Open Collections

UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Optical charge injection into a gallium arsenide acoustic charge transport device Beggs, Bruce Cameron 1987

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Item Metadata

Download

Media
831-UBC_1987_A7 B43.pdf [ 10.26MB ]
Metadata
JSON: 831-1.0097092.json
JSON-LD: 831-1.0097092-ld.json
RDF/XML (Pretty): 831-1.0097092-rdf.xml
RDF/JSON: 831-1.0097092-rdf.json
Turtle: 831-1.0097092-turtle.txt
N-Triples: 831-1.0097092-rdf-ntriples.txt
Original Record: 831-1.0097092-source.json
Full Text
831-1.0097092-fulltext.txt
Citation
831-1.0097092.ris

Full Text

OPTICAL CHARGE INJECTION INTO A GALLIUM ARSENIDE ACOUSTIC CHARGE TRANSPORT DEVICE By BRUCE CAMERON BEGGS B . S c . ( E n g . ) Queen's U n i v e r s i t y a t K i n g s t o n , 1983 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n THE FACULTY OF GRADUATE STUDIES ( D e p a r t m e n t 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 to the r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA A p r i l 1987 0 B r u c e Cameron Beggs, 1987 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 D a t e Apr>( .3?,, f???-DE-6(3/81) ABSTRACT There i s a need f o r m o n o l i t h i c d e v i c e s capable of s p a t i a l r e s o l u t i o n i n imaging and i o n i z i n g r a d i a t i o n d e t e c t i o n . In t h i s t h e s i s , a GaAs a c o u s t i c charge t r a n s p o r t device (ACT) was s t u d i e d f o r t h i s purpose. A new method of charge i n j e c t i o n has been demonstrated f o r the ACT. Using near-i n f r a r e d o p t i c a l p u l s e s i n c i d e n t through t h i n semi-t r a n s p a r e n t chromium windows, e l e c t r o n - h o l e - p a i r s were separ a t e d by the e l e c t r i c f i e l d i n a d e p l e t e d n-type channel r e g i o n of the d e v i c e . For l i g h t p e n e t r a t i o n l e s s than the depth of the e l e c t r o n p o t e n t i a l minimum, and f o r small i n j e c t i o n l e v e l s , c a l c u l a t i o n s i n d i c a t e d t h a t e l e c t r o n s and h o l e s were separated at t h e i r s a t u r a t i o n v e l o c i t i e s . Holes moving toward the s u r f a c e of the s u b s t r a t e c o u l d recombine w i t h e l e c t r o n s at an evaporated Schottky metal p l a t e . E l e c t r o n s moving toward the channel c e n t r e were bunched and t r a n s p o r t e d by the e l e c t r i c f i e l d coupled to a <110> p r o p a g a t i n g s u r f a c e a c o u s t i c wave (SAW) on (100) cut GaAs. Quantum e f f i c i e n c y , d e f i n e d as the number of e l e c t r o n s c o l l e c t e d at the output per i n c i d e n t photon on the GaAs s u r f a c e , was g r e a t e r than 9% at an o p t i c a l wavelength of 730 nm. When compensation was made f o r the l o s s and r e f l e c t i o n due to the chromium windows, the quantum e f f i c i e n c y was i n excess of 24%. Charge t r a n s f e r e f f i c i e n c y was g r e a t e r than 0.992 with the ACT c l o c k e d at 360 MHz. The demonstrated o p t i c a l i n j e c t i o n technique may be of use i n f u t u r e ACT imaging d e v i c e s . i i TABLE OF CONTENTS Page ABSTRACT i i LIS T OF SYMBOLS v L I S T OF TABLES x i L I S T OF FIGURES x i i LIS T OF ACRONYMS xv ACKNOWLEDGEMENTS x v i 1.0 INTRODUCTION 1 2.0 THEORY 2.1 C h a r a c t e r i s t i c s o f <110> P r o p a g a t i n g SAW 10 on (100) c u t GaAs 2.2 D e p l e t i o n P o t e n t i a l i n the ACT C h a n n e l 24 2.3 SAW T r a n s d u c t i o n on GaAs 30 2.4 The A c o u s t o e l e c t r i c E f f e c t 39 2.5 O p t i c a l Charge I n j e c t i o n 42 2.6 Charge C a p a c i t y and T r a n s f e r E f f i c i e n c y 48 3.0 EXPERIMENTAL DEVICE DESIGN AND FABRICATION 3 . 1 Mask L a y o u t 54 3.2 E p i t a x i a l GaAs S u b s t r a t e M a t e r i a l 56 3.3 B a s i c F a b r i c a t i o n S t e p s 59 3.4 T e s t S t r u c t u r e s and P r o c e s s C o n t r o l 68 4.0 MEASURED PERFORMANCE 4.1 F r o n t g a t i n g and B a c k g a t i n g B e h a v i o u r 75 4.2 I n t e r d i g i t a l T r a n s d u c e r RF Impedance 79 4.3 C a l c u l a t i o n o f Wave P o t e n t i a l 83 4.4 D e v i c e B i a s i n g 91 i i i 4.0 MEASURED PERFORMANCE (continued) 4.5 Pulse Response 4.5.1 E l e c t r i c a l Input 94 4.5.2 O p t i c a l Input 96 4.5.3 Pulse Response V a r i a t i o n with SAW 101 Magnitude and Charge I n j e c t i o n L e v e l 4.6 Charge T r a n s f e r E f f i c i e n c y 104 4.7 Device L i n e a r i t y C h a r a c t e r i s t i c s 109 and Input/Output Impedance 4.8 Quantum E f f i c i e n c y f o r 112 O p t i c a l S i g n a l I n j e c t i o n 5.0 CONCLUSION 114 REFERENCES 119 APPENDIX A: C a l c u l a t i o n of the Q u a s i s t a t i c P o t e n t i a l ... 127 f o r <110> Propagating SAW on (100) cut GaAs APPENDIX B: Tr a n s m i s s i o n of L i g h t through Chromium 136 onto GaAs i v L I S T OF SYMBOLS - e i g e n v e c t o r component ( A p p e n d i x A) A - p a r t i a l wave m a t r i x ( A p p e n d i x A) b - SAW beamwidth - e i g e n v e c t o r component ( A p p e n d i x A) c - s p e e d o f l i g h t i n vacuum c l l ' c 1 2 ' c 4 4 " s t i f f n e s s c o n s t a n t s ( a b b r e v i a t e d s u b s c r i p t s ) c ^ j k i - s t i f f n e s s c o n s t a n t s ( f u l l t e n s o r form) C s - low f r e q u e n c y c a p a c i t a n c e o f a s i n g l e IDT f i n g e r p a i r p e r u n i t SAW beamwidth - low f r e q u e n c y IDT c a p a c i t a n c e d - t h i c k n e s s o f chromium window l a y e r D - b o u n d a r y c o n d i t i o n s m a t r i x ( A p p e n d i x A) D^ - e l e c t r i c d i s p l a c e m e n t e14> e 2 5 ' e 3 6 " p i e z o e l e c t r i c c o n s t a n t s ( a b b r e v i a t e d s u b s c r i p t s ) e ^ j k - p i e z o e l e c t r i c c o n s t a n t s ( f u l l t e n s o r form) E, E^ - e l e c t r i c f i e l d E c - c o n d u c t i o n band e n e r g y l e v e l E C p - e l e c t r i c f i e l d i n ACT c h a n n e l E f m - m e t a l F e r m i l e v e l E s - s y n c h r o n o u s e l e c t r i c f i e l d E v - v a l e n c e band e n e r g y l e v e l f - f r e q u e n c y v a r i a b l e v L I S T O F S Y M B O L S ( c o n t i n u e d ) f g - c e n t r e f r e q u e n c y o f I D T ^ B U R S T " b u r s t f r e q u e n c y a p p l i e d t o A C T i n p u t g - e l e c t r o n - h o l e - p a i r g e n e r a t i o n r a t e g k - g e n e r a l i z e d i m m i t t a n c e s u s e d i n c a l c u l a t i o n o f o p t i c a l l o s s t h r o u g h c h r o m i u m f i l m G ( f ) - f r e q u e n c y r e s p o n s e o f A C T t o R F b u r s t s i g n a l s h - P l a n c k ' s c o n s t a n t H ( f ) - f r e q u e n c y r e s p o n s e o f S A W d e l a y l i n e I C p - c h a n n e l p l a t e c u r r e n t I j j - d r a i n c u r r e n t ^ O U T " A C T o u t p u t o h m i c c u r r e n t J ^ E - a c o u s t o e l e c t r i c c u r r e n t d e n s i t y ( A / m ) k - a c o u s t i c w a v e n u m b e r - S A W e l e c t r o m e c h a n i c a l c o u p l i n g c o e f f i c i e n t l c - t r a n s f e r l e n g t h ( o h m i c c o n t a c t ) L - o h m i c c o n t a c t t e s t s t r u c t u r e g a p w i d t h ( F i g u r e 3.8) L C r - f r a c t i o n o f o p t i c a l p o w e r l o s t i n c h r o m i u m f i l m L x - g a p - w i d t h a x i s i n t e r c e p t o f o h m i c c o n t a c t r e s i s t a n c e g r a p h ( F i g u r e 3.8) no - o p t i c a l E H P d e n s i t y a t d e v i c e s u r f a c e N - n u m b e r o f I D T f i n g e r p a i r s ^ A ' ^ D " d o p i n g d e n s i t i e s N e - n u m b e r o f e l e c t r o n s p e r c e n t i m e t e r c h a n n e l w i d t h v i L I S T O F S Y M B O L S ( c o n t i n u e d ' ) N ^ . - o p t i c a l c o n s t a n t s N t - n u m b e r o f e q u i v a l e n t c h a r g e t r a n s f e r s P ^ i - a c o u s t i c i n t e n s i t y ( W a t t s p e r A s b e a m w i d t h ) ^ C O N V " p o w e r l o s t i n c o n v e r s i o n b e t w e e n e l e c t r i c a l a n d a c o u s t i c e n e r g y P j N - e l e c t r i c a l p o w e r a p p l i e d t o I D T PQPX " o p t i c a l p o w e r i n c i d e n t o n A C T P R E F " e l e c t r i c a l p o w e r r e f l e c t e d f r o m I D T ^ S A W - f " c o m p o n e n t o f S A W p o w e r i n d i r e c t i o n o f A C T c h a n n e l ^ S A W - " c o m p o n e n t o f S A W p o w e r i n d i r e c t i o n o p p o s i t e t o A C T c h a n n e l q - e l e c t r o n i c c h a r g e QT_,/QT " f r a c t i o n o f c h a r g e l o s t f r o m l e a d i n g e d g e o f t h e A C T o u t p u t c u r r e n t p u l s e r2 - r e f l e c t i o n c o e f f i c i e n t f o r a i r - c h r o m i u m i n t e r f a c e r c - o h m i c c o n t a c t r e s i s t i v i t y R - r e f l e c t a n c e o f a i r - c h r o m i u m i n t e r f a c e R c - r e s i s t a n c e d e r i v e d f r o m o h m i c c o n t a c t g r a p h ( F i g u r e 3.8) R ^ n - r e a l p a r t o f t h e A C T i n p u t i m p e d a n c e R L - r e t u r n l o s s ( i n d e c i b e l s ) R s - s h e e t r e s i s t a n c e d e r i v e d f r o m o h m i c c o n t a c t g r a p h ( F i g u r e 3.8) v i i L I S T O F S Y M B O L S ( c o n t i n u e d ) S - a c o u s t i c p o w e r d e n s i t y ( W / m ) S Q - i n i t i a l a c o u s t i c p o w e r d e n s i t y a t x^=0 S ^ j - s t r a i n t e n s o r t - t i m e v a r i a b l e t 2 - t r a n s m i s s i o n c o e f f i c i e n t f o r c h r o m i u m - G a A s i n t e r f a c e t r - r i s e t i m e o f A C T o u t p u t c u r r e n t p u l s e u s e d i n c a l c u l a t i o n o f c h a r g e t r a n s f e r e f f i c i e n c y T - t r a n s m i t t a n c e o f c h r o m i u m - G a A s i n t e r f a c e T ^ j - s t r e s s t e n s o r u ^ , U 3 - s a g i t t a l p l a n e d i s p l a c e m e n t c o m p o n e n t s U2 - d i p l a c e m e n t n o r m a l t o s a g i t t a l p l a n e v ^ - c o m p o n e n t s o f p a r t i c l e v e l o c i t y v s - S A W p h a s e v e l o c i t y v g ^ - c a r r i e r s a t u r a t i o n v e l o c i t y f o r G a A s v s o - S A W v e l o c i t y w i t h u n m e t a l l i z e d s u r f a c e v s s - S A W v e l o c i t y w i t h m e t a l l i z e d s u r f a c e V g £ - s o u r c e v o l t a g e Vj) - d r a i n v o l t a g e Vps - d r a i n - s o u r c e v o l t a g e V Q - g a t e v o l t a g e V J D T - I D T b i a s v o l t a g e V m - S c h o t t k y c h a n n e l p l a t e v o l t a g e V s - a p p l i e d s u b s t r a t e v o l t a g e V s - e f f e c t i v e s u b s t r a t e v o l t a g e W - t o t a l d e p l e t i o n l a y e r t h i c k n e s s v i i i L I S T O F S Y M B O L S ( c o n t i n u e d ' ) W n - g e o m e t r i c t h i c k n e s s o f n - e p i t a x i a l l a y e r W p - w i d t h o f p - t y p e r e g i o n a t s u b s t r a t e / e p i t a x i a l l a y e r j u n c t i o n W j - w i d t h o f g a p i n o h m i c c o n t a c t t e s t s t r u c t u r e ( F i g u r e 3.8) - w e i g h t i n g f a c t o r s ( A p p e n d i x A ) x ^ , X 2 , X 3 - s p a t i a l c o o r d i n a t e s X 3 m - p o s i t i o n o f e l e c t r o n p o t e n t i a l m i n i m u m i n c h a n n e l Y , j g - m a g n i t u d e o f I D T a m p l i t u d e r e s p o n s e r e l a t i v e t o c e n t r e f r e q u e n c y v a l u e Z ^ Q _ a c o u s t i c c h a r a c t e r i s t i c i m p e d a n c e Zj: n(fo) - I D T c e n t r e - f r e q u e n c y i m p e d a n c e a - a c o u s t o e 1 e c t r i c a t t e n u a t i o n c o e f f i c i e n t a g - o p t i c a l a b s o r p t i o n c o e f f i c i e n t P - a n i s o t r o p y p a r a m e t e r u s e d i n d i f f r a c t i o n t h e o r y 7^ - a n g l e s a s d e f i n e d i n F i g u r e B . l SZp^ - c h a n g e i n a c o u s t i c c h a r a c t e r i s t i c i m p e d a n c e f r o m a f r e e t o a m a s s - l o a d e d s u r f a c e e - d i e l e c t r i c c o n s t a n t €Q - p e r m i t t i v i t y o f f r e e s p a c e e j ^ - p e r m i t t i v i t y ( t e n s o r f o r m ) - r e l a t i v e p e r m i t t i v i t y e t - c h a r g e t r a n s f e r i n e f f i c i e n c y r\ - q u a n t u m e f f i c i e n c y i x L I S T O F S Y M B O L S ( c o n t i n u e d ) 6 - p r o p a g a t i o n a n g l e r e l a t i v e t o <110> o n (100) s u r f a c e A p - w a v e l e n g t h o f i n c i d e n t l i g h t p u l s e A s - S A W w a v e l e n g t h H - c a r r i e r m o b i l i t y v - o p t i c a l f r e q u e n c y Pi - p h a s e c o n s t a n t o f c h r o m i u m f i l m p-Q - s p a c e - c h a r g e d e n s i t y d u e t o i o n i z e d d o n o r s i n A C T c h a n n e l PA - m a s s d e n s i t y - p i e z o e l e c t r i c s p a c e - c h a r g e d e n s i t y a, CT^ - d e c a y c o n s t a n t ( A p p e n d i x A ) <f> - b e a m - s t e e r i n g a n g l e 4>c - c h a n n e l p o t e n t i a l c o m p o n e n t s w i t h s p a t i a l d e p e n d e n c e <f>£ - d e p l e t i o n p o t e n t i a l i n A C T c h a n n e l <i>&m - e l e c t r o n p o t e n t i a l m i n i m u m i n c h a n n e l <f>ml - S c h o t t k y m e t a l w o r k f u n c t i o n ' ^ w " S A W q u a s i s t a t i c p o t e n t i a l 0p - G a A s w o r k f u n c t i o n 0g - p l a n e w a v e f o r m o f S A W w a v e p o t e n t i a l w - r a d i a n f r e q u e n c y o f S A W x L I S T OF TABLES Number Page 2.1 GaAs m a t e r i a l c o n s t a n t s 15 4.1 IDT l o s s components 90 4.2 T y p i c a l ACT d e v i c e b i a s v a l u e s 91 x i L I S T OF FIGURES Number Page 1.1a A b a c k s i d e i l l u m i n a t e d AlGaAs/GaAs CCD imager 4 1.1b A f r o n t s i d e i l l u m i n a t e d GaAs CCD imager 4 2.1 D e f i n i t i o n o f axes and p a r t i c l e d i s p l a c e m e n t s 13 f o r <110> p r o p a g a t i n g SAW on (100) c u t GaAs 2.2 SAW a t t e n u a t i o n due t o c o u p l i n g t o b u l k s h e a r 14 wave n e a r <110> p r o p a g a t i o n d i r e c t i o n 2.3a SAW d i s p l a c e m e n t components 16 2.3b SAW q u a s i s t a t i c p o t e n t i a l 16 2.4 SAW phase v e l o c i t y f o r p r o p a g a t i o n n e a r <110> 20 2.5 SAW s l o w n e s s c u r v e f o r p r o p a g a t i o n n e a r <110> 23 2.6 D e p l e t e d ACT c h a n n e l band d i a g r a m 25 2.7 ACT c h a n n e l p o t e n t i a l (a) - e x p e r i m e n t a l b i a s c o n d i t i o n 28 (b) - v a r i a t i o n o f f r o n t g a t i n g b i a s 28 ( c ) - v a r i a t i o n o f b a c k g a t i n g b i a s 29 (d) - v a r i a t i o n o f f r o n t g a t i n g 29 and b a c k g a t i n g b i a s 2.8 Q u a r t e r w a v e l e n g t h f i n g e r w i d t h IDT d e s i g n 31 2.9 F r e q u e n c y r e s p o n s e o f r e f l e c t i v e A/4 t r a n s d u c e r 33 d e l a y l i n e on GaAs 2.10 S p l i t - f i n g e r IDT d e s i g n 36 2.11 SAW t r a n s d u c e r e q u i v a l e n t c i r c u i t 37 2.12 S i m p l i f i e d a n a l y s i s o f o p t i c a l c h a r g e s e p a r a t i o n .... 45 2.13 C h a n n e l p l a t e waveform a f t e r a p p l i c a t i o n o f 46 o p t i c a l p u l s e x i i L I S T OF FIGURES (continued') Number Page 2.14 Charge p a c k e t shape f o r c h a r g e l o a d o f 1/2 51 2.15 Charge t r a n s f e r i n e f f i c i e n c y f o r d i f f e r e n t 53 SAW wave p o t e n t i a l s v e r s u s c h a r g e l o a d 3.1 E x p e r i m e n t a l ACT mask l a y o u t 55 3.2 SEM o f m e t a l "wings" on t r a n s d u c e r f i n g e r s 63 c a u s e d by p o o r p h o t o r e s i s t edge p r o f i l e 3.3 SEM o f 2 fj.m i n t e r d i g i t a l t r a n s d u c e r f i n g e r s 64 3.4 SEM o f "baked" p h o t o r e s i s t c a u s e d by e x c e s s i v e 66 h e a t d u r i n g e v a p o r a t i o n 3.5 SEM o f e t c h e d 1 (tm aluminum f i n g e r s c a u s e d by 67 m o i s t u r e i n Hunt m i c r o s t r i p (TM) 3.6 P h o t o g r a p h o f c o m p l e t e d ACT d e v i c e 69 3.7a P h o t o g r a p h o f ACT i n p u t f e a t u r e s 70 3.7b P h o t o g r a p h o f ACT o u t p u t f e a t u r e s . 70 3.8 Ohmic c o n t a c t t e s t s t r u c t u r e r e s i s t a n c e 71 v a r i a t i o n w i t h gap w i d t h 3.9 E p i t a x i a l w a f e r d o p i n g p r o f i l e m easured by 73 t h e c a p a c i t a n c e - v o l t a g e method 3.10 S c h o t t k y d i o d e c u r r e n t - v o 1 t a g e p l o t u s e d to 74 d e t e r m i n e t h e i d e a l i t y f a c t o r 4.1 E x p e r i m e n t a l s e t - u p f o r measurement o f 76 f r o n t g a t i n g and b a c k g a t i n g b e h a v i o u r 4.2a F r o n t g a t i n g c h a r a c t e r i s t i c 77 4.2b B a c k g a t i n g c h a r a c t e r i s t i c 78 x i i i L I S T OF FIGURES ( c o n t i n u e d )  Number Page 4.3 V a r i a t i o n o f IDT r e t u r n l o s s w i t h b i a s i n g (a) <V S = 0 V ; V I D T = 0 V) 80 (b) ( V s = 0 V ; V I D T = -8 V) 80 ( c ) ( V s = 0 V ; V I D T = -14 V) 81 (d) ( V s = -20 V ; V I D T - -5 V) 81 4.4 P o l a r p l o t o f IDT r e f l e c t i o n n e a r r e s o n a n c e 84 4.5 SAW g e n e r a t i o n power components 85 4.6 ACT b i a s i n g and o u t p u t a m p l i f i c a t i o n c i r c u i t r y ...... 