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Thin films of Gallium Antimonide by flash evaporation. Ryall, Patrick Randall 1968

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THIN FILMS OF GALLIUM ANTIMONIDE BY FLASH EVAPORATION by PATRICK RANDALL RYALL B . S c , U n i v e r s i t y o f Windsor, 1966 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTERS OF SCIENCE i n the Department of PHYSICS 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£«ndard THE UNIVERSITY OF BRITISH COLUMBIA J u l y , 1968 In presenting th is thesis in pa r t i a l fu l f i lment of the requirements for an advanced degree at the Univers i ty of B r i t i sh Columbia, I agree that the Library sha l l make i t f ree ly avai1ab1e for reference and Study. I further agree that permission for extensive copying of th is thesis for scholar ly purposes may be granted by the Head of my Department or by h ils representat ives. It is understood that copying or pub 1 i ca t ion of th is thesis for f inanc ia l gain shal l not be allowed without my wri t ten permission. Department of P h y s i c s  The Univers i ty of B r i t i sh Columbia Vancouver 8, Canada Date J u l y 2 2 . 1968 i i ABSTRACT A f l a s h e v a p o r a t i o n system i s c o n s t r u c t e d i n o r d e r to d e p o s i t t h i n f i l m s o f G a l l i u m A n t i m o n i d e . The system i n c l u d e s a s u b s t r a t e h e a t e r -h o l d e r , a f i l m t h i c k n e s s m o n i t o r i n g d e v i c e and a powdered evaporant f e e d e r . T h i n f i l m s of GaSb, 2 t o h m i c r o n s t h i c k are d e p o s i t e d on g l a s s , s a p p h i r e and s i l i c o n s u b s t r a t e s m a i n t a i n e d a t t e m p e r a t u r e s up to 3 6 5°C. The d e p o s i t e d f i l m s are o b s e r v e d to have many s t r u c t u r a l d e f e c t s . The •f i l m s e x h i b i t s e m i c o n d u c t o r - l i k e p r o p e r t i e s . O p t i c a l s t u d i e s o f the fundamental a b s o r p t i o n edge a t l i q u i d -n i t r o g e n temperature show t h a t the f i l m s d e p o s i t e d on heated s a p p h i r e s u b s t r a t e s have a p o l y c r y s t a l l i n e s t r u c t u r e and a band gap of O.78 eV. The n a t u r e o f the f i l m s i s dependent on the type and t e m p e r a t u r e o f the s u b s t r a t e . i i i TABLE OF CONTENTS Ab s t r a c t i i T a b l e o f C o n t e n t s i i i L i s t o f T a b l e s v L i s t o f F i g u r e s . . . v i Acknowledgement . v i i PAGE Ch a p t e r 1 - INTRODUCTION . . 1 C h a p t e r 2 - THE THIN FILM EVAPORATION SYSTEM I n t r o d u c t i o n . ; 6 2.A The b a s i c vacuum system and the changes and a d d i t i o n s to the system.... 6 2.B.1 Performance o f the vacuum system .8 2.B.2 P r e s s u r e d i f f e r e n c e between upper and lo w e r p o r t i o n s of the b e l l j a r 8 2.C O p e r a t i o n o f the b a s i c system 10 2.D B e l l j a r s h i e l d s . 12 2.E L i q u i d - n i t r o g e n f i l l e r c o n t r o l . . • 12 2.F . 1 S u b s t r a t e h e a t e r 13 2.F.2 Temperature c o n t r o l l e r 16 2.G T h i c k n e s s m o n i t o r i n g o f the f i l m s 16 2.H Powdered evaporant f e e d e r . . . . . 17 Ch a p t e r 3 - PREPARATION, STRUCTURE AND THICKNESS OF THE EVAPORATED FILMS I n t r o d u c t i o n 19 i v •PAGE 3«A C a l i b r a t i o n o f t h e t h i c k n e s s m o n i t o r 19 3.B E v a p o r a t i o n o f Ga and S b . . 22 3.C D e s c r i p t i o n o f t h e e v a p o r a n t a n d t h e s u b s t r a t e s . . . 24 3.D The e v a p o r a t i o n o f GaSb 25 3- .E S t r u c t u r e a nd d u r a b i l i t y o f t h e f i l m s 26 3.F T h i c k n e s s o f t h e GaSb f i l m s . ... 29 3 . G V o l t - a m p e r e c h a r a c t e r i s t i c s o f t h e f i l m s . 31 C h a p t e r 4 - ABSORPTION MEASUREMENTS 4 . A I n t r o d u c t i o n 34 4- .B T r a n s m i t t a n c e o f t h e - f i l m s . 35 4.C A b s o r p t i o n m e a s u r e m e n t s a t LN2 t e m p e r a t u r e . . . 37 4. D C a l c u l a t i o n s i n v o l v i n g E q u a t i o n 4 . 1 . . . . . . . . . . 40 C h a p t e r 5 - CONCLUSIONS AND SUGGESTIONS FOR FURTHER .VORK' 5«A C o n c l u s i o n s 4 1 5 . B S u g g e s t i o n s f o r f u r t h e r s t u d y . ...... 42 A p p e n d i x A ALTERATIONS AND ADDITIONS TO BASIC SYSTEM 43 A p p e n d i x B DESIGN OF THE' LN2 FILLER CONTROL. 45 A p p e n d i x C DESIGN OF THE SUBSTRATE HEATER AND SOLDER 48 A p p e n d i x D DESIGN OF THE PONDERED EVAPORANT FEEDER 50 B i b l i o g r a p h y . 52 V LIST OF TABLES TABLE PAGE 1.1 Summary o f the parameters f o r the d e p o s i t i o n o f t h i n f i l m s o f GaSb 3 1.2 Vapour p r e s s u r e o f Ga, Sb and GaSb a t the m e l t - ' ; p o i n t (MP) and 757°C .. 4 2 .1 C h a r a c t e r i s t i c s o f the i n t e r f e r e n c e f i l t e r s . . . 17 3 . 1 C a l i b r a t i o n o f the m o n i t o r o u t p u t f o r Ge...... 20 3 . 2 E v a p o r a t i o n parameters f o r Ga and Sb 24 3 . 3 A summary of t y p i c a l e v a p o r a t i o n p a r a m e t e r s . . . 25 J>.h Summary o f the e v a p o r a t i o n p arameters f o r the GaSb f i l m s . . . . . . . . . 27 3.5 Summary of the s t r u c t u r e and d u r a b i l i t y o f the f i l m s . 28 3.6 A comparison o f the v a l u e s o b t a i n e d f o r the t h i c k n e s s o f GaSb f i l m s b y . d i f f e r e n t methods o f measurement. 30 4 . 1 Nature o f the f i l m s 39 4 . 2 Energy gap c o r r e s p o n d i n g to the fundamental a b s o r p t i o n edge.. 39 v i L I S T OF FIGURES FIGURE PAGE 2.1 The f l a s h e v a p o r a t i o n system. 7 2.2 Measured pumpdown of the vacuum system com-pared t o a t y p i c a l C . V . C . pumpdown 9 2.3 Comparison o f the p r e s s u r e i n the upper and l o w e r p o r t i o n s o f the b e l l j a r 11 2.4 Temperature o f the f r o n t s u r f a c e of the sub-s t r a t e f o r a g i v e n back s u r f a c e t e m p e r a t u r e . . . 14 2.5 Comparison o f the temperature w i t h and w i t h o u t the f r o n t s u r f a c e thermocouple a t t a c h e d t o the s u b s t r a t e ( g l a s s ) f o r the h e a t e r v a r i a c s e t t i n g s o f 30 and 35 15 2.6 T r a n s m i t t a n c e o f the 1.75 H and the 2.30 \i f i l t e r s 18 3.1 M o n i t o r o u t p u t f o r Germanium. 21 3.2 Comparison o f the u n i f o r m i t y o f the Ge f i l m s on the m o n i t o r d i s k s e c t i o n s 23 3.3 A p l o t o f the conductance v e r s u s 1000/T(UA) f o r f i l m s 4 , 5, 6 and from d a t a o b t a i n e d from A i t k h o z h i n and S e m i l e t o v ( 1 9 6 6 ) 32 4 . 1 T r a n s m i t t a n c e v e r s u s wavelength 36 4.2 A b s o r p t i o n c o e f f i c i e n t v e r s u s energy 38 A. l The r e v i s e d e l e c t r i c a l s c h e m a t i c 4 4 B. l Diagram o f the f i l l i n g a p p a r a t u s i n the a u t o -m a t i c - f i l l i n g mode. 46 B. 2 Schematic o f the LN2 f i l l e r c o n t r o l i n the a u t o m a t i c f i l l i n g mode. 4? C. l The s t a i n l e s s - s t e e l s u b s t r a t e h e a t e r - h o l d e r and h o l d e r f o r the g l a s s s u b s t r a t e s . . . 4 9 D. l Diagram o f the powdered evaporant f e e d e r 50 v i i A C K N O W L E D G E M E N T S I would l i k e to thank my s u p e r v i s o r , Dr. R. C. B r a i s - W i l l i a m s f o r h i s i n t e r e s t and a d v i c e throughout these s t u d i e s . I would a l s o l i k e - to thank Dr. J . W.Bichard f o r many v a l u a b l e - d i s c u s s i o n s and h e l p f u l a d v i c e d u r i n g the p r e p a r a t i o n o f t h i s t h e s i s . The r e s e a r c h d e s c r i b e d i n t h i s t h e s i s was supported by the N a t i o n a l R e s e a r c h C o u n c i l o f Canada, Grant NRC A - V ? 0 0 , and the Defence R e s e a r c h Board o f Canada, Grant DRB 9 5 1 2 - 2 9 . The N a t i o n a l R e s e a r c h C 0un of Canada p r o v i d e d one b u r s a r y and one s t u d e n t s h i p f o r the a u t h o r . \ 1 CHAPTER I  I N T R O D U C T I O N I n r e c e n t y e a r s e x t e n s i v e r e s e a r c h has been devoted t o the p h y s i c s o f a c l a s s o f s e m i c o n d u c t i n g compounds formed from a c o m b i n a t i o n o f Group I I I and Group V e l e m e n t s . Most o f the s e I I I - V compounds, i n c l u d -i n g C~aSb, c r y s t a l l i z e i n the z i n c b l e n d e - t y p e s t r u c t u r e . T h i s g i v e s r i s e t o a s e r i e s o f s e m i c o n d u c t o r s w i t h v a r i e d e l e c t r i c a l and o p t i c a l p r o p e r t i e s . Many o f the s e I I I - V compounds have l a r g e energy gaps r e q u i r e d f o r h i g h t e m p e r a t u r e o p e r a t i o n o f t r a n s i s t o r s and d i o d e s and low m e l t i n g p o i n t s which s i m p l i f i e s t h e i r p r o d u c t i o n . Uardona and Harbeke (1962 and 1963) r e p o r t e d on the a b s o r p t i o n spectrum o f t h i n e v a p o r a t e d l a y e r s o f z i n c b l e n d e - t y p e s e m i c o n d u c t o r s above the f u n d a m e n t a l a b s o r p t i o n edge up to 5 . 5 e V . ^ e l e c t r o n v o l t s j . The d e p o s i t i o n r a t e and the s u b s t r a t e temperature were v a r i e d u n t i l p i n h o l e -f.ree f i l m s , showing w e l l d e f i n e d a b s o r p t i o n s t r u c t u r e , were o b t a i n e d . The a u t h o r s s t a t e d t h a t they o b t a i n e d r e l a t i v e l y s t i o c h i o m e t r i c f i l m s . R i c h a r d s e t a l , ( I 9 6 3 ) s t u d i e d the e p i t a x y o f z i n c b l e n d e - t y p e s e m i c o n d u c t o r s by f l a s h e v a p o r a t i o n . They found t h a t the c r y s t a l l i n e n a t u r e o f the d e p o s i t e d f i l m s depended s t r o n g l y on the s u b s t r a t e temper-a t u r e and t h a t a r e l a t i o n between s u b s t r a t e o r i e n t a t i o n and the c r y s t a l l i n e p e r f e c t i o n e x i s t e d . The s t r u c t u r e and c o m p o s i t i o n o f the d e p o s i t e d f i l m s were found t o be a f f e c t e d by the source t e m p e r a t u r e . They o b t a i n e d s i n g l e -c r y s t a l f i l m s w i t h the same o r i e n t a t i o n as the Ge s u b s t r a t e s . The c r y s t a l p e r f e c t i o n o f the f i l m s was not a f f e c t e d by d i f f e r e n t d e p o s i t i o n r a t e s and t h i c k n e s s e s . M u l l e r and R i c h a r d s (1964) s t u d i e s the m i s c i b i l i t y o f z i n c b l e n d e -type s e m i c o n d u c t o r s by f l a s h e v a p o r a t i o n . The d e p o s i t e d f i l m s had the same s t r u c t u r e and c o m p o s i t i o n as the i n i t i a l powder. E l e c t r o n d i f f r a c t i o n 2 s t u d i e s showed a p r e f e r r e d [ i l l ] f i l m o r i e n t a t i o n on f u s e d q u a r t z sub-s t r a t e s and p a r a l l e l c r y s t a l o r i e n t a t i o n on Ge s u b s t r a t e s . tfikolaichuk and Dutchak (1964) d e p o s i t e d t h i n f i l m s o f GaSb on s u b s t r a t e s which were a t room t e m p e r a t u r e . The f i l m s were removed from the s u b s t r a t e s and s u b j e c t e d t o heat t r e a t m e n t . From e l e c t r o n d i f f r a c t i o n s t u d i e s they found t h a t an amorphous homogeneous