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Studies on sublimed GaAs films, anodic A12O3 films and A12O3/GaAs interfaces Yan, George 1970

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STUDIES ON SUBLIMED GaAs FILMS, ANODIC A l 0 FILMS AND A l 2 0 3 / G a A s INTERFACES by GEORGE YAN B.A.Sc., U n i v e r s i t y o f B r i t i s h Columbia, 1965 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS OF THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department o f E l e c t r i c a l E n g i n e e r i n g We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o the r e q u i r e d s t a n d a r d R e s e a r c h S u p e r v i s o r . Members of the Committee. A c t i n g Head o f the Department Members o f the Department o f E l e c t r i c a l E n g i n e e r i n g THE UNIVERSITY OF BRITISH COLUMBIA June, 1970 I n 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 advanced degree 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 and s t u d y . I f u r t h e r agree 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 Head o f my Depar tment 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 . Depar tment o f 2_ The U n i v e r s i t y o f B r i t i s h Co lumbia Vancouver 8, Canada ABSTRACT The structural and electrical properties of sublimed GaAs films, the dielectric properties of anodic Al^O^ films and the electrical properties of A^O^/GaAs interfaces are of interest from the viewpoint of using GaAs and AJ^O^ films in thin-film integrated circuits. A new method, the close-spaced sublimation (CSS) method, was developed and used to deposit GaAs films on sapphire. The effects of growth conditions on the structural properties of the films were investigated using optical and electron microscopy, an electron microprobe and X-ray diffraction techniques. Crystallites increased in size with increasing substrate temperature, from about 0.7u to 20u for substrate temperatures from 480 to 670°C. The degree of preferred orientation of crystallites in the films increased with increasing substrate temperature. The films exhibited <111> textures when substrate temperatures were above about 600°C. Single-crystal diffraction patterns were obtained from films deposited on substrates held at 630 to 640°C. Electron microprobe analysis indicated that the ratio of Ga to As in the films was stoichiometric to less than 2 wt %. The as-grown heteroepitaxial films were p-type with room-temperature 2 , hole Hall mobility up to 42 cm /V-sec. The room-temperature resistivity ranged from 0.6ft -cm to 1.6 x 10^fi-cm. The resistivity of higher resistivity films was more temperature dependent than that of lower resistivity films. The electrical properties of the films are discussed in terms of the effects of space charge regions in the grains, potential barrier at the grain boundaries, deviation from stoichiometry, and compensation of impurities. Conductivity-type conversion of the as-grown films to n-type was done by postdeposition dopant diffusion. A 2 room-temperature electron Hall mobility of 77 cm /V-sec was obtained. While transistor action and rectification characteristics were observed in thin-film insulated-gate field-effect transistors and Au-Schottky barrier i diodes made with GaAs f i l m s , better films are required before devices with c h a r a c t e r i s t i c s competitive with bulk devices can be fabri c a t e d . An n-type homoepitaxial f i l m was deposited by using the CSS method. 3/2 The film's electron H a l l mobility varied with temperature as T , which i s the form predicted by the Brooks-Herring formula f o r ionized impurity s c a t t e r i n g . 2 The room-temperature electron H a l l mobility was 219 cm /V-sec. A.e. bridge and step response methods were used to study the d i e l e c -t r i c properties of anodic Al^O^ fi l m s . Metal/Al^O^/Al capacitors were made using evaporated A l films on glass which-had been anodized i n ammonium penta-borate dissolved i n ethylene g l y c o l . The d i e l e c t r i c constant and loss tangent of anodic kl^O^ decreased with increasing frequency over the range from 0.5 to 100 kHz. Step response currents followed a t n law. For l i n e a r d i e l e c t r i c response, t h i s corresponded to varying as (J1 \ E l e c t r i c a l properties of Al^O^/GaAs in t e r f a c e s were studied using the metal-insulator-semiconductor (MIS) capacitance technique. T h e o r e t i c a l curves r e l a t i n g the capacitance of metal/A^O^/GaAs capacitor to the d.e. voltage applied across the capacitor were calculated and p l o t t e d . These capacitance-voltage (C-V) curves then served as a basis f o r the i n t e r p r e t a t i o n of experimental C-V curves. "Fast" surface state d e n s i t i e s greater than 12 2 10 /cm -eV were obtained. i i TABLE OF CONTENTS Page LIST OF TABLES v i i . LIST OF ILLUSTRATIONS v i i i ACKNOWLEDGEMENT, . x i . 1. INTRODUCTION 1 2. CLOSE-SPACED SUBLIMATION (CSS) OF GaAs FILMS 4 2.1 I n t r o d u c t i o n 4 2.1.1 F l a s h - E v a p o r a t i o n Method 5 2.1.2 Diode S p u t t e r i n g : 5 2.1.3 L i q u i d Phase E p i t a x y 6 2.1.4 Vapour Phase E p i t a x y and C l o s e - S p a c e d Vapour T r a n s p o r t Method 7 2.1.5 The "Three Temperature" Method 8 2.1.6 N e a r - E q u i l i b r i u m S u b l i m a t i o n i n U l t r a - H i g h Vacuum 9 2.2 C l o s e - S p a c e d S u b l i m a t i o n (CSS) Method 9 2.2.1 I n t r o d u c t i o n - 9 2.2.2 Vapour P r e s s u r e s i n the GaAs System 10 2.2.3 P r i n c i p l e s o f O p e r a t i o n o f the CSS Method 10 2.2.4 D e s i g n and F a b r i c a t i o n o f the CSS Apparatus 12 2.3 E x p e r i m e n t a l P r o c e d u r e s 15 2.3.1 Source P r e p a r a t i o n 15 2.3.2 S u b s t r a t e P r e p a r a t i o n 16 2.3.3 S u b l i m a t i o n o f GaAs F i l m s 18 2.3.4 F i l m T h i c k n e s s D e t e r m i n a t i o n 20 3. STRUCTURAL PROPERTIES OF CSS GaAs FILMS ON SAPPHIRE 22 3.1 I n t r o d u c t i o n 22 3.2 O p t i c a l M i c r o s c o p e O b s e r v a t i o n s 22 i i i Page 3.3 Electron Probe Microanalysis.... 26 3.3.1 The Electron Microprobe 26 3.3.2 X-Ray Counts and Scans 2 7 30 3.3.3 Surface Topography 30 3.4 Reflection Electron Diffraction Observations 37 3.5 X-Ray Diffraction Analysis 37 3.5.1 Diffractometer Measurements 3.5.2 Back Reflection Laue 3 9 3.6 Summary and Discussion. 4. ELECTRICAL PROPERTIES OF CSS GaAs FILMS 4 6 4.1 Introduction ' 46 4.2 Carrier Transport Theory: A Brief Review 46 4.3 El e c t r i c a l Properties of Bulk GaAs 48 4.4 Modification of El e c t r i c a l Properties in Films 53 4.4.1 Effect of Sample Geometry 53 4.4.2 Effects of Surface Space Charge Layers and Surface Scattering . 55 4.4.3 Effects of Polycrystalline Structure 56 4.4.4 Effects of Compensation of Impurities 59 4.5 Experimental Procedures 60 4.5.1 Preparation of Ele c t r i c a l Contacts 60 4.5.2 Sample Holders 61 4.5.3 Hall Apparatus • 67 4.5.4 Accuracy of Measurements 68 4.5.5 Thermal-Probe Method . • 68 .4.6 Results and Discussion: CSS GaAs Films on Sapphire 69 4.6.1 Conductivity Type Determination by the Thermal-Probe Method • 69 iv Page 4.6.2 E f f e c t s o f F i l m T h i c k n e s s and S u r f a c e S c a t t e r i n g 70 4.6.3 E f f e c t s o f S u b s t r a t e Temperature and I m p u r i t i e s 70 4.6.4 E l e c t r i c a l P r o p e r t i e s o f "Low p" F i l m s 72 4.6.5 E l e c t r i c a l P r o p e r t i e s o f "High p" F i l m s 78 4.7 P o s t deposition Doping o f CSS GaAs " F i l m s on S a p p h i r e . . . 85 4.8 D e v i c e F a b r i c a t i o n U s i n g CSS GaAs F i l m s on S a p p h i r e 86 4.9 R e s u l t s and D i s c u s s i o n : CSS GaAs F i l m on S e m i - I n s u l a t i n g GaAs 8 7 5. DIELECTRIC PROPERTIES OF ANODIC A1 20 3 FILMS 90 5.1 I n t r o d u c t i o n 90 5.2 D i e l e c t r i c P r o p e r t i e s o f Amorphous F i l m s 91 5.2.1 A.c. B r i d g e Method 92 5.2.2 Step Response Method 94 5.3 Growth o f Oxide F i l m s 95 5.3.1 C o n s t a n t C u r r e n t F o r m a t i o n 95 5.3.2 Co n s t a n t V o l t a g e F o r m a t i o n 96 5.4 E x p e r i m e n t a l P r o c e d u r e s 96 5.4.1 S u b s t r a t e P r e p a r a t i o n 96 5.4.2 A l Source P r e p a r a t i o n 97 5.4.3 Vacuum D e p o s i t i o n o f A l 97 5.4.4 A n o d i z a t i o n o f A l F i l m s 97 5.4.5 D e p o s i t i o n o f C o u n t e r e l e c t r o d e s 99 5.4.6 A.c. B r i d g e , Step Response and d.e. Conduction" Measurements 100 5.5 A n a l y s i s o f R e s u l t s 101 5.5.1 A.c. B r i d g e 101 5.5.2 Step Response ' 102 6. CHARACTERISTICS OF THE A l ^ / G a A s INTERFACE 110 6.1 I n t r o d u c t i o n ; H O v Page 6.2 The Ideal MIS. Capacitor I l l 6.3 Capacitance-Voltage C h a r a c t e r i s t i c s of an Ideal MIS Capacitor H 2 6.3.1 The Depletion Approximation 113 6.3.2 The Low Frequency Approximation 115 6.3.3 The High Frequency Approximation 117 6.3.4 The E f f e c t s of Metal/Semiconductor Work Function D i f -ference, Slow Surface States and Charges i n the Insu-l a t o r : us 6.4 Capacitance-Voltage C h a r a c t e r i s t i c s of a MIS Capacitor with Frequency Dependent Traps .. 119 6.5 Computations • 122 6.6 Experimental Procedures ,. . 128 6.7 Results 129 7. CONCLUSION 135 BIBLIOGRAPHY .. 139 APPENDIX 4.1 C a r r i e r Transport i n Semiconductors 147 APPENDIX 5.1 P o l a r i z a t i o n Current of a D i e l e c t r i c with a Uniform D i s t r i b u t i o n of A c t i v a t i o n Energies . . 152 r APPENDIX 6.1 Computed Capacitance-Voltage Curves for an Ideal Metal/ A1„0 /GaAs MIS Capacitor 154 v i LIST OF TABLES Table Page 2.1 A t y p i c a l CSS deposition schedule f o r GaAs films i . jg 3.1 P a r t i c l e s i z e as a function of substrate temperature 3Q 3.2 The d-spacing of Debye rings . 3 4 4.1 Scattering mechanisms i n GaAs 52 4.2 E l e c t r i c a l properties of the source material at room temperature 7^ 4.3 E l e c t r i c a l properties of "low p" films and t h e i r corresponding source material at room temperature 7 3 4.4 Concentration of impurities i n "low p" films 77 4.5 E l e c t r i c a l properties of "high p" films and t h e i r corresponding source material at room temperature 7 3 4.6 Concentration of impurities i n "high p" films 84 4.7 E l e c t r i c a l properties of a converted f i l m 85 6.1 Density of surface states on GaAs 133 v i i LIST OF ILLUSTRATIONS Figure ' Page 2.1 Equilibrium pressures of As„, As,, and Ga over GaAs (from Arthur 1967) 11 2.2 A close-up photograph of the close-spaced sublimation (CSS) apparatus ... 13 2.3 A photograph of the CSS apparatus and the TNB vacuum system... 14 2.4 Displacement of the Fizeau fringes due to a film-substrate step 20 2.5 A t y p i c a l Talysurf scan across a GaAs-substrate step 21 3.1 Droplets of l i q u i d Ga on a GaAs f i l m 23 3.2 Optical photomicrographs of GaAs films deposited at different substrate temperatures........ '. 25 3.3 Optical photomicrograph of an etched f i l m 25 3.4 Optical photomicrograph of a f i l m with "120°" growth features. 26 3.5 Schematic i l l u s t r a t i o n of methods used i n obtaining informa-tion from a specimen using the electron microprobe 27 3.6 Electron microprobe topographs of films deposited at dif f e r e n t substrate temperatures 31 3.7 Reflection electron d i f f r a c t i o n patterns of GaAs films deposited at different substrate temperatures 32 3.8 Line drawings of RED patterns of fee and hep GaAs and free Ga (from Pankey and Davey 1966) 33 3.9 Single-crystal RED pattern of a 630°C f i l m 35 o 3.10 Kikuchi bands i n a 640°C f i l m 3 6 3.11a Back r e f l e c t i o n Laue d i f f r a c t i o n patterns of GaAs films 3.11b Back r e f l e c t i o n Laue d i f f r a c t i o n patterns of GaAs films 41 3.12a Single-crystal Laue d i f f r a c t i o n pattern of a 640°C f i l m 43 3.12b Single-crystal d i f f r a c t i o n pattern of a 630°C f i l m 44 4.1 Electron mobility i n GaAs versus temperature (from Ehrenreich I960)... 4 9 4.2 R e s i s t i v i t y and H a l l mobility of GaAs at 300°K versus impurity concentration (after Sze and I r v i n 1968) 51 v i i i Page 4.3 Shorting e f f e c t of large-area current contacts on H a l l c o e f f i c i e n t (after Isenberg, Russell and Greene 1948) 53 4.4 Schematic diagram of a van der Pauw sample ' 54 4.5 Schematic diagram of a mosaic f i l m 57 4.6 Energy band diagram of a p-type mosaic f i l m 58 4.7 Energy band diagram of a compensated p-type semiconductor.•••• 60 4.8 Photograph of a metallized CSS GaAs f i l m 62 4.9 Photograph of the sample-and-mask holder and the beryllium-copper mask used i n the deposition of e l e c t r i c a l contacts 63 4.10 Photograph of the sample holder (less p r o t e c t i v e cap) 64 4.11 Block diagram of the H a l l apparatus and a table of switch con-nections for the d i f f e r e n t measurement modes. 65 4.12 Magnetic f i e l d versus d.e. current of the Alpha 8500 e l e c t r o -magnet ' 66 4.13 Thermal-probe c i r c u i t 69 4.14 R e s i s t i v i t y of "low p" films versus temperature 73 4.15 H a l l m o b i l i t y of holes i n "low p" films versus temperature.... 74 4.15a H a l l voltage versus magnetic f i e l d for sample 21 75 4.16 Hole concentration i n "low p" films versus temperature 76 4.17 Conductivity of "high p" films versus temperature 79 4.18 H a l l m o b i l i t y of holes i n "high p" films versus temperature... 80 4.19 Hole concentration i n "high p" films versus temperature 81 4.20 Energy band diagram of a mosaic f i l m with p a r t i a l l y developed b a r r i e r s (from Sl a t e r 1956) 83 4.21 H a l l voltage versus magnetic f i e l d f o r current i n the "normal" and i n the "reverse" d i r e c t i o n 88 4.22 Temperature dependence of the H a l l m o b i l i t y of electrons i n a homoepitaxial GaAs f i l m °. 89 5.1 Schematic diagram of the anodization set-up 98 5.2 Schematic diagram of the d.e. conduction measurement set-up... 101 5.3 P l o t of tan 6/fC versus 1/f 103 P i x Page 5.4 Plot of C e' versus f 104 o 5.5 Plot of tan 6 versus f 104 c 5.6 Charging currents versus time as a function of applied d.e. voltage 105 5.7 Discharging currents versus time as a function of preapplied d.e-. voltage 1Q6 5.8 Frequency dependence of .e" as determined by a.e. bridge and step response methods 108 5.9 D.e. conduction current versus applied f i e l d . . . . 109 6.1 Energy band diagram of an i d e a l metal/insulator/p-type semiconduc-tor MIS capacitor . 114 6.2 Energy band diagram of p-type semiconductor with surface traps (af t e r H a l l and White 1965) 120 6.3 lo I, Iv I versus N for GaAs 123 ' scm' 1 sm1 6.4 c , c versus N for GaAs .. 124 s co s cm 1/2 6.5 Q /N versus V (from H a l l and White 1965) . 125 S C A . S O 6.6 C-V curves using the depletion approximation f o r A^O^/p-type GaAs 126 6.7 C-V curves using the low frequency and high frequency approxi-mations f or Al^O^/p-type GaAs 12 7 6.8 Th e o r e t i c a l FREDEP and experimental C-V curves f o r sample G13-B. 131 6.9 Th e o r e t i c a l FREDEP and experimental C-V curves f o r sample G15-B. 132 x ACKNOWLEDGEMENT I ^rn most g r a t e f u l to my supervisor, Dr. L. Young, for h i s invaluable guidance and supervision during the course of my work. I e s p e c i a l l y thank him for the unwavering personal support and encouragements he gave me. Gra t e f u l acknowledgement i s given to the National Research Council for scholarships which allowed me to carry out th i s work. The research was supported, i n part, by Defense Research Board contract //T79 and a National Research Council Grant //A3392. He l p f u l cooperation and assistance from Dr. L.C. Brown, Dr. B. Hawbolt and Mr. A. Lacis of the Department of M e t a l l u r g i c a l Engineering, U.B.C, i s g r a t e f u l l y acknowledged. I also thank Mr. R. H a l l i w e l l of the Physics Department, U.B.C, for h i s help i n obtaining Laue d i f f r a c t i o n pictures and Mr. J . Mercier, of the Physics Department, S.F.U., for h i s help i n doing the Talysurf measurements. I thank Mrs. J. Larcher, Messrs. G. Anderson, H. Black, V. Loney, A. Mackenzie, E. Voth and e s p e c i a l l y Mr. J . Stuber for t h e i r valuable t e c h n i c a l assistance and to Miss B. Harasymchuk f or typing t h i s t h e s i s . F i n a l l y , I thank my fellow graduate students i n the Solid-State E l e c t r o n i c s Group and my brother, James, for'innumerable useful discussions and for proof-reading my t h e s i s . xi. 1 1. INTRODUCTION Recent developments i n integrated c i r c u i t s have been p r i n c i p a l l y on monolithic s i l i c o n , hybrid and t h i n - f i l m integrated c i r c u i t s . In a mono-l i t h i c s i l i c o n integrated c i r c u i t , the c i r c u i t elements are connected to each other by unwanted e l e c t r i c a l paths through the s i l i c o n substrate. Reverse biased p-n junctions are commonly used to reduce the unwanted paths. The usefulness of th i s method i s l i m i t e d by undesirable high frequency capacitive coupling e f f e c t s and d.e. leakage at the junctions. D i e l e c t r i c i s o l a t i o n i s another method used. C i r c u i t elements of an integrated c i r c u i t are e l e c t r i c a l l y i s o l a t e d from each other and from the p o l y c r y s t a l l i n e s i l i c o n substrate by using a layer of SiO^- This technique involves many process steps. Good e l e c t r i c a l i s o l a t i o n between c i r c u i t elements i s achieved i n hybrid c i r c u i t s . . In a hybrid c i r c u i t , d i s c r e t e active elements are mounted on an Insulating substrate which has t h i n - f i l m or t h i c k - f i l m passive elements deposited on i t s surface. The disadvantages of hybrid c i r c u i t s are increased assembly cost due to the handling of many separate components and lower r e l i a b i l i t y due to the increased number of interconnections. T h i n - f i l m c i r c u i t s with both a c t i v e and passive elements deposited on the same i n s u l a t i n g substrate have the advantages of simpler f a b r i c a t i o n techniques and i d e a l i s o l a t i o n between c i r c u i t elements. Sputtered tantalum t h i n - f i l m c i r c u i t s have been s u c c e s s f u l l y made and are used i n increasing numbers i n telephone systems. The use of t h i n - f i l m c i r c u i t s has been l i m i t e d by the lack of an adequate t h i n - f i l m active device. The most promising t h i n - f i l m active device i s the t h i n - f i l m insulated-gate f i e l d - e f f e c t t r a n s i s t o r . The properties of the semiconductor f i l m , the i n s u l a t o r and the insulator/semi-conductor i n t e r f a c e l a r g e l y determine the performance of a t h i n - f i l m t r a n s i s t o r . Thus, the study of the properties of materials and of materials processes used i n device f a b r i c a t i o n i s v i t a l to the improvement of device c h a r a c t e r i s t i c s 2 and performance. The following objectives were chosen for this t h e s i s . (1) To f a b r i c a t e GaAs films on sapphire and study t h e i r e l e c t r i c a l and s t r u c t u r a l properties; (2) To study the d i e l e c t r i c properties of anodic A^O^; and (3) To study the i n t e r f a c e properties of A^O^/GaAs using the metal-insulator-semiconductor (MIS) capacitance technique. CdS and CdSe have been most commonly and most s u c c e s s f u l l y used i n the f a b r i c a t i o n of t h i n - f i l m t r a n s i s t o r s (see, f o r example, Weimer 1961; T i c k l e 1969). At room-temperature, the electron mobility of bulk, s i n g l e -2 c r y s t a l CdS and CdSe i s about 300 and 500 cm /V-sec, r e s p e c t i v e l y (Devlin 1967). In comparison, the room-temperature mobility of majority c a r r i e r s i n bulk, 2 s i n g l e - c r y s t a l n-type or p-type GaAs i s 6000 or 400 cm /V-sec, r e s p e c t i v e l y (Madelung 1964). Because of the higher c a r r i e r m o b i l i t i e s , devices fabricated using GaAs may have better high frequency c h a r a c t e r i s t i c s and higher trans-conductances. Presently there i s also i n t e r e s t i n a new class of microwave (Gunn) devices r.adc with bulk GaAs (Gunn 1963, Torrens 1969). The r e a l i z a -t i o n of t h i n - f i l m microwave devices w i l l further increase the importance of t h i n -f i l m c i r c u i t s . Up to now, not much information can be found i n the l i t e r a t u r e on the properties of GaAs films deposited by using methods compatible with e x i s t i n g t h i n - f i l m c i r c u i t f a b r i c a t i o n techniques. A reason for t h i s d e ficiency i s that good q u a l i t y GaAs films are d i f f i c u l t to make . In t h i s t h e s i s , a d i f f e r e n t method, the close-spaced sublimation (CSS) method, i s developed.. This method o f f e r s several advantages over other vacuum techniques reported * Since the i n i t i a t i o n of t h i s project (1967), s i g n i f i c a n t advances i n the f a b r i c a t i o n of homoepitaxial GaAs layers have been made (1968 Symposium: GaAs). Unfortunately, the methods used are not compatible with e x i s t i n g vacuum techniques used i n t h i n - f i l m c i r c u i t s (see Chapter 2). i n the l i t e r a t u r e . These t e c h n i q u e s and the CSS method are d i s c u s s e d i n Chapter 2 . The e l e c t r i c a l p r o p e r t i e s o f the CSS f i l m s a r e the most r e l e v a n t to the c h a r a c t e r i s t i c s o f any d e v i c e f a b r i c a t e d u s i n g these f i l m s . S i n c e the s t r u c t u r e o f the f i l m s a f f e c t s t h e i r e l e c t r i c a l p r o p e r t i e s , f u r t h e r i n s i g h t i n t o the e l e c t r i c a l p r o p e r t i e s of the f i l m s can be g a i n e d i f i n f o r m a t i o n on t h e i r s t r u c t u r a l p r o p e r t i e s i s a v a i l a b l e . Hence the 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 o f the CSS GaAs f i l m s a r e c o n s i d e r e d i n C h a p t e r s 3 and 4 , r e s p e c -t i v e l y . I n c l u d e d i n Chapter 4 i s a d i s c u s s i o n on the p o s t d e p o s i t i o n d o p i n g o f CSS GaAs f i l m s on s a p p h i r e and the growth o f h o m o e p i t a x i a l GaAs on semi-i n s u l a t i n g GaAs. The f a b r i c a t i o n of t h i n - f i l m t r a n s i s t o r s and A u - S c h o t t k y b a r r i e r d i o d e s u s i n g GaAs f i l m s on s a p p h i r e i s a l s o d i s c u s s e d . The methods used i n and the r e a s o n s f o r s t u d y i n g the d i e l e c t r i c p r o p e r -t i e s o f a n o d i c A l ^ O ^ a r e g i v e n i n C h a p t e r 5. The advantages and d i s a d v a n t a g e s o f u s i n g Al^O^ as an i n s u l a t o r i n s e m i c o n d u c t o r d e v i c e s a r e compared w i t h those o f S i 0 2 > The i D < i t a l - i n s u l a t o r - s e m i c o n d u c t o r (MIS) c a p a c i t a n c e t e c h n i q u e , as used i n the s t u d y of the p r o p e r t i e s o f i n s u l a t o r / s e m i c o n d u c t o r i n t e r f a c e s , i s a w e l l e s t a b l i s h e d t e c h n i q u e . I t i s s i m p l e and can be used to o b t a i n i n f o r m -a t i o n r e q u i r e d to e v a l u a t e the m e r i t s o f a p a r t i c u l a r i n s u l a t o r / s e m i c o n d u c t o r i n t e r f a c e i n so f a r as i t s f i e l d - e f f e c t p r o p e r t i e s a r e concerned. The use o f the MIS c a p a c i t a n c e t e c h n i q u e to study the A^O^/GaAs i n t e r f a c e has n o t been seen r e p o r t e d i n the l i t e r a t u r e . Such a s t u d y i s p r e s e n t e d i n C h a p t e r 6. The main r e s u l t s o f t h i s t h e s i s a r e summarized i n Chapter 7 . Areas f o r f u r t h e r r e s e a r c h a r e a l s o i n d i c a t e d . 4 2. CLOSED-SPACED SUBLIMATION (CSS) OF GaAs FILMS 2.1 Introduction One of the important points that must be considered i n the f a b r i c a t i o n of compound semiconductor films i s the maintenance of a s t o i c h i o m e t r i c r a t i o between the elements of the deposited f i l m . Scanlon (1953) reported that non-stoichiometry i n the l e a d - s a l t compound semiconductors af f e c t e d t h e i r e l e c t r i c a l p roperties. S i m i l a r l y , the o p t i c a l and e l e c t r i c a l properties of II-VI compound semiconductors depend upon the stoichiometry of t h e i r constituent elements (Albers 1967, Devlin 1967). In the case of III-V compound semi-conductors, previous work reviewed by Hilsum and Rose-Innes (1961) and Madelung (1964) indi c a t e d that no c l e a r - c u t evidence for nonstoichiometric behaviour has been observed i n these compounds. Recently, however, H a r r i s , Nannichi, Pearson and Day (1969) stated that t h e i r annealing experiments, photoluminescence data and contacting problems indicated that GaAs i s not a s t o i c h i o m e t r i c compound. The concentration of Ga and As vacancies depends upon the growth conditions and the thermal treatment of the c r y s t a l a f t e r growth. Deviation from stoichiometry affects the e l e c t r i c a l properties of GaAs. For example, a preponderance of As vacancies may change an i n i t i a l l y low r e s i s t i v i t y n-type sample i n t o high r e s i s t i v i t y n-type or i n t o p-type. From mass spectrometric and weight loss measurements, Arthur (1967) determined that at the melting point of GaAs (1238°C), the p a r t i a l pressures -4 of As2> As^, and Ga over GaAs are 0.32.8, 0.648 and 5 x 10 atm, r e s p e c t i v e l y . Because of the disparate As and Ga p a r t i a l pressures, the f a b r i c a t i o n of GaAs films by evaporation from a molten source i s complicated by problems of nonstoichiometry and the possible presence of excess Ga i n the f i l m s . In attempting to circumvent these complications, some of the methods that have been used are: 1. Flash Evaporation Method, 5 2. Diode Sputtering, 3. L i q u i d Phase Epitaxy, 4. Vapour Phase Epitaxy and Close-Spaced Transport Method, 5. The "Three Temperature" Method, and 6. Near-Equilibrium Sublimation i n Ultra-High Vacuum. The e s s e n t i a l features of each method w i l l be b r i e f l y described. 2.1.1 Flash-Evaporation Method Small grains of GaAs are continuously fed i n t o a heater, situated i n s i d e a vacuum chamber, hot enough to v o l a t i l i z e completely each i n d i v i d u a l grain as i t comes i n contact with the heater. As w i l l evaporate f i r s t because i t i s more v o l a t i l e than Ga. However, i f the grains are s u f f i c i e n t l y small or i f the source-to-substrate distance i s s u f f i c i e n t l y large, Ga and As vapours can condense i n alternate monolayers on the substrate and recombine to form GaAs, provided the substrate i s held at an appropriate temperature. For s t r u c t u r a l l y good f i l m s , the substrate temperature must be high. But i t must not be higher than the temperature at which As begins to re-evaporate. For Ge and GaAs substrates, t h i s temperature was 475°C and 535°C, r e s p e c t i v e l y ft (Richards 1963, 1966) 2.1.2 Diode Sputtering Sputtering as a method of making t h i n films i s an established technique (Holland 1956). Sputtering can be e i t h e r " r e a c t i v e " or " p h y s i c a l " , depending upon whether the bombarding gas ions and the cathode target chemically react with each other or not. There are many v a r i a t i o n s of the sputtering technique, with diode sputtering being the simplest (Campbell 1966). B r i e f l y , -2 the cathode (in t h i s case, GaAs) i s sputtered i n a low pressure (10 Torr) Ar * Recently, Farukhi and Charlson (1969) reported that m o b i l i t i e s near 0.01 cm^/V-sec were obtained from films made by using the flash-evaporation method. atmosphere by applying a 1 to 3 kV p o t e n t i a l difference between the cathode and the (usually grounded) anode, where the substrate i s situated. P o s i t i v e Ar ions created by the. discharge are accelerated toward and impinge on the surface of the. cathode, e j e c t i n g atoms which eventually condense on the sub-s t r a t e . Molnar, Flood and Francombe (1964) sputtered GaAs on CaF^, vitreous SiC^ and s i n g l e c r y s t a l Ge substrates. NaCl substrates were used by Evans and Noreika (1966). E l e c t r i c a l properties of sputtered GaAs films were not reported by any of these authors. 