92 4.7 ACT p u l s e r e s p o n s e f o r c o n t a c t i n j e c t i o n 96 4.8a O p t i c a l p u l s e g e n e r a t i o n and f o c u s s i n g 97 4.8b O p t i c a l i n j e c t i o n window p o s i t i o n s 98 4.9 P u l s e r e s p o n s e f o r o p t i c a l c h a r g e i n j e c t i o n (a) Over Window #1 99 (b) Over Window #2 99 ( c ) Over Window #3 100 (d) Over Window #4 100 4.10 P u l s e r e s p o n s e v a r i a t i o n w i t h SAW d r i v e power 102 4.11 P u l s e shape v a r i a t i o n w i t h i n j e c t i o n l e v e l 105 4.12 ACT RF b u r s t r e s p o n s e 106 4.13 Charge t r a n s f e r e f f i c i e n c y u s i n g f r e q u e n c y 107 r e s p o n s e method 4.14 ACT o u t p u t l i n e a r i t y f o r c o n t a c t i n j e c t i o n 110 4.15 ACT o u t p u t l i n e a r i t y f o r o p t i c a l i n j e c t i o n I l l B . l O p t i c a l model o f t h i n chromium window on GaAs 137 u s e d i n c a l c u l a t i o n o f quantum e f f i c i e n c y x i v L I S T O F A C R O N Y M S A C T a c o u s t i c c h a r g e t r a n s p o r t ( d e v i c e ) C C D c h a r g e - c o u p l e d d e v i c e C G C C D c a p a c i t i v e - g a t e c h a r g e - c o u p l e d d e v i c e C T E c h a r g e t r a n s f e r e f f i c i e n c y C T I c h a r g e t r a n s f e r i n e f f i c i e n c y C W c o n t i n u o u s w a v e E H P e l e c t r o n - h o l e - p a i r I D T i n t e r d i g i t a l t r a n s d u c e r I T O i n d i u m - t i n o x i d e L E D 1 i g h t - e m i t t i n g d i o d e L S A W l e a k y s u r f a c e a c o u s t i c w a v e M E S F E T m e t a l - s e m i c o n d u c t o r f i e l d - e f f e c t t r a n s i s t o r M B E m o l e c u l a r - b e a m e p i t a x y M O C V D m e t a l - o r g a n i c c h e m i c a l v a p o u r d e p o s i t i o n P I N p - t y p e / i n t r i n s i c / n - t y p e ( d i o d e ) R F r a d i o f r e q u e n c y R G C C D r e s i s t i v e - g a t e c h a r g e - c o u p l e d d e v i c e S A W s u r f a c e a c o u s t i c w a v e S E M s c a n n i n g e l e c t r o n m i c r o s c o p e S I s e m i - i n s u l a t i n g T E t r a n s v e r s e e l e c t r i c T M t r a n s v e r s e m a g n e t i c U H F u l t r a - h i g h f r e q u e n c y V H F v e r y - h i g h f r e q u e n c y V P E v a p o u r - p h a s e e p i t a x y X V A C K N O W L E D G E M E N T S I o w e m a n y t h a n k s t o m y g r a d u a t e s u p e r v i s o r , D r . L . Y o u n g , f o r h i s s u p p o r t , t h e o r e t i c a l i n s i g h t , a n d m a n y p r a c t i c a l s u g g e s t i o n s . I a m g r a t e f u l f o r t h e i n t e r e s t i n m y w o r k s h o w n b y D r . R . R . J o h n s o n o f T R I U M F a n d f o r t h e a s s i s t a n c e h e p r o v i d e d d u r i n g a r o u n d - u p o f t e s t e q u i p m e n t . I a p p r e c i a t e t h e u s e o f a n R F p o w e r a m p l i f i e r a n d n e t w o r k a n a l y z e r p r o v i d e d b y R . P o i r i e r o f T R I U M F . T h e s u p p o r t o f t h e s o l i d s t a t e l a b p e r s o n n e l a n d m y f e l l o w g r a d u a t e s t u d e n t s w a s i n v a l u a b l e . I n p a r t i c u l a r , I a m i n d e b t e d t o W . R u t h e r f o r d , w h o s e b e l i e f i n a c a u s a l u n i v e r s e p u t m e i n t h e p r o p e r f r a m e o f m i n d t o s o l v e t r o u b l e s o m e f a b r i c a t i o n p r o b l e m s ; P . T o w n s l e y f o r h i s " s y r i n g e " t e c h n i q u e t h a t w a s u s e d t o a p p l y a c o u s t i c a b s o r b i n g m a t e r i a l ; H . K a t o f o r s h o w i n g m e h o w t o e v a p o r a t e A u : G e w i t h o u t b a k i n g p h o t o r e s i s t ; N . J a e g e r f o r h e l p w i t h t h e o p t i c s ; a n d M . L e N o b l e f o r s h a r i n g h i s G a A s w a f e r d i c i n g t e c h n i q u e . I a m p a r t i c u l a r l y i n d e b t e d t o D r . M . J . H o s k i n s o f E l e c t r o n i c D e c i s i o n s I n c . , c o - i n v e n t o r o f t h e A C T d e v i c e , w h o s e k i n d i n v i t a t i o n t o v i s i t t h e U n i v e r s i t y o f I l l i n o i s A C T r e s e a r c h g r o u p r e s u l t e d i n a d e e p e r u n d e r s t a n d i n g o f t h e d e v i c e . S e v e r a l i n s i g h t s p r o v i d e d b y h i s g r a d u a t e s t u d e n t s s a v e d m e c o n s i d e r a b l e t i m e d u r i n g d e v i c e f a b r i c a t i o n . F i n a l l y a n d f o r e m o s t , I w a n t t o t h a n k m y w i f e , L o u i s e , f o r h e r l o v i n g s u p p o r t , f a i t h , a n d p e r s p e c t i v e t h r o u g h o u t m y g r a d u a t e r e s e a r c h . x v i 1.0 INTRODUCTION T h i s t h e s i s d e s c r i b e s e l e c t r o n i n j e c t i o n i n t o a GaAs a c o u s t i c c h a r g e t r a n s p o r t (ACT) d e v i c e u s i n g n e a r - i n f r a r e d o p t i c a l p u l s e s (Ap=730 nm). F r o n t s i d e i l l u m i n a t i o n was a c h i e v e d u s i n g a f o c u s s e d l i g h t s o u r c e i n c i d e n t w i t h i n the s u r f a c e a c o u s t i c wave (SAW) beamwidth t h r o u g h semi-t r a n s p a r e n t m e t a l l i c windows. I t w i l l be shown t h a t the e l e c t r o n - h o l e p a i r s (EHP's) c r e a t e d by t h e o p t i c a l p u l s e were q u i c k l y s e p a r a t e d by a s t r o n g e l e c t r o s t a t i c f i e l d i n the d e p l e t e d n - e p i t a x i a l c h a n n e l o f the ACT. The h o l e s d r i f t e d t o w a r d an e v a p o r a t e d S c h o t t k y c o n t a c t on t h e s u r f a c e where t h e y c o u l d r e c o m b i n e w i t h e l e c t r o n s i n t h e m e t a l . E l e c t r o n s moved t o w a r d t h e c h a n n e l c e n t r e where t h e y were b u n c h e d and t r a n s p o r t e d by the <110> p r o p a g a t i n g SAW on (100) c u t GaAs. The o p t i c a l i n j e c t i o n e x p e r i m e n t s were m o t i v a t e d by a n e e d t o s p a t i a l l y r e s o l v e r a d i a t i o n p a t t e r n s . F o r an ACT, the f u n d a m e n t a l r e s o l u t i o n l i m i t i s d e t e r m i n e d by the SAW w a v e l e n g t h : A t 360 MHz the w a v e l e n g t h i s 8 jum. As t h e d r i v e f r e q u e n c y i n c r e a s e s , t h e w a v e l e n g t h - l i m i t e d r e s o l u t i o n o f the ACT i m p r o v e s . An i m a g i n g d e v i c e b a s e d on c h a r g e - c o u p l e d d e v i c e (CCD) t e c h n o l o g y has s p a t i a l r e s o l u t i o n d e t e r m i n e d by the g a t e d i m e n s i o n s and number o f p h a s e s . F o r a two-phase CCD w i t h 5 nm g a t e w i d t h s and 1 /zm i n t e r g a t e gaps, the b e s t r e s o l u t i o n w o u l d be 12 /im, i n d e p e n d e n t o f c l o c k f r e q u e n c y . In a r a d i a t i o n d e t e c t i o n e n v i r o n m e n t s p a t i a l r e s o l u t i o n i s o n l y one measure o f p e r f o r m a n c e . F o r an o p t i c a l i m a g i n g d e v i c e , p a r a m e t e r s s u c h as quantum e f f i c i e n c y , s p e c t r a l r e s p o n s e , l i n e a r i t y , r e s p o n s e t i m e , a n d r e s p o n s i v i t y a r e i m p o r t a n t f o r m a n y a p p l i c a t i o n s . W i t h t h e e x c e p t i o n o f s p e c t r a l r e s p o n s e , t h e s e p a r a m e t e r s h a v e b e e n c o n s i d e r e d i n t h i s t h e s i s f o r a n e x p e r i m e n t a l A C T d e v i c e . A s a n u c l e a r p a r t i c l e d e t e c t o r , d e v i c e d e g r a d a t i o n a n d e v e n t u a l d e s t r u c t i o n c a u s e d b y r a d i a t i o n i n d u c e d s u b s t r a t e d a m a g e m u s t b e a s s e s s e d . A r e c e n t s t u d y o f G a A s C C D d e v i c e s i n d i c a t e d a n i n c r e a s e d c h a r g e t r a n s f e r i n e f f i c i e n c y a n d l e a k a g e c u r r e n t d u e t o r a d i a t i o n i n d u c e d t r a p s r e s u l t i n g f r o m t o t a l d o s e s o f =10^ r a d . [ 1 ] . O n e m i g h t e x p e c t s i m i l a r r e s u l t s t o a p p l y t o A C T d e v i c e s g i v e n a c o m p a r a b l e s u b s t r a t e c o n f i g u r a t i o n . T r a n s i e n t e f f e c t s h a v e b e e n r e p o r t e d f o r G a A s M E S F E T ' s e x p o s e d t o l o w - l e v e l ( = 1 0 0 r a d . ) i o n i z i n g r a d i a t i o n [ 2 ] . N o t e d w a s a s u d d e n d r o p i n M E S F E T d r a i n - s o u r c e c u r r e n t i m m e d i a t e l y f o l l o w i n g t h e r a d i a t i o n p u l s e f o l l o w e d b y a v e r y s l o w r e c o v e r y . T h e r e s p o n s e w a s a t t r i b u t e d t o e l e c t r o n t r a p p i n g i n t h e s e m i - i n s u l a t i n g ( S I ) s u b s t r a t e f o r d e v i c e s b u i l t o n e p i t a x i a l m a t e r i a l . T h e t r a p l i f e t i m e s w e r e s e v e r a l s e c o n d s a t r o o m t e m p e r a t u r e , r e s u l t i n g i n a s l o w r e l e a s e o f e l e c t r o n s . S i m i l a r t r a n s i e n t p h e n o m e n a h a v e b e e n o b s e r v e d i n G a A s M E S F E T p h o t o d e t e c t o r s [ 3 ] . T h e s e e f f e c t s c o u l d i m p a c t o n A C T p e r f o r m a n c e f o r o p t i c a l o r i o n i z i n g r a d i a t i o n p e n e t r a t i o n d e p t h s e x t e n d i n g i n t o t h e S I s u b s t r a t e . T h e u s e o f a b u r i e d p - l a y e r t o s h i e l d s u b s t r a t e t r a p p i n g e f f e c t s f r o m t h e n - t y p e t r a n s p o r t c h a n n e l h a s b e e n p r o p o s e d t o r e d u c e t h i s e f f e c t [ 4 ] . O p t i c a l c h a r g e i n j e c t i o n has b e en r e p o r t e d i n the p a s t f o r GaAs CCD's [5-6] . Two s e p a r a t e a p p r o a c h e s have been p r o p o s e d d e p e n d i n g upon whether the l i g h t was i n c i d e n t on, or o p p o s i t e , the g a t i n g s i d e o f the d e v i c e . A b a c k s i d e i l l u m i n a t e d s t r u c t u r e r e p o r t e d by L i u , A n d e r s o n , Deyhimy, and M i l a n o [5] i s shown i n F i g u r e 1.1a. A p - t y p e AlGaAs "window" was grown on a t r a n s p a r e n t s u b s t r a t e t o p a s s i v a t e the s u r f a c e s t a t e s o f a p + - G a A s a b s o r b e r l a y e r and p r o v i d e a l o n g w a v e l e n g t h c u t - o f f f o r the i n c i d e n t l i g h t . E l e c t r o n s g e n e r a t e d i n t h e p + - G a A s d i f f u s e d to t h e j u n c t i o n o f the wide bandgap n-AlGaAs l a y e r and were c o l l e c t e d i n a d e p l e t i o n r e g i o n c h a r g e w e l l , c r e a t e d by a p p r o p r i a t e l y b i a s e d m e t a l e l e c t r o d e s on the s u r f a c e . A f r o n t s i d e i l l u m i n a t e d d e v i c e d e s c r i b e d by S a h a i , P i e r s o n , A n d e r s o n , M a r t i n , S o v e r o , and H i g g o n s [6] i s shown i n F i g u r e 1.1b. I n t h i s s i m p l e r s t r u c t u r e , a semi-t r a n s p a r e n t i n d i u m - t i n - o x i d e (ITO) f i l m was s p u t t e r e d on the s u r f a c e o f the d e v i c e to f o r m th e g a t i n g e l e c t r o d e a r r a y . L i g h t i n c i d e n t t h r o u g h th e ITO e l e c t r o d e s c r e a t e d EHP's i n t h e c h a n n e l o f the d e v i c e . The EHP' s were s e p a r a t e d by the d e p l e t i o n f i e l d u n der the g a t e s t r u c t u r e . The d e v i c e s d e s c r i b e d by L i u e t a l . [5] and S a h a i e t a l . [6] u s e d A l G a A s and GaAs a c t i v e t r a n s p o r t r e g i o n s , r e s p e c t i v e l y . The use o f wide bandgap A l G a A s i s p r e f e r r e d f o r r e d u c t i o n o f d a r k c u r r e n t . To d a t e , no ACT d e v i c e s have been r e p o r t e d t h a t use an A l G a A s t r a n s p o r t c h a n n e l . V e r y few s t u d i e s have been c o n c e r n e d w i t h the SAW p r o p e r t i e s o f AlGaAs 0 1 » 0 2 <> 0 3 ° Opaque 6ates O O U T P U T n-Al6aAs Channel Layer GaAs 'Absorber' Layer P-A1B8A3 •Wndow' Layer Transparent Substrate F i g u r e 1.1a: A b a c k s i d e I l l u m i n a t e d A l G a A s / G a A s C C D i m a g e r Seal-transparent Gates O OUTPUT n-GaAs Channel Layer Sni-Insulating GaAs F i g u r e 1.1b: A f r o n t s i d e i l l u m i n a t e d G a A s C C D i m a g e r 4 w i t h some r e s u l t s r e p o r t e d by A d a c h i [7] and S a p r i e l , M i c h e l , T o l e d a n o , V a c h e r , K e r v a r e c , and R e g r e n y [8] . As a r e s u l t , ACT d e v i c e s on A l G a A s h e t e r o j u n c t i o n s t r u c t u r e s r e m a i n an a r e a r e q u i r i n g f u t u r e r e s e a r c h . D e v i c e s b u i l t f o r t h i s t h e s i s have been b a s e d on n-GaAs t r a n s p o r t c h a n n e l s . The ACT d e v i c e c o n c e p t f i r s t r e p o r t e d i n 1982 by H o s k i n s , Morkoc, and H u n s i n g e r [9] u t i l i z e d b o t h the s e m i c o n d u c t i n g and p i e z o e l e c t r i c n a t u r e o f GaAs. I n i t s s i m p l e s t form, the ACT d e v i c e o p e r a t e d as an a n a l o g d e l a y l i n e . E l e c t r o n s i n j e c t e d i n t o t h e ACT were b u n c h e d and t r a n s p o r t e d as d i s c r e t e c h a r g e p a c k e t s a t t h e s p e e d o f sound by t h e e l e c t r i c f i e l d c o u p l e d to a p r o p a g a t i n g s u r f a c e a c o u s t i c wave. P h y s i c a l l y , t h e ACT d e v i c e was c o n s i d e r e d as a GaAs MESFET w i t h a v e r y l o n g g a t e i n tandem w i t h a SAW i n t e r d i g i t a l t r a n s d u c e r . The MESFET p o r t i o n o f t h e d e v i c e was b i a s e d to d e p l e t i o n , p r o v i d i n g an e l e c t r i c f i e l d d i s t r i b u t i o n t h a t t e n d e d t o c o n f i n e e l e c t r o n s to t h e c e n t r e o f t h e c h a n n e l . The mechanism r e s p o n s i b l e f o r t h e b u n c h i n g and t r a n s p o r t o f e l e c t r o n s a t s o n i c v e l o c i t y was t h e l a r g e s i g n a l a c o u s t o e l e c t r i c e f f e c t [10, 1 1 ] . The i d e a o f t r a n s p o r t i n g c h a r g e by means o f the e l e c t r i c f i e l d a s s o c i a t e d w i t h a s u r f a c e a c o u s t i c wave i n a p i e z o e l e c t r i c m a t e r i a l i s n o t new. Work i n the p a s t c o n c e n t r a t e d on d e v i c e s b u i l t w i t h s i l i c o n , a non-p i e z o e l e c t r i c s e m i c o n d u c t o r , u s i n g a t h i n o v e r l a y o f p i e z o e l e c t r i c m a t e r i a l s u c h as ZnO i n a Zn0:SiC>2:Si c o n f i g u r a t i o n [ 1 2 - 1 5 ] . T r a n s d u c t i o n o f the SAW t o o k p l a c e i n t h e ZnO and the e l e c t r i c f i e l d was d i r e c t l y c o u p l e d i n t o the S i , where i n t e r a c t i o n s w i t h m i n o r i t y c h a r g e c a r r i e r s o c c u r r e d . A n o t h e r method i n v o l v e d p l a c i n g a p i e z o e l e c t r i c m a t e r i a l , s u c h as LiNb03 , i n c l o s e p r o x i m i t y t o a s i l i c o n s u b s t r a t e so t h a t t h e e x t e r n a l SAW e l e c t r i c f i e l d g e n e r a t e d i n the L i N b O j i n t e r a c t e d w i t h c h a r g e c a r r i e r s i n the S i t h r o u g h an a i r gap [16, 17] . F a b r i c a t i o n d i f f i c u l t i e s and p e r f o r m a n c e p r o b l e m s r e d u c e th e a p p e a l o f t h e s e d e v i c e s . T h r e e s u c h l i m i t a t i o n s on c h a r g e t r a n s f e r e f f i c i e n c y and d e v i c e l e n g t h a r e [15] : s u r f a c e c h a r g e t r a p p i n g e f f e c t s ; the d i f f i c u l t y o f o b t a i n i n g a u n i f o r m and r e p r o d u c i b l e a i r gap i n t h e LiNbC>3 d e v i c e and the a t t e n u a t i o n o f the SAW i n s p u t t e r e d ZnO f i l m s . R e c e n t l y , GaAs c h a r g e c o u p l e d d e v i c e s have been d e v e l o p e d [4, 1 8 - 2 6 ] . In the o r i g i n a l l y p r o p o s e d c a p a c i t i v e g a t e CCD s t r u c t u r e (CGCCD), c h a r g e was c a r r i e d i n w e l l s d e f i n e d by e l e c t r i c a l l y c l o c k i n g a m u l t i p h a s e a r r a y o f S c h o t t k y g a t e s on t h e s u r f a c e o f the d e v i c e . Good c h a r g e t r a n s f e r e f f i c i e n c y was a c h i e v e d by k e e p i n g t h e gaps between i n d i v i d u a l g a t e s v e r y s m a l l [20] . G e n e r a t i o n o f the m u l t i p h a s e c l o c k i n g waveforms and s u b s e q u e n t f i l t e r i n g o f c l o c k i n t e r f e r e n c e a t the o u t p u t a r e s u b s t a n t i a l t e c h n i c a l p r o b l e m s , e s p e c i a l l y a t h i g h c l o c k f r e q u e n c i e s . The e m e r g i n g r e s i s t i v e - g a t e GaAs CCD t e c h n o l o g y (RGCCD) [25] r e d u c e d the p h o t o l i t h o g r a p h y p r o b l e m s a s s o c i a t e d w i t h t h e o r i g i n a l l y p r o p o s e d c a p a c i t i v e g a t e GaAs CCD w h i l e p r o v i d i n g s t r o n g e r f r i n g i n g f i e l d s a t r e d u c e d t r a n s p o r t 6 d e p t h s . T h i s a l l o w e d e p i t a x i a l l a y e r t h i c k n e s s to d e c r e a s e to a v a l u e c o m p a t i b l e w i t h o t h e r GaAs a c t i v e d e v i c e s s u c h as microwave MESFET's, a s i g n i f i c a n t a d v a n t a g e . U n f o r t u n a t e l y , m u l t i p h a s e c l o c k i n g c i r c u i t r y was s t i l l r e q u i r e d . I n c r e a s e d l e a k a g e c u r r e n t s due to the n o n - i d e a l S c h o t t k y b e h a v i o u r o f the "Cermet" r e s i s t i v e g a t e s [25] was one d i s a d v a n t a g e o f the RGCCD. The ACT d e v i c e p r o v i d e s an e l e g a n t s o l u t i o n t o s e v e r a l CCD d e v i c e s h o r t c o m i n g s . I n p a r t i c u l a r , the ACT d r i v e s i g n a l i s an e a s i l y - g e n e r a t e d s i n g l e - p h a s e s i n u s o i d . E f f e c t i v e d e c o u p l i n g o f the c l o c k and i n p u t s i g n a l s i s a c h i e v e d s i n c e t h e SAW t r a n s d u c e r i s remote from t h e c h a r g e t r a n s p o r t c h a n n e l , n o t d i r e c t l y i n t e g r a t e d w i t h t h e c h a n n e l as i n a CCD. The c l o c k waveform can be e l i m i n a t e d a t the o u t p u t o f the ACT w i t h a n a r r o w b a n d n o t c h f i l t e r . ACT c h a r g e t r a n s p o r t o c c u r s c o n t i n u o u s l y i n time and sp a c e a t t h e sound v e l o c i t y u n d e r the i n f l u e n c e o f a c o n s t a n t e l e c t r i c f i e l d . E l e c t r o n s f o l l o w s t r a i g h t l i n e p a t h s from t h e d e v i c e i n p u t to o u t p u t . The s l o w e r p r o c e s s o f d i f f u s i o n i s n o t i n v o l v e d . As a r e s u l t , ACT c h a r g e t r a n s f e r e f f i c i e n c y (CTE) s h o u l d be s u p e r i o r t o t h a t a c h i e v e d w i t h GaAs CCD d e v i c e s , p a r t i c u l a r l y a t h i g h f r e q u e n c i e s . T h i s seems to be the c a s e , w i t h CTE i n e x c e s s o f 0.99994 r e p o r t e d i n the l i t e r a t u r e f o r an ACT d e v i c e o p e r a t e d a t 360 MHz [ 2 7 ] . Some a p p l i c a t i o n s o f CCD t e c h n o l o g y , s u c h as time e x p a n s i o n and c o m p r e s s i o n o f p u l s e s c a n n o t e f f e c t i v e l y be p e r f o r m e d by the ACT. T h i s i s b e c a u s e the d r i v e f r e q u e n c y i s f i x e d w i t h the l a t t e r d e v i c e by t h e n a r r o w b a n d i n t e r d i g i t a l t r a n s d u c e r ( I D T ) . S i n c e GaAs i s w e a k l y p i e z o e l e c t r i c , e f f i c i e n t g e n e r a t i o n o f a l a r g e a m p l i t u d e SAW r e q u i r e s a l o n g , and hence narrowband, IDT. T y p i c a l l y 100 f i n g e r p a i r s w i t h f i n e d i m e n s i o n s a r e n e c e s s a r y . F i n g e r w i d t h s o f 2 juin or 1 f*m a r e u s e d d e p e n d i n g upon the t y p e o f t r a n s d u c e r d e s i g n , a t 360 MHz. H i g h e r f r e q u e n c i e s n e c e s s i t a t e e i t h e r s m a l l e r d i m e n s i o n s o r the l a u n c h i n g o f SAW on a h a r m o n i c o f a l o w e r f r e q u e n c y IDT. D e v i c e p e r f o r m a n c e i s c r i t i c a l l y d e p e n d e n t on the s u r f a c e m o r p h o l o g y o f the GaAs s u b s t r a t e . A t t e n u a t i o n o f h i g h f r e q u e n c y SAW on GaAs i s s e n s i t i v e t o t o p o l o g i c a l i r r e g u l a r i t i e s , as may be p r o d u c e d d u r i n g a p o l i s h i n g or e p i t a x i a l l a y e r growth p r o c e s s [ 2 8 ] . The SAW a t t e n u a t i o n c h a r a c t e r i s t i c s have l i m i t e d the l e n g t h o f r e p o r t e d d e v i c e s to s e v e r a l m i l l i m e t e r s a t 360 MHz. The l a t t e r l i m i t a t i o n i s l i k e l y to improve as GaAs e p i t a x i a l w a f e r growth t e c h n i q u e s a d v a n c e . The f i r s t ACT d e v i c e s were d e l a y l i n e s . A s i g n a l was e l e c t r i c a l l y i n j e c t e d t h r o u g h an i n p u t d i o d e and was d e s t r u c t i v e l y s e n s e d by an o u t p u t d i o d e . S i n c e e l e c t r o n t r a n s p o r t o c c u r r e d n e a r th e s u r f a c e o f the s u b s t r a t e , an a r r a y o f e l e c t r o d e s p l a c e d a l o n g the p r o p a g a t i o n d i r e c t i o n c o u l d s e n s e the p a s s a g e o f i n j e c t e d c h a r g e by means o f image c u r r e n t s [29, 3 0 ] . By v a r y i n g the a p o d i z a t i o n o f t h e s e sense e l e c t r o d e s and summing t h e i r o u t p u t , s i g n a l p r o c e s s i n g d e v i c e s s u c h as t r a n s v e r s a l f i l t e r s have been formed. At 8 p r e s e n t , t h i s type of analog matched f i l t e r o p e r a t i o n has been achieved predominantly with CCD or SAW d e v i c e s . A f i r s t step toward e x t e n s i o n of ACT de v i c e concepts i n t o the areas of o p t i c a l or i o n i z i n g r a d i a t i o n imaging i s r e p o r t e d i n t h i s work. By s t u d y i n g the o p t i c a l charge i n j e c t i o n c h a r a c t e r i s t i c s of the de v i c e u s i n g near i n f r a r e d p u l s e s , experiments have i n d i c a t e d a charge t r a n s f e r e f f i c i e n c y i n excess of 0.992 and net quantum e f f i c i e n c y g r e a t e r than 9%. When compensation was made f o r the o p t i c a l s i g n a l l o s s through the chromium windows, a quantum e f f i c i e n c y g r e a t e r than 24% was c a l c u l a t e d . The r e s u l t s to date should p r o v i d e impetus f o r f u t u r e r e s e a r c h aimed at a p p l y i n g ACT technology i n an a c t u a l imaging device d e s i g n . T h i s t h e s i s c o n t a i n s f i v e c h a p t e r s . Chapter 2 deals with t h e o r e t i c a l c o n s i d e r a t i o n s of ACT d e s i g n and o p e r a t i o n . Chapter 3 d i s c u s s e s the experimental d e v i c e , o u t l i n e s the f a b r i c a t i o n p r o c e s s , and d e s c r i b e s t e s t s t r u c t u r e s added to the mask s e t f o r process c o n t r o l . Problems which had to be overcome while f a b r i c a t i n g these d e v i c e s are a l s o d i s c u s s e d . Chapter 4 p r o v i d e s measured performance data i n c l u d i n g the o p t i c a l charge i n j e c t i o n r e s u l t s . F i n a l l y , Chapter 5 p r o v i d e s a summary of experimental measurements, a d i s c u s s i o n of the device performance and o f f e r s some p o s s i b l e areas f o r f u t u r e study. 