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Properties of polycrystalline GaAs films grown by the close spaced vapour transport technique on Mo substrates 1976

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PROPERTIES OF POLYCRYSTALLINE GaAs FILMS GROWN BY THE CLOSE SPACED VAPOUR TRANSPORT TECHNIQUE ON Mo SUBSTRATES by BLAIR RUSSEL B.A.Sc., U n i v e r s i t y of B r i t i s h Columbia, 1972 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n the 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 re q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA September 1976 (S) Blair Russel 1976 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an a d v a n c e d d e g r e e a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e 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 r e f e r e n c e a n d s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e H e a d o f my D e p a r t m e n t o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . D e p a r t m e n t o f The U n i v e r s i t y o f B r i t i s h C o l u m b i a 2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5 ABSTRACT This t h e s i s i s a study of the p r o p e r t i e s of t h i n GaAs f i l m s grown on molybdenum substrates by the c l o s e spaced vapour t r a n s p o r t (CSVT) de p o s i t i o n technique w i t h the i n t e n t i o n that the GaAs/Mo s t r u c t u r e would be used as the semiconductor and s u b s t r a t e f o r economic s o l a r c e l l s . The GaAs f i l m s were p o l y c r y s t a l l i n e cubic c r y s t a l s w i t h no p r e f e r r e d o r i e n t a - t i o n . The c r y s t a l l i t e area increased w i t h the temperature at which the s u b s t r a t e was h e l d during growth and at 710°C g r a i n s i z e s of 100 ym were observed. The c r y s t a l l i t e s formed a columnar-like s t r u c t u r e w i t h c r y s t a l s i z e comparable to the f i l m t h i c k n e s s . No i m p u r i t i e s of f o r e i g n i n s t r u s - ions e x i s t e d i n the f i l m s i n q u a n t i t i e s observable on the e l e c t r o n micro- probe. The r e s i s t i v i t y of the GaAs f i l m s was 220 0, cm, hence acceptable f o r t h i n f i l m s o l a r c e l l s , however, the GaAs/Mo contact was m i l d l y r e c t i - f y i n g . Diodes were f a b r i c a t e d by the d e p o s i t i o n of Au onto the GaAs f i l m s and the r e s u l t i n g b a r r i e r s showed values of b a r r i e r h e i g h t of a p p r o x i - mately 0.8 eV, i d e a l i t y f a c t o r n = 1.5 to 2, and d e p l e t i o n - l a y e r m a j o r i t y c a r r i e r concentration of roughly 10 cm as measured by J-V and C-V methods. The GaAs f i l m s show promise f o r use i n s o l a r c e l l s provided that the Mo/GaAs i n t e r f a c e r e s i s t a n c e can be reduced. i i TABLE OF CONTENTS Page ABSTRACT . . . . i i TABLE OF CONTENTS . 1 1 1 LIST OF ILLUSTRATIONS v LIST OF TABLES . . . v i l LIST OF SYMBOLS v i i i ACKNOWLEDGEMENT x l i i I INTRODUCTION 1 A. F i l m P r e p a r a t i o n Techniques 2 B. Semiconductor S e l e c t i o n 4 C. Summary 7 I I THE CLOSE SPACE VAPOUR TRANSPORT TECHNIQUE 9 A. Close Space Vapour Transport Theory 9 B. Review of Previous Work 11 1. Flow Rate 11 2. Spacing 13 3. Temperature 13 4. Surface Topography and C r y s t a l O r i e n t a t i o n . 14 5. Doping 14 C. The Apparatus 14 D. Choice of Substrate M a t e r i a l 15 E. Experimental Conditions 21 I I I MICRO STRUCTURAL ANALYSIS OF THE GaAs FILMS 24 A. X-ray D i f f r a c t i o n 24 1. C r y s t a l l i n i t y 24 a. Apparatus 24 b. Re s u l t s 26 c. A n a l y s i s 29 2. P r e f e r r e d O r i e n t a t i o n , 29 a. Apparatus 29 b. Results 31 c. A n a l y s i s ° • 31 B. Scanning E l e c t r o n Microscopy 31 1. I n t r o d u c t i o n 31 2. Theory 33 3. Procedure 33 4. Results 35 i i i Page C. E l e c t r o n Mlcroprobe . . . . . . . 39 1. Theory and Apparatus 39 2. Results 40 D. D i s c u s s i o n 43 IV ELECTRICAL PROPERTIES OF THE GaAs FILMS 44 A. S o l a r C e l l Theory 44 B. E l e c t r i c a l Contacts to the GaAs Films 47 1. R e c t i f y i n g Contacts 47 a. Schottky B a r r i e r Theory . 47 b. Schottky B a r r i e r F a b r i c a t i o n 50 2. Ohmic Contacts 52 a. Ohmic Contact Theory 52 b. Ohmic Contact F a b r i c a t i o n - 54 C. E l e c t r i c a l Measurements 56 1. Measurement Apparatus 56 2. Current Voltage Results . . 57 a. cj)g, n and RgH 57 b. R s 59 3. Capacitance Voltage Results . . . 66 D. D i s c u s s i o n 71 V CONCLUSION 77 REFERENCES . 79 APPENDIX 83 A l l Review of the Energy S i t u a t i o n 8 3 " i l 2 Summary of Energy Conversion Versus Semiconductor Thickness C a l c u l a t i o n 86 A I V l Determination of Maximum E f f i c i e n c y of S o l a r C e l l s as a Function of S e r i e s Resistance . . . . . . . . . . . . 87 AIV2 Theories of Conduction i n P o l y c r y s t a l l i n e Films . . . . 91 i v LIST OF ILLUSTRATIONS Figure Page IA-1 Percent of Incident Energy Absorbed versus Semiconductor Thickness 6 IIA-1 S i m p l i f i e d Chemical Vapour Growth Chamber 10 IIA-2 Source and Substrate Arrangement f o r CSVT . . . . . . . . . 10 IIB-1 Amount of GaAs Transported as a Function of Water-Vapour Pressure 12 IIB-2 Amount of GaAs Transported as a Function of Spacing . . . . 12 IIB-3 Log Rate of Transport versus Temperature 12 IIC-1 The B a s i c CSVT Apparatus 16 IIC-2 The CSVT P e r i p h e r a l Equipment . . . . . . . 17 IID-1 R e l a t i v e L i n e a r Thermal Expansion of D i f f e r e n t Metals . . . 20 III A - 1 X-Ray D i f f r a c t i o n Goniometer 25 IIIA-2 X-Ray D i f f r a c t i o n of Poly, GaAs on Mo-Quartz . 27 II I A - 3 Texture Goniometer 25 IIIA-4 Texture Goniometer Chart of Po l y GaAs on Mo-Quartz . . . . 30 II I B - 1 Schematic Diagram of P o l y c r y s t a l l i n e Films 32 IIIB-2 Phenomena Generated by an Incident E l e c t r o n Beam 34 I I I B - 3 SEM Photomicrographs of GaAs F i l m Surfaces Grown on Mo-Quartz at D i f f e r e n t Substrate Temperatures 36 IIIB-4 SEM Photomicrograph of Noncontinuous GaAs Grown on Mo-Quartz at 725°C . • 37 IIIB-5 SEM Photomicrograph of Cleaved Side View of GaAs Grown on Mo-Quartz at 700 °C 3.7 IIIB-6 SEM Photomicrograph of Sample Etched i n HN03:HF:H20 . . . . 37 IIIB-7 GaAs D e n d r i t i c Growth on Mo F o i l 39 III C - 1 Microprobe A n a l y s i s of Oxygen Concentration 41 IIIC-2 Microprobe A n a l y s i s of Mo Concentration 42 v Figure Page IVA-1 Equivalent C i r c u i t of S o l a r C e l l . . . . . . . 45 IVB-1 Energy Band Diagram of a Metal/n-Type Semiconductor/Ohmic Contact S t r u c t u r e at Thermal E q u i l i b r i u m 47 IVB-2 P o l y GaAs S o l a r C e l l C o n f i g u r a t i o n 51 IVB-3 E f f e c t of Image Force on the P o t e n t i a l B a r r i e r at a M e t a l - Semiconductor I n t e r f a c e f o r n-Type GaAs w i t h Band Bending V b l = 1 eV . . . . . . . . 53 IVB-4 I l l u s t r a t i o n of the Current-Voltage R e l a t i o n s h i p f o r a Metal-Semiconductor Contact f o r P r o g r e s s i v e l y Higher C a r r i e r Concentrations 53 IVB-5 Ohmic Contact Annealing Furnace . . 55 IVC-1 S e r i e s Resistance Model Check 58 IVC-2 J-V C h a r a c t e r i s t i c s of Diodes on Sample B l / I I w i t h D i f f e r e n t *s • • •• • • 6 0 IVC-3 R e l a t i o n s h i p of P o l y c r y s t a l l i n e GaAs Chip w i t h Ohmic Con- t a c t s on Both Sides , 62 IVC-4 Current-Voltage C h a r a c t e r i s t i c of S i n g l e Ohmic Contact to GaAs/Mo 64 IVC-5 S e r i e s Resistance of B l / I I , S i n g l e Ohmic Contact Method . . 65 IVC-6 E q u i v a l e n t and A c t u a l C i r c u i t f o r a P o l y c r y s t a l l i n e M e t a l - Semiconductor Diode 67 IVC-7 (A/C) 2 vs. V of Specimen B l / I I , Diodes 2, 20 and 21 c o r r e c t e d f o r Rg and Approximated by a Chebychev Polynomial 67 A I V l E f f i c i e n c y as a f u n c t i o n of Series Resistance 90 AIV2-1 The Mosaic Model and I t ' s Band Diagram 9 2 AIV2-2 B a r r i e r Types . 9 2 AIV2-3 Energy Diagram of n-Type C r y s t a l l i t e at a Grain Boundary i n the Presence of Boundary States 96 v i LIST OF TABLES Table Page IA-1 Semiconductor Market P r i c e s . . . . . . . . 7 IID-1 Cost of Substrate M a t e r i a l s , S i and GaAs . . . . 23 IIE-1 Summary of CSVT Technique Conditions f o r Optimum Growth . . 23 III A - 1 Bragg R e f l e c t i o n Angles f o r Cubic GaAs and CSVT Grown GaAs. 28 II I B - 1 E f f e c t of Growth Temperature on C r y s t a l S i z e . . . . . . . 38 IVC-1 E l e c t r i c a l P r o p e r t i e s of Au Schottky P o l y c r y s t a l l i n e - G a A s Diodes . 70 AI1-1 World Estimated Annual P o t e n t i a l Energy . . . . . . . . . . 85 A I V l E f f i c i e n c y ri (%) vs. S e r i e s Resistance (fi) 89 v i i LIST OF SYMBOLS Symbol D e s c r i p t i o n U n i t AMO AMI A A A* a o '«*<»• B(S) B ( V ) C ce c* c ^Bu d E E C E F E M E V - S o l a r spectrum i n outer space - S o l a r spectrum at earth's s u r f a c e f o r optimum c o n d i t i o n s at sea l e v e l , sun at z e n i t h - contact area o f diode - f i r s t element of a compound - m o d i f i e d Richardson's constant = 4iTqm*k 2 c r y s t a l l a t t i c e constant a b s o r p t i o n c o e f f i c i e n t second element of a compound ( i n s o l i d phase) second element of a compound ( i n vapour phase) capacitance edge capacitance e q u i v a l e n t capacitance measured by capacitance meter -• speed o f l i g h t = 2.998 x 10 - d e n s i t y of sur f a c e s t a t e s 10 d e n s i t y of n e g a t i v e acceptor type boundary s t a t e s ( d i s c r e t e energy l e v e l ) n e t d e n s i t y of boundary s t a t e s (uniform d i s t r i b u t i o n ) c r y s t a l i n t e r p l a n a r spacing energy conduction band energy Fermi energy maximum e l e c t r i c f i e l d i n the d e p l e t i o n r e g i o n of MS diode valence band energy t e r r e s t r i a l s o l a r energy r a d i a t i o n cm' -2 -2 amps cm K I • cm -1 farads farads farads -1 cm sec eV^cm - 2 eV cm eV -^cm - 2 I eV eV eV v o l t s cm eV -1 watt sec m -2 v i i i Symbol Description "SC G I XM ^Ph J ^rec J s . "^Srec k I V X m* v V NC % N(X) - s t a t i c value of the semiconductor d i e l e c t r i c constant - solar c e l l e f f i c i e n c y - conductance - current - current delivered by solar c e l l at max. power - saturation current from i d e a l diode equation - photo generated current - current density - recombination current density - saturation current density i n diode equation - saturation current density for thermionic emission - saturation current density f o r recombination current - Boltzmann's constant = 8.62 x 10~-> - thickness - length of i n t e r c r y s t a l region - length of c r y s t a l region - wavelength - free electron mass = 9.108 x 1 0 ~ ^ - e f f e c t i v e mass of electrons - frequency of photons - concentration of acceptor stator - e f f e c t i v e density of states i n the conduction band - concentration of donor states - irradiance Unit farad cm~^ % mho amps amps amps amps amps cm amps cm amps cm amps cm -2 amps cm eV °K~ 1 ym o A o A I kg. kg. Hz -2 cm -3 cm -3 cm -3 watts m -2 xx Symbol D e s c r i p t i o n U n i t N V N. u n P *Bn M V QB q R R L *s RSH R ( V ) - e f f e c t i v e d e n s i t y of s t a t e s i n valence band - n e t number o f acceptor and donor s u r f a c e s t a t e s per u n i t energy - number of acceptor type surface s t a t e s per u n i t area at energy <j>g wrt - an i n t e g e r i n Bragg's Law - e m p i r i c a l f a c t o r i n the diode equation J = J s [ e x p _ i ] - perimeter o f metal contact on semiconductor - p o t e n t i a l height of b a r r i e r formed between 2 c r y s t a l s - metal-semiconductor b a r r i e r h e i g h t - metal-n type semiconductor b a r r i e r h e i g h t - i n t r i n s i c c a r r i e r c o n c e n t r a t i o n - angular frequency = 2irv - power across l o a d - maximum s o l a r c e l l power - energy l e v e l below which a l l s u r f a c e s t a t e s are f i l l e d f o r charge n e u t r a l i t y at the su r f a c e (before e q u i l i b r i u m ) - the boundary s t a t e s energy above E^ (Ey at the g r a i n boundary) charge at the g r a i n boundary a r i s i n g from su r f a c e s t a t e s magnitude o f e l e c t r o n i c charge = 1.6 x 10 r e s i s t a n c e l o a d r e s i s t a n c e s e r i e s r e s i s t a n c e o f semiconductor device " rSb + rC + rM shunt r e s i s t a n c e across diode r e a c t i v e agent i n vapour phase -19 cm ev cm „-l -2 ev cm cm eV eV eV -3 cm Hz watts watts eV eV c o u l . c o u l . ohms ohms ohms ohms Symbol Description Unit rC - resistance of back contact ohms rM - resistance of Schottky b a r r i e r metal f i l m ohms rSb - resistance of semiconductor bulk ohms P - r e s i s t i v i t y - a ̂  • ohm-cm P B r e s i s t i v i t y of i n t e r c r y s t a l region ohm—cm pC - r e s i s t i v i t y of c r y s t a l region ohm-cm PSb - r e s i s t i v i t y of semiconductor bulk ohm-cm cr conductivity = p mho cm ̂" °SC - conductivity across a single c r y s t a l mho cm "a - macroscopic conductivity (conductivity across a p o l y c r y s t a l l i n e sample) mho cm T - temperature °K TC - cooler temperature at which reaction proceeds to l e f t •c TH - . hotter temperature at which reaction proceeds to right °C T - e f f e c t i v e l i f e t i m e of carr i e r s due to traps sees V — voltage volts v b i - metal-semiconductor junction b u i l t i n voltage eV v i - voltage intercept of C-V curves vo l t s V L - voltage across a load v o l t s V n the energy difference between the Fermi l e v e l and the bottom of the conduction band i n the • semiconductor bulk eV Vph(^) spectral density of photons -2 m W - space charge depletion width cm X — displacement i n x direc t i o n cm x i ACKNOWLEDGEMENT I wish to thank my research s u p e r v i s o r , Dr. D.L. P u l f r e y , f o r h i s encouragement and guidance throughout the course of t h i s i n v e s t i g a t i o n . G r a t e f u l acknowledgement i s given to Mr. J . Lees f o r f a b r i c a - t i n g the quartz r e a c t i o n chamber, Mr. A r v i d L a c i s f o r h i s i n s t r u c t i o n and ass i s t a n c e w i t h the e l e c t r o n microscope instruments, P r o f . R.G. B u t t e r s f o r the x-ray spectrometer a n a l y s i s , and H. Stuber f o r h i s t e c h n i c a l a s s i s - tance. The work described i n t h i s t h e s i s was f i n a n c i a l l y supported by the N a t i o n a l Research C o u n c i l (Grant No. A-7248). 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 t h e i r h e l p f u l d i s c u s s i o n s and als o Miss S a n n i f e r Louie f o r t y p i n g t h i s t h e s i s . x i i 1. I INTRODUCTION The p h o t o v o l t a i c s o l a r c e l l i s being given s e r i o u s considera- t i o n as a s u p p l i e r of a s i g n i f i c a n t p o r t i o n of the world's f u t u r e energy demands (see appendix A l l ) . The success of t e r r e s t r i a l s o l a r c e l l s r e l i e s on developing new forms o f m a t e r i a l s and manufacturing processes which can b r i n g about g r e a t l y reduced costs and increased production r a t e s . The cost and production r a t e goals that w i l l have to be met i f p h o t o v o l t a i c e l e c t r i c i t y i s to be competitive w i t h other e x i s t i n g methods 2 of generation have been estimated by Wolf [75 WI] at $15-60/m and 8 2 2 5 x 10 m /year. In comparison the 1975 values were $2,000/m and 2 2,000 m /year r e s p e c t i v e l y . The two fundamental requirements f o r conventional p h o t o v o l t a i c devices are; 1) a m a t e r i a l that absorbs l i g h t w h i l e generating mobile charge c a r r i e r s , 2) a p o t e n t i a l b a r r i e r that separates the c a r r i e r s so that they cannot recombine. Semiconductors w i t h appropriate band gap f u l f i l the f i r s t requirement and j u n c t i o n s l i k e p-n, hetero, metal- semiconductor (MS), and metal-insulator-semiconductor (MIS) f u l f i l the second. The semiconductor i s the most expensive component of the s o l a r c e l l and v i a b l e t e r r e s t r i a l p h o t o v o l t a i c systems w i l l have to improve the present $/watt r a t i n g by using e i t h e r high p u r i t y , h i g h e f f i c i e n c y c e l l s w i t h concentrators, or reduced grade ( p u r i t y and c r y s t a l l i n i t y ) semiconductors i i i a t h i n f i l m form s u i t a b l e f o r l a r g e area coverage. In the former scheme S i and Ga, A l As are already a v a i l a b l e but are 1-x x prevented from immediate implementation by the h i g h production cost and the need f o r cheap, s u i t a b l e concentrator systems w i t h heat s i n k i n g . In the l a t t e r scheme s u i t a b l e m a t e r i a l s and c e l l c o n f i g u r a t i o n s are s t i l l 2. being developed and the e x i s t i n g problems are s t i l l of a fundamental, m a t e r i a l s science nature. J u n c t i o n formation i n t h i n f i l m c e l l s i s a l s o a major area of i n v e s t i g a t i o n as the conventional p-n diode c o n s t r u c t - i o n may hot be a p p l i c a b l e to the almost i n e v i t a b l y p o l y c r y s t a l l i n e form of the t h i n semiconductor f i l m s on account of g r a i n boundary d i f f u s i o n . This t h e s i s seeks to c o n t r i b u t e to the development of t h i n f i l m semiconductor s o l a r c e l l s by e v a l u a t i n g the p r e p a r a t i o n and proper- t i e s of a t h i n f i l m semiconductor grown d i r e c t l y onto a metal s u b s t r a t e . The f a c t o r s l e a d i n g to the s e l e c t i o n of the semiconductor m a t e r i a l and i t s growth process are o u t l i n e d i n the f o l l o w i n g s e c t i o n s . I-A F i l m P r e p a r a t i o n Techniques The p r i c e of s i n g l e c r y s t a l semiconductors that are normally used f o r s o l a r c e l l s i s from $2 to $4 f o r a 2 i n c h diameter'by 10 m i l t h i c k wafer. This l a r g e thickness i s only necessary f o r support not absorption. I f an inexpensive metal f o i l could be used as both a sub- s t r a t e and e l e c t r o d e onto which a t h i n semiconductor f i l m of only 5 ym. t h i c k could be deposited, t h i s would reduce the volume, and hence cost of semiconductor m a t e r i a l used by about 100 times. I t i s d o u b t f u l whether metal f o i l s can seed s i n g l e c r y s t a l f i l m s , but i t has been shown by Fang [74F1] and Vohl et a l [67V1] t h a t S i and GaAs can be grown on metals i n a p o l y c r y s t a l l i n e form that shows promise f o r use i n s o l a r c e l l s . The great advantage to t h i n f i l m growth i s the avoidance of the slow and c o s t l y C z o c h r a l s k i c r y s t a l growth method and the subsequent s l i c i n g , d i c i n g and p o l i s h i n g of wafers. Thin f i l m d e p o s i t i o n methods can be grouped i n t o three cate- g o r i e s ; vacuum evaporation, s p u t t e r i n g , and chemical d e p o s i t i o n . 