2.1.3 L i q u i d Phase Epitaxy Two types of s o l u t i o n growth systems have been used i n the l i q u i d phase epitaxy of GaAs. They are the h o r i z o n t a l t i l t tube system (Nelson 1963; Bolger, Franks, Gordon and Whitaker 1966) and the v e r t i c a l steady-state system (Kang and Greene 1968). In the former method, a GaAs substrate i s clamped at one end of a s u i t a b l e boat (either graphite or s i l i c a ) while a Sn-GaAs (or Ga-GaAs) mixture i s placed at the other end. The boat i s placed i n s i d e a constant temperature furnace which i s t i l t e d so that the source i s lower than the substrate. The temperature i n s i d e the furnace i s r a i s e d to about 640°C (or 850°C for Ga-GaAs) while i s passed continuously. The furnace i s tipped so that the melt runs onto the substrate and covers i t . The furnace i s then cooled. As the temperature drops, GaAs comes out of s o l u t i o n both as a homo-e p i t a x i a l deposit and as a p o l y c r y s t a l l i n e s o l i d which remains suspended i n the s o l u t i o n . At temperatures lower than 500°C, the substrate may be taken out and any excess Sn or Ga wiped o f f . In the v e r t i c a l method, a GaAs substrate i s lowered v e r t i c a l l y onto a Ga-GaAs s o l u t i o n . A thermal gradient between the source and the substrate i s subsequently maintained by slowly increasing the source temperature. Due to the increased s o l u b i l i t y of Ga at higher tem-perature, a concentration gradient i s established across the Ga s o l u t i o n . This 7 causes the transport of As from the source to the substrate. E p i t a x i a l layers may be grown at a constant temperature by using t h i s method. 2.1.4 Vapour Phase Epitaxy and Close-Spaced Vapour Transport Method The underlying p h y s i c a l mechanisms involved i n the vapour phase e p i t a x i a l growth of GaAs films are described by the following chemical reactions. Ga X 3 + 2GaAs"^3Ga X + ~ As^ (X=C1, Br, L) or 2GaAs + H 20 ^ f t Ga 20 + H 2 + A s 2 The reactions proceed from l e f t to r i g h t iv-ith increasing temperature and reverse upon cooling. Both closed tube (Moest and Shupp 1962) and open tube ( T i e t j e n and Amick 1966) systems have been used. In the closed tube system, a GaAs source and a s u i t a b l e substrate were sealed i n a quartz ampoule b a c k f i l l e d with HC1. The source was kept at a f i x e d temperature i n the range from 600 to 850°C. The substrate was kept at a temperature 20 to 150°C below the source temperature. The open tube system i s s i m i l a r to the closed tube system except that a continuov>>> £).ow of transport agent from the source to the substrate i s maintained. I t was found that t h i s provided a better c o n t r o l over the growth processes. Appropriate dopants can also be introduced during growth. E p i t a x i a l GaAs films with e l e c t r i c a l properties approaching that of bulk GaAs had been fa b r i c a t e d using the open tube system ( E f f e r 1965, T i e t j e n and Amick 1966). A disadvantage of t h i s method i s i t s low deposition e f f i c i e n c y ( t y p i c a l l y a few percent) due to the transported material condensing on surfaces other than the substrate. Deposition e f f i c i e n c y i s defined as the r a t i o of the substrate weight gain to the source weight l o s s . One way of improving the deposition e f f i c i e n c y i s by pla c i n g the source and substrate very close together. N i c o l l (1963) and Robinson (1963) deposited GaAs on GaAs and Ge substrates using the close-spaced arrangement with wet H as the transporting agent. Deposition efficiencies of up to 98% were reported by Robinson. 2.1.5 The "Three Temperature" Method Assume that the vapour phase inside a vacuum chamber consists of two components' A and B. Further assume that the vacuum is sufficiently high such that collisions between the component gases are negligible in the vapour phase and that interactions between them occur only within the adsorbed layer on the substrate, giving a stable compound AB. Gunther (1966) showed that at a to-be-determined substrate temperature, which has a limited range, only the stable compound can be formed at the substrate provided that V A B * < P e ( i " A ' B ) ( 2 - 1 } i as the incident fluxes of A, B, (N,.,N) are varied over a range between N + A \ B = constant and N + A - N + C A , N + g « N ^ . In the above equation, p . = equilibrium pressure of the ith component over i t s solid form, ex — P ("7"TT) ~ par t i a l pressure of the more volatile component i over the solid e AB-AB N = c r i t i c a l incident flux, above which the ith -component can condense in elemental form. As 2 For GaAs, p > p (-• - ); hence eqn. 2.1 is satisfied. Davey and As 2 Pankey (1964, 1966, 1968) had deposited GaAs on sapphire, Ge and semi-insulating GaAs. From their thermoelectric power measurements, they concluded that a l l the films were p-type. The lowest r e s i s t i v i t y values measured were between 20 and 30 fi-cm. Hall measurements were attempted. However, they did not observe Hall voltages which could be unambiguously interpreted as above the noise level of 9 their measuring system.. 2.1.6 Near-Equilibrium Sublimation in Ultra-High Vacuum Hudock (1967) deposited GaAs on semi-insulating GaAs by subliming a powdered charge in an essentially closed quartz vessel under near-equilibrium conditions. He showed that epitaxial films can be obtained at substrate temperatures as low as 600°C when depositions were performed in background -]0 pressures of 5 x 10 Torr or less. He also reported that the room-temperature 2 mobility of a homoeptiaxial layer was 4350 cm /V-sec. -2.2 Close-Spaced Sublimation (CSS) Method 2.2.1 Introduction The technology of vacuum-deposited passive.thin films is well established (see, for example, Holland 1956; Berry, Hall and Harris 1968; Chopra 1969). This knowledge may be put to f u l l use in thin-film integrated circuits applications i f high quality semiconducting films can be fabricated by using vacuum techniques. One of the purposes of this thesis i s to investigate the methods of fabricating semiconducting films which are compatible with existing vacuum techniques. For practical reasons, such methods must be adaptable to large scale production and must be economical on starting material. The close-spaced sublimation (CSS) method was developed for i t was fe l t that none of the reported methods satisfied the above requirements. Liquid phase epitaxy and vapour phase epitaxy have been used with considerable success in the fabrication of high quality homoepitaxial layers (1968 Symposium: GaAs). However, these methods are not compatible with existing vacuum techniques. An additional problem of "seeding" is also raised when the liquid phase epitaxy Recently Manasevit (1968) reported the deposition of single-crystal GaAs on insulating substrates using a chemical method. method i s used to f a b r i c a t e h e t e r o e p i t a x i a l f i l m s . Insulating substrates other than semi-insulating GaAs may be more a t t r a c t i v e for the following reasons. GaAs i s b r i t t l e ; greater mechanical strength, hence reduced s u s c e p t i b i l i t y of the f i n i s h e d devices to mechanical shocks, can be provided by substrates l i k e sapphire. Furthermore, sapphire substrates have low d i e l e c t r i c l o s s , are i n e r t to most chemicals and do not outgas, even at elevated temperatures. Also, at the present, s i n g l e - c r y s t a l semi-insulating GaAs costs US $38.50 per gram (Monsanto) while sapphire substrates cost about US $5.00 each (1/2" d i a , Adolf M e l i e r ) . 2.2.2 Vapour Pressures i n the GaAs System Upon heating up to temperatures below the melting point of GaAs, i t di s s o c i a t e s as 2GaAs, . •* 2Ga, . + ^ As„ , . + \ As. . (2.2) (s) (g) 2 2(g) 4 4(g) Arthur (1967) used mass spectrometric and weight loss measurements of the species e f f u s i n g from a Knudsen c e l l containing GaAs to obtain the p a r t i a l vapour pressures of Ga, As ^ j and As^ over GaAs f o r the temperature range 900°K to 1200°K. Thurmond (1965) had constructed a s i m i l a r P-T graph using the mass spectrometric data of Drowart and Goldfinger (1958) and Gutbier (1961), t o t a l pressure measurements near the GaAs melting point taken by Richman (1963) ? and s o l u b i l i t y data of Koster and Thomas (1955) and H a l l (1963). The point of i n t e r e s t i n the P-T p l o t i s the one which corresponds to the "congruent temperature". At t h i s temperature, Ga and As sublimes congruently, that i s p p = 2p + 4p . Thus, Ga and As vapours e x i s t i n a st< chiometric r a t i o . Arthur found that the congruent temperature was 910°K as com-pared to Thurmond's estimate of 933°K. 2.2.3 P r i n c i p l e s of Operation of the CSS Method The p r i n c i p l e s underlying the operation of the CSS method are as follows. A wafer of source material i s situated p a r a l l e l and close to the substrate. The source, i s held at or near the congruent temperature. If 11 A , A - — R i c h m a n ' s total pressure data x, + — M a s s spectrometer data for As., O — M a s s spectrometer data for G a ©, • — M a s s spectrometer data for A s 3 D3—Weight loss data Curves are calculated f rom measured enthalpies and free-energy data in low-temperature region and f rom Richman's data i n high-tempera-ture region. F i g . 2.1 Equilibrium pressures of AS2, As^ and Ga over GaAs(from Arthur 1967) a small temperature gradient i s maintained between the source and the substrate, near equilibrium conditions p r e v a i l i n the region bounded by them. Since Ga and As vapours e x i s t i n a stoichiometric r a t i o , they may condense s t o i c h i o m e t r i c a l l y on the substrate. Ga atoms were found to have a unity condensation c o e f f i c i e n t on GaAs and GaP substrates, while As atoms have a condensation c o e f f i c i e n t proportional to the Ga coverage of the substrate (Arthur 1968). Although no experimental data on the condensation c o e f f i c i e n t s of Ga and As on sapphire were found, i t i s reasonable to expect that the formation of GaAs on the cooler substrate surface i s enhanced because of multiple c o l l i s i o n s of Ga and As atoms with the substrate due to the close spacing, with a resultant increase i n the 12 adatom d e n s i t y a t the s u b s t r a t e . 2.2.4 Design and F a b r i c a t i o n o f the CSS Apparatus The f o l l o w i n g d e s i g n c r i t e r i a were adopted f o r the CSS a p p a r a t u s . (1) I t must be made from m a t e r i a l s c o m p a t i b l e w i t h e x i s t i n g h i g h vacuum t e c h n i q u e s . (2) The s o u r c e and s u b s t r a t e temperatures can be i n d e p e n d e n t l y m o n i t o r e d and c o n t r o l l e d . (3) The heat s o u r c e s must be " c l e a n " . That i s , i m p u r i t i e s emanating from the h e a t e r s must be k e p t t o a minimum. (4) A s m a l l s p a c i n g between the s o u r c e and the s u b s t r a t e i s d e s i r e d . (5) The s o u r c e and s u b s t r a t e h o l d e r s a r e t o be i n d e p e n d e n t l y s u p p o r t e d and r o t a t a b l e w i t h r e s p e c t to each o t h e r . The l a s t f e a t u r e a l l o w s the i n s i t u d e g a s s i n g o f t h e s o u r c e and s u b s t r a t e j u s t p r i o r t o the a c t u a l d e p o s i t i o n o f a f i l m . A t the same time, m u l t i p l e d e p o s i t i o n on d i f f e r e n t s u b s t r a t e s i s p o s s i b l e . F i g u r e s 2.2 and 2.3 show a c l o s e - u p photograph o f the CSS a p p a r a t u s and a photograph o f t h e CSS a p p a r a t u s i n the TNB vacuum system, r e s p e c t i v e l y . Type 304 s t a i n l e s s s t e e l , c l e a r f u s e d q u a r t z and Ta s h e e t s were used i n the c o n s t r u c t i o n o f the CSS a p p a r a t u s . Two q u a r t z i o d i d e lamps (650 w a t t s each., S y l v a n i a type DWY) were used as r a d i a n t h e a t s o u r c e s . Ta p a r a b o l i c r e f l e c t o r s were used i n c o n j u n c t i o n w i t h the h e a t s o u r c e s . Temperatures were m o n i t o r e d by ch r o m e l - a l u m e l thermocouples embedded i n the q u a r t z a s s e m b l i e s . The beads o f the thermocouples were s i t u a t e d l e s s than 3 mm. away from the s o u r c e and the s u b s t r a t e . The a l l s t a i n l e s s - s t e e l assembly can be e x t e r n a l l y r o t a t e d v i a a m a g n e t i c a l l y c o u p l e d d r i v e and a 100:1 worm-gear arrangement. A cam assembly was used t o a c h i e v e the c l o s e s p a c i n g w h i c h was l e s s than 0.3 mm. The s o u r c e -t o - s u b s t r a t e d i s t a n c e was about 0.7 mm. when a q u a r t z mask was used (Sec. 2 .3 .3) . F i g . 2.2 A close-up photograph of the close-spaced s u b l i m a t i o n (CSS) apparatus. F i g . 2.3 A photograph o f the CSS a p p a r a t u s and the TNB vacuum system. 15 The CSS apparatus was mounted i n s i d e a Ultek TNB vacuum system. Roughing of the vacuum chamber was accomplished by using a molecular sieve sorption pump. This avoids hydrocarbon contamination of the chamber which i s usually observed when a mechanical rotary pump i s vised. F i n a l pumping was accomplished by two 50 l i t / s e c ion pumps plus a T i sublimator pump, rated at 3600 l i t / s e c f o r a i r . Vacuum insi d e the chamber was monitored by using a Bayard Alpert ion gauge. T y p i c a l vacuum conditions during deposition are given i n Sec. 2.3.3. 2.3 Experimental Procedures 2.3.1 Source Preparation A wire saw was used to cut 0.5 to 1.2 mm thick s l i c e s o f f ingots of ei t h e r mono or p o l y c r y s t a l l i n e n-type GaAs c r y s t a l s . The e l e c t r i c a l properties of the sources are given i n Table 4.2. Each wafer was lapped u n t i l the two faces were p a r a l l e l . One face was mechanically polished, using Al^O^ powder, to a 0.3yfinish. P i c i e n wax, used i n mounting the sample during p o l i s h i n g , was dissolved away i n a hot t r i c h l o r e t h y l e n e bath. Just p r i o r to loading into the vacuum system, the wafers were rinsed and u l t r a s o r i i c a l l y agitated i n hot ft t r i c h l o r e t h y l e n e , hot acetone and hot propanol. This was followed by vapour degreasing f i r s t i n a Soxhlet extractor containing acetone and then i n another con-t a i n i n g propanol. The cleaning process following the hot t r i c h l o r e t h y l e n e bath s h a l l be c a l l e d the standard cleaning procedure. A f t e r a i r drying, the wafers were chemically etched i n 7 1^50^(95%):H 20 2(30%):H 20 by volume at 80°C (Northern E l e c t r i c ) f o r about 30 sec. Temperature of the etchant was maintained by using a water bath. o Immediately a f t e r etching, the wafers were rinsed thoroughly i n d o u b l y - d i s t i l l e d water and f i n a l l y placed i n s i d e a beaker containing reagent-grade propanol u n t i l ready for use. * Low u l t r a s o n i c power must be used as GaAs i s b r i t t l e . 16 2.3.2 Substrate. Preparation The e f f e c t s of substrates on the q u a l i t y of e i t h e r homo or hetero-e p i t a x i a l semiconducting films have been extensively studied, with emphasis placed on the Si-on-sapphire technology. (See, for example, F i l b y and Nielsen 1967; Manasevit and Morritz 1967). A c o r r e l a t i o n was found to e x i s t between the e l e c t r i c a l properties of a S i f i l m and the q u a l i t y of the sapphire surface: better S i films can be grown on s i n g l e c r y s t a l substrates with smooth, scratch-free surfaces. Similar r e s u l t s can be expected for GaAs films on sapphire. D i f f e r e n t methods of substrate preparation were investigated.-P o l i s h i n g of substrates by immersing i n a 400 to 500°C orthophosphoric acid bath was attempted. A Pt c r u i c i b l e was used to contain the hot orthophosphoric acid. The temperature of the acid was measured by using a thermocouple sheathed with Pt tubing and immersed i n the acid. I t was found that the dissolved Al^O^ redeposited on the sapphire surface, r e s u l t i n g i n a rough surface f i n i s h . The p o l i s h i n g rate was also d i f f i c u l t to c o n t r o l . Scheuplin and Gibbs (1960) found that while hot orthophosphoric acid removed the mechanically damaged layer s u c c e s s f u l l y , even surfaces with the best orientations showed a pronounced orange-peel structure. The surface f i n i s h was d i f f i c u l t to reproduce, probably because the p o l i s h i n g action of orthophosphoric acid was temperature dependent (Fil b y and Nielsen 1967). Robinson and Mueller (1966) polished sapphire using molten s a l t baths. They reported that at 1000°C, borax polished sapphire at the rate of about lOp/hr. However, a f i l m of residue was l e f t behind on the sapphire surface. This residue was d i f f i c u l t to remove and degraded the e l e c t r i c a l properties of the deposited S i layer. -Faktor, Fiddyment and Newns (1967) reported some preliminary r e s u l t s 17 on the polishing of sapphire using vanadium pentoxide at 900°C. Mechanically satisfactory surfaces were obtained. However, a pale violet film or bloom appeared on the sapphire surfaces after immersion in the ^ O,. melt. This bloom was resistant to chemical etching. Heating in air at 1200°C for about 24 hours resulted in the disappearance of the bloom. It was not clear whether the bloom was evaporated away or diffused into the sapphire as weight changes on heating were insignificant. There i s always concern that some residue is l e f t behind on the surfaces which have been chemically polished. For this reason, chemical polishing of sapphire surfaces i s undesirable (Filby and Nielsen 1967). Hydrogen pre-firing of sapphire at about 1200°C for several hours was found to give excellent surface finish. Furthermore, the problem of residue l e f t behind from chemical polishing is avoided. Robinson and Mueller (1967) obtained their best epitaxial Si layers on H pre-fired sapphire sub-strates. In a private communication to Davey and Pankey, Manasevit also indicated that H^  pre-firing of sapphire resulted in good surface finish (Davey and Pankey 1968). Hart, Etter, Jervis and Flanders (1967) found that Si films with mobilities similar to those grown on chemically polished sapphire substrates were obtained when high quality mechanically polished substrates were used. The substrates used in this study were Vernuille 0° sapphire (0.080" thick, 1/2" dia.; Adolf Melier) either in their as-received state or pre-fired in H^  at about 1200°C for several (1.5 to 4.5) hours. The as-o received sapphires were polished to a better than 200 A finish (manufacturer's specifications). Prior to use, these substrates were subjected to the standard cleaning procedure. Substrates were pre-fired in a 2.5" dia. quartz tube inside a JMC (115 volts 44 A) furnace. The temperature near the substrates was monitored by using a chromel-alumel thermocouple sheathed with a fused quartz tube. Standard grade l\ was introduced and monitored using a Roger Gilmart Inst. (RGI) flowmeter. The flow rate used was about 150 ml/min. The quartz tube was sealed and a positive pressure inside was maintained by venting H through a Pyrex gas washing bottle containing H^ O. Substrates were cleaned following the standard cleaning procedure before pre-firing. The pre-fired substrates were introduced directly into the vacuum chamber for deposition of GaAs. ' 2.3.3 Sublimation pf GaAs Films Parameters which affect the films during growth, are background pressure of the vacuum chamber, substrate temperature, contamination from outgassing of nearby materials and deposition rate. As a rule, the lower the background pressure, the less likely for the deposited material to getter impurities from i t s ambient surrounding. Typically, background pressures in -9 -8 the present experiments were 5 to 8 x 10 Torr, rising to 1 to 8 x 10 Torr during deposition. A possible contaminant was Si which may outgas from the quartz assemblies. However, quartz was s t i l l used for want of a better material. Because the heat lamps were sealed units, impurities emanating from the filaments may be expected to be excluded from the vacuum chamber. The vacuum gaskets and el e c t r i c a l leads used were made of Cu. There is always a possibility that Cu may contaminate the source material and the deposited film. This possibility was reduced by ensuring that Cu inside the vacuum chamber was not heated and away from the CSS quartz assemblies. Source and substrate temperatures were varied between the ranges 640 to 740°C and 480 to 670°C, respectively. Their effects on the electrical and structural properties of the films w i l l be discussed in the subsequent chapters. 19 The deposition rate is related to the source and the substrate temperatures and to the temperature gradient between them. Deposition rates were not monitored in situ due to experimental d i f f i c u l t i e s caused by the close source-to-substrate spacing. Nominal average deposition rate for each run was determined by measuring the total film thickness and the. total deposition time. A typical deposition schedule is given in Table 2.1. A quartz mask was used to define the shape of the deposited film. Time Action taken and comments Pressure Temperature of Temperature (Torr) source (°C) of Substrate ( ° C ) + 15 min. 16 min. Source and substrate holders are i n i t i a l l y misaligned. Heaters are turned on for outgassing of source and substrate. Heating power adjusted u n t i l source and sub-strate temperatures are near desired deposition temperatures. Source and substrate holders are aligned. Source and substrate temperatures are set at deposition tempera-tures. Start sublima-tion. 5x10 -9 8x10 -9 8x10 -9 8x10 to 5x10 -9 -8 T , + 50 sd sd sd T ,+100 sud = T sud sud 3 hrs. Source and substrate temperatures are low-ered at the rate of about 10 degrees per minute. T , and T , are the source and sd Sud substrate temperatures, respec-t i v e l y , chosen f o r the p a r t i -c u l a r run. 3.5 hrs. Heating power turned off. Table 2.1 A typical CSS deposition schedule for GaAs films. 2.3.4 Film Thickness Determination Three methods were used in determining the thickness of the deposited films. For films less than 0.2y thick, a sloan Angstrometer 0 (accurate to about +150 A) was used. A monochromatic light source (Na yellow, o X = 5890 A) is used and Fizeau fringes of equal spacings are produced by multiple reflections -from a highly reflecting surface. The reflecting surface was o obtained by depositing a thin (about 1000 A) layer of Al. The film thickness is determined by measuring the Fizeau fringe displacement across a film-substrate step and where t = i A D 2 (2.3) t = film thickness d = fringe displacement D = fringe step X = wavelength of monochromatic light This is illustrated in Fig. 2.4. J r r 1 Fig. 2.4 Displacement of the Fizeau fringes due to a film-substrate step The other two apparatuses used were the dial.gauge and the.Talysurf. They are primarily mechanical devices. The dial gauge is accurate to +5y. The Talysurf i s , from the manufacturer's specifications, accurate to +3% f u l l scale for the ranges from 0.5 to lOOu f u l l scale. This method consists of simply measuring, with respect; to a reference plane, the amplified mechanical movement of a stylus as i t traverses a film-substrate step. Because the surface of the f i l m and the reference plane are not necessarily coplanar, a correction procedure must be used to determine the true f i l m thickness. This correction method has been described by Berry, H a l l and Harris (1968). A l i m i t a t i o n on the use of the Talysurf l i e s i n that the thicknesses of a f i l m at a distance greater than the stylus to heel distance (about 2 mm) from the film-substrate step cannot be measured accurately since the heel w i l l ride on the f i l m rather than on the substrate surface. A t y p i c a l Talysurf scan of GaAs films i s shown i n Fig. 2.5. Fig. 2.5 A t y p i c a l Talysurf scan across a GaAs-substrate step 3. STRUCTURAL PROPERTIES OF CSS GaAs FILMS ON SAPPHIRE 3.1 Introduction The GaAs films were studied using o p t i c a l and e l e c t r o n microscopy, an electron microprobe and X-ray d i f f r a c t i o n techniques. The observed s t r u c t u r a l properties of these films are discussed and compared with the published r e s u l t s on sputtered films (Molnar, Flood and Francombe 1964, Evans and Noreika 1966), f l a s h evaporated films (Muller 1964, Richards 1966), vapour phase e p i t a x i a l films (Joyce and M u l l i n 1966; T i e t j e n , Abrahams, Dreeben and Gossenberger 1968) and films produced by the "three temperature" method (Davey and Pankey 1964, 1966, 1968). 3.2 O p t i c a l Microscope Observations M e t a l l i c droplets were observed at the edge of a 600°C f i l m when the source-to-substrate distance was greater than about 1 mm. This i s shown i n F i g . 3. Their appearance suggests a lack of As at the substrate surface due probably to As vapour escaping from the region bounded by the source and substrate. The As loss l e f t behind elemental Ga. Free Ga i s a metal which may e x i s t in the l i q u i d state near room-temperature (melting point = 29.78°C). I t adheres to the sapphire surface extremely w e l l . Molnar, Flood and Francombe (1964) noticed that a mixture of Ga and p o l y c r y s t a l l i n e GaAs was present on the surface of t h e i r sputtered films when substrate temperatures were higher than 580°C. This was a r e s u l t of As loss at the films due to re-evaporation. To reduce the loss of As vapour from the source-substrate region i n the CSS system, a source-to-substrate spacing of less than 0.7 mm was used i n a l l subsequent runs. No evidence of l i q u i d - l i k e , rounded droplets was present i n the films produced under the stated experimental conditions (see Sec. 2.3.3). Joyce and M u l l i n (1966) reported that growth features, which look l i k e "pyramids", were observed on homoepitaxial layers of GaAs. From electron 23 Fig. 3.1 Droplets of liquid Ga on a GaAs film. microprobe studies, they found that the "pyramids" formed at the sites of free Ga droplets. Similar growth features were not observed in the CSS films. This would suggest that Ga droplets sufficiently large enough to i n i t i a t e the formation of growth "pyramids" were not present at the substrate surface of the CSS films. Electron microprobe studies confirmed this observation (see Sec. 3.3.2). Films obtained with substrate temperatures in the range from about 600 to 670°C were shiny and metallic grey in colour. Dull grey films were obtained when substrate temperatures were in the range from 480 to about 580°C. The "shiny" films were composed of relatively large crystallites which were visible under low (x30) magnification. Faceting was well developed. The Due to the close spacing between the source and substrate, the actual surface temperature of the substrate depends on the intensity of radiation of the source heater. No sharp demarcation substrate temperature was found above which "shiny" films were produced and under which "du l l " films were pro-duced. The stated temperatures can only be taken as apparent rather than actual temperatures at the substrate surface and at the source material. 