D where N i s the trap d e n s i t y at the s u r f a c e , L i s some c h a r a c t e r i s t i c l ength and N t ( x ) i s the c o n c e n t r a t i o n of these traps at a d i s t a n c e x from the s u r f a c e . Both, L and N depend on the s p u t t e r i n g v o l t a g e and the 118. 200W 100W 50W F i g . 6.7 Dark and i l l u m i n a t e d (100 mW cm ) I-V c h a r a c t e r i s t i c s f o r Cu o0/n-Si diodes f a b r i c a t e d at d i f f e r e n t s p u t t e r i n g power l e v e l s . 119. n-type Si 0 50 100 mVolts - 2 F i g . 6.8 I l l u m i n a t e d (100 mW cm ) I-V c h a r a c t e r i s t i c s f o r C^O/n-Si h e t e r o j u n c t i o n diodes f a b r i c a t e d at d i f f e r e n t s p u t t e r i n g power l e v e l s . 120. L 1 . I I I : _ i 0 50 100 150 200 250 V p [mV] F i g . 6.9 Semilog p l o t of the forward I-V c h a r a c t e r i s t i c s from the diodes used to o b t a i n F i g . 6.8. 121. s p u t t e r i n g time. Berg, Andersson, Norstrom and B r u s s e l proposed [64] th a t t h i s high d e n s i t y of donor centres creates a t h i n r e g i o n of very h i g h e l e c t r i c f i e l d and thus forms a narrow surface b a r r i e r . Under such c o n d i t i o n s a s i g n i f i c a n t c o n t r i b u t i o n to the dark current can a r i s e from e l e c t r o n t u n n e l i n g a t some r e p r e s e n t a t i v e energy E n n s m a l l e r than the b a r r i e r h e i g ht energy of q$ , see F i g . 6.10. BU METAL SEMICONDUCTOR T-F EMISSION 0 d x F i g . 6.10 Schematic energy diagram of a reverse b i a s e d sputter-damaged Schottky b a r r i e r . The damaged r e g i o n i s 0 £ x £ d. The thermionic f i e l d emitted e l e c t r o n s are i n d i c a t e d by the arrow. E^ i s the Fermi l e v e l i n the metal and i s the quasi-Fermi l e v e l f o r e l e c t r o n s i n the semiconductor. The p r e c i s e p o s i t i o n of EQQ w i l l depend upon the magnitude of the surface trap d e n s i t y . A low sur f a c e trap d e n s i t y gives ^QQ^O^BO ^ U T i n c r e a s i n g the s u r f a c e d e n s i t y gives a dec r e a s i n g value of EQQ and thus leads to an i n c r e a s i n g value of the tunne l c u r r e n t which i s c o n t r i b u t e d by t h e r m i o n i c - f i e l d emission through the< narrow s u r f a c e b a r r i e r . Berg e t a l . [64] showed how t h i s c u r r e n t component was revealed i n reverse b i a s I-V measurements and very s i m i l a r c h a r a c t e r i s t i c s were obtained from the present diodes, see F i g . 6.11. The reverse b i a s current at any given b i a s increases as the s p u t t e r i n g power used during d e p o s i t i o n i n c r e a s e s , as would be expected from the as s o c i a t e d i n c r e a s e i n N t g and decrease i n E n n . F i g . 6.11 thus suggests that t h e r m i o n i c - f i e l d emission i s the dominating current transport mechanism at room temperature i n C^O/nSi diodes formed by r . f . r e a c t i v e s p u t t e r i n g , and that i t s c o n t r i b u t i o n to the t o t a l current increases w i t h the d e p o s i t i o n power. N w i l l a l s o depend, f o r a given d e p o s i t i o n power, on the d e p o s i t i o n time used. For the diodes w i t h c h a r a c t e r i s t i c s given by F i g s . 6.7-9 and 6.11 both the d e p o s i t i o n power l e v e l and time were v a r i e d i n order to produce approximately equal thicknesses of Cu^O i n a l l cases. I t i s conceivable that there i s an optimum c o n d i t i o n f o r the s p u t t e r i n g power and the time, but t h i s was not d e l i b e r a t e l y sought a t t h i s stage. However some i n d i r e c t evidence of t h i s e f f e c t can be i n f e r r e d from data taken on C^O/pSi diodes, f o r which the I-V c h a r a c t e r i s t i c s d i d not f o l l o w the s t r a i g h t f o r w a r d r e l a t i o n s h i p w i t h s p u t t e r i n g power shown by the diodes on n S i (see F i g s . 6.12 and 6.13 and compare w i t h F i g s . 