3. Vacuum evaporation of semiconductors such as S i , Ge, and GaAs i s usually done by electron beam evaporation. This method produces the purest results because interactions between evaporant and support material are greatly reduced (over the conventional resistance heater method). Sub- stra t e heaters are required to avoid amorphous films and after the evapora- t i o n a slow cool down cycle i s necessary to minimize formation of d i s l o c a - tions. This lengthy procedure i n conjunction with complex electron guns and related focusing equipment makes th i s method impractical i n the present context. In addition the electron beam emits x-rays and produces secondary electrons on s t r i k i n g the surface of the evaporant which can cause radia- tion damage to the semiconductor. Sputtering can also lead to radiation damage caused by ion beam bombardment. Fang ([74F1], p. 26) found, for S i i n p a r t i c u l a r , that sputtering would not be economical f o r solar c e l l s because of the slow rate of deposition 1 um/hr.). Other semiconductors are l i k e l y to sputter as slowly, leaving sputtering as an uneconomical process. Chemical methods form the largest group of f i l m preparative procedures, but most of them, e.g., e l e c t r o p l a t i n g , chemical reduction, anodization etc. are suitable only for amorphous f i l m growth. However, chemical vapour or l i q u i d phase growth does produce c r y s t a l l i n i t y . The most f a m i l i a r chemical vapour deposition (CVD) technique i s the pyrolysis of s i l a n e , but the r e s u l t i n g films are almost as expensive as-single c r y s t a l S i . The simplest and most direct method of growing GaAs by CVD i s the close spaced vapour transport technique developed by N i c o l l [63N1]. A preliminary investigation of this technique by Vohl et a l [67V1] showed that GaAs can be deposited by N i c o l l ' s method onto metals i n p o l y c r y s t a l - l i n e form. The well-known l i q u i d phase epitaxy of GaAs has only been 4. used to grow c r y s t a l s from other s i n g l e c r y s t a l seeds and i t i s uncer- t a i n what k i n d of f i l m s l i q u i d phase e p i t a x y would produce on metals. I-B Semiconductor S e l e c t i o n x .- Three semiconductors, CdS, S i and GaAs, are c u r r e n t l y being y i n t e n s i v e l y i n v e s t i g a t e d as bases f o r t h i n f i l m semiconductor s o l a r c e l l s . From a low c o s t , l a r g e area po i n t of view the CdS based device would seem to be an i d e a l c e l l f o r t e r r e s t r i a l a p p l i c a t i o n s . I t i s a t h i n f i l m p o l y c r y s t a l l i n e c e l l f a b r i c a t e d by e i t h e r evaporating or spraying a CdS l a y e r on an inexpensive su b s t r a t e and o v e r l a y i n g a C^S f i l m by a chemical d i p p i n g process. U n f o r t u n a t e l y , the c e l l has s e v e r a l major problems. The p h o t o v o l t a i c p r o p e r t i e s of C^S are a s t r o n g func- t i o n of i t s s t o i c h i o m e t r y which i s d i f f i c u l t to c o n t r o l i n f a b r i c a t i o n and degrades i n the presence of minute q u a n t i t i e s of moisture. A l s o , despite more than a decade of development, the e f f i c i e n c i e s are s t i l l only 5-7%. This i s s t i l l below the minimum value of 10% e f f i c i e n c y that a recent p r e l i m i n a r y cost a n a l y s i s has i n d i c a t e d to be r e q u i r e d f o r t e r r e s t r i a l a p p l i c a t i o n s (see Wolf [75W1] F i g . 2 ) . S i n g l e . c r y s t a l S i c e l l s have monopolized space a p p l i c a t i o n s f o r the past 20 years, and t h i s development has l e d to present AMI e f f i c - i e n c i e s of 19% as reported by Lindmayer and A l l i s o n [73L1], Inexpensive t h i n f i l m c e l l s have been only moderately developed. Fang [74F1] has deposited S i f i l m s on a v a r i e t y of substrates by evaporation and p y r o l y - s i s of s i l a n e and has i n v e s t i g a t e d t h e i r p r o p e r t i e s . Chu e t a l [75C1] have made p o l y c r y s t a l l i n e s o l a r c e l l s by the thermal r e d u c t i o n of t r i - c h l o r a s i l a n e on graphite and m e t a l l u r g i c a l 'grade s i l i c o n s u b s t r a t e s . E f f i c i e n c i e s were s t i l l very low, namely 2.5 and 4% r e s p e c t i v e l y . 5. In the past GaAs s o l a r c e l l s have been only a l a b o r a t o r y c u r i - o s i t y . However, r e c e n t l y the Ga, A l As h e t e r o s t r u c t u r e has been J ' 3 1-x x p reported [75E1] w i t h AMI e f f i c i e n c i e s of 21% and the good thermal s t a b i - l i t y of t h i s arrangement leads to an expectation of i t s use i n conjunc- t i o n w i t h concentrators. Vohl e t a l [67V1] are the only i n v e s t i g a t o r s who have worked w i t h t h i n f i l m GaAs c e l l s and t h e i r Pt/GaAs c e l l s on Mo f o i l produced 4.5% e f f i c i e n c i e s . GaAs t h i n f i l m s are the n a t u r a l choice over S i c o n s i d e r i n g t h e i r greater absorption of energy and higher p o t e n t i a l e f f i c i e n c i e s . F i g . 1-1 shows t h a t S i c e l l s g e n e r a l l y r e q u i r e more than 10 times as t h i c k m a t e r i a l as GaAs i n order to r e a l i s e comparable e f f i c i e n c i e s . Although Ga i s more expensive than S i at present market v a l u e s , i t i s s t i l l more p r o f i t a b l e to use GaAs f o r absorptions up to 95% of i n c i d e n t energy. P r i c e s can.be compared i n Tab^leTIA^lDDeta'ilsfjoftfthepprggram arid data used,in o b t a i n i n g F i g . IA-1 are given i n appendix A I 2 . Other advantages are: 1) GaAs has a b e t t e r high temperature performance compared w i t h m a t e r i a l s l i k e S i which have s m a l l e r band gaps. 2) MS s t r u c t u r e s are w e l l s u i t e d to t h i n f i l m GaAs because the high absorption i n GaAs generates most c a r r i e r s near to the metal i n the f i e l d r e g i o n . 3) In p o l y c r y s t a l l i n e form GaAs c e l l s have been estimated by Woodall and Hovel ([75W2], p. 1005) to have a 3:1 cost advan- tage over poly. S i at 10% e f f i c i e n c y . The authors a r r i v e d at t h i s f a c t o r by assuming that the c a r r i e r d i f f u s - i o n lengths were equal t o the g r a i n s i z e ( f o r the purpose of accommodat- i n g the e f f e c t of recombination at the g r a i n boundaries) and by modelling 6. PERCENT OF INCIDENT ENERGY ABSORBED Fig 7. IA-Ij Percent of Incident Energy Absorbed versus Semiconductor Thickness 7. C e l l Element U.S. Production ( i n 1,000 kgm) Cost $/kgm. :% P u r i t y S i S i 1250 (1974) 60 .2 PPB GaAs Ga .3 (1968) 750 10 PPB As 19 x 1 0 3 (1968) 1.21 pure Table IA-1 Semiconductor Market P r i c e s (Taken from [75W2] t a b l e 24) the c e l l as a s i n g l e v e r t i c a l c r y s t a l c e l l ( p a r a l l e l combination of f i l a - mentary c r y s t a l s ) . The r e s u l t i n g c a l c u l a t i o n s revealed t h a t poly. GaAs c e l l s were b e t t e r than p o l y . S i c e l l s i n s u s t a i n i n g t h e i r e f f i c i e n c i e s w i t h decreasing g r a i n s i z e . I-C Summary Vohl e t a l . ' s work showed that CSVT GaAs s o l a r c e l l s were f e a s i b l e but at th a t p r e l i m i n a r y stage inadequate. E f f i c i e n c i e s were only about 5% and contact and s t a b i l i t y problems were apparent. Nevertheless the technique shows promise and i t i s c l e a r that i n v e s t i g a t i o n s i n t o the m i c r o s t r u c t u r a l and e l e c t r i c a l p r o p e r t i e s of the GaAs f i l m s are r e q u i r e d before improvements can be made. I t i s the aim of t h i s t h e s i s to per- form these i n v e s t i g a t i o n s and to f u r t h e r evaluate the CSVT method as a p o s s i b l e method f o r f a b r i c a t i n g t h i n f i l m s o l a r c e l l s . Chapter I I reviews the CSVT technique and summarizes previous work i n t h i s area u s i n g GaAs. Chapter I I I analyses the m i c r o s t r u c t u r e of the chemical vapour deposited GaAs t h i n f i l m s and Chapter IV deals w i t h the e l e c t r i c a l p r o p e r t i e s of Schottky b a r r i e r s formed by deposi- t i o n of Au onto the t h i n f i l m GaAs. J-V and C-V measurements are used 8. to f i n d r e s i s t i v i t y , b a r r i e r h e i g h t , q u a l i t y f a c t o r , and m a j o r i t y c a r r i e r concentration i n the diodes. The conclusions to be drawn from t h i s work and recommendations f o r f u r t h e r research are given i n Chapter V. I I THE CLOSE SPACED VAPOUR TRANSPORT TECHNIQUE I I - A Close Spaced Vapour Transport Theory Vapour phase growth or vapour p l a t i n g i s the d e p o s i t i o n of a f i l m on a sub s t r a t e by means of a chemical r e a c t i o n i n v o l v i n g the vapour phase. The m a t e r i a l to be deposited i s vaporized and transported by convection or d i f f u s i o n to the substrate.where, under the c o r r e c t condi- t i o n s , a s o l i d phase f i l m can be seeded and grown (see e.g. F i g . I I A - 1 ) . The close spaced vapour t r a n s p o r t (CSVT) technique i s vapour phase growth w i t h the sub s t r a t e placed a short distance from the source as shown i n F i g . IIA-2. A slow flow of a c a r r i e r gas c o n t a i n i n g a r e a c t i v e agent i s introduced between the source and s u b s t r a t e . The source i s h e l d at a h i g h enough temperature (Tjj) i n order that the r e a c t i v e agent can o x i d i z e the source producing the vapours shown on the r i g h t of equation I I A - 1 H AB, N + R, . — A R . . + B, N (IIA-1) (s) (v) (v) (v) TC , source . compound These gases t r a n s p o r t e a s i l y bby d i f f u s i o n to a cooler s u b s t r a t e ( T £) where the r e a c t i v e agent i s reduced so that A and B can nucleate on the substrate as a f i l m of the o r i g i n a l source m a t e r i a l , i . e . r e a c t i o n proceeds to the l e f t i n eq. IIA-1. I d e a l l y , the source and su b s t r a t e should be independently heated, but t h i s i s d i f f i c u l t to achieve i n p r a c t i c e . A reasonable method i s to obtain the heat r e q u i r e d by absorp- t i o n of high i n t e n s i t y l i g h t i n c i d e n t on r e l a t i v e l y massive heater blocks which are i n thermal contact w i t h e i t h e r the source or the sub s t r a t e (see F i g . I I A - 2 ) . Non-conducting i n e r t spacers between source and sub- s t r a t e are used f o r separation and thermal i s o l a t i o n . Source and 10. VAPOURS OF LI COMPOUND TO BE 7 DEPOSI T E D FILM GROWTH SUBSTRATE f EXHAUST F i g . IIA-1 S i m p l i f i e d Chemical Vapour Growth Chamber THERMOCOUPLE THERMOCOUPLE (Tc) __M__M MOLY BLOCK 10 • .SOURCE SPACER ^SUBSTRATE F i g . IIA-2 Source and Substrate Arrangement f o r CSVT 11. substrate temperatures are monitored by thermocouples i n s e r t e d i n t o the heater b l o c k s . As a d e p o s i t i o n system t h i s technique's main advantage i s s i m p l i c i t y and the a b i l i t y to be used at atmospheric pressure. For s o l a r c e l l f a b r i c a t i o n the b e n e f i t from using c l o s e spacing i s the r e l a t i v e l y u n l i m i t e d area on which a f i l m may be grown. Only the spac- i n g i s c r i t i c a l so t h a t c e l l s could be mass-produced i n l a r g e sheets using a powdered source. Another advantage r e s u l t s from the e f f i c i e n t use of the r e a c t i v e agent. A f t e r each t r a n s p o r t R returns t o i t s o r i - g i n a l form a f t e r d e p o s i t i o n . I t t h e r e f o r e can be used again and again f o r t r a n s p o r t s i n c e the source i s c l o s e to the s u b s t r a t e . This m u l t i p l e use of r e a c t i v e agent reduces the r e q u i r e d amount of both t r a n s p o r t and r e a c t i v e agent y i e l d i n g reported m a s s - u t i l i z a t i o n e f f i c i e n c i e s of 90- 98% (see Robinson [63R1]). GaAs has been grown by the CSVT technique using a H2O r e a c t i v e agent transported on pure H 2 gas. The chemical r e a c t i o n i s b e l i e v e d to proceed as i n equation IIA-2 (from Frosch and Thurmond [62F1]). TH 2 G a A s ( g ) + H 2 0 ( v ) ^ G a 2 0 ( v ) + H 2 ( v ) + A s 2 ( v ) (IIA-2) T c II-B Review of Previous Work The CSVT technique f o r the d e p o s i t i o n of GaAs has been s t u d i e d i n some d e t a i l at RCA by Nidb'l©"[63N1'̂ V Ro^fhsotf'^KL-] » aai . GottliebaandcCarboy ^[6,361], The main t h r u s t of t h i s work was to i n v e s t i - gate the parameters a f f e c t i n g the growth of GaAs on s i n g l e c r y s t a l Ge. The f o l l o w i n g paragraphs are summaries of the r e s u l t s . 1. Flow Rate G o t t l i e b and Carboy i n v e s t i g a t e d the e f f e c t of the H 2 flow r a t e 12. V PH?0 mm Hg F i g . IIB-1 Amount of GaAs Transported as a Function of Water-Vapour Pressure (from G o t t l i e b and Carboy [63G1], p. 586) Q UJ h- Cr. _ 5T c. -c o 78 76" 14 12 10 8 TH=750C Tc =650°C H20at 0°C FL0WRATE-.4cc/mln 10 20 30 40 DISTANCE BETWEEN SOURCE AND SUBSTRATE (mils) TEMPERATURE I£ T('K) F i g . IIB-2 Amount of GaAs Transported as a Function of Spacing (from G o t t l i e b and Carboy [63G1], p. 591) F i g . IIB-3 Log Rate of Transport vs Temp. (from Robinson [63R1], p. 579) 13. on the amount of GaAs tra n s p o r t e d (weight l o s s of GaAs source). I t was found that the H2 flow r a t e had l i t t l e b e a r i n g on the transport r a t e . However, an increase i n water vapour pressure caused an increase i n t r a n s p o r t r a t e , see F i g . I I B - 1 . 2. Spacing As the name i m p l i e s t h e n 'close spaced vapour t r a n s p o r t ' tech- nique r e l i e s on the short distance between source and s u b s t r a t e f o r t r a n s p o r t a t i o n e f f e c t i v e n e s s . N i c o l l ([63N1], p. 1165) s t a t e d that a spacing of l e s s than l / 1 0 t h the diameter of the source area was necess- ary f o r e f f i c i e n t t r a n s f e r . G o t t l i e b and Carboy [63G1] grew samples w i t h d i f f e r e n t spacing and produced the p l o t i n F i g . IIB-2. I f most of the samples were about 1 cm. i n diameter (380 mils) i t can be seen from t h i s graph that t r a n s p o r t e f f i c i e n c y i n c r e a s e s once N i c o l l ' s c r i t e r i o n i s s a t i s f i e d . 