9 2.0 THEORY 2.1 C h a r a c t e r i s t i c s o f <110> P r o p a g a t i n g SAW on (100) c u t GaAs The p r o p e r t i e s o f SAW on GaAs have b e e n e x t e n s i v e l y s t u d i e d [ 3 1 - 3 9 ] . T h i s r e s e a r c h has d e t e r m i n e d t h a t the <110> p r o p a g a t i n g SAW on (100) c u t GaAs has the h i g h e s t e l e c t r o m e c h a n i c a l c o u p l i n g c o e f f i c i e n t (K^ ~ 6.4x10"^) d e f i n e d by [40] : Kg = 2 ( v s s - v s o ) / v s o (2.1) v s s = SAW v e l o c i t y w i t h m e t a l l i z e d s u r f a c e v s o = SAW v e l o c i t y w i t h u n m e t a l l i z e d s u r f a c e F o r t u n a t e l y , the (100) w a f e r o r i e n t a t i o n p o s s e s s i n g the h i g h e s t v a l u e o f i s commonly u s e d f o r f a b r i c a t i o n o f p r e s e n t GaAs a c t i v e d e v i c e s . The e l e c t r o m e c h a n i c a l c o u p l i n g c o e f f i c i e n t i s i m p o r t a n t i n t h e d e s i g n o f any SAW d e v i c e s i n c e i t r e l a t e s t r a n s d u c t i o n e f f i c i e n c y and b a n d w i d t h . H i g h e f f i c i e n c y and l a r g e b a n d w i d t h c a n n o t be s i m u l t a n e o u s l y a c h i e v e d w i t h weak p i e z o e l e c t r i c s s u c h as GaAs u s i n g an i n t e r d i g i t a l t r a n s d u c e r . A l o n g , and hence narrowband, IDT i s r e q u i r e d t o l a u n c h a SAW on GaAs w i t h s u f f i c i e n t wave p o t e n t i a l to d r i v e the ACT c h a r g e t r a n s p o r t p r o c e s s . However, i n c o n t r a s t w i t h most SAW d e v i c e s , the ACT b a n d w i d t h i s n o t d e t e r m i n e d by t h e a c o u s t i c t r a n s d u c e r . The c h a r g e i n j e c t i o n / d e t e c t i o n t e c h n i q u e and f i n i t e c h a r g e t r a n s f e r e f f i c i e n c y l i m i t o v e r a l l b a n d w i d t h to a v a l u e w i t h i t s uppe r bound p r e d i c t e d by t h e N y q u i s t s a m p l i n g theorem. 10 I n d e v i c e s where g r e a t e r c o u p l i n g t h a n t h a t a f f o r d e d by GaAs a l o n e i s r e q u i r e d , f o r example i n wide b a n d w i d t h SAW f i l t e r s , i t i s p o s s i b l e to d e p o s i t a p i e z o e l e c t r i c ZnO l a y e r e d s t r u c t u r e o v e r the t r a n s d u c e r to i mprove e f f i c i e n c y [41] . F o r t u n a t e l y , the d e s i g n o f the ACT d e c o u p l e s the IDT b a n d w i d t h from the s i g n a l p r o c e s s i n g b a n d w i d t h . As such, a n a r r o w b a n d t r a n s d u c e r d e s i g n s u f f i c e s . The <110> p r o p a g a t i n g SAW on (100) c u t GaAs i s sometimes r e f e r r e d t o as a p s e u d o - s u r f a c e a c o u s t i c wave o r t h e l i m i t o f a l e a k y s u r f a c e a c o u s t i c wave (LSAW). T h i s i s b e c a u s e the s u r f a c e wave has a h i g h e r v e l o c i t y t h a n a b u l k s h e a r wave whose d i s p l a c e m e n t i s p e r p e n d i c u l a r to t h e s a g i t t a l p l a n e , =2860 m/s f o r the f o r m e r compared to =2468 m/s f o r the l a t t e r [38] . The s a g i t t a l p l a n e i s d e f i n e d by t h e d i r e c t i o n o f p r o p a g a t i o n (phase v e l o c i t y ) and the s u b s t r a t e s u r f a c e n o r m a l . A l e a k y s u r f a c e a c o u s t i c wave has f i n i t e e n e r g y c o u p l i n g between i t s e l f and a b u l k wave. I n f a c t , i t has b e en shown [32] t h a t as t h e wave p r o p a g a t i o n d i r e c t i o n v a r i e s f r o m 0 t o 45 d e g r e e s f r om <100> towards <110> on (100) c u t GaAs, the s u r f a c e wave s t a r t s o u t as a p u r e R a y l e i g h mode w i t h no c o u p l i n g to the e l e c t r i c f i e l d , becomes c o u p l e d to the e l e c t r i c f i e l d and a s h e a r b u l k mode, and f i n a l l y ends up c o u p l e d t o the e l e c t r i c f i e l d b u t d e c o u p l e d from the b u l k s h e a r mode a l o n g <110>. Hence, f o r p r o p a g a t i o n e x a c t l y a l o n g <110>, the wave i s n o t " l e a k y " and b e h a v i o u r i s s i m i l a r to a " n o r m a l " R a y l e i g h mode. S u b s t r a t e o r i e n t a t i o n , p a r t i c l e m o t i o n i n the s a g i t t a l 11 p l a n e , and d e f i n i t i o n o f axes u s e d t h r o u g h o u t t h i s work a r e as g i v e n i n F i g u r e 2.1. M a t e r i a l " p a r t i c l e s " t r a c e out r e t r o g r a d e e l l i p t i c a l p a t h s i n t h e s a g i t t a l p l a n e f o r p r o p a g a t i o n i n t h e d i r e c t i o n i n d i c a t e d . The m a g n i t u d e s o f the d i s p l a c e m e n t d e c a y r a p i d l y i n t o the s u b s t r a t e w i t h most o f t h e e n e r g y c o n f i n e d t o w i t h i n a p p r o x i m a t e l y one a c o u s t i c w a v e l e n g t h o f the s u r f a c e . B u i l d i n g d e v i c e s whose SAW p r o p a g a t i o n i s a l i g n e d w i t h <110> i s a i d e d by the n a t u r a l GaAs c l e a v a g e p l a n e s a l o n g t h e s e d i r e c t i o n s . The l o s s due t o c o u p l i n g between the SAW and b u l k s h e a r mode i s a s t r o n g f u n c t i o n o f p r o p a g a t i o n d i r e c t i o n as shown i n F i g u r e 2.2 [ 3 6 ] . F o r t h i s r e a s o n , and beam s t e e r i n g c o n s i d e r a t i o n s t o be d i s c u s s e d l a t e r , p r e c i s e o r i e n t a t i o n a l o n g <110> i s h i g h l y d e s i r a b l e . An i m p o r t a n t c h a r a c t e r i s t i c o f t h e <110> SAW i s the v a r i a t i o n o f wave p o t e n t i a l as a f u n c t i o n o f d i s t a n c e i n t o the s u b s t r a t e . SAW d i s p l a c e m e n t components and q u a s i s t a t i c p o t e n t i a l m a g n i t u d e s have been c a l c u l a t e d i n t h e p r e s e n t work u s i n g t h e method d e v e l o p e d by T s e n g and White [42] and C a m p b e l l and J o n e s [ 4 3 ] . D e t a i l s o f t h e c a l c u l a t i o n a r e g i v e n i n A p p e n d i x A. An a l t e r n a t i v e method o f s o l u t i o n has r e c e n t l y b e en p r e s e n t e d by D a t t a and H u n s i n g e r [ 4 4 ] . The c a l c u l a t i o n s were made a s s u m i n g no f r e e c a r r i e r s i n the s u b s t r a t e . The s u r f a c e o f the c r y s t a l was assumed to be e l e c t r i c a l l y s h o r t e d by a m a s s l e s s , p e r f e c t l y c o n d u c t i n g s h e e t . The m a t e r i a l c o n s t a n t s w h i c h were u s e d a r e g i v e n i n 12 Rayleigh Wave Displacement F i g u r e 2.1: D e f i n i t i o n of axes and p a r t i c l e displacements f o r <110> p r o p a g a t i n g SAW on (100) cut GaAs 13 L e a k y " Wave L o s s i i i i I i i i i 1 i i i i I i i i i -5 o 5 0 (degrees) F i g u r e 2.2: SAW a t t e n u a t i o n due to c o u p l i n g to bulk shear wave near the <110> propaga t i o n d i r e c t i o n 14 T a b l e 2.1 [ 3 6 ] : T a b l e 2.1: GaAs m a t e r i a l c o n s t a n t s [36] C o n s t a n t Type V a l u e U n i t s S t i f f n e s s c l l 11 . 9 x l O i U N/m2 c 1 2 5 . 3 8 x l 0 1 0 N/m2 c 4 4 5 . 9 5 x l 0 1 0 N/m2 P i e z o e l e c t r i c e 1 4 -0.16 C/m2 e 2 5 -0.16 C/m2 e 3 6 -0.16 C/m2 R e l a t i v e P e r m i t t i v i t y €R 11 . 0 -Mass D e n s i t y Ptt 5 . 3 x l 0 3 kg/m 3 The r e s u l t s o f n u m e r i c a l c a l c u l a t i o n s a r e shown i n F i g u r e s 2.3a and 2.3b n o r m a l i z e d to an a c o u s t i c i n t e n s i t y o f 1 mW p e r w a v e l e n g t h beamwidth a t 360 MHz. The SAW v e l o c i t y c a l c u l a t e d was 2865.6 m/s. V a l u e s i n t h e l i t e r a t u r e r ange from 2856 m/s [38] to 2866 m/s [36] f o r an e l e c t r i c a l l y u n s h o r t e d s u r f a c e . S h o r t i n g the s u r f a c e l o w e r s the SAW v e l o c i t y by = 0.03%. C a l c u l a t i n g the SAW v e l o c i t y f o r b o t h a f r e e and s h o r t e d s u r f a c e a l l o w s the e l e c t r o m e c h a n i c a l c o u p l i n g c o e f f i c i e n t t o be d e r i v e d u s i n g e q u a t i o n ( 2 . 1 ) . The d i s a g r e e m e n t s a p p e a r i n g i n t h e l i t e r a t u r e r e g a r d i n g t h e c a l c u l a t e d SAW v e l o c i t y a r e p r e s u m a b l y due to the use o f s l i g h t l y d i f f e r e n t GaAs m a t e r i a l c o n s t a n t s i n t h e a n a l y s i s . F o r example, when m a t e r i a l c o n s t a n t s r e c e n t l y g i v e n by W e b s t e r and C a r r [38] were u s e d , a SAW v e l o c i t y o f 2854.4 m/s 15 X3/ - Normalized Distance into Substrate F i g u r e 2.3a: SAW d i s p l a c e m e n t components Quasistatic Potential 0 . 6 F i g u r e 2.3b: SAW q u a s i s t a t i c p o t e n t i a l 16 was found f o r an e l e c t r i c a l l y s h o r t e d s u r f a c e . The c a l c u l a t e d q u a s i s t a t i c p o t e n t i a l magnitude (# 0) u s i n g the c o n s t a n t s i n Table 2.1 i s ~ 6% h i g h e r than the curve g i v e n by H o s k i n s [45] page 8. When the m a t e r i a l c o n s t a n t s of Webster and C a r r [38] were used, the q u a s i s t a t i c p o t e n t i a l c a l c u l a t e d was =16% lower than the curve p r e v i o u s l y c a l c u l a t e d u s i n g the m a t e r i a l c o n s t a n t s g i v e n i n Table 2.1. The d i s p l a c e m e n t component magnitudes were v i r t u a l l y i d e n t i c a l f o r b o t h c a l c u l a t i o n s . The s t i f f n e s s , p i e z o e l e c t r i c and mass d e n s i t y g i v e n by Webster and C a r r do not d i f f e r g r e a t l y from the v a l u e s used by o t h e r a u t h o r s [35-37]. However, the v a l u e of r e l a t i v e p e r m i t t i v i t y used by the former was 13.1 w h i l e the l a t t e r a u t h o r s quote 11.0; a d i f f e r e n c e of —16%. I t i s t h e r e f o r e not s u r p r i s i n g t h a t the q u a s i s t a t i c p o t e n t i a l v a l u e s d i f f e r by a p p r o x i m a t e l y t h i s amount. To the p r e s e n t a u t h o r ' s knowledge, the a p p r o p r i a t e v a l u e of £R to be used i n the c a l c u l a t i o n has not been e s t a b l i s h e d . I n k e e p i n g w i t h the m a j o r i t y of p u b l i c a t i o n s d e s c r i b i n g SAW on GaAs, the r e l a t i v e p e r m i t t i v i t y of 11.0 was assumed f o r c a l c u l a t i o n s p r e s e n t e d i n t h i s t h e s i s . To use the c u r v e s shown i n F i g u r e 2.3 f o r o t h e r a c o u s t i c i n t e n s i t i e s , d e n o r m a l i z a t i o n based on the p r o p o r t i o n a l i t y below must be a p p l i e d : |* 0|, | U i | , | U 3 I « J P A l / 2 7 r v S (2.2) where: = A c o u s t i c i n t e n s i t y i n Watts/Ag beamwidth vg = SAW v e l o c i t y An i m p o r t a n t p r o p e r t y of the SAW p o t e n t i a l curve shown 17 i n F i g u r e 2.3b, i s t h e peak i n wave p o t e n t i a l t h a t o c c u r s =0.45 A below t h e s u b s t r a t e s u r f a c e . U s i n g a c l o c k f r e q u e n c y o f 360 MHz, maximum wave p o t e n t i a l i s a c h i e v e d a t =3.6 i n t o t h e s u b s t r a t e . S e c t i o n 2.2 d i s c u s s e s t h e method u s e d to c o n f i n e t r a n s p o r t e d c h a r g e so t h a t the d e p t h o f t r a n s p o r t c o i n c i d e s w i t h the d e p t h o f g r e a t e s t wave p o t e n t i a l . C o n s i d e r a t i o n o f the SAW p o t e n t i a l p r o f i l e i n t o the s u b s t r a t e s u p p o r t s t h e c h o i c e o f a 360 MHz d r i v e f r e q u e n c y . O p e r a t i o n a t much l o w e r c l o c k r a t e s i s i m p r a c t i c a l as the optimum d e p t h o f c h a r g e t r a n s p o r t moves f u r t h e r f r o m the d e v i c e s u r f a c e . T h i s r e d u c e s the b a n d w i d t h o f s u r f a c e e l e c t r o d e s d e s i g n e d t o sen s e the c h a r g e c a r r i e d by the SAW. E p i t a x i a l growth o f GaAs l a y e r s more t h a n s e v e r a l m i c r o n s i n t h i c k n e s s w i t h good s u r f a c e m o r p h o l o g y and d e p l e t i o n o f such l a y e r s c r e a t e a d d i t i o n a l c o m p l i c a t i o n s . C h o i c e o f h i g h e r c l o c k f r e q u e n c y , w h i l e i t a l l e v i a t e s t h e s e d i f f i c u l t i e s , i n c r e a s e s the p r o b l e m s a s s o c i a t e d w i t h SAW t r a n s d u c t i o n s i n c e t h e f i n g e r w i d t h s o f q u a r t e r and e i g h t h w a v e l e n g t h t r a n s d u c e r s q u i c k l y a p p r o a c h l i m i t s a c h i e v a b l e u s i n g s t a n d a r d o p t i c a l p a t t e r n i n g t e c h n i q u e s . C a r e f u l c o n s i d e r a t i o n must be g i v e n to a c o u s t i c d i f f r a c t i o n and beam s t e e r i n g i n SAW d e v i c e d e s i g n . F o r the ACT, t h e s e e f f e c t s p l a c e f u n d a m e n t a l l i m i t s on the p r a c t i c a l c h a n n e l l e n g t h due t o the s p r e a d i n g and s t e e r i n g o f the SAW beam p r o f i l e . Szabo and S l o b o d n i k [46] have r e v i e w e d SAW d i f f r a c t i o n t h e o r y . F o r t h e c a s e o f an a n i s o t r o p i c m a t e r i a l whose phase v e l o c i t y dependence on a n g l e a b o u t a p u r e mode 18 a x i s i s p a r a b o l i c , s i m p l e r e l a t i o n s h i p s and c u r v e s can be u s e d t o c a l c u l a t e the amount o f d i f f r a c t i o n and beam s t e e r i n g . The a l m o s t p a r a b o l i c phase v e l o c i t y r e l a t i o n s h i p w i t h a n g l e n e a r <110> f o r t h e SAW on GaAs i s shown i n F i g u r e 2.4 f o r a z e r o d e g r e e w a f e r ( i . e . t h e w a f e r n o r m a l d i r e c t i o n i s a l i g n e d w i t h the [001] c r y s t a l d i r e c t i o n ) [36, 39]. From the e x p e r i m e n t a l c u r v e , t h e a n i s o t r o p y p a r a m e t e r ft c a n be c a l c u l a t e d u s i n g t h e r e l a t i o n s h i p [47] : v s ( 0 ) / v s ( O r a d . ) * 1 + (/3/2)*2 (2.3) where 9 i s t h e a n g u l a r p r o p a g a t i o n d e v i a t i o n f r o m <110> i n r a d i a n s and v s i s t h e SAW phase v e l o c i t y . The v a l u e o b t a i n e d u s i n g F i g u r e 2.4 i s p ~ -1.6. The a n i s o t r o p y p a r a m e t e r , t r a n s d u c e r w i d t h , and p r o p a g a t i o n d i s t a n c e c a n be u s e d to c a l c u l a t e t h e p r o p a g a t i o n l o s s between two i d e n t i c a l t r a n s d u c e r s due to d i f f r a c t i o n o f the SAW beam. F o r the ACT d e v i c e b u i l t i n t h i s t h e s i s , t h e t r a n s d u c e r w i d t h and p r o p a g a t i o n d i s t a n c e were 75A and 250A r e s p e c t i v e l y . U s i n g t h e c u r v e s g i v e n by Szabo and S l o b o d n i k [46] a d i f f r a c t i o n r e l a t e d l o s s o f =0.4 dB was o b t a i n e d . A s e c o n d example c o u l d c o n s i s t o f a much n a r r o w e r t r a n s d u c e r (12.5A) and a l o n g e r p r o p a g a t i o n d i s t a n c e (1250A), c o r r e s p o n d i n g to 100 /j,m and 1 cm r e s p e c t i v e l y a t 360 MHz. The d i f f r a c t i o n i n d u c e d l o s s i n t h i s c a s e i s =7 dB . Where l o n g p r o p a g a t i o n d i s t a n c e s a r e d e s i r e d , t h e r e q u i r e m e n t f o r wide t r a n s d u c e r s i s e v i d e n t from t h e s e c a l c u l a t i o n s . In g e n e r a l , t h e d i f f r a c t i o n l o s s depends on the r a t i o o f the 19 2900 I I I I I I I I I 1 I I I I I I I I I ^ 2850 o o 5 2800 -r Experimental (0 degree wafer) (Hunt. M i l l e r , and Hunsinger) Theory (Penunuri and Lakin) 2750 1 I I I 1 1 I I I I I I I I I I I I I I - 1 0 - 5 0 5 0 (degrees) 10 F i g u r e 2.4: SAW phase v e l o c i t y f o r p r o p a g a t i o n near <110> 20 p r o p a g a t i o n d i s t a n c e to the s q u a r e o f t h e t r a n s d u c e r w i d t h where t h e s e q u a n t i t i e s a r e e x p r e s s e d i n w a v e l e n g t h s a t the d e s i r e d f r e q u e n c y . V a l u e s o f /3 u s e d t o c a l c u l a t e t h e amount o f beam s t e e r i n g u s i n g the p a r a b o l i c v e l o c i t y a p p r o x i m a t i o n have been t a b u l a t e d f o r many SAW m a t e r i a l s and o r i e n t a t i o n s [ 4 6 ] . The p r e s e n t a u t h o r was u n a b l e to f i n d a v a l u e t a b u l a t e d f o r <110> p r o p a g a t i n g SAW on (100) c u t GaAs even t h o u g h v a l u e s were q u o t e d f o r (110) c u t <100> and (211) c u t <111> p r o p a g a t i o n . B a s e d upon the shape o f the v e l o c i t y c u r v e s f o r s m a l l a n g l e s a b o u t <110> f o r (100) c u t GaAs, i t does n o t seem u n r e a s o n a b l e t h a t the p a r a b o l i c a p p r o x i m a t i o n c an be a p p l i e d to e s t i m a t e d i f f r a c t i o n r e l a t e d l o s s e s . T h i s a s s u m p t i o n i g n o r e s any e f f e c t o f c o u p l i n g to t h e b u l k s h e a r mode f o r d i r e c t i o n s o f f <110>. The a n i s o t r o p y p a r a m e t e r i s r e l a t e d to the beam s t e e r i n g a n g l e (<f>) t h r o u g h the r e l a t i o n s h i p [46] : t a n <f> = ( l / v s ( 0 r a d . ) ) d v s / d 0 (2.4a) where (j> i s t h e a n g l e between the <110> pu r e mode a x i s and the power f l o w v e c t o r . F o r s m a l l <j> , t a n <j> ~ <j> and (2.4a) can be s i m p l i f i e d : <j> « ( l / v s ( 0 r a d . ) ) d v s / d 0 (2.4b) F o r a m a t e r i a l w i t h phase v e l o c i t y a b o u t a p u r e mode a x i s s a t i s f y i n g ( 2 . 3 ) , e q u a t i o n (2.4b) can be u s e d to d e t e r m i n e t h e power f l o w a n g l e <j> : 4> » pB (2 . 4c) A s s u m i n g an a l i g n m e n t a c c u r a c y o f ±1° a l o n g <110>, from 21 e q u a t i o n ( 2 . 4 c ) t h e beam s t e e r i n g a n g l e i s ± 1 . 