6.7 and 6.8). In the case of the diodes on p S i i t i s l i k e l y that the s u r f a c e l a y e r of d o n o r - l i k e centres induces an i n v e r s i o n l a y e r i n the S i which could improve the p h o t o v o l t a i c performance of such diodes, as i s the case i n MIS s t r u c t u r e s [53]. However, f o r the sput t e r e d Cu^O/pSi d e v i c e s , there w i l l o b v i o u s l y be some trade o f f i n v o l v e d between t h i s p o s s i b l y advantageous e f f e c t and the ob v i o u s l y d e l e t e r i o u s e f f e c t of the in c r e a s e d dark current as N i n c r e a s e s . In the present work the h i g h e s t AMI conversion e f f i c i e n c y achieved was 1%, and was obtained on a p-type s u b s t r a t e w i t h a C^O l a y e r s p u t t e r e d at 100W f o r 5 minutes w i t h an oxygen p a r t i a l pressure of 0.4 123. i c f 3 10 2 10 1 10° 101 . VOLTS F i g . 6.11 Dark reverse I-V c h a r a c t e r i s t i c s f o r Ci^O/n-Si diodes f a b r i c a t e d at d i f f e r e n t s p u t t e r i n g power l e v e l s . 124. p - type Si 0 100 200 300 mVolts —2 F i g . 6.12 I l l u m i n a t e d (100 mW cm ) I-V c h a r a c t e r i s t i c s f o r Cu 20/p-Si h e t e r o j u n c t i o n diodes f a b r i c a t e d at d i f f e r e n t r . f . power l e v e l s . 125. F i g . 6.13 Semilog p l o t of the forward I-V c h a r a c t e r i s t i c s of Ci^O/p-Si diodes formed by r e a c t i v e s p u t t e r i n g . 126. -2 mTorr. For t h i s device the short c i r c u i t current J = 8.7 mA cm , open sc J f : c i r c u i t v o l t age V = 315 mV and f i l l f a c t o r FF = 0.40. I t i s noteworthy oc . J that the same e f f i c i e n c y has been obtained from ITO/Si c e l l s i n which r . f . s p u t t e r i n g was used f o r the ITO d e p o s i t i o n [65], but t h a t an e f f i c i e n c y of 12% has been a t t a i n e d i n ITO/Si c e l l s u s i n g n e u t r a l beam s p u t t e r i n g f o r the ITO p r e p a r a t i o n [57]. In the l a t t e r approach high energy e l e c t r o n bombard-ment of the su b s t r a t e can be avoided. 127. 7. CONCLUSIONS A comprehensive theory of r e a c t i v e s p u t t e r i n g has been developed which allows the absolute composition r a t i o of deposited f i l m s to be r e l a t e d to c o n t r o l l a b l e s p u t t e r i n g parameters. The theory i s based on the p o s t u l a t e that the composition r a t i o of deposited metal oxide f i l m s i s determined by the r e l a t i v e magnitudes of the f l u x e s of metal and oxygen atoms at the su b s t r a t e plane. These f l u x e s depend on the metal s p u t t e r i n g r a t e and on the oxygen p a r t i a l pressure P q r e s p e c t i v e l y . V a r i a t i o n of these parameters thus provides a convenient experimental arrangement f o r producing metal oxide f i l m s w i t h a range of compositional r a t i o s . Good agreement was obtained between the p r e d i c t i o n s from the theory and the composition of copper oxide f i l m s prepared e x p e r i m e n t a l l y by both d.c. and r . f . r e a c t i v e s p u t t e r i n g i n oxygen/argon mixtures. The trend i n f i l m composition r a t i o was Cu -> Cu + Cu 20 Cu 20 -> Cu 20 + CuO CuO oxygen p a r t i a l pressure copper s p u t t e r i n g r a t e <^  — Comparison of t h e o r e t i c a l and p r a c t i c a l data a l s o y i e l d e d a value f o r the s t i c k i n g c o e f f i c i e n t , namely a o^0.1148,of oxygen on copper-coated s u r f a c e s . Copper oxide f i l m s of each phase composition possessed d i f f e r e n t yet c h a r a c t e r i s t i c p r o p e r t i e s as regards o p t i c a l constants and r e s i s t i v i t y . The data on o p t i c a l constants are the f i r s t reported f o r r e a c t i v e l y s puttered copper oxide f i l m s . The r e s u l t s f o r CuO f i l m s are s i m i l a r to those reported f o r thermally-grown f i l m s . Cu 20 f i l m s show a c h a r a c t e r i s t i c peak i n the r e a l p a r t of the r e f r a c t i v e index (n ^  3.36) centred at X = 0.48um. There i s evidence of f r e e c a r r i e r absorption i n conductive Cu 20 and acceptor l e v e l (copper i o n vacancy)- conduction band t r a n s i t i o n s i n n e a r - s t o c h i o m e t r i c 1 2 8 . ( h i g h l y r e s i s t i v e ) m a t e r i a l . The r e s i s t i v i t y of CuO f i l m s i s u n u s u a l l y low ( 0 . 