3. Temperature Temperature a f f e c t s the r e a c t i o n and i t would be expected that higher source to substrate temperatures would speed up the k i n e t i c s of the f i l m growth. Robinson [63R1] p l o t t e d the l o g ra t e of t r a n s p o r t versus r e c i p r o c a l of source temperature (substrate temperature h e l d constant) and as expected mass t r a n s p o r t increased w i t h temperature (see F i g . IIB-3) and from t h i s p l o t an energy of a c t i v a t i o n of 43 k c a l per mole can be c a l c u l a t e d . Substrate temperatures tended to c o n t r o l the surface nature of the f i l m s ; rough and d e n d r i t i c f o r h i g h tempera- tures and smooth f o r low temperatures. Growth r a t e s were r e s t r i c t e d by an upper bound imposed on the source-substrate temperature d i f f e r e n c e by the heater arrangement. 1 14. 4. Surface Topography arid C r y s t a l O r i e n t a t i o n The authors d i d not report i n d e t a i l on surface topography and c r y s t a l o r i e n t a t i o n ; however, there i s some evidence t h a t the smoothest l a y e r s were obtained on the smoothest s u b s t r a t e s . I t was found that the best deposits were on chemically p o l i s h e d and vapour phase etched Ge wafers. Robinson [63R1] observed some source o r i e n t a - t i o n e f f e c t s and found t h a t . a [100] GaAs surface f a c i n g the Ge s u b s t r a t e y i e l d e d f i l m s w i t h the smoothest s u r f a c e s . 5. Doping The f i l m s reported by the RCA workers [63N1], [63R1], and [63G1] were i n v a r i a b l y n-type regardless of any attempt to o b t a i n the c o n t r a r y , i . e . p type. This was l i k e l y due to the low vapour pressure of the common p-type dopant oxides and to. Ga.autodbping.A Zn-doped source a l s o f a i l e d to produce p-type GaAs [63G1]. C o n t r o l l i n g the amount of n-type doping was attempted w i t h T e and H£S i n c l u d e d i n the H2O vapour, but no q u a n t i t a t i v e r e s u l t s were reported. A l l the authors d i d some work w i t h source m a t e r i a l s other than GaAs, not a b l y GaP, InAs, CdS, ZnS and Ge. These sources a l l gave r e s u l t s s i m i l a r to GaAs when deposited on substrates w i t h s i m i l a r c r y s - t a l s i z e and shape. However, a l l the above source m a t e r i a l s , i n c l u d i n g GaAs, deposited as p o l y c r y s t a l l i n e f i l m s onto quartz or metal f o i l s . S t r u c t u r a l p r o p e r t i e s l i k e p r e f e r r e d o r i e n t a t i o n , c r y s t a l s i z e and shape were not i n v e s t i g a t e d . II-C The Apparatus The design of the apparatus used to grow GaAs by the CSVT technique f o r the present i n v e s t i g a t i o n stemmed from a c o n s i d e r a t i o n of the apparatus and r e s u l t s p r e v i o u s l y discussed ( I I - B ) . The r e a c t i o n 15. chamber was made from quartz male and female 1 i n c h diameter ground j u n c t i o n s c l o s e d at the ends (see F i g . I I C - 1 ) . The female end could be removed to replace the source, su b s t r a t e o r spacers. A quartz tube connected through the male end su p p l i e d the source and sub s t r a t e w i t h the r e a c t i v e agent as w e l l as f u r n i s h i n g support. Two 625 watt S y l v a n i a "Sun Guns" s u p p l i e d the necessary energy to heat the molybdenum heater blo c k s which were i n thermal contact w i t h source and s u b s t r a t e . The p e r i p h e r a l equipment i s shown i n F i g . IIC-2. A Med i c a l grade hydrogen source s u p p l i e d the gas which was blown over i c e , i n order to absorb water vapour,and then was introduced i n t o the quartz r r e a c t i o n chamber. The i c e , which was kept cl o s e to 0°C by a s a l t i c e bath, produced a c o n t r o l l e d p a r t i a l pressure of i n the H2 gas stream. Two thermocouples attached to the molybdenum b l o c k s monitored the source and s u b s t r a t e temperature w i t h the a i d of as s o c i a t e d voltmeters. A water trap was connected onto the r e a c t i o n chamber exhaust f o r p r o t e c t - i o n against backup, as a scrubber, and as a means of checking H2 flow r a t e s . The l a t t e r f u n c t i o n was achieved by counting the number of bubbles per minute and c a l i b r a t i n g t h i s r a t e w i t h a soap bubble flow meter. The bubble count method was convenient because the rat e s used were very slow and clos e to the l i m i t of s a t i s f a c t o r y continuous opera- t i o n of the soap bubble flow meter. II-D Choice of Substrate M a t e r i a l The c r i t e r i a f o r choosing a s u i t a b l e s u b s t r a t e i n v o l v e d i t s c o s t , s t a b i l i t y , and thermal expansion c o m p a t i b i l i t y w i t h GaAs. The l i s t below i n c l u d e s a l l the common subs t r a t e m a t e r i a l s that were considered. MALE GROUND JUNCTION EXHA UST THERMOCOUPLE WIRES FEMALE GROUND JUNCTION SYLVAN IA SUN GUNS 625 W. F i g . IIC-1 The B a s i c CSVT Apparatus r-> SOAP BUBBLE FLOW METER- SALT ICE BATH WATER TRAP FOR PROTECTION AND BUBBLE COUNT F i g . IIC-2 The CSVT P e r i p h e r a l Equipment 18. 1. Aluminum 9. Invar 2. T i n 10. N i c k e l - I r o n A l l o y s 3. Copper 11. Tantalum 4. Bronze (95% Cu, 5% Sn) 12. Tungsten 5. Brass (70% Cu, 30% Zn) 13. Molybdenum 6. S t e e l 14. Alumina (coated w i t h metal e l e c t r o d e ) 7. S t a i n l e s s S t e e l 15. Fused S i l i c a 8. Kovar (coated w i t h metal e l e c t r o d e ) The f i r s t and most important task was to f i n d s t a b l e s u b s t r a t e s . The h i g h growth temperatures of the GaAs CSVT technique (pi700°C) make the GaAs f i l m p a r t i c u l a r l y s e n s i t i v e to d i f f u s i o n or r e a c t i o n s w i t h elements from w i t h i n the s u b s t r a t e . A l s o the m e l t i n g p o i n t s of the base m a t e r i a l s must be greater than approximately 750°C. 'Tin and aluminum have values below t h i s and t h e r e f o r e would n o t , alone, support a GaAs f i l m . Substrates t h a t are "wetted" by Sn and A l could be used but most of the p o s s i b l e substrates c o u l d be used by themselves and t h e r e f o r e t i n and aluminum are excluded. Copper i s a known dopant i n GaAs (see Madelung [64M1], pp. 229, 230). F u l l e r and Whelan [58F1] found i t d i f f u s e d very r a p i d l y e s p e c i a l l y along d i s l o c a t i o n s or s u r f a c e s . This type of d i f f u s i o n would create s h o r t i n g of a s o l a r c e l l from h e a v i l y doped g r a i n boundaries, t h e r e f o r e Cu and Cu a l l o y s are to be avoided. The p r i n c i p a l component of s t e e l s and other Fe a l l o y s i s i r o n which acts as a deep acceptor i n GaAs (Cunnell et a l . [60C1], p. 103). Impurity d i f f u s i o n i n f o r m a t i o n i s u n a v a i l a b l e f o r i r o n but i t i s thought not to be as r a p i d a d i f f u s e r as Gu. D i f f u s i o n could be"prevented by coating these Fe a l l o y s w i t h p a s s i v a t i o n l a y e r s of SiO or metals such as 19. Ta» Mo, or W. In the case of SiO, a second f i l m would be r e q u i r e d to f u r n i s h the metal e l e c t r o d e . A l l these f i l m formations r e q u i r e expensive e l e c t r o n beam evaporation or s p u t t e r i n g equipment and added processing steps which reduce t h e i r economic f e a s i b i l i t y . Kovar, Invar and other Ni-Fe a l l o y s contain appreciable amounts of n i c k e l and c o b a l t which have i o n i z a t i o n energy l e v e l s deep i n the band gap and thus form e f f e c t i v e t r a p p i n g centers ([64M1], p. 231). Therefore prevention of N i and Go d i f f u s i o n i s a l s o mandatory (and probably expensive). The remaining metals Ta, W, and Mo are the most c o s t l y i n the l i s t . However, as o u t l i n e d above, the inexpensive metals r e q u i r e p a s s i v a t i o n and thus t h e i r f i n a l cost would a l s o not be cheap. There are two methods that would reduce the h i g h p r i c e of Ta, W and Mo sheets. The f i r s t r e q u i r e s c o l d r o l l i n g the metals to t h e i r thinnest p o s s i b l e thickness (&.02 mm), thereby s t r e t c h i n g them and reducing cost to a l e v e l where the s u b s t r a t e p r i c e could become comparable to that of the t h i n f i l m semiconductor (see Table IID-1) . The second method i n v o l v e s d e p o s i t i o n of t h i n l a y e r s of i n e r t metals onto cheap s u b s t r a t e s . A 2000 A l a y e r sputtered or evaporated onto a smooth, high temperature m a t e r i a l would f u r n i s h an e l e c t r o d e w i t h e l e c t r i c a l p r o p e r t i e s s i m i l a r to that of the b u l k metal. Using the f i r s t s u b s t r a t e formation technique, i t seems reason- able to consider Ta, W and Mo f o r use i n the present i n v e s t i g a t i o n and molybdenum (.1 mm t h i c k f o i l ; 99r9% .pure^wwas^finaMytchoserifas i t s c o e f f i - c i e n t :ofcthermal expansion resembles" most c l o s e l y G t h a t of GaAs.(see graph i n F i g . IID-1). An i n i t i a l attempt to smooth the r o l l marks i n the molybdenum f o i l by e l e c t r o p o l i s h i n g i n 175 ml. methyl a l c o h o l and 25 ml. s u l f u r i c 20. mo 0 200 400 600 TEMPERATURE (°C) F i g . IID-1 R e l a t i v e Thermal Expansion of D i f f e r e n t Metals 21 a c i d at current d e n s i t i e s v a r y i n g from 0.6 to 1.0 amp/sq. cm r e s u l t e d i n the p i t t i n g and e t c h i n g of a l l s u b s t r a t e s . This was probably a r e s u l t of contaminants present i n the Mo r e a c t i n g , unfavourably, w i t h the a c i d . The smoothest and cleanest Mo surfaces were prepared by sp u t t e r e t c h i n g the f o i l s f o r 20 min. at 100 watts i n argon a t a p r e s s - -2 ure of 1J.5 x 10 t o r r , The f o i l s were precleaned i n hot detergent and water, r i n s e d f i r s t i n d i s t i l l e d water then hot methanol, and blown dry before i n s e r t i o n i n a Perkin-Elmer r . f . s p u t t e r i n g system f o r the s p u t t e r e t c h i n g . The second method of s u b s t r a t e formation c o n s i s t e d of Mo (of the type and p u r i t y described above) sputtered onto fused s i l i c a s l i d e s . The same Perkin-Elmer s p u t t e r i n g system was used w i t h a 6 i n c h Mo f o i l t a r g e t . Fused s i l i c a s l i d e s were precleaned as o u t l i n e d above and out- -6 9 gassed i n the s p u t t e r i n g chamber at 10 t o r r f o r 10 min. 2000 A of Mo (as l a t e r measured on a Sloan Angstrometer) was sputtered onto the quartz s l i d e s at 1.5 x 10 t o r r argon pressure w i t h 100 watts forward power f o r 30 minutes. Films prepared under these c o n d i t i o n s had a sheet r e s i s t a n c e (measured by the 4 p o i n t probe method) of roughly 0.3 ft/square This value i s equivalent to a f i l m r e s i s t i v i t y of 6 x 10 ^ Q cm and i s only about 10% higher than the bulk value. II-E Experimental Conditions The experimental c o n d i t i o n s used to grow GaAs by the CSVT t e c h n i q u e 1 i n t h i s present work were derived from the r e s u l t s of the RCA workers [63N1], [63R1], [63G1] summarized i n Chapter I I s e c t i o n C, and a l s o from experience gained by using the p a r t i c u l a r experimental arrangement at U.B.C. Table IIE-1 l i s t s the parameters used during each growth unless otherwise s t a t e d i n the t e x t . 221 A H_2 gas flow r a t e of 4 cc/min. gave a steady flow w i t h adequate vapour f o r growth. The i c e bath was h e l d at 0°C f o r convenience and to give h i g h t r a n s p o r t a t i o n y i e l d (see F i g . I I B - 1 ) . 2 Substrates and sources were roughly 1 cm i n area, t h e r e f o r e by N i c o l l ' s c r i t e r i o n the spacing should be l e s s than 1 mm. 500 um was chosen. For the p a r t i c u l a r experimental arrangement w i t h Mo on fused s i l i c a s u b s t r a t e s , temperatures were adjusted to e s t a b l i s h optimum growth. The source temperature was v a r i e d from 650°C to 750°C w h i l s t m a i n t a i n i n g a 100°C temperature d i f f e r e n c e between source and s u b s t r a t e . At 650°C the GaAs f i l m was not continuous but grew i n i s o l a t e d i s l a n d s . At 750°C the f i l m peeled or co n s i s t e d of l a r g e d e n d r i t i c growth-. A sub s t r a t e temperature of 710°C gave the smoothest r e s u l t s w i t h an acceptable t r a n s - port r a t e . The tr a n s p o r t of GaAs reduces w i t h lower source to substrate temperature. Therefore the maximum obtainable temperature d i f f e r e n c e * was used ( i . e . 100°C f o r t h i s p a r t i c u l a r experimental arrangement). For the experiments using s p u t t e r etched Mo f o i l s ubstrates the maximum temperature d i f f e r e n c e a t t a i n a b l e was only 90°C. The GaAs sources were obtained from Monsanto and were i n the form of boat grown s i n g l e c r y s t a l wafers 15 m i l s t h i c k . They were n-type 17 -3 -2 doped w i t h S i to a c a r r i e r c o ncentration of 2.0 x 10 cm (10 fi cm). 23. M a t e r i a l (.25 mm t h i c k ) Cost (10 6$/km 2) Reference Kovar 400 ([74F1], p. 15) Molybdenum 240 Tantalum 68 (42% Ni.) Fe a l l o y 1.1 S t e e l .3 " Cu (and Cu a l l o y ) 7.8 Aluminum .2 Fused S i l i c a > 400 Alumina > 400 S i (100 y t h i c k ) 14. ([74F1], p. 15) GaAs (10 y t h i c k ) 22.8 ([75W2], p. 1005) Table IID-1 Cost of Substrate M a t e r i a l s , S i , and GaAs D e s c r i p t i o n of Parameter Quantity or Remarks H 2 Flow Rate 4-5 cc/min. Ĥ O Vapour Pressure ( i c e bath temp.) 5.3 mmHg (0°C) Source and Substrate Area '2 1 cm Spacing 500 ym Source Temperature 810-°-C Substrate Temperature 710 °C Time 1 hour GaAs Source O r i e n t a t i o n [100] f a c i n g s u b s t r a t e Substrate Mo. on fused s i l i c a Table IIE-1 Summary of CSVT Technique Conditions f o r Optimum Growth 2 4 . I l l MICROSTRUCTURAL ANALYSIS OF THE GaAs FILMS In this chapter the structure and topography of the polycrys- t a l l i n e GaAs films are examined by x-ray d i f f r a c t i o n and scanning elec- tron microscopy. X-ray d i f f r a c t i o n techniques were employed to determine c r y s t a i l i n i t y and preferred orientations. The SEM was used to study the c r y s t a l sizes and shapes as a function of growth temperature. In addi- tion -to the c r y s t a l investigations ,r.electron microprobe analyses of impurities were performed on the surfaces and at the cross sections exposed from cleaving the santples ,5_ t t . - i » ,- t . III-A X-ray D i f f r a c t i o n Ordinary x-ray diffractometry can be used to analyse GaAs 2 films up to 10 ym thick and several mm i n surface area. This technique was chosen to investigate the structure of the films on a macrovolume s cale. In a cubic l a t t i c e , the interplanar spacing d corresponding to a set of M i l l e r indices (h k I) i s d = a ° (IIIA-1) 4 2 + k 2 + I2 where a Q i s the l a t t i c e constant. Crystalline GaAs i s generally cubic with a l a t t i c e constant of a D = 5.6535 %. ([06Hl], p.333). The Bragg angles, 26, that produce constructive interference i n cubic GaAs are l i s t e d i n Table IIIA-1. 1. C r y s t a l l i n i t y a. . Apparatus A P h i l i p s type 12-04'6 x-ray d i f f r a c t i o n machine with CuKa radiation of wavelength X = 1.542A was used to study the c r y s t a l l i n i t y . X-RAY TUBE F i g . I I I A - 1 X-ray D i f f r a c t i o n Goniometer SOURCE F i g . I IIA-3 Texture Goniometer 26. X-rays emitted from the CuKa source were r e f l e c t e d from the GaAs c r y s t a l planes and c o l l e c t e d at a de t e c t o r . The x-ray d e t e c t o r was a xenon f i l l e d p r o p o r t i o n a l counter capable of ha n d l i n g extremely high count rat e s without need f o r any dead time c o r r e c t i o n . The detector and specimen were f i t t e d to a goniometer ( P h i l i p s type 42202), i . e . the detector wias made to move i n a c i r c l e around the sample at twice the angular speed of the sample as shown i n F i g . I I I A - 1 . Bragg's Law i s s a t i s f i e d whenever nA = 2d s i n 9 (IIIA-2) where o X = wavelength of r a d i a t i o n source (CuK6ti=- 1.54051 A) n = an i n t e g e r > 1 0 = angle of i n c i d e n t r a d i a t i o n on specimen • Pulses from the detector were conveyed v i a a Tennelec TC 214 L i n e a r a m p l i f i e r to a Tennelec TC 592P D i g i t a l Ratemeter that summed the pulses every 2 seconds. Each sum was tr a c e d on a c a l i b r a t e d chart recorder (Minneapolis-Honeywell Reg. Co. model no. 153X17V-X-6A3P4). The chart recorder r a t e was set at 1/2 i n c h per min. The goniometer revolved esofc that 29 moved 1° i n 1 min. Therefore the chart s c a l e was 2 degrees per i n c h (or d i v i s i o n ) . b. R e s u l t s Two specimens were t e s t e d , each grown on a d i f f e r e n t s u b s t r a t e , namely: i ) moly f o i l , i i ) moly sputtered on q u a r t z , and the r e s u l t s obtained were i d e n t i c a l . A d i f f r a c t o g r a m of the GaAs on Mo/quartz specimen i s shown i n F i g . I I I A - 2 . The observed angles of the peaks are l i s t e d alongside the t h e o r e t i c a l values i n Table I I I A - 1 . (Note when reading the graph of F i g . IIIA-2 t h a t the chart recorder pen r e f l e c t e d on overshoot and thus the r e l a t i v e i n t e n s i t y magnitudes cannot be 'd i r e c t l y compared to Table III A - 1 ) ) Polu Gate or, Mo-Quartz 3s~ tt-j < IF 2 sec. c o u n t j I F i g . IIIA-2 X-ray D i f f r a c t i o n of P o l y c r y s t a l l i n e GaAs on Mo-Quartz Theory for a Q = 5.6535 Poly GaAs Results Miller Indices hkl Bragg Reflection 29 in degrees Relative Intensity I / I D 26 (tolerences ±.05°) Calculated Lattice Constant a Q (A) 111 27.33 100 27.35 5.643 220 45.38 90 45.40 5.645 311 53.78 80 53.80 5.646 400 66.12 60 66.15 5.646 331 72.95 70 72.95 5.648 422 83.84 70 83.85 5.648 511,333 90.25 50 90.25 . 