phase was o b t a i n e d i n -dependent o f the d e p o s i t i o n r a t e . T h i s s t r u c t u r e which was s i m i l a r t o GaSb i n the c r y s t a l l i n e s t a t e , d i d not change f o r t e m p e r a t u r e s below 200°0. There was o n l y an o r d e r i n g o f the s t r u c t u r e as the temperature i n c r e a s e d . Above 200°0 the s t r u c t u r e t r a n s f o r m e d i n t o the c r y s t a l l i n e s t a t e w i t h a z i n c b l e n d e - t y p e s t r u c t u r e . A i t k h o z h i n and S e m i l e t o v (1965 and 1966) made t h i n f i l m s o f p-type GaSb by f l a s h e v a p o r a t i o n and. s t u d i e d 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 v e r s u s t e m p e r a t u r e . At a s u b s t r a t e temperature o f about 200°C, depending on the d e p o s i t i o n r a t e and the s u b s t r a t e t y p e , the f i l m s t r u c t u r e was found to change from amorphous t o p o l y c r y s t a l l i n e , a m i x t u r e o f c u b i c and hex a g o n a l phases. E l e c t r o n d i f f r a c t i o n s t u d i e s showed t h a t the m e t a l s Ga and Sb were not p r e s e n t , t h a t the f i l m s were s t i o c h i o m e t r i c and t h a t t h e r e were many s t r u c t u r a l d e f e c t s . The a u t h o r s noted t h a t the p e r f e c t i o n f o f the f i l m i n c r e a s e d w i t h i n c r e a s i n g s u b s t r a t e temperature which r e q u i r e d h i g h d e p o s i t i o n r a t e s t o pr e v e n t a p p r e c i a b l e r e - e v a p o r a t i o n o f Sb. M i c a s u b s t r a t e s had an o r i e n t i n g i n f l u e n c e on the f i l m s . C r y s t a l l i t e s i z e was found t o i n c r e a s e w i t h i n c r e a s i n g s u b s t r a t e t e m p e r a t u r e . At 350°C the s i z e was 0.6 u w h i l e a t 620°C the s i z e was 6.OH and i n c r e a s i n g r a p i d l y w i t h t e m p e r a t u r e . The e l e c t r i c a l parameters o f the f i l m s a pproximated those o f s i n g l e - c r y s t a l GaSb. From c o n d u c t i v i t y measurements they found t h a t a t room temperature the energy c o r r e s p o n d i n g t o the fundamental a b s o r p t i o n edge was 0.68+0.07 eV. The parameters used by the above a u t n o r s f o r the e v a p o r a t i o n 3 o f t h i n f i l m s o f GaSb a n d t h e t y p e s o f f i l m s o b t a i n e d a r e s u m m a r i z e d i n T a b l e 1.1. R e f e r e n c e M e t h o d o f e v a p . B o a t B o a t temp. (°C) S u b s t r a t e T h i c k n e s s F i l m s t r u c t u r e C a r d o n a a n d H a r b e k e (1962 and 1963) B f u s e d q u a r t z s u b s t r a t e t emp. (°C) 215 P r e s s u r e (mjx) D e p o s i t i o n r a t e ( j i / m i n . ) 1.2-1.8 0.08 PC R i c h a r d s e t a l . (1963) t a n t a l u m 1650 Ge jlodj f [110] , (11JJ 500 10 M u l l e r and R i c h a r d s (1964) t a n t a l u m 1300-1400 Ge £l00) , [lioj , [lii3 , f u s e d q u a r t z , t i t a n i u m 355 10 0 .017-0.17 0.018 0 . 2 - 1 0 . 0 .5 M i k o l a i c h u k a n d D u t c h a k (1964) B c e l l u l o s e n i t r a t e 25 50-100 v a r i e d A i t k h o z h i n and S e m i l t o v (1965 and 1966) 450-500 g l a s s , q u a r t z , m i c a , r o c k s a l t v a r i e d 10 1 .-12. 0 . 0 3 - 0 . 0 7 0.01-100, SC SC AM, PC AM, PC T a b l e 1.1 Summary o f t h e p a r a m e t e r s f o r t h e d e p o s i t i o n o f t h i n f i l m s o f GaSb. M e t h o d o f e v a p o r a t i o n i s f l a s h (F) o r p l a c i n g a q u a n t i t y o f e v a p o r a n t i n t h e b o a t ( B ) . P r e s s u r e s a r e i n m i l l i m i c r o n s o f Hg (mu.). D e p o s i t i o n r a t e s a nd t h i c k n e s s e s c o r r e s p o n d t o a s i n g l e f i l m o r s e v e r a l f i l m s . S y m b o l s u n d e r s t r u c t u r e c o r r e s p o n d t o am o r p h o u s ( A M ) , p o l y c r y s t a l l i n e ( P C ) o r s i n g l e c r y s t a l ( S C ) . 4 T h i n f i l m s o f GaSb, i n p a r t i c u l a r , a r e s t u d i e d i n t h i s t h e s i s because o p t i c a l s t u d i e s i n b u i l t GaSb are b e i n g made a t p r e s e n t i n t h i s l a b o r a t o r y . lor t h i s t h e s i s a f l a s h e v a p o r a t i o n system i s b u i l t . T h i n f i l m s o f GaSb are d e p o s i t e d on a v a r i e t y o f s u b s t r a t e s by f l a s h e v a p o r a t i o n . These f i l m s are examined w i t h a monochromator a t l i q u i c V * n i t r o g e n ( L N 2 ) t e m p e r a t u r e . The c r y s t a l s t r u c t u r e and the p r o p e r t i e s o f the f i l m s i n r e l a t i o n t o b u l k GaSb a r e d i s c u s s e d from i n f o r m a t i o n about the fundamental a b s o r p t i o n edge and the energy gap. GaSb decomposes i n t o i t s c o n s t i t u e n t s a t t e m p e r a t u r e s used f o r e v a p o r a t i o n (1300°C). The v o l a t i l e component, Sb, as shown i n T a b l e 1 .2 , e v a p o r a t e s f i r s t and w i l l be almo s t c o m p l e t e l y v a p o u r i z e d b e f o r e any a p p r e c i a b l e e v a p o r a t i o n of Ga t a k e s p l a c e . The d e p o s i t e d f i l m i s c h e m i c a l l y inhomogeneous because the c o m p o s i t i o n o f the vapour v a r i e s d u r i n g the course o f the e v a p o r a t i o n . Component Temperature Vapour ( C) P r e s s u r e ( H o f Hg) Ga 30 (MP) 10 Sb .630 (MP) 10 Ga 757 0 .01 Sb 757 1000 GaSb 720 (MP) 10 T a b l e 1.2 Vapour p r e s s u r e o f Ga, Sb and GaSb a t the m e l t i n g p o i n t (MP) and 757 C -Reference B a n a r d (1959) and Dushman Q 9 6 2 ) . S i e g e l The method of f l a s h e v a p o r a t i o n , f i r s t d e s c r i b e d by H a r r i s and (19^8), c o n s i s t s i n d r o p p i n g a f i n e powder of GaSb, a few g r a i n s 5 a t a t i m e i n t o a r e f r a c t o r y b o a t . The b o a t i s h e a t e d w e l l a bove t h e t e m p e r a t u r e r e q u i r e d t o v a p o u r i z e t h e c o n s t i t u e n t s c o m p l e t e l y . The f i l m d e p o s i t e d on t h e s u b s t r a t e c o n s i s t s o f a l t e r n a t i n g l a y e r s o f Ga a n d S b . I f t h e g r a i n s i z e i s s m a l l and t h e 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 l a r g e , t h e i n c r e m e n t o f e a c h l a y e r w i l l be l e s s t h a n a m o n o l a y e r . S u r f a c e d i f f u s i o n p r o v i d e d by t h e s u b s t r a t e h e a t i n g and t h e t h e r m a l e n e r g y of t h e i m p i n g i n g a t o m s h o m o g e n i z e t h e g r o w i n g f i l m , r e f o r m i n g t h e o r i g i n a l compound. The c r y s t a l l i n e s t a t e a n d t h e p e r f e c t i o n o f t h e d e p o s i t e d f i l m d e p e n d s on t h e n a t u r e and t e m p e r a t u r e o f t h e s u b s t r a t e a n d t h e d e c o m p o s i t i o n t e m p e r a t u r e o f t h e d e p o s i t e d f i l m s . E x p e r i m e n t a l l y , more t h a n one g r a i n i s i n t h e b o a t a t a n y t i a e . I f t h e r e i s no n e t a c c u m u l a t i o n o f GaSb i n t h e b o a t and t h e r a t e o f f l o w o f e v a p o r a n t i s c o n t i n u o u s , t h e v a p o u r h a s t h e same c o m p o s i t i o n a s t h e i n i t i a l e v a p o r a n t . The d e p o s i t e d f i l m i s homogeneous an d o f t h e same c o m p o s i t i o n a s t h e i n i t i a l e v a p o r a n t i f t h e s u b s t r a t e t e m p e r a t u r e i s l o w e n o u g h t o a v o i d r e - e v a p o r a t i o n o f any c o n d e n s i n g c o m p o n e n t . A v e r y h o t s o u r c e c a n a v o i d f i l m i n h o m o g e n e i t i e s t h a t r e s u l t f r o m f l u c t u a t i o n s i n t h e r a t e o f f l o w o f e v a p o r a n t t o t h e b o a t . I f t h e s o u r c e t e m p e r a t u r e i s t o o h i g h , p a r t i c l e s l e a v e t h e b o a t e x p l o s i v e l y , w i t h o u t com-p l e t e l y d e c o m p o s i n g . I f t h e t e m p e r a t u r e i s l o w , Ga a c c u m u l a t e s i n t h e b o a t r e s u l t i n g i n a n o n - s t i o c h i o m e t r i c f i l m . 6 CHAPTER I I  THE THIN FILM EVAPORATION SYSTEM INTRODUCTION The b a s i c vacuum system and a l t e r a t i o n s to the system are d e s c r i b e d i n S e c t i o n A. I n S e c t i o n B pumping performance i s d i s c u s s e d and a comparison of the p r e s s u r e s i n s i d e the b e l l j a r i s made. S e c t i o n C i n c l u d e s a d i s c u s s i o n of the o p e r a t i o n and c l e a n i n g o f the system and the performance o f i o n i z a t i o n gauge. The t y p e s o f b e l l j a r s h i e l d s are i n c l u d e d i n S e c t i o n D. I n S e c t i o n E t h e L N 2 ( l i q u i d - n i t r o g e n ) f i l l e r c o n t r o l i s d e s c r i b e d . S u b s t r a t e h e a t e r d e s i g n , temperature measurement and temperature c o n t r o l are g i v e n i n S e c t i o n F. The t h i c k n e s s m o n i t o r i n g d e v i c e i s d e s c r i D e d i n S e c t i o n G. I n S e c t i o n H the d e s i g n and performance of the d e v i c e used to f e e d p a r t i c l e s to the boat i s d e s c r i b e d . The e v a p o r a t i o n system i s i l l u s t r a t e d i n F i g . 2.1. 2.A THE BASIC VACUUM SYSTEM AND THE CHANGES A N D ADDITIONS TO THE SYSTEM The vacuum system used i n these e x p e r i m e n t s was a C o n s o l i d a t e d Vacuum C o r p o r a t i o n (C.V.C.) High Vacuum C o a t e r (type CVE-15). I t was equipped w i t h a 4" o i l d i f f u s i o n pump, a LN2 c o l d t r a p , a l 4 " b e l l j a r , a h o t - f i l a m e n t i o n i z a t i o n gauge w i t h thermocouple a c c e s s o r y , a 2 k i l o w a t t power t r a n s f o r m e r and v a r i a c , and a "Welch m e c h a n i c a l pump (Type 1 4 0 2 B ) . The e l e c t r i c a l c i r c u i t o f the d i f f u s i o n pump was a l t e r e d t o prevent the pump from t u r n i n g cn a g a i n a f t e r a power f a i l u r e . A t h e r m o s t a t pro-t e c t e d the d i f f u s i o n pump a g a i n s t a f a i l u r e of the water f l o w i n the c o o l i n g c o i l s . The r e v i s e d e l e c t r i c a l s c h s m a t i c and d e t a i l s of the changes are g i v e n i n Appendix A. Monitor disk Monitor support Light source i 1 and detector | |~ Multipin feedthrough Platform D i f f u s i o n ^ J J J pump Substrate, heater Substrate holder Shutter P a r t i c l e feeder Electrode Boat F i g . 2.1 The f l a s h evaporation system 8 2.B.1 PERFORMANCE OF THS VACUUM SYSTEM A t y p i c a l pumpdown c u r v e f o r t h e s y s t e m w i t h t h e LN2 t r a p f i l l e d i s g i v e n i n F i g , 2.2. A C.V.C. c u r v e i s i n c l u d e d f o r c o m p a r i s o n . The b e l l j a r was b a c k f i l l e d w i t h a i r f o r t h i s pumpdown. Dow C o r n i n g 705 d i f f u s i o n pump o i l was i n u s e . When t h e b e l l j a r was b a c k f i l l e d w i t h n i t r o g e n s i m i l i a r p e r f o r m a n c e was o b t a i n e d . The l o w e s t p r e s s u r e e v e r o b t a i n e d was 0.09+0.01 mu ( m i l l i m i c r o n s of Hg). T y p i c a l u l t i m a t e p r e s s u r e was 0.15 mu. I t was t h o u g h t t h a t t h e r e was a l e a k i n t h e d i f f u s i o n pump s e c t i o n of t h e s y s t e m . When t h e s y s t e m was s h u t down w i t h e a c h s e c t i o n i s o l a t e d , t h e p r e s s u r e i n s i d e t h e d i f f u s i o n pump r o s e t o a t m o s p h e r i c p r e s s u r e o v e r n i g h t . P r e s s u r e i n s i d e t h e b e l l j a r r o s e no h i g h e r t h a n 300|i o f Hg e v e n a f t e r t h r e e w e e k s . 