24 shininess of these films may be due to the high r e f l e c t i v i t y of the facet faces. No surface features were d i s c e r n i b l e i n the " d u l l " films under the same magnification. The surfaces of the " d u l l " films appeared to be smoother than the surfaces of the "shiny" f i l m s . I t was quite evident that the p a r t i c l e s i z e of the "shiny" films was l a r g e r than the p a r t i c l e s i z e of the " d u l l " • f i l m s . This i s expected because the p a r t i c l e s i z e of most films condensed from the vapour phase increases with increasing substrate temperature which increases the surface mobility of adatoms and c l u s t e r s during deposition. Microphotographs of both as-grown and chemically etched surfaces were, taken using a Reichert m e t a l l u r g i c a l microscope. Not many surface features were seen on a 580°C f i l m when magnified 1420 times. However, faceting was developed and was seen on films grown at substrate temperatures higher than 600°C. These are shown i n F i g . 3.2. The c r y s t a l l i t e s were quite uniform i n each"CSS f i l m . This would suggest that the temperature was f a i r l y uniform --across the substrate surface where the GaAs was„deposited during each sub-limation run. The grain s i z e of the 580°C f i l m was estimated to be about 0.4 to 2u. The 600°C f i l m had grains about twice as large. Twinning i n some of the l a r g e r c r y s t a l l i t e s was evidenced by the presence of p a r a l l e l l i n e s within the grains. This i s i l l u s t r a t e d i n F i g . 3.3, which i s a photograph of an etched 670°C f i l m . The as-grown f i l m was mechanically polished to a 0.5u f i n i s h and then etched for about 15 sec. i n a f r e s h l y prepared g r a i n -boundary-revealing etch . The composition of the etchant was SH^OiH^O^ (30%): H 2S0^ (95%) by volume (Cunnell, Edmond and Harding 1960). The p a r a l l e l edges of the twin bands i n d i c a t e that these were growth twins which arose probably due to stacking f a u l t s i n the f i l m during growth. A 630°C f i l m which manifested numerous~"120°" growth features i s shown i n F i g . 3.4. This structure appears s i m i l a r to the top view of an 580°C (#2) 600°C (#4) 10M Fig. 3.2 Optical photomicrographs of GaAs films deposited at different substrate temperatures. 670°C (#42) j | lOy Fig. 3.3 Optical photomicrograph of an etched film. 26 e q u i l a t e r a l tetrahedron which has i t s basal plane p a r a l l e l to the plane of the photograph. Three l i g h t e r coloured ridges i n t e r s e c t at the apex and each two adjacent ones subtend a 120° angle. These growth features appear to possess a three-fold symmetry which would be consistent with the symmetry properties of the <111> d i r e c t i o n of a zincblende structure. 3.3 Electron Probe Microanalysis 3.3.1 The Electron Microprobe The electron microprobe allows the X-ray spectrochemical analysis of small samples. (See, for example, McKinley, Heinreich and Wittry 1966; Castaing, Deschamps and P h i l i b e r t 1965). Colby (1968) had analyzed and determined o the stoichiometry of thin (>350 A) d i e l e c t r i c films such as the oxides and n i t r i d e s of S i , A l and Ta. Kyser and Wittry (1966) studied the cathodoluminescence of GaAs using the electron microprobe. Lubkin and Sutkowski (1966) determined the composition of e p i t a x i a l layers of AlAs-GaAs. Joyce and M u l l i n (1966) used the electron microprobe to confirm the presence of Ga droplets at growth "pyramids" 2 on vapour phase e p i t a x i a l layers of GaAs. A description of the. electron microprobe was given by Birks (1963). The electron microprobe consists e s s e n t i a l l y of three systems. The f i r s t i s an electron optics system, similar to that of an electron microscope, which focuses a beam of electrons (about 0.1 to 0.3p i n diameter) onto the surface of the sample. Due to the impinging electrons, the emission of X-rays and other physical processes occur. These processes, together with the methods of obtaining information from the specimen are depicted p i c t o r i a l l y i n Fig. 3.5. The second i s an X-ray optics system which i s used to analyse the X-rays emitted by the sample. Both q u a l i t a t i v e and quantitative results can be obtained. The t h i r d i s a viewing system which allows the operator to see and to choose the sample area under test. Secondary Elec t ron Me thod Back -sca t t e red / E lec t ron M e t h o d ^ / ' - - . . c"A Q"sr ' ., C « , e > ^ M e t h o d A b s o r b e d .Electron M e t h o d E l e c t o n Method Fig. 3.5 Schematic i l l u s t r a t i o n of methods used i n obtaining information from a specimen using the electron microprobe • - • The electron microprobe was used to obtain X-ray counts and scans, and surface topographies of CSS GaAs films. 3.3.2 X-Ray Counts and Scans Under ide a l operating conditions, the detection s e n s i t i v i t y of a probe The detection s e n s i t i v i t y i s derived from s t a t i s t i c a l considerations. For s i g n i f i cant re s u l t s , 90% of the X-ray counts, for a t y p i c a l peak to background count r a t i o of 1000 to 1, must l i e within N - 3 / N and N+3/N, where N i s the number of counts. 28 i s 0.03%. The p r a c t i c a l detection s e n s i t i v i t y for elements with atomic numbers 11 to 21 and 33 to 37 i s 0.1 to 1%, and i s 0.01 to 0.1% for elements with atomic numbers 22 to 33, 38 to 74 and 82 to 92. The atomic numbers of Ga and As are 31 and 33, r e s p e c t i v e l y . The accuracy of the JX3-3A probe was placed at 1 to 2% (Dept. of Metallurgy, U.B.C.). The presence of trace impurities i n concentrations of les s than 1 part per 100 cannot be adequately detected by using the el e c t r o n probe. However, gross inhomogeneities such as impurity i n c l u s i o n s , dust p a r t i c l e s or Ga 3 droplets with volumes >_2u can be detected. CSS films were polished to a 0.5u f i n i s h and scanned. No gross . . . • • - , n 2 0 , 3 inhomogeneities and impurities m concentrations greater than 10 /cm were observed. A l l the i d e n t i f i a b l e peaks were due to Ga and As. For purposes of comparison, a s i n g l e c r y s t a l GaAs standard was scanned. Quartz and mica c r y s t a l s were used to detect the X-ray emissions of Ga and As, r e s p e c t i v e l y . An i n d i c a t i o n of the r e l a t i v e composition of Ga and As i n the CSS films can be obtained by recording t h e i r respective X-ray counts and comparing them with those obtained from a GaAs standard. Correction factors due to matrix absorption, fluorescence and atomic number cor r e c t i o n were assumed to be the same for both the CSS fil m s and the GaAs standard. Correction due to the "dead time", tp, of the proportional flow counters was taken i n t o account by applying C = C /(1-C t ) (3.1) true meas meas D where c = recorded counts per second meas t = dead time of counter (4y sec.) c = true counts true * The accuracy of the probe i s defined as I(concentration of element A) -(concentration of element A) I '. actual meas' , o r, %accuracy = — — — — x 100 (concentration of element A) , actu a l Assume that I_, , I. and 1° , 1° are corrected (for dead time) Ga As Ga As average number of Ga and As counts of the f i l m and standard, r e s p e c t i v e l y . A number, r, can be defined as r = (I_ 11. )/(I° IV ) (3.2) Ga As Ga As where r = 1 implies that Ga and As e x i s t i n a stoichiometric r a t i o i n the f i l m ; r < 1 implies that the f i l m i s Ga d e f i c i e n t ; and r > 1 implies that the f i l m i s As d e f i c i e n t . One of the reasons for defining r as above i s that i t i s not necessary to know the e f f e c t s of matrix absorption, .fluorescence and atomic number of the elements on the X-ray counts. Their e f f e c t s a l l cancel out. The bounds on the values of r f o r the d i f f e r e n t cases, however, must be modified to take into account equipment and operator e r r o r s . As was mentioned, the accuracy of the probe i s about 2%. Apart from t h i s , X-ray counts depend on surface f l a t n e s s A 1% change i n f l a t n e s s r e s u l t s i n a 3% change i n the calculated weight percent ( B r o x i m , Dept. of Metallurgy, U . B . C . ) . Errors due to surface conditions can be reduced by using adequate preparation techniques and by taking the average of several consecutive counts at each area over several areas on the same sample. Errors due to i n s t a b i l i t i e s i n the probe i s reduced by counting the standard before and a f t e r probing a f i l m . Within the l i m i t s of the equipment, the modified bounds on r are 1 + 0.02, r < 0.98, r > 1.02 f o r the cases of stoichiometry, Ga d e f i c i e n t and As d e f i c i e n t , r e s p e c t i v e l y . CSS GaAs films were examined, taking i n t o account the precautions mentioned. A l l the films were found to be stoichiometric within the l i m i t s imposed.by t h i s method. * Apart from e l e c t r i c a l measurements, a standard method of determining the presence of trace impurities i n semiconductors i s emission spectroscopy. The stoichiometry of most compounds can be determined by q u a n t i t a t i v e chemical analysis (Kroger 1964). These methods e n t a i l the destruction of the specimen under t e s t . 30 3.3.3. Surface Topography The surface topography of films grown with substrates held at d i f f e r e n t temperatures i s shown i n F i g . 3.6. The electron microprobe topographs revealed more surface features than the o p t i c a l photomicrographs i n Sec. 3.1. The "120°" growth features discussed i n conjunction with F i g . 3.4 are apparent i n F i g . 3 . 6 c P a r t i c l e s i z e of the films as a function of substrate temperature was estimated and tabulated i n Table 3.1. Sample Substrate Temperature (°C) P a r t i c l e s i z e (u) Obtained from 41 480 0.3 to 1 electron probe topograph 40 580 1 to 2.5. electron probe topograph 2 580 0.4 to 2 o p t i c a l photomicrograph 4 600 1.5 to 4 o p t i c a l photomicrograph 12 630 3 to 8 o p t i c a l photomicrograph 39 650 3 to 10 electron probe topograph 42 670 16 to 20 electron probe topograph Table 3.1 P a r t i c l e s i z e as a function of substrate temperature 3.4 R e f l e c t i o n Electron D i f f r a c t i o n Observations R e f l e c t i o n electron d i f f r a c t i o n (RED) photographs of CSS films were taken using a H i t a c h i HU-11B ele c t r o n microscope. The camera constant, XL, o o was determined to be 2.3198 A-cm at 50 kV and 1.6035 A-cm at 100 kV. An o evaporated gold f i l m was used as the standard. The degree of preferred o r i e n t a t i o n of c r y s t a l l i t e s i n the films increased with increasing substrate temperature. Arcing i n the Debye rings of a. 480°C (#41) b. 580°C (#40) c. 650°C (#39) d. 670°C (#42) Electron microprobe topographs of films deposited at different sub strate temperatures. a. 580°C (#5) b. 600°C (#4) c. 610°C (#15) d. 620°C (#3 ) Fig. 3.7 Reflection electron diffraction patterns of GaAs films deposited at different substrate temperatures. ho 33 films deposited at substrate temperatures higher than 600°C indicated that these films were preferentially oriented. Single crystal diffraction patterns were observed on 630 and 640°C films. Fig. 3.7 shows the RED patterns of CSS films deposited at different substrate temperatures. For purposes of comparison, line drawings of the RED patterns of fee (face centred cubic) and hep (hexa-gonal close pack) GaAs and free Ga are reproduced in Fig. 3.8. GaAs CUBIC GaAs HEXAGONAL" GaAs ' CUBIC FREE 222 / 1200 3 l i 2 2 0 •o.ol"°^gT3 Fig. 3.8 Line drawings of RED patterns of fee and hep GaAs and free Ga (from Pankey and Davey 1966). Debye rings found in the RED patterns were indexed using the relationship (Hall 1966) XL = r d hk£ (3.3) where = radius of Debye rin£ ^hk£ = ^ spacing of the diffracting plane (hk£). The d values for most of the lower order {hk£} planes are tabulated in ASTM standards. Alternatively, the d value for a specific (hk£) plane of a cubic crystal can be calculated from 2 2 2 2 ? d = a /(h + k + I) (3.4) o a = l a t t i c e constant of the unit c e l l . (For GaAs, a = 5.65 A at room temperature). Results of the indexed films are tabulated in Table 3.2. Since GaAs has a zincblende structure, the diffraction spots were indexed by proceeding as follows (Howie 1965,. Alderson and Halliday 1965) 34 Sample Substrate Ring number d spacing temperature(°C) 580 1 111 2 (200) 3 220 4 311 5 4 0 0 600 1 111 2 220 3 311 15 610 1 111 2 220 3 311 4 331 5 422 620 1 I H 2 220 3 311 4 (222) 5 400 * Rings are numbered consecutively beginning with the f i r s t v i s i b l e r i n g nearest the beam spot. Table 3.2 The d-spacing of Debye rings Three noncollinear spots, i n c l u d i n g the centre (baam) spot, were chosen. The lengths of the vectors g^ and g^, from the centre spot to spots 1 and 2, were measured. The d values were calculated using the r e l a t i o n s h i p XL = d|g|, where XL i s known. A t o t a l of 48 combinations of (h^ k^ l^) and (h^ k^ l^} a r e possible. The correct sets were obtained by comparing the measured and the calculated planar angle fy^' cos cf>^2 81* 82 A g - L ^ ) (g2-g2) h l h 2 + k l k 2 + hh ? ? ? ? ? ? / ( h l + k l + V ( h 2 + k2 + V (3.5) 35 Having determined (h^ l^) and (h^ k^ I ), the normal to the cross-grating n = [u, v, w] can be obtained from g*n = h u + k v + I w = 0 (3.6) g^n = h 2u + k ?v + £ w = 0 where n, g , g 2 form a right-handed set of axes and u = k ^ 2 - V l * mhS-h\ (3.7) w = h xk 2 - h ^ The RED pattern of a 630°C film is shown in Fig. 3.9. The dominant spots were indexed and are shown in the accompanying schematic drawing. The zone axis was [0 1 1]. This is one of the prominent cross-grating patterns of an fee structure. (#12) Fig. 3.9 Single-crystal RED pattern of a 630°C film. 0 : [on] / a m 2 : (022) 3 :(Jt1) 4 :(3tt) 5 .(222) 6 : (222) Kikuchi bands, which indicate that the film has a high order of angular perfection, were observed in the RED pattern of a 640°C film (see Fig. 3.10). A hexagonal modification of GaAs was reported by Muller (1964) and by Davey and Pankey (1966, 1968). Precipitation of free Ga on A120^ was also reported by the latter authors. Extra diffraction lines which can only be indexed as due to free Ga were observed by them. Examination of the RED patterns of the CSS films did not reveal any lines other than those which belong to and were indexed as zincblende GaAs. Since wurtzite GaAs is metastable, i t s absence in the CSS 36 (#14) Fig. 3.10 Kikuchi bands i n a 640°C f i l m . films may be due to the following reasons. The substrate temperatures used were r e l a t i v e l y high (480 to 670°C) compared to those used by Muller (410 to 500°C) and Davey and Pankey (450 to 500°C). The higher substrate temperatures result in higher surface mobility of adatoms thus preventing the formation of the meta-stable structure. Because of the small temperature gradient between the source and substrate, no sharp quenching occurs at the substrate surface. Quenching may occur i f the deposition rate i s s u f f i c i e n t l y fast. This further minimizes the formation of the metastable wurtzite phase. Martinuzzi (1966) found ref l e c t i o n s due to excess Ga i n a l l the films he deposited at substrate temperatures above 470°C. Laverko, Marakhonov and Polyakov (1966) found that GaAs whiskers grown i n an excess of Ga exhibited RED rings due to pol y c r y s t a l l i n e wurtzite GaAs. On the basis of these and their own observation that p o l y c r y s t a l l i n e films deposited above 450°C exhibited extra RED lines due to free Ga and that GaP films and whiskers grown under excess Ga exhibited the wurtzite modification, Davey and Pankey stated that the existence of the wurtzite modification may be causally related to a non-equilibrium structure associated with an excess of Ga either i n the deposition beam and/or due to special conditions at the substrate. As an 37 example of the latter case, free Ga may nucleate preferentially at scratches on the substrate surface. In the case of the CSS films, near-equilibrium conditions established in the region bounded by the source and substrate due. to their close spacing may reduce the loss of As, resulting in less free Ga. Thus, the absence of the wurtzite structure in CSS films is consistent with electron microprobe and (see later) X-ray results that no free Ga was to be found in the CSS films. 3.5 X-Ray Diffraction Analysis Information obtained from standard RED techniques i s limited by the fact that only the surface layers of the films are characterized. Differences in geometrical arrangements between the RED and X-ray apparatuses and longer penetration depths of X-rays enable X-ray measurements to present a picture of the structure of the underlayers of the films. 3.5.1 Diffractometer Measurements Diffractometer scans of the films indicated that the strength of the <111> texture increased with increasing substrate temperature. A l l the identifiable peaks were Indexed to be those of zincblende GaAs. If a wurtzite modification is present, a weak (10 10) peak should appear at 29 =25.75° (Pankey and Davey 1966). Because of i t s weak intensity, this peak may be swamped by the background noise and i t s presence undetected when diffraction scans are taken at high speeds. Consequently, low speed (0.25°/min) scans were taken in the 29 range between 24.5° and.27°. No (1 0 1 0) peaks were seen. Since the (10 10) reflection was absent, higher order reflections would probably be also absent. This was true for these films. The ratio, I , of the intensities of the (111) and (220) reflections of r a powdered GaAs sample is 1.1 (ASTM standards). For the CSS films, I ranged from 2.35 to i n f i n i t y (i.e. 220 peak absent). I may be taken as an index of the extent of texturing i n the films. In contrast with sputtered and "three tempera-ture" films, which exhibited either <110> or <111> textures, depending upon 38 deposition conditions, only <111> textures were observed i n the CSS films produced under the present experimental conditions. From the appearance of the GaAs growth form (Wolff 1956) and from c a l c u l a t i o n s for the diamond structure (Stranski and Kaischew 1931), Molnar, Flood and Francombe determined that the GaAs (111) plane has the lowest s p e c i f i c surface energy. Occurrence of the <111> o r i e n t a -ti o n may be explained by the theory of Kaischew and Bliznakow (1948) that c r y s t a l s growing on a s t r u c t u r e l e s s base must present one of t h e i r equilibrium surfaces to the substrate. The c r y s t a l must be able to grow by two-dimensional nucleation i n a plane p a r a l l e l to the substrate. The i n i t i a l l a yer a r i s e s as two-dimensional n u c l e i of the equilibrium face. Subsequent c r y s t a l growth w i l l rest on such a face. Since the order of preference of the equilibrium faces does not appear to be temperature dependent, i t i s supposed that GaAs c r y s t a l l i t e s nucleate on i n a c t i v e substrates {111} planes p a r a l l e l to the surface. However, surface adsorption at the substrate may modify the order of importance of the equilibrium faces. Since the (110) face i s also an e q u i l i b r i u m face of GaAs, the e f f e c t of adsorption may be s u f f i c i e n t to produce <110> o r i e n t a t i o n s at lower substrate -9 temperatures. The background pressure used i n the CSS method was 5x10 Torr as compared with 10 ^ and 10 7 Torr for the sputtering (Molnar, Flood and Francombe 1964) and "three temperature" (Davey and Pankey 1968) methods, r e s p e c t i v e l y 4 The absence of <110> orientations i n the CSS films may i n d i c a t e that the substrates used i n t h i s work were "cleaner" than those used i n sputtered and "three tempera-ture" f i l m s . The p a r t i c l e s i z e , t, can be estimated by measuring the width, B (in radians), at -^ I of the most intense peak of a diffractometer scan and applying 2 max the Scherrer formula ( C u l l i t y 1959) t = 0.9 X/(B cos GO (3.8) * Another method that i s commonly used i s a Fourier-Stokes type analysis (Warren and Averbach 1950). 39 o where X i s the wavelength of the incident X-ray. For a Cu target, i t i s 1.54 A. The corresponding Bragg angle i s Q . The p a r t i c l e sizes were estimated to range o o • from about 250 A to 770 A for substrate temperatures i n the range 480 to 600°C. Under normal operating conditions, no incident beam i s p e r f e c t l y p a r a l l e l or mono-chromatic. These deviations from i d e a l i t y and the mosaic structure of the films render the Scherrer formula, at best, a rough, estimate of the true p a r t i c l e s i z e s . A better estimate can be obtained from o p t i c a l photo-micrographs or electron microprobe topographs. This was done i n Sec. 3.3. 3.5.2 Back R e f l e c t i o n Laue The Bragg angle, 0 , of a Debye r i n g i n back r e f l e c t i o n Laue pinhole B photographs i s given by ( C u l l i t y 1959) Tan( w - 2© B) = ~ (3.9) where V = diameter of the Debye r i n g D = specimen to f i l m distance (3.0 cm.) .Because of the p o s i t i o n of the photographic p l a t e with respect to the sample i n the back r e f l e c t i o n Laue method, only high angle r e f l e c t i o n s are recorded. This l i m i t s i t s usefulness i n the analysis of texturing i n the GaAs fi l m s . Fo^ poly-c r y s t a l l i n e samples, back r e f l e c t i o n Laue pinhole photographs can give q u a l i t a t i v e information regarding the r e l a t i v e "degree of c r y s t a l l i n i t y " of the f i l m s . The more c r y s t a l l i n e the f i l m s , the les s d i f f u s e the Debye rings. Arcing of the Debye rings implies preferred o r i e n t a t i o n i n the specimen. As the c r y s t a l l i t e s increase i n s i z e , the Debye rings become more "grainy". F i g . 3.11 show Debye rings of films deposited at d i f f e r e n t conditions. As expected, the Debye rings of lower substrate temperature (480 to 580°C) films were d i f f u s e . "Grainy" Debye rings were observed i n a 650°C f i l m while arced rings were observed i n a 660°C f i l m . A 630°C and a 640°C f i l m exhibited s i n g l e - c r y s t a l Laue patterns having most of the prominent spots consistent with those of the cross-grating of [111] 480°C (#41) 580°C (#40) Fig. 3 . i i a Back r e f l e c t i o n Laue d i f f r a c t i o n patterns of GaAs films. 650°C (#39) 660°C (#24) F i g . 3.11b Back r e f l e c t i o n Laue d i f f r a c t i o n p a t t e r n s of GaAs f i l m s . 4 2 GaAs perpendicular to the substrate surface. These were compared with a [111] p r o j e c t i o n for GaAs adapted from Manasevit (1968) and are shown i n Figs. 3.12a and 3.12b.. 3.6 Summary and Discussion Results from RED X-ray analysis indicated that texturing i n the CSS GaAs films increased with increasing substrate temperature. <111> textures were observed i n films deposited on substrates held at temperatures above 600°C. S i n g l e - c r y s t a l Laue patterns, with most of the prominent spots consistent with the pattern of a (111) GaAs plane p a r a l l e l to the substrate, were observed on 630 to 640°C f i l m s . In contrast with GaAs films deposited using other.vacuum techniques, only <111> textures were observed i n the CSS GaAs fi l m s . An explana-t i o n f o r the occurrence of <111> textures i s that i t i s e n e r g e t i c a l l y most favourable for {111} GaAs planes to form on a substrate since these planes have the lowest s p e c i f i c surface energy. Because of the constraints imposed by the present CSS. apparatus, the deposition rate cannot be varied independently nor monitored e a s i l y . For t h i s reason, substrate temperature was the main parameter used to i d e n t i f y the d i f -ferent f i l m s . C r y s t a l l i t e s ranged from 0.3 to 20p wide. Larger c r y s t a l l i t e s were obtained with higher substrate temperatures. This i s because higher substrate temperature lends i t s e l f to providing a c t i v a t i o n energy for adatoms to occupy postions of p o t e n t i a l minima;.enhancing r e c r y s t a l l i z a t i o n due to the coalescence of islands by increasing surface and volume d i f f u s i o n ; and lowering supersaturation which allows the adatoms s u f f i c i e n t time to reach equilibrium p o s i t i o n s (Chopra 1969). Higher substrate temperature also aids i n the desorp-ti o n of adsorbed surface contaminants. A l l CSS films were deposited on substrates which had been outgassed i n s i t u by r a i s i n g the temperature about 100°C higher than the deposition temperature. The abnormal wurtzite phase was not observed i n the CSS f i l m s . Excess 640°C (#21) Fig . 3.12a S i n g l e - c r y s t a l 313 ti213 312*. 21Uo 535xo *335 x ° »123 533 (g> o "331 °*133 *132 9 231 121 o FROM SAMPLE * FROM MANASEVIT (1968) d i f f r a c t i o n pattern of a 640°C f i l m . 45 Ga was not found when the source-to-substrate distance was less than about 0.7 mm. No excess Ga, due to re-evaporation of As from the deposit, was observed i n a l l the.CSS film's. The highest substrate temperature used was 670°C. In comparison, Richards (1966) reported loss of As i n flash-evaporated films at substrate temperatures above 535°C while Molnar, Flood and Francombe (1964) reported that As re-evaporated from sputtered films at substrate temperatures above 580°C. The formation of the abnormal, metastable wurtzite structure has been observed i n vacuum deposited GaAs films (Muller 1964, Davey and Pankey 1966, Farukhi and Charlson 1969). Nonstdichiometry, due to excess Ga, can cause the formation of the metastable wurtzite phase i n GaAs (Davey and Pankey 196.8, Chopra 1969). It would appear that the non-existence of the wurtzite phase and the undetected presence of excess Ga i n the CSS films indicated that these films were s t o i c h i o m e t r i c a l l y better than the films produced by other vacuum techniques. 46 4. ELECTRICAL PROPERTIES OF CSS GaAs FILMS 4.1 Introduction In t h i s chapter, a b r i e f review i s f i r s t given on the theory of c a r r i e r transport c o e f f i c i e n t s of bulk semiconductors based on the Boltzmann transport equation. The e l e c t r i c a l properties and the e f f e c t s ' o f d i f f e r e n t s c a t t e r i n g mechanisms on the c a r r i e r m o b i l i t y i n bulk GaAs are discussed. The e l e c t r i c a l properties of CSS GaAs films are then discussed i n terms of the important s c a t t e r i n g mechanisms i n GaAs, the f i l m s ' p o l y c r y s t a l l i n e s t r u c t u r e , compensation of i m p u r i t i e s a n d of deviation from stoichiometry. T h i n - f i l m insulated-gate f i e l d - e f f e c t t r a n s i s t o r s are fa b r i c a t e d on the as-grown f i l m s . Conversion of the as-grown p-type films to n-type i s considered. Au-Schottky b a r r i e r diodes are made on a converted f i l m . The reasons f o r studying GaAs films on sapphire were given i n Sec. 2.2. In order to f i n d out the e f f e c t s of using a d i f f e r e n t substrate,-a CSS GaAs f i l m on semi-insulating GaAs was grown and i t s e l e c t r i c a l properties studied. 