5.648 440 100.97 40 100.90 5.651 531 107.57 107.50 5.651 Table IIIA-1 Bragg Reflection Angles for Cubic GaAs and CSVT Grown GaAs 29. c. A n a l y s i s The p o s i t i o n s of the observed x-ray d i f f r a c t i o n peaks were, w i t h i n the bounds of experimental e r r o r , i d e n t i c a l to cubic l a t t i c e GaAs. At higher Bragg angles, where the accuracy of the machine was b e s t , the c a l c u l a t e d l a t t i c e constant approached i t s expected value w i t h i n .04%. No hexagonal phase r e f l e c t i o n s were observed as was the case f o r the vacuum deposited GaAs f i l m s of Pankey and Davey ([66P1], p. 1513). These authors had encountered a weak hexagonal close packed (1010) r e f l e c t i o n at 26 = 25.90°. No other major peaks were observed i n the present work. The f i l m s were t h e r e f o r e GaAs cubic c r y s t a l s . 2. P r e f e r r e d O r i e n t a t i o n a. Apparatus The specimens were inspected f o r p r e f e r r e d c r y s t a l o r i e n t a t i o n using a Schulz type texture goniometer. The te x t u r e goniometer i n c o r - porated the same P h i l i p s x-ray diffTactometer ( P h i l i p s type 12046) as described above but w i t h a more complex sample holder. The sample holder r e s t e d on a motorized gyro w i t h two axes of r o t a t i o n . The f i r s t a x i s r o t a t e d about the sample's center and perpe n d i c u l a r to i t s f a c e , i . e . a x i s A'A shown i n F i g . I I I A - 3 . The second a x i s B'B revolved about a l i n e i n the plane formed by the i n t e r s e c t i o n of the primary and d i f f r a c t e d beams and at an angle 6 t o the primary beam. When the primary beam i s at an angle 26 from the d i f f r a c t e d beam where 26 corresponds to a Bragg r e f l e c t i o n from an i n t e r p l a n a r spacing, x-ray i n t e n s i t y peaks occur at the detector. These peaks appear only when the sample i s o r i e n t a t e d so that the per- p e n d i c u l a r to c r y s t a l planes b i s e c t and are i n the plane formed by the primary and d i f f r a c t e d beams. The gearing was arranged so that f o r every 360° r e v o l u t i o n of F i g . IIIA-4 Texture Goniometer Chart of P o l y c r y s t a l l i n e GaAs on Mo-Quartz LO O 31. a , 3 advanced 5°. In t h i s way the primary beam searched through a wide range of angles w i t h respect to the surface of the sample. For f i l m s w i t h p r e f e r r e d c r y s t a l o r i e n t a t i o n s , peaks occur at the detector f o r c e r t a i n a Q and B Q when Bragg's Law i s s a t i s f i e d . The r e s u l t s are s i m i l a r to those from Laue analyses but e a s i e r to i n t e r p r e t . When a peak occurs, one set of a Q and $ 0 are defined which y i e l d the d i r e c t i o n of t h a t p r e f e r r e d c r y s t a l o r i e n t a t i o n . The s t r e n g t h of any peak com- pared to another gives a q u a l i t a t i v e i n d i c a t i o n of the r e l a t i v e magnitude of the p r e f e r r e d o r i e n t a t i o n . b. Results No peaks were observed. The r e s u l t s comprised a s l i g h t l y decreasing waving output that corresponded to the r o t a t i o n of the sample about AA'. This was due to the p r o j e c t i o n of the x-ray beam on the sample becoming l a r g e r than the surface area of the GaAs f i l m f o r low 20 angles. The chart from the GaAs on Mo/quartz substrate i s shown i n F i g . IIA-4. Again the GaAs on Mo f o i l specimen y i e l d e d i d e n t i c a l r e s u l t s . c. A n a l y s i s Since no d i s t i n c t peaks were observed the f i l m s were random c r y s t a l l i t e s w i t h no p r e f e r r e d o r i e n t a t i o n . I I I - B Scanning E l e c t r o n Microscopy 1. I n t r o d u c t i o n For good s o l a r c e l l e f f i c i e n c y u s i n g p o l y c r y s t a l l i n e semi- conductor f i l m s , i t i s necessary t h a t the p h o t o c a r r i e r s avoid the p o l y - c r y s t a l l i n e g r a i n boundaries so that conduction i s not hindered by g r a i n boundary b a r r i e r s . In other words, growth co n d i t i o n s should be arranged to produce a f i l m of semiconductor c r y s t a l s w i t h p r e f e r a b l y one c r y s t a l between the base su b s t r a t e and the top e l e c t r o d e . This type of c e l l , 32. a) b) •SUBSTRATE F i g . I I I B - 1 Schematic Diagram of P o l y c r y s t a l l i n e F i l m s : a) m u l t i p l e v e r t i c a l c r y s t a l s b) s i n g l e v e r t i c a l c r y s t a l s 33. the single v e r t i c a l c r y s t a l p o l y c r y s t a l l i n e solar c e l l i s shown i n Fi g . I I I B - l b . The. most important parameter affecting the c r y s t a l l i t e size and shape i s growth temperature. In the following experiments, GaAs films were grown on Mo on quartz substrates at different tempera- tures ranging from 675°C to 725°C. The scanning electron microscope (SEM) was used to study the c r y s t a l size arid shape p r i n c i p a l l y because of i t s large depth of f i e l d . 2. Theory The scanning electron microscope (SEM) uses a fine electron beam to s t r i k e the specimen. At the point of impact, a variety of phenomena occur as shown i n Fig. IIIB-2. The electron beam i s scanned i n a raster pattern across the specimen surface i n synchronization with a cathode ray tube (CRT). Secondary electrons, which are dependent on topography, are detected and used to produce an image of the specimen on the CRT. 3. Procedure The temperature experiment u t i l i z e d 5 samples a l l grown on Mo sputtered on quartz substrates. A substrate temperature range of 675 to 725°C was covered i n increments of 12.5°C. The temperature difference between substrate and source was kept at 100°C. After the CSVT f i l m growth step, the quartz substrates were scribed, then diced, i n order to produce cross sections of the GaAs films. (Attempts to view cross sections of the films on Mo f o i l were un- successful because the shearing chipped away the films at the cut edge of the substrate.) The exposed side views were too f l a t to distinguish the grains on the SEM e a s i l y so the following grain boundary etches were investigated. CHARACTERISTIC XRAY PHOTONS BREMSSTRAHLUNG VISIBLE LIGHT (CATHODO- LUMINE S CEN CE ) ABSORBED ELECTRONS INCIDENT ELECTRON BEAM ELASTICALLY SCATTERED ELECTRONS BACKSCATTERED ELECTRONS SECONDARY ELECTRONS AUGER ELECTRONS TRANSMITTED AND INELASTICALLY SCATTERED ELECTRONS IIIB-2 Phenomena Generated from an Incident E l e c t r o n Beam ( i ) HN0 3 : HF : H 20 i n r a t i o 1 : 1 : 1 f o r 10 sees ( i i ) H 2 0 2 : HF : H 20 i n r a t i o 2 : 1 : 6 f o r 1.5 mins ( i i i ) H 2 0 2 : H^O^ i n r a t i o 1 : 1 : 8 f o r 10 sees Etch number 1 exposed grains that could be seen on the SEM. I t etched deepest at the c r y s t a l boundaries and was l e a s t s u b j e c t to f l u c t u a t i o n s i n temperature, freshness and-time. No. 2 etch d i d not att a c k the grains and no. 3 l e f t a worm-eaten surface that was l e s s comprehensible than the o r i g i n a l . An Etec Autoscan SEM w i t h 20 kV a c c e l e r a t i n g voltage and 2000 l i n e s per frame p i c t u r e r e s o l u t i o n was used to produce the photomicro- graphs shown i n F i g . I I I B - 3 through to F i g . I I I B - 7 . A t h i n l a y e r , l e s s - o than 100 A, of gold-palladium had to be sputtered onto the quartz surfaces to reduce charge b u i l d up and haze. 4. R e s u l t s The topography of the samples grown at d i f f e r e n t growth tem- p e r a t u r e s , as shown i n F i g s . I I I B - 3 and 4, i l l u s t r a t e d t h a t the c r y s - t a l l i t e surface areas increased w i t h temperature and t h i c k n e s s . The mean c r y s t a l l i t e areas at the surface arid t h e i r standard d e v i a t i o n s are l i s t e d i n Table I I I B - 1 . The f i l m thicknesses were measured on the SEM from the c r o s s - s e c t i o n a l view of the diced specimens; see f o r example F i g s . IIIB-5 and 6. The etched sample c l e a r l y revealed that the f i l m commenced growth w i t h the n u c l e a t i o n of sm a l l c r y s t a l s that grew t o a 2-5 ym diameter. Further growth c o n s i s t e d of every second or t h i r d c r y s t a l outgrowing i t s neighbours and covering the surface w i t h l a r g e r c r y s t a l s i n the 10-100 ym diameter range. The f i l m s grown on Mo f o i l had surface features s i m i l a r i n shape but not as l a r g e as the f i l m s on Mo/quartz s u b s t r a t e s . The F i g . IIIB-3 SEM Photomicrographs of GaAs F i l m Surfaces Grown on Mo-Quartz at D i f f e r e n t Substrate Temperatures 37. I 10 ym F i g . IIIB-4 SEM Photomicrograph of Noncontinuous GaAs Grown on Mo-Ouartz at 725 8 C 10 um F i g . IIIB-5 SEM Photomicrograph o f Cleaved Side View o f GaAs Grown on Mo- Quartz at 700°C ( I n t e r f a c i a l l a y e r i s 3000A of s p u t t e r e d Mo.) I 1 ym F i g . IIIB-6 SEM Photomicrograph of Sample Etched i n HN0-i:HF:H90 38 . average c r y s t a l size of the films on the former substrates was 60 ym and 100 ym2 for films on Mo/quartz. The larger crystals on the l a t t e r substrates were probably due to a higher temperature at the quartz surface r e s u l t i n g from a temperature gradient across the i n s u l a t i n g quartz. The surface roughness on the bulk of a l l the samples was approximately 10% of the thickness. The exceptions occurred on high temperature growths and selected areas of films grown on Mo f o i l . In the former case (see e.g. F i g . IIIB-4), discontinuous films resulted. In the l a t t e r case, occasional clusters of dendrites would grow out of the f i l m s , see for example F i g . IIIB-7. This phenomenon was probably related to inhomogeneous seeding from the Mo f o i l . Crystal Size at Surface Crystal height at Cleaved Edge Substrate Temperature <°c) Mean Area (ym^) Standard Deviation (ym̂ ) Mean Height (ym) Standard Deviation Film Thickness (ym) 675 687.5 700.0 712.5 725.0 20 45 105 100 not continuous 33 52 88 , 440 6.7 8.8 9.2 10 2.9 4.7 5.4 10 / i 10 22 22 19 Table IIIB-1 Effect of Growth Temperature on Crystal Size (For films grown on Mo on quartz.) 3 9 . 10 ym F i g . I I I B - 7 GaAs D e n d r i t i c Growth on Mo F o i l I I I - C E l e c t r o n Microprobe A semi q u a n t i t a t i v e a n a l y s i s o f the i m p u r i t y content of the CSVT GaAs f i l m s was made u s i n g an e l e c t r o n microprobe. 1. Theory and Apparatus The e l e c t r o n microprobe c o n s i s t s of an e l e c t r o n beam probe and an x-ray spectrometer d e t e c t o r . The specimen can be brought i n t o p o s i - t i o n under the e l e c t r o n beam w i t h the a i d of an o p t i c a l microscope. C h a r a c t e r i s t i c x-rays and other phenomena r e s u l t from the e l e c t r o n bombardment (see F i g . I I I B - 1 ) . The angle o f the spectrometer i s s e t f o r maximum i n t e n s i t y d i f f r a c t i o n of c h a r a c t e r i s t i c x-rays from a known standard. The i n t e n s i t y o f the x-rays from the specimen b e i n g analysed i s then compared w i t h t h a t of the standard. The mass conc e n t r a t i o n of the standard element i n the specimen can be determined from the r a t i o of these i n t e n s i t i e s and c a l i b r a t i o n curves. (A d e t a i l e d d e s c r i p t i o n of the microprobe i s given by B i r k s [63B1].) The a n a l y s i s was performed on a JEOL Co. JXA-3A microprobe w i t h a v a r i a b l e f r a c t i o n of a percent s e n s i t i v i t y f o r elements of atomic number eleven and above. The r e s o l u t i o n of the probe was 1 to 2% 2. Results The CSVT f i l m specimens were encapsulated i n p l a s t i c and cross s e c t i o n s were diamond-polished down to a 1/4 ym f i n i s h . The surfaces were scanned and no peaks occurred other than those a s s o c i a t e d w i t h Ga and As. P a r t i c u l a r i m p u r i t i e s that might be expected on account of t h e i r presence i n the r e a c t i o n chamber were oxygen, molybdenum and aluminum. S p e c i f i c scans were made to look f o r each of these p o s s i b l e i m p u r i t i e s . The oxide t e s t involveddthe scanning of a 1 ym diameter e l e c - t r o n beam across the specimen w i t h a count every second. The a c c e l e r a - t i n g voltage was 5 kV and a l e a d s t e a r a t e c r y s t a l d i f f r a c t e d the oxygen Koi2 r a d i a t i o n . The r e s u l t s are superimposed over an absorbed e l e c t r o n image of the f i l m (see F i g . I I I C - 1 ) . The counts were low enough to make i t unnecessary to c o r r e c t f o r dead time. No oxygen was present i n con- c e n t r a t i o n s greater than one p a r t i n 100 (probe i s about 1% s e n s i t i v e f o r 0 2 ) . The i n t e n s i t y increased only at the p l a s t i c i n d i c a t i n g oxygen was present there. In the Mo t e s t , the e l e c t r o n beam stepped 1.25 ym every 10 sees. The a c c e l e r a t i n g voltage was 25 kV and a mica c r y s t a l d i f f r a c t e d the Mo Kai r a d i a t i o n . The r e s u l t s show ( F i g . IIIC-2) that no Mo e x i s t e d 41. across the thickness of the f i l m i n concentrations greater than 1 par t i n 1000 (probe i s .01-.1% s e n s i t i v e f o r Mo). Aluminum was not found i n q u a n t i t i e s greater than 0.1% using an a c c e l e r a t i n g voltage of 25kV and a mica c r y s t a l d i f f r a c t i n g the Ka r a d i a t i o n . X-RAY INTENSITY IN COUNTS/SEC. W0 0J Mo Plastic T ff ^ELECTRON BEAM PATH ^ X-RAY INTENSI T Y GaAs Plastic F i g . IIIC-1 Microprobe A n a l y s i s of Oxygen Concentration. 42. POSITION (IN MICRONS) F i g . IIIC-2 Microprobe A n a l y s i s of Mo Concentration 43. III-D Discussion The films prepared i n this present work were cubic crystals of GaAs with no preferred orientation. Continuous c r y s t a l growth on Mo substrates was found to take place over the substrate temperature range of 675-725°C with the grain size increasing with temperature up to a maximum of around 100 ym2. The growth rates varied from 15 to 40 ym/hr. Films were essenti a l l y a layer of columnar single crystals 10-100 ym2 i n area with a sparse bed of smaller (stunted) crystals at the Mo-GaAs interface. Hovel [75H1] has shown using a 'filament l i f e t i m e ' model for GaAs single v e r t i c a l solar c e l l s 1 ym thick, that e f f i c i e n c i e s , short c i r c u i t current, and open c i r c u i t voltage a l l have maximum values com- parable to single c r y s t a l values for grain sizes i n excess of 2 ym. This columnar growth, then, would be highly suitable for thin f i l m solar c e l l s . The surface topography indicates a roughness on the scale of 10% of the f i l m thickness arid t h i s would be an advantage i n subsequent solar c e l l fabrication as such a surface would present a textured plane to incoming radiation, leading to the p o s s i b i l i t y of improved radiation absorption. Such surfaces are being deliberately introduced into S i single c r y s t a l c e l l s and have led to a marked reduction i n r e f l e c t i o n losses. No impurities resided i n the films i n quantities observable on the electron microprobe. In p a r t i c u l a r Mo, O2, and A l could have perhaps been expected i n the films but i n fact were not present, at least not i n concentrations greater than 1 part per 1000, 100, and 1000, respectively. IV ELECTRICAL PROPERTIES OF THE GaAs FILMS The m i c r o s t r u e t u r a l p r o p e r t i e s of the GaAs CSVT-prepared f i l m s discussed i n chapter I I I i n d i c a t e that f i l m s prepared by t h i s method have a g r a i n s i z e and c r y s t a l s t r u c t u r e w e l l s u i t e d to p h o t o v o l t a i c a p p l i c a t i o n s . -For s o l a r c e l l use the semiconductor must be i n c o r p o r a t e d i n a diode s t r u c t u r e . This can be achieved by doping the semiconductor to form a homojunction or by the a p p l i c a t i o n of a d i s s i m i l a r m a t e r i a l onto the semiconductor to form a h e t e r o j u n c t i o n . C e r t a i n semiconductors, metals or m e t a l - t h i n i n s u l a t o r s t r u c t u r e s can be used f o r t h i s l a t t e r f u n c t i o n . For the GaAs f i l m s under c o n s i d e r a t i o n , and indeed