2.B.2 PRESSURE DIFFERENCE BETWEEN UPPER AND LO.VSR PORTIONS OF THE BELLJAR F o u r s t e e l r o d s 0.375" d i a m . X 19" , were m o u n t e d i n t o t h e t a p p e d h o l e s i n t h e b a s e p l a t e . T h e s e r o d s s u p p o r t e d an A l . d i s k 11.5" d i a m . X 0 . 2 5 " . T h i s d i s k was u s e d t o s h i e l d t h e u p p e r p o r t i o n o f t h e b e l l j a r . A C.V.C. m u l t i p i n f e e d - t h r o u g h ( t y p e AE-009-155) was i n s t a l l e d i n t h e b a s e p l a t e i n o r d e r t o c o n n e c t t o t h e gauge a s e c o n d C.V.C. i o n i z a t i o n t u b e p l a c e d i n s i d e t h e b e l l j a r . The i o n i z a t i o n t u b e f o r t h e s y s t e m was moun t e d a b o u t 8" b e l o w t h e b a s e . S u b s t r a t e s were p l a c e d a b o u t 8" a b o v e t h e b a s e p l a t e . The s e c o n d i o n i z a t i o n t u b e was p l a c e d on t h e A l d i s k w i t h i t s o r f i c e 15" a b o v e t h e b a s e p l a t e a n d 2.3" f r o m t h e c e n t r e o f t h e d i s k . By s w i t c h i n g t h e i o n i z a t i o n g a u g e b e t w e e n t h e t u b e s t h e r a t i o o f u p p e r p r e s s u r e t o l o w e r p r e s s u r e was c a l c u l a t e d . 9 o o o o o o o a o (n I ED o LU QT O ==N a —' o cn -LU 1 cn Q_ o — I CO. I o CD o I o o o CD. I BELLJRR OPENED TO DIFFUSION-PUMP r-MEflSURED PUMPDOWN L _ TYPICAL C . V . C . PUMPDOWN • I I I i 1 -.oao 20.00a yo.ooa 60-ooa 80-oaa loa.oao 120. oao TIMECMIN.) F i g . 2.2 Measured pumpdown of the vacuum system compared to a t y p i c a l C.V.C. pumpdown. 10 S u c c e s s i v e p r e s s u r e r e a d i n g s o f t h e two t u b e s a r e g i v e n i n F i g . 2.3. The d i f f u s i o n pump was t u r n e d o n 4 h o u r s b e f o r e t h e s t a r t o f t h e p r e s s u r e m e a s u r e m e n t s . S y s t e m p r e s s u r e i s s l o w l y d e c r e a s i n g . P r e s s u r e r a t i o s f o r s e v e r a l d i f f e r e n t t i m e s a r e g i v e n . S i n c e t h e r a t i o , a b o u t 1.6, i s c l o s e t o u n i t y a n d t h e s u b s t r a t e i s l o c a t e d midway b e t w e e n t h e t u b e s , t h e l o w e r p r e s s u r e r e a d i n g i s t a k e n t o be t h e p r e s s u r e o f t h e s y s t e m . 2.C OPERATION OF THE B A S I C SYSTEM The s y s t e m was l e f t o p e n t o t h e a t m o s p h e r e o n l y a s l o n g a s r e q u i r e d . The m e c h a n i c a l pump was l e f t r u n n i n g c o n t i n u o u s l y t o k e e p t h e s y s t e m a s c l e a n a s p o s s i b l e . When t h e s y s t e m was b r o u g h t up t o a t m o s p h e r i c p r e s s u r e , a i r was p a s s e d t h r o u g h t u b i n g i m m e r s e d i n LN2 b e f o r e e n t e r i n g t h e b e l l j a r . A f t e r t h e s y s t e m h a d b e e n r e - e v a c u a t e d t h e n o i s e i n t h e i o n i z a t i o n gauge i n c r e a s e d . * I f t h e s y s t e m was d i r t y o r t h e d i f f u s i o n pump o i l was c h a n g e d t h e n o i s e i n t h e i o n i z a t i o n gauge r e a d i n g was a b o u t 0.1 mu. A f t e r c l e a n i n g a l l r e m o v e a b l e p a r t s o f t h e s y s t e m w i t h KOH and b a s e p l a t e and b e l l j a r w i t h a c e t o n e , t h e s y s t e m was c o n d i t i o n e d w i t h t h e l a r g e C.V.C. f i l a m e n t p l a c e d i n s i d e t h e b e l l j a r . A f t e r a b o u t 24 h o u r s n o i s e was a b o u t 0.G2 mu.. When t h e s y s t e m v/as pumped on f o r s e v e r a l h o u r s a t i t s u l t i m a t e p r e s s u r e , t h e a v e r a g e p r e s s u r e v a r i e d b y 0.01 mu. N o i s e i n t h e gauge s o m e t i m e s i n -c r e a s e d . T h ese c h a n g e s were a t t r i b u t e d t o a m p l i f i e r d r i f t o f t h e gauge s i n c e t h e gauge was s u p p l i e d b y a c o n t r o l l e d v o l t a g e s o u r c e . The i o n i z a t i o n gauge, a f t e r o u t g a s s i n g , t o o k 15 o r more m i n u t e s b e f o r e i n d i c a t i n g a r e l i a b l e p r e s s u r e r e a d i n g . T h i s e f f e c t c a n be s e e n i n F i g , 2,3 where t h e s y s t e m was c l o s e t o i t s u l t i m a t e p r e s s u r e . T h i s o c c u r r e d e s p e c i a l l y when t h e s y s t e m was c l e a n . 11 -,000 50-000 100.000 150.000 TIMECMIN.) 200.000 250.000 F i g . 2.3 Coiaparison o f the p r e s s u r e i n the upper and l o w e r p o r t i o n s o f the b e l l j a r . Numbers c o r r e s p o n d t o the r a t i o o f the two p r e s s u r e s . 12 2.D BELLJAR SHIELDS A l s h i e l d s were made t o keep the w a l l s of the b e l l j a r c l e a n * . Sheet A l , 19" h i g h , was r o l l e d i n t o a c y l i n d e r 12" diam. and cut i n t o two s e c t i o n s . The s h i e l d s were d e s i g n e d w i t h windows to h o l d m i c r o s c o p e s l i d e s so t h a t the boat c o u l d be viewed d u r i n g the e v a p o r a t i o n . The s h i e l d s were used f o r a l l e v a p o r a t i o n s of Ge. I t was d e c i d e d t h a t f o r the e v a p o r a t i o n o f GaSb no s h i e l d s would be used. The A l s h i e l d s d i r t i e d e a s i l y and took about one day t o o u t g a s s a f t e r c l e a n i n g . Aluminum was used to c o a t p a r t s o f the Ge and GaSb f i l m s . A h i g h l y r e f l e c t i n g s u r f a c e was r e q u i r e d f o r the p o s t - g r o w t h f i l m t h i c k n e s s measur-i n g d e v i c e . Handiwrap, a c l e a r p l a s t i c f i l m , was used as a d i s p o s a b l e b e l l j a r s h i e l d f o r these e v a p o r a t i o n s ( H o h e n s t e i n 1967). The handiwrap was p u l l e d t i g h t l y around the s t e e l r o d and h e l d i n p l a c e by an a l l i g a t o r c l i p . There was a 1" space between the handiwrap and the b a s e p l a t e . The handiwrap was about 6" from the f i l a m e n t . With a v a r i a c s e t t i n g o f 50 the handiwrap began to s t r e t c h and t e a r w i t h no i n c r e a s e i n p r e s s u r e . A comparison of the pumpdown o f the system, w i t h and w i t h o u t h a n d i -wrap, was made. The system was a l l o w e d to pump down s i x t i m e s from atmos-. p h e r i c p r e s s u r e f o r about 70 m i n u t e s , a l t e r n a t e l y w i t h and w i t h o u t handiwrap. The system t o o k a minimum o f 10 minutes and a maximum of 20 minutes l o n g e r to r e a c h 0.3 mu . This.was a c c e p t a b l e . 2.E -LIQUID-NITROGEN FILLER CONTROL The c a p a c i t y o f the LN2 t r a p i s 3 L which l a s t s about -3 h o u r s . A d e s c r i p t i o n o f the a u t o m a t i c LN2 f i l l e r t h a t was b u i l t t o a l l o w the d i f f u s i o n Commerical A l f o i l was too d i r t y t o be used as s h i e l d s . 13 pump to be o p e r a t e d w i t h the t r a p f i l l e d f o r extended p e r i o d s i s g i v e n i n Appendix B. When the t r a p i s warm 6 1. o f LN2 are r e q u i r e d t o f i l l i t . LN2 b o i l s o f f so q u i c k l y a t t h i s time t h a t the t r a p may o v e r f l o w s l i g h t l y . The f l o a t c a n , d u r i n g t h i s i n i t i a l f i l l , s t i c k i n the up p o s i t i o n which s t o p s the f l o w of LN2 due to f r e e z i n g of water vapour i n the t r a p . A f t e r the i n i t i a l f i l l , the c o n t r o l o p e r a t e s about 6 t i m e s an hour- u s i n g a t most 2 1, per hour. 2.F . 1 SUBSTRATE HEATER A s u b s t r a t e h e a t e r and h o l d e r was d e s i g n e d and b u i l t (Appendix C ) . The h e a t e r - h o l d e r assembly was s m a l l , e a s i l y moveable and c a p a b l e o f t e m p e r a t u r e s above 300°C. An i r o n - c o n s t a n t a n (Fe-Con.) thermocouple w i t h an i c e water r e f e r e n c e j u n c t i o n was used t o measure the temperature of the back s u r f a c e of the s u b s t r a t e . Once the h e a t e r v a r i a c was set, the h e a t e r reached e q u i l i b r i u m i n about 2 hours and m a i n t a i n e d t h i s temperature to w i t h i n 2°C. I t was thought t h a t the temperature o f the f r o n t s u r f a c e of the s u b s t r a t e d u r i n g the e v a p o r a t i o n of the f i l m was more i m p o r t a n t . A thermo-c o u p l e c o u l d be a t t a c h e d d i r e c t l y t o the s u b s t r a t e w i t h g l u e or h e l d between the l i p of the s u b s t r a t e h o l d e r and the s u b s t r a t e . The f i r s t method was not a c c e p t a b l e d u r i n g the d e p o s i t i o n o f the f i l m because g l u e c o v e r e d a l a r g e p o r t i o n o f the s u b s t r a t e . T h i s method was used to measure the temperature o f the c e n t r e of the f r o n t s u r f a c e i n comparison t o the b a c k s u r f a c e temp-e r a t u r e . The second method was used d u r i n g f i l m d e p o s i t i o n . A comparison between the temperature o f the f r o n t and back s u r f a c e o f the s u b s t r a t e was made u s i n g two Fe-Con. t h e r m o c o u p l e s . When they were p l a c e d i n LN2 t o check t h e i r c a l i b r a t i o n b o t h t h e r m o c o u p l e s r e a d between -192°0 and - 1 9 5°C The thermocouple was h e l d to the f r o n t s u r f a c e of the s u b s t r a t e w i t h S a u e r e i s e n g l u e . o o a o Q Q O a G i n . O LU (_> a: g — > ° O o DC o LL- ° a L U L D -cr DC o o LU O &= d LU OJ O o a o IT). SUBSTRRTE ©-©-© GLRSS A - ^ A GLRSS 4- SRPPHIRE T T 200.000 250.000 300.000 350.000 400.000 450.000 TEMPERATURE BR'CK. SURFACE (°C) i sea.oao 550.000 F i g . 2.4 Temperature of the front surface of the substrate f o r a given back surface temperature. The dashed l i n e i s f o r the sapphire substrate. The r e s u l t s f o r two g l a s s s u b s t r a t e r u n s and one s a p p h i r e s u b s t r a t e run are g i v e n i n F i g . 2.4. The temperature d i f f e r e n c e between the two s u r f a c e s i s v e r y s i m i l a r f o r b o t h t y p e s of s u b s t r a t e s . Heater variac at 30 35 O 22 -O 25 -without 21 -O 24 -20 with <> 23 7 ? () 19 - 22 -F i g . 2.5 Comparison of the temperature with and without the front surface thermocouple attached to the substrate (glass) for the heater variac settings of 30 and 35. Temperatures are i n m i l l i v o l t s . I t appeared t h a t g l u i n g the thermocouple to the f r o n t s u r f a c e l o w e r e d the temperature o f the back s u r f a c e * . T h i s i s shown i n F i g . 2.5 w i t h a comparison o f the back s u r f a c e temperature w i t h and w i t h o u t the f r o n t s u r -f a c e thermocouple g l u e d to the s u b s t r a t e ( g l a s s ) . The t e m p e r a t u r e s g i v e n i n F i g . 