6 ° . Over a p r o p a g a t i o n d i s t a n c e o f 2 mm, t h i s y i e l d s a l a t e r a l s h i f t o f the SAW beam e q u a l l i n g ±56 /urn. S i n c e the e x p e r i m e n t a l ACT b u i l t f o r t h i s work has a t r a n s d u c e r w i d t h o f 600 pm and a c h a n n e l w i d t h o f 400 fim, the l a t e r a l s h i f t i n SAW c a u s e d by beam s t e e r i n g e f f e c t s s h o u l d n o t be a p r o b l e m as l o n g as the a l i g n m e n t a c c u r a c y o f ±1° i s m a i n t a i n e d . F o r l o n g e r p r o p a g a t i o n d i s t a n c e s , beam s t e e r i n g becomes i n c r e a s i n g l y i m p o r t a n t r e q u i r i n g v e r y a c c u r a t e a l i g n m e n t o f SAW p r o p a g a t i o n d i r e c t i o n w i t h <110>. By p l o t t i n g the r e c i p r o c a l o f the SAW phase v e l o c i t y as a f u n c t i o n o f p r o p a g a t i o n d i r e c t i o n , the s o - c a l l e d s l o w n e s s s u r f a c e i s c r e a t e d [ 4 7 ] . SAW e n e r g y f l o w i s p e r p e n d i c u l a r to the s l o w n e s s s u r f a c e . F i g u r e 2.5 shows t h e c u r v e g e n e r a t e d f o r SAW p r o p a g a t i o n n e a r <110> on (100) c u t GaAs. I n c o m p a r i s o n t o t h e c a s e shown f o r an i s o t r o p i c m a t e r i a l , the <110> p r o p a g a t i n g SAW on GaAs i s s l i g h t l y f o c u s s i n g . A g i v e n d i f f r a c t i o n p a t t e r n w i l l o c c u r a t a g r e a t e r d i s t a n c e from the a c o u s t i c t r a n s d u c e r f o r <110> p r o p a g a t i o n on GaAs t h a n f o r an i s o t r o p i c m a t e r i a l . The c u r v a t u r e o f t h e e x p e r i m e n t a l and t h e o r e t i c a l r e s u l t s shown i n F i g u r e 2.5 a r e q u i t e d i f f e r e n t . T h i s i s due to t h e c u r v a t u r e d i f f e r e n c e i n t h e v e l o c i t y r e s u l t s shown i n F i g u r e 2.4. The c a u s e o f t h e d i f f e r e n c e was n o t e x p l a i n e d by Hunt, M i l l e r , and H u n s i n g e r [39] who p u b l i s h e d the e x p e r i m e n t a l r e s u l t s . 22 o i o l (x 10 seconds/meter) F i g u r e 2.5: SAW slowness curve f o r p r o p a g a t i o n near <110> 23 O t h e r p r o p a g a t i o n e f f e c t s t h a t must be c o n s i d e r e d f o r SAW d e v i c e d e s i g n i n c l u d e v i s c o u s l o s s e s , a i r l o a d i n g and s u r f a c e s c a t t e r i n g by d e f e c t s [ 4 8 ] . The l a t t e r e f f e c t i s c o n s i d e r e d i n S e c t i o n 3.2. 2.2 D e p l e t i o n P o t e n t i a l i n t h e ACT C h a n n e l Charge t r a n s p o r t i n t h e ACT d e v i c e u n d e r c o n s i d e r a t i o n i s a c c o m p l i s h e d i n the d e p l e t e d n - e p i t a x i a l r e g i o n o f an n-GaAs:semi - i n s u l a t i n g GaAs s u b s t r a t e c o n f i g u r a t i o n . The assumed band d i a g r a m u n d e r c o m p l e t e d e p l e t i o n o f the n - e p i t a x i a l l a y e r i s shown i n F i g u r e 2.6. An "ohmic" c o n t a c t a l l o y e d t o t h e b a c k o f the d e v i c e b i a s e s t h e j u n c t i o n between t h e e p i t a x i a l l a y e r and the s e m i - i n s u l a t i n g s u b s t r a t e . The c o n t a c t and s u b s t r a t e a c t as a l a r g e n o n - l i n e a r r e s i s t a n c e . Hence, t h e e f f e c t i v e b i a s a p p e a r i n g a t t h e j u n c t i o n |V S| i s much s m a l l e r t h a n the b i a s a p p l i e d to the s u b s t r a t e c o n t a c t |V S| . P o t e n t i a l i n the c h a n n e l i s c a l c u l a t e d u s i n g P o i s s o n ' s e q u a t i o n and the d e p l e t i o n a p p r o x i m a t i o n s u b j e c t t o the f o l l o w i n g b o u n d a r y c o n d i t i o n s : E x 3 ( x 3 = W) = 0 (2. , 5a) E x 3 i s c o n t i n u o u s a t x 3 = W n (2. ,5b) ^ d ( x 3 = 0) = v m + ^p " ^ml (2. ,5c) <t>& i s c o n t i n u o u s a t x 3=W n (2. .5d) «£d( x3 = W) = v s (2. ,5e) 24 / S c h o t t k y Contact y ^ n - e p i t a x i a l GaAs: N D Ohnic Contact / / 'p-type" N A « 1 0 1 6 c r * 3 I x 3 - 0 A : 1 q(*il-*) \ i i i Semi-Insulating \ 6aAs - ^ W Non-Linear R i I N n + Hp •sv (-) T M II' F i g u r e 2.6: D e p l e t e d ACT c h a n n e l band d i a g r a m I n t h e c a l c u l a t i o n i t i s assumed t h a t t h e semi-i n s u l a t i n g s u b s t r a t e i s p - t y p e w i t h an a c c e p t o r d o p i n g o f -10^-^ cm" 3 i n p r o x i m i t y to t h e e p i t a x i a l l a y e r j u n c t i o n . T h i s d o p i n g d e n s i t y c o r r e s p o n d s to the b u l k t r a p d e n s i t y o f the s u b s t r a t e . T r a p s a r e t h o u g h t to be r e s p o n s i b l e f o r the c a p t u r e o f e l e c t r o n s and r e l e a s e o f h o l e s , making the s u b s t r a t e l o o k p - t y p e as f a r as t h e s t a t i c d e p l e t i o n p o t e n t i a l c a l c u l a t i o n i s c o n c e r n e d [20, 45, 4 9 ] . The d y n a m i c s o f t h e c a p t u r e and r e l e a s e p r o c e s s a r e s l o w . Thus, w h i l e e l e c t r o s t a t i c a l l y the s u b s t r a t e c a n be t r e a t e d as p-t y p e , f o r time v a r y i n g e l e c t r i c f i e l d s s u c h as the p r o p a g a t i n g SAW, the s u b s t r a t e a p p e a r s semi - i n s u l a t i n g . C harge s c r e e n i n g e f f e c t s on the SAW c a n be i g n o r e d as a r e s u l t . The d e p l e t i o n p o t e n t i a l c a l c u l a t i o n has b e en r e p o r t e d by Deyhimy, Eden, and H a r r i s [20] f o r CCD d e v i c e s and by H o s k i n s [45] f o r ACT d e v i c e s . A l t h o u g h th e g e o m e t r i e s o f the s t r u c t u r e s u nder c o n s i d e r a t i o n were i d e n t i c a l f o r each c a l c u l a t i o n , t h e r e s u l t s d i f f e r e d s l i g h t l y s i n c e Deyhimy e t a l . d i d n o t a c c o u n t f o r t h e GaAs and S c h o t t k y m e t a l work f u n c t i o n s . H o s k i n s ' r e s u l t s f o r the c o n f i g u r a t i o n shown i n F i g u r e 2.6 a r e g i v e n below: F o r 0 < x 3 < Wn : ^d< x3> = ^dm " q N D / 2 £ ( x 3 - x 3 m ) 2 (2.6) ^dm = < l N D x L / 2 e + vm + ^p " Kl (2.7) x3m " Wn - N AW p/N D (2.8) 26 F o r Wn < x 3 < W : ^ d ( x 3 ) = qN A / 2 e[x$ - 2Wx 3 + w2(N D/N A + 1)] (2.9) " ^ m l + v m + ^ p W = [2e/q^A]1'2 [ q w 2(N D+N A)/26 - ^ m l + V m - V s + ^ p ] 1 ' 2 (2.10) Wp = W - Wn (2.11) where: q = e l e c t r o n i c c h a r g e Nj) = e p i t a x i a l l a y e r donor d e n s i t y N A = e q u i v a l e n t a c c e p t o r d e n s i t y a t i n t e r f a c e between the semi - i n s u l a t i n g s u b s t r a t e and t h e n - e p i t a x i a l l a y e r ( u n o c c u p i e d deep t r a p d e n s i t y ) € = p e r m i t t i v i t y o f GaAs v m - b i a s on S c h o t t k y c h a n n e l p l a t e e f f e c t i v e s u b s t r a t e b i a s *v " semi - i n s u l a t i n g GaAs work f u n c t i o n = S c h o t t k y m e t a l work f u n c t i o n w n = t h i c k n e s s o f n - e p i t a x i a l l a y e r w p = t h i c k n e s s o f d e p l e t e d ' p - t y p e ' l a y e r a t i n t e r f a c e between t h e s e m i - i n s u l a t i n g s u b s t r a t e and n - e p i t a x i a l l a y e r W = t o t a l d e p l e t i o n l a y e r t h i c k n e s s F i g u r e s 2.7a to 2.7d show p l o t s o f the d e p l e t i o n p o t e n t i a l f o r v a r i o u s v a l u e s o f s u b s t r a t e and c h a n n e l p l a t e b i a s c o m b i n a t i o n s . The p o t e n t i a l d i s t r i b u t i o n i s p r e s e n t e d f o r t y p i c a l b i a s v a l u e s u s e d w i t h d e v i c e s b u i l t f o r t h i s r e s e a r c h ( F i g u r e 2 . 7 c ) . S i n c e c h a r g e t r a n s p o r t o c c u r s a t d e p t h x 3 m where t h e e l e c t r i c f i e l d i s z e r o , t h e s e g r a p h s 27 1.00 -6.00 I i • • » • ' • ' I I ' ' 0 0.4 0.6 1.2 1.6 2 2.4 Diatonce from Surface (pm) x 3 F i g u r e 2.7a: ACT channel p o t e n t i a l - experimental b i a s c o n d i t i o n _i.oo T • 1 Distance from Surface (urn) X3 F i g u r e 2.7b: ACT channel p o t e n t i a l - v a r i a t i o n of f r o n t g a t i n g b i a s 28 -1.00 -6.00 2.4 F i g u r e 2.7c: ACT c h a n n e l p o t e n t i a l - v a r i a t i o n o f b a c k g a t i n g b i a s -1.00 -8.00 1.2 1.6 2 Distance from Surface (^ m) x ^ 2.4 F i g u r e 2.7d: ACT c h a n n e l p o t e n t i a l - v a r i a t i o n o f f r o n t g a t i n g and b a c k g a t i n g b i a s 29 show t h a t the d e p t h o f c h a r g e t r a n s p o r t i s i n c r e a s e d u s i n g l a r g e r c h a n n e l p l a t e or s m a l l e r s u b s t r a t e b i a s m a g n i t u d e s . I n c r e a s i n g t h e c h a n n e l p l a t e and s u b s t r a t e b i a s s i m u l t a n e o u s l y by e q u a l amounts had no e f f e c t on p o t e n t i a l v a r i a t i o n i n the c h a n n e l . By i n c r e a s i n g |V m| to 8 V o l t s w i t h | V g| = 2.2 V o l t s , the p o t e n t i a l w e l l d i s a p p e a r s and i n j e c t e d e l e c t r o n s e s c a p e t o the s u b s t r a t e . S i m i l a r l y , f o r l a r g e enough b a c k g a t i n g b i a s ( V s ) w i t h f r o n t g a t i n g b i a s ( V m ) h e l d c o n s t a n t , i n j e c t e d c h a r g e f l o w s i n t o the c h a n n e l p l a t e . The assumed d e p l e t i o n c o n d i t i o n e x i s t s p r o v i d e d the e p i t a x i a l l a y e r o u t s i d e the d e p l e t e d r e g i o n i s b i a s e d a t a p o t e n t i a l e q u a l to or g r e a t e r t h a n t h e v a l u e i n d i c a t e d a t X 3 m , t h e e l e c t r o n p o t e n t i a l minimum i n t h e c h a n n e l . The i n p u t and o u t p u t ohmic c o n t a c t s o f the ACT must be b i a s e d a t a p o t e n t i a l e q u a l to o r more p o s i t i v e t h a n t h e v a l u e i n d i c a t e d a t to a c h i e v e d e p l e t i o n o f the c h a n n e l r e g i o n . 2.3 SAW T r a n s d u c t i o n on GaAs G e n e r a t i o n o f the l a r g e a m p l i t u d e SAW on GaAs i s a c c o m p l i s h e d w i t h an i n t e r d i g i t a l t r a n s d u c e r . The ACT d e v i c e s b u i l t f o r t h i s t h e s i s u s e d a A/4 t r a n s d u c e r d e s i g n w i t h f i n g e r and gap w i d t h s o f 2/^ m as shown i n F i g u r e 2.8. The p e r f o r m a n c e o f s u c h t r a n s d u c e r s has b een w e l l documented; f o r example by M i l s o m , Redwood, and R e i l l y [ 5 0 ] . One c o m p l i c a t i o n t h a t a r i s e s when i n t e r d i g i t a l t r a n s d u c e r s a r e u s e d to g e n e r a t e SAW on s e m i c o n d u c t i n g s u b s t r a t e s i s s c r e e n i n g o f t h e SAW f i e l d s by f r e e c a r r i e r s . 30 =SAW Beamwidth H H -X/4 F i g u r e 2.8: Q u a r t e r w a v e l e n g t h f i n g e r w i d t h IDT d e s i g n 31 By f a b r i c a t i n g the t r a n s d u c e r u s i n g a m e t a l t h a t forms a S c h o t t k y b a r r i e r w i t h the s e m i c o n d u c t o r , a r e v e r s e b i a s a p p l i e d t o t h e f i n g e r s r e l a t i v e to a n e a r b y ohmic c o n t a c t w i l l c a u s e d e p l e t i o n o f the r e g i o n u n d e r n e a t h the f i n g e r s [51] . The a p p l i c a b i l i t y o f t h i s a p p r o a c h depends on the t r a n s d u c e r d e s i g n , s u b s t r a t e d o p i n g , and s u b s t r a t e t h i c k n e s s s i n c e c o m p l e t e d e p l e t i o n t o a d e p t h a t w h i c h SAW RF f i e l d s a r e s m a l l must be a c h i e v e d b e f o r e r e v e r s e v o l t a g e breakdown o f t h e S c h o t t k y d i o d e s f o r m e d by the f i n g e r s o c c u r s . A s i g n i f i c a n t p r o b l e m w i t h the l o n g A/4 t r a n s d u c e r s i s l o s s i n d u c e d by i n t e r n a l a c o u s t i c r e f l e c t i o n s . F o r t h i s IDT d e s i g n , maximum i n t e r n a l r e f l e c t i o n s a r e p r e s e n t a t the same f r e q u e n c y g r e a t e s t r a d i a t i o n c o n d u c t a n c e i s a c h i e v e d . The i n t e r n a l r e f l e c t i o n s have two p r i n c i p a l o r i g i n s . One o f t h e s e i s due to t h e SAW e l e c t r i c f i e l d b e i n g s h o r t - c i r c u i t e d u n d e r n e a t h th e t r a n s d u c e r f i n g e r s . T h i s e f f e c t i s s i g n i f i c a n t f o r s t r o n g p i e z o e l e c t r i c s s u c h as L i N b 0 3 , b u t s m a l l f o r weak p i e z o e l e c t r i c s s u c h as GaAs. The o t h e r mechanism i s r e l a t e d to the p e r i o d i c mass l o a d i n g o f the s u r f a c e by the IDT. The l a t t e r e f f e c t i s s i g n i f i c a n t f o r GaAs a t h i g h f r e q u e n c i e s as t r a n s d u c e r f i n g e r h e i g h t becomes a l a r g e r p r o p o r t i o n o f the e l e c t r o d e w i d t h . The f r e q u e n c y r e s p o n s e o f a GaAs SAW d e l a y l i n e c o n s i s t i n g o f two 100 f i n g e r p a i r t r a n s d u c e r s w i t h phase c e n t r e s s e p a r a t e d by 4.38 mm i s shown i n F i g u r e 2.9. The w a f e r u s e d was semi - i n s u l a t i n g and the f i n g e r s c o n s i s t e d o f 50 nm o f chromium f o l l o w e d by 300 nm o f aluminum. The 32 INPUT-200 Finger IDT ^•OUTPUT 3.57 mm 4.38 mm .Return L o s s Magni tude R esponse F i g u r e 2.9: F r e q u e n c y r e s p o n s e o f r e f l e c t i v e A/4 t r a n s d u c e r d e l a y l i n e on GaAs 33 a c o u s t i c r e f l e c t i o n s c o m p l e t e l y d e s t r o y e d t h e s i n e f u n c t i o n f r e q u e n c y r e s p o n s e e x p e c t e d f o r an u n a p o d i z e d a r r a y . Bogus, H o s k i n s and H u n s i n g e r [52] d e s c r i b e d a method to r e d u c e i n t e r n a l r e f l e c t i o n s i n A/4 t r a n s d u c e r s u s i n g a l a y e r e d m e t a l s t r u c t u r e . The t e c h n i q u e c o n s i d e r s the s i g n and m a g n i t u d e o f a c o u s t i c r e f l e c t i o n s i n an IDT s t r u c t u r e g i v e n by a n a l y s i s o f the m e t a l and s u b s t r a t e m a t e r i a l p r o p e r t i e s . The m a g n i t u d e o f the a c o u s t i c r e f l e c t i o n depends on t h e e l e c t r o d e h e i g h t . By d e p o s i t i n g two d i f f e r e n t m e t a l s d u r i n g i n t e r d i g i t a l t r a n s d u c e r f a b r i c a t i o n , a r e d u c t i o n o f t h e i n t e r n a l r e f l e c t i o n s o c c u r s t h r o u g h a c a n c e l l a t i o n o f the e f f e c t s c a u s e d by e a c h m e t a l . The b a s i c t h e o r y i s b a s e d on a t r a n s m i s s i o n - 1 i n e model f o r t h e a c o u s t i c impedance mismatch c r e a t e d by t h e p e r i o d i c IDT a r r a y . The a c o u s t i c c h a r a c t e r i s t i c impedance o f the GaAs w i t h o u t any d e p o s i t e d m e t a l i s Z A U c o r r e s p o n d i n g to the gap r e g i o n s o f t h e IDT. The a c o u s t i c c h a r a c t e r i s t i c impedance o f t h e m e t a l l i z e d r e g i o n s i s ( Z A Q + 5 Z A ) c o r r e s p o n d i n g t o the IDT f i n g e r s . S i n c e 6 Z A i s f o u n d t o v a r y i n s i g n and m a g n i t u d e w i t h the d e p o s i t e d m e t a l , t h e method s e e k s t o make S Z A = 0 t o e l i m i n a t e r e f l e c t i o n s . The a s s u m p t i o n u n d e r l y i n g t h i s method i s t h a t t h e e f f e c t s o f two m e t a l s p l a c e d on top o f e a c h o t h e r l i n e a r l y s u p e r i m p o s e . The t h e o r y was e x p e r i m e n t a l l y v e r i f i e d u s i n g an IDT a r r a y w i t h 28 ;um f e a t u r e s and m e t a l t h i c k n e s s o f 250 nm. H o s k i n s [45] s u g g e s t e d t h a t a c e r t a i n amount o f t r a n s d u c e r r e f l e c t i v i t y may i n c r e a s e t h e SAW t r a n s d u c t i o n 34 e f f i c i e n c y f o r m a t e r i a l s w i t h low e l e c t r o m e c h a n i c a l c o u p l i n g c o e f f i c i e n t s . H i s argument was b a s e d on t h e f a c t t h a t r a d i a t i o n c o n d u c t a n c e i n c r e a s e s f o r a t r a n s d u c e r i n a SAW r e s o n a t o r c o n f i g u r a t i o n [ 5 3 ] . I n the extreme c a s e o f p e r f e c t r e f l e c t i v i t y , the SAW w i l l be " t r a p p e d " w i t h i n t h e IDT i n the f o r m o f a s t a n d i n g wave. The i n t e r n a l l o s s e s o f the IDT i n c r e a s e as r e s o n a n c e i s a p p r o a c h e d a l o n g w i t h the magnitude o f t h e SAW. As s u c h , H o s k i n s s t a t e d t h a t some v a l u e o f e l e c t r o d e r e f l e c t i v i t y f o r a t r a n s d u c e r o f a g i v e n l e n g t h s h o u l d r e s u l t i n m a x i m i z e d SAW g e n e r a t i o n e f f i c i e n c y . A n o t h e r a p p r o a c h t o r e d u c e th e p r o b l e m o f i n t e r n a l r e f l e c t i o n s i s t h e A/8, d o u b l e - f i n g e r , o r s p l i t - f i n g e r t r a n s d u c e r [54] shown i n F i g u r e 2.10. Geometry o f the IDT e l i m i n a t e s a c o u s t i c r e f l e c t i o n s i n t h i s d e s i g n w h i c h i s e f f i c i e n t a t g e n e r a t i n g b o t h the f u n d a m e n t a l and t h i r d h a r m o n i c . A l t h o u g h l a u n c h i n g t h e f u n d a m e n t a l a t 360 MHz w o u l d i n v o l v e 1 pm geometry, the t h i r d h a r m o n i c d e s i g n would r e q u i r e 3 jum f e a t u r e s . T h e r e i s no r e p o r t e d use o f t h i s t y p e o f t r a n s d u c e r i n the l i t e r a t u r e f o r ACT d e v i c e s , a l t h o u g h t h e y a r e c u r r e n t l y b e i n g u s e d i n r e c e n t ACT d e v i c e r e s e a r c h [55] . Whether t h e s e IDT's a r e b e i n g o p e r a t e d a t t h e i r f u n d a m e n t a l o r t h i r d h a r m o n i c g e n e r a t i o n f r e q u e n c y i s unknown. An a p p r o x i m a t e e q u i v a l e n t c i r c u i t f o r t h e t r a n s d u c e r i s shown i n F i g u r e 2.11. 35 b=SAW Beamwidth H | ^ X/8 F i g u r e 2.10: S p l i t - f i n g e r IDT design 36 o o B(f) = B a ( f ) + & r f C T F i g u r e 2.11: SAW t r a n s d u c e r e q u i v a l e n t c i r c u i t 37 The r a d i a t i o n c o n d u c t a n c e and s u s c e p t a n c e a r e g i v e n f o r a A/4 des i g n [ 5 6 ] : G(f) ~ G§ s i n 2 X N / X $ = r a d i a t i o n c o n d u c t a n c e (2.12) B a ( f ) « Gg(sin2X N - 2 X N ) / 2 X § (2.13) B ( f ) = B a ( f ) + 27rfC T (2.14) = t r a n s d u c e r s u s c e p t a n c e Gg = 8 f 0 K § C s N 2 b (2.15) X N = Nir(f - f 0 ) / f 0 (2.16) K2; = SAW e l e c t r o m e c h a n i c a l c o u p l i n g c o e f f i c i e n t C s = C T/Nb (2.17) C-j- = DC c a p a c i t a n c e o f t r a n s d u c e r N = number o f f i n g e r p a i r s b = a c o u s t i c beamwidth f = f r e q u e n c y fQ = c e n t r e f r e q u e n c y o f t r a n s d u c e r F o r the t r a n s d u c e r u s e d i n ACT d e v i c e s b u i l t f o r t h i s t h e s i s : N = 100 f 0 = 360.7 MHz K§ = 6.4X10' 4 b = 600 /xm C T = 7.67 pF C a l c u l a t i o n o f the t r a n s d u c e r impedance a t t h e r e s o n a n t f r e q u e n c y o f 360.7 MHz y i e l d e d : G ( f 0 ) = 1 . 4 2 x l 0 " 3 Siemens B ( f 0 ) = 1 . 7 4 x l 0 " 2 Siemens 38 T h i s c o r r e s p o n d s t o an i n p u t impedance o f : z I N < f 0 ) - < 4 - 6 6 - J'57.1) a The l a c k o f agreement between t h i s v a l u e and t h e measured RF impedance o f t h e t r a n s d u c e r g i v e n i n s e c t i o n 4.2 i s b e l i e v e d t o be due to the combined e f f e c t o f p a c k a g e p a r a s i t i c s and l a r g e i n t e r n a l a c o u s t i c r e f l e c t i o n s w h i c h make the e x p r e s s i o n s f o r t r a n s d u c e r impedance p r e s e n t e d above i n v a l i d . 