1 - 1.0 ohm.cm) and th a t of Cu^O f i l m s e a s i l y c o n t r o l l a b l e , by v a r y i n g P q or R^, w i t h i n the range of 2 0 - 60 ohm cm. C^Q. i s a defe c t - c o n d u c t i n g , p-type semiconductor but has been l i t t l e used outside the l a b o r a t o r y on account of the f a c t that usual methods of p r e p a r a t i o n (thermal o x i d a t i o n and e l e c t r o d e p o s i t i o n ) cannot r e p r o d u c i b l y y i e l d low r e s i s t i v i t y m a t e r i a l . The r e a c t i v e s p u t t e r i n g method of d e p o s i t i o n developed i n t h i s work o f f e r s a way around t h i s problem. One p a r t i c u l a r a p p l i c a t i o n of promise i s i n the area of t h i n f i l m s o l a r c e l l s and devices u s i n g a C^O/Si h e t e r o j u n c t i o n s t r u c t u r e were f a b r i c a t e d . R e c t i f i c a t i o n w i t h both n- and p-type S i s u b s t r a t e s was observed and a s o l a r energy conversion e f f i c i e n c y of 1 % was obtained. The s o l a r c e l l s were c l e a r l y adversely a f f e c t e d by hig h energy e l e c t r o n bombardment from the s p u t t e r i n g environment and i t i s suggested t h a t the employment of n e u t r a l beam s p u t t e r i n g could a l l e v i a t e t h i s problem yet s t i l l m aintain the a t t r a c t i v e features of t h i s d e p o s i t i o n method. APPENDIX A THERMAL OXIDATION OF COPPER TO COPPER OXIDE At the outset of t h i s study the formation of copper oxide f i l m s by both r e a c t i v e s p u t t e r i n g and the more t r a d i t i o n a l method [66-86] of thermal o x i d a t i o n was c a r r i e d out. When i t became c l e a r that the r e a c t i v e s p u t t e r i n g method was going to y i e l d new and i n t e r e s t i n g data the work on thermal o x i d a t i o n was d i s c o n t i n u e d . However a b r i e f review of t h i s l a t t e r work i s i n order and i s given i n t h i s appendix. Oxidations were performed i n the temperature range of 1020 -1065°C i n the v e r t i c a l quartz furnance shown i n F i g . A . l . The e l e c t r i c a l l y c o n t r o l l e d v alves i n the system allowed easy changing of the furnace environment from n i t r o g e n (used when lo a d i n g and quenching the samples) to a i r (used at 1.48 1/min f o r o x i d a t i o n ) . Rapid quenching was achieved by a l l o w i n g the quartz rod supporting the sample to s l i d e through i t s r e t a i n i n g r i n g at the top of the furnance u n t i l the sample was outside the furnace hot zone. P r i o r to an o x i d a t i o n samples (6x4 cm Cu f o i l s w i t h t h i c k n e s s i n the range 5-25 m i l ) were u l t r a s o n i c a l l y degreased i n chloroform, etched i n d i l u t e d HNO^ and r i n s e d i n d e i o n i z e d water. Before a sample was loaded i n t o the furnace the quartz tube was throughly r i n s e d by n i t r o g e n to avoid CuO formation. A f t e r l o a d i n g , when the temperature of the sample reached the d e s i r e d o x i d a t i o n temperature, a i r i n s t e a d of n i t r o g e n was allowed i n t o the quartz tube. Because a i r was used as an o x i d a t i o n atmosphere, the o x i d a t i o n temperature was r e s t r a i n e d to a range of 1020-1065°C. The lower temperature l i m i t was given by the s t a b i l i t y o f Cu^O i n a i r and the higher l i m i t was determined by the e u t e c t i c temperature of the Cu/Cu ?0 system. 130. F i g . A . l V e r t i c a l furnace f o r thermal o x i d a t i o n and s i n g l e c r y s t a l growth of Cu„0. 