2.5 are those o b t a i n e d w i t h the h e a t e r a t v a r i a c s e t t i n g s of 30 and 35 on s e v e r a l d i f f e r e n t days. I t was assumed f o r the purposes of t h i s t h e s i s t h a t the temperature d i f f e r e n c e between the two s u r f a c e s was the same w i t h or w i t h o u t the thermo-c o u p l e a t t a c h e d . T h i s was a l s o assumed to be the case f o r the temperature change observed d u r i n g e v a p o r a t i o n . * S i l v e r p a i n t was used t o t r y to improve the t h e r m a l c o n t a c t between the f r o n t s u r f a c e thermocouple and the s u b s t r a t e but then, the thermocouple would not s t i c k . 16 2.F.2 TEMPERATURE CONTROLLER For a g i v e n h e a t e r v a r i a c s e t t i n g the back s u r f a c e temperature o f the s u b s t r a t e remained c o n s t a n t t o 2°C. D u r i n g e v a p o r a t i o n the temperature u s u a l l y i n c r e a s e d . A f t e r e v a p o r a t i o n the s u b s t r a t e temperature d e c r e a s e d (up t o 40°C). A Thermo E l e c t r i c P r e c i s i o n I n d i c a t i n g C o n t r o l l e r (Model 5626) was used t o m a i n t a i n a g i v e n temperature on the f r o n t s u r f a c e o f the s u b s t r a t e . The c o n t r o l l e r had a temperature range from 20°C to 650°C a t 300 w a t t s . Temperature was measured w i t h a c r o m e l - a l u m e l thermocouple p l a c e d between the s u b s t r a t e and the l i p o f the s u b s t r a t e h o l d e r * . The temperature was kep t c o n s t a n t to w i t h i n 5°C (except i n one case where a f t e r an e v a p o r a t i o n the h e a t e r c o u l d not s u p p l y enough power t o m a i n t a i n the t e m p e r a t u r e ) . Due t o i t s l a t e a r r i v a l the c o n t r o l l e r was used f o r the l a s t t h r e e GaSb f i l m s . The c o n t r o l arm of the c o n t r o l l e r was v e r y s u s c e p t i b l e to g e t t i n g bent and s t i c k i n g i n the low temperature end of s c a l e . T h i s made c a l i b r a t i o n and z e r o s e t d i f f i c u l t . 2.G THICKNESS MONITORING OF THE FILMS An O p t i c s Technology M o n i t o r / P h o t o m e t e r ( m o d e l 240} was used to monitor the t h i c k n e s s of the f i l m s as they were b e i n g d e p o s i t e d . A s t a i n l e s s - s t e e l m o n i t o r d i s k s u p p o r t was mounted on the b a s e p l a t e as shown i n F i g . 2.1. The g l a s s m o n i t o r d i s k c o u l d be r o t a t e d from o u t s i d e the b e l l j a r e x p o s i n g new g l a s s s e c t i o n s to the boa t . A chopped l i g h t source and a d e t e c t o r were mounted on a p l a t f o r m o u t s i d e the b e l l j a r next to the s u p p o r t . An a m p l i f i e r / p o w e r s u p p l y was used to d e t e c t the l i g h t r e f l e c t e d from the d i s k . The output o f the a m p l i f i e r was connected to a c h a r t r e c o r d e r . * U s u a l l y t h i s temperature was h i g h e r than g i v e n i n F i g . 2.4 s i n c e the thermocouple was i n c o n t a c t w i t h m e t a l . 17 S e v e r a l i n t e r f e r e n c e f i l t e r s were s u p p l i e d f o r the d e t e c t o r . The f i l t e r s i s o l a t e d a p a r t i c u l a r s p e c t r a l r e g i o n and reduced the e f f e c t o f ambient l i g h t . For a g i v e n w avelength the o u t p u t of the m o n i t o r c o u l d be r e l a t e d to the t h i c k n e s s o f the f i l m b e i n g d e p o s i t e d . The t r a n s m i t t a n c e v e r s u s wavelength of the 1.75 H and the 2.30 u f i l t e r s i s g i v e n i n F i g . 2 . 6 . The r e s u l t s a r e summarized i n T a b l e 2 . 1 . F i l t e r Maximum Wavelength H a l f w i d t h Width at 5% o f maximum T r a n s m i t t a n c e (u) 1.75 ji 26% 1.753 2.8% 12.6% 2 . 3 0 u hk% 2.310 2.7% Table 2 .1 C h a r a c t e r i s t i c s of the i n t e r f e r e n c e f i l t e r s The 2.30 p. f i l t e r was used i n the GaSb t h i c k n e s s measurements because GaSb has a h i g h t r a n s m i t t a n c e a t t h i s w a v elength. The g a i n o f the m o n i t o r a m p l i f i e r c o u l d be reduced t o m i n i m i z e a m p l i f i e r n o i s e . A f t e r growth, the t h i c k n e s s o f the f i l m s ( c f . s e c t i o n 2.D) was measured u s i n g TolansKy i n t e r f e r e n c e t e c h n i q u e w i t h a S l o a n i i n g s t r o m e t e r (model M-lOO). The s h i f t and s e p a r a t i o n o f the f r i n g e s were measured w i t h a r u l e r from a photograph t a k e n of the f r i n g e s . The average t h i c k n e s s and s t a n d a r d d e v i a t i o n o f the f i l m s were then c a l c u l a t e d . 2.H POWDERED EVAPORANT FEEDER The GaSb f i l m s v/ere d e p o s i t e d u s i n g a f l a s h e v a p o r a t i o n s o u r c e . An evaporant f e e d e r ( E c k a r d t 1966) u s i n g u l t r a s o n i c v i b r a t i o n s and r e q u i r i n g no m e c h a n i c a l f e e d t h r o u g h i n t o the b e l l j a r was c o n s t r u c t e d (Appendix D). The f l o w o f evaporant from the f e e d e r tended t o be i n t e r m i t t e n t which, l e d t o i n h o m o g e n e i t i e s i n the d e p o s i t e d f i l m s . 18 o C D in. o o LU o co 0 z: to CE ro. DC * o o o D 2.30|J FILTER s\ 12.31Q|J,0.1438) \ HRLFWIDTHz 0.062U o C D CO. LU P CE CO o CE DC CD o o 1.6DD 1 i T 1 I I r 2.150 2.2O0 2.250 2-300 2.350 2-400 2.450 WAVELENGTH(MICRONS) 1.75P FILTER (l.'753|J, 0.262) HRLFWIDTHz 0.049JJ T T T 1.900 1.650 1.700 1.750 1.800 1.850 WRVELENGTH(MICRONS) Fig. 2.6 Transmittance of the 1.75 u and the 2.30 u f i l t e r s . 19 CHAPTER III PREPARATION, STRUCTURE AND THICKNESS OF THE SVAPORTA3D FILMS  INTRODUCTION Section A deals with the calibration of tne monitor for Ge and the uniformity of the films. The properties of thin films of Ua and Sb are dis-cussed in Section B. The evaporant and the type and preparation of the sub-strates are given in Section C. The evaporation of GaSb i s described in Section D. The properties of the films and their durability are discussed in Section U. In Section F a comparison of the different methods for measuring the thickness of the GaSb films i s made. Semiconducting pro-perties of the films are described in Section G. 3.A CALIBRATION OF THE THICKNESS MONITOR Ge was deposited on monitor disk sections to calibrate the monitor. Two different tungsten boats were used for these evaporations (C.V.C. 267793-1 and 267793-2). Ge melted at a variac setting of 35 and was evaporated at settings of 50 and 60 at a current of about k amps*. Pressures ranged from 0 .5 to 5 . 0 mu. The rate of deposition averaged 0.015 [i /min. Ge wet tungsten very well but lowered the boat temperature in the wetted regions. At the edge of the pan of the boat, where there was no Ge, the temperature rose high enough to burn out the boat +. This problem was partially alleviated by reducing the amount of Ge in the boat. Several runs were then required to obtain a given thickness. * This was the current i n the' primary of the power transformer. The current through the Doat was about go amps. + With no Ge in the boat 8 amps v/ere passed through the primary of the power transformer with no sign of deterioration. 20 A t y p i c a l monitor output u s i n g the 2.3 u. i n t e r f e r e n c e f i l t e r i s shown i n F i g . 3*1. ^he o s c i l l a t i o n s decrease due to i n c r e a s i n g a b s o r p t i o n i n the growing f i l m and s u r f a c e i n h o m g e n e i t i e s (Dumin 1967)* At a wave-l e n g t h o f 2.3 |i , Ge has a r e f r a c t i v e index of 4.08 ( B a l l a r d e t a l . 1959) so t h a t the t h i c k n e s s per monitor output o s c i l l a t i o n i s 0.28 u. A summary of a comparison of the t h i c k n e s s of 6 monitor s e c t i o n s i s gi v e n i n Table 3 .1 . '^he t a b l e i n c l u d e s the number of runs to o b t a i n a given t h i c K n e s s and whether t"he run was stopped at a maximum or a ndnimum o f tne monitor output. The t h i c K n e s s o b t a i n e d from the monitor output and the average post-growth t h i c k n e s s measurements with t h e i r standard d e v i a t i o n s are a l s o g i v e n . F i l m Kuns Evaporate T h i c k n e s s t o from Bionitor iiionitor output output (|i) 5 6 Maximum Minimum Maximum riaximum Minimum Maximum 0 . 4 2 3 0 . 2 8 2 0 . 1 4 1 0.705 0 .282 0 . 1 4 1 Angstormeter t h i c K n e s s (,fi) RM 0.359+0.023 RT 0.291+0.010 LM 0.276+0.016 RM 0.114+0.006 LM O .096+O .OO7 RM 0.719+0.027 LB 0.684+0.012 RB 0.281+0.013 LM 0.273+0.008 RM 0.155+0.006 LT 0.157+0.012 Comments boat broke before maximum reached not sure o f maximum, iiionitor went o f f s c a l e evaporated s l i g h t l y past the maximum evaporated s l i g h t l y past the maximum Table 3.1 C a l i b r a t i o n of the monitor output f o r Ge. Symbols i n column 5 r e l a t e to the l e f t ( L ) , r i g h t ( R ) , t o p ( T ) , middle(M) or bottom(B) of the monitor d i s k s e c t i o n ( c f . F i g . 3 . 2 ) . 2 1 o o CD C3 O CO o o o CD > - O rr~ o CE ° I — o . to 03 CC CE I — o ZD o 0- d . 1— =r ZD g o ,t-J o o . CD O CD WRVELENGTHz 2.30U TRRT EVRPQRRTING )<} I :—I 1 --ODD 2.0D0 4.000 6.0D0 TIME(MIN.) 8.000 10.000 F i g . 3.1 Monitor output f o r Gerraaniura, 22 I t i s seen i n Table 3.1 t h a t the t h i c k n e s s o b t a i n e d from the m o n i t o r output most c l o s e l y c o r r e s p o n d s ( t o w i t h i n 3%) t o the p o s t - g r o w t h measurement when the e v a p o r a t i o n was stopped at a minimum of the m o n i t o r o u t p u t . As shown i n F i g . 3.1 minima are sharp w h i l e maxima are broad which. makes d e t e r m i n a t i o n o f the p o s i t i o n , o f the maxima^ w h i l e e v a p o r a t i n g , d i f f i -" o f -c u l t . A s m a l l o v e r s h o o t ^ minimum r e s u l t s i n a s m a l l e r e r r o r than an o v e r -s h o o t of a maximum. Even when the e v a p o r a t i o n i s stopped a t a maximum the m o n i t o r o u t p u t can be used to g i v e the t h i c k n e s s t o w i t h i n . 30%. There i s no o b s e r v a b l e d i f f e r e n c e between the f i l m s where s e v e r a l r u ns were made t o o b t a i n a g i v e n t h i c k n e s s and where one run was made. A t h i c k n e s s g r a d i e n t i s o b s e r v e d i n the a n g s t r o m e t e r measurements of f i l m s 2 t h r o u g h 5. I n o r d e r to t e s t the u n i f o r m i t y o f the f i l m s , Ge was d e p o s i t e d on two d i s k s e c t i o n s . S e v e r a l a n g s t r o m e t e r measurements were then made around the d i s k s e c t i o n s . The average t h i c k n e s s and s t a n d a r d d e v i a t i o n a t s e v e r a l p o i n t s are g i v e n i n F i g . 3-2. Assuming t h a t the t h i c k n e s s of the f i l m i s i n v e r s e l y p r o p o r t i o n a l to the square of the d i s t a n c e from the boat s u g g e s t s , f o r an extended s o u r c e , a t h i c k n e s s g r a d i e n t o f l e s s than 1%. The s t a n d a r d d e v i a t i o n o f the average t h i c k n e s s o f the d i s k i s about 1%. I t i s assumed t h a t the f i l m s are u n i f o r m s i n c e the s t a t e d a c c u r a c y o f a n g s t r o m e t e r i s 5%. 3.B EVAPORATION OF Ga AND Sb Because GaSb d i s s o c i a t e s i n t o i t s c o n s t i t u e n t s on m e l t i n g , Ga (99.99%) and Sb(99.9%) were d e p o s i t e d s e p a r a t e l y on unheated g l a s s s u b s t r a t e as summarized i n Table 3.2. The p r o p e r t i e s of t h i n f i l m s o f the two element were observed. A molybdeneum boat (Edwards High Vacuum type H 1401/42) was use d . 