4.2 C a r r i e r Transport Theory: A B r i e f Review The Boltzmann Transport Equation (BTE) i s solved for the case of n-type c a r r i e r s i n homogeneous semiconductors. Under c e r t a i n r e s t r i c t i v e assumptions, expressions f o r the one-dimensional isothermal e l e c t r i c a l c onductivity, isothermal H a l l c o e f f i c i e n t and H a l l m o b i l i t y are (see Appendix 4.1) .T _ 4 n q 2 a ( k T ) T « Y+3/2 - n . a = — ^ r j j — / q n' e dp (4.1) 3TT m 3 T r 1 / 2 . 1 r< 2T + f> *H 4 nq r 2 ( Y + | ) ( 4- 2) 47 . t H2y +|) y, = \ - «(kT) Y (4.3) m" r(y+{) A major assumption i s that the c a r r i e r s are e l a s t i c a l l y scattered and the rate of change of the d i s t r i b u t i o n function due to s c a t t e r i n g i s given by . f - f scat Y where the re l a x a t i o n time, x, has the form otE . In the case where more than one s c a t t e r i n g mechanism are operative, t h e i r e f f e c t s may be taken into account by using an e f f e c t i v e r e l a x a t i o n time given by - = Z — (4.5) T ' . T. X 1 The corresponding e f f e c t i v e m o b i l i t y i s 1 E l y i v± (4.6) For the s p e c i a l case where a mean free path, £, can be defined indepen-dent of energy ^ H = T v (4.7) -1/2 x = aE 1 (4.8) Equation 4.1 reduces to the same expression as obtained from the Drude-Lorentz theory and i s cr - nq u d r . f t (4.9) where 4 q a. ' 3 ( 2 7 Im*kT) 1 / 2 The corresponding H a l l c o e f f i c i e n t and H a l l mobility are given by rl • 8 nq nq d i I _ 3TT H 1 ~ 8 " d r i f t P w = | R 7 I | a = ~ u ^ . ^ ( 4 . H ) 48 Equations 4.1, 4.2, 4.3 and 4.9, 4.10, 4.11 are also applicable to p-type semi-conductors when the appropriate expressions of T and m are used. 4.3 E l e c t r i c a l Properties of Bulk GaAs The assumption x = aE^ i s v a l i d only i f the change i n the energy of the scattered c a r r i e r i s small compared to i t s i n i t i a l energy. Howarth and Sondheimer (1953) showed that i n the case of polar semiconductors such as GaAs, no r e l a x a t i o n time can be assumed i f the energy absorbed or emitted by an e l e c t r o n i s not small when compared with i t s i n i t i a l energy. Since c a r r i e r s i n very 15 -3 pure ( c a r r i e r concentration less than 10 cm ) GaAs are strongly scattered by o p t i c a l phonons, no re l a x a t i o n time can be assumed for the o p t i c a l phonon s c a t t e r i n g process except at high f i e l d s and/or at temperatures higher than the Debye temperature 0 ^ (Conwell 1967). The Debye temperature i s defined by d = k where co^  i s the l o n g i t u d i n a l fundamental o p t i c a l frequency of the l a t t i c e . Equation 4.5 i s also not applicable. The m o b i l i t y - l i m i t i n g mechanism for both electron and holes i n pure GaAs i s polar o p t i c a l s c a t t e r i n g . At room temperature, 2 the c alculated electron and hole m o b i l i t y are 10,400 and 520 cm /V-sec, respec-t i v e l y (Hilsum 1966). 16 —3 For r e l a t i v e l y pure n-type samples, (n= 2.2 x 10 cm ), Ehrenreich (1960) showed that a combination of polar o p t i c a l and io n i z e d impurity s c a t t e r i n g l i m i t s the mobility of electrons. The graph of electron m o b i l i ty versus temperature i s given i n F i g . 4.1. The m o b i l i t y can be approximated by y ~ ( m * ) " 3 / 2 T 1 / 2 (4.12) Rosi, Meyerhofer and Jensen (1960) analyzed the dependence of hole 1 6 — 3 mobility on temperature i n pure (p ~ 4 x 10 cm ) GaAs. A good f i t of t h e i r experimental data was obtained when the m o b i l i t y due to o p t i c a l phonons was 4 9 Calculated curves: (1) deformation potential scattering; (2) screened polar scattering; (3) combined polar and charged impurity scattering (n^ . = 2.2 x 10 cm ); (4) combined polar, charged impurity, and deformation potential scat-tering, including.effects of non-parabolic conduction band. T I 'K) Fig. 4.1 Electron mobility in GaAs versus temperature (from Ehrenreich 1960), given by % t = 4 3 5 ( 2 f 5 ) _ 2 ' 3 c m 2 / V - s e c C4.13) Little information is available on p-type GaAs while extensive investigation on n-type GaAs has been and is being conducted. The main reason for focussing attention on n-type GaAs is that electrons are much more mobile than holes in GaAs. This makes n-type GaAs attractive as a starting material for devices. For impure samples* of either conductivity type, the dominant scattering mechanism is ionized impurity scattering. The mobility due to this process was given by Brooks (1955) as 5 0 7 _3 _3 _1 ,2" ••!„ m i 2 - 3 , * 2 . .* 2 i rq f i n(2 - — ) D An equivalent expression of the mobility u_^, which i s more convenient for l a t e r use, was given by Hilsum and Rose-Innes (196 1) as 1 3 = 3 . 2 x l 0 1 5 ( m n / m * ) 2 e 2 T 2 r • 1 . 3 x l 0 1 4 T 2 F , ( m 7 m o ) -1 y i N + N 1 & i n > J " -"-^  / A D where n i s the c a r r i e r concentration, £ the d i e l e c t r i c constant, N„ and N, are D A the concentration of ionized donors and acceptors and m i s the ef f e c t i v e mass. 3 The concentrations are per cm . The ef f e c t i v e mass of electrons, l i g h t and heavy holes of GaAs at 300°K i s 0.068 m , 0.12m and 0.5m , respectively o o o (Sze 1969). The mass of an electron i s m . o The r e s i s t i v i t y and the H a l l mobility of carriers i n p-type and n-type single c r y s t a l GaAs as functions of impurity concentration at 300°K were given by Sze and I r v i n (1968). They analyzed the most accurate and up-to-date exper-imental data using a least-square method. Their graphs are reproduced i n Fig. 4.2. A +25% scatter i n the H a l l m o b i l i t i e s was quoted. I t can be seen that the mo b i l i t i e s decrease with increasing impurity content i n the semiconductor, as predicted i n eqn. 4.15. Acoustic mode in t r a v a l l e y scattering i s r e l a t i v e l y unimportant i n GaAs. Other scattering mechanisms l i k e electron-hole, electron-electron and neutral scattering usually can also be neglected (Conwell 1967). The dependence of the d r i f t mobility on m* and T, the energy dependence of x and the value of y f° r the different scattering mechanisms which can be found i n GaAs are shown i n Table 4.1. F i g . 4.2 R e s i s t i v i t y and H a l l m o b i l i t y of GaAs at 300°K versus impurity concentra-tion (after Sze and'Irvin 1968). Scattering Mechanism d r i f t m o b i l i t y p<*(m*)XTy R j = - r / n q Remarks Reference X y Y r o p t i c a l phonon 3 2 exponential independent 1.00 to 1.14 r i s temperature dependent = —^— = Debye temperature Hilsum and Rose-Innes (1961) Putley (1960) T > 0 D 3 2 1 2 1 2 for GaAs, © D = 418°K ionized impurity 1 2 3 2 , 3 2 315 TT 512 Hilsum and Rose-Innes (1961) combined polar and ionized impurity . - c - f ) _ 1 2 Ehrenreich (1960) Sze (1969) acoustic phonon 5 2 3 2 1 2 3TT 8 These s c a t t e r i n g mechan-isms are usually less impor-tant i n GaAs. Hilsum and Rose-Innes (1961) neu t r a l impurity 1 independent independent 1 electron-hole 1 2 3 2 - 3 2 315TT 512 Table 4.1 Scattering mechanisms i n GaAs 5 3 4 . 4 M o d i f i c a t i o n o f E l e c t r i c a l P r o p e r t i e s i n F i l m s 4 . 4 . 1 E f f e c t o f S a m p l e G e o m e t r y O n e o f t h e a s s u m p t i o n s u s e d i n t h e d e r i v a t i o n o f t h e t r a n s p o r t c o e f -f i c i e n t s i s t h a t t h e e l e c t r i c a l c o n t a c t s t o t h e s a m p l e a r e s u f f i c i e n t l y f a r a w a y s o t h a t t h e i r e f f e c t s o n t h e f i e l d s i n t h e r e g i o n o f i n t e r e s t a r e n e g l i -g i b l e . . I n p r a c t i c a l s y s t e m s , h o w e v e r , c o n t a c t e f f e c t s m u s t b e c o n s i d e r e d . F o r a s t a n d a r d H a l l b a r c o n f i g u r a t i o n , I s e n b e r g , R u s s e l l a n d G r e e n e ( 1 9 4 8 ) c a l c u l a t e d t h e e f f e c t s o f c u r r e n t e l e c t r o d e s h o r t i n g o n t h e H a l l c o e f f i c i e n t . I t c a n b e s e e n f r o m F i g . 4 . 3 t h a t i n o r d e r t o e l i m i n a t e t h i s s h o r t i n g e f f e c t , t h e s a m p l e l e n g t h - t o - w i d t h r a t i o m u s t b e g r e a t e r t h a n 3 . 7.0-Q: X io -Q O Q: F i g . 4 , 3 S h o r t i n g e f f e c t o f l a r g e - a r e a c u r r e n t c o n t a c t s o n H a l l c o e f f i c i e n t ( a f t e r I s e n b e r g , R u s s e l l a n d G r e e n e 1 9 4 8 ) . F o r a r b i t r a r i l y s h a p e d f l a t s a m p l e s w h o s e s u r f a c e i s s i n g l y c o n n e c t e d , v a n d e r P a u w ( 1 9 5 8 ) s h o w e d t h a t t h e s a m p l e e l e c t r i c a l r e s i s t i v i t y a n d H a l l m o b i l i t y a r e g i v e n b y i r d 1 2_ £ n 2 2 R l f ( R }  R 2 ( 4 . 1 6 ) d A R B p ( 4 . 1 7 ) 54 provided that the ele c t r i c a l contacts are sufficiently small and are at the circumference of the sample. The resistance R is the ra.tio of the voltage V^  which appears across contacts 4 and 3 when a current I is passed through contacts 1 and 2 (see Fig. 4.4). Similarly, the resistance R^ is the ratio of the voltage V across contacts 1 and 4 when a current I i s passed through contacts . R± 2 and 3. The function fGjr -) was given in graphical form by van der Pauw. The 2 change in resistance AR, is the ratio of AV to I. The extra voltage, Av, which appears across contact 2 and 4, i s due to a magnetic f i e l d B perpendicular to the plane of the sample when a current I is passed between contacts 1 and 3. .4 2 Fig. 4.4 Schematic diagram of a van der Pauw sample. If the contacts are at the circumference of the sample but are of fin i t e size with width w and i f the diameter of the sample is D, then the relative errors introduced are given by van der Pauw as p 16 D £n 2 • —- Z - - (4.19) V u2D 55 4.4.2 Effects of Surface Space Charge Layers and Surface Scattering The BTE considered in Appendix. 4.1 is for a medium of i n f i n i t e extent. No boundary conditions are imposed on f(k~, r ) . In the case of thin films, however, surface space, charge layers can arise due to surface states. Tamm and Shockley states can act to trap or to repel free carriers; so do impurities adsorbed at the surface. Hence charge neutrality at the surface is destroyed. The space charge region is of the order of Debye length. The effective Debye length of a semiconductor is given by JL e e kT 2 L = [ \ ° ] (4.20) q (nb+pb) If eqn. 4.6 holds, then for the two cases (1) thick films in the f l a t band approximation and (2) sufficiently thin films such that d/L << 1, the average mobility is given by M = — ^ - T (4.21) where is the mobility of carriers in the bulk and d is the film thickness (Many, Goldstein and Grover 1965). In the more general cases where accumulation or depletion layers can exist, Greene, Erankl and Zemel (1960) gave a complete formulation of the effective carrier mobility in surface space charge layers within the framework of Schrieffer's (1955) assumptions that the carriers have constant effective mass, constant relaxation time and obey Maxwell-Boltzmann st a t i s t i c s . Surface scattering reduces the mobility of carriers which are in a region less than one mean free path away from the boundary of a semiconductor. A perfect surface is expected to act as a specular scatterer. A surface is a specular scatterer i f the magnitude and the parallel component of the momentum of the carrier which has collided with the surface remain unchanged; only the sign of the component of momentum perpendicular to the scattering surface has changed. Increasing surface roughness tends to make the surface, behave more like a diffuse scatterer. A diffuse scatterer causes the carrier momentum to become random after c o l l i s i o n . If films are much thicker than the mean free path of the carriers in the bulk, then i t is expected that the effect of surface scattering is negligible. The mean free path is given by Chopra (1969) as _1 1 = y h 7 ( ir n h ) 3 ( 4 - 2 2 ) b q gn o where = mobility of carriers in the bulk, n^ = carrier concentration in the bulk, and h = Planck's constant. A discussion on the effects of surface scattering was given by Many, Goldstein and Grover (1965). 4.4.3 Effects of Polycrystalline Structure The carrier transport theory that was presented in the preceding sections is for single-crystal semiconductors. Results from Chapter 3 indicated that the CSS GaAs films were polycrystalline. The polycrystalline structure of the films must be taken into account in the analysis of the e l e c t r i c a l properties of the films. In particular, the effects of grain boundaries must be considered. Associated with the grain boundaries are defects which may act as scatterers, thus reducing the mobility of the carriers at the boundaries. Defects can also act as dopants or trapping centres, which may result in a difference in carrier concentration between the interior of a c r y s t a l l i t e (to be called region I) and i t s boundaries (to be called region II). Due to this difference, a potential barrier may exist between these regions. It is also possible, for example, that region I is n-type while region II is converted to p-type due to a pre-ponderance of acceptors at the boundaries. A representation of a polycrystalline film by a mosaic structure was proposed by Volger (1950). Based on a similar model, Slater (1956) considered the 57 effects of space charge regions i n the c r y s t a l l i t e s on the e l e c t r i c a l conductivity. P e t r i t z (1956) and Waxman, Henrich, Shallcross, Borkan and Weimer (1965) con-sidered the effects of grain boundaries on the mobility of the c a r r i e r s . The concentration of grain boundaries and their potential b a r r i e r height are the parameters expected to l i m i t the c a r r i e r mobility and to influence the temperature dependence of the e l e c t r i c a l conductivity and c a r r i e r mobility. Expressions for the e l e c t r i c a l conductivity and the H a l l mobility of c a r r i e r s i n a p o l y c r y s t a l l i n e semiconducting f i l m may be derived by con-sidering the mosaic f i l m model. Assume that the f i l m i s composed of c r y s t a l l i t e s of length separated by grain boundaries of width SL^- The c a r r i e r con-centration i n region I, p^, i s assumed to be larger than the c a r r i e r concentration i n region I I , p^. Also, i s larger than a^, where and are the e l e c t r i c a l conductivities of regions I and I I , respectively. A schematic diagram of the model i s shown i n Fig. 4 . 5 . For a p-type sample, the energy band diagram i s as shown i n Fig. 4 . 6 . ja3 2 \^ Fig. 4 . 5 Schematic diagram of a mosaic f i l m . 58 T Fig. 4.6 Energy band diagram of a p-type mosaic film. At thermal equilibirum, a potential barrier q<j> exists between region I and region II. For a>potential barrier of the type shown in Fig. 4.6, where i t is assumed that the barrier width is thin compared with the mean free path of the carriers, Torrey and Whitmer (1948) showed that where J = i q <V> p e - q + / k T ( e q V b / k T _ n ) ( 4 . 2 3 ) 2kT 1/2 <v> = 2(—£-) and is the average thermal velocity of the. carriers TT"m (Shockley 1963) M - V  Vb ~ -^S. n l L . n^ = number of barriers per unit length L = sample length between electrodes. At low voltages, eqn. 4.23 may be linearized and a macroscopic conductivity defined a ^ z (4-24) where H = - f 2 (4.25) a = pq y o e " ^ / k T (4.26) q ,2kT.l/2 % = 2nfkT- ( — } < 4' 2 7 ) 1 irm Equation 4.26 may be written as a = pqu* ; y*. = y e"q*/kT ( 4 > 2 g ) o 59 o - p* q u o ; P * - P e ' ^ / k T (4.29) In eqn. 4.28, i t i s i m p l i e d t h a t a l l of the h o l e s i n the c r y s t a l l i t e s take p a r t i n the c o n d u c t i o n p r o c e s s b u t w i t h a r e d u c e d m o b i l i t y p . On t h e o t h e r hand, eqn. 4.29 i m p l i e s t h a t the m o b i l i t y o f the c a r r i e r s i n the c r y s t a l l i t e s remain * unchanged; the number o f c a r r i e r s t a k i n g p a r t i n the c o n d u c t i o n p r o c e s s i s p • E q u a t i o n 4.28 i s used when i t i s assumed t h a t the measured H a l l c o e f f i c i e n t g i v e s the t o t a l c a r r i e r c o n c e n t r a t i o n i n the c r y s t a l l i t e s . E q u a t i o n 4.29 i s used when space charge r e g i o n s , due to t r a p p i n g o f c a r r i e r s , a r e p r e s e n t i n r e g i o n I . 4.4.4- E f f e c t s Of Compensation o f I m p u r i t i e s Compensation i n the CSS GaAs f i l m s may be e x p e c t e d t o be l a r g e because o f t h e i n c r e a s e d number o f d e f e c t s and i m p u r i t i e s which were i n c o r p o r a t e d d u r i n g growth. F r e e c a r r i e r c o n c e n t r a t i o n w i l l be s m a l l e r than the t o t a l i m p u r i t y c o n c e n t r a t i o n . I n p - t y p e s e m i c o n d u c t o r s , the c o n c e n t r a t i o n o f a c c e p t o r s , N , i s more than the c o n c e n t r a t i o n o f donors, N . An e s t i m a t e o f N and N may be made from the f o l l o w i n g c o n s i d e r a t i o n s . Assume t h a t , a t T = 0°K, the N c e n t r e s c o n t a i n N = N - N h o l e s . A t T g r e a t e r than 0°K, the t o t a l number of h o l e s a v a i l a b l e i s ^ B " 1 **(E^-Ep)/kT + » « - 3 0 > .„ 1 + — e g where - E F / k T P = N v e a 2irm* kT 7 / 0 N £ 2 ( — ) 3 / 2 ( 4 > 3 1 ) h The energy l e v e l s E and E a r e measured from the v a l e n c e band edge, g i s the A r o degeneracy f a c t o r , i s the e f f e c t i v e d e n s i t y o f s t a t e s i n the v a l e n c e band and m* i s the d e n s i t y - o f - s t a t e e f f e c t i v e mass of h o l e s . The energy band diagram i s . dh shown i n F i g . 4.7. I f i t i s assumed t h a t p <<N , then V D 4£ V K T (4.32) 6 0 The energy l e v e l or a c t i v a t i o n energy E may be obtained from the slope of a log p versus 1/T p l o t . NA ~EA ' EV F i g . 4.7 Energy band diagram of a compensated p-type semiconductor In the case of c a r r i e r s i n GaAs with a m o b i l i t y V_^ , the corresponding t o t a l impurity concentration may be obtained ei t h e r from F i g . 4.2 or from eqn. 4.15. Thus and may be obtained. For a semiconductor with a given N + N , eqn. 4.15 shows that the c a r r i e r m o b i l i t y would s t i l l depend upon the r a t i o N /N . The semiconductor i s more compensated the c l o s e r NA/N approaches unity. The c a r r i e r mobility of a compensated semiconductor w i l l be smallest when the free c a r r i e r concentration i s at a minimum. 4.5 Experimental Procedures In t h i s section, the methods of putting e l e c t r i c a l contacts on GaAs films are described. The sample holders, the H a l l apparatus and the thermal-probe method used i n measuring the f i l m s ' e l e c t r i c a l properties are also described. 4.5.1 Preparation of E l e c t r i c a l Contacts E l e c t r i c a l contacts to the p-type GaAs films were made by the following methods. The sample was f i r s t cleaned following the standard cleaning procedure, 61 flash-etched in 711 SO^  (95%) iH^OO/O :H20 by volume, rinsed thoroughly in doubly-distilled water and then with isopropanol. An 80 Ag : 10 In : 10 Zn (wt %) alloy (Cox and Strack 1967) was evaporated from a Ta boat through photo-etched beryllium-copper mask onto the GaAs film. Evaporation was done in a Veeco 400 vacuum system. This system is pumped by a conventional 4-inch o i l diffusion pump backed by a mechanical rotary pump. ' A liquid nitrogen cold trap was used to minimize backstreaming of the diffusion pump o i l . Typical background pressure was 5 x 10 ^ Torr rising to about 1 x 10 ~* Torr during deposition. Film thickness was monitored using a quartz crystal controlled oscillator. Films about 0.1 to 0.2u thick were used. A special specimen-and-mask holder was used (see Sec. 4.5.2). The eutectic was alloyed in a H 2 ambient at about 600°C for about 1.5 min. The eutectic temperature is 540°C. The alloying temperature was monitored using a chromel-alumel thermocouple. Melting of the evaporated contact lands was observed by using a Bausch and Lomb zoom microscope (x30 magnification). Gold contact wires were attached to the lands either by using silver paint or soldered by using In. For high resistance samples, silver paint gave erratic contacts. High purity (99.99%) indium which had been degreased by ultrasonic agitation in trichlorethylene was used for these cases. For p-type samples fresh out of the CSS chamber, the whole cleaning procedure was by-passed. The rest of the procedure, however, was the same. For n-type samples, a Au-Ge alloy (12 wt % Ge, eutectic temperature . = 356°C) was used (Braslau, Gunn and Staples 1967). A photograph showing a GaAs film with 4 contact lands is shown in Fig. 4.8. 4.5.2 Sample Holders Figure 4.9 shows the beryllium-copper mask and the sample-and-mask holder used in the deposition of el e c t r i c a l contacts. The.sapphire substrate 6 2 F i g . 4.8 Photograph of a metallized CSS GaAs f i l m . i s located on a stage which has independent x-y movements. Photoetched beryllium-copper masks can be clamped between two s t a i n l e s s s t e e l p l a t e s , which make up the mask-mount assembly. The mask-mount assembly can be moved i n the z-d i r e c t i o n . Alignment between sample and mask i s achieved by suitably manipulating the x-y posit i o n e r s . After alignment, the mask-mount assembly i s lowered u n t i l the mask touches the GaAs f i l m surface. This feature provides f o r ease i n alignment while avoiding scratches on the GaAs f i l m surface due to r e l a t i v e motions between f i l m and mask. Because the mask i s touching the f i l m surface, penumbra e f f e c t s are reduced. DC CURRENT SOURCE L_ 2 -o SHIEL DING B H I SAMPLE TU F E C A 7 ~o o~— POTENTIOMETER 7 (PI) CURRENT METER POTENTIOMETER 2(P2) J Function Switch connections (sample terminals to measurement c i r c u i t terminals) Meas. of sample conductivity by 4-probe method A - l , B-2, F-4, C-3 Meas. of H a l l voltage i n a standard H a l l bar or i n a van der Pauw sample A - l , B-2, E-4, H-3 Meas. of i n a van der Pauw sample A - l , H-2, B-4, E-3 Set P2 = 0 Meas. of R^ i n a van der Pauw sample H-l, B-2, E-4, A-3 Set P2 = 0 F i g . 4.11 Block diagram of the H a l l apparatus and a table of switch connections for the d i f f e r e n t measurement modes. 6 6 .0.6r © — - — • > 1 1 1 i i i i j 0 2.0 4.0 6.0 8.0 10.0 I/°amp F i g . 4.12 Magnetic f i e l d versus d.e. current of the Alpha 8500 electromagnet 6 7 The specimen holder (less protective cap) shown i n Fig. 4.10 was used when e l e c t r i c a l measurements on the films were taken. Contacting of the films was described i n Sec. 4.5.1. A s t r i p heater and a chromel-alumel ther-mocouple were provided. The thermocouple was arranged so that i t s t i p pushed against the sapphire substrate that was mounted i n the holder. 4.5.3 H a l l Apparatus A block diagram of the H a l l apparatus, together with the switch connections for the different measurement modes, are shown i n Fig. 4.11. This apparatus i s a modified version of the apparatus of Fischer, Greig and Mooser (1961). Vibron model 33B, Keithley models 600-A and 602 electrometers were used as n u l l de-tector. .A Fluke d.e. d i f f e r e n t i a l voltmeter (model 881 AB) shunted by a r e s i s t o r was used to measure the current. Leakage paths to ground were minimal as teflon insulation was used i n areas indicated. Double-shielding and common-mode suppression were used to reduce noise pick-up. Kelvin-Varley potentiometers, i n conjunction with dry c e l l s , were used to n u l l out offset voltages. The magnetic f i e l d was supplied by an Alpha 8500 electromagnet powered by an Alpha P8500 power supply. The f i e l d at the centre of the region between the pole pieces was measured using a Radio Frequency Laboratories (RFL) model 1890 H a l l Probe Gaussmeter. The measured magnetic f i e l d as a function of d.e. current i s shown i n Fig. 4.12. The specimen was cooled by immersing the entire t i p of the specimen holder into a Dewar containing l i q u i d nitrogen. Temperatures between room temperature and l i q u i d nitrogen temperature were maintained by heating the specimen with a s t r i p heater. The heating power, hence the temperature T, was controlled by an amplified signal of the difference between the thermocouple voltage and a predetermined voltage, set equivalent to the desired temperature T as obtained from a chromel-alumel thermocouple c a l i b r a t i o n chart. The tem-perature was maintained at T +2°C during the time i t took to complete one voltage and one current measurement-Polarity of the Hall voltage for p and n-type samples, for the present circuit and geometrical arrangement, was checked by testing with known samples of Ge and GaAs. 4•^• 4 Accuracy of Measurements The accuracy of measurements of the 4-probe sample resistances was limited by the accuracy of the Keithley electrometers. This was +2% f u l l scale on a l l ranges exclusive of d r i f t . To reduce errors due to d r i f t , the meters were usually l e f t on for several hours prior to use and their zero settings were checked before every reading. Accuracy of the calculated r e s i s t i v i t i e s was limited by the accuracy of the measured film thickness. The resultant accuracy of the r e s i s t i v i t y was estimated to be about +5%. Measurements of Hall voltages of high resistance (10^ to lO^fi) 2 and low mobility (~ 1 to 5 cm /V-sec) films were the least accurate. The magnitudesof the Hall voltages were typically about 1 mV. The peak-to-peak low-frequency noise superimposed on the Hall voltage was found to be within this range. The accuracy of the measured Hall voltages of lower resistance films was estimated to be about +15%. Errors due to existence of temperature gradients across the sample were checked by averaging the following four sets of readings: current in "normal" direction with magnetic f i e l d in "normal" and "reverse" direction, and current in "reverse" direction with magnetic f i e l d in "normal" and "reverse" direction (Putley 1960). From eqns. 4.18 and 4.19, the relative errors in el e c t r i c a l conductivity and Hall mobility of the CSS films due to the f i n i t e size o°f the contacts A -3 ^H were — ~ -1.84 x 10 and ~ -0.058, respectively. p yH 4.5.5 Thermal-Probe Method The thermal-probe cir c u i t i s shown in Fig. 4.13. If a temperature difference between two probes placed in contact with the sample is maintained, 6 9 a potential difference w i l l exist between them. The polarity of the hotter probe is positive for an n-type and negative for a p-type material (Hunter 1 9 6 2 ) . The thermal probe circuit was checked by testing with known samples of Si, Ge and GaAs. r SHORT/NG SWITCH COLD PROBES SAMPLE LEED AND NORTHRUP GALVANOMETER Fig. 4 . 1 3 Thermal-probe circuit. 4 . 6 Results and Discussion: CSS GaAs Films on Sapphire In this section, experimental results obtained from Sec. 4 . 5 are discussed in terms of the theoretical considerations given in Sees. 4 . 2 , 4 . 3 and 4 . 4 . 4 . 6 . 1 Conductivity Type Determination by Thermal-Probe Method .The determination of the conductivity type of every CSS film was attemp-ted. Knowledge of the conductivity type of a film is necessary before an appropriate alloy can be chosen and evaporated on to the film as contact lands for e l e c t r i c a l measurements. Most of the as-grown "shiny" GaAs films on sapphire were p-type. The conductivity type of some of the "shiny" and a l l of 70 the " d u l l " films could not be determined by using the thermal-probe method. 