2 . 4 The A c o u s t o e l e c t r i c E f f e c t The t r a n s f e r o f e n e r g y and momentum between a c o u s t i c waves and c a r r i e r s i n a s e m i c o n d u c t o r i s c a l l e d the a c o u s t o e l e c t r i c e f f e c t . W e i n r e i c h [57] d e r i v e d a r e l a t i o n s h i p between the m a g n i t u d e o f the a c o u s t o e l e c t r i c c u r r e n t d e n s i t y ( J A E ) a n a t h e a c o u s t i c power d e n s i t y ( S ) : J A E - -2/»aS/v s (2.18) where: S = a c o u s t i c power d e n s i t y (W/m) fi = c a r r i e r m o b i l i t y a = a t t e n u a t i o n c o e f f i c i e n t V g = a c o u s t i c wave v e l o c i t y J A E = a c o u s t o e l e c t r i c c u r r e n t d e n s i t y (A/m) S o l v i n g f o r t h e a c o u s t o e l e c t r i c a t t e n u a t i o n c o e f f i c i e n t : a = - J A E V S / 2 ^ S (2.19) The SAW power l o s s as a f u n c t i o n o f p r o p a g a t i o n a l o n g x-^  i s t h e n g i v e n by: S = S 0 e x p ( 2 a x 1 ) (2.20) where: S Q = i n i t i a l a c o u s t i c i n t e n s i t y a t x^ = 0. The W e i n r e i c h r e l a t i o n i s v a l i d f o r a r b i t r a r y a c o u s t i c 39 waveforms and a m p l i t u d e s w i t h no a p p l i e d m a g n e t i c f i e l d s , no c a r r i e r t r a p p i n g e f f e c t s and o n l y one t y p e o f c a r r i e r p r e s e n t . F o r low v a l u e s o f SAW power, the a c o u s t o e l e c t r i c c u r r e n t i s p r o p o r t i o n a l to the a c o u s t i c power i n t e n s i t y . T h i s i s the l i n e a r o r s m a l l s i g n a l r e g i m e i n w h i c h c h a r g e t r a n s p o r t o c c u r s a t s u b s o n i c v e l o c i t y . L a r g e v a l u e s o f SAW power l e a d to b u n c h i n g and t r a n s p o r t o f t h e f r e e c a r r i e r s a t a c o u s t i c v e l o c i t y i n the w e l l s d e f i n e d by t h e p e r i o d i c SAW p o t e n t i a l . T h i s i s t e r m e d the l a r g e s i g n a l or n o n - l i n e a r r e g i m e and r e s u l t s when the SAW i n d u c e d p i e z o e l e c t r i c s p a c e c h a r g e d e n s i t y a p p r o a c h e s , or e x c e e d s , th e s p a c e c h a r g e d e n s i t y o f t h e f r e e c a r r i e r s . In t h i s c a s e i t has b e en shown t h a t t h e a c o u s t o e l e c t r i c c u r r e n t s a t u r a t e s [ 5 8 ] . The t r a n s p o r t e d c a r r i e r s a r e u n d e r the i n f l u e n c e o f a c o n s t a n t " s y n c h r o n o u s " e l e c t r i c f i e l d [59] g i v e n by: E S = -v s/M (2.21) T h i s i s s i m p l y t h e f i e l d r e q u i r e d t o move the c a r r i e r s a t a c o n s t a n t v e l o c i t y e q u a l to the sound v e l o c i t y i n a m a t e r i a l w i t h m o b i l i t y p. F o r SAW, most o f t h e a c o u s t i c e n e r g y i s c o n f i n e d t o w i t h i n a w a v e l e n g t h o f t h e s u b s t r a t e s u r f a c e . T h i s f a c t , and the h i g h e l e c t r o n m o b i l i t y i n GaAs, p e r m i t m o d e r a t e SAW d r i v e powers t o be u s e d t o a c h i e v e the s y n c h r o n o u s f i e l d . As an example o f the m a g n i t u d e o f SAW l o s s c a u s e d by the a c o u s t o e l e c t r i c e f f e c t an e x p e r i m e n t a l d e v i c e w i t h c h a r g e 40 i n j e c t i o n l e v e l a d j u s t e d to p r o d u c e a t i m e a v e r a g e d o u t p u t c u r r e n t o f 6 pA i s c o n s i d e r e d . The a c o u s t o e 1 e c t r i c c u r r e n t d e n s i t y i s 15 1A/1 t a k i n g i n t o a c c o u n t a 400 c h a n n e l w i d t h . The a c o u s t o e l e c t r i c a t t e n u a t i o n c o e f f i c i e n t c a l c u l a t e d f r o m (2.19) i s a = -0.43 m"^ a s s u m i n g a wave power i n t e n s i t y o f 1 mW/A ( c o r r e s p o n d i n g to a —0.5 V o p t i m i z e d wave p o t e n t i a l ) and an e l e c t r o n m o b i l i t y v a l u e o f 4000 cm^/Vs. The c a l c u l a t e d a t t e n u a t i o n c o e f f i c i e n t c o r r e s p o n d s to a SAW power l o s s o f o n l y 3.7 dB/m. The s m a l l m a g n i t u d e o f t h i s a t t e n u a t i o n means t h a t v e r y l i t t l e power i s r e q u i r e d to " d r i v e " t h e t r a n s p o r t p r o c e s s . The p r a c t i c a l d e v i c e l e n g t h b a s e d on t h i s l o s s v a l u e c o u l d be e x t r e m e l y l a r g e w i t h o u t s e v e r e d e g r a d a t i o n o f the SAW p o t e n t i a l . U n f o r t u n a t e l y , i n an a c t u a l d e v i c e , t h e SAW a t t e n u a t i o n w i l l be d o m i n a t e d by l o s s mechanisms a s s o c i a t e d w i t h s u b s t r a t e i m p e r f e c t i o n s and d i f f r a c t i o n e f f e c t s . T hese c o n s i d e r a t i o n s l i m i t c u r r e n t d e v i c e l e n g t h s to s e v e r a l m i l l i m e t e r s . I t i s i n t e r e s t i n g t o c o n s i d e r l o s s c a u s e d by the gap b e tween the i n t e r d i g i t a l t r a n s d u c e r and t h e ACT c h a n n e l where the e p i t a x i a l l a y e r i s n o t d e p l e t e d . I n t h i s c a s e , the e q u i v a l e n t s h e e t c h a r g e d e n s i t y o f the 4 /im t h i c k e p i t a x i a l r e g i o n was c a l c u l a t e d as 8 x l o H cm" 2 a s s u m i n g a donor d e n s i t y o f 2x10-'-^ cm~3. F o r the p u r p o s e o f t h i s c a l c u l a t i o n , i t i s assumed t h a t the c a r r i e r s c o u l d be t r a n s p o r t e d a t the sound v e l o c i t y by a SAW whose power d e n s i t y i s 1 mW/A a c o u s t i c beamwidth. F o r a 100 pm gap, the a t t e n u a t i o n u n d e r t h e s e c o n d i t i o n s i s o n l y 0.1 dB. 41 2.5 O p t i c a l Charge I n j e c t i o n L i g h t i n c i d e n t t h r o u g h s e m i - t r a n s p a r e n t windows on the ACT d e v i c e c r e a t e s e l e c t r o n - h o l e - p a i r s (EHP's) i n the d e v i c e c h a n n e l whose i n i t i a l s p a t i a l d i s t r i b u t i o n c a n be d e s c r i b e d by: n ( x 3 , t 0 ) = n 0 e x p ( - a o x 3 ) (2.22) p ( x 3 , t 0 ) = n 0 e x p ( - a 0 x 3 ) (2.23) a s s u m i n g u n i f o r m o p t i c a l i n t e n s i t y o v e r the window i n the x^ and X 2 d i r e c t i o n s and m o n o c h r o m a t i c l i g h t . The p a r a m e t e r aQ i s t h e o p t i c a l a b s o r p t i o n c o e f f i c i e n t f o r the g i v e n w a v e l e n g t h o f i l l u m i n a t i o n . The ACT c h a n n e l p o t e n t i a l i s g i v e n by H o s k i n s [45] page 61: ^ c ( x 1 , x 3 l t ) = - ( q N D / 2 e ) ( x 3 - x 3 m ) 2 +17o < x3) I c ° s < k x l " w t > (2.24) where t h e f i r s t t erm d e s c r i b e s t h e p a r a b o l i c d e p l e t i o n p o t e n t i a l e x c l u d i n g components w i t h no s p a t i a l dependence. The s e c o n d term d e s c r i b e s the t r a v e l l i n g wave p o t e n t i a l due to t h e SAW. In p r a c t i c e , the x 3 - c o m p o n e n t o f the d e p l e t i o n e l e c t r i c f i e l d i s much l a r g e r t h a n the x 3 - c o m p o n e n t o f t h e SAW e l e c t r i c f i e l d . Hence, a s i m p l e a n a l y s i s d e s c r i b i n g c h a r g e s e p a r a t i o n i n the x 3 - d i r e c t i o n can i g n o r e t h i s component to o b t a i n an a p p r o x i m a t i o n o f the c h a r g e s e p a r a t i o n b e h a v i o u r . The c h a n n e l p o t e n t i a l ( i g n o r i n g the SAW) i s t h e r e f o r e g i v e n by: * c ( x 3 , t ) « - ( q N D / 2 e ) ( x 3 - x 3 m ) 2 (2.25) The e l e c t r i c f i e l d due to the d e p l e t i o n p o t e n t i a l a l o n e i s g i v e n by: 42 E c p ( x 3 , t ) = - d ^ c ( x 3 , t ) / d x 3 * ( q N D / € ) ( x 3 - x 3 m ) (2.26) U s i n g some t y p i c a l v a l u e s : N D = 2 x l 0 1 5 c m - 3 e = 11 . 0 e 0 x 3 m - 3.5 Mm y i e l d s : E c p ( x 3 , t)«3 . 2 9 x l 0 6 ( x 3 - 3 . 5) V/m where x 3 i s i n /zm . (2.27) The s a t u r a t i o n d r i f t v e l o c i t y (vg^T^ f ° r c a r r i e r s i n GaAs i s » 7 x l 0 ^ cm/s [ 6 0 ] . I n the a n a l y s i s t h a t f o l l o w s , i t has b e e n assumed t h a t v e l o c i t y s a t u r a t i o n i s a c h i e v e d f o r e l e c t r o n s and h o l e s w i t h an e l e c t r i c f i e l d o f 3.3 kV/cm and 25 kV/cm r e s p e c t i v e l y . F o r e l e c t r o n s , t h e d e p l e t i o n f i e l d e x c e e d s t h e t h r e s h o l d o v e r 0<x 3<3 . 4 /zm. Hence, t o an a p p r o x i m a t i o n , t h e e l e c t r o n s move t o t h e c e n t r e o f the ACT c h a n n e l a t or above t h e i r s a t u r a t e d d r i f t v e l o c i t y . F o r h o l e s , the f i e l d f o r v e l o c i t y s a t u r a t i o n i s a c h i e v e d o v e r 0<x 3<2 . 7 /zm. Hence, f o r c a r r i e r i n j e c t i o n where the o p t i c a l p e n e t r a t i o n d e p t h i s l e s s t h a n 2.7 /zm, the h o l e s and e l e c t r o n s a r e s e p a r a t e d a t t h e i r s a t u r a t e d d r i f t v e l o c i t i e s . T h i s i s a h i g h l y d e s i r a b l e c o n d i t i o n f o r r e d u c t i o n o f r e c o m b i n a t i o n and t r a n s i t t i m e . A t Ap = 730 nm, the o p t i c a l a b s o r p t i o n c o e f f i c i e n t i s aQ = 1.92x10^ cm"-'- [ 6 1 ] . A t a d e p t h i n t o the s u b s t r a t e o f ( 4 / a Q ) = 2.1 /zm, o v e r 98% o f i n c i d e n t p h o t o n s have been a b s o r b e d by t h e GaAs. T h i s i s w i t h i n the 2.7 /zm l i m i t j u s t c a l c u l a t e d f o r v e l o c i t y s a t u r a t i o n f o r h o l e s . 43 A q u a l i t a t i v e a n a l y s i s o f the c h a r g e s e p a r a t i o n p r o c e s s w i l l be d i s c u s s e d . I n i t i a l e l e c t r o n and h o l e d i s t r i b u t i o n s i n t h e c h a n n e l a r e g i v e n by e q u a t i o n s (2.22) and ( 2 . 2 3 ) . I t i s assumed t h a t the e l e c t r i c f i e l d i n t h e c h a n n e l e x c e e d s the e l e c t r o n s a t u r a t i o n f i e l d s from the d e v i c e s u r f a c e a t x 3 = 0 to the e l e c t r o n p o t e n t i a l minumum a t x 3 = x 3 m . The o p t i c a l p e n e t r a t i o n d e p t h i s assumed to be s m a l l enough t h a t h o l e v e l o c i t y s a t u r a t i o n a p p l i e s . F o r x 3 > x 3 m t h e e l e c t r i c f i e l d c h anges s i g n . I g n o r i n g d i f f u s i o n and r e c o m b i n a t i o n , the i n i t i a l e l e c t r o n and h o l e c h a r g e d i s t r i b u t i o n s move i n o p p o s i t e d i r e c t i o n s a t v g A x u n t i l the e l e c t r o n s r e a c h x 3 = x 3 m and t h e h o l e s r e a c h x 3 = 0 as shown i n F i g u r e 2.12. I n t h i s s i m p l e a n a l y s i s , the f i n a l e l e c t r o n and h o l e c h a r g e d i s t r i b u t i o n s c an be r e g a r d e d as f o r m i n g a p a r a l l e l p l a t e c a p a c i t o r w i t h t h e e l e c t r o n s i n t h e c h a n n e l f o r m i n g one p l a t e and t h e h o l e s , or p o s i t i v e c h a r g e , on t h e S c h o t t k y m e t a l a t the s u r f a c e f o r m i n g the o t h e r p l a t e . A c u r r e n t f l o w s i n the c h a n n e l p l a t e when the e l e c t r o n s a r e removed from the c h a n n e l , e f f e c t i v e l y d i s c h a r g i n g the c a p a c i t o r . T h i s l a s t r e s u l t has been v e r i f i e d w i t h an a c t u a l ACT d e v i c e by m o n i t o r i n g the c h a n n e l p l a t e c u r r e n t b e f o r e , d u r i n g , and a f t e r a p p l i c a t i o n o f the o p t i c a l p u l s e . The r e s u l t i s shown i n F i g u r e 2.13. A c u r r e n t p u l s e a p p e a r e d when e l e c t r o n s were removed from the c h a n n e l o f t h e d e v i c e a t the o u t p u t node. The p o l a r i t y o f t h i s c u r r e n t p u l s e i n d i c a t e d e l e c t r o n f l o w i n t o the c h a n n e l p l a t e , i n agreement w i t h t h i s a n a l y s i s . 44 '0 t. t 4 Optical Penetration Depth t» 4/«b) Channel Electron y Potential Minumum X31J1 I n i t i a l Distribution ^ > p ( x 3 . t f l ; E>ESAT n (X-. tg) p (x3, t 2 ) P (x3, t3) p ( x 3 , t 4 ) " ^ n f o y t j ) n ( x 3 , t ^ n (X3, tg) 11 C (Xg.tJ t 2*'4/«DV S A T V ^ n A s A T -^X< Holes Collected by Channel Plate Electrons reach Channel Centre F i g u r e 2.12: S i m p l i f i e d a n a l y s i s o f o p t i c a l c h a r g e s e p a r a t i o n 45 100 n s / d i v > - V . O W v r hv/ MIDDLE TRACE O XJP Non-Inverting Channel Plate Output Ohmic LOWER TRACE Substrate F i g u r e 2.13: Channel p l a t e waveform a f t e r a p p l i c a t i o n of o p t i c a l pulse 46 In summary, t h i s s i m p l i f i e d theory p r e d i c t s that at t » 3.4 i*m/ v g A T » 50 ps , the charge has been completely s e p a r a t e d and the p h o t o - e x c i t e d e l e c t r o n s have d r i f t e d to the cen t r e of the channel where they are bunched and t r a n s p o r t e d by the SAW analogous to e l e c t r i c a l l y i n j e c t e d c a r r i e r s . V e l o c i t y overshoot has been ignored, but i n p r a c t i c e c o u l d reduce even f u r t h e r the charge s e p a r a t i o n times. Comparing the 50 ps charge s e p a r a t i o n with the m i n o r i t y c a r r i e r l i f e t i m e i n GaAs, s e v e r a l ns, the amount of recombination should be s m a l l , j u s t i f y i n g an assumption made e a r l i e r . The c a l c u l a t i o n d i d not account f o r changes i n s t a t i c d e p l e t i o n p o t e n t i a l due to the presence of i n j e c t e d c a r r i e r s . The e f f e c t of t h i s a d d i t i o n a l charge on the channel p o t e n t i a l i s expected to be sm a l l f o r low i n j e c t i o n l e v e l s . For example, i n j e c t i o n of a s i n g l e c a r r i e r s p e c i e s i n t o the ACT and comparison of the d e n s i t y of i n j e c t e d c a r r i e r s with the background charge d e n s i t y due to the i o n i z e d donors was co n s i d e r e d . For an i n j e c t i o n l e v e l of 1.74x10-' e l e c t r o n s / w e l l c o r r e s p o n d i n g to a 10/iA output c u r r e n t , the maximum charge d e n s i t y due to the i n j e c t e d e l e c t r o n s i s 7 x l 0 ^ 3 cm"^ f o r a s l i t dimension of 16 pm x 300 /im and an o p t i c a l a b s o r p t i o n c o e f f i c i e n t of a D - 1.92x10^ cm~^ assuming a l l e l e c t r o n -h o l e - p a i r s are c r e a t e d at t - t g • T h i s charge d e n s i t y value a p p l i e s o n l y near the s u b s t r a t e s u r f a c e s i n c e charge i n j e c t i o n f o l l o w s an e x p o n e n t i a l r e l a t i o n s h i p with depth. The maximum i n j e c t e d charge d e n s i t y i s r e l a t i v e l y small 47 compared to the b a c k g r o u n d d o p i n g d e n s i t y o f ~ 2x10-'-^ cm" 3 and i s r e d u c e d f u r t h e r by a c o m p e n s a t i n g e f f e c t o f the h o l e s i n an a c t u a l o p t i c a l i n j e c t i o n s i t u a t i o n and by t h e f i n i t e t i m e d u r i n g w h i c h the o p t i c a l p u l s e c a n i n j e c t c h a r g e i n t o a g i v e n w e l l , a p p r o x i m a t e l y 2.8 ns c o r r e s p o n d i n g to the amount o f t i m e t h a t a SAW c h a r g e w e l l i s u n d e r t h e i n j e c t i o n window. D i f f u s i o n e f f e c t s have n o t been i n c l u d e d i n t h i s a n a l y s i s . T h i s a p p e a r s j u s t i f i e d i n l i g h t o f the h i g h d r i f t f i e l d s r e s p o n s i b l e f o r c h a r g e s e p a r a t i o n and the s m a l l s e p a r a t i o n t i m e . T r a p p i n g e f f e c t s c a u s e d by s u r f a c e s t a t e s have b e e n i g n o r e d s i n c e t h e s e t r a p s a r e e x p e c t e d t o o c c u p y a s m a l l f r a c t i o n o f the t o t a l e l e c t r o n - h o l e - p a i r g e n e r a t i o n volume. 2.6 Charge C a p a c i t y and T r a n s f e r E f f i c i e n c y The p e r f o r m a n c e o f a c h a r g e t r a n s f e r d e v i c e i s q u a n t i f i e d i n terms o f t h e amount o f c h a r g e t h a t c a n be c a r r i e d i n e a c h p o t e n t i a l w e l l a t a g i v e n t r a n s f e r e f f i c i e n c y . I n t h e c a s e o f a CCD, the c h a r g e t r a n s f e r e f f i c i e n c y i s the f r a c t i o n o f c h a r g e t h a t i s s u c c e s s f u l l y c a r r i e d t h r o u g h one c o m p l e t e c l o c k i n g c y c l e . An a n a l o g o u s d e f i n i t i o n f o r an ACT w o u l d be t h e f r a c t i o n o f c h a r g e t r a n s p o r t e d one a c o u s t i c w a v e l e n g t h . I n g e n e r a l , t h e amount o f c h a r g e l o s t d u r i n g each c l o c k i n g i n t e r v a l i s a n o n - l i n e a r f u n c t i o n o f the t o t a l c h a r g e l o a d . I n p r a c t i c e , a p r o p o r t i o n a l l o s s mechanism i s o f t e n assumed, where the t o t a l amount o f c h a r g e l o s t i s 48 p r o p o r t i o n a l to the t o t a l amount b e i n g t r a n s p o r t e d . R e s u l t s o f the t h e o r y p r e s e n t e d by H o s k i n s t o d e s c r i b e c h a r g e t r a n s f e r b e h a v i o u r o f the ACT d e v i c e w i l l be b r i e f l y d e s c r i b e d . A d e t a i l e d d e v e l o p m e n t o f t h e t h e o r y i s g i v e n by H o s k i n s [45] and H o s k i n s and H u n s i n g e r [ 6 2 ] . The a s s u m p t i o n s i n t h e a n a l y s i s a r e l i s t e d below: the c h a n n e l c e n t r e to image c o n d u c t o r s e p a r a t i o n i s a t l e a s t one q u a r t e r o f an a c o u s t i c w a v e l e n g t h o n l y e l e c t r o n s a r e assumed t o e x i s t i n t h e c h a n n e l h o t e l e c t r o n e f f e c t s a r e i g n o r e d no c a r r i e r g e n e r a t i o n - r e c o m b i n a t i o n i s assumed the a c o u s t o e l e c t r i c a t t e n u a t i o n o f t h e SAW i s n o t cons i d e r e d c h a n n e l edge e f f e c t s a r e i g n o r e d the f i e l d r e q u i r e d to move t h e e l e c t r o n s a t t h e SAW v e l o c i t y i s assumed s m a l l compared t o t h e SAW e l e c t r i c f i e l d m a g n i t u d e a p l a n e wave a p p r o x i m a t i o n t o t h e SAW wave p o t e n t i a l i s assumed Most o f t h e a s s u m p t i o n s a p p e a r j u s t i f i e d i n l i g h t o f the d e v i c e g eometry and n o r m a l o p e r a t i n g c o n d i t i o n s . One p o i n t o f i m p o r t a n c e i n a p r a c t i c a l d e v i c e w h i c h has been i g n o r e d i n the a n a l y s i s , i s the SAW a t t e n u a t i o n . A l t h o u g h a c o u s t o e l e c t r i c a t t e n u a t i o n o f SAW f o r h i g h m o b i l i t y m a t e r i a l s i s s m a l l , as d i s c u s s e d i n s e c t i o n 2.4, the a t t e n u a t i o n due to s c a t t e r i n g f r o m s u r f a c e i m p e r f e c t i o n s and d i f f r a c t i o n e f f e c t s i s n o t i n s i g n i f i c a n t e ven f o r r e a s o n a b l y 49 s m a l l p r o p a g a t i o n d i s t a n c e s . I n c o r p o r a t i o n of a propagation d i s t a n c e dependent parameter i n t o the model would perhaps be j u s t i f i e d . The charge packet shape i s determined by a two-dime n s i o n a l s o l u t i o n of Laplace's e q u a t i o n i n the device channel t a k i n g i n t o account the e l e c t r o s t a t i c d e p l e t i o n f i e l d , t h a t due to the SAW, and the f i e l d due to t r a n s p o r t e d charge. Shape of the charge boundary i s shown In F i g u r e 2.14 f o r a