7 8 9 10 11 12 13 14 15 d i s t a n c e x from top of the furnace [inches] F i g . A.2 Temperature p r o f i l e of the o x i d a t i o n furnace at 1.48 l/min. of a i r flow. The d i s t a n c e i s measured from p o i n t L, see F i g . A . l . i—1 i - 1 132. Because of the thermal gradient i n the furnace, the upper end of the sample was kept at a temperature of 1062 ± 2°C. Having a thermal gradient along the sample proved b e n e f i c i a l i n o b t a i n i n g l a r g e c r y s t a l s and the d e s i r e d thermal gradient could be chosen by p o s i t i o n i n g the sample s l i g h t l y o f f the center of the furnace (see F i g . A.2). The length of time f o r a complete sample o x i d a t i o n depended on the thickness of the s t a r t i n g copper f o i l . For 5 m i l t h i c k copper t y p i c a l l y 10 minutes were r e q u i r e d f o r complete o x i d a t i o n . Oxidation times f o l l o w e d the expected d i f f u s i o n theory [96], given by aK ( A . l ) where a i s the r a t i o of the i n i t i a l copper th i c k n e s s t o the t h i c k n e s s of r e s u l t i n g C^O, d i s the i n i t i a l Cu f o i l t h i c k n e s s and K an i s o b a r i c thermal constant. A f t e r completion of the Cu to Cu^O conversion, the r e s u l t i n g cuprous oxide had a f i n e grained p o l y c r y s t a l l i n e s t r u c t u r e w i t h an average 2 g r a i n s i z e of 0.02 mm , see F i g . A.3. Cuprous oxide g r a i n s , d u r i n g t h e i r thermal growth, form a columnar s t r u c t u r e , see F i g . A.4. The d i s o r d e r e d region which can be seen i n Fig.A.5, across the middle p a r t of the sample, i s caused by the f a c t that o x i d a t i o n proceeds from both s i d e s of the sample. Note that i n preparing samples f o r scanning e l e c t r o n microscropy the c r y s t a l s t r u c t u r e was revealed by a s e l e c t i v e etch i n s o l u t i o n of 25% HNO^, 10% H 2 0 2 and 65% H 20. I t was found that the c r y s t a l g r a i n s i z e could be c o n s i d e r a b l y increased by annealing i n a i r at the growth temperature. Presumably t h i s p r e v i o u s l y unreported phenomenon stems from the temperature gr a d i e n t present i n the furnace which allows s i g n i f i c a n t s t r a i n - a n n e a l i n g growth to F i g . A.4 Cross s e c t i o n SEM p r i n t of p a r t i a l l y o x i d i z e d Cu p l a t e . M a g n i f i c -a t i o n x 125. 134. F i g . A.6 Photograph of cuprous' oxide p l a t e prepared from 99.999% Cu f o i l 20 m i l t h i c k and annealed f o r 14 hours. 135. (a) (b) F i g . A.7 Photographs of the Cu 20 samples prepared from 99.99% Cu w i t h 63 ppm of N i content. The thic k n e s s of the i n i t i a l copper i n both cases was 5 m i l . (a) - sample annealed f o r 1% hours; (b) - sample annealed f o r 5 hours. F i g . A.8 Photograph of the Cu 20 sample prepared from 99.9% Cu 5 m i l t h i c k , annealed f o r 7 hours. occur [94,95]. Annealing times used were r e l a t e d to the thickness of the samples and there was some evidence that the u l t i m a t e g r a i n s i z e depended on the p u r i t y of the copper f o i l . This i s i l l u s t r a t e d by F i g s . A.6-8. The o r i e n t a t i o n and p e r f e c t i o n of c r y s t a l s was examined by usi n g Laue-pattern X-ray d i f f r a c t i o n techniques [93]. In a l l cases the Laue d i f f r a c t i o n p i c t u r e s taken from w e l l annealed samples had sharp d i f f r a c t i o n s p o t s , thus i n d i c a t i n g good c r y s t a l p e r f e c t i o n without s i g n i f i c a n t c r y s t a l d e f e c t s . I t was found that l a r g e s i n g l e c r y s t a l g r a i n s had c e r t a i n pre-f e r r e d o r i e n t a t i o n s , namely a few degrees from [ 110] , [211], [311], [511] and [231] normal d i r e c t i o n s . In some cases the [210] o r i e n t a t i o n was a l s o found. During the g r a i n growth process g r a i n s grow not. only l a t e r a l l y across the sample but a l s o through the thickness of the sample. Thus the l a r g e grains extend through the whole sample t h i c k n e s s , however the memory of the disordered r e g i o n could never be completely o b l i t e r a t e d , see F i g . A.9. A f t e r annealing a l l samples were a b r u p t l y cooled from