4.6.2 E f f e c t s of Film Thickness and Surface Scattering The thicknesses of the CSS GaAs films ranged from 16 to 95u. The e f f e c t s of surface space charge layers w i l l have to be considered i f f i l m thicknesses'were i n the order of a Debye length. With the exception of the 650 and 670°C films at T < 220°K, i t was calculated that d/L >_ 20 for the rest. From eqn. 4.17 V'b P = 3 ^ 0 5 : °- 9 5 l Jb Surface s c a t t e r i n g i s expected to be n e g l i g i b l e since the thinnest sample (16M) i s much larger than the mean free path of c a r r i e r s i n bulk GaAs o o (I ~ 600 A, Z, ~ 40 A)", e h 4.6.3 E f f e c t s of Substrate Temperature and Impurities In Chapter 3, i t was shown that the s t r u c t u r a l properties of films were l a r g e l y determined by the substrate temperature. It i s expected that the e l e c t r i c a l properties of the films are affected by the f i l m s ' structure, and thus by the substrate temperature. In general, lower substrate temperature f i l m s , 2 which appeared " d u l l " , had very low c a r r i e r m o b i l i t i e s (< 1 cm /V-sec). Higher substrate temperature f i l m s , which appeared "shiny", had higher c a r r i e r m o b i l i t i e s . Because the e l e c t r i c a l properties of " d u l l " films were les s i n t e r -esting, t h e i r study was not pursued. Two types of films were observed among the "shiny" films;these were the "high p" and the "low p" f i l m s . At room 2 5 temperature, t h e i r r e s i s t i v i t i e s were i n the range 4 x 10 to 10 fi-cm, and 0.6 to 86ft-cm, r e s p e c t i v e l y . Films deposited above 650°C had lower m o b i l i t i e s and higher r e s i s t i v i t i e s than most of the films deposited at 630 to 640°C. Since, as shown i n Fig . 2.2.1 that As i s much more v o l a t i l e than Ga, i t i s expected that at higher temperatures, more As was reevaporated from the f i l m s , r e s u l t i n g i n a larger 71 amount of As vacancies. Due to the increased number of As vacancies, the concentration of a c c e p t o r - l i k e impurities may have increased, with a re su lt ant r i s e i n impurity s c a t t e r i n g and hence, lower mob i l i t y . H a r r i s , Nannichi, Pearson and Day (1969) found that As vacancies themselves can act as acceptors, they can also form acceptor complexes with impurity atoms, or there may be amphoteric dopants such as Si which can move into the As vacancies and act as acceptors. The As loss i n the CSS films could s t i l l be l e s s than what would be detected by the methods used i n Chapter 3. The three source materials were a l l n-type. Their room temperature r e s i s t i v i t y , c a r r i e r concentration and c a r r i e r m o b i l i t y are given i n Table 4.2. Source p/Q-cm y /cm V sec rl Dopant , "3 n/cm Supplier 1 0.063 6,820 "undoped" 14 1.48x10 Monsanto 2 0.07 1,800 Te 5 x l 0 1 6 . Monsanto 3 0.0038 3,300 Te 5 x l 0 1 7 ASARCO Table 4.2 E l e c t r i c a l properties of the source material at room temperature. From thermal-probe and H a l l measurements, the as-grown films were found to be p-type. There was also a big d i f f e r e n c e between the r e s i s t -i v i t y of the films and t h e i r corresponding source m a t e r i a l . These can be due to one or a combination of the following reasons. P a r t i a l pressures of the source dopants may d i f f e r with the p a r t i a l pressures of Ga and As. Hence, the concentration of source dopants at the films has changed. Impurities l i k e S i and Cu may have been introduced during growth. Under As d e f i c i e n t condi-tions, S i can-behave i n an a c c e p t o r - l i k e fashion since i t i s a Group IV element and i s a known amphoteric dopant of GaAs (Sze and I r v i n 1968). Even at moderate 72 (600°C) temperatures, Cu i s known to convert n-GaAs to p-type (Weisberg, Rosi and Herkart -I960). Cu may also act to a c t i v a t e l a t t i c e defects r e s u l t i n g i n the occurrence of donor-acceptor p a i r s . According to Blanc, Bube and Weisberg (1962, 1964), the possible donor-acceptor p a i r s are Ga ( i n t e r s t i t i a l ) - As .(vacancy) pairs or As ( i n t e r s t i t i a l ) - Ga (vancancy) pa i r s which can e x i s t i n the c r y s t a l as microprecipitates, or Ga (vacancy) - As (vacancy) p a i r s , which are b u i l t i n as microvoids. Defects associated with grain boundaries can further increase the concentration of acceptors and donors. I f the concentration of acceptors dominate, then the f i l m would become p-type. For convenience, the e l e c t r i c a l properties of "low p" and "high p" films w i l l be discussed separately. 4.6.4 E l e c t r i c a l Properties of "Low p" Films The temperature dependence of the r e s i s t i v i t y of three "low p" films which were studied i n d e t a i l i s shown i n F i g . 4.14. At room temperature the r e s i s t i v i t y of the films was 1 to 2 orders of magnitude higher than that of the sources. The r e s i s t i v i t y of the films increased slowly with decreasing temperature. This would be expected of semiconductors wherein most of the dopants are shallow. As already stated, thermal-probe and H a l l measurements indicated that these films were p-type. The temperature dependence of the H a l l mobility of holes i n these films i s given i n F i g . 4.15. The magnetic f i e l d dependence of H a l l voltage for sample 21 i s shown In F i g . 4.15a. The hole concentration which was calculated from the H a l l constant R^, was p l o t t e d against temperature and i s shown i n F i g . 4.16. The room temperature r e s i s t i v i t y , H a l l mobility and hole concentration of four f i l m s , together with t h e i r cor-responding source material are given i n Table 4.3. 7 3 F i g . 4.14 R e s i s t i v i t y of "low p" films versus temperature. Sample Substrate Temperature (°C) Source e l e c t r i c a l properties of films at room temperature p/ft-cm conducti-v i t y type u„/cm V -sec rl p/cm 12 630 3 86 P 19 4 . 6 x l 0 1 5 21 640 2 6 P 42 2 . 5 x l 0 1 6 22 630 ' 2 0.6 P 26 4.1 x l 0 1 7 25 630 1 14 P 12 4 x l 0 1 6 Table 4.3 E l e c t r i c a l properties of "low p" films of room temperature. 7 4 F i g . 4.15 H a l l mobility of holes i n "low p" films versus temperature. T.293 K I = 0.268 mA 0-5 0-6 0-4 k 0 ~0.2 -0.5 '-7,01 0 0.2 0.4 B/Web ni T.133 K I = 0.124 m A 0.6 0-4 0 1.0 \ 0 0.2 - 0 . 5 1 -7.0 0 0.2 0-4 B/Web m F i g . A.15a H a l l voltage versus magnetic f i e l d for sample 21. 7 6 10 500 400 18\ «— 10 17 i o \ 16 10 300 200 r/°f< 125 $ cj W3oK T 8 Fig. 4.16 Hole concentration in "low p" films versus temperature. The temperature dependence of the hole mobility in samples 22. and -120/T 21 were fitted by straight lines satisfying the equations y^ = 44e and y = 64e ^ 4 4 ^ cm^/V-sec, respectively. These equations have the same form H as eqn. 4.28. Because these films are polycrystalline, i t is likely that one of the scattering mechanism is due to the potential barrier at the grain boundaries. If this scattering mechanism is the dominant one then the experi-mentally determined y Q should agree with that calculated by using eqn. 4.27. 7 7 S a m p l e s 2 1 a n d 2 2 w e r e c o m p o s e d o f c r y s t a l l i t e s a b o u t 1 0 p w i d e ( C h a p t e r 3 ) . 5 - 1 H e n c e t h e n u m b e r o f b o u n d a r i e s p e r u n i t l e n g t h , n ^ , i s a b o u t 1 0 m . A t r o o m t e m p e r a t u r e t h e m a s s o f l i g h t h o l e s i s 0 . 1 2 m . S u b s t i t u t i n g t h e s e v a l u e s i n t o e q n . 4 . 2 7 g i v e s ( 2 k T _ ) 2 o 2 n ^ k T m 3 x 1 0 ^ m 2 4 2 . = — = 3 0 x 1 0 c m / V - s e c , n ^ V - s e c . a v a l u e t h a t i s u n l i k e l y t o b e o b s e r v e d i n G a A s f i l m s . A c o n v e r s e c a l c u l a t i o n c o u l d b e m a d e . I f t h e h o l e m o b i l i t y w e r e l i m i t e d b y g r a i n b o u n d a r y s c a t t e r i n g " 9 9 - 1 ' a l o n e , t h e n n ^ f o r s a m p l e s 2 1 a n d 2 2 i s ,-. . 4 . 7 x 1 0 a n d 6 . 8 x 1 0 m , r e s p e c t i v e l y . T h e c o r r e s p o n d i n g g r a i n s i z e i s 2 . 1 x 1 0 ^ a n d 1 . 5 x 1 0 ^ . m , r e s p e c t i v e l y , T h e s e r e s u l t s d o n o t a g r e e w i t h t h e f i n d i n g s o f C h a p t e r 3 . I n o r d e r t o e x p l a i n t h e l o w m o b i l i t y o f t h e s e f i l m s , o t h e r s c a t t e r i n g m e c h a n i s m s m u s t b e t a k e n i n t o a c c o u n t . O f t h e i m p o r t a n t s c a t t e r i n g m e c h a n i s m s i n G a A s , i o n i z e d i m p u r i t y s c a t t e r i n g i s t h e o n e m o s t l i k e l y t o l i m i t t h e h o l e m o b i l i t y . D u e t o c o m p e n s a t i o n , t h e m e a s u r e d h o l e c o n c e n t r a t i o n w a s l e s s t h a n t h e c o n c e n t r a t i o n o f a c c e p t o r s . N A _ N D E s t i m a t e s o f — a n d N . + N w e r e d o n e b y u s i n g t h e m e t h o d d e s c r i b e d i n N D A D S e c . 4 . 4 . 4 . T h e s e q u a n t i t i e s , t o g e t h e r w i t h E , p a n d u ^ , a r e g i v e n i n T a b l e 4 . 4 . S a m p l e E A / e V p / c m V N D 2 N D / 2 * 7 - 1 - 1 P j j / c m V - s e c N A + N / c m 3 A D 1 2 - 4 . 6 x l 0 1 5 - 1 9 1 9 8 . 7 x 1 0 y 2 1 0 . 9 7 x l 0 ~ 2 2 . 5 x l 0 1 6 0 . 0 1 0 4 4 2 1 9 5 . 0 x 1 0 2 2 2 . 3 2 x l 0 ~ 2 4 . 1 x l 0 1 7 0 . 2 8 5 2 6 1 . 4 x l 0 2 ° 2 5 2 . 4 9 x l 0 ~ 2 4 x l 0 1 6 • 0 . 2 9 7 1 2 1 . 9 x l 0 2 0 T a b l e 4 . 4 C o n c e n t r a t i o n o f i m p u r i t i e s i n " l o w p " f i l m s . 78 4.6.5 Electrical Properties of "High p" Films Figure 4.17 shows the temperature dependence of the conductivity of 4 "high p" films. For convenience, either the el e c t r i c a l conductivity or re s i s t i v i t y w i l l be discussed, whichever is more applicable. These films differed markedly with the "low p" films in two respects. F i r s t l y , the room temperature r e s i s t i v i t y was 4 to 10 orders of magnitude larger than the source r e s i s t i v i t y . In comparison, the r e s i s t i v i t y of the "low p" films was 1 to 2 ft orders of magnitude larger. Secondly, the r e s i s t i v i t y of these films was strongly temperature dependent, but leveling off at temperatures less than 200°K. The mobility of the "high p" films, however, was lower than the "low p" ones by a factor varying from 20 to'40, at room temperature. The room tem-perature electrical properties of 5 "high p" films and their corresponding source' material are given in Table 4.5. The temperature dependence of u and ri p are given in Fig. 4.18 and 4.19, respectively. Sample Substrate temperature (°c) Source Elec t r i c a l properties . of . films.at . room temperature p/ft-cm conduct-i v i t y type / K-1 -1 u /cm V -sec rl p/cm 14 640 3 9.2xl0 4 P - -23 ' 636 2 7.9xl0 3 .P 13 14 6.1x10 24 660 1 3.9xl0 2 P 3 5.3xl0 1 5 38 660 1 7.2xl0 4 P 2 13 4.3x10 39 650 1 1.6xl0 5 P 3 13 1.3x10 42 670 2 6.4xl0 2 P 3 3.3xl0 1 5 Table 4.5 Ele c t r i c a l properties of "high p" films. From Fig. 4. ,17, i t can be seen that for a l l o the samples, the curves of The photovoltaic effect was observed in the "high p" films. Photovoltages in the "low p" films may have been shorted out by low resistance paths in the films. Fig.. 4.17 Conductivity of "high p" films versus- temperature. 500 400 500 400 701 300 u CD o OJ 2 3 W3°K T 200 T/°K 125 $3Q 7 8 103°K T F i g . 4.18 H a l l mobility of holes i n "high p" films versus temperature. i o 3 a v e r s u s — — a r e n e a r l y s t r a i g h t l i n e s o v e r s e v e r a l p o w e r s o f 1 0 . H o w e v e r , a t l o w t e m p e r a t u r e s , t h e y a l l t e n d t o s a t u r a t e . A p o s s i b l e e x p l a n a t i o n f o r t h i s i s . a s f o l l o w s . V o l g e r ( 1 9 5 0 ) h a d s h o w n t h a t i f t h e c r y s t a l l i t e s w e r e a s s u m e d t o b e s q u a r e s a n d i f £ ^ >>^2 a n C * p 2 > > P l t ' i e n , l2 ( 4 . 3 4 ) p ~ p l + I ^ P 2 H o w e v e r , i t i s c o n c e i v a b l e t h a t t h e g r a i n b o u n d a r i e s h a v e a l o w e r r e s i s -t i v i t y t h a n t h e c r y s t a l l i t e s . T u c k e r ( 1 9 6 6 ) s h o w e d t h a t i f p ^ > > p ^ , t h e n ll+!i2 P Z P 2 — - — ( 4 . 3 5 ) B o t h V o l g e r a n d T u c k e r d i d n o t c o n s i d e r t h e t e m p e r a t u r e d e p e n d e n c e o f p ^ a n d P ^ - A t h i g h e r t e m p e r a t u r e s , t h e a s s u m p t i o n s o f V o l g e r m a y h o l d a n d t h e m a c r o s c o p i c r e s i s t i v i t y i s t h e n g i v e n b y e q n . 4 . 3 4 . F u r t h e r m o r e , t h e t e m p e r a t u r e d e p e n d e n c e o f p ^ m a y b e s t r o n g . T h u s , t h e e f f e c t s o f t h e g r a i n b o u n d a r i e s a r e c o m p l e t e l y s w a m p e d b y t h e m a i n c o n d u c t i v i t y t h r o u g h t h e b u l k o f t h e m a t e r i a l . A t l o w e r t e m p e r a t u r e s t h e c o n d u c t i v i t y o f t h e c r y s t a l -l i t e s d e c r e a s e s d u e t o a d e c r e a s e i n f r e e c a r r i e r c o n c e n t r a t i o n . T h e s h u n t i n g e f f e c t o f t h e b o u n d a r i e s w i l l n o w d o m i n a t e . S i n c e t h e d e f e c t d e n s i t y a t t h e b o u n d a r i e s i s e x p e c t e d t o b e h i g h e r t h a n a t t h e c r y s t a l l i t e s , g r e a t e r c o m p e n s a t i o n m a y h a v e t a k e n p l a c e . T h i s c o u l d r e s u l t i n a r e s i s t i v i t y v a r y i n g s l o w l y w i t h t e m p e r a t u r e a n d w o u l d r e s u l t i n t h e f l a t t e n i n g o u t o f t h e r e s i s t i v i t y v e r s u s t e m p e r a t u r e p l o t s a t l o w t e m p e r a t u r e s . A s i m i l a r e f f e c t i n P b S f i l m s w a s o b -s e r v e d b y M a h l m a n ( 1 9 5 6 ) a n d w a s d i s c u s s e d b y S l a t e r ( 1 9 5 6 ) . I t c a n b e s e e n f r o m F i g . 4 . 1 7 a n d 4 . 1 8 t h a t w h i l e a o f t h e f i l m s c h a n g e d b y 6 o r d e r s o f m a g n i t u d e i n t h e t e m p e r a t u r e r a n g e u n d e r c o n s i d e r a t i o n , t h e c o r r e s p o n d i n g y r e m a i n e d n e a r l y t h e s a m e . T h i s o b s e r v a t i o n i s c o n s i d e r e d i n t h e f o l l o w i n g d i s c u s s i o n , w h i c h f o l l o w s f r o m S l a t e r ' s b a r r i e r t h e o r y ( 1 9 5 6 ) . T h e c o n d u c t i v i t y o f t h e " h i g h p " f i l m s m a y b e r e p r e s e n t e d b y e q n . 4 . 2 9 o r 83 0 = o e (4.36) o 0Q = pqii where AE i s the b a r r i e r height of the grain boundaries measured from the Fermi l e v e l . The c a r r i e r s i n the c r y s t a l l i t e s must overcome t h i s b a r r i e r i n order to p a r t i c i p a t e i n the conduction process. I t i s assumed that the mobility of the c a r r i e r s was not changed s i g n i f i c a n t l y by these b a r r i e r s and that t h e i r value i s l i m i t e d by ionized impurity s c a t t e r i n g . The b a r r i e r height may range from zero, for no grain boundaries, to a value equal to the bandgap, i . e . the b a r r i e r s are f u l l y developed. The b a r r i e r height i n the films may be estimated from the following considerations. The energy band diagram that i s used i n conjunction with the following discussion i s shown i n F i g . 4 . 2 0 . Assume that the net impurity concentration i n both the n-type and p-type regions are equal and i s N . The dotted l i n e s o F i g . 4 . 2 0 Energy band diagram of a mosaic f i l m with p a r t i a l l y developed b a r r i e r s (from S l a t e r 1956). 84 s e p a r a t e t h e t w o r e g i o n s . T h e e n e r g y E ( x ) i s m e a s u r e d f r o m t h e F e r m i l e v e l . I t i s m e a s u r e d u p w a r d s t o t h e c o n d u c t i o n b a n d e d g e i n t h e n - r e g i o n a n d d o w n -w a r d s t o t h e v a l e n c e b a n d e d g e i n t h e p - r e g i o n . T h u s , E ( x ) i s p o s i t i v e i n b o t h c a s e s . W i t h t h e s e a s s u m p t i o n s , S l a t e r ( 1 9 5 6 ) s h o w e d t h a t t h e b a r r i e r h e i g h t , A E , i s g i v e n b y • „ N e A E - ( ~ — ) X 2 ( 4 . 3 7 ) 4 e e r o T h e t h i c k n e s s o f t h e b a r r i e r i s X . I n t h e c a s e o f CSS G a A s f i l m s , e = 1 1 , r 1 5 - 3 N ~ 10 . c m a n d X - l y . W i t h t h e s e v a l u e s , o AE - 0 . 3 7 e V io 3 A p l o t o f l o g o v e r s u s w i t h A E = 0 . 3 7 e V w a s i n c l u d e d i n F i g . 4 . 1 7 , f o r c o m p a r i s o n . T h e r o o m t e m p e r a t u r e c o n d u c t i v i t y w a s a s s u m e d e q u a l t o t h e 6 7 0 ° C f i l m . I t c a n b e s e e n t h a t t h e a c t i v a t i o n e n e r g i e s o f t h e e x p e r i m e n t a l c u r v e s a g r e e d r e a s o n a b l y w e l l w i t h t h a t c a l c u l a t e d b y u s i n g S l a t e r ' s s i m p l e m o d e l . T h e t o t a l i m p u r i t y c o n c e n t r a t i o n ( N + N ) o f 5 " h i g h p " f i l m s w e r e Pi. D e s t i m a t e d u s i n g t h e m e t h o d d e s c r i b e d i n S e c . 4 . 4 . 4 . T h e s e q u a n t i t i e s t o g e t h e r w i t h t h e c o r r e s p o n d i n g p a n d y o f t h e f i l m s a r e g i v e n i n T a b l e 4 . 6 . n S a m p l e E A / e V , - 3 p / c m A D y / 2N H D c m V - 1 - 1 - s e c N + N / c m 3 A D 2 3 0 . 3 3 1 1 4 6 . 1 x 1 0 5 7 1 3 l . l x l O2 0 2 4 - 5 . 3 x l 0 1 5 - 3 5 . 6 x l 0 2 0 3 8 0 . 3 9 8 1 3 4 . 3 x 1 0 5 6 2 2 0 5 . 2 x 1 0 3 9 0 . 3 9 8 1 3 1 . 3 x 1 0 1 5 . 8 3 3 . 2 x l 02 ° 4 2 0 . 3 0 3 3 . 3 x l 0 1 5 1 2 6 3 5 . 9 x l 02 0 T a b l e 4 . 6 C o n c e n t r a t i o n o i P i m p u r i t i e s i n " h i g h p " f i l m s 85 4.7 Postdeposition Doping of CSS GaAs Films on Sapphire The surface layer of sample 2 2 was converted into n-type by doping with Ge. The sample was cleaned following the standard procedure and then f l a s h -etched i n 7H2SO (95%):H 20 2(30%):H 20 (by volume) at about 80°C. A layer of Ge, o about 700 A thick, was deposited on the CSS f i l m . The deposition was carried out i n the V-eeco 400 vacuum system at a background pressure of 5x10 ^ Torr. A wafer of p o l y c r y s t a l l i n e GaAs was placed on top of the CSS f i l m and the "sandwich" was then placed inside a quartz furnace. The "sandwich" arrangement was used i n order to avoid excessive As loss from the f i l m surface. The temperature inside the furnace was measured by using a chromel-alumel thermocouple whose t i p touched the sapphire substrate. Diffusion of the deposited Ge was carried out at 700°C -4 i n a vacuum of 10 Torr for about 8 hours. The sample was then contacted using a Au-Ge allo y and mounted i n a manner as described i n Sec. 4.5. Thermal probe and H a l l measurements indicated that the f i l m was n-type. The e l e c t r i c a l properties of the converted f i l m i s tabulated i n Table 4.7. temperature (°K) p/fi-cm u /cm V sec . H ... n/cm 293 0.183 77 4.44xl0 1 7 273 0.201 58 17 5.37x10 133 0.214 56 5.22xl0 1 7 Table 4.7 E l e c t r i c a l properties of a converted f i l m Unsuccessful attempts to convert the CSS films into n-type by Ge or Sn d i f f u s i o n were made on 3 other samples. Since Ge and Sn are amphoteric dopants of GaAs, they can occupy' vacant As s i t e s and become acceptor-like 86 impurities. The amphoteric dopants w i l l act as donors when they occupy Ga. vacancies. In order to convert a p-type sample into n-type, the concentration of donors must be made larger than the concentration of acceptors. For the CSS f i l m s , t h i s would mean that the concentration of the i n t e n t i o n a l l y added 20 -3 dopant must be greater than 10 cm . The mobility of c a r r i e r s i n the converted layers would be small due to ionized impurity s c a t t e r i n g . The group IV elements were used f o r , so f a r , they are the best n-type dopants to be used i n the s o l i d -t o - s o l i d d i f f u s s i o n of impurities into GaAs layers. The more common group VI dopants react with Ga and corrode the GaAs surface (Goldstein 1962, Muench 1966). 4.8 Device Fab r i c a t i o n Using CSS GaAs Films on Sapphire T h i n - f i l m insulated-gate f i e l d - e f f e c t t r a n s i s t o r s and Au-Schottky b a r r i e r diodes were fabricated using the CSS f i l m s . The t r a n s i s t o r structure used was s i m i l a r to the ones used by Weimer (1964) and by Salama (1966). The source-drain gap was about 70u. The o as-grown GaAs films were about 1000 to 3000 A thick. Evaporated SiO^ served as o the i n s u l a t o r . Thicknesses of about 1500 to 2000 A were used. Two out of 12 t r a n s i s t o r s f a b r i c a t e d had transconductances of O.lu-b". The e f f e c t i v e 2 mobility was 4 cm /V-sec. These devices operated i n the p-type depletion mode. Source-drain current modulation by gate voltage was not observed i n the rest of the devices. This may be due to a high density of surface states at the SiO /GaAs i n t e r f a c e , x Five Au-Schottky b a r r i e r diodes were fab r i c a t e d on an n-type layer converted by Ge doping. The diodes a l l had " s o f t " c h a r a c t e r i s t i c s . - The d.e. r e c t i f i c a t i o n r a t i o and the voltage |v| for diodes 1, 2 and 3 were 40 (1.2 V), 12 (1.2 V) and 42 (0.6 V) , res p e c t i v e l y . 8 7 4 . 9 R e s u l t s a n d D i s c u s s i o n : CSS G a A s f i l m o n S e m i - I n s u l a t i n g G a A s A h o m o e p i t a x i a l l a y e r o f G a A s w a s d e p o s i t e d o n s e m i - i n s u l a t i n g G a A s u s i n g t h e CSS m e t h o d . T h e G a A s w a s s u p p l i e d b y M o n s a n t o a n d a c c o r d i n g t o t h e m a n u f a c t u r e r ' s s p e c i f i c a t i o n s w a s C r d o p e d w i t h a r o o m t e m p e r a t u r e r e s i s t i v i t y o f g 5 . 3 2 t o 5 . 5 x 1 0 fi-cm. T h e w a f e r s u r f a c e w a s o r i e n t e d i n t h e [ 1 1 1 ] d i r e c t i o n a n d w a s c u t f r o m a C z o c h r a l s k i s i n g l e c r y s t a l . T h e s u b s t r a t e w a s m e c h a n i c a l l y p o l i s h e d t o 0 . 3 u . f i n i s h a n d t h e n c l e a n e d f o l l o w i n g t h e s . t a n d a r d c l e a n i n g p r o c e d u r e . J u s t p r i o r t o i n t r o d u c t i o n i n t o t h e U l t e k v a c u u m c h a m b e r , t h e s u b s t r a t e w a s c h e m i c a l l y p o l i s h e d f o r a b o u t 3 0 s e c , a t r o o m t e m p e r a t u r e , i n a s o l u t i o n o f 4 0 H C 1 ( 3 8 % ) : 4 H ^ O ^ ( 3 0 % ) i l - ^ O ^ ( b y v o l u m e ) ( S h a w 1 9 6 8 ) . T h e s u b s t r a t e w a s t h e n t h o r o u g h l y r i n s e d i n d o u b l y - d i s t i l l e d w a t e r a n d i n p r o p a n o l . . T h e s u b s t r a t e s u r f a c e w a s e x a m i n e d w i t h a n o p t i c a l m i c r o s c o p e . T h e p o l i s h e d s u r f a c e w a s s h i n y a n d s m o o t h a n d a p p e a r e d t o b e u n i f o r m l y e t c h e d b y t h e c h e m i c a l p o l i s h . T h e s o u r c e m a t e r i a l w a s n - t y p e G a A s w i t h t h e f o l l o w i n g r o o m t e m p e r a t u r e 1 6 — 3 2 e l e c t r i c a l p r o p e r t i e s : p = 0 . 0 7 f i - c m , n = 5 x 1 0 c m , a n d u = 1 » 8 0 0 c m / V - s e c . H D u r i n g d e p o s i t i o n , t h e s o u r c e a n d s u b s t r a t e t e m p e r a t u r e s w e r e m a i n -- 9 t a i n e d a t 6 6 6 a n d 5 8 5 ° C , r e s p e c t i v e l y . T h e b a c k g r o u n d p r e s s u r e w a s 9 . 5 x 1 0 —8 T o r r r i s i n g t o 1 . 4 x 1 0 T o r r d u r i n g d e p o s i t i o n . T h e n o m i n a l d e p o s i t i o n t i m e w a s 1 h o u r . T h e r m a l - p r o b e m e a s u r e m e n t s i n d i c a t e d t h a t t h e f i l m w a s n - t y p e . T h u s , t h e f i l m w a s c o n t a c t e d b y e v a p o r a t i n g A u - G e a l l o y l a n d s . T h e d e p o s i t i o n , a l l o y i n g a n d s a m p l e m o u n t i n g p r o c e d u r e s w e r e d e s c r i b e d i n S e c . 4 . 5 . 1 . A p l o t o f H a l l v o l t a g e v e r s u s m a g n e t i c f i e l d f o r c u r r e n t i n t h e " n o r m a l " a n d i n t h e " r e v e r s e " d i r e c t i o n s i s g i v e n i n F i g . 4 . 2 1 . T h e t e m p e r a t u r e d e p e n d e n c e o f p i s g i v e n i n 3 / 2 F i g . 4 . 2 2 . I t c a n b e s e e n t h a t t h e e l e c t r o n m o b i l i t y f o l l o w e d a T l a w , w h i c h i s p r e d i c t e d i n e q n . 4 . 1 5 . f o r i o n i z e d i m p u r i t y s c a t t e r i n g . A t h i g h e r t e m p e r a t u r e s , t h e m e a s u r e d m o b i l i t y t e n d e d t o s a t u r a t e . A s t h e t e m p e r a t u r e 88 increases, the electrons are scattered more by o p t i c a l phonons. There w i l l be a temperature range where electrons are scattered by a combination of ionized impurities and o p t i c a l phonons. Ehrenreich (1960) showed that the m o b i l i t y , 1/2 due to a combination of these mechanisms, va r i e s as T . A t even higher temperatures, polar o p t i c a l s c a t t e r i n g dominates and t h i s gives a m obility -1/2 varying as T (Sec. 4.3). F i g . 4.21 H a l l voltage versus magnetic f i e l d f o r current i n the "normal" and i n the "reverse" d i r e c t i o n . F i g . 4.22 Temperature dependence of the H a l l m o b i l i t y of electrons i n homoepitaxial GaAs f i l m . 9 0 5. DIELECTRIC PROPERTIES OF ANODIC A^O FILMS 5.1 Introduction The most commonly used and most extensively studied i n s u l a t o r i n semiconductor, p a r t i c u l a r l y S i , device technology i s S i 0 2 produced e i t h e r by thermal oxidation of S i ( A t a l l a 1960) or by p y r o l y t i c deposition, e.g. e t h y l t r i e t h o x y - s i l a n e [C 2H S i (C2E ) ] deposited a t 650 to 750°C (Klerer 1961). The other commonly used i n s u l a t o r i n semiconductor device technology i s Si^N^ (Chu, Lee and Gruber 1967). Other i n s u l a t o r s which are being investigated f or use i n semiconductor devices are CaF 2 (Haering and O'Hanlon 1967), evaporated Sio (Johansen 1965), p y r o l y t i c A l 0 (Aboaf 1967), r e a c t i v e l y evaporated A l 0 (Ferrieu and Pruniaux 1969), wet (Mier and Buvinger 1969) and plasma (Waxman 1968) anodized A l . This study i s concerned p r i m a r i l y with the d i e l e c t r i c properties of evaporated A l films anodized i n ammonium pentaborate dissolved i n ethylene g l y c o l . The objectives of t h i s study are to develop a procedure of f a b r i c a t i n g A1 20^ films and to i n v e s t i g a t e t h e i r properties which w i l l be used l a t e r i n the study of metal-insulator-semiconductor (MIS) systems. A.e. bridge, step response and d.e. conduction measurements were done to determine the d i e l e c t r i c constant as a function of frequency and d.e. r e s i s t i v i t y of the f i l m s . In comparison with S i 0 2 , anodic A^O^ may o f f e r several advantages when used as the i n s u l a t o r i n semiconductor devices. Anodic A^O^ can be formed at room temperature while thermal S i 0 2 i s formed at 1000 to 1300°C and p y r o l y t i c S i 0 2 i s deposited at 650 to 750°C. High temperature treatment, which may degrade device performance, i s avoided i f anodic A^O^ i s used. Waxman and Zaininger (1968, 1969) found that devicesmade with plasma anodized A l as the i n s u l a t o r were stable and r e s i s t a n t to r a d i a t i o n damage. They also found that A1 20^ i s less susceptible to impurity ion migration r e s u l t i n g i n le s s d r i f t i n the device c h a r a c t e r i s t i c s . On the other hand, ease with which ions, p a r t i c u l a r l y 91 Na ions, migrate across SiO^ i s w e l l known (Yon, Ko and Kuper 1966). The d i e l e c t r i c constant of SiC^ and Al^O^ are 3.9 and 7 to 9, r e s p e c t i v e l y . More f l e x i b i l i t y i n the design of insulated-gate devices i s a v a i l a b l e when Al^O^ instead of SiC^ i s used. For example, a higher gate-voltage device can be designed by using an Al^O^ f i l m twice as thick as a SiC^ f i l m . The amount of induced charges would s t i l l be about the same for both cases. As another example, because a change i n conductivity of the surface space charge layer i s proportional to a change i n the free c a r r i e r concentration, an insulated-gate device with a higher transconductance may be f a b r i c a t e d i f Al^O^ rather than SiO^ i s used. The two major disadvantages of using wet anodized Al^O^ are as follows. The anodization of A l i s s e n s i t i v e to C l ions (Harkness and Young 1966). Extreme care must be used to avoid C l ion contamination. The e n t i r e device i s exposed to the e l e c t r o l y t e during anodization. This may be an added source of contamination. Viscous e l e c t r o l y t e , however, may be used only on areas where desired. 