charge l o a d of 1/2. Packet shape depends on the channel constant pj)/p\j d e f i n e d by [45, 62]: PD/PV " q N D / ^ 0 « k 2 (2.28) where q i s the e l e c t r o n i c charge, Nn Is the i o n i z e d donor d e n s i t y i n the channel, 4>0 *-s t ^ e SAW p o t e n t i a l , c i s the p e r m i t t i v i t y of GaAs, and k i s the a c o u s t i c wavenumber. The channel constant i s the r a t i o of the space charge d e n s i t y due to i o n i z e d donors to the p i e z o e l e c t r i c p o l a r i z a t i o n space charge d e n s i t y . I n t u i t i v e l y , one would expect l a r g e channel c o n s t a n t s to i n c r e a s e the e l e c t r o n c o n f i n i n g f i e l d i n the dev i c e channel r e s u l t i n g i n a f l a t t e n i n g of the charge packets i n the d i r e c t i o n of t r a n s p o r t . T h i s i s indeed the case ( F i g u r e 2.14). Under c o n d i t i o n s of maximum charge load, the packet dimension i n the p r o p a g a t i o n d i r e c t i o n i s approximately one h a l f of an a c o u s t i c wavelength. The mechanism of charge t r a n s f e r i n e f f i c i e n c y i s assumed to be the d i f f u s i o n of packet charge over the p o t e n t i a l b a r r i e r imposed by the SAW. T h i s i s a symmetric e f f e c t with 50 -02+ From: H o s k i n s [45] F i g u r e 2.14: Charge p a c k e t shape f o r c h a r g e l o a d o f 1/2 51 e q u a l c h a r g e b e i n g d i s p e r s e d i n t o l e a d i n g and t r a i l i n g w e l l s . As t h e c h a r g e l o a d i s i n c r e a s e d , t h e p o t e n t i a l b a r r i e r p r o v i d e d by t h e SAW i s r e d u c e d , l e a d i n g to i n c r e a s e d d i f f u s i o n i n d u c e d s p r e a d i n g o f the c h a r g e . To c a l c u l a t e the m a g n i t u d e o f t h i s e f f e c t , a w o r s t c a s e a s s u m p t i o n o f t h e r m i o n i c e m i s s i o n o f the p a c k e t c h a r g e o v e r t h e b a r r i e r p r o v i d e d by t h e SAW was u s e d . R e s u l t s o f t h i s c a l c u l a t i o n a r e g i v e n i n F i g u r e 2.15, showing c h a r g e t r a n s f e r i n e f f i c i e n c y as a f u n c t i o n o f c h a r g e c a p a c i t y , N e = number o f e l e c t r o n s p e r c e n t i m e t e r o f c h a n n e l w i d t h , w i t h t h e SAW p o t e n t i a l as a p a r a m e t e r . A u s e f u l e x p r e s s i o n p r o v i d e d by H o s k i n s , Brophy, D a l l e s a s s e , M i l l e r , and P e t e r s o n [27] r e l a t e s t h e maximum c h a r g e c a p a c i t y ( N e ) t o the wave p o t e n t i a l (4Q) and i s g i v e n by: N e ( e l e c t r o n s / c m ) « 53xlO 6 (<0 O - 0.26 V o l t s ) (2.29) T h i s e x p r e s s i o n i n d i c a t e s t h a t the f i r s t l O k T / q = 260 mV o f wave p o t e n t i a l i s r e q u i r e d to p r o v i d e c h a r g e c o n f i n e m e n t , w i t h a d d i t i o n a l wave p o t e n t i a l c o n t r i b u t i n g l i n e a r l y to the c h a r g e c a p a c i t y . 52 From: H o s k i n s [45] F i g u r e 2.15: Charge t r a n s f e r i n e f f i c i e n c y f o r d i f f e r e n t SAW wave p o t e n t i a l s v e r s u s c h a r g e l o a d 53 3.0 EXPERIMENTAL DEVICE DESIGN AND FABRICATION 3.1 Mask L a y o u t F i g u r e 3.1 shows mask l e v e l s f o r the e x p e r i m e n t a l ACT d e v i c e . P a t t e r n s were g e n e r a t e d on a Metheus A700 computer a i d e d d e s i g n s t a t i o n and f a b r i c a t e d a t P r e c i s i o n Photomasks I n c . i n S t . L a u r e n t , Quebec. The d e v i c e shown i s s i m i l a r i n l a y o u t to t h e s e c o n d g e n e r a t i o n ACT d e v i c e d e s c r i b e d by H o s k i n s [45, 63] w i t h the a d d i t i o n o f windows i n the c h a n n e l p l a t e so t h a t o p t i c a l c h a r g e i n j e c t i o n c o u l d be a c h i e v e d . To s i m p l i f y f a b r i c a t i o n and r e d u c e c o s t s , t h e number o f mask l e v e l s n e e d ed t o implement t h e d e v i c e was m i n i m i z e d . The g u a r d r i n g i s o l a t i o n t e c h n i q u e [19] was c h o s e n s i n c e i t c o u l d be i m p l e m e n t e d a t t h e same time as o t h e r S c h o t t k y f e a t u r e s . I s o l a t i o n methods s u c h as mesa e t c h i n g [64] and p r o t o n bombardment [65] r e q u i r e a d d i t i o n a l mask l e v e l s and f a b r i c a t i o n s t e p s . D i s a d v a n t a g e s o f t h e g u a r d r i n g a p p r o a c h i n c l u d e t h e f a c t t h a t , by s u r r o u n d i n g t h e a c t i v e d e v i c e , i t becomes d i f f i c u l t t o r o u t e a d d i t i o n a l m e t a l l i z a t i o n p a t t e r n s i n t o the c h a n n e l a r e a w i t h o u t r e s o r t i n g to m u l t i l e v e l t e c h n i q u e s . The g u a r d r i n g a l s o r e q u i r e s an a d d i t i o n a l power s u p p l y f o r b i a s i n g . An a d d i t i o n a l mask to b u i l d up d e v i c e b o n d i n g a r e a s was e l i m i n a t e d i n the e x p e r i m e n t a l d e v i c e . W h i l e t h i s w ould be a p o o r d e c i s i o n f o r a p r o d u c t i o n ACT t h a t must w i t h s t a n d e n v i r o n m e n t a l and v i b r a t i o n a l s t r e s s , i t d i d n o t seem to c r e a t e any p r o b l e m s i n the l a b o r a t o r y e n v i r o n m e n t . 54 F i g u r e 3.1: E x p e r i m e n t a l ACT mask l a y o u t 55 The mask l e v e l s shown i n F i g u r e 3.1 i n c l u d e : 1) Ohmic f e a t u r e s 2) S c h o t t k y f e a t u r e s 3) Window f e a t u r e s A f o u r t h l e v e l l a b e l l e d " d i e l e c t r i c " was a l s o p r e s e n t f o r e x p e r i m e n t s d e s i g n e d to i n t e g r a t e o p t i c a l l y i n d u c e d c h a r g e o u t s i d e t h e SAW beamwidth. T h i s p a r t i c u l a r mask l e v e l was d e s i g n e d s u c h t h a t i t s o m i s s i o n w o u l d n o t a f f e c t the o p e r a t i o n o f the b a s i c d e v i c e , as i t was n o t u s e d f o r work p r e s e n t e d i n t h i s t h e s i s . 3.2 E p i t a x i a l GaAs S u b s t r a t e M a t e r i a l The s u b s t r a t e m a t e r i a l f o r t h e e x p e r i m e n t a l ACT d e v i c e s was a v a p o u r phase e p i t a x y (VPE) n-GaAs l a y e r grown on a Cr-doped h o r i z o n t a l Bridgman s u b s t r a t e . The g e o m e t r i c t h i c k n e s s and d o p i n g c o n c e n t r a t i o n o f the e p i t a x i a l l a y e r were 4 /nm and 1 c O 2 x l 0 J - J cm' J r e s p e c t i v e l y . The l i g h t d o p i n g was n e a r the l o w e r l i m i t a c h i e v a b l e u s i n g s t a n d a r d GaAs VPE growth t e c h n i q u e s when no i n t e n t i o n a l i m p u r i t i e s a r e i n t r o d u c e d i n the growth p r o c e s s . R e s i d u a l d o p i n g i s b e l i e v e d to be a r e s u l t o f s i l i c o n " c o n t a m i n a t i o n " f r o m the q u a r t z w a r e o f the e p i t a x i a l growth chamber [ 6 6 ] . The f i r s t r e p o r t e d ACT d e v i c e was b u i l t u s i n g m o l e c u l a r -- b e a m - e p i t a x i a l (MBE) m a t e r i a l [ 6 7 ] . P r o b l e m s o f e x c e s s i v e SAW a t t e n u a t i o n (~ 6 dB / i i s ) were a t t r i b u t e d to the poor s u r f a c e m o r p h o l o g y o f MBE l a y e r s t h a t were s e v e r a l m i c r o n s t h i c k . The d e f e c t s were f o u n d to be o f a p p r o x i m a t e l y the 56 same d i m e n s i o n as the SAW w a v e l e n g t h (~ 8 /zm) . I n t h i s g r o w t h t e c h n i q u e , s u c h s u r f a c e i m p e r f e c t i o n s a r e known as o v a l d e f e c t s and t y p i c a l l y number 1 0 3 - 1 0 ^ cm" 2 [68] . More r e c e n t ACT d e v i c e s have been b u i l t on VPE grown n - e p i t a x i a l l a y e r s u s i n g an undoped 1 i q u i d - e n c a p s u l a t e d C z o c h r a l s k i (LEC) sub s t r a t e . The s u r f a c e o f the VPE w a f e r s r e c e i v e d f r o m a c o m m e r c i a l s u p p l i e r f o r t h i s work had s m a l l s u r f a c e r i p p l e s t h a t were v i s i b l e w i t h an o p t i c a l m i c r o s c o p e . The w a v e l e n g t h o f t h e s e u n d u l a t i o n s was g e n e r a l l y l o n g e r t h a n th e SAW w a v e l e n g t h . A c c o r d i n g t o H o s k i n s [45] pages 41-42, VPE grown e p i t a x i a l l a y e r s gave c o n s i d e r a b l y l e s s SAW s c a t t e r i n g (~ 3 dB//zs) t h a n MBE grown e p i t a x i a l l a y e r s . E p i t a x i a l m a t e r i a l was f o u n d to have h i g h e r SAW a t t e n u a t i o n t h a n w e l l - p o l i s h e d b u l k semi-i n s u l a t i n g s u b s t r a t e s . I n a s t u d y o f [111] p r o p a g a t i n g SAW on (211) c u t and [100] p r o p a g a t i n g SAW on (110) c u t GaAs a t 1 GHz, S l o b o d n i k [69] c o n c l u d e d t h a t s u r f a c e i m p e r f e c t i o n s much s m a l l e r t h a n the a c o u s t i c w a v e l e n g t h do n o t a f f e c t SAW a t t e n u a t i o n . From t h e s e r e s u l t s , i t may be p o s s i b l e to c o n c l u d e t h a t s u r f a c e i m p e r f e c t i o n s much s m a l l e r or much l a r g e r t h a n the SAW w a v e l e n g t h do n o t a d v e r s l y a f f e c t p r o p a g a t i o n . S h o r t l y a f t e r the VPE grown m a t e r i a l had b e en r e c e i v e d f o r t h i s p r o j e c t , c o r r e s p o n d e n c e w i t h M.J. H o s k i n s r e v e a l e d t h a t l i g h t l y doped e p i t a x i a l l a y e r s grown on C r - d o p e d s u b s t r a t e s p r e s e n t s e v e r a l p r o b l e m s when u s e d to b u i l d ACT d e v i c e s [55] . One p r o b l e m r e l a t e s to the o u t - d i f f u s i o n o f 57 chromium from the s u b s t r a t e i n t o the e p i t a x i a l l a y e r d u r i n g t h e g r o w t h p r o c e s s , c a u s i n g a p a r t i a l c o m p e n s a t i o n o f the n-d o p i n g [ 7 0 ] . In d e e d , a l t h o u g h an n - e p i t a x i a l l a y e r g e o m e t r i c t h i c k n e s s o f 4 /zm was s p e c i f i e d i n t h e m a t e r i a l o r d e r , the d e l i v e r e d w a f e r s were o n l y e f f e c t i v e l y doped a t 2x10-'-^ cm" 3 t o a d e p t h o f = 2.2 /zm. The r e d u c t i o n o f t h e n - e p i t a x i a l l a y e r t h i c k n e s s i m p a c t e d the d e p t h o f c h a r g e t r a n s p o r t and hence t h e wave p o t e n t i a l a c h i e v e d i n the d e v i c e . T h i s i n t u r n l e d t o a r e d u c t i o n i n the c h a r g e l o a d t h a t c o u l d be t r a n s p o r t e d s i n c e c h a r g e t r a n s p o r t o c c u r r e d a t a d e p t h l e s s t h a n the optimum =0.45 w a v e l e n g t h s below t h e s u r f a c e o f the s u b s t r a t e a t w h i c h wave p o t e n t i a l p e a k s . An a d d i t i o n a l p r o b l e m w i t h the use o f C r - d o p e d s u b s t r a t e s i n the c o n t e x t o f ACT d e v i c e p e r f o r m a n c e i s t h e i r t e n d e n c y t o e x h i b i t l e s s b a c k g a t i n g b e h a v i o u r [ 4 9 ] . S i n c e b a c k g a t i n g i s u s e d t o v a r y the d e p t h o f c h a r g e t r a n s p o r t , e x c e s s i v e l y l a r g e v o l t a g e s c o u l d be r e q u i r e d w i t h some Cr-doped s u b s t r a t e s [ 5 5 ] . T e s t i n g p e r f o r m e d on d e v i c e s made f o r t h i s r e s e a r c h ( s e c t i o n 4.1) r e v e a l e d t h a t b a c k g a t i n g v o l t a g e s a p p r o x i m a t e l y 40% l a r g e r t h a n t h o s e m e a s u r e d by H o s k i n s [45] page 118 were r e q u i r e d . However, t h i s i n c r e a s e was n o t so g r e a t as to p l a c e the v o l t a g e s o u t o f t h e ra n g e o f a s t a n d a r d 50 v o l t l a b o r a t o r y s u p p l y . The w a f e r s u s e d were r e l a t i v e l y t h i n (16 m i l s ) . T h i s may have h e l p e d r e d u c e b a c k g a t i n g v o l t a g e s . W h i l e n o t c o n s i d e r e d i n t h i s t h e s i s , t h e use o f m e t a l -o r g a n i c c h e m i c a l v a p o u r d e p o s i t i o n (MOCVD) grown e p i - l a y e r s 58 m i g h t be p r e f e r a b l e f o r f u t u r e ACT d e v i c e work. Good s u r f a c e m o r p h o l o g y has been o b t a i n e d u s i n g l o w e r growth t e m p e r a t u r e s t h a n VPE ( = 65 0 ° C e l s i u s v e r s u s =750 - 8 5 0 ° C e l s i u s [ 7 1 ] ) . The l o w e r growth t e m p e r a t u r e s m i g h t r e s u l t i n fewer s u b s t r a t e i m p u r i t y o u t - d i f f u s i o n p r o b l e m s . S i n c e MOCVD growth u s e s a c o l d - w a l l v e s s e l , S i c o n t a m i n a t i o n s h o u l d be r e d u c e d [ 7 2 ] , a l l o w i n g e a s i e r growth o f the t h i c k e p i t a x i a l l a y e r s w i t h the low d o p i n g d e n s i t y u s e d f o r ACT d e v i c e s . 3.3 B a s i c F a b r i c a t i o n S t e p s F a b r i c a t i o n o f the ACT d e v i c e s i n v o l v e d t h e f o l l o w i n g s t e p s : 1) P r e - c l e a n o f the w a f e r i n a 1% f i l t e r e d A l c o n o x (TM) s o l u t i o n . 2) D e p o s i t i o n o f a Au:Ge/Ni/Au ohmic m e t a l l i z a t i o n on the semi - i n s u l a t i n g s u b s t r a t e s i d e o f t h e w a f e r . T h i c k n e s s : Au:Ge = 96 nm N i = 2 0 nm Au = 130 nm 3) P a t t e r n i n g and d e p o s i t i o n o f ohmic f e a t u r e s on the d e v i c e f r o n t s i d e u s i n g t h e Au:Ge/Ni/Au m e t a l l i z a t i o n s y s t e m . (Same t h i c k n e s s e s as above) 59 A l i g n m e n t o f t h e p r o p a g a t i o n d i r e c t i o n was made w i t h a {110} GaAs c l e a v a g e p l a n e . 4) A l l o y i n g o f t h e ohmic f e a t u r e s i n an open tube f u r n a c e a t 450 d e g r e e s C e l s i u s f o r 1.5 m i n u t e s i n a N i t r o g e n a t m o s p h e r e . 5) P a t t e r n i n g and d e p o s i t i o n o f t h e S c h o t t k y f e a t u r e s u s i n g a C r / A l : C u ( l % ) m e t a l l i z a t i o n s y s t e m . T h i c k n e s s : Cr = 50 nm A l : C u ( l % ) = 300 nm 6) P a t t e r n i n g and d e p o s i t i o n o f t h e t h i n s e m i - t r a n s p a r e n t Cr m e t a l window f e a t u r e s . T h i c k n e s s : Cr = 25 nm 7) D i c i n g and m o u n t i n g o f t h e c o m p l e t e d d e v i c e s i n a 16 p i n d u a l - i n - l i n e p a c k a g e . RTV s i l i c o n r u b b e r was a p p l i e d to each end o f t h e c h i p and w i r e b o n d i n g p e r f o r m e d w i t h 0.8 m i l g o l d w i r e u s i n g a thermo-compress i o n w i r e b o n d e r . A l l bonds u s e d one w i r e b o n d / p a d e x c e p t f o r the i n t e r d i g i t a l t r a n s d u c e r pads where two w i r e b o n d s were made. The ohmic c o n t a c t s u s e d f o r t h i s t h e s i s d i f f e r e d s l i g h t l y from t h o s e r e p o r t e d by H o s k i n s [45, 63] i n w h i c h a Au:Ge/Ni s y s t e m was d e s c r i b e d . The a d d i t i o n o f a Au o v e r -60 l a y e r t e n d s to p r o d u c e ohmic c o n t a c t s w i t h smoother s u r f a c e t e x t u r e and may y i e l d b e t t e r s p e c i f i c c o n t a c t r e s i s t i v i t y v a l u e s [73] . S i n c e the ohmic b i a s r i n g and i n p u t / o u t p u t c o n t a c t s a r e i n the p r o p a g a t i o n p a t h o f the SAW, i t was f e l t t h a t a smooth s u r f a c e f o r low s c a t t e r i n g was d e s i r a b l e . Copper was added to t h e aluminum S c h o t t k y m e t a l l i z a t i o n i n o r d e r to r e d u c e the e f f e c t o f " a c o u s t o m i g r a t i o n " o f m e t a l w h i c h c o u l d l e a d to d e s t r u c t i o n o f the i n t e r d i g i t a l t r a n s d u c e r [ 5 5 ] . The C r / A l : C u m e t a l l i z a t i o n s y s t e m has been u s e d i n s i l i c o n work to r e d u c e the e f f e c t s o f e l e c t r o m i g r a t i o n [74, 7 5 ] . Copper i s an u n d e s i r a b l e m e t a l to have i n d i r e c t c o n t a c t w i t h GaAs s i n c e i t c a n d i f f u s e i n t o t h e s u b s t r a t e , g e t t e r o n t o c r y s t a l l i n e d e f e c t s , and has a h i g h c h e m i c a l r e a c t i v i t y as p o i n t e d o u t by W i l l i a m s [ 7 6 ] . Hence, a chromium l a y e r was e v a p o r a t e d f i r s t t o a c t as a d i f f u s i o n b a r r i e r . The e x c e l l e n t a d h e s i o n o f chromium to GaAs was an a d d i t i o n a l b e n e f i t o f a d d i n g t h i s l a y e r . The l i f t - o f f t e c h n i q u e [77] was u s e d f o r a l l m e t a l l i z a t i o n p a t t e r n i n g . F a b r i c a t i o n e x p e r i e n c e was f i r s t d e v e l o p e d u s i n g undoped GaAs w a f e r s and a s c a n n i n g e l e c t r o n m i c r o s c o p e (SEM) to c h e c k p h o t o r e s i s t and m e t a l edge p r o f i l e s . S e v e r a l p r o b l e m s t h a t were e n c o u n t e r e d and s o l v e d d u r i n g t h i s phase o f the work a r e d i s c u s s e d b elow. To a c h i e v e u n d e r c u t p h o t o r e s i s t p r o f i l e s e s s e n t i a l f o r good l i f t - o f f , the c h l o r o b e n z e n e method was u s e d [ 7 8 ] . A f t e r s o f t - b a k i n g t h e p h o t o r e s i s t , t h e w a f e r i s d i p p e d i n an a r o m a t i c s o l v e n t s u c h as c h l o r o b e n z e n e e i t h e r b e f o r e o r a f t e r 61 e x p o s u r e to t h e u l t r a v i o l e t l i g h t i n the mask a l i g n e r . I t i s b e l i e v e d t h a t l o w - m o l e c u l a r - w e i g h t r e s i n and s o l v e n t i s l e a c h e d o u t o f t h e p h o t o r e s i s t by a d i f f u s i o n p r o c e s s . T h i s r e s u l t s i n a t o u g h e n e d s u r f a c e l a y e r t h a t d i s s o l v e s a t a g r e a t l y r e d u c e d r a t e i n t h e d e v e l o p e r s o l u t i o n r e l a t i v e to e x p o s e d p h o t o r e s i s t . The h a r d e n e d s u r f a c e l a y e r g i v e s the d e s i r e d u n d e r c u t edge p r o f i l e a f t e r d e v e l o p m e n t . F o r d e v i c e s b u i l t d u r i n g t h i s r e s e a r c h , soak t i m e s were 7 m i n u t e s j u s t p r i o r to p l a c i n g the w a f e r i n t h e d e v e l o p e r s o l u t i o n . I n i t i a l l y , a p h o t o r e s i s t s o f t - b a k e t e m p e r a t u r e o f 95° C e l s i u s was u s e d b u t t h i s l e d to "wings" on t h e l i f t e d -o f f m e t a l p a t t e r n s shown i n F i g u r e 3.2. L o w e r i n g the s o f t -bake t e m p e r a t u r e t o 70° C e l s i u s e l i m i n a t e d t h i s p r o b l e m and y i e l d e d a c c e p t a b l e m e t a l edge p r o f i l e s , ( c . f . F i g u r e 3.3). T h i s o b s e r v a t i o n i s i n agreement w i t h r e s u l t s r e p o r t e d by H a t z a k i s , C a n a v e l l o and Shaw [ 7 8 ] . They e x p l a i n e d t h a t the p h o t o r e s i s t r e s i n m o l e c u l e s a r e more t i g h t l y p a c k e d a f t e r a h i g h e r t e m p e r a t u r e s o f t - b a k e r e d u c i n g t h e d i f f u s i o n r a t e o f s o l v e n t and l o w - m o l e c u l a r - w e i g h t r e s i n . No u l t r a s o n i c a g i t a t i o n was u s e d d u r i n g the l i f t - o f f p r o c e s s f o r f e a r o f s h a t t e r i n g the w a f e r . I n s t e a d , a s p r a y -b o t t l e o f a c e t o n e was f o u n d to be an e f f e c t i v e l i f t - o f f a i d and p o s s i b l y s a f e r . Good m a s k - t o - w a f e r c o n t a c t was r e q u i r e d f o r p r o p e r p h o t o r e s i s t edge p r o f i l e b e c a u s e o f o p t i c a l d i f f r a c t i o n e f f e c t s . To a c h i e v e a c c e p t a b l e c o n t a c t , a p r e c i s i o n mask a l i g n e r , w h i c h o p e r a t e s by p l a c i n g a s m a l l vacuum between the 62 . s „ o n t r a n s d u c e r r i n g e r s . a t . f t i "wings on . 