23 0.454+0.002 0.453+0.007 0.450+0.007 0.442+0.005 Point closest Q.-39"-fr to filament 4* 0.56" Point farthest from filament Average thickness^ 0.450+0.005 0.294+0.002 0.282+0.004 0.283+0.009 0.304+0.005 0.292+0.006 Average thickness= 0.291+0.006 F i g . 3.2 Comparison of the uniformity of the Ge fi l m s on the monitor disk sections. Thicknesses are i n microns. 24 Evaporant Evaporant M e l t e d Evaporate a t P r e s s u r e Rate of a t V a r i a c S e t t i n g V a r i a c S e t t i n g (nyx) E v a p o r a t i o n Ga 20 30 3 Slow Sb 20 20 1 R a p i d T a b l e 3.2 E v a p o r a t i o n parameters f o r Ga and Sb« I n i t i a l l y the dep.oaited Ga f i l m was h i g h l y r e f l e c t i n g but as the f i l m t h i c k n e s s i n c r e a s e d the t e x t u r e became c o a r s e - w i t h a grey c o l o u r . No a n g s t r o m e t e r measurements o f the t h i c k n e s s were made because A l would not c o a t the f i l m w i t h a h i g h l y r e f l e c t i n g s u r f a c e . A l i g h t g r e y d e p o s i t remained on the boat a f t e r e v a p o r a t i o n . Antimony e v a p o r a t e d v e r y q u i c k l y , a l m o s t as soon as i t m e l t e d w i t h a f a s t r i s e and f a l l i n p r e s s u r e . The f i l m was d e p o s i t e d so r a p i d l y t h a t t h e r e was no change i n the m o n i t o r o u t p u t . The d e p o s i t e d f i l m s were h i g h l y r e f l e c t i n g . 3.C DESCRIPTION OF THE EVAPORANT AND SUBSTRATES The GaSb used i n these e x p e r i m e n t s was Semi-Elements I n c . , Sevac grade (40 mesh). The p u r i t y o f the s t a r t i n g elements was 9 9 * 9 9 9 9 % Ga a n ^ 9 9 * 9 9 9 % Sb*. S i n g l e - c r y s t a l GaSb was used.as an evaporant to e l i m i n a t e h a v i n g to mix the two elements i n the r i g h t p r o p o r t i o n s . The s u b s t r a t e s used i n the experiment were g l a s s , s a p p h i r e and s i l i c o n . The amorphous s o f t g l a s s was c u t 1 . 0 " square from C o r n i n g M i c r o s l i d e s ( t y p e 2 9 ^ 7 ) . Randomly o r i e n t e d , s i n g l e - c r y s t a l , p o l i s h e d s a p p h i r e windows, 0 . 7 5 " diam. X 0.04" were o b t a i n e d from A d o l f M e l l e r Co. S a p p h i r e was used as a s u b s t r a t e because i t had a t h e r m a l c o n d u c t i v i t y double t h a t o f g l a s s a t 300°C * P r i v a t e communication w i t h Semi-Elements 25 and 1000 t i m e s t h a t of g l a s s a t LN2 temperature (used f o r o p t i c a l s t u d i e s d e s c r i b e d i n Chpt. 4 ) . S a p p h i r e was used t o t r y to o b t a i n a s i n g l e -c r y s t a l f i l m . I n t r i n s i c s i l i c o n , 1 .0" diam. X 0.16" was used as a s u b s t r a t e t o t r y to o b t a i n a s i n g l e - c r y s t a l f i l m . The g l a s s and s a p p h i r e s u b s t r a t e s were washed w i t h d e t e r g e n t and r i n s e d w i t h d e i o n i z e d water. They were then wiped w i t h i s o p r o p y l a l c o h o l and p o l i s h e d b e f o r e they were p l a c e d i n the h e a t e r . S i l i c o n was p o l i s h e d m e c h a n i c a l l y u n t i l the s u r f a c e was h i g h l y r e f l e c t i n g , c l e a n e d w i t h t o l u e n e i n an u l t r a s o n i c c l e a n e r and r i n s e d w i t h e t h y l a l c o h o l . J.D THE EVAPORATION OF GaSb E v a p o r a t i o n was begun when the s u b s t r a t e h e a t e r reached e q u i l i b r i u m . A f t e r e v a p o r a t i o n the h e a t e r temperature was a l l o w e d t o r e a c h e q u i l i b r i u m a g a i n . The f i l m s were an n e a l e d a t t h i s temperature i n the hope t h a t the a n n e a l i n g would f u r t h e r homogenize the f i l m s . When the s u b s t r a t e h e a t e r was t u r n e d o f f the f i l m c o o l e d t o room temperature i n about 4 h o u r s . P r e s s u r e b e f o r e e v a p o r a t i o n 1 mu P r e s s u r e d u r i n g e v a p o r a t i o n 3 mu P r e s s u r e a f t e r e v a p o r a t i o n 0 .5 mp Amount o f GaSb used 1 gm» .Boat temperature a t a v a r i a c s e t t i n g of 35 * > 1300°C Evaporate to m o n i t o r o u t p u t minimum Average d e p o s i t i o n r a t e 0 . 2 u /min. Ta b l e 3 . 3 A summary o f t y p i c a l e v a p o r a t i o n p a r a m e t e r s . With f l a s h e v a p o r a t i o n the Mo boat was o u t g a s s e d w i t h the s u b s t r a t e s h i e l d e d b e f o r e GaSb was dropped i n t o the b o a t . To keep the boat temperature high., the amount o f evaporant dropped i n t o the boat was a d j u s t e d so t h a t the evaporant wet o n l y a s m a l l p o r t i o n of the boat a t any t i m e . The e v a p o r a t i n g GaSb caused o t h e r p a r t i c l e s o f evaporant f a l l i n g i n t o the boat to be s c a t t e r e d b e f o r e h i t t i n g the boat a l l over t h e . i n s i d e o f the b e l l j a r . I t a l s o Measured w i t h an o p t i c a l pyrometer 26 s t u c k on the s u r f a c e of the g r o w i n g f i l m . The c o n d i t i o n s under which the f i l m s were d e p o s i t e d a r e summarized i n Table 3 .^. The t e m p e r a t u r e s i n columns 7i 8 and 9 o f the f r o n t s u r f a c e are o b t a i n e d from the back s u r f a c e t e m p e r a t u r e . The t e m p e r a t u r e d u r i n g e v a p o r a t i o n i s u s u a l l y the extreme t e m p e r a t u r e . Temperatures f o r f i l m s 8, 9 and 10 are average t e m p e r a t u r e s o b t a i n e d w i t h the temperature c o n t r o l l e r . The a n n e a l i n g time i s measured from the c o m p l e t i o n of e v a p o r a t i o n . Except f o r f i l m s 1 and 6yall t e m p e r a t u r e s o f the f r o n t s u r f a c e were chosen n e a r 3 2 5 ° c - T h i s was about the h i g h e s t temperature t h a t c o u l d be o b t a i n e d from the h e a t e r w i t h o u t b u r n i n g i t o u t . Temperatures changed d u r i n g the e v a p o r a t i o n due t o r a d i a t i o n from the f i l a m e n t and a f t e r the e v a p o r a t i o n due t o h e a t c o n d u c t i o n away from the f i l m because o f the d e p o s i t e d f i l m . 3.E STRUCTURE AND DURABILITY OF THE FILMS The f i l m s were examined w i t h a 150X microscope i m m e d i a t e l y a f t e r r emoval from the b e l l j a r . As summarized i n column 2 o f Table 3.5 a l l f i l m s e x h i b i t e d i m p e r f e c t i o n s s u c h as p i n h o l e s , h a i r l i n e c r a c k s and g r o v e s . A l l f l a s h - e v a p o r a t e d f i l m s had c r y s t a l l i n e - l i k e p a r t i c l e s of the evaporant c o v e r -i n g about 3% of the s u r f a c e a r e a . Those s u b s t r a t e s c l o s e s t to the source had the most p a r t i c l e s on the s u r f a c e . F i l m s t h a t had h i g h l y r e f l e c t i n g s u r -f a c e s appeared t o have g r a i n s a t the l i m i t o f r e s o l u t i o n of the m i c r o s c o p e . The g r e y i s h c a s t on f i l m s 1, 2, 4, 8, 9 and 10 was Ga ( c f . s e c t i o n 3.B). F i l m s 1 and 2, which were not f l a s h evaporated, c o n s i s t e d o f a l a y e r of Sb covered by a l a y e r o f Ga. The g r e y i s h c a s t o f f i l m s h and 8 r e s u l t e d from not c o v e r i n g the s u b s t r a t e w i t h the s h u t t e r i m m e d i a t e l y a l t e r t e r m i n a t i o n o f the e v a p o r a t i o n . The s l i g h t excess Ga i n the boat would then evaporate F i l m Method of Subs. Substrate to boat Duration of evap. Filament variac Front surface temperature (°C) Annealing time Comment s evap. distance (in.) (min.) setting Before During After (min.) 1 B GL 3.5 5 .5 30,35 260 265 255 35 no change in monitor output 2 B GL 12 25,30 310 315 300 40 3 F GL 4 22 30,40 290 300 290 35 / 4 F GL 2 . 3 17 4o 310 335 300 80 5 F SA 3A 19 35,^0 330 300 70 6 F SA 4 . 8 o 23 4o 25 35 25 7 F SA 5 . 0 18 4o 305 325 310 330 305 315 50 heater l e f t on with diffusion pump off for 16 hr. 8 F SA 5.8 19 4o 355 405 365 410 360 355 30 used TC 9 F SA 4 . 8 9 35 325 345 3^5 320 3^5 90 used TC, flow of evaporant coarse, too much in boat 10 F Si 5 17 ; 35 295 295 295 320 295" 40 used TC, flow of evaporant coarse, too much in boat Table 3 . ^ Summary of the evaporation'parameters for the GaSb films. The substrates are glass (GL), sapphire (SA) or silicon. Method of evaporation i s flash (F) or placing a quantity of evaporant in the boat ( r O , '-Variac settings are the highest before the boat began to deteriorate. The second set of temperatures is obtained from a front surface thermocouple (cf. section 2.G.1). A temper-ature controller (TC) i s used for films 8 , 9 and 10. 23 onto the s u r f a c e . I n f i l m s 9 and 10 the r a t e of f l o w o f evaporant i n t o the boa t was too f a s t and l a r g e amounts o f Ga were l e f t i n the boa t . As t h i s Ga e v a p o r a t e d i t formed a c o a r s e grey l a y e r on the s u r f a c e o f the f i l m . F i l m 1 3 k 5 6 7 S t r u c t u r e IK, two l a y e r s , HR l o w e r l a y e r t h e n LGPS IM, t h r e e l a y e r s , b l a c k , d ark G, and a LGPS IM, HRS IM, HRS, BGS, s l i g h t LG c a s t t o s u r f a c e IM, HRS, BGS _ IM, HRS, l e s s BCS than 5 IM, HRS, some BCS, n o t i c e a b l e h a i r l i n e c r a c k s IK, HRS, much uCS, s l i g h t G c a s t on s u r f a c e D u r a b i l i t y s t a i n s on the s u r f a c e a f t e r c u t t i n g and c l e a n i n g d ark s t a i n s on s u r f a c e a f t e r c l e a n i n g and c u t t i n g most of the f i l m l i f t e d o f f edges o f s u b s t r a t e i n c l e a n i n g h a l f o f the f i l m l i f t e d o f f s u b s t r a t e i n the u l t r a s o n i c c l e a n e r 10 IM, GPS, l i t t l e BCS s h i n y d o t s on S s h i n y LGPS, v e r y s h i n y d o t s s t a n d out from S most o f the f i l m l i f t e d up w h i l e i t was b e i n g c u t T a b l e 3 .5 Summary o f the s t r u c t u r e and the d u r a b i l i t y o f the f i l m s . The symbols are i m p e r f e c t i o n s (IM), l i g h t g r e y s u r f a c e (LGS), pebbly s u r f a c e ( P S ) , h i g h l y r e f l e c t i n g s u r f a c e (HRS), b u l k c r y s t a l l i n e - l i k e p a r t i c l e s on s u r f a c e (BCS) and s u r f a c e ( S ) . The f i l m s were c u t i n t o r e c t a n g l e s 15.5 mm. X 7 . 5 - m m , f o r o p t i c a l measurements to be d e s c r i b e d i n Ch a p t e r 4 . The s u b s t r a t e was mounted on a m e t a l b l o c k w i t h A p i e z o n W wax and c u t w i t h a diamond saw. A f t e r the sub-s t r a t e was removed from the b l o c k , e x c e s s wax was removed w i t h t o l u e n e . 29 The f i l m and s u b s t r a t e were c l e a n e d i n a b a t h o f t o l u e n e w i t h an u l t r a -s o n i c c l e a n e r . The t o l u e n e was d i s s o l v e d i n e t h y l a l c o h o l . I n t h i s c u t t i n g and c l e a n i n g p r o c e s s the f i l m s were f u r t h e r s c r a t c h e d and more p i n h o l e s d e v e l o p e d . Many o f the c r y s t a l l i n e - l i k e p a r t i c l e s were removed by the u l t r a s o n i c c l e a n e r . F o r c e r t a i n f i l m s noted i n column 3 o f Ta b l e 3 . 