5.2 D i e l e c t r i c Properties of Amorphous Films The complex p e r m i t t i v i t y e(w,T) i s used to describe the d i e l e c t r i c properties of amorphous f i l m s . e (OJ,T) = e o [ e l ( a ) > T ) - j e"(a),T)] (5.1) d = e e o r . Conventionally, e" i s defined by the magnitude of the inphase current flowing through a capacitor when a s i n u s o i d a l voltage i s applied across the capacitor. In terms of the phase angle, 6, between the t o t a l current, i , and the out-of-phase current, i 92 r.2 .2,1/2 U - 1 J tanfi = ' • — 1 = e'Ve* (5.2) T-he d i e l e c t r i c properties of amorphous films have been the subject of many papers. (See, for example, Cherki and Coelko 1967; A r g a l l and Jonscher 1968; Pul f r e y , Wilcox and Young 1969). I t has been determined that, f o r most amorphous, i n s u l a t o r s , e" and tan6 are r e l a t i v e l y independent of frequency i n ft the audio range and that e"/T i s constant f o r . c e r t a i n temperatures. The i o n i c r e l a x a t i o n model with a f a i r l y f l a t d i s t r i b u t i o n of a c t i v a t i o n energies, W, was used by Gevers and Dupre (1946) to describe the near independence of e" on ca. They showed that W E' = e + (e -e ) / ° G(W)dW (5.3) oo g oo O e" = (e -e ) ^  kT G(W ) (5.4) s 0 0 2 o o x ' W /kT . . . . 2TT _ _o o (5.4a) co 2 6 where £ = e (OJ=0) s r £ = £ ((0=°°) oo r — T q = a constant," inverse of jump frequency. Eqn. 5.3 implies that only those processes with a c t i v a t i o n energies le s s than W o respond to an applied frequency OJ. Equation 5.4 implies that losses occur only for those processes with the c h a r a c t e r i s t i c frequency u. The angular frequency to i s r e l a t e d to the a c t i v a t i o n Wq by equation 5.4a. 5.2.1 A.c. Bridge Method 2 The r e l a t i v e p e r m i t t i v i t y of i n s u l a t o r s i n the frequency range 10 to 10 7 Hz can be e a s i l y measured using one of the many conventional a.c. bridges. * A r g a l l and Jonscher (1968) found that f o r t h e i r Al^O^ films constancy i n e" and E"/T was a s p e c i a l case at room temperature. T h e l o s s e s m e a s u r e d b y t h e c a p a c i t a n c e b r i d g e m u s t b e c o r r e c t e d f o r l o s s e s d u e t o t h e s e r i e s r e s i s t a n c e R ^ , w h i c h c a n b e d u e t o c o n t a c t a n d l e a d r e s i s t a n c e s , O n c e R ^ i s ' k n o w n , t h e l o s s t a n g e n t d u e t o e " ' a l o n e c a n b e c a l c u l a t e d . C o n -s i d e r t h e p a r a l l e l R - C e q u i v a l e n t c i r c u i t m o d e l o f a c a p a c i t o r . Z(s). T h e l o s s t a n g e n t i s t a n 6 R, ( 1 + c o V c 2 ) + R x coR 2 C P o o r * ' R , + R t a n o „ . L _JJ coC A s co a p p r o a c h e s ° ° , — - — 9 a p p r o a c h e s R ^ . T h u s R ^ c a n b e c a l c u l a t e d o n c e t h e U P 1 v e r s u s — p l o t i s k n o w n . T h e l o s s t a n g e n t d u e t o e " i s = R T + V r 2 co R C P P (5.5) i n t e r c e p t o f a t a n 6 coC t a n < 5 = t a n S - t a n 6 T c L (5.6) T h e f r e q u e n c y d e p e n d e n c e o f e " c a n b e c a l c u l a t e d i m m e d i a t e l y f r o m C ( c o ) = C E ' (co) o t a n S e " ( c o ) c E'(CO) t h e r e f o r e , C e " ( c o ) = C ( c o ) t a n S (co) o c (5.7) 94 where C q i s the capacitance of the capacitor when the d i e l e c t r i c i s replaced by a i r or vacuum and C i s the measured capacitance. 5.2.2 Step Response Method -2 -4 At low frequencies (10 to 10 Hz), losses i n the d i e l e c t r i c can be calculated by measuring the p o l a r i z a t i o n current as a function of time upon the a p p l i c a t i o n or removal of a step voltage across the d i e l e c t r i c . Hamon (1952) and l a t e r , Baird (1968) analyzed measurements of t h i s type by assuming a time dependence of the form A t n for the p o l a r i z a t i o n current i ( t ) . For n=l, t h i s assumption i s equivalent to saying that the d i s t r i b u t i o n of a c t i v a t i o n energies i s f l a t . I f l i n e a r response theory holds, then e' and e" are given by £'(") = T r - [ C + f°° A t " n cos cot dt] (5.8) KJ O . O £"(w) = ~ [ £ + /" A t " n s i n cot dt] (5.9) L» tu o o where G i s the d.e. conductance, C^ i s the capacitance of the sample at very high frequencies, C q i s the corresponding value of capacitance with vacuum or a i r between the electrodes and co i s the angular frequency. By a simple s u b s t i t u t i o n , i t can be shown that e'<") = \~ [ + A co 1 1" 1 r(l-n) c o s ( l - n ) | ] (5.10) £ , , ( a ) ) = Ir [ S + A ajn_1 r ( 1 - n ) c o s ] ( 5 - n ) o where r(l-n) i s the gamma function. Usually, discharge currents are considered, then G = 0 and the e f f e c t of leakage current can be eliminated. Two methods may be used i n c a l c u l a t i n g In the f i r s t one, A i s i ( t ) t n obtained from the intercept of a — versus V p l o t . By extrapolating to zero.V, the non-linear e f f e c t s , which may be due to space charge, are minimized. C e"(w) can be obtained immediately from eqn. 5.11. In the second one, Hamon o (1952) showed that for 0.3 < n < 1.2, e" i s given by i ( t . ) e" : — ^ (5.12) o where i ( t ^ ) i s the value of the discharge current at t^ and the corresponding angular frequency i s u. = 0.63/t. (5.13) I I 5.3 Growth of Oxide Films The properties of anodic f i l m s , the p h y s i c a l mechanisms involved i n t h e i r growth and t h e i r k i n e t i c s have been studied extensively (see, for example, Young 1961; Goruk, Young and Zobel 1966; Dell'Oca 1969). , Only the r e s u l t s r e l e -vant to the preparative techniques w i l l be quoted here. 5.3.1 Constant Current Formation An oxide may be grown to a predetermined thickness by applying a constant current u n t i l an amount of charge corresponding to the desired thickness has been passed through the o x i d e / e l e c t r o l y t e i n t e r f a c e . The thickness of an oxide with formula M 0 and density p i s given by x y D = (QMn)/(2AyFp) (5.14) where Q = / i(x)dT = t o t a l charge passed M = molecular weight of metal A = area of oxide F = the Faraday (96500 coulombs) n = current e f f i c i e n c y = the proportion of charge not used i n other reactions. At a fixed temperature and current density, the rate of change of voltage i s 96 d t J,T d D J,T d t J , T = E J n X (5.15) To a f i r s t a p p r o x i m a t i o n x = M/(2yF p) J = J e B £ (5.16) t h e r e f o r e = ' 7 £11(7-) J 1, x (5.17) d t J,T B J o The r a t e of change o f V i s almost p r o p o r t i o n a l to the c u r r e n t d e n s i t y . 5.3.2 C o n s t a n t V o l t a g e F o r m a t i o n D u r i n g f o r m a t i o n a t c o n s t a n t v o l t a g e , the r a t e o f change of c u r r e n t d e n s i t y may be g i v e n by ( D r e i n e r 1964) A = ± _ BV/D M t ; v dt 1 o 6 J __BV , dD = _ pv j 2 o g } D 2 d t D 2 I f D i s .assumed c o n s t a n t , _ / dJ = 3V a d t J D t h e r e f o r e i = c o n s t + ^ t (5.19> J D d M n  a 2yFp P l o t s o f \ v e r s u s t w i l l be l i n e a r u n t i l such time t h a t the l e a k a g e c u r r e n t dominates the f o r m a t i o n c u r r e n t . Thus, a y v e r s u s t p l o t can be used as a s i m p l e and u s e f u l check t h a t the o x i d e f o r m a t i o n i s p r o c e e d i n g p r o p e r l y . 5.4 E x p e r i m e n t a l Procedures 5.4.1 S u b s t r a t e P r e p a r a t i o n P r e c l e a n e d m i c r o s c o p e s l i d e ( F i s h e r 12-550) were u l t r a s o n i c a l l y 97 agitated i n reagent grade acetone for about 3 min. Then they were vapour degreased f i r s t i n a Soxhlet extractor containing acetone and then i n another one containing propanol. Next, they were b o i l e d i n d o u b l y - d i s t i l l e d water for about 5 min , dried, i n a stream of N^ gas and immediately placed i n s i d e the Veeco vacuum chamber for A l deposition. 5.4.2 A l Source Preparation High p u r i t y (99.999%) A l wires, cut i n about 1 cm lengths, were f i r s t degreased by dipping i n te c h n i c a l grade t r i c h l o r e t h y l e n e , i n reagent grade acetone u l t r a s o n i c a l l y agitated i n a KOH. s o l u t i o n (1 gram i n about 15 ml of d o u b l y - d i s t i l l e d water) for about 2 min., and then thoroughly rinsed i n doubly-d i s t i l l e d water. A piece of W wire, about 7 cm long, was cleaned i n a s i m i l a r manner. The A l wires were clipped onto the W wire heater and excess water was blown dry by a stream of N gas. The A l source was then mounted i n s i d e the vacuum chamber. 5.4.3 Vacuum Deposition of A l T y p i c a l l y , the background pressure i n s i d e the vacuum b e l l j a r was about -6 -4 5 x 10 Torr and r i s i n g to about 0.5 to 1 x 10 Torr during deposition. The Veeco vacuum system has been described elsewhere i n the thesis (see Sec. 4.5). As a further precaution against contamination, a shutter was used during the i n i t i a l outgassing of the A l source. A quartz c r y s t a l c o n t r o l l e d o s c i l l a t o r was used to monitor the thickness of the deposited f i l m s . The o s c i l l a t o r was f i r s t c a l i b r a t e d by measuring the change i n frequencies for a se r i e s of films deposited on d i f f e r e n t glass substrates. The thicknesses of these films were measured using o O a Sloan M-100 Angstrometer (see Sec. 2.3.4). T y p i c a l l y , films of about 1000 A o thick were deposited at a rate of about 7 A/sec. 5.4.4 Anodizatio'n of A l Films A l films were anodized by using the experimental set-up as shown 98 schematically i n Fig. 5.1. A piece, of Pt wire was used as the cathode and the ele c t r o l y t e was ammonium pcntaborate dissolved i n ethylene gl y c o l (33.4 gm/lit). The glass beaker and Pt wire were boiled i n d o u b l y - d i s t i l l e d water for at least 10 min and subsequently dried i n an oven. Extreme care was taken during the entire anodization procedure to avoid Cl ion contamination. ANODE + CATHODE CONSTANT CURRENT SOURCE HIGH INPUT IMPEDANCE VOLTMETER Fig. 5.1 Schematic diagram of the anodization set-up. A method of estimating the oxide thickness i s as follows. Assume that, under constant current conditions, an A l f i l m i s completely anodized i n t ^ sec. The oxide thickness, D^ , i s measured using a Sloan Angstrometer. From eqn. 5.14, oxide thickness i s proportional to time. By applying the same current density, the oxide thickness of another A l f i l m which has been anodized to t i s given by D l = ( t l / t f ) D f (5.20) A second method of estimating the oxide thickness i s from capacitance measurements. The area, A, of the counterelectrode i s calculated. By assuming that e' = 8.4+0.2 (Bernard and Cook 1959), the thickness i s simply given by D = A e E'/C (5.21) o The thicknesses determined by the two methods agreed to within 15%. This discrepancy may be due to errors i n obtaining and A. Accuracy i n Angs-trometer readings are l i m i t e d by the r e s o l u t i o n of the fri n g e linewidths. This o -was estimated to be about + 150 A. Accuracy i n A depends upon the accuracy i n measuring the diameter of the counterelectrode. This i s dealt with i n Sec. 5.4.5. The r e l a t i v e p e r m i t t i v i t y e 1 was measured by Bernard and Cook at 120 Hz. The e l e c t r o l y t e they used was ammonium pentaborate dissolved i n ethylene g l y c o l (30% by weight of s a l t ) . I t was assumed here that e' i s independent of the e l e c t r o l y t e concentration and that i t i s also f a i r l y independent of frequency at room temperature (see Sec. 5.5.1). 5.4.5 Deposition of Counterelectrodes Counterelectrodes of A l or Au dots were evaporated. Their shapes were defined by using photoetched beryllium-copper masks. The dots ranged from 0.1 t 0.7 mm i n diameter. Diameters of the dots were measured by using a t r a v e l l i n g microscope. Several sources of errors can a r i s e i n the determination of counter electrode area. Due to imperfections i n the beryllium-copper masks or to d i s t -ortions i n the mask during deposition from being heated by the source, the evaporated counterelectrodes deviated from the c i r c u l a r shape. Several readings with the t r a v e l l i n g microscope were taken and an average value of the diameter of each p a r t i c u l a r dot was determined. A penumbra e f f e c t was also observed. I f the kl^O^ f i l m w a s n o t situated p a r a l l e l and close to the mask, the boundary of the dots w i l l appear smeared, rendering the determination of the diameters of the dots d i f f i c u l t . The counterelectrode areas were estimated to be accurate to +5%. 100 5.4.6 A.c. Bridge, Step Response and d.e. Conduction Measurements Capacitances and d i s s i p a t i o n factors were measured using e i t h e r a GR 1615A (frequency range: 50 Hz to 100 kHz) or a Boonton 75C (frequency range: 5 to 500 kHz) capacitance bridge, depending on the frequency used. The GR 1615A i s a transformer arm r a t i o bridge with the following rated accuracies: C = C + 2 x 10~ 5 f/kHz% + 2 x 10 _ 3 ( C / I J F ) (f 2/kHz 2)%, D = D + 0.1 x (D ) % meas - v ' meas - meas + 0.001. The Boonton 75C i s e s s e n t i a l l y a modified transformer arm r a t i o bridge and i s equipped with i t s own s i g n a l generator, d.e. supply and n u l l dector. Its rated accuracies depend upon the range used. For the m u l t i p l i e r range C = 1, G = 10 kb", C = C + (0.25% + l00.® P F + 0.2 pF) meas - R /" r r • P G = G + (10% + -r§7- % + O.OOlUo") meas - 500 1 = (2TTf/mHz)(C/pF) D G /VU meas The a.c. s i g n a l frequency was c a l i b r a t e d using a Beckman 6146 frequency counter. Stray and lead capacitances were minimized by using the three terminal configura-ti o n f o r a l l bridge measurements. The sample was placed i n s i d e a grounded, l i g h t -t i g h t metal box. E l e c t r i c a l contacts to be electrodes were made by using Au plated beryllium-copper springs mounted on Kulicke and Soffa micromanipulators. Step response and d.e. conduction currents were measured by using a Keithley 417 high-speed Picoammeter (accuracy: +3% f u l l scale f o r a l l ranges except the lowest, which i s +5%). The c i r c u i t used i s shown i n F i g . 5.2. Co-axial go and return leads were used and other usual precautions for measure-ment of low currents were also taken. The d.e. voltage supply was c a l i b r a t e d using a Fluke d.e. D i f f e r e n t i a l Voltmeter model 881 AB. A fi x e d voltage of 10.000 + 0.002 V in conjunction with a high p r e c i s i o n GR voltage d i v i d e r was used. D r i f t i n the d.e. supply was found to be n e g l i g i b l e during an 8-hour period. The c i r c u i t was checked by measuring the current flowing through a 101 SAMPLE J r 1 i_ PICO AM-METER r ~ ! 1 P CHART RECORDER 6 \ SHORTING SWITCH Fig. 5.2 Schematic diagram of the d.e. conduction measurement set-up. standard 10"^°, r e s i s t o r . Under open c i r c u i t conditions (sample absent) , no measurable leakage current was recorded even when a voltage about 5 volts higher than the highest used i n the actual experiments was applied. Charging and discharging currents were recorded using a Moseley 7100 BM s t r i p chart recorder connected to the recorder output of the Picoammeter. Steady-state conduction currents were estimated by extrapolating the measured charging currents to long periods of time. The sample was mounted i n a manner si m i l a r to that during bridge measurements and was l e f t on overnight with the terminals shorted before taking any d.e. conduction measurements. A l l capacitance and d.e. measurements were done i n an a i r ambient and at room temperature. 5.5 Analysis of Results 5.5.1 A.e. Bridge The following discussions refer p a r t i c u l a r l y to sample //9C43. Similar 102 r e s u l t s were obtained from -Al 0 f i l m s that were made i n conjunction w i t h the metal/Al^O /GaAs MIS experiments (Sec. 6.6). Following the procedure d e t a i l e d o i n Sees. 5.4.1 to 5.4.3, an A l f i l m 900 A t h i c k was deposited onto a glass substrate. 2 The oxide was formed at a constant current density of 1 mA/cm to 15V (t = 15.5 sec) and l e f t at constant v o l t a g e f o r almost 24 hr. The oxide thickness was o o •estimated to be 280 + 40 A. Counterelectrodes of A l , about 600 A t h i c k , were -3 -3 2 deposited. The e l e c t r o d e area of sample #9C43 was 3.91 x 10 + 0.2 x 10 cm . Measured values of the l o s s tangent were co r r e c t e d f o r s e r i e s r e s i s t a n c e using the procedure o u t l i n e d i n Sec. 5.2.1. A p l o t of ~^ n^ versus 1/f i s given P i n F i g . 5.3. At 1/f = 0, the i n t e r c e p t gives 2-rrR^; t a n 6 c was c a l c u l a t e d using eqn. 5.6. F i g s . 5.4 and 5.5 show p l o t s of C e' and t a n $ c versus frequency, r e s -p e c t i v e l y , i t can be seen that e' v a r i e d l e s s than 3% i n the frequency range 0.5 to 100 kHz, r i s i n g at lower frequencies. TanS^ was found to be a slox-7 func-t i o n of frequency w i t h higher values at lower f r e q u e n c i e s . 5.5.2 Step Response The current d e n s i t y , n e g l e c t i n g d i f f u s i o n c u r r e n t , f l o w i n g through a c a p a c i t o r i s given by J T = If + J(6) (5.22) In the case of an a p p l i e d step f i e l d £ = £ q u ( t ) , J T = e S 6(t) +|| + J[g u ( t ) ] (5.23) T O O dt o I f i t i s assumed that the p o l a r i z a t i o n processes i n the d i e l e c t r i c can be represented by a s u p e r p o s i t i o n of Debye -type processes i n v o l v i n g i o n hopping w i t h a 1/x d i s t r i b u t i o n of r e l a x a t i o n times, JP - f - A i f <5-24> Equation 5.24 i s d e r i v e d i n Appendix 5.1. 2 The input r e s i s t a n c e of the PicoAmmeter increases from 10 at the -5 10 -13 10 A range to 10 Q at the 10 A range i n decade steps. I f the capacitance 103 400 c: 300. 200 0 j-xW5 Hz 2 Fig. 5.3 Plot of tan 6/fC versus 1/f. P of the. sample is 1000 pF, the transient currents may be neglected provided that t > 10 ^ sec to t > 1.0 sec spanning the extreme current ranges in decade steps. From Fig. 5.4, C Z 1000 pF, and at the highest applied voltage (5V), the current range used was 10 ^ A. Since RC = 0.1 sec, C QS o 6(t) may be neglected for t > 1 sec and JT = A l f + J [ $ u ( t ) ] Figs. 5.6 and 5.7 show the charging arid discharging currents as functions of time with the applied or removed voltages as parameters, respectively. The charging currents tended to saturate at large t and their slopes were less than ^ 10-8 o • 6 10-7 70.5-o . I I 10.5 10-4 CD O 10-3 10 2 103 10 f/Hz 4 F i g . 5.4 Plot of C e' versus f o 0.009r 0-OOSl 0.006 70 2 JO3 w' f/Hz F i g . 5.5 Plot of tan 6 versus c 105 106 -70,' log (I /sec ) F i g . 5 . 7 D i s c h a r g i n g c u r r e n t s v e r s u s t i m e a s a f u n c t i o n o f p r e a p p l i e d v o l t a g e . 107 that for the discharging currents (-0.70 as compared to -0.95). This may be due to the leakage current J becoming more dominant because J approaches 0 as t approaches 00. On the other hand, for discharging currents, J = 0 and JT = V The frequency dependence of e" at low frequencies was determined using the two methods discussed in Sec. 5.2.2 and is shown in Fig. 5.8. The equation of the heavy line (eqn. 5.25) was calculated from eqn. 5.11, with G = 0 and n = 0.95. log[C oe"/pF] = 1.40 - 0.05 log(f/Hz) (5.25) The points represented by open circles were calculated using Hamon's method. This method may be used for n was 0.95 which lie s within the limits of n imposed by Hamon. In the same plot were points (represented by f i l l e d circles) calculated using eqn. 5.7 to determine the frequency dependence of e" in the audio range. As shown in Fig. 5.8, e" increased with decreasing frequency. This may be due to increased losses at low frequencies arising from flaws. D.e. conduction currents were obtained from charging currents by extrapolating to long periods of time. Fig. 5.9 shows a Log (J) versus £"^ 2 plot. The straight line was drawn by assuming a Schottky law with - 4. The theoretical Schottky slope is given by r o 13 The d.e. r e s i s t i v i t y for low fields was about 10 fi-cm. The breakdown 6 ° f i e l d was about 5 x 10 V/cm. eqn, 5.25 o° o 16 •1.2 11.0 0-8 o FROM BRIDGE MEASUREMENT. o CALCULATED USING HAMON S ME THOD © © < 2 o 5 • 4 -3 -2 0 1 log (f/Hz) 2 3 4 5 F i g . 5.8 F r e q u e n c y dependence o f e " as d e t e r m i n e d by a . c . b r i d g e and s t e p r e s p o n s e me thods . 110 6. CHARACTERISTICS OF THE A l 0 /GaAs INTERFACE 6.1 Introduction The properties of semiconductor surfaces and insulator-semiconductor i n t e r f a c e s have been extensively studied. (See, for example, Many, Goldstein and Grover 1965; Frankl 1967). The two p r i n c i p a l methods used are f i e l d - e f f e c t conductance and metal-insulator-semiconductor (MIS) capacitance measurements. An advantage of the MIS capacitance method i s that a knowledge of the surface * mobility i s not required. The MIS technique consists of measuring the capacitance of a metal-insulator-semiconductor sandwich as a function of an applied d.e. voltage and a small a.e. s i g n a l . Conclusions on the c h a r a c t e r i s t i c s of the insulator/semicon-ductor i n t e r f a c e are a r r i v e d at by i n t e r p r e t i n g the deviations of the experimental C-V curve from a t h e o r e t i c a l C-V curve based on an i d e a l i z e d model of the MIS capacitor. The most extensively studied system to date i s the SiO^/Si i n t e r f a c e system. A recent review was published by Revesz and Zaininger (1968). Very l i t t l e * The surface m o b i l i t y , u , i s defined by ns J j Al d nx yns q AN £ w x where AI = increase i n electron current over the flatband value for one ns unit width of a surface AN = / (n-n^)dz = excess surface c a r r i e r density with respect to the conditions at flatband. £ = e l e c t r i c f i e l d x See Many, Goldstein and Grover (1965) for further discussion. I l l work had been published on the insulator/GaAs system. H a l l and White (1965) reported some r e s u l t s on the p y r o l y t i c a l l y deposited SiO^/GaAs system. Since A^O^ appears to have c e r t a i n advantages over SiO^ (Sec. 5.1), the c h a r a c t e r i s t i c s of the Al^O^/GaAs i n t e r f a c e system would be of i n t e r e s t from the f i e l d - e f f e c t devices f a b r i c a t i o n viewpoint. The reasons for studying GaAs were mentioned i n chapter 1. 6.2 The Ideal MIS Capacitor The capacitance of the MIS structure i s defined by A W d .u C " W (6.1) g where = surface charge density induced on the metal electrode by an applied voltage V . For the moment, neglect metal-semiconductor contact p o t e n t i a l d i f f e r e n c e s , space charge i n the i n s u l a t o r and ohmic drop across the semiconductor bulk. Their e f f e c t w i l l be discussed i n Sec. 6.3.4. The applied voltage V i s V = V + V (6.2) g ox s where V = voltage drop across the i n s u l a t o r ox V g = voltage drop across the space charge layer near the i n s u l a t o r / semiconductor i n t e r f a c e . The applied voltage V i s defined to be p o s i t i v e when the metal plate i s p o s i t i v e l y biased with respect to the semiconductor. From Gauss' theorem, and the following d e f i n i t i o n s d 9 Q s c C s c = I' 3V I = d i f f e r e n t i a l capacitance of space charge (6.3a) s layer. * Some preliminary work was done on a plasma anodized A^O^/GaAs system. 112 d 3 Q s s c = I—r~ I = d i f f e r e n t i a ] , capacitance clue to surface states. (6.3b) ss 1 3V I and the fact that Q = -(Q + Q ) the normalized MIS capacitance c/c. can p ss sc 1 be written as c_ C s s + °sc (6.4) c. c + c + c. I ss sc i The quantities Q , Q are the t o t a l surface state and surface space charge ss sc d e n s i t i e s , r e s p e c t i v e l y . The capacitance per unit area of the i n s u l a t o r alone i s given by c^. For an i d e a l MIS capacitor, Q g g i s equal to zero. Hence = ± — (6.5) c. c. 1 + 1 c sc The capacitance c^ i s assumed to be independent of temperature and the magnitude, and frequency of the applied voltages. I t can be calculated once the d i e l e c t r i c constant and the thickness of the i n s u l a t o r are known. Depending upon the approximations used, an expression for c can be calculated. Thus, using eqns. 6.2, 6.5 and the appropriate expression for c > t h e o r e t i c a l C-V curves may be obtained and used i n the analysis cf the experimentally measured C-V curves. 6.3 Capacitance-Voltage C h a r a c t e r i s t i c s of an Ideal MIS Capacitor Many p h y s i c a l models and equivalent c i r c u i t s have been proposed as attempts i n explaining the c h a r a c t e r i s t i c s of a r e a l MIS capacitor. The MIS structure, as a voltage v a r i a b l e capacitor, was analyzed by Lindner (1962). Lehovec and Slobdskoy (1964) analyzed the a.c. behaviour of the MIS capacitor as a function of the frequency of the applied a.c. s i g n a l and of the d.e. b i a s . Terman (1962) was the f i r s t to use the MIS capacitor i n the study of thermally oxidized S i surfaces. Later, Grove, Deal, Snow and Sah (1965) presented, i n a 113 u n i f i e d form, simple- p h y s i c a l models of a MIS structure and used these models to explain the c h a r a c t e r i s t i c s of thermally oxidized S i surfaces. The models of the MIS capacitor discussed i n this section follow from Lindner and from Grove, Deal, Snow and Sah. 6.3.1 The Depletion Approximation Assume for t h i s , and for a l l subsequent discussions, that the semi-conductor i s p-type, that c l a s s i c a l s t a t i s t i c s hold and that a l l impurity atoms are ionized. From the energy band diagram (Fig. 6.1), i t can be seen that holes are depleted from the insulator/semiconductor i n t e r f a c e I f V y 0. The t o t a l space g charge per unit area i s given by Q = -q N x, (6.6) &c n A d where = density of impurity ions X j = width of the depletion layer d I m p l i c i t i n eqn. 6.6 are the assumptions that N i s uniform throughout the space charge regions i n the semiconductor and that the minority c a r r i e r s are t o t a l l y absent even i n the bias range where E_^  becomes smaller than E^. This s i t u a t i o n can a r i s e i f the i n s u l a t o r i s leaky or i f the capacitance i s measured under transient conditions. That i s , the d.e. bias i s switched r a p i d l y and the capacitance measured before minority c a r r i e r s can accumulate at the i n t e r f a c e . 14 -3 For impurity concentrations of l e s s than 10 cm , the depletion layer i s i n the order of lu thick. T y p i c a l l y , the metal counterelectrodes used are about 0.3 to 0.7 mm i n diameter. Hence, to a good approximation, the one-dimensional Poission's equation may be used. & = _ JL = < L N . ( 6.7) • 3 x 2 £ € 114 INSULATOR SEMICONDUCTOR E 4 F l l T d E. c — t v F i g . 