2 . SEM o f m e t a l p r o f i l e F i g u r e 3.2. p h o t o r e s i s t edge P c a u s e d by P ° ° r P h 63 F i g u r e 3.3: SEM o f 2 Mm i n t e r d i g i t a l t r a n s d u c e r f i 64 w a f e r c huck and the mask, was u s e d to p u l l t h e w a f e r and mask i n t o i n t i m a t e c o n t a c t . To p r o v i d e a c l e a n GaAs s u r f a c e p r i o r t o a l l d e p o s i t i o n s t e p s , a 2 0 s e c o n d I N H 4 O H : I O H 2 O d i p was u s e d i n a c c o r d a n c e w i t h r e s u l t s p r e s e n t e d by M i e r s [ 7 9 ] . I m m e d i a t e l y a f t e r t h i s f i n a l c l e a n i n g s t e p , the w a f e r was l o a d e d i n t o the e v a p o r a t i o n chamber and pump-down commenced. P h o t o r e s i s t b a k i n g was o b s e r v e d d u r i n g t h e r m a l e v a p o r a t i o n o f chromium from t u n g s t e n o r molybdenum b o a t s as shown i n F i g u r e 3.4. T h i s i n t u r n d i s t o r t e d t h e p a t t e r n i n g and made l i f t - o f f v e r y d i f f i c u l t . S w i t c h i n g t o chromium c o a t e d t u n g s t e n r o d s e l i m i n a t e d t h i s e f f e c t , p r o b a b l y b e c a u s e b e t t e r t h e r m a l c o n t a c t between the chromium and t u n g s t e n a l l o w e d e v a p o r a t i o n to p r o c e e d w i t h o u t e x c e s s i v e l y h e a t i n g t h e s u b s t r a t e . A s i m i l a r p r o b l e m n o t i c e d w h i l e t h e r m a l l y e v a p o r a t i n g g o l d f r o m t u n g s t e n or molybdenum b o a t s was e l i m i n a t e d u s i n g an i n t e g r a t e d t u n g s t e n - a l u m i n a c r u c i b l e . As s u c h , t h e h e a t r e q u i r e d t o l i q u i f y the g o l d was more c l o s e l y c o n f i n e d to the a r e a where t h e m e l t was l o c a t e d . D u r i n g some o f the l i f t - o f f p r o c e s s e s , Hunt m i c r o s t r i p (TM) was u s e d t o remove r e s i d u a l p h o t o r e s i s t . I f any w a t e r was p r e s e n t i n the m i c r o s t r i p s o l u t i o n , as m i g h t o c c u r i f a wet b e a k e r were u s e d , e t c h i n g o f aluminum m e t a l l i z a t i o n o c c u r r e d ( F i g u r e 3.5). Care i n e n s u r i n g t h a t b e a k e r s were c o m p l e t e l y d r y b e f o r e a d d i n g m i c r o s t r i p s o l v e d t h i s p r o b l e m . 65 F i g u r e 3.4: SEM o f "baked" p h o t o r e s i s t h e a t d u r i n g e v a p o r a t i o n 66 c a u s e d by e x c e s s i v e F i g u r e 3.5: SEM o f e t c h e d 1 pm aluminum f i n g e r s c a u s e d by m o i s t u r e i n Hunt m i c r o s t r i p (TM) 67 A d i c e d ACT d e v i c e , p r i o r t o a p p l i c a t i o n o f a c o u s t i c a b s o r b e r m a t e r i a l , m o u n t i n g and w i r e b o n d i n g i s shown i n F i g u r e 3.6. C l o s e r v i e w s o f the i n p u t and o u t p u t s t r u c t u r e s a r e d i s p l a y e d i n F i g u r e s 3.7a and 3.7b r e s p e c t i v e l y . 3.4 T e s t S t r u c t u r e s and P r o c e s s C o n t r o l I n o r d e r t o a s s e s s t h e s u c c e s s o f each s t a g e i n the f a b r i c a t i o n p r o c e s s from an e l e c t r i c a l v i e w p o i n t , t e s t s t r u c t u r e s were i n c l u d e d i n t h e ohmic and S c h o t t k y mask l a y e r s. Ohmic c o n t a c t r e s i s t a n c e was measured u s i n g the t e s t p a t t e r n f o u n d i n the i n s e t o f F i g u r e 3.8 [73] . By g r a p h i n g t h e r e s i s t a n c e v e r s u s gap w i d t h f o r t h i s s t r u c t u r e , the c u r v e shown i n F i g u r e 3.8 was o b t a i n e d . A p p r o x i m a t e v a l u e s f o r the GaAs s h e e t r e s i s t a n c e and ohmic c o n t a c t r e s i s t a n c e were d e r i v e d by m e a s u r i n g the s l o p e and r e s i s t a n c e a x i s i n t e r c e p t o f t h i s g r a p h r e s p e c t i v e l y . By making a r o u g h a p p r o x i m a t i o n t h a t the s h e e t r e s i s t a n c e o f t h e GaAs u n d e r the a l l o y e d c o n t a c t was t h e same as t h e s h e e t r e s i s t a n c e o f t h e n - e p i t a x i a l w a f e r a l o n e , v a l u e s o f s p e c i f i c c o n t a c t r e s i s t i v i t y and t r a n s f e r l e n g t h were o b t a i n e d . W i t h d a t a d e r i v e d f r o m t h e g r a p h , s p e c i f i c c o n t a c t r e s i s t i v i t y ( r c ) and t r a n s f e r l e n g t h ( I t ) were ~ 2.7x10"-* ohm-cm 2 and 0.75 /zm r e s p e c t i v e l y . A more a c c u r a t e e s t i m a t e o f s p e c i f i c c o n t a c t r e s i s t i v i t y and t r a n s f e r l e n g t h w o u l d r e q u i r e the measurement o f end r e s i s t a n c e . However, f o r c o n t a c t s whose d i m e n s i o n s a r e o f the o r d e r o f 150 izm, 68 F i g u r e 3.6: P h o t o g r a p h o f c o m p l e t e d ACT d e v i 69 I n p u t Gate ~~ Channe1 P l a t e n I n p u t Ohmic Guard ^ R i n g B i a s jS R i n s F i g u r e 3.7a: P h o t o g r a p h o f ACT i n p u t f e a t u r e ! Channe1 P l a t e O u t p u t — Ohmi c F i g u r e 3.7b: P h o t o g r a p h o f ACT o u t p u t f e a t u r e s 70 g u r e 3 . 8 : Ohmic c o n t a c t t e s t s t r u c t u r e r e s i s t a n c e v a r i a t i o n w i t h gap w i d t h t h i s i s a d i f f i c u l t measurement s i n c e t h e end r e s i s t a n c e r a p i d l y a p p r o a c h s z e r o f o r c o n t a c t l e n g t h s w h i c h a r e s e v e r a l t r a n s f e r l e n g t h s l o n g [ 8 0 ] . The d o p i n g p r o f i l e o f t h e n - e p i t a x i a l l a y e r was measured by t h e c apac i t a n c e - vo 1 t a g e or C-V method [81] . ACT d e v i c e c h a n n e l p l a t e and s u r r o u n d i n g ohmic b i a s r i n g were u s e d f o r t h i s measurement. R e s u l t s shown i n F i g u r e 3.9 were i n good agreement w i t h d a t a p r o v i d e d by the w a f e r m a n u f a c t u r e r . The S c h o t t k y i d e a l i t y f a c t o r was m e asured u s i n g the d i o d e shown i n F i g u r e 3.10. The c u r r e n t - v o 1 t a g e c h a r a c t e r i s t i c y i e l d e d an i d e a l i t y o f 1.05, i n d i c a t i n g a c c e p t a b l e s u r f a c e p r e p a r a t i o n and e v a p o r a t i o n t e c h n i q u e . 72 D O P I N G P R O F I L E SRMPLE- RCT1.RUN1 .3V TO -8V SWEEP 1E+1B r 1E+15 j-m 6 1E+14 u s u M 1E+13 Q. O a 1E+12 1E+11 0 10 DISTANCE (urn) F i g u r e 3.9: E p i t a x i a l w a f e r d o p i n g p r o f i l e m e asured by the c a p a c i t a n c e - v o l t a g e method 73 LNI ( ) •100.0 E-03 CURSOR ( 250E-03 . -452E-03 MARKER ( 448E-03 .-264E-03 295.8uA ) 39.54UA ) (mA) 50.00 /div -600 -440 . 1.000 1000 /div 0000 550.0 V 11 O.O/div (V ) E-03 GRAD 1/GRAD Xintercept Yintercept LINE1 951E-03 1.05E+00 725E-03 -689E-03 LINE2 LNI ( ) - 26E-3MLN (ABS (I) ) V (V ) - VF-WSB gure 3.10: S c h o t t k y d i o d e c u r r e n t - v o 1 t a g e p l o t u s e d to d e t e r m i n e t h e i d e a l i t y f a c t o r 4.0 MEASURED PERFORMANCE 4.1 F r o n t g a t i n g and B a c k g a t i n g B e h a v i o u r O m i t t i n g t h e i n t e r d i g i t a l t r a n s d u c e r , the ACT d e v i c e can be v i e w e d as a MESFET w i t h a v e r y l o n g g a t e . The i n p u t ohmic forms t h e s o u r c e , the c h a n n e l p l a t e t h e g a t e , and t h e o u t p u t ohmic the d r a i n . U s i n g t h e b i a s i n g a r r a n g e m e n t shown i n F i g u r e 4.1, t h e MESFET d r a i n - s o u r c e c u r r e n t was m o d u l a t e d by a p p l y i n g b i a s v o l t a g e s to the c h a n n e l p l a t e ( f r o n t g a t i n g ) or s u b s t r a t e ( b a c k g a t i n g ) . The m e asured c h a r a c t e r i s t i c s f o r a t y p i c a l ACT a r e shown i n F i g u r e s 4.2a and 4.2b. F r o n t g a t i n g p i n c h - o f f v o l t a g e was between -5 t o -6 V. T h i s was i n good agreement w i t h the c a l c u l a t e d v a l u e o f -5.6 V a s s u m i n g a donor d e n s i t y o f l.QxlO-'-^ cm" 3 and e p i t a x i a l l a y e r t h i c k n e s s o f 2.2 ^m. B a c k g a t i n g v o l t a g e s were much l a r g e r to a c h i e v e the same amount o f s o u r c e - d r a i n c u r r e n t change. T h i s c a n be a t t r i b u t e d to a l a r g e v o l t a g e drop between the "ohmic" c o n t a c t a l l o y e d to t h e semi - i n s u l a t i n g s u b s t r a t e and the a c t i v e d e v i c e c h a n n e l . C u r v e s shown i n F i g u r e s 4.2a and 4.2b p r o v i d e d a t a u s e d to c a l c u l a t e t h e d e p l e t i o n p o t e n t i a l i n t h e c h a n n e l o f the d e v i c e ( s e c t i o n 2.2) and hence the d e p t h o f c h a r g e t r a n s p o r t . F o r i n s t a n c e , f r o m the g r a p h s , i t c a n be s e e n t h a t -2 to -3 V o f f r o n t g a t i n g b i a s has t h e same e f f e c t as -18 V o f b a c k g a t i n g b i a s on the d r a i n - s o u r c e c u r r e n t . 75 I n t e r d i g i t a l Transducer Input 6 a t e Channel P l a t e o (GATE) [7 A Input Ohmic H i ' (SOURCE) B i a s R i n g • 6rounded 6uard R i n g B -14 V o l t s A Output Ohmic -o (DRAIN) B i a s R i n g and Guard R i n g not shown f o r c l a r i t y Y g » S e m i - i n s u l a t i n g s u b s t r a t e "ohmic" t i e d t o Ground f o r F r o n t g a t i n g and made n e g a t i v e f o r B a c k g a t i n g C h a r a c t e r i s t i c F i g u r e 4.1: Experimental set-up f o r measurement of t f r o n t g a t i n g and bac k g a t i n g behaviour 76 ID (MA) 100.OL .0000 1.500 VDS .1500/div ( V) V S R - 0 V o l t s F i g u r e 4.2a: F r o n t g a t i n g c h a r a c t e r i s t i c 77 V G = 0 V o l t s F i g u r e 4.2b: B a c k g a t i n g c h a r a c t e r i s t i c 78 4.2 I n t e r d i g i t a l T r a n s d u c e r RF Impedance The i n t e r d i g i t a l t r a n s d u c e r r e t u r n l o s s m a g n i t u d e and phase were measured as a f u n c t i o n o f r e v e r s e b i a s a p p l i e d to the t r a n s d u c e r f i n g e r s u s i n g a n e t w o r k a n a l y z e r . The r e s u l t s a r e shown i n F i g u r e s 4.3a to 4.3d. W i t h no b i a s a p p l i e d to t h e IDT t h e peak i n the r e t u r n l o s s i n d i c a t i v e o f SAW g e n e r a t i o n was a l m o s t n o n - e x i s t e n t and the o u t - o f - b a n d r e t u r n l o s s was g r e a t e r t h a n 0.5 dB i n d i c a t i n g a w i d e - b a n d power l o s s . As t h e r e v e r s e b i a s on the t r a n s d u c e r f i n g e r s was i n c r e m e n t e d , r e t u r n l o s s i n c r e a s e d m o n o t o n i c a l l y i n m a g n i t u d e to a maximum v a l u e o f -5 dB a t -14 V. F u r t h e r i n c r e a s e s i n the f i n g e r b i a s p r o d u c e d no change i n the t r a n s d u c e r impedance, i n d i c a t i n g d e p l e t i o n o f the e p i t a x i a l l a y e r a t = -14 V ( F i g u r e 4 . 3 c ) . The d e p l e t i o n c o n d i t i o n c o u l d be o b t a i n e d by a c o m b i n a t i o n o f f r o n t g a t i n g and b a c k g a t i n g . F o r example, F i g u r e 4.3d shows the r e t u r n l o s s and phase r e s p o n s e w i t h o n l y -5 V o f f r o n t g a t e b i a s combined w i t h -20 V o f s u b s t r a t e b i a s . The r e v e r s e b i a s r e q u i r e d f o r c o m p l e t e d e p l e t i o n b e n e a t h the IDT was much l a r g e r t h a n t h e p i n c h o f f v o l t a g e o f the MESFET p o r t i o n o f the ACT measured e a r l i e r ( c . f . s e c t i o n 4 . 1 ) . T h i s i s n o t s u r p r i s i n g when one c o n s i d e r s t h a t d e p l e t i o n o f the IDT gap r e g i o n s i s a c h i e v e d s o l e l y by the f r i n g i n g f i e l d s o f t h e f i n g e r r e g i o n s . To c a l c u l a t e the m a g n i t u d e o f d e p l e t i o n p o t e n t i a l w o u l d r e q u i r e a two-d i m e n s i o n a l f i e l d a n a l y s i s o f the p e r i o d i c IDT s t r u c t u r e . 79 C H l i A / R MAG. -MEM 1 dB/DIV CH2lA/R PHASE-- 154 DEC 45 OEO/DIV (1 C H l i A / R MAG. -MEM CM2:A/R PHASE— 131 DEC 1 dB/DIV 45 DEG/OIV < 0 dB V s = 0 V v I D T = 0 V 0 dB V V, IDT 0 V = -8 V Note : F i g u r e 4.3 (b) Top t r a c e i s r e t u r n l o s s m a g n i t u d e Bottom t r a c e i s r e t u r n l o s s p h a s e V a r i a t i o n o f IDT r e t u r n l o s s w i t h b i a s i n g 80 CHliA/R MAO. -«** i de/oiv DClA/R PHASE*- 132 DEC 45 OEO/OIV <- 0 dB V s = 0 V V I D T = " 1 4 V ( c ) < 0 dB V s =-20 V V I D T = -5 V (d) N o te: Top t r a c e i s r e t u r n l o s s m a g n i t u d e Bottom t r a c e i s r e t u r n l o s s p h a s e F i g u r e 4.3: V a r i a t i o n o f IDT r e t u r n l o s s w i t h b i a s i n g 81 A s i m i l a r d i s c r e p a n c y between MESFET p i n c h o f f v o l t a g e and v o l t a g e r e q u i r e d f o r IDT d e p l e t i o n a p p e a r e d i n H o s k i n s ' r e s u l t s [45] pages 32 and 118 a l t h o u g h no e x p l a n a t i o n was p r o v i d e d . B e h a v i o u r o f the o u t - o f - b a n d l o s s r e q u i r e s c l a r i f i c a t i o n . I n c r e a s i n g t h e IDT f i n g e r b i a s from 0 V i n i t i a l l y c a u s e d an i n c r e a s e i n the o u t - o f - b a n d r e t u r n l o s s t o a maximum o f ~ -1 dB a t -8 V. F u r t h e r i n c r e a s e s i n the b i a s c a u s e d the o u t - o f - b a n d r e t u r n l o s s to d e c r e a s e to i t s minimum v a l u e a t -14 V. One 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 b e h a v i o u r m i g h t be the l a r g e c o n c e n t r a t i o n o f f r e e c a r r i e r s s c r e e n i n g t h e t r a n s d u c e r a t 0 V r e v e r s e b i a s w h i c h made i t l o o k l i k e an e f f e c t i v e RF s h o r t c i r c u i t o u t - o f - b a n d . As r e v e r s e b i a s was i n c r e a s e d , t h e d e n s i t y o f f r e e c a r r i e r s was r e d u c e d and t h e t r a n s d u c e r l o o k e d r e s i s t i v e , hence t h e f i n i t e r e t u r n l o s s . I n c r e a s i n g s t i l l f u r t h e r t h e IDT r e v e r s e b i a s , a c o n d i t i o n o f c o m p l e t e d e p l e t i o n was a p p r o a c h e d i n d i c a t i n g an o p e n - c i r c u i t c o n d i t i o n and a r e t u r n l o s s n e a r 0 dB. D i f f e r e n t b e h a v i o u r was n o t e d w i t h some t r a n s d u c e r s . T h e s e had o u t - o f - b a n d l o s s t h a t d e c r e a s e d m o n o t o n i c a l l y w i t h i n c r e a s i n g IDT r e v e r s e b i a s . T h i s i s i n agreement w i t h the r e s u l t s p r e s e n t e d by H o s k i n s [45] page 31. F o r r e a s o n s n o t c l e a r to the a u t h o r , the r e t u r n l o s s peak i n t h e r e s u l t s p r e s e n t e d by H o s k i n s [45] page 31 was much d e e p e r , w i t h -12.5 dB r e t u r n l o s s a t f u l l d e p l e t i o n . The v a l u e i n d i c a t e d by the g r a p h [45] page 31 was not 82 c o n s i s t e n t w i t h the c e n t r e f r e q u e n c y impedance q u o t e d [45] page 33, from w h i c h a r e t u r n l o s s o f -8 dB was c a l c u l a t e d . Even w i t h a modest r e t u r n l o s s o f -5 dB, «68% o f the i n c i d e n t power was a b s o r b e d by t h e t r a n s d u c e r . I n f u t u r e d e v i c e s , the r e f l e c t i o n l o s s c o u l d be r e d u c e d u s i n g m a t c h i n g t e c h n i q u e s t o t r a n s f o r m the IDT impedance shown i n F i g u r e 4.4 to 50 n. The low f r e q u e n c y IDT c a p a c i t a n c e m easured u s i n g an LCR meter was 7.9 pF compared to a c a l c u l a t e d v a l u e o f 7.7 pF and one o b t a i n e d by H o s k i n s [45] page 33 o f = 7 pF. 4.3 C a l c u l a t i o n o f Wave P o t e n t i a l The r e l a t i o n s h i p between RF power a p p l i e d to the i n t e r d i g i t a l t r a n s d u c e r and t h e r e s u l t i n g SAW wave p o t e n t i a l w i l l i n i t i a l l y be d i s c u s s e d f o r an IDT w i t h no i n t e r n a l a c o u s t i c r e f l e c t i o n s . I n t h i s c a s e , t h e f r e q u e n c y r e s p o n s e o f t h e t r a n s d u c e r i s a s i n e f u n c t i o n w i t h n u l l s e v e r y 3.6 MHz a b o u t 360 MHz f o r a 100 f i n g e r p a i r u n i t . By m e a s u r i n g the r e t u r n l o s s a t c e n t r e f r e q u e n c y and a t one o f t h e f i r s t f r e q u e n c y n u l l s , the SAW power c a n be c a l c u l a t e d a s s u m i n g l o s s mechanisms i n t h e t r a n s d u c e r a r e c o n s t a n t between t h e s e two f r e q u e n c i e s . F i g u r e 4.5 shows the power components under c o n s i d e r a t i o n . Power i n c i d e n t on the t r a n s d u c e r ( p I N ) ^ s p a r t i a l l y r e f l e c t e d (PREF^- ^REF c a n D e c a l c u l a t e d f r o m t h e measured r e t u r n l o s s v a l u e . Of t h e n e t power i n c i d e n t on the t r a n s d u c e r , a c e r t a i n p r o p o r t i o n i s l o s t i n the c o n v e r s i o n 83 F i g u r e 4.4: P o l a r p l o t o f IDT r e f l e c t i o n n e a r r e s 84 p c o n v SAW+ IDT Frequency Response First Frequency Nulls (no SAW launched) (f - T Q ) M H Z Y Nrrtf " f 0 ) X N " 1 — E x p e r i m e n t a l D e v i c e N = 100 f 0 - 360 MHz F i g u r e 4.5: SAW g e n e r a t i o n power components 85 p r o c e s s between e l e c t r i c a l and a c o u s t i c e n e r g y . T h i s l o s s i s r e p r e s e n t e d by PcONV-S i n c e t h e t r a n s d u c e r i s sy m m e t r i c i n the d i r e c t i o n o f p r o p a g a t i o n , i t w i l l l a u n c h SAW e q u a l l y e i t h e r way ( i . e . PSAW+ = PSAW-)- S i n c e no r e f l e c t o r a r r a y i s b e i n g u s e d to r e d i r e c t the PSAW- component, t h i s amount o f power ( i . e . 3 dB) i s l o s t . A t the c e n t r e f r e q u e n c y the power b a l a n c e e q u a t i o n can be w r i t t e n : PSAW+ + PSAW- ~ P I N " P R E F " PCONV (4.1) At t h e f i r s t f r e q u e n c y n u l l , s i n c e PgAW+ = PSAW- = ^, we can w r i t e : 0 = p I N " p R E F ' " pCONV ( 4 - 2 ) where P R E F ' t^[ie n e w v a l u e o f r e f l e c t e d power a t t h i s f r e q u e n c y . E q u a t i o n s (4.1) and (4.2) can be s o l v e d to y i e l d t he v a l u e o f PsAW+' ^ e P°wer i n the f o r w a r d l a u n c h e d wave i n terms o f measured q u a n t i t i e s . W i t h t h e c a l c u l a t e d SAW power v a l u e , the q u a s i s t a t i c p o t e n t i a l c u r v e ( F i g u r e 2.3b) i s us e d , a f t e r a p p r o p r i a t e d e n o r m a l i z a t i o n , to g i v e t h e d i s t r i b u t i o n o f SAW p o t e n t i a l i n t o t h e s u b s t r a t e . The d e p t h o f c h a r g e t r a n s p o r t i s c a l c u l a t e d from the known b i a s v a l u e s and s u b s t r a t e p a r a m e t e r s u s i n g the a n a l y s i s p r e s e n t e d i n s e c t i o n 2.2. Knowing t h e c h a r g e t r a n s p o r t d e p t h , the wave p o t e n t i a l c a n be r e a d d i r e c t l y from the d e n o r m a l i z e d q u a s i s t a t i c p o t e n t i a l c u r v e . 