the o x i d a t i o n temperature to room temperature i n a stream of c o l d n i t r o g e n , as proposed by Olsen [ 8]> to achieve the lowest p o s s i b l e r e s i s t i v i t y . R e s i s t i v i t i e s and H a l l m o b i l i t i e s of samples were determined by the Van derPauw method [97] . Ohmic contacts to the sample were made by gold •evaporation. E l e c t r i c a l connections between the probe and the sample were 2 - 1 -1 made by tungsten s p r i n g s . M o b i l i t i e s were i n the range 50-75 cm V sec R e s i s t i v i t i e s were i n the range 5,000 - 60,000 ohm-cm. There was some i n d i c a t i o n that samples prepared from the l e a s t pure copper f o i l (99.9%) gave lower values of r e s i s t i v i t y than samples r e s u l t i n g from 99.999% m a t e r i a l . D e l i b e r a t e doping (by d i f f u s i o n ) of C^O by Zn and Cd has been reported to g i v e , i n some cases, enhanced p-type c o n d u c t i v i t y [ 7 ,89 1 a n d var i o u s attempts were made i n the present work to achieve t h i s e f f e c t . / 137. Zinc was i n c o r p o r a t e d i n or on the s t a r t i n g copper f o i l , e i t h e r by d i r e c t e vaporation o r by u t i l i z i n g f o i l s o f a l l o y (5% Zn). In both cases v a p o r i z a t i o n of the z i n c occured d u r i n g o x i d a t i o n and, although r e s i s t i v i t i e s around 2,000 ohm-cm could be obtained, the r e p r o d u c i b i l i t y and u n i f o r m i t y was not s a t i s f a c t o r y . As a r e d u c t i o n i n r e s i s t i v i t y i s e s s e n t i a l i f t h e r m a l l y grown cuprous oxide f i l m s are to f i n d any widespread use i n the semiconductor i n d u s t r y i t f o l l o w s t h a t more work might be done i n t h i s area of doping. Replacing oxygen by Br, CI or F may prove more s a t i s f a c t o r y than attempting metal i o n doping, although the l a t t e r might be achieved by the n u c l e a r transmutation of Cu to Zn us i n g thermal neutrons. F i g . A.9 Cross s e c t i o n SEM p r i n t of completely annealed Cu^O sample. M a g n i f i c a t i o n x 260. 138. A P P E N D I X B PROGRAM FOR COMPUTATION OF O P T I C A L CONSTANTS 1 C FORTRAN (FTtg) PROGRAM 2 C PROGRAM DESIGNED TO COMPUTE THE OPTICAL CONSTANTS N , K , A L P H A F R O M 3 C REFLECTANCE AND TRANSMITTANCE DATA GIVEN IN UNITS OF OPTICAL it c '" OTNSI TIE s FOR MAXIMUM" SO VAVE L E N G T H P O I N T S , C O M P U T A T I O N i s BASED 5 C O N THE NUMERICAL ITERATION METHOD UTILIZING " N D I N V T " ROUTINE 6 C (SECANT METHOD) SUPPLIED BY T HE UBC COMPUTING CENTRE.  7 C EQUATIONS WHICH RF.|.ATp N,K TO MEASURED R,T VALUES WERE ADOPTED 8 C FROM PUBLICATION R Y P.0.NILSSON,APPLIE0 OPTICS V0L.7,«i5 ( 1 9 0 8 ) , 9 C PROGRAM IS DESIGNED To RE RUN FROM A CONVERSATIONAL TERMINAL, . „ . f H K i ^ P T j r !<A~f A A«'f:i F r R s r i l N E - F l L M thickness IN M I C R O N S I N FORMAT 11 C (F) (G10.6),FILM IDENTIFICATION F ( 5 A O ) ; A L L NEXT SUBSEQUENT LTNES-1 2 C WAVELENGTH F(G3.2),REFERENCE MIRROR NE F LLC T A Nr. E F (G5 . a ) , K E F R ACT 1 VE ~ T j C INDEX OF THE SllFJSTRATF. F (G6 , 0),0.D.FOR REFLECTANCE FROM THE h I L M \U C STOE F ( G 5 . 3 ) , O . P . F o R TRANSMITTANCE FCG. 3 ) , CONTROL NUMBER: BLANKS 15 C WHEN NEXT POINT EXpECTED,01 FOR LAST POINT,ALL OTHER DATA ARE 16 C STTP PT TOT' BY "0 P f R AT I I R o U RI NT," THEPROGRAM EXECUTION,THEIR MEANING 17 C AND FORMATS ARE SEL^EXPLANATORY FROM THE MESSAGES WRITTEN ON THE 1 8 C CRT .  ~T9 C DATA ARE EXPECTED TO BE READ FROM UNIT 1, AND RESULTS TO BE 20 C WRITTEN ON UNIT 2. 21 C INSTRUCTION TCi RUN THE PROGRAM ARE A S FOLLOWS j ""22 C "RUNPRTGRA'HT*Nrn"MX'lBIs 1 NP(|T 0 A T AF I L E 2 = 0UT P;jTD AT A F I L E 23 C THE RESULTS WRITTEN ON UNIT 2 ARE SELFEXPLANAT ORY, 2o C * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * i t c : ' 26 C DEFINING OF ARRAYS, TYPES OF VARIABLES AND BLOCKS O F MEMORY COMMON 27 C TO THE MAIN PROGRAM AMD THE SUBPROGRAM; ? a ' — 1 Mf-'i i r. rr R E A r*>. CA - H , n _ n - - - — . - - • -29 DIMENSION A c C E s T ( 2 ) » F f 2 ) . X ( 2 ) , R O C 5 0 ) , R E C 5 0 ) , T R ( 5 0 ) , W L ( 5 0 ) , Y N ( 5 0 ) « Y 30 1 K (50 ), S M (50 1 , A I . P H A ( S O ) • '  32 C 33_ C CONSTANTS DECLARATION ._ 35 1 = 0 3b API=^0000.