6.1 Energy band diagram of an i d e a l metal/insulator/p-type semiconductor MIS capacitor. with the boundary conditions V ( 0 ) = V V ( x d ) = 0 Thus where v = v [ i - ~ ] 2 s x , a s I z (6.8) From equations 6.2, 6.3, 6.5 and 6.8, i t can be shown that 1 [ i + 2 c i 2 - y ] 1 / 2 q N A zvZo g (6.9) which predicts that the normalized capacitance varies inversely as the square root of the applied voltage while the semiconductor surface i s being depleted of majority c a r r i e r s . For non-positive gate voltages, no depletion region e x i s t s and eqn. 6.9 does not apply. 6.3.2 The Low Frequency Approximation For t h i s case, i t i s assumed that the space charge region and the semiconductor bulk are i n thermal equilibrium. This means that the a.c. s i g n a l must have a frequency low enough so that the minority c a r r i e r s can follow i t s v a r i a t i o n . The charge density p i s given by p = q[N D - N A + p - n] (6.10) From the condition that charge n e u t r a l i t y holds i n the bulk and that c l a s s i c a l s t a t i s t i c s i s applicable, p . q [ % - p b + p b e ^ k T - n b e < ^ k T ] (6.11) In terms of the normalized p o t e n t i a l , v = qV/kT, Poission's equation becomes 4 - - K - p b + p b - v ' ] (6.12) 3x subject to the boundary conditions ^ (.) = 0 3x (6.13) V ( o o ) = 0 In p r a c t i c e , the boundary conditions stated i n eqn. 6.13 are v a l i d provided that x >> L, the e f f e c t i v e Debye length which i s defined as 0 L = / - y - ^ I . (6.14) q (n b+p b) Since Q = p(x) dx (6.15) P sc o K 1 1 6 I t c a n b e s h o w n t h a t . . v Q = - y ^ - r q ( n , + p , ) L F ( 6 . 1 6 ) s c v b b s 1 s 1 w h e r e c o s h ( u -l-v ) , F = / 2 [• - 2 — _ v t a n h u . - 1 ( 6 . 1 7 ) s c o s h u ^ s D d q < ( , b a n d u , = r — - w h e r e q A, = - E . i n t h e b u l k . I n t e r m s o f t h e i n t r i n s i c b k T b F l c o n c e n t r a t i o n , v , 12 Q = - 2 - r - ^ - r q n . L „ { 2 [ c o s h ( v +i.L ) - c o s h u ^ - v s i n h u , ] } ( 6 . 1 8 ) H s c v I 1 i D s b b s b 1 s 1 1 2 L = [ £ ^ T ] , t h e i n t r i n s i c D e b y e l e n g t h . ( 6 . 1 9 ) 2 q n . l T h e t o t a l m i n o r i t y c a r r i e r s i n t h e i n v e r s i o n l a y e r , Q , i s g i v e n b y Q n - - q A n ( x ) d x - - q T b ^ d x s — d x V+U, v v b , s T , s e d v | v | q n i L D f - % ' " 1 / 2 ( 6 . 2 0 ) ( 2 [ c o s h ( v + u ^ ) - c o s h u ^ - v s i n h u ^ ] } w h e r e E . = E „ a t t h e p o i n t x . , a n d i s t h e p o i n t a t w h i c h t h e s e m i c o n d u c t o r l F . • l i s i n t r i n s i c . S i n c e , 3Q s c 1 8 v 1 s e q r ~( V s + U b ) - n . e ^ % + N - N ] ( 6 . 2 1 ) c = [ n . e x D A ' s c Q i s c f o r p - t y p e s e m i c o n d u c t o r , ®^>> N ^ , a n d . • = [ p - n . N J ( 6 . 2 2 ) ' s c Q L F s s A J s c - u , - ( v + u ) v + u w h e r e N = n e b a n d p - n = n . [ e s b - e S ] . I n t e r m s o f t h e s p a c e A i s s l 117 charge density, eqn. 6 . 2 can be written as • Q__ since V = V — ' ( 6 . 2 3 ) 8 s c . . Q + Q = 0. p sc When the surface potential is zero, the corresponding "flat-band" space charge capacitance, c , may be obtained from eqn. 6 . 2 2 , by expanding p S C O and Q in their power series and neglecting higher order terms If the impurities are completely ionized Csco ~ [ f e / N / / 2 ' ( 6 - 2 5 ) The minimum space charge capacitance, c , is not necessarily equal to c scm n sco The former is found by setting the diff e r e n t i a l of eqn. 6.22 equal to zero. For v > 0.3V, c was.given by Hall and White (1965) as s scm ,q NA scm 2|v | The minimum space charge capacitance occurs when the space charge layer is strongly inverted. Strong inversion begins at V g > 2^. 6.3.3 The High Frequency Approximation At sufficiently high frequencies, the minority carriers cannot follow the applied a.c. signal and thermal equilibrium cannot be maintained in the space charge layer. When a gate bias which corresponds to the depletion to inversion range is applied, the space charge density is given by Q s c = Q n + q [ N D - N A ] x d ' • (6.27) F ° r V s y > ±' v (v +u V2 r> o s , ~ S b Q - -2 - i — ~ r q n.L e sc v l D 1 s 1 118 n v i D 1 s 1 The ra t i o Q /Q approaches 1 and x, tends towards a saturation value given sc n d b y 4.e.£n(N /n.) 1/2 x d = [ (6.28) max A The c r i t e r i o n used for determining the onset of' strong inversion i s that the minority c a r r i e r concentration i n the space charge layer i s greater than or equal to the impurity ion concentration. This means that V g = 2^, which when substituted into eqn. 6.8 gives eqn. 6.28. The space charge capacitance i s now given by c = — (6.29) S C X , • a For applied voltages which give r i s e to accumulation layers, the high frequency approximation becomes i n v a l i d . For p-type semiconductors, this approximation can be used only i f V > 0. The C-V characteristics may now be calculated from -C- = 1 , (6.5) c. 1 + c./c 1 1 sc and Q s c V = - + V (6.23) g c ± s where c i s given by eqn. 6.29. When V >> 1, c/c. reaches a minimum and i s sc g 1 given by (•£-) = (6.30) c. c. l mm , l 1 H x , e d max 6.3.4 The Effect of Metal/Semiconductor Work Function Difference, Slow  Surface States and Charges i n the Insulator The effect of the metal/semiconductor work function difference, slow surface states and charges i n the oxide, on the C-V characteristics of the MIS capacitor can be conveniently taken into account by simply replacing V by 119 V - V^, i n the equations derived i n Sec. 6.3.1 to 6.3.3. The voltage V,., g fb fb i s defined as the "flat-band" p o t e n t i a l and i s given by V f, = > - — - - / ' j p . C x J d x - (6.31) f b Yms c. c. o d M i l l where d> = <f> - <i> = metal/semiconductor work function d i f f e r e n c e Tms m rs Q ^ = density of slow surface s t a t e s , i . e . states that cannot follow the applied a.e. s i g n a l . p^(x) = density of f i x e d space charge i n the i n s u l a t o r d = i n s u l a t o r thickness The term "flat-band" was derived from the fact that when V = , V =0. g f b ' s That i s the energy bands i n the semiconductor are f l a t r i g h t up to the i n s u l a t o r / semiconductor i n t e r f a c e and no space charge layer e x i s t s . 6.A Capacitance-Voltage C h a r a c t e r i s t i c s of a MIS Capacitor with Frequency Dependent Traps As opposed to the "slow" surface states considered i n Sec. 6.3.4, the insulator/semiconductor i n t e r f a c e may have traps that can follow the applied a.e. s i g n a l . These are known as " f a s t " surface states. H a l l and white's (1965) treatment of the e f f e c t of " f a s t " surface states on the C-V c h a r a c t e r i s t i c s of an MIS capacitor w i l l now be considered. Assume that P(E) i s the surface state density per electron v o l t , with the energy E measured from the i n t r i n s i c Fermi l e v e l at the surface of the semiconductor. (See F i g . 6.2) I f the traps are f i l l e d i n the energy range from E^ to E^ then E Q s 2 = q P(E)dE (6.31) The energy E^ i s assumed to be independent of V , while E^ = e C ^ - V ^ . ' The quantity "e" i n t h i s case i s taken to mean "one e l e c t r o n " and has a magnitude 120 of one. To avoid confusion, the symbol "e" i s used as opposed to the magnitude i i -19 of the charge of an electron which i s q = 1.6 x 10 couloumbs. E E. 2 E< I i c Ei E. E V Fig. 6.2 Energy band diagram of p-type semiconductor with, surface traps (after H a l l and Write 1965). From eqn. 6.3 and by applying Leibniz' rule for d i f f e r e n t i a t i o n of integrals to eqn. 6.31, c = |q e P(E )|' (6.32) ss ' 1 1 I m p l i c i t i n eqn. 6.32 i s the assumption that a l l the fast surface states are able to follow the applied a.c. sign a l . In general, this i s not true. The frequency dependence of c^^ can be accounted for by assuming a dimensionless frequency dependent factor, f(co), where 0 < -f((o) < 1 1 lim f(cj) a^ O lim f(w) 0 (6.32a) (6.32b) (6.32c) and eqn. 6.32 becomes ss |q e f(o)) P(E].) (6.33) In order to proceed further, the form of P(E) must be known. There are no a p r i o r i reasons why P(E) should be constant across the energy gap 121 of the semiconductor. In fact, for SiO^/Si, N i c o l l i a n and Goetzberger (1967) 1] -2 -1 found that P(E) varied from about 1.5 to 4 x 10 cm eV near midgap. However, by assuming that P(E) i s constant equal to N > the mathematics i s sim p l i f i e d and quantitative results can be obtained which may give some indica-tion as to the nature of the insulator/semiconductor interface. Equations 6.31 and 6.33 become %i = q [ V E i ] N s s = M N s s q [ V V V ( 6- 3 4 ) C = Iq e f ( u ) N I (6.35) ss 1 ss 1The normalized MIS capacitance and applied voltage (eqns. 6.4, 6.2 and 6.31) (6.36) become qfN + c c sc or where c q f N + c + c. i sc l c. c qfN = 1 1 n - c (6.37) 1 - c sc n N = e N ss d c c = -— n c. l and Q +Q -i j V = -[-§£_££.]•+ V + 4> - — r jp.(x)dx g c. s ms c. o d x " X X Q (V -<j> +V ) V -V £ L = q- 1 b 5 N + V (6.38) g fb c. c. s ° X X V „ — / d ^ p . ( x ) d x - V (6.39) fb c. ms c. o d x so x x Q s s = Q s l + Q s 2 ( 6 ' 3 9 a ) The C-V characteristics can be obtained from eqn. 6.36 and 6.38. The voltage ^ s l 1 d x V i s the surface potential when V and [ 1- d> - — / -j p. (x) dx] are so E c. ms c. o d x b x x both equal to zero. or where and 122 6.5 Computations Theoretical C-V characteristics were calculated with the aid of an IBM 360 computer and plotted using a Calcomp p l o t t e r . The following calculations and plots were done, s p e c i f i c a l l y for the Al^O^/p-type GaAs system. For n-type GaAs, simply.replace V by -V . The d i e l e c t r i c constants used were 8.4 (Bernard and Cook 1959) for A1 20 and 12 (Grove 1967) for GaAs. The i n t r i n s i c 1 2 - 3 c a r r i e r concentration at 300°K for GaAs was assumed to be 9 x 10 m (Grove 1967).. Unless otherwise stated, the MKS system of units was used. The computer programmes and plot subroutines were f i r s t tested and checked for errors by p l o t t i n g C-V curves for known values of SiO^ and Si and comparing them with published results (Goetzberger 1966). The following sets of curves were plotted. (1) The normalized capacitance versus applied d.e. voltage with the insulator thickness and dopant concentration as parameters i n the depletion, low frequency and high frequency approximations. (2) The flat-band and the minimum capacitances as functions of dopant concentration i n the low frequency approximation. (3) The normalized capacitance versus applied d.e. voltage with the frequency factor, surface state density, insulator thickness and dopant concentration as parameters. (4) The minimum surface potential as a function of dopant concentration. (5) The minimum surface space charge density as a function of dopant concentration. These curves are given i n the next pages. More C-V curves as functions of dopant concentration and insulator thickness are given i n Appendix 6.1. 123 F i e . 6 . 3 Q I, |v I v e r s u s N f o r G a A s 6 s c m 1 1 s m 1 124 F i e . 6.4 c , c versus N for GaAs. sco sera -6. F i g . 6.5 Q /N versus V (from H a l l and White 1965). s c /\ s o 128 6.6 Experimental Procedures Slices of single c r y s t a l n-type GaAs were mechanically polished to a 0.3 u f i n i s h and then were subjected to the standard cleaning procedure. After which they were flash-etched i n 7 H 2 S ° 4 : H 2 ° 2 ; H 2 ° ^ b y v o l u m e ) a t 8 0 ° c - T n e v were then rinsed i n d o u b l y - d i s t i l l e d water, i n propanol and dried i n a stream of N 2 gas. The wafers were immediately introduced into the Veeco vacuum system for the deposition of A l . A l sources were prepared and evaporated i n the same manner and vacuum conditions as described i n Chapter 5. The source and samples were allowed to cool for about an hour before breaking vacuum. This may .reduce the oxidation of the deposited A l f i l m due to exposure to the atmosphere. The GaAs wafers were then placed on suitable masks for the deposition of Au-Ge contacts on the opposite faces. Alloying of the eutectic has been described elsewhere i n the thesis. Each wafer, except the A l surface, was painted w i t h Apiezon wax which had been dissolved i n xylene, and l e t dried i n a i r . Apiezon wax acts as an insulator against the ele c t r o l y t e during a'nodization. The A l f i l m was anodized i n a manner sim i l a r to that described i n Chapter 5. P a r a l l e l runs using precleaned glass slides were always done as a means of checking and measuring the d i e l e c t r i c properties and the thickness of the A l 0 films. Au dot counterelectrodes were evaporated onto the A^O films. At 1 kHz, loss tangent ranging from 0.006 to 0.010 was obtained. Capacitances were measured using either the GR 1615A or the Boonton 75 C capacitance bridge, depending upon the test frequency used. The experi-mental set-up was as described i n Chapter 5. 1 2 9 6 . 7 R e s u l t s T h e m e a s u r e d M I S c u r v e s w e r e a n a l y z e d u s i n g a p r o c e d u r e s i m i l a r t o t h e o n e f o r m u l a t e d b y H a l l a n d W h i t e ( 1 9 6 5 ) . H o w e v e r , t h e e f f e c t s o f s l o w s t a t e s a n d c o n t a c t p o t e n t i a l d i f f e r e n c e s , w h i c h t h e y n e g l e c t e d , w e r e t a k e n i n t o a c c o u n t i n t h i s a n a l y s i s . E q u a t i o n 6 . 3 6 c a n b e r e w r i t t e n a s c . c q f N = - 1 n m + c ( 6 . 4 0 ) 1 - c s o n m n w h e r e c , c a r e t h e m i n i m u m n o r m a l i z e d c a p a c i t a n c e a n d m i n i m u m s p a c e c h a r g e n m s c m c a p a c i t a n c e , r e s p e c t i v e l y . T h e v a l u e o f c a n d t h e c o r r e s p o n d i n g V c a n b e n m g m e s t i m a t e d f r o m t h e e x p e r i m e n t a l c u r v e . A t z e r o s u r f a c e p o t e n t i a l , c c a n b e c a l c u l a t e d f r o m n o c + q f N SCO C n o ~ c + q f N + c . ( 6 . 4 1 ) SCO 1 T h e v a l u e s o f c a n d c c a n b e d e t e r m i n e d f r o m t h e g r a p h s o n s c m s c o F i g . 6 . 4 o n c e t h e c a r r i e r c o n c e n t r a t i o n o f t h e s e m i c o n d u c t o r i s k n o w n . H a v i n g c a l c u l a t e d c , t h e c o r r e s p o n d i n g v a l u e o f V c a n b e e s t i m a t e d n o g o f r o m t h e e x p e r i m e n t a l c u r v e . F r o m e q n . 6 . 3 8 a n d t h e k n o w n v a l u e s o f V a n d V g o g m [ V - V - V ] c . + Q q N = _ g m g o s m i s c m . ( 6 < 4 2 ) s m w h e r e V a n d Q a r e t h e s u r f a c e p o t e n t i a l a n d s u r f a c e c h a r g e , d e n s i t y w h e n s m s c m c = c . T h e s e c a n b e d e t e r m i n e d f r o m t h e g r a p h s i n F i g s . 6 . 3 a n d 6 . 4 . T h e s c s c m f r e q u e n c y f a c t o r i s , u s i n g e q n . 6 . 4 0 a n d 6 . 4 2 , g i v e n b y [ c . c - c • ( 1 - c )] V , i n m s c m n m s m 1 ~ [ 1 - c ] [ ( V - V - V ) c . + Q ] n m g m g o s m l s c m Q s l B y a s s u m i n g t h a t V = 0 a n d t h a t , f o r t h e m o m e n t , [ - + i> + / \ g ' c . Y m s 1 d p i ^ 1 — / — — — — d x ] i s n e g l i g i b l e , t h e f o l l o w i n g n o r m a l i z e d e x p r e s s i o n i s o b t a i n e d . i 130 V c c.+qN Q -SJP + V = = ^ ( 6 . 4 4 ) y N A ^ A so ^ • Q s c The p l o t o f - — v e r s u s V , which was adapted from H a l l and White ( 1 9 6 5 ) , i s •M. SO r A g i v e n i n F i g . 6 . 5 . The v a l u e of V i s o b t a i n e d by g r a p h i n g the LHS e x p r e s s i o n 1/2 and r e a d i n g o f f the c o o r d i n a t e a t i t s i n t e r s e c t i o n w i t h the graph o f Q / N. sco A Thus can be c a l c u l a t e d by u s i n g c N v = [v + v ] 4 + — <6-45> 2 so go qN q n^ The f r e q u e n c y f a c t o r , " f a s t " s u r f a c e s t a t e d e n s i t y and the " f i l l i n g " l e v e l o f two MIS c a p a c i t o r s were dete r m i n e d and t a b u l a t e d . U s i n g these v a l u e s t h e o r e t i c a l C-V c u r v e s were computed and p l o t t e d by u s i n g the FREDEP ( f r e q u e n c y dependence a p p r o x i m a t i o n ) s u b r o u t i n e . These are g i v e n i n F i g . 6 . 8 and 6 . 9 . The h o r i z o n t a l d i s p l a c e m e n t between the e x p e r i m e n t a l and the t h e o r e t i c a l c u r v e s g i v e s . . Q s l 1 d P i ( x ) V - V = -cf> - - s i + — / ~ — dx ( 6 . 4 6 ) f b so ms c. c o d l l where Ks = *m- [* + 2 V V ( 6 - 4 7 ) The work function of Au i s 4.8 eV (Sze 1969). The room-temperature electron a f f i n i t y and bandgap of GaAs are 4.07 eV and 1.4 eV, r e s p e c t i v e l y (Sze 1969). Using these values and V ^ - V as obtained g r a p h i c a l l y , the slow states 16 —2 —1 including charges i n the oxide, i f any, were found to be 9.4 x 10 m eV 17 -2 -1 and 1.6 x 10m eV . A comparison between r e s u l t s obtained i n t h i s work and those reported i n the l i t e r a t u r e i s given i n Table 6.1. From Table 6.1, i t can be seen that the " f a s t " surface state density of A^O^/GaAs i n t e r f a c e was comparable to that of p y r o l y t i c Si02/GaAs i n t e r f a c e and of " f r e e " and "treated" GaAs surfaces. The experimental MIS curves did not ex h i b i t any minima. From the Low Frequency Approximation and the Frequency Dependence Approximation models a minimum i n the C-V curve i s predicted provided F i g . 6.8 T h e o r e t i c a l FREDEP and e x p e r i m e n t a l C-V c u r v e s f o r sample G13-B. 0.9-o o 0.8 0.7 T — i — r — j — i — i — i — i — j — i — i — i — i — | i i i i j -30 -2P -10 0 VG/VOLT N = 2.0E18 XO =1.1SE-7 MS - 1.2E17 — j 1 1 ! 1 j 1 1 1 1 1 1 1" 10 20 30 . F i g . 6.9 T h e o r e t i c a l FREDEP and experimental C-V curves for sample. G15-Substrate i n s u l a t o r used f "slow" " f a s t " V2 Method(s) used Reference or surface con- Surface states surface states (Volts) d i t i o n . . p-GaAs p y r o l i t i c s i o 2 0.86 - 1.4xl0 1 6/m 2-eV -1.25 MIS H a l l and White (1965) n-GaAs p y r o l i t i c S i 0 2 0.79 - 1.0xl0 1 6/m 2-eV 1.15 MIS H a l l and White (1965) GaA^ P " f r e e " - - > 10 1 6/m 2 - f i e l d e f f e c t F l i n n and Briggs (1964) conductance and surface photovoltage n and p GaAs "tre a t e d " i n 1 5 / 2 10 /m pulsed f i e l d e f f e c t Kawaji and Gatos (1964) n and p GaAs "clean" i n uhv i n 1 7 / 2 10 /m contact poten- van Laar and Scheer t i a l d ifference (1967) and p h o t o e l e c t r i c emission n-GaAs . (G 13-B) n-GaAs (G 15-B) anodic A l ^ 0.95 9.4x10 /m"eV 1.9x10 /m eV -2.24 MIS anodic A 1 2 0 3 0.97 ~1.6xl0 1 7/m 2eV ~1.2xlO L 7/ m 2eV —0.61 MIS present work Table 6.1 Density of Surface States on GaAs 134 that the minority carriers in the space charge region are able to follow the applied a.c. signal. Thus, experimental results of the Al^O^/GaAs interface implied that the minority ca r r i e r s did not respond to the applied a.c. signal when the space charge layer was biased i n the inversion region. This result agreed with the results of H a l l and Write (1965, 1966) and Fl i n n and Briggs (1964). The l a t t e r authors measured the f i e l d effect conductance of p-type and n-type GaAs. They f a i l e d to observe any conductance minimum. As far as active f i e l d - e f f e c t devices are concerned, the need for an insulator/GaAs interface with better i n t e r f a c i a l properties (e.g. low " f a s t " surface state density) i s indicated. Although GaAs transistors were one of the f i r s t I l l - V compound active devices fabricated, as yet no p r a c t i c a l device has been made. I t was found that surface states l i m i t e d the performance of GaAs MIS f i e l d - e f f e c t transistors (1966 and 1968 Symposia: GaAs) . Improvements on the properties of the Al^O^/GaAs interface are necessary before i t can be seriously considered i n the fabrication of devices. * A solution.to this problem may l i e i n the use of a reverse-biased Schottky barr i e r as the gate (HowerHooper, Tremere, Lehrer and Bittmann 1968). A disadvantage of using a Schottky gate i s that the transistor can be operated only i n the depletion mode. 135 7. CONCLUSION A new method, the close-spaced sublimation (CSS) method, was developed and used to deposit GaAs films on sapphire. Advantages of the CSS method are as follows. F i r s t , the close source-to-substrate spacing produces near-equilibrium conditions, which lends i t s e l f to the deposition of s t o i c h i o m e t r i -c a l l y better fil m s . Second,most of the material i s deposited on where i t i s needed. This i s economical on source material. Third, because both the source and the substrate holders are independently supported and moveable with respect to each other, films can be deposited on more than one substrate i n one pumpdown. •The s t r u c t u r a l properties of the CSS GaAs films on sapphire were studied using o p t i c a l and electron microscopy, an electron microprobe and X-ray d i f f r a c -t i o n techniques. Growth features which resemble e q u i l a t e r a l tetrahedrons with t h e i r basal planes p a r a l l e l to- the surface of the substrate were observed i n most of the films which were deposited on substrates held at 600°C or higher. These growth features appeared to possess a thre e - f o l d symmetry which would be consistent with the symmetry properties of the <111> d i r e c t i o n of a zincblende structure. An i n d i c a t i o n of the r e l a t i v e composition of Ga and As i n a CSS f i l m was obtained by comparing, t h e i r respective X-ray counts to those obtained from a s i n g l e - c r y s t a l GaAs standard. The CSS films were found to be s t o i c h i o m e t r i c within the l i m i t s imposed by the probe, which i s less than 2 wt %. No gross inhomogeneities and excess Ga were detected. C r y s t a l l i t e s increased i n s i z e with increasing substrate temperature, from about 0.7u to 20u f o r substrate temperatures from 480 to 670°C. The c r y s t a l l i t e s i z e of CSS GaAs films compares favourably with the reported c r y s t a l l i t e sizes of GaAs films depostied by using other vacuum techniques. o Results from electron microscope r e f l e c t i o n - e l e c t r o n - d i f f r a c t i o n (RED) and X-ray d i f f r a c t i o n studies indicated that texturing of the films increased with increasing substrate temperature. <111> texture was observed on films 136 deposited at 600°C or higher. The theory of Kaischew and Bliznakow as applied by Molnar, Flood and Francombe was presented as an explanation for the occurrence of the <111> texture. Single-crystal Laue patterns, with most of the prominent spots consistent with those found i n the pattern, of (111) GaAs p a r a l l e l to the substrate, were observed on 630 to 640°C films. No abnormal metastable wurtzite phases were observed i n the CSS films. In the case of III-V compounds, nonstoichio-metry can lead to the formation of a metastable wurtzite phase. Lower substrate temperature fi l m s , which appeared " d u l l " , had very low 2 ( <1 cm /V-sec) c a r r i e r m o b i l i t i e s while higher substrate temperature films, which appeared "shiny", had higher c a r r i e r m o b i l i t i e s . Two types of "shiny" films were observed - the "high p" and the "low p" films. At room temperature, the i 2 "high p" films had low H a l l m o b i l i t i e s (~2 to 13 cm /V-sec) and high r e s i s t i v i t i e s 2 5 ' (4 x 10 to 10 jj-cm). At the same temperature, the "low p" films had higher H a l l 2 mob i l i t i e s (12 to 42 cm /V-sec) and lower r e s i s t i v i t i e s (0.6 to 86£2-cm). A l l the as-grown films were p-type. The e l e c t r i c a l properties of the films were discussed i n terms of the films' p o l y c r y s t a l l i n e structure, deviation from s t o i -chiometry and compensation of impurities. Ionized impurity scattering' was the dominant mobility-reducing mechanism. The films were heavily compensated, with free c a r r i e r concentrations several orders of magnitude lower than the t o t a l 19 20 -3 impurity concentrations, which were i n the order of 10 to 10 cm .' The t o t a l impurity concentration i n each f i l m was estimated by using Brooks' formula for ionized impurity scattering. The room-temperature electron mobility of a 2 f i l m that was converted to n-type by postdeposition Ge doping was 77 cm /V-sec. Although the c a r r i e r mobilities of the CSS films were low, they compare favourably with the c a r r i e r m o b i l i t i e s of heteroepitaxial GaAs films deposited by using other vacuum techniques. A homoepitaxial layer was grown on a semi-insulating GaAs substrate. 2 The room-temperature electron mobility of the as-grown n-type f i l m was 219 cm /V-sec. 3/2 The electron H a l l mobility varied with temperature as T , which i s the form 137 predicted by Brooks-Herring formula f o r ionized impurity s c a t t e r i n g . T h i n - f i l m insulated-gate f i e l d - e f f e c t t r a n s i s t o r s were fa b r i c a t e d using as-grown CSS f i l m s . T r a n s i s t o r s that operated i n the p-type depletion mode with transconductance of O.lp-y were observed. The e f f e c t i v e m o b i l i t y 2 was 4 cm /V-sec. Diodes fabricated on a converted f i l m showed " s o f t " charac-t e r i s t i c s . While t r a n s i s t o r action and r e c t i f i c a t i o n c h a r a c t e r i s t i c s were observed, the q u a l i t y of the CSS films needs to be improved before devices with better c h a r a c t e r i s t i c s can be made. Evaporated A l films on glass were anodized i n ammonium pentaborate d i s -solved i n ethylene g l y c o l . The loss tangent was found to be a slow function of frequency with higher values at lower frequencies. Step response currents followed a t n law, where n was 0.95 and 0.7 for the discharging and charging currents, r e s p e c t i v e l y . For a l i n e a r d i e l e c t r i c response, t h i s corresponded to e"(w) varying as co11 The d.e. r e s i s t i v i t y at low f i e l d s was about 10^"3fi-cm. The breakdown f i e l d was about 5 x 10^ V/cm. Capacitances of Au/Al^O^/GaAs MIS capacitors were measured as functions of applied d.e. f i e l d . T h e o r e t i c a l capacitance-voltage (C-V) curves, based on i d e a l i z e d models, were computed and pl o t t e d . The experimental C-V curves were analyzed by using a procedure s i m i l a r to the one developed by H a l l and White. The e f f e c t s of contact p o t e n t i a l d i f f e r e n c e s , charges i n the oxide and "slow" surface states, which H a l l and White neglected, were considered i n t h i s study. "Fast" surface state d e n s i t i e s i n the order of 10"^ to 10^ /m" eV were obtained. These were comparable to the density of surface states at the GaAs surface as obtained by H a l l and White and by other authors. The presence of surface states i n the insulator/semiconductor i n t e r f a c e at such concentration l e v e l s would degrade the f i e l d - e f f e c t action at the i n t e r f a c e and render i t unsuitable i n the f a b r i c a t i o n of f i e l d - e f f e c t devices. 138 Interesting topics related to the present work which merit further re-search are given as follows. A systematic investigation of the R^-prefiring of the sapphire substrate i s important i n determining the effects of substrate surface on the properties of CSS GaAs films. How di f f e r e n t substrates, i n p a r t i c u l a r semi-insulating GaAs, affect the properties of CSS GaAs films i s of interest. The us.e of semi-insulating GaAs substrates i s of less interest from the t h i n - f i l m c i r c u i t ' point of view. A necessary condition for obtaining a tunnel diode i s that both p-type and n-type regions of a p-n junction must be degenerately doped (dopant 18 — 3 concentration of greater than about 10 cm for GaAs at room temperature). The high density of dopants i n the CSS GaAs films may actually prove to be an asset i n the fabrication of tunnel diodes. However, other factors such as the degradation of the tunnel diode due to defects and to d i f f u s i o n of impurities l i k e Cu must be considered. Insulating films produced by anodizing a metal f i l m i n an oxygen plasma i s a r e l a t i v e l y new technique. As yet, very l i t t l e work has been reported on the properties of the plasma-anodized-metal-oxide/GaAs interface. Such studies may y i e l d an insulator/GaAs interface with characteristics suitable for f i e l d -effect devices. 