f o r most t h i n f i l m non-single c r y s t a l semiconductors, homojunction f a b r i c a t i o n i s d i f f i c u l t to achieve on account of the non-uniform doping l i k e l y to r e - s u l t from d i s s i m i l a r rates of d i f f u s i o n along the g r a i n boundaries and i n the bulk of the c r y s t a l s . Of the remaining h e t e r o j u n c t i o n p o s s i b i - l i t i e s the metal/semiconductor s t r u c t u r e was s e l e c t e d f o r t h i s i n v e s t i - gation on account of i t s s i m p l i c i t y and relevance to economical t e r r e s - t r i a l p h o t o v o l t a i c systems. In t h i s chapter, the important features of metal semiconductor s o l a r c e l l s are enumerated and the r e s u l t s of measurements on Au/GaAs Schottky b a r r i e r diodes presented. IV-A S o l a r C e l l Theory E f f i c i e n c y , n, of s o l a r c e l l s i s a matter of prime concern f o r commercial t e r r e s t r i a l a p p l i c a t i o n s and, as has been mentioned i n chapter I I , a minimum requirement of 10% has been suggested [75W1] as being necessary f o r p h o t o v o l t a i c systems to be competitive with.the other methods of power generation. The important f a c t o r s t h a t c o n t r o l the s o l a r c e l l e f f i c i e n c y n can be both process- and material-dependent and 45, are shown i n the e q u i v a l e n t c i r c u i t of F i g . IVA-1. They are the s e r i e s r e s i s t a n c e R n, shunt r e s i s t a n c e R_„, diode i d e a l i t y f a c t o r n and b a r r i e r o o n h e i g h t <f>B. The l a t t e r term i s the p r i n c i p a l component i n J g , see eq. IVA-1. -o F i g . IVA-1 Equivalent C i r c u i t of a S c l a r C e l l The s e r i e s r e s i s t a n c e i n p o l y c r y s t a l l i n e GaAs i s not expected to be n e g l i g i b l e . Grain boundaries w i l l cause recombination and b a r r i e r s that w i l l reduce the e f f e c t i v e number of c a r r i e r s i n the f i r s t case and hinder t h e i r flow through the thickness of the f i l m i n the second case. Ohmic and t h i n r e c t i f y i n g contacts a l s o c o n t r i b u t e t o s e r i e s r e s i s t a n c e . The e f f e c t of R g on n has been evaluated i n appendix IVA-1 u s i n g Au/GaAs Schottky diode parameters (see appendix IVA-1). The r e s u l t s , as i l l u s - t r a t e d i n F i g . AIV1 show that s e r i e s r e s i s t a n c e s g r e a t e r than 1.0 ft begin t o degrade s o l a r c e l l e f f i c i e n c y markedly, and values g r e a t e r than 10 ft would not be acceptable f o r commercial c e l l s . Shunt r e s i s t a n c e i s expected to occur due to s h o r t i n g between e l e c t r o d e s v i a low r e s i s t a n c e paths such as h e a v i l y doped g r a i n bound- a r i e s o r m e t a l l i c bridges i n microcracks. The e f f e c t of R ^ on n f o r AMO r a d i a t i o n on s i n g l e c r y s t a l s o l a r c e l l s has been evaluated by Hovel 3 ([75H1], F i g . 51) where i t was shown t h a t R g H l e s s than 10 ft j u s t begins to degrade e f f i c i e n c y u n t i l the p o i n t where Rgjj % 100 ft at which c e l l s would become uneconomic (n < 10%). The e f f e c t of the diode j u n c t i o n on n depends, i n the c a l c u l a t i c on the model chosen. For the Au/GaAs Schottky diode format u t i l i z e d i n t h i s i n v e s t i g a t i o n , the Thermionic Emission theory was considered most appropriate as o u t l i n e d i n chapter IV-B. The diode equation can be expressed as (see equations IVB-8,9,11) I = A*T 2 e x p [ - ^ | ] [exp ̂  _ ; i ] ( I V A _ 1 } The b a r r i e r height and diode i d e a l i t y f a c t o r " n are the two main B parameters dependent on f a b r i c a t i o n processes. P u l f r e y and McOuat [74P1] have c a l c u l a t e d the e f f e c t of a) on n f o r n-type GaAs using a model w i t h zero r e f l e c t i o n and r e s i s t a n c e l o s s e s , and a quantum e f f i c i e n c y of u n i t y . The r e s u l t s showed that e f f i c i e n c y increased monotonically from 10% at <j>B = .9 eV to as hig h as 25% at <f>B = E = 1.43 eV. Au-Schottky b a r r i e r s have already been f a b r i c a t e d w i t h a <|L = .9 eV by Padovani and Sumner a ([65P1], p. 3747). S t i l l h igher e f f e c t i v e b a r r i e r s and e f f i c i e n c i e s are a c t u a l l y p o s s i b l e using the m e t a l - t h i n insulator-semiconductor s t r u c t u r e as evidenced by the 15% e f f i c i e n t s o l a r c e l l announced by the J e t P r o p u l s i o n Laboratory [75S1]. The e m p i r i c a l f a c t o r n has the same e f f e c t on e f f i c i e n c y as <j) because i t also r e s i d e s i n an exponential term of the diode equation B and e f f i c i e n c y can t h e r e f o r e be expected to increase monotonically as n incre a s e s from 1. However, i t must be r e a l i s e d that n > 1.2 means other conduction mechanisms are present i n the metal-semiconductor b a r r i e r besides thermionic emission, f o r example, recombination and i n t e r f a c i a l l a y e r e f f e c t s . Therefore, a c e l l w i t h h i g h n does not n e c e s s a r i l y mean a high n w i l l r e s u l t . -Rc , R p n, n and "cbB can be e x t r a c t e d from J-V measurements'" of Au/GaAs Schottky b a r r i e r diodes and used as a p r e l i m i n a r y e v a l u a t i o n of the s u i t a b i l i t y of the CSVT GaAs f i l m s f o r t e r r e s t r i a l s o l a r c e l l s . 47. IV-B E l e c t r i c a l Contacts to the GaAs Films In t h i s s e c t i o n , the theory of metal-semiconductor contacts i s b r i e f l y reviewed and the f a b r i c a t i o n procedures used to make ohmic and r e c t i f y i n g contacts to the CSVT-grown GaAs f i l m s are presented. 1. R e c t i f y i n g Contacts a. Schottky B a r r i e r Theory The fundamental theory of Schottky diodes has been reviewed i n many books ([69S1], chapter 8 ) , ([72M1], chapter 6 and 7), and ([57H1], chapter 7). The energy band diagram f o r a metal i n contact w i t h an n-type semiconductor at 0 bi a s i s shown i n F i g . IVB-1. The b a r r i e r h e i g h t f o r a RECTIFYING SEMICONDUCTOR OHMIC CONTACT CONTACT F i g . IVB-1 Energy Band Diagram of a Metal/n-type Semiconductor/ Ohmic Contact S t r u c t u r e at Thermal E q u i l i b r i u m 48. semiconductor w i t h a doping concentration i s : ((>„ = V. . + V - A<(> (IVB-1) Y B n b i n r where i s the b u i l t i n v o l t a g e , V n i s the d i f f e r e n c e between the fermi l e v e l and the bottom of the conduction band i n the semiconductor b u l k , and A<f> i s the b a r r i e r lowering due to the image f o r c e . The image force lowering at b i a s V i s r e l a t e d to the maximum e l e c t r i c f i e l d E m by /q E A* =V 4^f ( I V B~ 2 ) where , „ E m = 2(V. . - V - ̂ -)/W (IVB-3) m b i ,q and e Q i s the s t a t i c value of the semiconductor d i e l e c t r i c - c o n s t a n t , q i s the e l e c t r o n charge, and kT/q a r i s e s from the e f f e c t of the reserve l a y e r ([57H1], p. 177). The width of the space charge region i s denoted by W: flll- (V. , - V - 1% (IVB-4) qN n b i • q D Therefore the Schottky capacitance r e l a t i o n becomes 4 = 2 ( v b i - v - ] f ) / ( ( i e s v ( i v b - 5 ) c 2 2 and can thus be determined from a p l o t of A /C vs V, so en a b l i n g c a l c u l a t i o n of <J>g.from eq. IVB-1. I t a l s o f o l l o w s that the doping con- c e n t r a t i o n i n the depleted r e g i o n can be given by N = - — ( ^ / C ) 2 ) " 1 (IVB-6) D q s s d V A 2 For non-uniform i m p u r i t y content, the slope of —~- versus V i s no longer C a s t r a i g h t l i n e . Equation IVB-6 can s t i l l be used; however,. N D(W) i s the value of the doping concentration a t x = W (the space charge w i d t h ) , 49. where e gA W = — (IVB-7) The current transport i n metal-semiconductor b a r r i e r s i s mainly due to m a j o r i t y c a r r i e r s . Two t h e o r i e s have been proposed to describe the process, Schottky's d i f f u s i o n theory and Bethe's thermionic emission theory. Both have s i m i l a r J-V c h a r a c t e r i s t i c s . J = J s [ e x p ^ - 1] (IVB-8) However, the thermionic emission theory has a s a t u r a t i o n current d e n s i t y Jg that i s more temperature dependent and l e s s s e n s i t i v e to voltage than J of the d i f f u s i o n theory. Sze has shown th a t the thermionic emission theory i s v a l i d f o r t r a n s p o r t i n GaAs Schottky b a r r i e r diodes at room 3 temperature w i t h an e l e c t r i c f i e l d range between 9 x 10 V/cm and 1 x 10 5 V/cm ([69S1], p. 390). This encompasses the u s e f u l forward b i a s \J 16 —3 range of Schottky b a r r i e r s f o r doping concentrations l e s s than N D ^ 10 cm (which are g e n e r a l l y used i n p h o t o v o l t a i c s ) . The s a t u r a t i o n current d e n s i t y f o r thermionic emission i s given by q < ) ) J g T = A*T 2 exp(- - ^ p ) (IVB-9) where A* i s the modified Richardson's constant A* = 4 7 T m * k 2 (IVB-10) ti where h i s Planck's constant and m* i s the c a r r i e r e f f e c t i v e mass. For -2 -2 GaAs m* = 0.068 mQ ([69S1], p. 20) t h e r e f o r e A* = 8 Amp cm °K . In p r a c t i c e , both A* and <(>g are f u n c t i o n s of v o l t a g e ; t h e r e - fore, the forward J-V c h a r a c t e r i s t i c s ( f o r V > 3 kT/q) should be repre- sented by J = JST e X? %S> ( I V B- 1 1 ) 50. when 1 < n < 1.2 The b a r r i e r height can be obtained from the s a t u r a t i o n current J ^ T (found by e x t r a p o l a t i n g J to zero voltage) i n s e r t e d i n the f o l l o w i n g equation. I fT A * T 2 <f>_ = — In (V^-) (IVB-13) B n <» J S T b. Schottky B a r r i e r F a b r i c a t i o n In order to prepare a r e c t i f y i n g contact to the p o l y c r y s t a l l i n e f i l m s grown on Mo by the present CSVT technique the f o l l o w i n g procedure was adopted. Immediately a f t e r removal from the growth chamber, hydro- carbons and other gross contaminants were removed by an u l t r a s o n i c wash i n hot d i s t i l l e d water and detergent. The GaAs was then dipped f o r a minute i n chloroform, r i n s e d f o r 5 minutes i n hot methyl a l c o h o l , etched f o r 10 seconds i n f r e s h l y prepared H^SO^'H^O.H202 i n a 16'3-1 r a t i o , r i n s e d i n b o i l i n g d i s t i l l e d water and blown dry. Each sample was put d i r e c t l y i n t o a Veeco vacuum chamber which was then pumped down to a —6 base pressure of 1 x 10 t o r r . The GaAs was heated to s l i g h t l y above 100°C i n order to evaporate water vapour from the s u r f a c e . Au b a r r i e r f i l m s were deposited by evaporation through a metal -3 2 mask, forming c i r c u l a r Au contacts of 7.85 x 10 cm i n area i n order to i n v e s t i g a t e d i f f e r e n t regions of the f i l m . The edge capacitance C.g from Goodman ([63G2], eq. 7) C <-e„ir p/2 = .4pf (IVB-14) e S i s n e g l i g i b l e f o r t h i s s i z e of contact (p i s the perimeter of the contact) As t h i s area of contact was not s u i t a b l e f o r l i g h t c o l l e c t i o n purposes, no attempt was made to fashion p h o t o s e n s i t i v e diodes and, indeed, the Au 51. a , FRONT WALL CELL STRUCTURE METAL GRID RECTIFYING. ^ CONTACT OHMIC CONTACT (TRANSPARENT) METAL FOIL SUBSTRATE b. BACK WALL CELL STRUCTURE F i g . IVB-2 Poly GaAs S o l a r C e l l C o n f i g u r a t i o n s 52. f i l m was made t h i c k 2000 A* as measured by an I n f i c o n 321 f i l m t h i c k - ness monitor) to ensure that there would be no c o n t r i b u t i o n to Rg from t h i s r e g i o n . 2. Ohmic Contacts Schottky b a r r i e r s o l a r c e l l s are normally f u r n i s h e d w i t h ohmic contacts to the non r e c t i f y i n g s i d e of the semiconductor. The CSVT GaAs f i l m s could be used i n e i t h e r a f r o n t w a l l or a backwall c e l l c o n f i g u r a - t i o n as shown i n F i g . IVB-2. The f r o n t w a l l c e l l r e q u i r e s an ohmic con- t a c t between the substrate and GaAs f i l m . The b a c k w a l l c e l l depends on the s u b s t r a t e - GaAs i n t e r f a c e being r e c t i f y i n g and an ohmic contact i s f i t t e d to the surface of the GaAs. The next s e c t i o n contains the theory of ohmic contacts followed by a s e c t i o n o u t l i n i n g the f a b r i c a t i o n process used to place ohmic contacts on the GaAs f i l m s . a. Ohmic Contact Theory Any conductor (metal) brought i n contact w i t h a semiconductor w i l l produce an e l e c t r o s t a t i c b a r r i e r . This i s due to the presence of surface s t a t e s and the d i f f e r e n c e between the work f u n c t i o n of the metal and the e l e c t r o n a f f i n i t y of the semiconductor. The equation t h a t des- c r i b e s the p o t e n t i a l energy d i s t r i b u t i o n of the b a r r i e r i s q 2 N n x 2 2 from Rideout and Crowell ([70R1], p. 996). In order that t h i s contact become ohmic, the b a r r i e r must be reduced so t h a t adequate current d e n s i t i e s can flow w i t h a voltage drop n e g l i g i b l e compared to t h a t i n the Schottky b a r r i e r r e g i o n . An obvious way to lower the b a r r i e r i s by the image force e f f e c t . Note that f o r low b a r r i e r s the contact voltage w i l l be near zero and the b a r r i e r lowering i s 53. / w B F i g . IVB-4 I l l u s t r a t i o n of the Current-Voltage R e l a t i o n - ship f o r a Metal-Semicon- ductor Contact f o r Pro- g r e s s i v e l y Higher C a r r i e r Concentrations A) N < 1 0 1 7 c m - 3 Thermionic Emission Dominates Ij N % 10 1 8-10 1 9cm-3, Thermionic-Field Tunnel- i n g Dominates C) N > 1 0 1 9 c m _ 3 J F i e l d Emission Tunneling Dominates •54. 8ir s s kT Equation IVB-16 i s derived from equations IVB-1,2,3 with — assumed ne g l i g i b l e . The effect of image force on the shape of the potential b a r r i e r i s shown i n Fig. IVB-3. The image force lowering equals the band bending V, . for GaAs when b i _ 3 N = 2.7 x 1 0 2 2 C m V. . (IVB-17) D volts b i For most GaAs contacts V j ^ ̂  .7 volts and zero barriers would require impurity concentrations greater than the s o l u b i l i t y l i m i t of GaAs. Besides the image force, the e l e c t r i c f i e l d i n the b a r r i e r controls conduction and Sze has shown that for E m > IO"* v/cm ([69S1], p. 388) thermionic f i e l d emission occurs. In effect the current com- ponent due to tunneling rapidly increases for f i e l d s above 10"* v/cm and b a r r i e r width less than 3000 A. The,:,field and the width are related to N D by E m « and W « ( v 7 ^ ) - (IVB-18) 19 -3 Rideout specified > 10 cm as the doping density required for f i e l d emission tunneling ([75R1], p. 545). Fig. IVB-4 i l l u s t r a t e s the I-V characteristics of the three modes of metal-semiconductor conduction. In conclusion, very heavily doped semiconductors produce a tunneling s i t u a - tion that gives symmetric I-V relationships and low contact resistance, i.e . an ohmic contact. b. Ohmic Contact Fabrication The ohmic contacts were formed by evaporation of gold german- ium n i c k e l and annealing at 450°C i n H2 gas for a short period of time. * The image force d i e l e c t r i c constant e,D % the s t t i c valu  e.g from Sz 's argument ([69S1], p. 367)}. 55. The films of GaAs were cleaned for 1 minute i n b o i l i n g chloroform then 5'minutes i n hot methyl alcohol, then blown dry. The clean pieces were placed i n contact with a mask i n a Veeco vacuum chamber that was pumped down to a maximum pressure of 10 ̂  t o r r . 100 mg. of Au Ge al l o y i n a 88% to 12% r a t i o with 8 mg. of 99.98% pure Ni added was evaporated through -2 2 the mask to form round dots roughly 1 x 10 cm i n area. A f t e r removal from the vacuum system, the sample was placed i n the furnace shown i n Fig. IVB-5. The furnace was designed to produce rapid thermal cycling required for good ohmic contacts ([73H1], p. 836). The fast annealing was made possible by the use of a s p e c i a l l y made magnetized t r o l l e y . The specimens were attached to the end of a long quartz rod which was supported at the nose of a truck. This truck BURNING j TRACK MAGNET THERMOCOUPLE WIRES F i g . IVB-5 Ohmic Contact Annealing Furnace 56. could be moved along a t r a c k l y i n g i n the quartz furnace tube but out- s i d e the furnace (see F i g . IVB-5). The specimen was run i n t o the hot s e c t i o n of the furnace and up to a thermocouple by p u l l i n g the truck down to the t r a c k w i t h a magnet. The thermocouple measured the r i s e i n temperature of the GaAs to 450°C, 20 seconds a f t e r which the truck was p u l l e d back and the GaAs allowed to c o o l . Good ohmic contacts w i t h contact r e s i s t a n c e r l e s s than c -2 2 3 x 10 ft cm have been reported by Robinson ([75R2], p. 335) using t h i s method. IV-C E l e c t r i c a l Measurements In the f i r s t s e c t i o n of t h i s chapter, the parameters R, R c u, on. n and <(> were defined and shown to be important i n determining s o l a r c e l l 15 performance. Information on a l l these parameters can be gained from measurement of the dark I-V r e l a t i o n s h i p of a Schottky b a r r i e r diode. Such measurements were performed on Schottky b a r r i e r diodes f a b r i c a t e d from the CSVT grown GaAs f i l m s and are described below. As a check on the b a r r i e r height determination and a l s o to ob t a i n i n f o r m a t i o n on the doping density of the GaAs f i l m s C-V measure- ments were performed on the completed Schottky b a r r i e r diodes. A l l the e l e c t r i c a l measurements were made on GaAs f i l m s grown at a su b s t r a t e temperature of 710°C. Diodes were f a b r i c a t e d on three such f i l m s and a range of behaviour was observed. Results are presented from 8 diodes which serve to i l l u s t r a t e the range of the data obtained. 