*PI _ 3 7 ___ . . 3fl C CONSTANTS REQUIRED BY NDINVT ROUTINE 39 N = 2_ «l E RRsl.0-06 42 EXTERNAL FC.N • - - T 3 C" t i i c • ••• US C READING OF FILM T H I C K M E S S AND FILM DESCRIPTION DATA tit, " C DATA A R E E X P E C T E D T ' O R E O N F I L E ( U N I T 1 ) «7 R E A D d , I 00 )FILMTH,ED1 , F D 2 , F D 3 , F D ' 4 F F D 5 UH DTsFILMTH  50 C LOOP DESIGNED TO C O f ' T « O L READING A D A T A FROM F I L E 51 2 1=1+1 52 C READ FROM' FILE.'WAVELENGTH, A(.UMIHUMMIRROR REFLECTANCE, SUBSTRATE N, 53 C OPTICAL DENSITY '(0.0.) FOR REFLECTANCE FROM FILM SIDE, O.D. FOR 5a C TRANSMITTANCE, CONTINUATION N U M R E R ; N E X T D A f A s Q ( B L A N K ) , L A S T DATA =1 ~ 5 5 READ( 1 , 1 0 1 ) « L ( I 1 » R O C I ) , S N ( I ) , R t ( I ) , T R ( I ) « J ' ~~ 56 I F ( J . E Q . O ) G O T O 2 57 NP=I —58 C 7 " " ' ~ 7 "'"'"T'""'~"" 59 C WRITING T H E F I L M THICKNESS, FILM DESCRIPTION A N D NUMBER O F POINTS O N 6 0 C FI L E (UNIT 2) . 139. 61 w R t T E (2, 2 0 <J) F 11 M T H, Y D \ , F 0 2","TD 3, F D"«T, F OYTfTP' ' 62 WRITE.(6,20'»)Fl_HTH,FDi , FD2,FD3.FD4,FD5,NP 63 C 6 4 ' ' " o n " i o i ) 6 1=1,NP : ' " ' " "' 65 C CALCULATION OF ABSOJ-UTE RK.FLECATNCE FROM 0,0, DATA 66 P E ( I ) = R O ( r ) « 1 0 . * « ( - R E M 1) _ 67 C CALCULATION OF ABSOLUTE I RA NSTHT ' f Wl .TY~fR 0 M 0~7D , DATS : 68 T R ( I ) = 1 0 . * * ( - T R ( I ) ) 70 1 oa FORMAT ( 1 r= ' ,C-l2,6,'«x, IR = i ,G12.6,2X, ,12) 71 1000 CnMTI• !U£ 72 C ^ . 7'i J = 0 75 1 = 0 '* 7 6 " C " RE A 0 POINT NUMBERS FOR WHICH THE OPT ICAl.'" CONSTANTS A R E " T/0 HE C A LCD LA T E 0 77 3 WRITE(6,200) 7 fl READ(5, 1 02 )H, J ____ _ -79 C HEAP GUESS FOR N -80 WRITE(6,201 ) Bl PE'AOCS, 103)X(2) 8 2 " C R E A D GUESS F O R K " " , : ' " " 7 ~ " ; «3 teRlTt(6,202) __B'i ^E AD (5, I «3 ) X (1 ) ^ _ &~5 "' 'IF ( J . G T , i l ) X o T O 5 ~ -£6 IF (M.GT.O) I=M-1 87 G O T O 10 3<J C 90 C ' LOOP DESIGNED FOR ITERATION COMPUTING OF N,K FOR GIVEN R, T DATA ' • > I C r n T T W " 7 l 7 / t ^ A~ EOT? ~"CT VFN RAW(VE 0F—WAVEV,ENGTH'T-01 N T S • -9H • 5 DO 100 1 I 'ti RMS = SM(I) < > > • > : " P O N S = R : < S >»2 "'" "'' •' "" "»S SONSM1 =33NS-1 , 96. SQNSP1=SONS+ 1 , ~ * n r R=RE~m — : ; • '— 9fl T=TR(I) "9 w=WL(I3 " T O O " ' C A L L f N V T (M , x , F, A C C E S T / MAX I T / ERR » FCN , & 20 ) " • 101 YN ( I ) = X ( 2 ) 102 YK( I )=X(1) • 10 3 GO TO 1 0 00 I 10H C ERROR OUTPUT FQR NDlNvT 105 20 YN(IJ = 0.0 _ _ _ _ 107 X ( l ) = . l _10ri X(2) = 3. '  109 C" ~ '~ 110 C COMPUTATION OF THE ABSORPTION COEFFICIENT: 111 10001 A L P H A ( I ) = API*YK ( I )/wL (I 1 1 1 > " C " T>1 S P L A Y IN G"" T H E R t S U L 7 9 F QU GIVEN PO I NT ' H i w o l T E ( 6 , 2 0 3 ) i ' L ( I ) , Y N ( i ) , Y K ( I ) , A L P H A ( I ) , R E ( n » T R ( I ) , I I •' 100 1 CONTINUE ' ~11 <i c '-• _ ~ ~ ~ : l i b C IF ALL POINTS HAVE BEEN CALCULATED, GO TO I t 117 « IF (I.EO.NP) GO TO i l II a " " " " " " " r,o TO I " ~ ' ~ " ' • " " • ~ " : ' • " " " ' " — • 11^) C 120 C N AND K SINGLE POINT CALCULATION L O q P 140. ~T2l To i = I + i ~ : 122 RNSS'SN(I) 123 S0N-S = RNg**2 i2a " SONSM 1=SON3-i, ~ ~ " ~ " " " " " 125 S0NSrM=3QNS*-l,, . __2_ R s R E ( I )  127 f s f R T I l ~ ~ " ~ 12H WsWLCI) 123 CAUL NOUJVT 0N,x,F,ACCEST,MAXlT,eRR,FC»,&2t) l*30 " YN(! ) = x ( 2 5 " • - : ~ - " ~ 131 V K : D = X M ) 132 _?_J° >i'0o2 •_ ______ T l i " '• C~ ERK'OTTTSiJYPTJT FOR NOIfTvT ' ~ 13a. 21 YM(I)=0.0 135 YK(I}=0.0 i 3 S x d ) = . i • " ~ : ; " ' • " - ~ — " " 137 X ( 2 i=3, 13* C m z~ co rTPTTTsr rofTTyt THE ATTJ^TIRFTTD"T-COITFFT CTETTI 1«0 10002 ALPHA CI ) = A('I *YK ( I ) / " L ( I ) I'D WRITF. ( 6 , 2 0 3 ) W L ( I ) , Y N ( T ) , Y K ( I ) , A L P H A ( I ) , R t ( I ) , T R ( I ) / I 1 4 2 G O T O "4" " " ~ " " : U 3 C • % i a n c nnrs c ""iTRTrTWG THE~RFSOCTS O N - F I L E CETNTT 2 5 — :  lUo 11 1 = 1 1«7 12 WRITf(2 , 2 0 3 ) W L(I),yN(j),YK( I ) , A L P H A(I),RE CI)> TR CI ) , I r a a 1 = 1 v i ~ " : " " : ~ " ~ ~ _ _ la9 I K ('I ,UT. (Nf ' t l ) ) GO T() 12 IS 0 c 152 READ (5, J05)K 153 _ I F C K . E U . l ) GO TO 3 ' _. 