3 the c l e a n i n g p r o c e s s l e f t s t a i n s and removed p o r t i o n s o f the f i l m s . 3.F THICKNESS OF THS GaSb FILMS The r e m a i n i n g p o r t i o n s o f the f i l m s and the m o n i t o r s e c t i o n s were c o a t e d w i t h A l . T h i c k n e s s measured by the a n g s t r o m e t e r was compared w i t h t h a t g i v e n by the m o n i t o r o u t p u t i n Table 3 . 6 . F i l m s 1 and 2 were not i n c l u d e d because A l would not coat these f i l m s w i t h a h i g h l y r e f l e c t i n g l a y e r . Column 2 of Tab l e 3 .6 shows the r a t i o o f the t h i c k n e s s o f the f i l m d e p o s i t e d on the s u b s t r a t e to t h a t o f the m o n i t o r s e c t i o n . T h i s r a t i o i s found assuming the boat i s a p o i n t source and t h a t the t h i c k n e s s o f the f i l m i s i n v e r s e l y p r o p o r t i o n a l t o the square o f the s u b s t r a t e - t o - b o a t o r mo n i t o r s e c t i o n - t o - b o a t d i s t a n c e and the c o s i n e o f the a n g l e o f i n c i d e n c e . The t h i c k -n e s s e s found from the o s c i l l a t i o n s o f the m o n i t o r o u t p u t are quoted i n column 3. A t a wavel e n g t h o f 2.3p., the r e f r a c t i v e i n d e x o f GaSb i s 3.758 ( B a l l a r d e t al, 1959) which g i v e s a t h i c k n e s s o f O.316.U per m o n i t o r output o s c i l l a t i o n . T h i c k n e s s from the m o n i t o r o u t p u t was d i f f i c u l t t o e s t i m a t e s i n c e the o u t p u t went through a minimum i n t i a l l y r a t h e r than the e x p e c t e d maximum. The t h i c k n e s s o f the f i l m s , the pro d u c t of columns 2 and 3 are g i v e n i n column 4 . The a n g s t r o m e t e r measurement o f the m o n i t o r s e c t i o n and the c o r r e s p o n d i n g f i l m . t h i c K n e s s are i n columns 5 and 6. The second t h i c k n e s s i n column 6 i s the a n g s t r o m e t e r measurement o f the d e p o s i t e d f i l m . The upper l i m i t o f the u s e f u l n e s s of the angstrometer i s about 2 u. S i n c e the f i l m s were o f t h i s t h i c k n e s s o r more, the d e t e r m i n a t i o n o f the e x a c t s h i f t o f the i n t e r f e r e n c e f r i n g e s i s d i f f i c u l t . T h i s c o u l d i n t r o d u c e an T h i c k n e s s ( m i c r o n s ) o f the GaSb f i l m s F i l m 3 4 5 6 Ratio 19-30 8.58 4.03 3.7^ 2.82 M o n i t o r measure of f i l m d e p o s i t e d on D i s k S u b s t r a t e Angstometer measurement of the f i l m d e p o s i t e d on D i s k S u b s t r a t e 0.43+0.02 0.237 *+.57 0.257+0.005 4.96+0.10 >3 " 0.474 4.07 0.273+0.018 2.34+0.15 3.0 0.553 2.23 0.3^9+0.027 1.41+0.11 2.04+0.04 0,948 3.55 0.353+0.004 I.32+O.OI 1.62+0.05 I .58 4.46 0.808+0.015 2.28+0.04 >3 " I n t e r f e r e n c e T r a n s m i t t a n c e f r i n g e s s t u d y 3.31 2.07 3.^1 3.^6 1.92 3.^0 OJ o 4.14 0.948 3.92 0.521+0.013 2.16+0.05 2.13+0.04 Table 3.6 A comparison of the v a l u e s o b t a i n e d f o r the t h i c k n e s s o f GaSb f i l m s by d i f f e r e n t methods o f measurement. Column 2 l i s t s the t h e o r e t i c a l r a t i o of the t h i c K n e s s o f the f i l m d e p o s i t e d on the s u b s t r a t e t o t h a t d e p o s i t e d on the m o n i t o r s e c t i o n . The t h i c k n e s s l i s t e d i n column 4 i s the pr o d u c t of columns 2 and 3 w h i l e the t h i c k n e s s l i s t e d i n column 6 i s the p r o d u c t of columns 2 and 5. The second t h i c K n e s s i n column 6 i s the a n g s t r o m e t e r measurement of the f i l m d e p o s i t e d on the s u b s t r a t e . T h i c k n e s s e s o b t a i n e d from o p t i c a l s t u d i e s are g i v e n i n columns 7 and 8. 31 e r r o r of a multiple of 0.29u . Columns 7 and 8 contain thickness estimates obtained from o p t i c a l experiments discussed i n Chpt. 4. E x c e p t for f i l m 4, there appears to be no correspondence, between the m o n i t o r and angstrometer measurements of the thickness. A l l estimates give the thickness to within the same order of magnitude. The angstrometer measurements of the substrate and monitor section f i l m thicKnesses are only approximately related by the r a t i o of column 2. These discrepancies may a r i s e because the re f r a c t i v e index of the films i s not the same as'that for b u l k GaSb and that the films deposited on the monitor sections are not GaSb because the sections are not heated and the monitor sections are glass while the substrates for films 5 through 9 are sapphire. The thickness given i n column 7 i s taken for the thicKness of films 5, 7 and 8. F o r films 6 and 9 the angstrometer measurement of the thickness i s used. 3.G VOLT-AMPERE CHARACTERISTICS OF THE FILMS To determine whether the films had semiconducting properties,volt-ampere ( V - l ) were measured. Reagent grade A l was deposited on the cut films 4,5 and 6 leaving an uncoated area of about 9 mm. X 7*5 mm. The films were deposited onto a heated glass, unheated sapphire and heated sapphire substrate, A l pressure contacts were used on the f i l m . The V-I curves were measured with an oscilloscope. The curves exhibited some hysteresis and neglecting t h i s , were l i n e a r for the films i n b o i l i n g water ( 3 7 3 ° A ) . ice water (273°A), and dry ice (178°A). The slope for the curve measured at LN2 temperature (78°A) decreased slowly with increasing voltage. A plot of the logarithm of the conductance (in units of u amps/volt) versus 1000/T (°A) i s given i n F i g . 3.3". A l l films exhibit semiconductor-like conductances. I n c l u d e d i n F i g . 3,3 lis a curve obtained from conductivity 32 O ID SURSTRRTE U7' A A A I 1 I HERTED GLR3S (FILM 4) HERTED 5RPPHIRE (FILM 5) UNHERTED 5RPPHIRE (FILM 6) HERTED R.RSS C J o >• o o CD O Q tn o a CD O -..000 I ~ Z.5Q0 5-000 7.500 1000/T(CR) 1D.O0Q ~ I — -12.500 F i g . 3 . 3 A p l o t of the conductance versus 1000/T(°A) f o r f i l m s 4 , 5 , 6 and from data o b t a i n e d frora A i t k h o z h i n and Serailetov ( 1 9 6 6 ) . 33 d a t a * m e a s u r e d by A i t k h o z h i n and S e m i l e t o v ( 1 9 6 6 ) f o r a t h i n f i l m o f p - t y p e GaSb d e p o s i t e d o n h e a t e d g l a s s a t 550°C. The c o n d u c t a n c e a t a g i v e n temp-e r a t u r e i s n o t a s g r e a t a s o b t a i n e d f r o m A i t k h o z h i n a n d S e m i l e t o v , n o r a r e t h e c o n d u c t a n c e c u r v e s o f t h e same s h a p e . F i l m 5 w h i c h was d e p o s i t e d on h e a t e d s a p p h i r e h a s t h e g r e a t e s t c o n d u c t a n c e a t a g i v e n t e m p e r a t u r e w h i l e f i l m 6 w h i c h was d e p o s i t e d o n u n h e a t e d s a p p h i r e h a s t h e s m a l l e s t c o n d u c t a n c e . The h e a t i n g and t y p e o f s u b s t r a t e d e t e r m i n e t h e p r o p e r t i e s o f t h e f i l m s . The d i m e n s i o n s c h o s e n f o r t h e c a l c u l a t i o n o f c o n d u c t a n c e a r e 0 . 9 cm. X 0.75 cm. X 0.0033 cm. so t h a t t h e c o n d u c t a n c e s g i v e n i n F i g . 3 * 3 a r e f o r f i l m s o f a p p r o x i m a t e l y t h e same d i m e n s i o n s . 34 CHAPTER IV ABSORPTION MEASUREMENTS k.A INTRODUCTION The a b s o r p t i o n spectrum o f the f i l m s was i n v e s t i g a t e d a t LN2 temperature i n the s p e c t r a l range 0.5 t o 1.0 eV. u s i n g an E b e r t monochromator. The monochromator was o p e r a t e d i n the second o r d e r a t a r e s o l v i n g power o f about 20000. The m e c h a n i c a l s l i t w i d t h was 0.13 mm. The t r a n s m i t t a n c e (the r a t i o of i n t e n s i t y through f i l m and s u b s t r a t e t o the i n t e n s i t y t h r o u g h the s u b s t r a t e a l o n e ) a t normal i n c i d e n c e t h r o u g h a p l a n e p a r a l l e l f i l m o f t h i c K n e s s e s d and a t wavelength X i s g i v e n by T = - i l - R ) 2 **B-1=B*1 (1 + 2 £ R N exp (-Nad) CosN9) 1 - R 2 exp (- 2 ad) N ^ where a i s the a b s o r p t i o n c o e f f i c i e n t R i s the r e f l e c t i v i t y and -R = ( ( n - l ) 2 + k 2 ) / U n + l ) 2 + k 2 ) h.2 n i s the r e f r a c t i v e i n d e x k i s the e x t i n c t i o n c o e f f i c i e n t and k = a\/kTi 8 = 2(2irdn/A. + 2k/ ( n 2 - l ) ) (Johnson 1965) I f t h e r e i s no i n t e r f e r e n c e then the sum of c o s i n e terms i n equa-t i o n 4.1 ave r a g e s to zer o and (1 - R ) 2 e x p ( _ a d ) T = : : 2 k'^> 1 - R exp v-2ad) 35 An interative proceedure is used to solve equation 4 .1 or 4 . 3 for a . An approximate value for a i s obtained from the formula T = exp (-ad). This allowed k, R and G to be determined and substituted into the equations. The sum of cosine terms in equation 4 . 1 i s evaluated with the approximate a. Both equations are now quadratic in exp (-ad) and can be solved for a new approximation to a. The proceedure i s repeated until convergence i s obtained ( |« new *" « 0 i d | < 0 . 0 1 cm."1). 4.B TRANSMITTANCE OF THE FILMS The transmittance of the films was measured. Film 1 was opaque which indicated metallic layers (cf. section 3.B). Film 3 had a transmittance of about 20% in the range of measurement. Film 4 was almost opaque with a transmittance similiar to that of film 6. The transmittance of films 5 through 9 at LN2 temperature i s given i n Fig.4 . 1 . When film 5 was rotated 20° to the incident light direction the Oscillations disappeared. This indicated that the oscillations in the trans-mittance date of films 5, 7 and 8 were due to multiple reflections and that the^ films were uniform. The absence of fringes with film 6 may be due to a non-uniform deposition because of the unheated substrate. With film 9 a non-perpendicular orientation of the substrate to the source may have been used which would result in a wedge-shaped film or d i f f i c u l t i e s in evaporant flow (cf. section 3.D) may result in a non-uniform deposition. The thickness of films 5 , 7 and 8 i s found from the interference fringes in the transparent region using the formula t =(A.-1 - X 2 ) / 2 n \ A-2 (Heavens 1965) where X^ and are the wavelength of two successive maxima and n i s the refractive index. Heavens states that this formula i s accurate to about 10%. The thicknesses are listed in column 7 of Table 3 . ^ . .36 o CD F i g . 4.1 T r a n s m i t t a n c e v e r s u s w a v e l e n g t h . 37 The r e f r a c t i v e .index o f GaSb i s not t a b u l a t e d f o r w a v e l e n g t h s below 1 , 8 j i . T h i c k n e s s e s a r e measured i n the r e g i o n 1 . 6 to 1 . 8 Ji . Cardona (1961) has measured a t room temperature the r e f l e c t i v i t y o f n-type GaSb i n the range 0.25 to 2 .6 JJL . The r e f l e c t i v i t y s l o w l y d e c r e a s e s w i t h i n c r e a s i n g w avelength i n the range 1 . 6 to 1 . 