1. Measurement Apparatus The c u r r e n t - v o l t a g e measurements were made i n a grounded metal chamber. The Mo s i d e of the diodes were pressed i n t o contact w i t h a gold p l a t e d d i s c using micrometer probes on the su b s t r a t e edges. The top contact was made v i a a gold wire probe. The t e s t f i x t u r e was connected to the measuring instrument by RG-58 c o a x i a l cables. Forward and reverse voltage b i a s was s u p p l i e d by an A m b i t r o l regulated ,»DC power supply model 5005R. Voltages were measured across the specimen w i t h a Fluke DCC d i f f e r e n t i a l voltmeter to ensure i n f i n i t e r e s i s t a n c e at the meter f o r low b i a s e s . A K e i t h l e y 417 high speed picoammeter and a Fluke d i g i - t a l multimeter model 8000A measured currents l e s s than and greater than 3 x 10 "* amps r e s p e c t i v e l y . The capacitance-voltage measurements were made i n the same t e s t f i x t u r e as f o r the J-V c h a r a c t e r i s t i c s . The contact probes were connected to the t e s t terminals o f a Boonton Capacitance/inductance meter model 71A by RG-58 c o a x i a l cables. The combined l e n g t h of the cables was l e s s than 2 f t . The c o r r e c t i o n f o r e r r o r i n capacitance measure- ment owing to s e r i e s inductance f o r t h i s l e n g t h of cables was l e s s than 0.1 p f . (0.1% of r e s u l t s ) . The model 71A instrument provided capacitance measurements by means of a 15 mv t e s t s i g n a l at 1 MHz. The DC b i a s was a p p l i e d to the specimen by an e x t e r n a l source and a Fluke d i f f e r e n t i a l voltmeter model 881AB measured the b i a s voltage to greater than 2 s i g n i - f i c a n t d i g i t accuracy. Measurements were performed i n the dark 30 minutes a f t e r any exposure to l i g h t . These precautions prevented l i g h t induced t r a p p i n g e f f e c t s . 2. Current-Voltage R e s u l t s a. <|>B, n and R g H The forward b i a s semilog J-V p l o t s of a l l the diodes t e s t e d e x h i b i t e d three c h a r a c t e r i s t i c regions arid F i g . IVC-1 curve c shows a t y p i - c a l example.-? The c e n t r a l region of the curve r e l a t e s to the expected 58. Vf (VOLTS) F i g . IVC-1 Series Resistance Model Check experimental behaviour and from t h i s p o r t i o n and i t s e x t r a p o l a t i o n to zero b i a s the parameters n and <j> were determined, n values i n the B range of 1.58-2.29 were encountered, w h i l s t <f>g values f e l l w i t h i n a range of 0.67 to 0.80. The r e s u l t s are t a b u l a t e d i n Table IVC-1. n and <j) d i d not vary a p p r e c i a b l y over the surface o f the B f i l m s except f o r diodes w i t h l a r g e values of s e r i e s r e s i s t a n c e . High s e r i e s r e s i s t a n c e tended to reduce the range of the l i n e a r region of the in J-V p l o t to the p o i n t where i t was d i f f i c u l t to make accurate s t r a i g h t l i n e approximations. See, f o r example, F i g . IVC-2. The low b i a s (< about 0.2-0.3v) p o r t i o n of the £n J-V curves i n d i c a t e d the presence of recombination or other shunting c u r r e n t s . As described i n s e c t i o n IV-A, these phenomena can be represented i n the equiv a l e n t c i r c u i t by a shunt r e s i s t a n c e R c r r. Values of R were com? o n o n puted f o r the various diodes by measuring the slope of the I-V curve a t values of reverse b i a s close to the o r i g i n . In the eq u i v a l e n t c i r c u i t of F i g . IVA-1, J * J s + A i r : o n f o r the reverse b i a s case where JAR « V. The above c o n d i t i o n s l e a d to o n L u = 4rl • Th e r e s u l t s are summarized i n Table IVC-1 and show t h a t S H d J v R k o Rgg values f o r the diodes were hig h and i n the range of 3.9 x 10 5 to 3.2 x 1 0 7 a. b - R S The d e v i a t i o n from l i n e a r i t y of the in J-V p l o t s at high b i a s values was accounted f o r by the presence of s e r i e s r e s i s t a n c e i n the diodes. In t h i s region of the curve, the curre n t - v o l t a g e r e l a t i o n can be expressed as 60. F i g . IVC-2 J-V C h a r a c t e r i s t i c s of Diodes on Samples B1/II w i t h D i f f e r e n t Rg- •* ' • Rg(2) = 120ft, R s(20) = 550ft, R g(21) = 35kft 61. J = J s exp - j j f (V - JAR S) (IVC-2) Thus JARg i s simply the v o l t a g e d i f f e r e n c e (AV) between the experimental curve and the s t r a i g h t l i n e approximation to the i d e a l diode equation ( J = Js exp (qV/nkT)) and therefore Rg = AV/JA. Where a p p r e c i - able dependence of t h i s c a l c u l a t e d s e r i e s r e s i s t a n c e on diode current occurred the h i g h e s t value (worst case) of Rg was recorded. The r e s u l t s are summarized i n Table IVC-1 from which i t can be seen that Rg values covering the wide range of 20.5 fi to 15.8 kfi were obtained. F i g . IVC-2 shows some t y p i c a l r e s u l t s . These values of Rg would be too high i n a s o l a r c e l l . As the gold b a r r i e r f i l m s used here were at l e a s t 2000 A t h i c k the c o n t r i b u t i o n to Rg from the b a r r i e r metal sheet r e s i s t a n c e can be neglected. The h i g h s e r i e s r e s i s t a n c e i s thus r e l a t e d t o the Mo contact and semiconduc- t o r f i l m r e s i s t a n c e s and i n an attempt to d i s c e r n the r e l a t i v e magnitude of these components the f o l l o w i n g t e s t s were made. In the f i r s t t e s t , GaAs chips were s c r i b e d and broken from s e v e r a l of the f i l m s and t h e i r substrates i n order to measure the f i l m r e s i s t i v e l y alone. In only one i n s t a n c e was t h i s method s u c c e s s f u l (part of sample A5) and to t h i s f i l m ohmic contacts were deposited on opposing faces. The r e s u l t i n g c urrent-voltage p l o t was l i n e a r over four decades i n voltage as shown i n F i g . IVC-3. The r e s i s t a n c e R i s the i n v e r s e of the slope and therefore the f i l m r e s i s t i v i t y i s given by p = — ^ — . 4 = 220 fificm (IVC-3) slope I -3 2 For a contact area of 7.8 x 10 cm (as used f o r the diodes reported i n Table I V - 3 ) , t h e " s e r i e s r e s i s t a n c e due to the GaAs f i l m would be 33 ohms. This i s close to the lowest reported value of Rg, suggesting that the higher values of Rg may have been due to h i g h r e s i s t a n c e between F i g . IVC-3 R e l a t i o n s h i p o f . P o l y c r y s t a l l i n e GaAs Chip w i t h Ohmic Contacts on Both Sides 63. the GaAs f i l m and Mo s u b s t r a t e . To i n v e s t i g a t e t h i s p o s s i b i l i t y , the f o l l o w i n g t e s t was c a r r i e d out. The sample used f o r t h i s t e s t was p a r t of sample B l / I I I which had p r e v i o u s l y been s c r i b e d and broken i n t o two pieces (the other of the pieces was used to o b t a i n some of the data reported i n Table IVC-1). Ohmic, i n s t e a d of r e c t i f y i n g , contacts were a p p l i e d to the free surface of the f i l m and the range of J-V c h a r a c t e r i s t i c s obtained i s represented by the curves shown i n F i g . IVC-4. None of the curves i s e i t h e r l i n e a r o r symmetrical about the o r i g i n . A v a r i e t y of degrees of r e c t i f i c a t i o n i s demonstrated and the more ex p o n e n t i a l nature of the c h a r a c t e r i s t i c s when the Mo e l e c t r o d e was p o s i t i v e i m p l i e s t h a t the Mo-GaAs i n t e r f a c e was r e c t i f y i n g . The e q u i - v a l e n t r e s i s t a n c e s of the contacts were c l e a r l y voltage dependent,, although over the range o f -0.1 to +0.1 v o l t s r e l a t i v e l y constant values o f s e r i e s r e s i s t a n c e could be i m p l i e d (see F i g . IVC-5). I t i s apparent that the Mo/GaAs i n t e r f a c e exerts a l a r g e and v a r i a b l e i n f l u e n c e on the t o t a l Au/GaAs/Mo diode c h a r a c t e r i s t i c . To f u r t h e r i l l u s t r a t e t h i s p o i n t , i t i s i n t e r e s t i n g to p l o t on one graph the I-V c h a r a c t e r i s t i c s from the three t e s t s reported above. This i s done i n F i g . IVC-1 where the diode c h a r a c t e r i s t i c s (c) came from contacts to sample B l / I I I and the ohmic contact c h a r a c t e r i s t i c s (b) came from sample B l / I I used to construct F i g . IVC-4 (both samples o r i g i n a t e d from the same f i l m ) . The d i f f e r e n c e between curves (a) which i s F i g . IVC-3 f o r dimensions eq u i v a l e n t to B l / I I I and Kjb) represents the c o n t r i b u t i o n of the GaAs/Mo i n t e r f a c e to the conduction c h a r a c t e r i s t i c . By s u b t r a c - t i n g t h i s d i f f e r e n c e from curve (c) the l i n e a r region of curve (c) i s extended to high bia s l e v e l s . This shows the e f f e c t of the back contact F i g . IVC-4 Current-Voltage C h a r a c t e r i s t i c of Single Ohmic Contact to GaAs Mo  66. i n causing the onset of a series resistance-dominated regime i n the over- a l l Schottky bar r i e r diode conduction characteristic. 3. Capacitance Voltage Results The zero-bias capacitance values for the eight diodessreferred to i n section IV-C covered the range 8 - 250 picofarads, see Table IVC-1, and this v a r i a t i o n i s undoubtedly related to the non-uniformity i n the back contact conditions described e a r l i e r . Because of the.ihigh effective series resistance of many of the diodes most of the measured C-V charac- t e r i s t i c s had to be corrected, after the fashion described below, for -2 the presence of Rg before interpretation of the C -V characteristics could be attempted. The correction to the capacitance readings due to series resistance i s i l l u s t r a t e d through Fig. IVC-7a which shows how the Boonton capacitance meter characterizes the specimen as a p a r a l l e l combination of conductance and capacitance. When the series resistance of the semiconductor bulk and Mo-GaAs contact i s taken into account the c i r c u i t of Fig. IVC-7b results with the p a r a l l e l branches representing the actual r e c t i f y i n g contact (Au/GaAs). The value Rg would be expected to vary only s l i g h t l y over the normal range of C-V biasing (see Fig. IVC-5 and note that the voltage drop due to the Mo contact was much less than the r e c t i f y i n g contact over this range). The steady state small signal equivalent capacitance of Fig . IVC-6 for angular frequency co i s given by equation IVC-4.. C = C/[(R SG + l ) 2 + co 2R s 2C 2]. (IVC-4) Goodman ([63G2], p. 330) introduced a useful approximation for the case where 0.005 < (coRsC)2 < 0.1 and RgG « 1 by inse r t i n g equation IVC-4 (with RqG = 0) into IVB-5 giving 67. J CZXEQXK3K G=l/V a. METER MEASURES THIS CIRCUIT b. POLYCRYSTALLINE DIODE CIRCUIT F i g . IVC-6 Equivalent and A c t u a l C i r c u i t f o r a P o l y c r y s t a l l i n e Metal-Semiconductor Diode - 2 . 6 l - 2 . 1 9 -1.7a -1.37 -0.95 -D.55 -C.:-, BIHS VOLTAGE C V X T S ) 0.J7 1.5 F i g . IVC-7a (A/C) vs V of Specimen B l / I I , Diode 2, Corrected f o r R and Approximated by a 5tfi Degree Chebychev Polynomial b) Diode 20, Corrected for Rs and Approximated c) Diode 21, Not Corrected for Rg and Approxi- by a 4th Degree Chebychev Polynomial mated by a 5th Degree Chebychev Polynomial Fig. IVC-7b & c (A/C) vs V of Specimen B l / l l ON 00 69, [ A / C ] 2 = 2 ( V b l - V - kT/q)/(qE sN D) + 2A 2co 2Rg 2 (IVC-5) The measured (A/C') 2 vs V curve i s t r a n s l a t e d upward from the Schottky 2 2 2 r e l a t i o n by a constant 2A co Rg . This c o r r e c t i o n was a p p l i e d to the re l e v a n t diodes and used to o b t a i n the r e s u l t s summarized i n Table IVC-1. To demonstrate the recorded range i n C-V data, r e s u l t s are shown i n F i g . IVC-7 f o r the three diodes that were used to i l l u s t r a t e the J-V r e s u l t s ( F i g . IVC-2). For diode B2/II p t . 21, the s e r i e s r e s i s t a n c e was so high that no c o r r e c t i o n could be attempted. I t i s c l e a r i n a l l cases that the diode doping d e n s i t y p r o f i l e was not uniform and that the doping density was i n c r e a s i n g away from the Au/GaAs i n t e r f a c e . To f a c i l i t a t e comparison of the various c h a r a c t e r i s t i c s , the doping d e n s i t y was c a l c u l a t e d from the C-V data using equation IVB-6 w i t h V s e t to zero. The r e s u l t i n g doping d e n s i t i e s are shown i n Table IVC-1. 2 Because of the n o n - l i n e a r i t y i n the .(A/C) -V p l o t s , the e x t r a - p o l a t i o n of the data to the a b s c i s s a (to f i n d <j>g using equation IVB-3 and 5) i s of very d o u b t f u l value. However, an attempt to do t h i s was made by f i t t i n g the data to Chebychev polynomials of degree 6 or l e s s . The c o e f f i c i e n t s were c a l c u l a t e d by numerical methods on an IBM 370 computer. Both the o r i g i n a l data and the Chebychev approximation were p l o t t e d on a Calcomp p l o t t e r and the c r i t e r i o n f o r choosing the polynomial degree was the c l o s e s t f i t w i t h the l e a s t e r r o r . The <(>JJ r e s u l t s so c a l c u l a t e d and shown i n Table IVC-1 were computed f o r zero b i a s c o n d i t i o n s t o allow comparison w i t h the values c a l c u l a t e d from the J-V method. To do t h i s the image force lowering Acj) was c a l c u l a t e d at V = 0 and V n was c a l c u l a t e d f o r the doping concentration that c o i n c i d e d w i t h V = 0 where, N C V_ = fcTUn TT (IVC-6) ND(V=0) Device RS ( n ) RSH ( n ) J-V C-V Ch eb yc he v Po ly - no mi al  D eg re e C Co rr ec te d fo r Rg ? n (eV) *B (eV) ND(V*0) (cm-3) C(V=0) (pf) Vi (volts) Ch eb yc he v Po ly - no mi al  D eg re e C Co rr ec te d fo r Rg ? B2/II pt. 4 20.5 7.9 x 106 1.58 .78 .77 4.0 x IO 1 6 .200 .72 5 N Bl/III pt. 2 87.9 6.2 x IO6 1.58 .74 .89 1.0 x IO 1 6 146 .80 5 Y B3/I pt. 2 2010. 3.9 x 105 2.02 .67 - 1.1 x 10 1 6 64 1.05 4 U Bl/I P t . 9 196. 1.0 x IO 6 2.29 .72 1.04 1.4 x IO 1 6 165 .96 6 Y B2/III pt. 17 22.9 5.7 x IO 6 . 1.50 .73 .77 5.8 x IO 1 6 250 .73 5 Y B2/II pt. 2 54.0 1.1 x IO 7 1.66 .77 .83 3.9 x IO 1 6 192 .78 5 Y B2/II pt. 20 248. 2.3 x IO 7. 1.64 .80 .97 3.9 x IO 1 5 52 .85 4 Y B2/II pt. 21 15,800. 