155 C 156 C_ _ REAP _(im 1AT STATEME N T 3  157 " rn (.• F 0 R M A T ffH'oTti r 5 .A AT " " ~~ : 150 101 FnRMAT(G3.2,G5,a,G6.<».2G5.3,l2) 159 102 FORMAT(212) • 1 6 0 " " " " " 1 0 3 F O R M A T (Gin . 5 ) " " " " " ' " 161 105 FORMATU1 J lt>2 C " F6 3 C " : : " 16a C WRITE FORMAT STATEMENTS lt>5 200 FORMAT('SINGLE P="lTSS«r GROUP=#tfXXj UEFAULT=N£XT P » ) ' 1 6iS 20 iFOr!MA T ( • GOES3 "N = ?C"t 0"„ 5 ' ) " " " " ' " " " . " " " "~ ""' 167 202 FORMAT ( 'GUt-SS K = ? G 1 0 . 5 ' ) 163 205 _F 0 R H A T ( > . t , F 6 . 3 i 2__ '•)__» F_o ,_3 , 2_X ,__ = 1 ,-F6» 3 ,2 X, ' ALP H A = ' , \ PO 1 p , 3 , ax , low ..'t •«"=«, OPF75 . 2X , • TH » , FT. 673X # r2"j 170 20a F O R M A T ( IF I L H T H ICK= ' , G20 .8 , 5X , • s A M P L F-: '»5A a, 3X / • H •» 12 ) 171 205 FORMAT I 1 DO YOU WANT To REPEAT THIS CALCULATION? YES=1,DEFAULT=NO») 1 72 it NO " " " " • ' : " " _ " " 173 C 17a SU8R 01ITIME FCNfX,Fl J 175 IMPLICIT RtAL*8(A"H,G r_Z) • . . ~ 17h DIMENSION X ( 1 ) ,F( 1 ) 177 _ _ _ • COMHON/CONUM/O.T_ . P I, R ,RN,RN3 f RK , S0NS,SQNSM1 ,SUNSP1 ,_W,T _ i ft ~ " ~ ~ R K = X ( U : V " " " " " " " * ~ 179 RN=x(2) 180 c 141. 181 C CALCULATION OF SUBTtRMS 182 C NEXT Two STATEMENTS ARE DESIGNER TO CATCH ANY NUMERICAL OVERFLOW 183 C WHICH CAN OCCUR IN TH F ITERATION SUBROUTINE! i a « IF (OA-as(RN) .GT.50.) RN = 3. 185 IF (DA8srRK).t?T.50.) RK=,5 ' ' 186 RNHl S0= fRN-1 , ).* (RN-1 , ) 187 RNP1Sa=(R.N+l .)*(RN+l,) 188 SOK=RK*SK 189 SQN=RN»RN 190 SONh:P = SQNtSQK 191 'fJNKNSP = 50NKp.+ SQHS 192 QK:KNSM = 30*'*P-S0NS 193 QWPlKP = RriPlSO + SQK 194 OK.'MtKP = RNMlstl + S0K W5 S0NKM1=SQMKP-1. i 9 t , - : SnNSN'J = SO"i>*RN*a, 197 0HNS8=SrtK*SQNS*8, 19fi C 199 A=S0NKP*SQNS*32. 200 C 201 ARGP = 4,*Pt*[>T/W 202 ARN=ARCP*RN 203 ARK=ARGP*RK <• 20 4 A 1 12 = S!iNSPl*QNKNSP 205 A 1 2 = A 1 J 2 + S U ;>i 3 N 4 206 Al=0NMlKP*Al2 207 A2 = 0NPUP*A12 208 C. \ I 2'=S0NSR t Vu'NkN'S'M 2 09 I112-C 1 12*S:.MKMt 210 HI =-2.*(QKfJSS + nl2) 2l 1 n2 = 2.*(0Kfib8-0l2) 212 C 213 RK4--4,*RK 214 Cl22 = ?.+S'l"S*SaWKMi 215 C1=RKU*(C122-C112) 216 C2 = RK'l*rCl22 + Cl 12) 217 C 2U1 CE11=A112-SgNSNU 219 CF 1 =«NP|KP*CFt1 220 CF2-0NMlKP*CFil 221 c 222 c 223 OENOM=(A2*fEXP(AHK)+B2*0C0S(ARN)+C2*0SIN(AHN)+CF2*DEXPC -ARK)) 224 F(1)=A/OENOM-T 225 Ff2)=(Al*OExP(ARK)+Ul*DC0S(ARN)+Cl*0SIN(ARN)+CFl*0EXP(- ARK) J/DENOM 226 A-R 227 RETURN 22* END ENO OF y ILE Copy of the input data set (example) 1 ,105 CU20#3 •: 2- .80.054 i .45 .5'* ,?tS 0 3 ,78,857 1.450 ,41 ,_0 . 0 4 .76.859 1.-SO ,44 ,105 0 5 1 - ^ ^ ^ - - K - ^ ^ O - V i 5-rt- W> — 0 - ^ 6 ,72.864 1.455 .54 . 155 0 7 .7 0.867 1.455 ,61 .1.4 • 8 v 6 9 . 6 6 7 5 1 „ 4 5 5 , 6 5 .13 0 9 -,68.66s 1.456 ,70 t t 2 0 10 ,67.66951.^56 .75 „ j J 0 H r ^ P 7 - M T y - * > - t 8 f l i i - - i - 1 - F R — °-— 12 ',65.87 151.457 .80 .r,95 0 13 .64.872 1 .457 ,94 ,o*>0 0 - 4 4 ^>3.873 1 .457 1 .00 .085 0 15 ,62.674 1.457 1.05 .085 0 16 ,61.875 1.458 1.06 ,0«5 0 — 1-7 r6^ r8-7-6—1^56-H-«-?-5- r^ a^--0— 18 .59.877 1 .458 ,91 , ) i 0 19 ,58.B7B 1.458 .83 .135 0 20 ,57.878 1.459 .73 , 57 0 21 ,56.878 1 .459 .65 ,?1 . 0 22 ,55,87855.46 ,545 .25 0 2 3 — — r 5 4- -8-7- -6—-.41.7-—.-3.;-5—-Oy-24 ,53.87851.46 ,41 ,_75 0 25 ,52.878 1.461 ,37 .03 0 26 • *51 .87751 *461 , 3 5 „ 5 t 0 27 .,50.877 1.462 .35 .585 0 28 ,49.876 1.46 .33 .67 0 2-9 -^4.8-^ 7-55-1-^  6 -3—3 5—V'H> a - -" 30 .47.075 1.464 .555 .«6 0-31 ,46.87451.465 .375 .06 0 32 * 45.87 0 1 .466 ,V>S 1.07 0 33 ,44.87351.4665,415 1.18 •> 34 ,4 3.873 1.407 .43 ' 1.28 0 3£ r — l - r « 68—.-44 1--3-9 0 36 ,41.867 1.469 ,445 1.52 0 37 .40.864 .1 .470 ,445 1 .66 0 - - - 3 8 .39.060 1 .471 ' .435 1 .65 0 39 .38.858 1.4725,43 2.0 1 END OF FRF. Copy of the output data s e t (example) 78 76 a o 3! 32 -8 3-8 a 35 86 • 37 88 — 8 9 -90 91 92 93 in -95-96 97 9)j 99 100 4-04-102 105 -1 04-i 05 106 - w -108 1 0 9 •••HO-1 1 S US F 1 L M 7 H T C X = W= 0,300 •« = W = 0 . 7 8 0 N = 0,760 Ms w= 0.740 N = -ws-O-j-7-2-0—-f-i *- 0 . 700 M = W = 0 , 6 9 0 Ni = • * = 0,6^0 N = = 0 , fa 7 0 H = w= 0,660 N = -w-=—O-j-65-0 W= 0,640 . N = W= 0.630 f-l: •-W= 0.62 0 N: V'= 0,610 N: .