8 jl , S i n c e the e x t i n c t i o n c o e f f i c i e n t i s s m a l l (found t o be a maximum o f 0.14) then from e q u a t i o n 4.2 the r e f r a c t i v e i n d e x s l o w l y d e c r e a s e s w i t h d e c r e a s i n g w a v e l e n g t h . T h i s t r e a t m e n t a l s o assumes t h a t the f i l m s have the same r e f r a c t i v e i n d e x as b u l k GaSb. 4.C ABSORPTION MEASUREMENTS AT LN2 TEMPERATURE For the c a l c u l a t i o n o f the a b s o r p t i o n c o e f f i c i e n t , a « the i n t e r f e r e n c e f r i n g e s i n the t r a n s m i t t a n c e c u r v e s o f f i l m s 5, 7 and 8 were avera g e d . T h i s l i m i t e d the a c c u r a c y o f a t o about 15%. The s u b s t r a t e temperature was taken t o be the extreme t e m p e r a t u r e d u r i n g the e v a p o r a t i o n , which i s l i s t e d i n column 8 o f T a b l e 3-^. E q u a t i o n 4 . 3 was used t o c a l c u l a t e a by the method o f s e c t i o n 4 ,A assuming a r e f r a c t i v e i n d e x f o r GaSb of 3 . 8 2 . F o r f i l m s 5 t h r o u g h 9» & a t LN2 temperature i n g i v e n i n F i g . 4.2. F i l m 6 has a v e r y broad edge w h i l e f i l m s 5, 7, 8 and 9 have broad edges compared to s i n g l e - c r y s t a l GaSb. Because f i l m . 6 shows a v e r y broad edge and i t was d e p o s i t e d a t room temperature i t i s thought to be amorphous w h i l e the o t h e r f i l m s are p o l y c r y s t a l l i n e ( A i t k h o z h i n and S e m i l e t o v 1966). The l a r g e d i f f e r e n c e i n a between f i l m s 6, 7 and 9 and f i l m s 5 and 8 may be due t o f r e e c a r r i e r a b s o r p t i o n i n a m e t a l l i c l a y e r of Ga or Sb ( f o r example, f i l m 9 had a grey c a s t on the s u r f a c e ) . I n the range 310 t o 365°C the s u b s t r a t e temperature appears to have no e f f e c t on the edge. There e x i s t s some temperature between room temperature and 300°C a t which the f i l m s become p o l y c r y s t a l l i n e ( A i t k h o z h e n and S e m i l e t o v 1966) . 38 o o o FILM THICKNESS -600 .650 .700 .750 .800 .850 .900 ENERGY (EVJ F i g . 4 .2 A b s o r p t i o n c o e f f i c i e n t v e r s u s e n e r g y . T e m p e r a t u r e s r e f e r t o s u b s t r a t e t e m p e r a t u r e d u r i n g t h e d e p o s i t i o n o f t h e f i l m s . 39 I n f i l m 3 and 4, d e p o s i t e d , on heated g l a s s , no a b s o r p t i o n edge and a v e r y broad edge r e s p e c t i v e l y , were o b s e r v e d . The type and temperature o f the s u b s t r a t e determined the f i l m p r o p e r t i e s as summarized i n Table 4 . 1 , S u b s t r a t e Type o f f i l m Heated g l a s s Amorphous Unheated s a p p h i r e Amorphous Heated s a p p h i r e P o l y c r y s t a l l i n e Table 4 . 1 Nature o f the f i l m s . As i s d i s c u s s e d i n s e c t i o n 3.S, the f i l m s were cut to f i t the sample h o l d e r o f the monochromator. I n the c u t t i n g and c l e a n i n g p r o c e s s the f i l m s were damaged. I t would have been b e t t e r to perform the o p t i c a l e x p e r i m e n t s b e f o r e c u t t i n g by d e s i g n i n g a new sample h o l d e r . The energy gap a t LN2 temperature c o r r e s p o n d i n g to the fundamental a b s o r p t i o n edge i s g i v e n i n T a b l e 4.2. T h i s energy i s taken as the i n t e r -s e c t i o n o f the asymptotes o f the h o r i z o n t a l and r i s i n g s e c t i o n s o f the c u r v e s . B e c k e r et a l . ( l 9 6 l ) l i s t t h i s energy as 0 .80 eV. f o r s i n g l e - c r y s t a l GaSb. The presence of d e f e c t s i n the f i l m s may account f o r why the measured v a l u e s o f the energy gap are l e s s t h a n t h a t g i v e n by Becker (Cardona and Harbeke 1963). F i l m T h i c k n e s s S u b s t r a t e Energy gap ( u ) temperature (eV.) (°C) 5 3.31 315 0.78+0.02 6 2.04 35 7 2.07 310 0.77+0.02 8 5 . 4 i 365 0.78+0.02 9 2.13 320 0.77+0.02 T a b l e 4 .2 Energy gap c o r r e s p o n d i n g to the f u n d a m e n t a l a b s o r p t i o n edge. 40 4.D CALCULATIONS INVOLVING EQUATION 4.1 E q u a t i o n 4.1 was s o l v e d u s i n g the proceedure o u t l i n e d i n s e c t i o n 4.A f o r a u s i n g the t r a n s m i t t a n c e data f o r f i l m s 5, 7 and o. O s c i l l a t i o n s s t i l l r e mained i n a . I t was thought t h a t t n i s c o u l d be due t o an e r r o r i n the t h i c k n e s s . The averaged t r a n s m i t t a n c e d a t a f o r each f i l m was used t o c a l c u l a t e a w i t h e q u a t i o n 4.3 f o r d i f f e r e n t t h i c k n e s s e s . These c o e f f i c i e n t s were i n t u r n s u b s t i t u t e d i n t o e q u a t i o n 4.1 t o f i n d t r a n s m i t t a n c e s . The t r a n s m i t t a n c e c u r v e s f o r d i f f e r e n t t h i c k n e s s e s were compared to the c u r v e s g i v e n i n F i g . 4.1 . The t h i c K n e s s a t which the t r a n s m i t t a n c e maxima and minima o f b o t h c u r v e s matched most c l o s e l y v/as d e t e r m i n e d . The a m p l i t u d e o f the o s c i l l a t i o n s d i d not agree between e x p e r i m e n t a l and c a l c u l a t e d c u r v e s and t h e r e were o s c i l l a t i o n s i n the a b s o r b i n g r e g i o n o f the c a l c u l a t e d c u r v e s w h ich d i d not appear i n the e x p e r i m e n t a l c u r v e s . U s i n g t h i s t h i c k n e s s and the t r a n s m i t t a n c e d a t a , e q u a t i o n 4.1 was s o l v e d f o r a w i t h the method g i v e n i n s e c t i o n 4.A. The r e s u l t was a curve w i t n o s c i l l a t i o n s l80° out o f phase w i t n the o s c i l l a t i o n s i n the unaveraged t r a n s m i t t a n c e d a t a . I f the t h i c k n e s s was changed s l i g h t l y from t h i s v a l u e , the a m p l i t u d e o f the o s c i l l a t i o n s i n c r e a s e d and the phase s h i f t e d . The l i g h t i n c i d e n t on the f i l m s i n the monochromator i s not p e r p e n d i c u l a r as was assumed f o r e q u a t i o n 4 .1 , but i s d i v e r g e n t by about 10° (Cobb 196l) . T h i s may account f o r the r e a s o n why the i n t e r f e r e n c e f r i n g e s were not removed from the a b s o r p t i o n c u r v e s when e q u a t i o n 4.1 was s o l v e d f o r a. The t h i c k n e s s o b t a i n e d are l i s t e d i n column 8 o f T a b l e J>.k. These t h i c K n e s s e s are w i t h i n 8% of those g i v e n i n column 7 of t h i s t a b l e . 4 1 CHAPTER V CONCLUSIONS AND SUGGESTIONS FOR FURTHER WORK 5.A CONCLUSIONS The vacuum system performed s a t i s f a c t o r i l y . Normal u l t i m a t e p r e s s u r e was 0.15 m |i . When handiwrap was used f o r b e l l j a r s h i e l d s , a p r e s s u r e o f 0.3 n|i was r e a c h e d i n a maximum of 20 m i n u t e s l o n g e r t han w i t h -out the handiwrap p r e s e n t . The p r e s s u r e a t the s u b s t r a t e l o c a t i o n was l e s s t h a n 1.6 t i m e s g r e a t e r than t h a t measured by the i o n i z a t i o n gauge. The LN2 f i l l e r m a i n t a i n e d s u f f i c i e n t LN2 i n the c o l d t r a p t o a l l o w o v e r n i g h t o p e r a t i o n o f the d i f f u s i o n pump. T h i n f i l m s o f GaSb were f l a s h e v a p o r a t e d from 2 to k \i t h i c k on g l a s s , s a p p h i r e and s i l i c o n s u b s t r a t e s a t s u b s t r a t e t e m p e r a t u r e s up to 365°C. Fi n e c o n t r o l over the r a t e o f f l o w o f evaporant i n t o the boat was not p o s s i b l e w i t h the evaporant f e e d e r used i n t h i s t h e s i s . A t t i m e s , too much GaSb was i n the boat which r e s u l t e d i n m e t a l l i c l a y e r s o f Ga and Sb b e i n g d e p o s i t e d on the s u b s t r a t e s . The output o f the t h i c K n e s s m o n i t o r which was c a l i b r a t e d s a t i s f a c t o r i l y w i t h Ge f i l m s , d i d not pe r f o r m as ex p e c t e d w i t h GaSb f i l m s . The o u t p u t c o u l d be used t o e s t i m a t e , d u r i n g the d e p o s i t i o n , the t h i c K n e s s t o w i t h i n a f a c t o r o f 3. Volt-ampere measurements a t v a r i o u s t e m p e r a t u r e s showed t h a t the f i l m s e x h i b i t e d s e m i c o n d u c t o r - l i k e p r o p e r t i e s . The r e s u l t s were dependent on the type and temperature o f the s u b s t r a t e . O p t i c a l s t u d i e s showed t h a t no s i n g l e - c r y s t a l f i l m s were d e p o s i t e d . The f i l m s d e p o s i t e d on s a p p h i r e s u b s t r a t e s were amorphous w i t h a s u b s t r a t e m a i n t a i n e d a t room temperature and p o l y c r y s t a l l i n e when the s u b s t r a t e temp-e r a t u r e was above 300°C. The f i l m s d e p o s i t e d on heated g l a s s s u b s t r a t e s 42 were amorphous. An attempt to d e p o s i t a f i l m on a s i l i c o n s u b s t r a t e was not s u c c e s s f u l . The energy gap c o r r e s p o n d i n g to the fundamental a b s o r p t i o n edge was about O.78 eV. a t LN2 temperature f o r f i l m s 5, 7, 8 and 9. T h i s was l o w e r t h a n the v a l u e o f the gap f o r b u l k s i n g l e - c r y s t a l GaSb. E q u a t i o n 4.1 c o u l d not account s a t i s f a c t o r i l y f o r the i n t e r f e r e n c e f r i n g e s i n the t r a n s m i t t a n c e data o f f i l m s 5. 7 and 8 when t h i s e q u a t i o n was usffid to c a l c u l a t e a b s o r p t i o n c o e f f i c i e n t s , 5B SUGGESTION FOR FURTHER STUDY A f t e r p r o l o n g e d e v a p o r a t i o n the temperature, i n s i d e the b e l l j a r i n c r e a s e d due to r a d i a t i o n from the f i l a m e n t e l e c t r o d e s . T h i s caused a r i s e i n p r e s s u r e due to o u t g a s s i n g from the w a l l s o f the b e l l j a r . T h i s problem c o u l d be c o r r e c t e d by the use o f w a t e r - c o o l e d - e l e c t r o d e s . The s u b s t r a t e h e a t e r c o u l d j u s t m a i n t a i n the temperature (.325°C) r e q u i r e d f o r the d e p o s i t i o n o f s i n g l e - c r y s t a l f i l m s . A h e a t e r w i t h a h i g h e r power r a t i n g would make t h i s temperature e a s i e r to m a i n t a i n and a l l o w f i l m s t o be d e p o s i t e d a t h i g h e r t e m p e r a t u r e s . I t i s suggested t h a t Ge s u b s t r a t e s o r i e n t e d i n the [l O O ] , [no] and { l l l [ d i r e c t i o n s be used t o o b t a i n s i n g l e - c r y s t a l f i l m s as was done by M u l l e r e t a l . (1963) and R i c h a r d s and M u l l e r (1964). (Ge and GaSb have the same c r y s t a l s t r u c t u r e and s i m i l i a r l a t t i c e c o n s t a n t s ) . 4 3 APPENDIX A ALTERATIONS AND ADDITIONS TO BASIC SYSTEM • A DPDT r e l a y i s i n s t a l l e d i n s e r i e s w i t h the e l e c t r i c a l c i r c u i t of the d i f f u s i o n pump t o p r e v e n t i t from t u r n i n g on a g a i n i n event o f a power f a i l u r e . when c l o s e d , a monentary c o n t a c t p u s h - b u t t o n s w i t c h i n s e r i e s w i t h the r e l a y e n e r g i z e s the r e l a y . when the power t o the pump i s t u r n e u on the r e l a y r e g a i n s d e e n e r g i z e d , i n d i c a t e d by a r e a d y l i g h t , u n t i l the pu s h - b u t t o n s w i t c h i s c l o s e d . A n o r m a l l y open s o l e n o r d a l V3lve i s con n e c t e d i n the water l i n e f o r the quench c o i l s o f the.pump. When the d i f f u s i o n pump e l e c t r i c a l c i r c u i t opens water f l o w s t h r o u g h t h e s e c o i l s . A t h e r m o s t a t I C V . C . 