3.2 x IO 7 1.85 .77 - 1.6 x IO 1 5 8 1.27 5 U *N - Not necessary: (coRgC)z << 1 Y - Yes: RgG « 1 and 0.005 < ( RgC)2 < 0.1 U - Unable to correct for Rg: RgG \ 1 Table IVC-1 Ele c t r i c a l Properties of Au Schottky Polycrystalline-GaAs Diodes 71. N p i s the e f f e c t i v e density of s t a t e s i n the conduction band and i s equal to 4.7 x 1 0 7 cm - 3 ([69S1], p. 57). IV-D D i s c u s s i o n Two of the major problems encountered i n making s o l a r c e l l s from p o l y c r y s t a l l i n e semiconductors i n v o l v e the formation of a s u i t a b l e e l e c t r o s t a t i c inhomogeneity f o r p h o t o c a r r i e r separation and production of low r e s i s t i v i t y semiconductor m a t e r i a l . The work presented i n t h i s chapter has shown that these problems need not n e c e s s a r i l y a r i s e i n Schottky b a r r i e r s formed by the d e p o s i t i o n of Au onto GaAs f i l m s grown by the CSVT method. The r e s i s t i v i t y of f i l m s grown at a substrate temperature - of 710°C on Mo f o i l s u b s trates has been measured to be 220 ohm-cm, which -2 can be compared w i t h the value of 10 fi cm appropriate to the GaAs source m a t e r i a l . From the C-V data presented, i t i s c l e a r t h a t the doping density p r o f i l e i n the f i l m s i s not uniform but that a value of around 16 3 1 x 10 doners per cm i s present i n the b e t t e r f i l m s . The e q u i v a l e n t s i n g l e c r y s t a l r e s i s t i v i t y f o r GaAs doped to t h i s extent would be 0.15 fi cm, i n d i c a t i n g a reduced conduction i n the p o l y c r y s t a l l i n e f i l m . This i s to be expected and a number of models have been proposed to e x p l a i n the phenomenon (see appendix AIV2)'., I f the f i l m conduction i s p r i m a r i l y c o n t r o l l e d by the g r a i n boundaries, which may be a p p r o p r i a t e l y modelled as a two s i d e d abrupt j u n c t i o n , then the incomplete b a r r i e r h e i g h t would be 0.3 eV. A d e t a i l e d i n v e s t i g a t i o n of the temperature and magnetic dependence of the c o n d u c t i v i t y would be necessary, however, t o y i e l d f u r t h e r i n s i g h t i n t o the mechanisms c o n t r o l l i n g conduction. The metal-semiconductor b a r r i e r height of the A u / p o l y c r y s t a l l i n e GaAs system was found to l i e i n the range of 0.67-0.80 eV. These values 72. are a l i t t l e lower than the value of 0.89 eV which i s g e n e r a l l y accepted to represent the b a r r i e r h e i g ht i n the Au/single c r y s t a l GaAs case. The lower value found here i s most l i k e l y due to c o n d i t i o n s at the surface of the GaAs f i l m where surface s t a t e d i s t r i b u t i o n s might d i f f e r from the s i n g l e c r y s t a l case. Another p o s s i b i l i t y , ( P u l f r e y [76P1]) i s the pre- sence of 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 between the metal and semi- conductor, although i n the present i n v e s t i g a t i o n care was taken to avoid i n t e r f a c i a l l a y e r formation by ensuring that the t r a n s f e r of the GaAS from the e t c h i n g s o l u t i o n to the evaporation chamber was r a p i d (% 20 sees). Under these c o n d i t i o n s , an oxide i n excess of 10 A would not be expected to grow (see Pruniaux and Adams [72P1]). The d i f f e r e n c e i n b a r r i e r height of .09 eV or so i s l i k e l y due to p o s i t i v e surface s t a t e d e n s i t i e s that might form on n-type GaAs f i l m s . Using a value f o r (jig of 0.8 eV and a f i l m r e s i s t i v i t y of 220 ohm cm, i t can be computed from the t a b l e i n appendix IVA-3 t h a t a 5 um t h i c k GaAs s o l a r c e l l w i t h n = 1.5 has a maximum AMI conversion e f f i c - i ency of 10%. Such a t h i c k n e s s of f i l m i s e a s i l y grown by the present method and comprises e s s e n t i a l l y columnar c r y s t a l s of g r a i n s i z e averag- i n g around 5 ym. Hovel ( F i g . 68 (75H1]), u s i n g a s p e c i f i c p o l y c r y s t a l l i n e semiconductor model, has c a l c u l a t e d t h a t an AMI e f f i c i e n c y of 80% of the s i n g l e c r y s t a l case i s p o s s i b l e under these circumstances. I f such an e f f i c i e n c y could be r e a l i z e d i n a p r a c t i c a l c e l l then t h i s would be &• s u b s t a n t i a l improvement over the best f i g u r e s so f a r reported f o r t h i n f i l m s o l a r c e l l s . The only work reported on s o l a r c e l l f a b r i c a t i o n using t h i n f i l m CSVT GaAs was by Vohl et a l . [67V1] who obtained AMI e f f i c i e n c i e s of 4.5 %, but encountered s t a b i l i t y and s e r i e s r e s i s t a n c e problems. The 73. l a t t e r i s a l s o very much i n evidence i n the c e l l s prepared i n t h i s work and has been found to be a s s o c i a t e d w i t h the contact between the GaAs f i l m and the Mo s u b s t r a t e on which i t was grown. This contact i s r e c t i - f y i n g to a degree t h a t apparently v a r i e s w i t h p o s i t i o n on the s u b s t r a t e . The reason f o r the non-uniformity i s not obvious although i t may be due to i m p u r i t i e s i n the Mo f o i l or to v a r i a t i o n s i n surface s t a t e and i n t e r - f a c i a l l a y e r p r o p e r t i e s . An ohmic contact i s d e s i r a b l e at t h i s i n t e r f a c e as backwall c e l l s are prone to u n i f o r m i t y problems (backwall c e l l s using d i r e c t band gap semiconductors need to be very t h i n , 1-2 ym). In any event, the Mo/GaAs i n t e r f a c e i s not s u f f i c i e n t l y r e c t i f y i n g f o r consider- a t i o n of t h i s mode of operation. Even the most r e c t i f y i n g contact (see F i g . IVC-4) only had an estimated b a r r i e r h e i g ht of .6 eV, which would only give an AMI conversion e f f i c i e n c y of 2% (from F i g . 1 [74P1]). To improve t h i s back contact and make i t more n e a r l y ohmic, the doping"density i n t h i s region of the semiconductor must be i n c r e a s e d . Vohl e t a l . [67V1] achieved t h i s by p r e c o a t i n g the Mo f o i l w i t h a l a y e r of tin-germanium p r i o r to GaAs f i l m formation. The subsequent f i l m growth presumably allowed s h a l l o w - l e v e l donor d i f f u s i o n i n t o the i n t e r - f a c i a l region of the semiconductor. The d i f f u s i o n process i s apparently very c r i t i c a l as was, discovered when an attempt was made i n the present i n v e s t i g a t i o n to repeat t h i s r e s u l t (using a 500 A tin-germanium r 1layer) , no r e c t i f y i n g a c t i o n was observed. Presumably, the t i n and/or germanium d i f f u s e d through the f i l m causing s h o r t i n g paths. P o s s i b l y , movement of these metals can proceed along g r a i n boundaries by e l e c t r o m i g r a t i o n which may account f o r the s t a b i l i t y problems noted by Vohl e t a l . In the present work, the back contact r e s i s t a n c e has been shown to account f o r the d e v i a t i o n from l i n e a r i t y at high b i a s of the dark £nJ-V 74/ c h a r a c t e r i s t i c s . The c o r r e c t e d c h a r a c t e r i s t i c s show l i n e a r i t y over n e a r l y f i v e decades of c u r r e n t , w i t h a slope that gives a diode i d e a l i t y f a c t o r n t y p i c a l l y i n the range of 1.5 to 2.0. These values are too h i g h to be accounted f o r s o l e l y by the voltage-dependent parameters i n the thermionic emission model, see equation IVB-11, but are t y p i c a l of the values r e - ported i n the l i t e r a t u r e f o r Schottky b a r r i e r s o l a r c e l l s , n values of t h i s magnitude can be a s s o c i a t e d w i t h the presence of an i n t e r f a c i a l oxide (see Card and Roderick [71C1]) o r w i t h the dominance of a recombina- t i o n - g e n e r a t i o n component of current. In the recent s o l a r c e l l work by Anderson and Milano [75A1], i t has been suggested t h a t h i g h n values would be advantageous i n Schottky b a r r i e r s o l a r c e l l s on account of t h e i r apparent a f f e c t on the open c i r c u i t v o l t a g e , e.g. from equation IIA-2 of appendix IVA-2 at 1^ = 0, we have V = n — £n ( I p h T + I S ) (IVD-1) oc q XS However, i t must be r e a l i z e d t h a t f o r n-values greater than about 1.1, the dark current can probably no longer be adequately expressed by the usual expression (see equation IVA-1) and thus the Ig term i n equation IVD-1 needs m o d i f i c a t i o n , almost c e r t a i n l y to a higher value. This e f f e c t w i l l cause a reduction i n V q c that may w e l l o f f s e t the gain that might otherwise be expected from the h i g h n value. I t i s thus unreason- able to t r e a t n as an independent parameter and even i n d e l i b e r a t e l y - formed MIS s o l a r c e l l s , high n values cannot be automatically, used i n an equation such as IVD-1, as f o r example, Anderson and Milano [75A1] have suggested, because the i n t e r f a c i a l l a y e r a f f e c t s the m i n o r i t y c a r r i e r flow and the m a j o r i t y c a r r i e r flow i n d i f f e r e n t ways and i t i s t h i s e f f e c t that c o n t r o l s V (see Card and Yang [76C1]). n values greater than u n i t y can als o be as s o c i a t e d w i t h recom- b i n a t i o n - g e n e r a t i o n current flow and there i s evidence f o r the presence of such currents i n the present work. Such currents can be expected to occur on account of deep l e v e l traps formed at d i s l o c a t i o n s and i m p u r i t i e s i n the p o l y c r y s t a l l i n e f i l m . The usual expression f o r recombination current i s where J = J exp [eV/2kT] (IVD-2) rec r J = qn.W/x r n 1 and 4 kT/q < V < V,_. - 10 kT/q (from Roderick [70R2], p. 1163). n^ i s the i n t r i n s i c c a r r i e r concentra- t i o n = ZN^N^ exp(-Eg/2kT) and T i s the e f f e c t i v e l i f e t i m e . For s e v e r a l trapping centers at d i f f e r e n t energy p o s i t i o n s , and p o s s i b l y i n t e r a c t i n g , J could be the sum of the i n d i v i d u a l recombination currents and thus rec the t o t a l current could be of the form J = J C T exp (qV/kT) + J exp (qV/2kT) (IVD-3) sum f o r moderate b i a s . When both currents are comparable n has a value between 1 and 2. Such behaviour has r e c e n t l y been reported i n MIS s o l a r c e l l s on p-type s i l i c o n (see P u l f r e y [76P2]). In the present work, the recombination-generation currents can be represented i n the equ i v a l e n t c i r c u i t (see F i g . IVA-1) by a p a r a l l e l r e s i s t a n c e R c u. The values reported i n t h i s work (see Table IVC-1) are, o n however, acceptably high. I f such values could be maintained i n a l a r g e 2 4 area s o l a r c e l l , say 5 cm , R C I J magnitudes of about 10 ohms would r e s u l t o n and these would not cause any s i g n i f i c a n t r e d u c t i o n i n s o l a r conversion e f f i c i e n c y (Hovel [75H1], F i g . 52). Shunt r e s i s t a n c e i n p o l y c r y s t a l l i n e s o l a r c e l l s can a l s o be caused by l a r g e i m p u r i t y concentrations d i f f u s e d along d i s l o c a t i o n s or g r a i n boundaries, or by the presence of f i n e metal- l i c bridges through microcracks i n the f i l m . Although microcracks were observed i n some f i l m s using the SEM (see F i g s . IIIB-3) the high recorded values of Rgjj i n d i c a t e t h a t these imperfections would not adversely a f f e c t s o l a r c e l l performance. 77. V CONCLUSION GaAs f i l m s grown by the close-spaced vapour t r a n s p o r t method d i r e c t l y onto Mo substrates have been found to be p o l y c r y s t a l l i n e w i t h no p r e f e r r e d o r i e n t a t i o n . The c r y s t a l l i t e area increased w i t h the temp- erature at which the substrate was h e l d during growth and at 710°C g r a i n 2 s i z e s of 100 ym were observed. The c r y s t a l l i t e s formed a columnar-like s t r u c t u r e w i t h c r y s t a l s i z e comparable to the f i l m t h i c k n e s s . This p a t t e r n i s advantageous f o r t h i n f i l m s o l a r c e l l s i n that c a r r i e r s need to cross few g r a i n boundaries and high conversion e f f i c i e n c i e s are p o s s i b l e . No i m p u r i t i e s or f o r e i g n i n t r u s i o n s e x i s t e d i n the f i l m s i n q u a n t i t i e s observable on the e l e c t r o n microprobe. The r e s i s t i v i t y of the GaAs f i l m s was greater than would be 16 expected f o r s i n g l e c r y s t a l GaAs of the same doping d e n s i t y ft 1 x 10 -3 atoms cm ) , but s t i l l low enough f o r c o n s i d e r a t i o n f o r use i n t h i n f i l m s o l a r c e l l s . However, the GaAs-Mo contact was a m i l d l y r e c t i f y i n g con- t a c t , v a r y i n g i n the degree of r e c t i f i c a t i o n across the sample but never s u f f i c i e n t f o r use as a backwall c e l l . F r o n t w a l l c e l l o peration would a l s o be adversely a f f e c t e d by t h i s arrangement and the problem was thought to be a s s o c i a t e d w i t h surface s t a t e s , i n t e r f a c i a l l a y e r s or low doping d e n s i t i e s i n the r e s u l t i n g GaAs f i l m s . A t h i n l a y e r of Ge or Sn precoated on the Mo substrates would probably enhance the doping d e n s i t y of the f i l m and experimentation w i t h these and other dopants i s suggested to t r y and r e v e a l a combination that y i e l d s good ohmic contacts but does not a f f e c t the s t a b i l i t y of the diode. Diode j u n c t i o n formation was achieved by the d e p o s i t i o n of gold onto the GaAs f i l m s and the r e s u l t i n g barrierssshowed acceptable values of b a r r i e r height and diode i d e a l i t y f a c t o r . The l a t t e r values and the 78. low b i a s p o r t i o n of the dark J-V c h a r a c t e r i s t i c s suggested the presence of recombination-generation c u r r e n t s , but the a s s o c i a t e d value of shunt r e s i s t a n c e was s u f f i c i e n t l y high not to present a problem i n subsequent s o l a r c e l l f a b r i c a t i o n . 79. 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E l E l e c t r o n i c s . " S o l a r C e l l O f f e r s 21% E f f i c i e n c y w i t h GaAs", V o l . 47, No. 11. 82. 1975 (Continued) HI Hovel, H.J., " S o l a r C e l l s " , .Semiconductors and Semimetals,edited by Beer and W i l l a r d s o n , Academic P r e s s , V o l . 11. R l Rideout, V.L., "A Review of the Theory and Technology f o r Ohmic Contacts to Group I I I - I V Compound Semiconductors, S o l i d State E l e c t r o n i c s , V o l . 18, pp. 541-550. R2 Robinson, G.Y., " M e t a l l u r g 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 of A l l o y e d Ni/Au-Ge Films on n-Type GaAs", S o l i d State E l e c t r o n i c s , V o l . 18, pp. 331-342. S l S t i r n , R.J. and Yeh, Y.C.M. , "A 15% E f f i c i e n t A n t i r e f l e c t i o n - Coated Metal-Oxide-Semiconductor S o l a r C e l l " , A p p l i e d Physics L e t t e r s , V o l . 27, No. 2, pp. 95-9.7. 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PI P u l f r e y , D.L., " B a r r i e r Height Enhancement i n p - S i l i c o n MIS S o l a r C e l l s " , IEEE Transactions on E l e c t r o n Devices, June, pp. 587-589. P2 P u l f r e y , D.L., "Surface I n s u l a t i o n Solar C e l l s Using p-Type S i l i c o n " , to be published. 83. APPENDICES A l l . REVIEW OF THE ENERGY SITUATION The future for photovoltaic solar cells is shining brighter as the energy crisis darkens. The deeper one looks into energy resources the better solar cells appear for many reasons. They use a non depletable fuel (the sun's radiation), do not emit, "poTlutants or produce dangerous wastes, can use less surface area than hydroelectric reservoirs for pro- ducing equivalent power, are not biaseddtowards large scale conversion, are mass production oriented and require low maintenance. So why are they s t i l l years in the future? I t is the cost, which must be reduced by 100 to 1000 times. This factor is not so large as i t looks consider- ing that mass production techniques have not been implemented arid research and development has, in the past, been concerned with space not terrestrial uses. I f an intense program is initiated for photovoltaic power systems, such as for the transistor in the 50 ' s , the necessary breakthroughs could result in an economic ce l l . But one might.ask is such a large scale effort necessary? Are there not other easier methods? For present trends of energy consumption o i l w i l l be depleted in less than half a century. A f t e r o i l , coal is estimated to last two centuries but with increased cost per watt and sulfer polutants ([74M1], p .5). Even a shorter lifetime lies in store for present day nuclear reactors using uranium 238. New sources of energy must be found to replace the exhausted ones. I f the drain of fuels to extinction is to be avoided, energy for the future must come from the following non depletable sources 1. Hydro 84. 2. Wind 3. Tidal 4. Geothermal 5. Deuterium deuterium (fusion) 6. Directesolar radiation Hydro power is the only substantial method presently in use. But i t has been exploited considerably, for example in Western Europe 57% of the potential hydro is already harnessed. 8.5% of a l l the world&s potential Hydro power was developed by 196 7. Even i f a l l sites are even- tually utilized, s t i l l 2/3 of today's industrialized society's demand.must be furnished by other means [74M1], p. 14). Substantial energy sources must be found to meet the ever increasing consumption (double every 7 years). Wind, t i d a l and geothermal power sources are intermittant or limited to certain geographical locations. They therefore require storage and transmission f a c i l i t i e s which reduce efficiencies and increase expenses. The worst problem with tid a l and geothermal sites are their relative .scarcity.