* = 0 .600 N: -«-=—G-,-5-9-0-—^ N: Ni; 2.797 2 , ? 5 i ?,77fi 0 , 1 0 50-PoOO 2.79; K= 0.02a K: 0.02B K = 0,033 K= 0,0 3s = - ? T 7 9 - 3 X-=--0 v 03v 2,80 3 K= 0.05^ K= 0.0 37 K= 0,0.37 K'= 0 . 0 3 h K= 0 . 0 3/j k= 0,580 W= 0,57 0 '•'.= 0.560 w= 0,550 W = 0 , 5 'J 0 -^^—O-vS^O-2.809 2.822 2.827 2,852 ..  . . --?v3-b3—xs-O-.-O-Ja... 2,85ft K=. 0.039 2,866 K= 0.039 •••?,a7g ... K= 0.0^2 ?,95o K= o . o a i 2,945 K= 0 . 0 a s -?-,-9^9 K-=—0---0 46-2,99a K= 0.055 5 . 0 I a 3,028 3,069 3.V08 -3 TW-w 114 115 1 1 6 W= 0,5?0 W= 0,510 v;=- 0,500 W= C . i!9 0 N-W = 0 , 'J 8 0 N -w-s—o-v-4-7-0—K N N •-N N N -N N N •N 3 , 1 7 7 3 , 2 0 9 • 3 , 2 5 7 3 . 2 ^ 0 ' 3 , 3 3 3 K = K = 0 , 067 0.OH2 0.087 0.111 -0.5 3?-0,15/4 0.203 0,247 0.31 ! 0.382 0,460 0 , a 5 0 w= o , una-0,030 '*= 0,420 O-^J-Hh W= 0.4 00 W= 0,39 0 w = 0 , 3 8 0-i__--3-i.i) K-—0 i ^ <*7-: 3,330 K= 0,54$ : 3,30? KS 0.635 : -3,234 K"-0,717 = 3.170 KS 0.786 : 3. i U K=- 0 .856 s—3-i-0-?-3—H<-=-4-,-9-35-: 3.05o K= 1.013 = 3 , 0 7 3 K5 1.116 s -3,063 KS 1 , 18a At PHA: A I. P HA: AL pHA; ALPHA: -ALPHA: ALPHA: A L ALPHA ALPHA ALPHA -ALPHA ALPHA: ALPHA: ALPHA: ALPHA: Al.PHA: -ALPHA.: A I PriA: A I. P H A : ALPHA: ALPHA ALPHA _AlrP-HA ALPHA ALPHA ALPHA ALPHA ALPHA -ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA —Ai,PilA ALPHA ALPHA ALPHA SAMPLE: L'U20*3-'l,61?L + 03 K : ' i , 5 8 » t V 0 3 R = 6 , 3 3 0 f c + 0 3 Rs : 5 . 9 3 7 M + 0 3 H= - 5 , 7 7 o t + 0 3 - - - - H z 6 , 7 3 3 ^ + 0 3 « : 6 , 7 0 6 E + 0 3 ; 6 , 8 5 ? K + 0 3 H: : 6 . 7 2 3 ^ + 0 3 Rs : fc.Slfof. + O i R = - 7,4 4 1£ + 0 3 R = = 7 . 7 5 2 t : . + 0 3 R = = 7 , 8 6 l f c + 0 3 K: = 8 . 5 2 7 K . + 0 3 •'••• R = 8 . 3 5 R E + 0 3 R' s 9,a6afc+ 0 3 R •=- -9,72n£ + . 0 3 R. = t , 198fc + OU ' R r. J , <l <i 0 E + 0 U R i , M a a t + o a R l,983t'-»0« R 2 . 5 8 3 E + 0 4 R 3.1 3 ? L + 0tt R 3 . 7 l 9 t + 0 y k a . 9 9 a t + 0 « R fc,?19fc..+ 0 i l « 7 , 9 B f t t + 0iJ R 1 , 0 u 01 + 0 5 R • -1-, 2 « 9 E t 0 5 R 1 ,5 0 o I + 0 5 R 1 . 7 7 3 L + 0 5 R 2,0aAL + 0 5 R 2 . 2 9 6 ^ + 0 5 H 2 . 5 6 U + 0 5 ' R -2 , 86 F. •* 0 5 — • — K 3 . 1 8 3 L + 0 5 . R 3 . 5 9 7 E + 0 5 R 3 , 9 ^ 9 t + 0 5 R 0 . 3 5 6 0 0 6 0 , 3 3 3 ^ 1 2 0 , 3 1 0 , 2 8 2 le . 8 0 , ? « 9 . 1 8 0 -0 . 2 1 2 8 2 3 0. 1 9 ^ 2 0 9 0. 1 7 3 1 8 9 0 , 1 5 4 6 2 1 0, 1 3 6 4 64 o , i-i a s 6 6 -0 . 1 0 0 1 1 9 = 0 . 0 8 7 3 0 0 = 0 , 0 7 7 8 9 5 = 0 . 0 7 6 2 0 9 = 0 , 0 8 2 7 0 0 = 0 , 1 0 7 8 . 9 5 -= 0 , 1 2 9 8 6 6 . = 0 , 1 6 3 4 9 1 = 0 , 1 9 6 5 6 0 = 0 , 2 5 0 4 6 2 s 0 , 2 9 7 8 u a = 0 , - 3 a i 7 7 - 6 -= 0 . 3 7 4 5 3 7 = 0 . 3 9 1 9 6 5 = o . a i o 2 o a = 0 , 4 0 9 7 3 6 = 0 , 4 0 4 8 1 5 = 0 ,-3 B 63-74-= 0 , 3 6 8 7 7 4 = 0 , 3 5 1 9 7 5 = 0 , 3 3 5 9 a i = 0 . 3 2 4 3 5 0 = 0 , 3 1 5 8 / 8 •s a-,-3 -VI1-8-5-s O , 3 1 0 1 0 9 0 , 3 1 5 8 6 3 0 . 3 1 8 7 7 7 8 39 T=0.609537 T=t',fe30957--r = 0 . 6 3 8 2 6 3 T=0,668344 _T-sO-, 69.9842— T=0.724436 T = U . 7 4 l 3 i O •TSO,758578 T=0,776247 T = 0 , 7 9 4 3 2 8 _T-0.^6035-2 6-1=0.6128-31 T = 0 , 8 2 2 2 4 3 T = 0,6222-a.3.... T = 0 . 8 2 2 2 4 3 T = 0 , 8 0 3 5 2 6 _.t=0.,.7-X624.7_ T = 0 . 7 3 2 8 2 5 1=0,676083 T = 0,616595.• T = 0,562341 T = 0 . 484 1 72 _J..=.0-,4.2.V4)AJ_ T s O . 3 7 1 5 3 5 T = 0 , 309030 -TsO.260016-T = 0, 2 1 3 7 9 6 r = 0, 1 73780 _.T-=.0-.r.t.3A038-T = 0, 109648 T = 0 . 0 8 5 l 14 1=0 , 066069... T = 0,05248.1 T = 0 , 0 4 0 7 3 8 ._T_u0-,_0^0-2-OO-T = 0 , 0 2 1 8 7 8 T s O . 0 1 4 1 2 5 T=0,010000 6 7 8 9 10 •44-12 13 - !«. 15 16 — 1 - 7 -18 19 2 0 21 22 24 25 26 27 28 -2-9-30 31 32 33 34 —35-36 37 38 144 REFERENCES » [I] D.L. Pulfrey,"Photovoltaic Power Generation'1,Van Nostrand Reinhold: New York (1978), chapter 1. [2] See reference 1, chapter 4. [3] L.C. Olsen, Quarterly Progress rept. NSF/RANN/AER75-20501/PR/75/4 (1975). [4] L.C. Olsen, Research repts. NSF/RANN/AER/75-20501/PR/76/l,2,3 (1976). [5] D. Trivich, E.Y. Wang, R.J. Komp and F. Ho, Proc. IEEE Photo. Spec. Conf. 12th, 875 (1976). [6] D. Trivich, E.Y. Wang, R.J. Komp and A. Kakar, Proc. IEEE Photo. Spec. 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