264727) p l a c e d i n c o n t a c t w i t h the l o w e s t c o o l i n g c o i l o f tne pump opens a t about 75°C. I f water i s not f l o v / i n g the t h e r m o s t a t t u r n s the pump o f f . The ready l i g h t r emains out and the p u s h -b u t t o n s w i t c h w i l l not e n e r g i z e the r e l a y u n t i l the t h e r m o s t a t c l o s e s a t about 65°C. j The r e v i s e d e l e c t r i c a l s c h e m a t i c i s g i v e n i n F i g . A . l . The C.V.C. sch e m a t i c shows two f u s e s i n w i r e s 9 and 10 o f the v a r i a c w h i c h were not i n s t a l l e d . I t i s suggested t h a t t h e y be i n s t a l l e d t o p r o t e c t the v a r i a c . E l a p s e d time i n d i c a t o r s are con n e c t e d i n p a r a l l e l v/ith the m e c h a n i c a l and d i f f u s i o n pumps to keep a r e c o r d o f the usage of the pump o i l s . M Q 'O-4-12 A. 6-© Symbols A/ 15 A. 9 -Thermostat Push-button iwitch Ready l i g h t s v Voltage •10 MP Mechanical pump DP D i f f u s i o n pump SV Solenoidal valve R Relay T Elapsed time indicator A Ammeter 1-2 220 V. 60 *J 1- 3 117 Vi 60 2- 3 117 V. 60 rj 8 A. - Variac ©1 Power ——— transforme 10 V. To filament 0 V. F i g . A . l The revised e l e c t r i c a l schematic. 45 APPENDIX B DESIGN OF THE LN2 FILLER CONTROL A cork f l o a t h o l d i n g a t h i n - w a l l s t a i n l e s s - s t e e l tube i s placed i n one of the f i l l i n g p o r ts of the trap as shown i n F i g . B . l . When s u f f i c i e n t LN2 h a s b e e n allowed i n t o the tr a p through the other port the tube makes contact w i t h a n adjustable screw, i h i s turns on a t r a n s i s t o r , which energies a r e l a y , which i n turn deenergies two s o l e n o i d a l valves. A normally closed (NC) valve c u t s o f f . t h e compressed a i r (set a t 5 l b s . / s q . i n . ) to a 50 l i t r e LN2 f l a s k and a normally open (NO) valve vents the f l a s k , stopping the flow of LN2. kVhen the tube breaks contact with the screw, LN2 flows again. The trap can be f i l l e d manually with a push-button switch. The e l e c t r o n i c s of the f i l l e r are i l l u s t r a t e d i n F i g . B.2. The hOOO ufd. c a p a c i t o r i n p a r a l l e l with the r e l a y i s used to .stop the f i l l e r from operating s e v e r a l times i n a short period of time. Because of the c a p a c i t o r , contact with the screw must be made or broken f o r about 2 seconds before LN2 stops or s t a r t s f l o w i n g . Due to t h i s delay and the excess pressure i n the f l a s k , the trap f i l l s above the f l o a t l e v e l . I t i s suggested that the f i l l e r c o n t r o l be modified to i n c l u d e a c i r c u i t that w i l l shut i t o f f . i f i t t r i e s to f i l l f o r longer than some given p e r i o d of time. I f a hose breaks or the f l a s k empties than the compressed a i r w i l l be shut o f f . The use of dry ni t r o g e n rather than compressed a i r to f o r c e the LN2 from the f l a s k i s a l s o suggested. .Vhen the f l a s k empties the water vapour i n the a i r freezes which blows o f f the a i r l i n e . F i g . B . l Diagram of the f i l l i n g apparatus i n the automatic f i l l i n g mode. Diagram of the f l o a t assembly i s drawn approximately to scale. Float leads L7 Screw (10X32) Ple x i g l a s s ( 3 . 8 M X l M d i a B j . ) S t a i n l e s s - s t e e l tubing (5.3"Xl/16"diam.) Vent •Trap port "7~ ON Cork f l o a t (2.8"X0.5"diam.) F i g . B . 2 Schematic of the L N 2 f i l l e r c o n t r o l in„the automatic f i l l i n g mode. 48 APPENDIX C DESIGN OF THE SUBSTRATE HEATER AND "HO LP LOR A diagram of the s u b s t r a t e h e a t e r used i n the experiment i s g i v e n i n F i g . C . l . The h e a t e r c o n s i s t s o f t h r e e p i e c e s of s t a i n l e s s - s t e e l 3 " square. The upp3r two p l a t e s , 0 . 1 2 5 i n . t h i c k , c o n t a i n a r e s i s t a n c e h e a t e r . The h e a t e r w i r e i s 8 ' o f number 2 6 nichrome with a room temperature r e s i s t a n c e o f about 2 0 ohms and a r a t i n g o f about 5 0 w a t t s . The w i r e i s wrapped around a p i e c e o f m i c a , 1 . 5 " X 2 1 ' , and i s p l a c e d between two p i e c e s o f m i ca. The i n s i d e s u r f a c e o f the top p l a t e i s m i l l e d to t i g h t l y h o l d the h e a t e r w i r e . The middle p l a t e i s tapped so t h a t the l o w e r p l a t e , the s u b s t r a t e h o l d e r , can be removed w i t h o u t d i s t u r b i n g the h e a t e r . Number 2 0 n i c k e l l e a d s a r e s p o t -welded t o the h e a t e r w i r e and pass through h o l e s i n the upper p l a t e . These l e a d s are anchored w i t h S a u e r e i s e n g l u e . A 0 . 0 1 5 " grove i s m i l l e d to the c e n t r e of the bottom s u r f a c e of the m i d d l e p l a t e . I n t h i s way a thermocouple can be p l a c e d between the heater, and the back s u r f a c e of the s u b s t r a t e . Two s u b s t r a t e h o l d e r s are made. One, 0 . 1 2 5 : I t h i c k , h o l d s s u b s t r a t e s , 1 . 0 " sq. X 0.04". The h o l d e r exposes t o the boat an a r e a of 0 . 7 5 " s q . of s u b s t r a t e . The o t h e r h o l d e r , O . O 8 3 " t h i c k , h o l d s s u b s t r a t e s , 0 . 7 5 " diam. X 0.04". A d i s k 0 . 5 0 " diam. i s exposed t o the b o a t . The h o l d e r s are h e l d t o the upper p l a t e s w i t h 4 b o l t s . The temperature o f the h e a t e r i s c o n t r o l l e d by v a r y i n g the power s u p p l i e d to i t w i t h a v a r i a c . Power and thermocouple c o n n e c t i o n s are made thr o u g h the m u l t i p i n f e e d - t h r o u g h . T e f l o n i n s u l a t i o n i s used o n l y when l e a d s are i n danger o f s h o r t i n g . 4 9 The h e a t e r i s s u p p o r t e d by a U-shaped p i e c e o f s t a i n l e s s s t e e l 6" X 0 . 5 " X 0 . 0 3 " . T h i s i s f a s t e n e d to a s t e e l rod 0 . 3 7 5 " diam. X 6 " . The ro d i s clamped h o r i z o n t a l l y to one of the v e r t i c a l s t e e l r o d s i n s i d e the b e l l j a r as shown i n F i g . 2 . 1 . The h e a t e r can be p l a c e d i n many p o s i t i o n s w i t h ' r e s p e c t t o the boat w h i l e k e e p i n g heat c o n d u c t i o n t o a minimum. A C.V.C. r o t a r y s e a l (type A 3 - 0 0 2 ) i s i n s t a l l e d i n the base p l a t e . The s e a l a l s o a l l o w s l o n g i t u d i n a l motion o f 4 " . A s t a i n l e s s - s t e e l s h u t t e r 5 " X 2 " X 0 . 0 3 " i s f i x e d t o the s h a f t o f the s e a l . The s h u t t e r , as shown i n F i g . 2 . 1 , i s used to mask the s u b s t r a t e from the bo a t . U-shaped holder e JUL -Substrate heater -Substrate holder Heater leads -^-Screw (10X32) III* /// 1 II 1 II f r>* / / ' i -Hole for thermocouple Substrate holder for glass substrates F i g . C . l The s t a i n l e s s - s t e e l substrate heater-holder and the holder for the glass substrates. 50 APPENDIX D DESIGN OF THE POWDERED EVAPORANT FEEDER A t h i n - w a l l s t a i n l e s s - s t e e l tube 0 .375" diam. X 6" was clamped as shown i n * ' i g . D . l t o a p i e z o e l e c t r i c t r a n d u c e r made o f barium t i t a n a t e The c l a m p i n g p r e s s u r e was a d j u s t a b l e . The f e e d e r was s u p p o r t e d by one of the v e r t i c a l r o d s on the base p l a t e as shown i n F i g . 2 . 1 . The t r a n s d u c e r was c y l i n d r i c a l 0 . 5 " X 0 . 5 " diaa. A 0.125" w a l l s . I t had a resonance a t 157 KHZ w i t h a f«i o f about 150. When the t r a n s d u c e r was mounted w i t h i t s a x i s p a r a l l e l t o the tube t h e r e was b e t t e r c o n t r o l over the r a t e o f f l o w o f eva p o r a n t i n t o the b o a t . The tube was a d j u s t e d t o about 15° from the h o r i z o n t a l w i t h the tube end 1" t o 2" above the b o a t . The r a t e o f evaporant f l o w was a d j u s t e d by v a r y i n g the a m p l i t u d e o f the s i g n a l o f a r a d i o f r e q u e n c y g e n e r a t o r . Radio f r e q u e n c y c o n n e c t i o n s were made th r o u g h the m u l t i p i n f e e d - t h r o u g h . Slow f l o w r a t e s which were d e s i r a b l e f o r c o n t r o l l e d e v a p o r a t i o n u s u a l l y s t o p p e d . Heat from the boat made the eva p o r a n t s t i c k y which stopped the f l o w or maue the f l o w i n t e r m i t t e n t . Heat s h i e l d s d i d not h e l p . A t t i m e s i t was n e c e s s a r y t o v a r y the f r e q u e n c y above and below resonance t o r e s t a r t p a r t i c l e f l o w . F i g . D . l Diagram o f the powdered e v a p o r a n t f e e d e r . 51 A t r a n s d u c e r w i t h more s u r f a c e area and a more p o w e r f u l s i g n a l g e n e r a t o r would g i v e b e t t e r c o n t r o l over r a t e o f f l o w . A l e a d z i r c o n a t e t i t a n a t e t r a n d u c e r which has a breakdown temperature o f about 360°C compared t o 120°C f o r b a r i u m . t i t a n a t e would p r o v i d e b e t t e r p r o t e c t i o n a g a i n s t the h i g h t e m p e r a t u r e s i n s i d e the b e l l j a r w h i l e e v a p o r a t i n g . 52 BIBLIOGRAPHY A i t k h o z u i n , i>. A. and S e m i l e t o v , S . A. 1965. S o v i e t Phys. C r y s t a l l o g r a p h y . 9, 488. . 1966. S o v i e t Phys. C r y s t a l l o g r a p h y , JLO, 409. B a l l a r d , S. S. , McCarthy, K. A. and W o l f e , W. L. 1959. I R I A s t a t e - o f - t h e -a r t r e p o r t O p t i c a l P r o p e r t i e s f o r I n f r a r e d I n s t r u m e n t a t i o n , R e p o r t No. 2 3 9 O - I I - S ( U n i v e r s i t y o f M i c h i g a n , Ann A r b o r , M i c h i g a n ) . B e c k e r , W. M . , Ramdas, A. K. and Fan, H.'Y. I 9 6 I . J . A p p l . Phys. 32, 2094. Cardona, M . 196I. Z . P h y s i k , l 6 l , 99. Cardona, M, and Harbeke, G. 1962. Phys. Rev. L e t t e r s , 8, 90, 1963. J . A p p l . Phys. 3jt» 813. Cobb, R. W. 1 9 6 I . M. Sc. t h e s i s , 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 . Dumin, D. J . 1967. Rev. S c i . I n s t r . 38, 1107. Dushman, S. 1962. S c i e n t i f i c F o u n d a t i o n s o f Vacuum Technique ( W i l e y , Ne?/ Y o r k ) . S c k a r d t , J . R. and Peacock, R. N. 1966. J . Vac. S c i . Tech. 3, 356. H a r r i s , L. and S i e g e l , B. M. 1948. J . A p p l . Phys. 19, 739. Heavens, 0. S. 1965. O p t i c a l P r o p e r t i e s of T h i n S o l i d F i l m s (Dover, New Yor k ) pp. 113-4. H o h e n s t e i n , J . 1967. Rev. S c i . I n s t r . 38, 1170. Johnson, F. A. 1965. Prog, i n S e m i c o n d u c t o r s , _9, 179. K i k o l a i c h u k , A. G. and Dutchak, Y. I . 1964. S o v i e t Phys. C r y s t a l l o g r a p h y , 9, 86. M u l l e r , 3 . K. and R i c h a r d s , J . L. 1964. J . A p p l . Phys. 35, 1233. R i c h a r d s , J . L., H a r t , P. B. and M u l l e r , 2. K. 1963. Conference on S i n g l e -C r y s t a l F i l m s a t the P h i l c o S c i e n t i f i c L a b o r a t o r y , " B l u e b e l l , P e n n s y l v a n i a , p u b l i s h e d i n S i n g l e - C r y s t a l F i l m s ( M a c M i l l i a n , New York, 1964) p. 2 4 l , see a l s o J . A p p l . Phys. 34, 34l8 (1963)• 

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