139 BIBLIOGRAPHY J.A. Aboaf "Deposition and Properties of Aluminum Oxide Obtained by Pyrolytic Decomposition of An Aluminum Alkoxide", 'J. Electroche.m. Soc. vol. 114, 948 (1967). — W. Albers, "Physical Chemistry of Defects", Physics and Chemistry of II-VI Compounds M. Aven and J.S. Prener (editors), Interscience (1967). R.H. Alderson and J.S. Halliday, "Electron Diffraction" Techniques for Electron  Microscopy, D.A. Kay (edit) Blackwell (1965). J.R. Arthur, "Vapour Pressures and Phase Equilibria In The Ga-As System", J. Phys.  Chem. Solids vol. 28, #11, 2257 (1967). J.R. Arthur, "Interaction of Ga and As 2 Molecular Beams with GaAs Surfaces", J. App. Phys. vol. 39, #8, 4032 (1968). F. Argall and A.K. Joncher, "Dielectric Properties of Thin Films of Aluminium Oxide and Silicon Oxide", Thin Solid Films vol.2, 185 (1968). M.M. Atalla, "Semiconductor Surfaces and Films; The Si-Si02 System", Properties of Elemental and Compound Semiconductors, H. Gatos (edit), Interscience (1960). M.E. Baird, "Determination of Dielectric Behaviour at Low Frequencies from Measure-ments of Anomalous Charging and Discharging Currents", Rev. Mod. Phys. vol. 40 #1, 219 (1968). W.J. Bernard and J.W. Cook, "The Growth of Barrier Oxide Films on Al" , J. Electrbchem.  Soc. vol. 106, 643 (1959). R.W. Berry, P.M. Hall and M.T. Harris, Thin Film Technology, Bell Tel. Lab. Series, North-Holland (1968). F. Berz, "Variation with Frequency of the Transverse Impedance of Semiconductor Surface Layers", J. Phys. Chem. Solids vol. 23, 1795 (1962). J. Blanc, R.H. Bube and L. Weisberg, "E l e c t r i c a l Acitivty of Cu in GaAs", J. Phys. Chem. Solids vol. 25, 221 (1964). J. Blanc, R.H. Bube and L. Weisberg, "Evidence for the Existence of High Concen-trations of Lattice Defects in GaAs", Phys. R.ev. Letters, vol. 9, 252 (1962). D.E. Bolger, J. Franks, J. Conrad and J. Whitaker, "Preparation and Characteristics of GaAs", 23, GaAs: 1966 Symposium Proceedings, Inst. of Phys. and Phys. Soc. Conf. Series #3 (1967). L.S. Birks, Electron Probe Microanalysis, Wiley (1963). N. Braslau, J.B. Gunn and J.L. Staples, "Metal-Semiconductor Contacts for GaAs Bulk Devices", Solid State Elec. vol. 10, 381 (1967). 140 H. Brooks, "Theory of • the E l e c t r i c a l Properties -of 'Germanium and S i l i c o n " , Advances i n Electronics and'Electron Physics, v o l . 7, 85 (1955). D. S. Campbell, "Methods of Preparing Thin Films", The Use of Thin Films in Physical Investigations, J. C. Anderson (edit) AP (1966). R. Castaing, P. Deschamps and J . P h i l i b e r t (editors) X-Ray Optics and Microanalysis (1965). C. Cherki and R. Coelho, "On Charge Storage i n Anodic Tantal Oxide Layers", Phys. Stat. S o l i d i , v o l . 19, K91 (1967). K.L. Chopra, Thin Film Phenomena, McGraw-Hill (1969). T.L. Chu, CH. Lee and G.A. Gruber, "The Preparation and Properties of S i l i c o n N i t r i d e " , J. Electrochem. Soc. v o l . 114 #7, 718 (1967). J.W. Colby, "Quantitative Microprobe Analysis of Thin D i e l e c t r i c Films", private communication. E. M. Conwell, High F i e l d Transport i n Semiconductors, Solid State Physics Supp 9, Academic Press (1967). E. M. Conwell and V.F. Weisskopf, "Theory of Impurity Scattering i n Semiconductors", Phys. Rev, v o l . 77 #3, 388 (1950). R.H. Cox and H. Strack, "Ohmic Contacts for GaAs Devices", Solid-State Elec. vol 10 #12, 1213 (1967). B. D. C u l l i t y , Elements of X-Ray D i f f r a c t i o n , Addision-Wesley (1959). F. A. Cunnell, T. Edmond and W.R. Harding, "Technology of GaAs", Solid-State Elec. v o l . 1, 97 (1960). J.E. Davey and T. Pankey, "Structural and Optical Characteristics of Thin GaAs Films", J. App. Phys. v o l . 35 #7, 2203 (1964). J.E. Davey and T. Pankey, " E p i t a x i a l GaAs Films Deposited by Vacuum Evaporation", J. App. Phys. v o l . 39 #4, 1941 (1968). C. J. Dell'Oca, D.L. Pulfrey and L. Young, "Anodic Oxide Films", review a r t i c l e i n Physics of Thin Films, i n press. S.S. Devlin, "Transport Properties", Physics and Chemistry of II-VI Compounds, M. Aven and J.S. Prener (editors), Interscience (1967). R. Dreiner, "The Temperature Dependence of the F i e l d Coefficient for the Anodization of Tantalum", J. Electrochem. Soc. v o l . I l l , 1350 (1964). J.' Drowart and P. Goldfinger, J. Chim. Phys. v o l . 55, 721 (1958) (in French). D. Eff e r , " E p i t a x i a l Growth of Doped and Pure GaAs i n an Open Flow System", J. Electrochem. Soc. v o l . 112, 1020 (1965). 141 H. Ehrenreich, "Band Structure and Electron Transport of GaAs", Phys. Rev., vol. 120 #6, 1951 (1960). " T. Evans and A. Noreika, "Effect of Gaseous Environment on the Structure of Sputtered GaAs Films on NaCl Substrates", Phil. Mag, vol. 13, 717 (1966) M.R. Farukhi and E.J. Charlson, "Structural and El e c t r i c a l Properties of Flash-Evaporated Thin GaAs Films", J. App. Phys. vol. 40 #13, 5361 (1969). M.M. Faktor, D.G. Fiddyment and G.R. Newns, "Preliminary Study of the Chemical Polishing of a-Corundum Surfaces with Vanadium Pentoxide", J. Electrochem.  Soc. vol. 114 #4, 356 (1967). E. Ferrieu and B. Pruniaux, "Preliminary Investigations of Reactively Evaporated A1 20 3 Films on S i " , J. Electrochem. Soc, vol. 116, #7, 1008 (1969). J.D. Filby and S. Nielsen, "Single-Crystal Films of Silicon on Insulators", B r i t . J. App. Phys. vol. 18, 1357 (1967). G. Fischer, D. Greig and E. Mooser, "Apparatus for the Measurement of Galvanomagnetic Effects in High Resistance Samples", Rev. Sci. Inst, vol. 32 #7, 842 (1961). I. Flinn and M. Briggs, "Surface Measurements on GaAs",. Surf. Sci. vol. 2, '136 (1964). D.R. Frankl, E l e c t r i c a l Properties of Semiconductor Surfaces, Pergamon Press (1967). H. Frbhlich, Theory of Dielectrics, Oxford (1949). M. Gevers and F. Dupre, "The Relation Between the Power Factor and the Temperature Coefficient of the Dielectric Constant of Solid Dielectrics", Phil. Res.  Rept. vol. 1, 279 (1945). M. Gevers and 7. Dupre, "Power Factor and Temperature Coefficient of Solid (Amorphous) Dielectrics", Trans. Farad. Soc. vol. 42A, 47 (1946). A. Goetzberger, "Ideal M0S Curves for Silicon", Bell System Tech. J. vol. 45 #7, 1097 (1966). B. Goldstein, "Self and Impurity Diffusion in Gallium Arsenide", Compound Semi-conductors , R.K. Willardson and H.L. Goering (editors), Reinhold (1962). W.S. Goruk, L. Young, F.G.R. Zobel, "Ionic and Electronic Currents at High Fields in Anodic Oxide Films," Modern Aspects of Electrochemistry, vol. 5, Plenum (1966). R.F. Greene, D.R. Frankl and J.N. Zemel, "Surface Transport in Semiconductors", Phys. Rev, vol. 118, 967 (I960). A.G. Grove, B.E. Deal, E.H. Snow and C T . Sah, "Investigations of Thermally Oxi-Oxidized Si Surfaces Using MOS Structures", Solid State E l e c vol. 8, 145 (1965). A.G. Grove, Physics and Technology of Semiconductor Devices, Wiley (1967). J.B. Gunn, "Microwave Oscillations of Current in III-V Semiconductors", Solid-State E l e c Comm. vol. 1, 88 (1963). 142 K.G. Gunther, "Interfacial and Condensation Processes Occurring with Multicom-ponent Vapours", The Use of Thin Films in Physical Investigations, J.C. Anderson (edit) AP(1966). H. Gutbier, Z. Naturf ,vol. 169, 268 (1961) (in German). R.R. Haering and J.F. 0'HanIon, "Control of the Surface Potential of Evaporated CdS Layers", Proc. IEEE vol. 55, 692 (1967). R.N. Hall, "Solubility ofHI-V Compound Semiconductors in Column III Liquids", • J. Electrochem. Soc. vol. 110, 385 (1963). C.E. Hall, Introduction to Electron Microscopy, McGraw-Hill (1966). R. Hall and J.P. White, "Surface Capacity of Oxide Coated Semiconductors" Solid-State Elec. vol. 8, 211 (1965). B. Hamon, "An Approximate Method for Deducing Dielectric Loss Factor from dc . Measurements", Proc IEE vol. 99, 151 (1952). A.C. Harkness and L. Young, "High Resistance Anodic Oxide Films on Aluminium", Can. J. Chem. vol. 44, 2409 (.1966). J. S. Harris, Y. Nannichi, G.L. Pearson and G.F. Day, "Ohmic Contacts to Solution-Grown Gallium. Arsenide", J. Appl Phys. vol. 40 #11, 4575 (1969). P.B. Hart, P.J. Etter, B.W. Jervis and J.M. Flanders, "E l e c t r i c a l Properties of Epitaxial Silicon Films on ct-Aluroina", Brit. J. App. Phys. vol. 18, 1389 (1967). C. Hilsum, '.'Some Key Feature of III-V Compounds", Semiconductors and Semimetals , vol.1, R.K. Willardson and A.C. Beer (editors) AP (1966). C. Hilsum and A.C. Rose-Innes, Semiconductor III—V Compounds, Pergamon (1961). S.R. Hofstein and G. Warfield, "Physical Limitations on the Frequency Response of a Semiconducting Surface Inversion Layer", Solid State Elec. vol. 8, 321 (1965). L. Holland, Vacuum Deposition of Thin Films, Chapman...and..Hall (1956). D. J. Howarth and E.H. Sondheimer, "The Theory of Electronic Conduction in Polar Semiconductors", Proc. Royal Soc. vol. A 219, 53 (1953). P.L. Hower, W.W. Hooper, D.A. Tremere, W. Lehrer and CA. Bittmann, "The Schottky Barrier GaAs FET", 1968 Symposium on GaAs, Dallas (1968). A. Howie, "Interpretation of Micrographs of Metal Foils and Other Objects", Techniques for Electron Microscopy, D.H. Kay (edit) Blackwell (1965). P. Hudock, "Epitaxial GaAs Films Deposited Under Near-Equilibrium Conditions in Ultra-High Vacuum", Extended Abst. 1-3, Electronics Div. vol. 16 #1, Electrochem. Soc. Meeting, Dallas (1967). L.P. Hunter, Handbook of Semiconductor Electronics. (2nd edition), McGraw-Hill (1962). I. Isenberg, B.R. Russell and R.F. Greene, "Improved Method of Measuring Hall Ceofficierits", Rev. Sci. Instrum., vol. 19, 685 (1948). 143 I.T. Johansen, " E l e c t r i c a l Conductivity i n Evaporated S i l i c o n Oxide Films", J. App. Phys. v o l . 37, ill, 499 (1966). B.D. Joyce and J.B. M u l l i n , "Growth 'Pyramids' i n E p i t a x i a l GaAs", Solid State Comm. vol . 4, 463 (1966). B. D. Joyce and J.B. M u l l i n , "Pyramid Formation in E p i t a x i a l GaAs Layers", 23, GaAs: 1966 Symposium Proceedings, Inst, of Phys. and Phys. Soc. Conf. Series #3 (1967). R. Kaischew and G. Bliznakow, Compt. Rend. L'Acad. Bulgure S c i . , v o l . 1 #2-3, 23 (1948). C. S. Kang and P.E. Greene, "Tin and Tellurium Doping Characteristics i n Gallium Arsenide E p i t a x i a l Layers Grown from Ga Solution", 1968 Symposium on  GaAs, Dallas Inst, of Phys. and Phys. Soc- Conf. Series #7 (1968). D. H. Kay (ed i t o r ) , Techniques for Electron Microscopy, Blackwell (1965). S. Kawaji and H.C. Gatos, "Gallium Arsenide Surface States", Surf. S c i . v o l . 1, 407 (1964). J. Klerer, "A Method.for the Deposition of Si0„ at Low Temperatures", J. Electrochem. Soc. v o l . 108, 1070 f1961). W. Kb:ster and B. Thomas, Z. Mettalk, v o l . 46, 291 (1955) (in German). F. A. Kroger, Chemistry of Imperfect Crystals, North Holland (1964). D. F. Kyser and D.B. Wittry, "Cathodoluminescence i n Gallium Arsenide", The Electron Microprobe T.D. McKinley, K.F.J. Heinreich and D.G. Wittry (editors), Wiley (1966). E. N. Laverko, V.M. Marakhonov and S.M. Polyakov, "Structure of Gallium Arsenide • Whiskers on Germanium", Sov. Phys. C r i s t , v o l . 10, 611 (1966). K. Lehovec and A. Slobodskoy, "Impedance of Semiconductor-Insulator-Metal Capacitors", Solid-State Elec. v o l . 7, 59 (1964). R. Lindner, "Semiconductor Surface Varactor", B e l l Syst. Tech. J. Vol. 41, 803 (1962). P. Lublin and W.J. Sutkowski, "Application of the Electron Probe to Electronic Materials", The Electron Microprobe T.D. McKinley, K.F.J. Heinreich and D.B. Wittry (editors), Wiley (1966). G. W. Mahlman, "Photoconductivity of Lead Sulfide.Films", Phys. Rev, v o l . 103 #6, 1619 (1956). H. M. Manasevit, "Single-Crystal Gallium Arsenide on Insulating Substrates", App. Phys. Lett, v o l . 12 # 4 , 156 (1968). H.M. Manasevit and F.L. Morritz, "Gas Phase Etching of Sapphire with Sulfur Fluorides", J. Electrochem. Soc. v o l . 114, 204 (1967). 144 0. Maclelung, Physics of III--V Compounds, Wiley (1964). A. Many, Y. Goldstein and N.B. Grover, Semiconductor Surfaces, North-Holland (1965). S. Martinuzzi, Ph.D. Thesis, U. of Marseille (1966). T.D. McKinley, K.F.J. Heinreich and D.B. Wittry (editors), The Electron Microprobe, Wiley (1966). M.G. Mier and E.A. Buvinger, "A Comparative Study of Anodized Evaporated and Sputtered Aluminum Oxide Thin Films", Vac. Sci. and Tech. vol. 6 //4, 727 (1969). R.R. Moest and B.R. Shupp, "Preparation.of Epitaxial. GaAs and GaP Films by Vapour Phase Reaction", J. Electrochem. Soc. vol. 109 #11, 1061 (1962). B. Molnar, J. Flood and M.. Francombe, • "Fibered and Epitaxial Growth in Sputtered • Films of GaAs", J. App. Phys. vol. 35 #12, 3554 (1964). W. von Muench, "Gallium Arsenide Planar Technology", IBM Journal, 438, (Nov. 1966). E. K. Miiller, "Structure of Oriented, Vapor-Deposited GaAs Films Studied by Electron Diffraction", vol. 35 #3, 580 (1964). H. Nelson, "Epitaxial Growth from the Liquid State and Its Application to the Fabrication of Tunnel and Laser Diodes", RCA Review, vol. 24 (1963). F. H. N i c o l l , "The Use of Close Spacing in Chemical-Transport Systems for Growing Epitaxial Layers of Semiconductors", J. Electrochem. Soc. vol. 110 #11, 1165 (1963). E.H. Nicollian and A. Goetzberger, "The Si-Si02 Interface - El e c t r i c a l Properties as Determined by the Metal-Insulator-Silicon Conductance Technique", Bell Syst. Tech. J. vol. 46 #6 , 1055 (1967). T. Pankey and J.E. Davey, "Structural and Optical Characteristics of Thin GaAs Films. II", J. App. Phys. vol. 37 #4, 1507 (1966). R.L. Petritz, "Theory of Photoconductivity in Semiconductor Films", Phys. Rev. vol. 104 #6, 1508 (1956). D. L. Pulfrey, P.S. Wilcox and L. Young, "Dielectric Properties of Ta205 Films", J. App. Phys. vol. 40 #10, 3891.(1969). E. H. Putley, The Hall Effect and Related Phenomena, Butterworths (1960). F. D. Rosi, D. Meyerhofer and R.V. Jensen, "Properties of p-Type GaAs Prepared by Copper Diffusion", J. App. Phys. vol. 31 #6, 1105 (1960). A.G. Revesz and K.H. Zaininger, "The Si-Si02 Solid-Solid Interface System", RCA Rev, vol. 29 #1, (1968). J.L. Richards, P.B. Hart and L.M. Gallone, "Epitaxy of Compound Semiconductors by Flash Evaporation", J. App. Phys. vol. 34, 3418 (1963). J.L. Richards, "Flash Evaporation", The Use of Thin Films in Physical Investigations J.C. Anderson (edit) AP(1966). D. Richman, "Dissociation Pressures of GaAs, GaP and InP and the Nature of III-V • Melts", J. Phys. Chem. Solids vol. 24, 1131 (1963). 145 P.H. Robinson, "Transport of Gallium Arsenide by a Close-Spaced Technique", RCA Rev, v o l . 24 , 574 (.1963). P.H. Robinson and CW. Mueller, "The Deposition of S i l i c o n Upon Sapphire Substrates", Trans. AIME vo l . 236, 268 (1966). C.A.T. Salama, "Evaporated S i l i c o n Thin-Film Transistors", Ph.D. Thesis, Dept. of E l e c t r i c a l Engineering, U.B.C. (1966). W.W. Scanlon, "Interpretation of Hall Effect and R e s i s t i v i t y Data i n PbS and Similar Binary Compound Semiconductors", Phys. Rev, v o l . 92, 1573 (1953). R. Scheuplin and P. Gibbs, "Surface Structure i n Corundum: I, Etching of Dislocations", J. Am. Ceramic Soc. v o l . 43, 458 (1960). J.R. Schrieffer, "Effective Carrier Mobility i n Surface-Space Charge Layers", Phys. Rev, v o l . 97, 641 (1955). W. Shockley, Electrons and Holes i n Semiconductors, van Nostrand (1963). J.C. Slater, "Barrier Theory of the Photoconductivity of Lead Sulfide", Phys. Rev. v o l . 103 #6, 1631 (1956). I.N. Stranski and R.Z. Kaischew, Z. K r i s t . v o l . 78, 373 (1931) (in German). S.M. Sze, Physics of Semiconductor Devices, Wiley (1969). S.M. Sze and J.C. I r v i n , " R e s i s t i v i t y , Mobility and Impurity Levels i n GaAs, Ge and Si at 300°K", Solid-State Elec. v o l . 11, 599 (1968). L.M. Terman, "An Investigation of Surface States at a S i l i c o n / S i l i c o n Oxide Interface Employing Metal-Oxide-Silicon Diodes", Solid-State Electronics v o l . 5, 285 (1962)^ CD. Thurmond, "Phase E q u i l i b r i a i n the GaAs and GaP Systems" J. Phys. Chem.  Solids v o l . 26, 785 (1965). A.C Tickl e , Thin-Film Transistors A New Approach to Microelectronics, Wiley (1969). J.J. Tietjen and J.A. Amick, "Preparation and Properties of Vapour Deposited GaAs P Using Arsine and Phosphine", J. Electrochem. Soc. v o l . 113 //7, 72 X, X(1966) J.J. Tietjen, M.S. Abrahams, A.B. Dreeben and H.F. Gossenberger, "The Origin of Macroscopic Surface Imperfections i n Vapour-Grown GaAs", 1968 Symposium on GaAs, Dallas, Inst. Phys. and Phys. Soc. Conf. Series ill, (1968). N.J. Tighe, "Jet Thinning Device for Preparation of AI2O3 Electron Microscope Specimens", Rev. Sci. Inst, v o l . 35, 520 (1964). A.B. Torrens, "Negative D i f f e r e n t i a l Conductivity Effects i n Semiconductors", Ph.D. Thesis,Dept. of E l e c t r i c a l Engineering,,U.B.C (1969). 146 H.C. Torrey and CA. Whitner, Crystal Rectifiers, McGraw-Hill (1948). T.W. Tucker, "The Electrical Properties of Evaporated Silicon Films", M.A.Sc. Thesis, Dept. of Electrical Engineering, U.B.C. (1966). H.J.vanDaal, "Mobility of Charge Carriers in SiC", Philips Res. Rept. Supp. (1965). L.J. van der Pauw, "A Method of Measuring Specific Resistivity and Hall Effect of Discs of Arbitrary Shape", Philips Res. Rept. vol. 13 #1, (1958). J. van Laar and J.J. Scheer, "Influence of Volume Dope on Fermi Level Position at Gallium Arsenide Surfaces", Surf. Sci. vol. 8, 342 (1967). J.R. Volger, "Note on the Hall Potential Across an Inhomogeneous Conductor", Phys. Rev, vol. 79, 1023 (1950). B.E. Warren and B.L. Averbach, "The Effect of Cold-Work Distortion on X-Ray Patterns", J. App.Phys.,vol. 21, 595 (1950). A. Waxman, V.E. Henrich, F.V. Shallcross, H. Borkan and P.K. Weimer, "Electron Mobility Studies in Surface Space-Charge Layers in Vapour-Deposited CdS Films", J. App. Phys. vol. 36 ill, 168 (1965). A. Waxman and K.H. Zaininger, "A^O^-Silicon Insulated Gate Field Effect Transistors", App. Phys. Lett, vol. 12 #13, 109 (1968). L.R. Weisberg, F.D. Rosi, P.G. Herkart, Properties of Elemental and Compound  Semiconductors, Met. Soc. Conf. vol.5, Interscience (I960). P.K. Weimer, "The Insulated-Gate Thin-Film Transistor", Physics of Thin Films vol. 2, G. Hass and R. Thun (editors), AP (1964) G.A. Wolff, "Faces and Habits of Diamond Type Crystals", Am. Mineralogist vol. 41, 60 (1956), E. Yon, W.H. Ko and A.B. Kyser, "Sodium Distribution in Thermal Oxide on Silicon by Radiochemical and M0S Analysis", IEEE vol. ED-13, 276 (1966). L. Young, Anodic Oxide Films, AP (1961). K.H. Zaininger and A.S. Waxman, "Radiation Resistance of AI2O3 M0S Devices", IEEE vol. ED-16 #4, 333 (1969). 1 4 7 A P P E N D I X 4 . 1 C A R R I E R T R A N S P O R T I N S E M I C O N D U C T O R S A 4 . 1 . 1 T h e B o l t z m a m i T r a n s p o r t E q u a t i o n A t s t e a d y s t a t e , t h e B o l t z m a n n e q u a t i o n f o r t h e d i s t r i b u t i o n f u n c t i o n f ( k , r ) o f c a r r i e r s i n a s o l i d i s g i v e n b y - £ [ £ + v * ¥ ] v ^ f (7,7) + v - v 7 f (7,7) = [ ^ r l ] s c a t ( A 4 . 1 ) F o r s o l i d s w h i c h h a v e a s i n g l e s p h e r i c a l e n e r g y s u r f a c e , - n k = m v , a n d e q n . A 4 . 1 c a n b e w r i t t e n a s ~ £ *[£+ v x B]V-f(v,r) + v-V-fCv,r)= [MCLil] ( M ' 2 ) 1 1 1 • s c a t I n t h e c a s e o f s e m i c o n d u c t o r s t h a t a r e a t t h e r m a l e q u i l i b r i u m , t h e d i s t r i b u t i o n f u n c t i o n f i s f = f o = T + ( E - E p ) / k T ' ( M - 3 ) 1 + e ^ f o r d e g e n e r a t e s e m i c o n d u c t o r s o r - [ ( E - E ) / k T ] f = f = e ( A 4 . 4 ) o f o r n o n d e g e n e r a t e s e m i c o n d u c t o r s . W h e n c a r r i e r s s u f f e r e l a s t i c s c a t t e r i n g o n l y , t h e r a t e o f c h a n g e o f f ( k , r ) d u e t o s c a t t e r i n g i s g i v e n b y [ 8 f ( ^ r ) ] = / { f ( k',7 ) [ l - f ( k,7)] - f ( k,7 ) [ l - f ( k\7)]} Q ( k , k ' ) d k ' ( A 4 . 5 ) s c a t T h e t r a n s i s t i o n p r o b a b i l i t y Q ( k , k ' ) m e a s u r e s t h e r a t e o f t r a n s i t i o n f r o m s t a t e k t o s t a t e k ' . B y t h e p r i n c i p l e o f m i c r o s c o p i c r e v e r s i b i l i t y , Q ( k , k ' ) = Q ( k ' , k ) 3' f ( k 7) T h e e x p r e s s i o n f o r [ r 1 - — ] g i v e n i n e q n . A 4 . 5 c a n n o t b e s i m p l i f i e d a n y o t s c a t f u r t h e r . I t i s u s u a l t o a s s u m e t h a t ( l l ) = ° o ( A 4 . 6 ) 9 t s c a t x w h e r e T i s t h e r e l a x a t i o n t i m e . T h e p h y s i c a l s i g n i f i c a n c e o f t h i s a s s u m p t i o n i s t h a t f w o u l d a p p r o a c h i t s e q u i l i b r i u m v a l u e e x p o n e n t i a l l y i f t h e e x t e r n a l f o r c e s w h i c h w e r e i n i t i a l l y p r e s e n t w e r e s u d d e n l y r e m o v e d . A l l t r a n s p o r t p r o b l e m s a r e s i m p l i f i e d w h e n a r e l a x a t i o n t i m e d u e t o t h e s c a t t e r i n g p r o c e s s e s 1 4 8 c a n b e a s s u m e d . F o r m o s t s e m i c o n d u c t o r s , a r e l a x a t i o n t i m e c a n b e a s s u m e d p r o v i d e d t h a t t h e c h a n g e i n e n e r g y o f t h e c a r r i e r s d u e t o s c a t t e r i n g i s s m a l l w h e n c o m p a r e d t o t h e i r t o t a l e n e r g y . A 4 . 1 . 2 I s o t h e r m a l E l e c t r i c a l C o n d u c t i v i t y T h e i s o t h e r m a l e l e c t r i c a l c o n d u c t i v i t y o f a u n i f o r m m a t e r i a l i s d e f i n e d b y B = 0 ( A 4 . 7 ) i l = v - T = 0 3 t r 2 F o r s m a l l e l e c t r i c f i e l d , f - f < < 1 a n d t e r m s w i t h P c a n b e n e g l e c t e d . T h e o d i s t r i b u t i o n f u n c t i o n f c a n b e o b t a i n e d b y r e p l a c i n g f w i t h f i n t h e L H S o f e q n . A 4 . 1 . _ • f • = f + £ ' V r f ( A 4 . 8 ) o fi k o T h e e l e c t r i c c u r r e n t d e n s i t y i s g i v e n b y J = - / v f d k ( A 4 . 9 ) 4TT = - q / v f ( E ) N ( E ) d E ( A 4 . 1 0 ) w h e r e , , N ( E ) = / d S 4 i r ' k 1 a n d d S d E d k I n t h e c a s e o f a n o n d e g e n e r a t e s e m i c o n d u c t o r w i t h a s i n g l e p a r a b o l i c e n e r g y b a n d , t h e o n e - d i m e n s i o n a l c o n d u c t i v i t y i s 2 4 n q .«> 3 / 2 - n , /i/ n \ a = — T / 0 \ . . / x n e d r i ( A 4 . l l ) x 1 / 2 o 3TT m w h e r e d E 149 n = / f(E) N(E) dE o It i s usual to assume that: T = ctEY = a ( k T ) Y n Y (A4.12) Thus, eqn. A 4 . l l becomes 4nq a ( k T ) Y ,» -y+3/2 -n , a = V / o — ~ J n e dn x 1/2 " o 3TT m - 4 n q3/2 k Tr r ( Y + |) • (A4.13) 3TT " m For the s p e c i a l case where a mean free path, £, can be defined indepen-dent of energy, £ = vx, a = C ^ ) 1 ' 7 2 ». T - - 1 From eqn. A4.13, a = n q u d r . f t (A4.13) . x where q£ d r i f t 3 * , „ x l/2 (3im kT) A4.1.3 Isothermal H a l l C o e f f i c i e n t The isothermal H a l l c o e f f i c i e n t i s defined by R -H B J z x |£ = V- T = 0 (A4.14) dt r J = 0 y and . _ e = £ A + £ j x y B = B k z Equation A4.1 can be solved by assuming a so l u t i o n of the form 1 5 0 3f f = f - v • C ° a n d t h a t f - f Q < < 1 a n d n ( £ , B , ~ ~ ) a r e n e g l i g i b l e . T h u s o 3 E ( A 4 . 1 5 ) 3f SI 1 + s [ s £ + £ ] C = - q x — ^ 2 _ ( A 4 . 1 6 ) v H 2 y 1 + s , d ^ z w h e r e s = — — ; — m F r o m e q n s . A 4 . 1 0 , A 4 . 1 4 , a n d A 4 . 1 6 , K a . H „ 2 „ 2 , 2 w h e r e R u = - ~ — - ^ — ^ ( A 4 . 1 7 ) 2 q B a , + a „ z 1 2 9 f a = / T 9 N ( E ) d E 1 o - . 2 9E 1 + s 9f » s E x o , „ a 2 = ' < > T T 7 ^ WN(E) d E 1 + s ( A 4 . 1 8 ) F o r a n o n d e g e n e r a t e s e m i c o n d u c t o r s w i t h a s i n g l e p a r a b o l i c b a n d a n d i f s < < 1 2 ( t e r m s i n B ^ o r h i g h e r a r e n e g l i g i b l e ) e q n . A 4 . 1 7 b e c o m e s 3 T r l / 2 F ( 2 Y + | ) R „ = " "^ S - — V" ( A 4 . 1 9 ) 4 n q r 2 ( Y + | ) T h e H a l l m o b i l i t y i s d e f i n e d b y y R = ( A 4 . 2 0 ) •• r ( 2 Y + | ) = \ a ( k T ) Y ^ - ( A 4 . 2 1 ) m r (y + f ) F o r t h e s p e c i a l c a s e y = - zj, V = ~ h • (A4-22> 8 n q n q ( A 4 . 2 3 ) .151 References E.H. Putley, The H a l l Effect and Related Phenomena, Butterworths (1960). S. Wang, Solid-State Electronics, McGraw-Hill (1966). A.H. Wilson, The Theory of Metals, Cambridge (1965). J.M. Ziman, Pr i n c i p l e s of the Theory of Solids, Cambridge (1965). 152 APPENDIX 5.1 POLARIZATION CURRENT OF A DIELECTRIC WITH A UNIFORM DISTRIBUTION OF ACTIVATION ENERGIES . . The p o l a r i z a t i o n , P, of a d i e l e c t r i c i n which ions make f i e l d - a s s i s t e d thermally-activated jumps between contiguous equilibrium s i t e s separated by a p o t e n t i a l b a r r i e r of height W i s given by (Frohlich 1949) f = i ( P s - P ) (A5.1) W -To kT where x = — e (relaxation time of the process) (A5.2) T q = a constant equal to the inverse of the jump frequency W = a c t i v a t i o n energy P = s t a t i c p o l a r i z a t i o n . s r Assume that only one process with a c t i v a t i o n energy W i s operative and that a s i n u s o i d a l f i e l d represented by £ = £ q e3wt i s applied. From eqn. A5.1 and from D = E ( W ) £ = E 6 + P(w) (A5.3) o the p o l a r i z a t i o n P(OJ) i s given by P(oj) = [s(u>) - E ] £ (A5.4) oo and where P = (E - £ ) £ (A5.5) s s °° e = e(w = 0) s e = e (u = 00) 00 The d i e l e c t r i c constant e(w) i s obtained.by s u b s t i t u t i n g eqns. A5.4 and A5.5 in t o A5.1 and £ - e E(OJ) = e + — " (A5.6) 0 0 1 + JOJX o r E' = E + — (A5.7) 1 T a .T ( £ - £ )0JX 153 Equations A5.7 and A5.8 are the Debye equations. Gevers and Dupre (1946) showed that i f a nearly flat distribution of activation energies G(w) is present in the dielectric then W e' = e + (e -e ) / °G(w) dW (A5.9) oo S 0 0 O e" = (e -E ) | kT G(W ) (A5.10) S oo I o where W is defined by o 1 To W /kT , ... — = — e o (A5.ll) and f is the frequency of the applied field. The polarization current due to a single relaxation process is given b y v = § ( A 5 , 1 2 ) Integrating eqn. A5.1 from t = 0 gives s therefore P(t) = P (1 - e t / T ) (A5.13) P . . J = — e" t / T (A5.14) P T For a G(W) distribution of activation energies G(W)P e" t / x J = 5 dV (A5.15) P o T Since G(W) is assumed nearly flat, i t can be evaluated at W = W and taken o kT out at the integral sign. From eqn. A5.2, dW = — dx and by making a change of variable, u = — , eqn. A5.15 can be simply integrated to give T kTG(W ) J = P (A5.16) p t s If a step field £ = £ u(t) is applied or, using eqn. A5.10, kTG(W ) £ u(t) _ _ o o  JP = t = f £Qu(t) (A5.17) m 2 ^ c u ( t ) ( A 5 . 1 8 ) P TT t O 154 APPENDIX 6.1 COMPUTED CAPACITANCE-VOLTAGE CURVES FOR AN IDEAL METAL/Al^O /GaAs MIS CAPACITOR The following plots are capacitance-voltage (C-V) curves for an idea l metal/Al^O /GaAs MIS capacitor computed by using the equations given i n Chapter 6 for the 4 .approximations. The acronyms for the different approximations are (1) DEPLET - the depletion approximation (2) LOFREQ - the low frequency approximation (3) HIFREQ - the high' frequency approximation (4) FREDEP - the low frequency approximation modified by the presence of frequency dependent traps. Dopant concentration i n GaAs(N) and insulator thickness (XO) were the parameters used i n cases 1, 2 and 3. In case 4, the added parameters were frequency factor (f) and the density of " f a s t " surface states (NS). 

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