- To date t i d a l power has only been uti l i z e d at the Ranee Estuary in France where the plant has a 240 MW capacity. Hubbert estimates that the global potential t i d a l power is less than 2% of water power ([71H1], p. 67). Geothermal energy has been more widely developed around the world with a total of over 1000 MW produced in Italy, Japan, New Zealand and U..S.S.R. But this method is just potentially comparable to t i d a l power and can furnish only a small fraction of our energy needs in the future [71H1, p. 67). On the other hand, energy available in the winds is more abundant (see Table A I l - 1 ) . It has been noted that periods of high winds and intense solar radiation occur at different times during the 85. Energy Source kWh References H y d r o e l e c t r i c 2.0 x 1 0 1 3 [74R1] p. 13 Winds over land 5.7 x 1 0 1 5 [74R1] p. 13 Tides and t i d a l currents 1.1 x 1 0 1 1 [71H1] p. 67 Geothermal 8.9 x 1 0 1 2 * [74R1] p. 13 Deuterium deuterium e s s e n t i a l l y i n f i n i t e S o l a r R a d i a t i o n i n c i d e n t 2.0 x 1 0 1 4 [74R1] p. 13 on la n d P r o j e c t e d energy use i n year 2000 A.D. 2.0 x 1 0 1 4 [74R1] p. 13 Table A l l — 1 World Estimated Annual P o t e n t i a l Energy *USA alone. day and year. Therefore R. Ramakumar e t a l . [74R1] suggest combined wind generation and s o l a r c e l l i n s t a l l a t i o n s which would increase the energy supply w h i l e reducing the s i z e of the storage system. In the n u c l e a r power f i e l d the f a s t breader r e a c t o r s could be l a b e l l e d as consumers of non depletable f u e l . However, t h e i r ever i n - cre a s i n g cost due to ser i o u s c o o l i n g problems and danger from using h i g h l y enriched weapons grade f u e l render them l e s s promising than o r i g i n a l l y expected. Fusion re a c t o r s are f u r t h e r behind i n the research stage and cannot be regarded as an energy s o l u t i o n w i t h c e r t a i n t y . No one person, now, can p r e d i c t that a l l the fundamental s c i e n t i f i c , t e c h n i c a l and engineering breakthroughs, r e q u i r e d to produce net energy, w i l l be found. So, f o r the moment, other methods must be i n v e s t i g a t e d i n case f u s i o n power i s u n a t t a i n a b l e . Besides, no one source of power has s u p p l i e d a l l our 86. energy needs i n the past and i t would be foolhardy t o suppose an e x c l u s - i v e method could i n the f u t u r e . The f i n a l non depletable energy source i s s o l a r r a d i a t i o n . I t can be tapped without danger or d i s r u p t i o n to the e c o l o g i c a l system. Next to f u s i o n f u e l s i t i s the most abundant source of energy known. The sun r a d i a t e s 22800 x l O ^ 2 watts i n c i d e n t on land which i s many orders of magnitude gr e a t e r than present human consumption of 8 x lO^- 2 watts. U n l i k e f u s i o n i t can be and i s already being harnessed. The two prime methods of converting the sun's power to u s e f u l energy are thermal and p h o t o v o l t a i c . Both are expensive, but p h o t o v o l t a i c i s the most d i r e c t method f o r conver- s i o n to e l e c t r i c i t y . Since e l e c t r i c i t y i s the most adaptable form of energy e x i s t i n g today, research should be d i r e c t e d to produce cheaper p h o t o v o l t a i c converters. AI2. SUMMARY OF ENERGY CONVERSION VERSUS SEMICONDUCTOR THICKNESS CALCU- of p h o t o v o l t a i c energy conversion of AMI i r r a d i a n c e i n GaAs and S i c e l l s . The program used Simpson's Rule to evaluate the i n t e g r a l of equation AI2-1 below, f o r semiconductor thickness from 0.1 ym to 100 ym LATION The curves i n Fig.IA-1 were p l o t t e d from computer c a l c u l a t i o n s (AI2-1) where Wavelength of r a d i a t i o n i n m' ,-1 6M I r r a d i a n c e i n Wm~z m' a(X) Absorption c o e f f i c i e n t i n cm' -1 x Thickness of semiconductor i n cm 87. Shult z ' s AMI i r r a d i a n c e data ([7211], p. 47) was used f o r i n c i d e n t energy i n a form converted to 100, .02 ym s p e c t r a l bands by a cubic s p l i n e i n t e r - p o l a t i o n w i t h t e n s i o n (see McOuat [76M1]). The absorption data f o r GaAs came from Seraphin and Bennett ([67S1], pp. 518-519) and the energy was summed from \ ± = 0.3 to Xf = .904 ym. The S i absorption i n f o r m a t i o n was taken from Sze ([69S1], p. 54) f o r wavelengths between X^ = .3 ym and X^ = 1.13 ym. AIV1. DETERMINATION OF MAXIMUM EFFICIENCY OF SOLAR CELLS AS A FUNCTION OF SERIES RESISTANCE So l a r c e l l e f f i c i e n c i e s are l i m i t e d by t h e i r s e r i e s r e s i s t a n c e , e s p e c i a l l y f o r the case of hi g h r e s i s t a n c e m a t e r i a l s such as p o l y c r y s t a l - l i n e GaAs. To give an i d e a of the extent of t h i s e f f e c t , a r e l a t i o n between e f f i c i e n c y and s e r i e s r e s i s t a n c e has been derived. The s o l a r c e l l has been modelled by a simple photo diode, shown i n F i g . IVA-1. Both shunt r e s i s t a n c e and recombination processes are considered n e g l i g i b l e compared to high s e r i e s r e s i s t a n c e and therefore these e f f e c t s were omitted i n the c a l c u l a t i o n s . From F i g . IVA-1 I L = l p h - I s { e x p [ e ( V L .+ R g I L ) / n k T ] - 1} (AIV1-1) where f o r thermionic emission over the metal/semiconductor b a r r i e r Ig. = A(A* T 2 exp(-e<()B/kT)) . For (V L+R gI L)» .03volts (AIV1-2) (AIV1-3) = 0 (AIV1-4) 88. S u b s t i t u t i n g AIV1-2 i n AIV1-3 and performing"the operation of equation AIV1-4 y i e l d s , I - I T + I L n ^ [ n ( - ^ ^ 2.) - T T L , - ] - 2 I L R = 0 (AIV1-5) S ph ~ \ S The s o l u t i o n to AIV1-5 can be obtained by i t e r a t i o n . Let the value of the current at maximum power be given by 1^, then the maximum power PM i s n kT PM " XM 2 [ I - I + I + RS 3 (AIV1-6) Using AMI value of 0.112 W/cm2 ([7211], p. 47) as i n c i d e n t s u n l i g h t power, e f f i c i e n c y i s 2 kT v • oni2 W J U + T + R s ] ( A I V 1 - 7 ) ph M S The generated photoccurrent I ̂  can be approximated by assuming a c o l l e c t i o n e f f i c i e n c y of 100% f o r a l l photons w i t h energy greater than the band gap (Eg) of GaAs. In r e a l i t y the absorption by the f r o n t con- t a c t and transmission through the thickness of the semiconductor would reduce the c o l l e c t i o n e f f i c i e n c y to between 70 and 90%. However, the r e s u l t a n t lowering i n e f f i c i e n c y i s s m a l l compared to t h a t due to s e r i e s r e s i s t a n c e , and therefore 100% c o l l e c t i o n e f f i c i e n c y was used f o r s i m p l i c i t y . For <£(X) r e p r e s e n t i n g AMI s o l a r energy r a d i a t i o n according to Schulze (see t a b u l a t i o n i n CIE p u b l i c a t i o n No. 20 (TC 2.2) [7211], p. 47), then &(\) = hv V h ( X ) (AIV1-8) where ^ p h ^ = s P e c t r a l d e n s i t y of photons v = frequency of photons Therefore the number of photons w i t h energy greater than Eg i s n 0.3Vm ch where and c = speed of l i g h t he E< T . 0.3um For GaAs E g = 1.43 eV IgC = 33.2 ma/cm2 Various values of Rg, n, <|>g were i n s e r t e d i n t o equation AIV1-7 and i t e r a t e d using an IBM 370/168 computer y i e l d i n g the r e s u l t s of Table AIV1. Richardson's constant and the temperature were as f o l l o w s A* = 8 amps ° K - 2 T _ 2 T = 300 °K (fi) R g .8 .8 .8 .9 .9 .9 J*a ( 6 V ) 1 2 3 1 2 3 n .1 6.6 13.2 19.8 9.3 18.7 28.0 .5 6.2 12.9 19.5 8.9 18.3 27.7 1.0 5.8 12.5 19.1 8.5 17.9 27.3 5.0 3.3 9.5 16.0 5.5 14.6 23.9 10. 1.9 6.7 12.6 3.3 11.1 20.0 50. .4 1.6 3.6 .7 2.9 6.3 100. .2 .8 1.8 .4 1.5 3.2 500. .0 .2 .4 .1 .3 .7 Table AIV1 E f f i c i e n c y n(%) vs Ser i e s Resistance R q (fi)  91. AIV2 THEORIES OF CONDUCTION IN POLYCRYSTALLINE FILMS The resistance of p o l y c r y s t a l l i n e semiconductors i s usually found to be greater than for that of the single c r y s t a l case [72J1], No completely satisfactory theories have evolved to explain t h i s , however, i t i s generally accepted that grain boundaries consist of large quantities of imperfections, and these imperfections somehow reduce conduction. Volger [50V1] proposed a mosaic model for inhomogeneous conduc- tors consisting of high conductance c r y s t a l l i t e s separated by thin layers (grain boundaries) of lower conductivity (see Fig. AIV2-la). The macro- scopic r e s i s t i v i t y p becomes ZG P *'P B + ( j - ) p.c (AIV2-1) B for the non t r i v i a l case of p^ >> p ,̂. P e t r i t z [56P1] working with photo- conductive PbS, has suggested that the high resistance region could arise from potential barriers of the type shown i n Fig. AIV2-lb. In t h i s model the f i l m was composed of c r y s t a l l i t e s of n type PbS whilst the t h i n i n t e r - c r y s t a l barriers were p type oxides of lead. The conductivity was assumed to be l i m i t e d by the potential b a r r i e r formed by the n-p-n junctions. From the current voltage relationship of a b a r r i e r l i k e the one shown i n Fig. AIV2-lb, the macroscopic conductivity a was expressed i n terms of the single c r y s t a l conductivity a = e kT a (AIV2-2) where <f i s the potential height of the barriers above the conduction band edge i n the bulk of the c r y s t a l l i t e s . Slater [56S1], assuming, a complete b a r r i e r model (Fig. AIV2-2b), showed that i n the case of PbS f i l m s , a b a r r i e r height <}> of 0.37 eV would be necessary to f i t the experimental data. 92. - - - CRYSTAL BOUNDARIES &= RESISTIVITY OF BOUNDARY <£s RESISTIVITY OF CRYSTAL Is = WIDTH OF BOUNDARY lc= LENGTH CF CRYSTAL CONDUCTION BAND VALENCE BAND F i g . AIV2-1 The Mosaic Model and I t s Band Diagram a. INCOMPLETE BARRIER p n p n p F i g . AIV2-2 B a r r i e r Types 93 . Incidently, p o l y c r y s t a l l i n e GaAs films have been found to exhibit n-type character [63R1, 63G1, and 63N1]. This could be due to large n type crystals that contribute the bulk of the carriers separated by only thin p-type i n t e r c r y s t a l regions. Note that the forbidden energy band for GaAs i s 1.43 eV, several times larger than that for PbS (of .4eeV). I f complete barriers are formed, as i n P e t r i t z ' s mosaic model i l l u s t r a t e d i n Fig. IVA-2b, <|> would be approximately 1.4 eV and a would be orders of magnitude lower than the observed results for p o l y c r y s t a l l i n e GaAs. But o o note that &c > 10,000 A and Jig < 200-A (beam diameter) as was seen on the SEM and complete barriers cannot form i n such a narrow i n t e r c r y s t a l space. Therefore incomplete barriers of the type shown i n Fig. AIV2-2a are expected for the case of p o l y c r y s t a l l i n e GaAs. I f the grain boundary to c r y s t a l region i s modelled by a two sided abrupt junction, the shape and size of the barriers would be governed as i n Sze's ([69S1], p. 89) equations 10 and 15 from which the barri e r height follows d i r e c t l y , * = Vf+ N D (AIV2-3) 2 e (-̂  —) V N AN D ; From the r e s i s t i v i t y measurements of chapter IVG, i t was found that <)>, by equation AIV2-2, would be approximately 0.3 eV. I f i t i s assumed that 1 8 — 3 ° N^ = N^ = 10 cm then W = 270 A (width of the p-type region). Keeping to this model, for any narrower p-type regions, N^ would have to be greater than N n and q W2N <j> = 2 e for N A >> N D (AIV2-4) s Grain boundaries are generally believed to be of the order of 10 A to 100 A thick (Tucker [66T1], p. 15). This suggests large v a r i a - tions i n impurity concentrations between boundary and c r y s t a l regions. Perhaps a more r e a l i s t i c model f o r t h i n g r a i n boundaries would o i n v o l v e a very narrow, say 10 A, region of s t r u c t u r a l i m p e r f e c t i o n s which might i n c l u d e dangling bonds, d i s l o c a t i o n s and i m p u r i t y atoms. An under- standing of the e f f e c t of such a g r a i n boundary can be achieved by c o n s i - d e r i n g the Kronig-Penney r e p r e s e n t a t i o n of a p e r i o d i c l a t t i c e . Consider two a d j o i n i n g c r y s t a l s , each of which c o n s i s t s of a p e r i o d i c square w e l l p o t e n t i a l , w h i l e the contact area between them c o n s i s t s of atoms that at l e a s t have p e r i o d i c i t i e s other than that of the c r y s t a l l i t e s . Allowed energy l e v e l s i n the forbidden band would then be produced at the j o i n t ( i . e . g r a i n boundary). A s i m i l a r model was used by Tamm and Shockley ([65M1], pp. 165-182) to account f o r surface s t a t e s on semiconductor sur f a c e s . However, i n t h i s work these allowed energy l e v e l s w i l l be c a l l e d g r a i n boundary s t a t e s . Because the exact nature of the boundary s t a t e s i s not known, t h e i r d e n s i t y d i s t r i b u t i o n w i t h respect to energy i s impossible to p r e d i c t . However, l i m i t i n g cases f o r the d i s t r i b u t i o n of b a r r i e r s t a t e s can be p o s t u l a t e d and so i n d i c a t e the range of p o s s i b l e i n t e r a c t i o n between the boundary regions and adjacent space-charge regions. At one l i m i t , the dens i t y of s t a t e s would be uniformly d i s t r i b u t e d over a l l energy l e v e l s w i t h i n the band gap. At the other l i m i t , narrow bands or d i s c r e t e energy l e v e l s would e x i s t w i t h i n the band gap. For an n-type semiconductor, a c c e p t o r - l i k e boundary s t a t e s ( n e g a t i v e l y charged when occupied by e l e c - trons and n e u t r a l when empty) would form a d e p l e t i o n region at the edge of the c r y s t a l s and donor l i k e boundary s t a t e s ( p o s i t i v e l y charged when empty and n e u t r a l when occupied by e l e c t r o n s ) would form an accumulation l a y e r . A net d e n s i t y of occupied acceptor s t a t e s would form a b a r r i e r p o t e n t i a l t h a t would reduce conduction. S i m i l a r l y f o r p-type semiconductors donor s t a t e s i n the g r a i n boundaries would reduce conduction. Let us now consider i n d e t a i l the charges and p o t e n t i a l s i n the g r a i n boundary region that might r e s u l t from the presence of acceptor- l i k e s t a t e s i n the g r a i n boundaries. We s h a l l assume an n-type semicon- ductor i n e q u i l i b r i u m w i t h no e x t e r n a l l y a p p l i e d b i a s . The s i t u a t i o n at the g r a i n boundaries i s represented i n F i g . AIV2-3,for both boundary s t a t e cases. For the s i n g l e d i s c r e t e energy s t a t e s case, where i s the num- -1 -2 ber of acceptor type boundary s t a t e s per u n i t area ( i n eV cm ) , the gr a i n boundary charge i s EC(Q) q N { «(E - ^ - E v ( 0 ) ) Q b = " i v ( G ) 1 + e x P ( E - E F ) / k T d E ') - - q J j f o r <j>6 + E y ( 0 ) < E p * 0 «>6 + E y ( 0 ) > Ep (AIV2-5) For the uniform d e n s i t y of s t a t e s case, where N i s the net J u number of acceptor and donor surface s t a t e s per u n i t area, per u n i t energy -1 -2 ( i n eV cm ) , and <j>o s p e c i f i e s the energy l e v e l ( i n eV) below which a l l surface s t a t e s are f i l l e d f o r charge n e u t r a l i t y at the surface (before e q u i l i b r i u m ) , the gr a i n boundary charge i s r E c ( 0 ) - q N u Q B i o + E v ( 0 ) 1 + e X * < * - E F ) / k T ^ - - q N u ( E g - <f,B - V n - (f,o) * (AIV2-6) For charge n e u t r a l i t y to e x i s t at the g r a i n boundaries the gra i n boundary charge must be balanced by a p o s i t i v e space charge at * 1 The approximation that — . „ = 1 f o r E < E-r. and 0 f o r v 1 + exp (E - E j,) /kT F E > E F was used si n c e kT i s small at T = 300°K. GRAIN BOUNDARY CRYSTALLITE \ SINGLE DISCRETE STATES AT ENERGY 0S UNIFORM DENSITY OF S TA TES 0 POSITIVE SPACE CHARGE REGION F i g . AIV2-3 Energy Diagram of a n-Type C r y s t a l l i t e at a Grain Boundary i n the Presence Boundary States 97. the c r y s t a l l i t e s u r f a ce. Assuming complete i o n i z a t i o n i n the c r y s t a l l i t e space charge r e g i o n of width W, then Q B = - qNDW (AIV2-7) The one dimensional Poisson's equation y i e l d s an i n t e r c r y s t a l b a r r i e r h e i g h t (see p. 88 Sze [69S1]) K = q N D (W) 2 Therefore, * B + V n = 2 ^ 7 ~ TA 57 ( A I V 2" 8 ) Do U where N i s the e f f e c t i v e d e n s i t y of s t a t e s i n the conduction band and u 17 -3 i s equal to 4.7 x 10 cm . For the d i s c r e t e case, <j>B f o l l o w s d i r e c t l y from equations 5 a i d 8 f o r a given N^, tf^ and N^. For the uniform case, equations 6 and 8 can be s o l v e d g r a p h i c a l l y f o r a given N u, <|> and N^. I t can be seen t h a t f o r a g r a i n b a r r i e r h e i g h t of about .3 eV (as was found by r e s i s t i v i t y measurements and by equation AIV2-2) the d e n s i t y of boundary s t a t e s f o r e i t h e r d i s c r e t e or uniform d i s t r i b u t i o n 11 13 -1 -2 must be about 10 tto 10 eV cm . The conclusion to be drawn here i s that the e x i s t i n g c o n d u c t i v i t y i n p o l y c r y s t a l l i n e GaAs could be explained i n the f o r g o i n g model i f boundary-state d e n s i t i e s of 10,H to 10 eV cm" o were present. This number would seem reasonable on account of the magni- 14 2 tude of the surface atom;density of GaAs (6 x 10 atoms/cm ).

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