86 U n f o r t u n a t e l y , i n t e r n a l r e f l e c t i o n s i n l o n g t r a n s d u c e r s o f t h e t y p e u s e d f o r t h e d e v i c e i n t h i s r e s e a r c h i n t r o d u c e a d d i t i o n a l f r e q u e n c y d e p e n d e n t l o s s n o t a c c o u n t e d f o r i n the a n a l y s i s j u s t p r e s e n t e d . The i n t e r n a l r e f l e c t i o n s a l t e r the s i n e f u n c t i o n f r e q u e n c y dependence o f t h e t r a n s d u c e r , as p r e v i o u s l y d i s c u s s e d i n s e c t i o n 2.3. H o s k i n s [45] a c c o u n t e d f o r t h i s a d d i t i o n a l l o s s by c o m p a r i n g o n e - p o r t IDT impedance w i t h t h e f r e q u e n c y r e s p o n s e o f the t r a n s d u c e r , as measured w i t h a l a s e r p r o b e [82] . An a l t e r n a t e a p p r o a c h to o b t a i n t h e SAW p o t e n t i a l i n the p r e s e n c e o f i n t e r n a l r e f l e c t i o n s w i l l be d e r i v e d . The f r e q u e n c y r e s p o n s e o f a two t r a n s d u c e r d e l a y l i n e i s c o n s i d e r e d . A t some m a g n i t u d e v a l u e -Y<jg b e l o w t h e peak i n the r e s p o n s e a t the c e n t r e f r e q u e n c y fQ, the f r e q u e n c y w i d t h c a n be measured Sf. S i n c e the l o s s o f a s i n g l e t r a n s d u c e r i s d e s i r e d , a c o r r e c t i o n f o r the d i f f e r e n c e i n f r e q u e n c y r e s p o n s e between the l a t t e r and a d e l a y l i n e c o n f i g u r a t i o n c o n s i s t i n g o f a t r a n s d u c e r p a i r must be made. I n the f r e q u e n c y domain, the t r a n s f e r f u n c t i o n s o f e a c h t r a n s d u c e r m u l t i p l y to g i v e the d e l a y l i n e r e s p o n s e ( i g n o r i n g a phase term a c c o u n t i n g f o r the d i s t a n c e between t h e t r a n s d u c e r s ) . Hence, t h e s i n g l e t r a n s d u c e r m a g n i t u d e i n d e c i b e l s Y£g i s s i m p l y h a l f the d e l a y l i n e m a g n i t u d e i n d e c i b e l s Y^g . At f 0 ± 5 f : Y d B = Y d B / 2 < 4- 3) 87 The r e t u r n l o s s e s RL and RL' a r e measured a t f g and fg±5f/2 r e s p e c t i v e l y . The r e f l e c t e d powers a r e t h e n : P R E F ( f 0 ) = P I N 1 0 " R L / 1 0 ( 4 . 4 ) P £ E F ( f 0 ± 5 f / 2 ) = P I N 1 0 - R L ' / 1 0 ( 4 5 ) A t fQ, c o n s e r v a t i o n o f e n e r g y r e q u i r e s : PSAW+ + PSAW- = P I N " P R E F " PCONV (4.6) Fo r a b i d i r e c t i o n a l IDT, ?SAW+ = PSAW- a n c * ( 4 - 6 ) can be r e d u c e d t o : 2 PSAW+/ PIN = 1 " p R E F / p I N " P C O N v/ pIN ( 4-?> At f Q ± 5 f / 2 , c o n s e r v a t i o n o f e n e r g y r e q u i r e s : PSAW+ + PSAW- = P I N " P R E F " PCONV ( 4 - 8 ) where 5 f / 2 i s assumed s m a l l enough so t h a t the l o s s e s (PcONv) a r e t h e s ame. But, t h e f o r w a r d wave SAW power a t f 0 ± 5 f / 2 (P&AW+) i s r e l a t e d t o the f o r w a r d wave SAW power a t f g ( pSAW+) t h r o u g h : pSAW+ " 1 0 - Y ' d B / 1 0 ( p s A w + ) ( 4 9 ) Hence, a t fg±5f/2 f o r a b i d i r e c t i o n a l a r r a y , u s i n g (4.8) and ( 4 . 9 ) : 2 x l 0 - Y ' d B / 1 0 p S A W + / P i N . 1 - P £ E F / P I N - P C 0 N V / P I N (4.10) E q u a t i o n s (4.7) and (4.10) c an be u s e d t o s o l v e f o r PSAW+/ PIN' t h e f r a c t i o n o f i n c i d e n t e l e c t r i c a l power c o n v e r t e d to t h e f o r w a r d t r a v e l l i n g SAW i n terms o f measured q u a n t i t i e s : P S A W + ( P R E F / P I N • P R E F / P I N ) = 2 ( 1 - 1<>-1'<"»/1U ) ( 4 - 1 1 ) 88 I n d i v i d u a l l o s s components a t f g c a n be c a l c u l a t e d from the f o l l o w i n g e x p r e s s i o n s : E l e c t r i c a l = - l O l o g [ < P I N - P R E F ) / P I N ] (4.12) M i s m a t c h L o s s C o n v e r s i o n = - 1 0 1 o g [ 2 P S A W + / ( P I N - P R E F ) ] (4.13) Lo s s B i d i r e c t i o n a l = 3 dB L o s s The l o s s due to a c o u s t i c r e f l e c t i o n s c o u l d be c a l c u l a t e d by m e a s u r i n g t h e t r a n s d u c e r r e t u r n l o s s f a r enough away from f g s u c h t h a t the na r r o w b a n d r e f l e c t i o n l o s s e s were s m a l l r e l a t i v e t o t h o s e due t o o t h e r f a c t o r s s u c h as f i n g e r c o n d u c t i v i t y , s h u n t c o n d u c t a n c e between t h e f i n g e r s , b u l k mode g e n e r a t i o n , and so on. A t a f r e q u e n c y n u l l f a r enough away f r o m f g so t h a t r e f l e c t i o n l o s s c an be i g n o r e d b u t c l o s e enough t o f g so t h a t the o t h e r l o s s components have n o t ch a n g e d a p p r e c i a b l y , t h e power b a l a n c e c a n be w r i t t e n a s : 0 = P I N " P R E F - P 6 0 N V (4-14) where: P R E F = P I N 1 0 - R L " / 1 0 (4.15) and RL" i s the r e t u r n l o s s i n d e c i b e l s m e asured a t the f r e q u e n c y n u l l . The d i f f e r e n c e i n power l o s t between t h e l a t t e r c a s e ( PC0NV) and t h e f o r m e r c a s e (^CONV^ -*-s t h a t due t o a c o u s t i c r e f l e c t i o n s . The a n a l y s i s above c o u l d n o t be u s e d to e s t i m a t e the t r a n s d u c e r c o n v e r s i o n l o s s f o r h i g h l y r e f l e c t i v e IDT's employed i n ACT d e v i c e s b u i l t f o r t h i s work. T h i s i s b e c a u s e 89 t h e v a l i d i t y o f the a n a l y s i s depends on: 1) i d e n t i c a l t r a n s d u c e r s f o r the d e l a y l i n e and 2) no i n t e r t r a n s d u c e r r e f l e c t i o n s . Measurement o f the r e t u r n l o s s f o r each t r a n s d u c e r i n the d e l a y l i n e c o n f i g u r a t i o n y i e l d e d a c e n t r e f r e q u e n c y d i f f e r e n c e o f 0.1-0.3 MHz between the IDT's. Thus, c o n d i t i o n 1) above was v i o l a t e d . A l s o , t h e h i g h l y r e f l e c t i v e n a t u r e o f t h e IDT's b u i l t f o r t h i s r e s e a r c h v i o l a t e d c o n d i t i o n 2 ) . The method w o u l d be v a l i d f o r t r a n s d u c e r s w h i c h a r e l e s s r e f l e c t i v e , s u c h as s p l i t - f i n g e r t y p e s . A l s o , t h e a n a l y s i s c o u l d be u s e d to d e r i v e l o s s components f o r a r e f l e c t i v e a r r a y i f a l a s e r p r o b e were a v a i l a b l e . One w o u l d have to m o d i f y e q u a t i o n (4.3) s u c h t h a t Y ' ^ g = Y ^ B • U n f o r t u n a t e l y , t h e l a s e r p r o b e i s n o t a s t a n d a r d p i e c e o f e q u i p m e n t i n most l a b o r a t o r i e s . A s s u m i n g a c o n v e r s i o n l o s s v a l u e s i m i l a r t o t h a t q u o t e d by H o s k i n s [45] page 33, and c a l c u l a t i n g m i smatch l o s s u s i n g t h e r e t u r n l o s s shown i n F i g u r e 4.3c and e q u a t i o n (4.12) the l o s s components a r e as g i v e n i n T a b l e 4.1: T a b l e 4.1: IDT l o s s components E l e c t r i c a l M i s m a t c h L o s s = 1.7 dB C o n v e r s i o n L o s s = 3.1 dB B i d i r e c t i o n a l L o s s = 3 dB T o t a l L o s s = 7.8 dB Us i n g d e v i c e b i a s the a n a l y s i s v a l u e s , the p r e s e n t e d i n e s t i m a t e d d e p t h 90 s e c t i o n 2.2 and the o f c h a r g e t r a n s p o r t was » 1.7 /*m. A f t e r a p p r o p r i a t e d e n o r m a l i z a t i o n o f the q u a s i s t a t i c p o t e n t i a l c u r v e ( F i g u r e 2.3b), the SAW p o t e n t i a l i n the ACT c h a n n e l was c a l c u l a t e d from the i n p u t e l e c t r i c a l power. The r e l a t i o n s h i p d e r i v e d was: 4>Q « 13Vp I N(mW) ' mV (-4.16) 4 . 4 D e v i c e B i a s i n g F i g u r e 4.6 shows the ACT b i a s i n g scheme i n c l u d i n g the o u t p u t a m p l i f i e r s t a g e . The e n t i r e c i r c u i t was i m p l e m e n t e d w i t h s t a n d a r d RF " b r e a d b o a r d " t e c h n i q u e s u s i n g c o a x i a l c a b l e to r e d u c e s t r a y n o i s e p i c k u p . T y p i c a l b i a s v a l u e s a r e l i s t e d i n T a b l e 4.2: T a b l e 4.2: T y p i c a l ACT d e v i c e b i a s v a l u e s V ( g u a r d r i n g ) = -11 3 V V ( s u b s t r a t e ) = -18 0 V V(IDT b i a s ) -18 9 V V ( c h a n n e l p l a t e ) = - 5 .; > V V C i / p e a t e ) = - 5 . f > V The o u t p u t a m p l i f i e r was c o n f i g u r e d to have 49 dB g a i n when matched to 50 ohms a t i t s i n p u t and o u t p u t and was powered by a 9 V b a t t e r y to r e d u c e power s u p p l y n o i s e . The m e a s u r e d 3 dB b a n d w i d t h o f the a m p l i f i e r was 70 MHz. P i c k u p o f the 360 MHz SAW d r i v e s i g n a l by the o u t p u t c i r c u i t was removed w i t h a s i m p l e c o a x i a l s t u b n o t c h f i l t e r . 91 SAW RF Drive (50 Ohss) 10 nF 10 k —| n ^ i T ^ n 1 0 nF1 i I O I F I I » I/P Gate (-) 10 k Guard Ring (-) 10 nF 10 k Channel Plate (-) 1 10 nFT 10 k I 10 nF[ ZfZlO nF substrata IDT H/C I*" DEYICEI N/C|— 81M Ring 81M Ring I/P 6att I/P0tadc Surd Mug Guard Ring Chamal Piatt 0/P Ohalc N/C H/C — H/C H/C— Substrate IDT Bias (-) 10 k 10 nF ' (10 nF 47 nF -| |—-(jbl/P Ohiic 51 47 nF OUTPUT (To Oscilloscope) (50 ohn) -^ j> ( 360 MHz Trap Figure 4.6: ACT b i a s i n g and output a m p l i f i c a t i o n c i r c u i t r y The MESFET p o r t i o n o f the ACT d e v i c e i s b i a s e d i n t o p i n c h - o f f u n d e r n o r m a l o p e r a t i n g c o n d i t i o n s . Hence, the ACT i n p u t and o u t p u t impedances a r e l a r g e . V i e w i n g the ACT o u t p u t as a h i g h - i m p e d a n c e c u r r e n t s o u r c e , t h e o u t p u t v o l t a g e r i s e t i m e i s c o n t r o l l e d by the l o a d r e s i s t a n c e and any s t r a y c a p a c i t a n c e t h a t m i g h t e x i s t . Measurement o f the d e v i c e p a c k a g e c a p a c i t a n c e a l o n e w i t h an LCR meter y i e l d e d a v a l u e o f =0 . 5 pF. Hence, a s s u m i n g a c a p a c i t a n c e f o r t h e o u t p u t a m p l i f i e r o f 4 pF and u s i n g a l o a d r e s i s t o r o f 51 Q, an RC time c o n s t a n t o f ~ 0.2 ns was c a l c u l a t e d . A l t h o u g h t h e o u t p u t pad o f the ACT was i n d i r e c t c o n t a c t w i t h the n-e p i t a x i a l l a y e r , t h e p r e s e n c e o f t h e g u a r d r i n g s h o u l d i s o l a t e t h i s a r e a . The e f f e c t i v e c a p a c i t a n c e i s t h e s e r i e s c o m b i n a t i o n o f the d e p l e t i o n l a y e r c a p a c i t a n c e and the c a p a c i t a n c e o f t h e u n d e p l e t e d n - e p i t a x i a l l a y e r t o g r o u n d t h r o u g h t h e semi - i n s u l a t i n g s u b s t r a t e . C a l c u l a t i o n o f t h i s l a t t e r c a p a c i t a n c e y i e l d e d a v a l u e o f « 18 f F , much s m a l l e r t h a n t h e measured package v a l u e . The p a ckage p l u s ACT d e v i c e o u t p u t c a p a c i t a n c e was q u a n t i f i e d u s i n g an LCR meter w i t h t h e g u a r d r i n g and c h a n n e l p l a t e v o l t a g e s s e t t o t y p i c a l b i a s v a l u e s and t h e b i a s r i n g g r o u n d e d . The measured v a l u e was «0.5 pF, i n d i c a t i n g t h a t t h e o u t p u t node c a p a c i t a n c e was due a l m o s t e n t i r e l y to the p a c k a g e u n d e r n o r m a l b i a s i n g c o n d i t i o n s . T h i s c a p a c i t a n c e d i f f e r s w i t h t h e «4 pF v a l u e q u o t e d by H o s k i n s [45] page 137, w h i c h he a t t r i b u t e d m a i n l y to the d e v i c e b o n d i n g pad. The i n t e r d i g i t a l t r a n s d u c e r was d r i v e n w i t h p u l s e s o f 93 360 MHz a t d r i v e l e v e l s up to 30.5 dBm and t y p i c a l d u t y r a t i o s o f 5-10%. The p u l s e r e s p o n s e s u f f e r e d when a t t e m p t s were made to d r i v e the t r a n s d u c e r i n a c o n t i n u o u s wave manner. T h e r e was a t i m e c o n s t a n t o f s e v e r a l s e c o n d s d u r i n g w h i c h th e o u t p u t p u l s e r i s e and f a l l t i m e s were o b s e r v e d to i n c r e a s e d r a m a t i c a l l y from t h e i r minimum v a l u e s o f 10 ns . I t was s u b s e q u e n t l y f o u n d t h a t h e a t i n g the ACT s u b s t r a t e c a u s e d the r e t u r n l o s s peak o f t h e IDT t o s h i f t down i n f r e q u e n c y . The amount o f t h i s s h i f t c o u l d be c a l c u l a t e d t a k i n g i n t o a c c o u n t the e x p a n s i o n c o e f f i c i e n t s o f the s u b s t r a t e , IDT m e t a l , and t h e v a r i a t i o n o f t h e SAW v e l o c i t y o v e r t e m p e r a t u r e . Webster and C a r r [38] have g i v e n a t e m p e r a t u r e c o e f f i c i e n t o f f r e q u e n c y d r i f t o f -52 p p m / ° C . Whether or n o t t h i s v a l u e i s s e n s i t i v e t o t h e t h e IDT m e t a l c h o s e n i s u n c l e a r to the p r e s e n t a u t h o r . By l o w e r i n g t h e g e n e r a t i o n f r e q u e n c y to a c c o u n t f o r the IDT t e m p e r a t u r e r i s e , i t was p o s s i b l e t o o p e r a t e the d e v i c e a t h i g h e r d u t y r a t i o s . S i n c e no a t t e m p t was made to h e a t s i n k the e x p e r i m e n t a l d e v i c e , c o n t i n u o u s wave (CW) o p e r a t i o n was n o t u s e d . However, CW o p e r a t i o n o f ACT d e v i c e s has been d e m o n s t r a t e d u s i n g RF d r i v e s i g n a l s up to 33 dBm [ 5 5 ] . 4.5 P u l s e Response 4.5.1 E l e c t r i c a l I n p u t A p p l i c a t i o n o f a n e g a t i v e p u l s e to the i n p u t ohmic c o n t a c t i n j e c t s e l e c t r o n s i n t o the SAW p o t e n t i a l w e l l s . F i g u r e 4.7 shows the r e s p o n s e o f the d e v i c e when a 1 V i n p u t 94 100 n s / d i v > Top t r a c e : ACT i n p u t ohmic ( l V / d i v ) Bottom t r a c e : ACT o u t p u t ohmic (= 4.1 p A / d i v ) F i g u r e 4.7: ACT p u l s e r e s p o n s e f o r c o n t a c t i n j e c t i o n 95 p u l s e was a p p l i e d . The 700 ns time d i f f e r e n c e between the i n p u t and output p u l s e agreed w e l l with the v a l u e c a l c u l a t e d u s i n g the known channel l e n g t h of the d e v i c e and the SAW v e l o c i t y . The measured r i s e and f a l l time were both 10 ns. The amount of charge i n j e c t e d , and hence the input impedance of the d e v i c e , was s t r o n g l y a f f e c t e d by the s u b s t r a t e , channel p l a t e and input gate v o l t a g e f o r e l e c t r i c a l charge i n j e c t i o n . T h i s e f f e c t was not observed when o p t i c a l charge i n j e c t i o n experiments were attempted. 4.5.2 O p t i c a l Input U s i n g the f o c u s s i n g scheme shown i n F i g u r e 4.8a o p t i c a l p u l s e s were a p p l i e d to the ACT d e v i c e . The spot was focussed over each of the four t h i n chromium windows shown i n F i g u r e 4.8b to get uniform charge i n j e c t i o n a c r o s s the SAW beamwidth. Pulse response of the ACT device under these c o n d i t i o n s i s shown i n F i g u r e s 4.9a to 4.9d. The delays of 0.61 ns, 0.52 ns , 0.43 ns and 0.34 ns were i n good agreement with v a l u e s c a l c u l a t e d from the window p o s i t i o n s and SAW v e l o c i t y . The r e t e n t i o n of p u l s e shape along the d e v i c e was an i n d i c a t i o n of good charge t r a n s p o r t e f f i c i e n c y . R i s e and f a l l times f o r the o p t i c a l l y induced pulses were ~ 20 ns, double the v a l u e s f o r e l e c t r i c a l i n j e c t i o n . An independent measurement of the o p t i c a l r i s e and f a l l time f o r the AlGaAs LED u s i n g a high speed S i PIN photodiode i n d i c a t e d 96 Biasing Dni-t-ted f o r Clarity Figure 4.8a: O p t i c a l pulse g e n e r a t i o n and f o c u s s i n g I n t e r d i g i t a l T r a n s d u c e r Bias Ring and Guard Ring not shown for c l a r i t y F i g u r e 4.8b: O p t i c a l i n j e c t i o n window p o s i t i o n s 98 gBBgKgSK I B O B B a D O B B 100 n s / d i v > (a) : Over Window #1 100 n s / d i v > (b) : Over Window #2 Top t r a c e : V o l t a g e to LED c i r c u i t B ottom t r a c e : ACT o u t p u t c u r r e n t (« 1.5 p A / d i v ) F i g u r e 4.9: P u l s e r e s p o n s e f o r o p t i c a l c h a r g e i n j e c t i o n 99 100 n s / d i v ( c ) : Over Window # 3 1HK1HI 100 n s / d i v > (d) : Over Window #4 Top t r a c e : V o l t a g e to LED c i r c u i t B ottom t r a c e : ACT o u t p u t c u r r e n t ( ~ 1.5 / i A / d i v ) F i g u r e 4.9: P u l s e r e s p o n s e f o r o p t i c a l c h a r g e i n j e c t i o n 100 a v a l u e o f ~ 10 ns . O t h e r c o n t r i b u t i o n s to the r i s e time i n c l u d e : o u t p u t ohmic s t r a y c a p a c i t a n c e ; f i n i t e b a n d w i d t h o f t h e o u t p u t a m p l i f i e r ; n o n - z e r o c h a r g e t r a n s f e r i n e f f i c i e n c y and l i g h t l e a k a g e i n t o the gap r e g i o n between the g u a r d r i n g and c h a n n e l p l a t e . Any s t r a y l i g h t i n t h e c h a n n e l p l a t e / g u a r d r i n g gap r e g i o n w i l l i n j e c t c a r r i e r s b e yond the l i m i t s d e f i n e d by s e m i - t r a n s p a r e n t windows and d e g r a d e the p u l s e r e s p o n s e . 4.5.3 P u l s e Response v s . SAW M a g n i t u d e and Charge I n j e c t i o n Le ve 1 F i g u r e s 4.10a to 4.10c i l l u s t r a t e t h e o u t p u t p u l s e waveform as the i n p u t RF power to the IDT, and h ence the wave p o t e n t i a l , was i n c r e a s e d f o r o p t i c a l c h a r g e i n j e c t i o n . W i t h low v a l u e s o f SAW p o t e n t i a l , a g r e a t d e a l o f i n j e c t e d c h a r g e was s p i l l e d i n t o t r a i l i n g w e l l s . D i f f u s i o n i n d u c e d c h a r g e t r a n s f e r i n e f f i c i e n c y t h e o r y p r e s e n t e d by H o s k i n s [45, 62] p r e d i c t s an e q u a l amount o f c h a r g e s p i l l a g e i n t o b o t h l e a d i n g and t r a i l i n g SAW w e l l s , r e f e r r e d to as " b a l l o o n i n g " . T h i s s y m m e t r i c e f f e c t was n o t o b s e r v e d e i t h e r i n the p r e s e n t or p r e v i o u s work [ 4 5 ] . H o s k i n s [45] page 123 a t t r i b u t e d a s y m m e t r i c b e h a v i o u r w h i c h he o b s e r v e d t o s m a l l p o t e n t i a l w e l l s c r e a t e d i n 2 fim s u r f a c e gaps p r e s e n t i n the c h a n n e l p l a t e o f h i s d e v i c e s . I n i t i a l l y i t was t h o u g h t t h a t t h e a s y m m e t r i c b e h a v i o u r w i t h t h e p r e s e n t d e v i c e c o u l d be e x p l a i n e d i n terms o f a v o l t a g e drop a c r o s s t h i n chromium windows on the s u r f a c e o f 101 P e l e c " + 1 5 d B m (<f>o ~ 70 mV) (a) 100 n s / d i v > ••••••• • • B 9 B B B P e l e c = +18.5 dBm (<t>Q m 110 mV) (b) 100 n s / d i v - --> P P e l e c " + 2 4 d B m (^ 0 « 2 00 mV) ( c ) 100 n s / d i v > V e r t i c a l : ACT o u t p u t c u r r e n t (-1.5 ^ A / d i v ) ( O p t i c a l c h a r g e i n j e c t i o n ) F i g u r e 4.10: P u l s e r e s p o n s e v a r i a t i o n w i t h SAW d r i v e power 102 the device c r e a t i n g a p o t e n t i a l h i l l over which the SAW would need to push t r a n s p o r t e d charge. Measurement of sheet r e s i s t i v i t y f o r evaporated 25 nm chromium f i l m s y i e l d e d a value of « 740 fl/square. Assuming a form of the SAW p o t e n t i a l given by: rfs(xlft) - tfosindcx! - wt) (4.17) where: ^ Q — magnitude of SAW p o t e n t i a l (assume no X3 dependence) k - a c o u s t i c wavenumber u> — r a d i a n frequency t — time v a r i a b l e A p p l i c a t i o n of Poisson's equation to gi v e n by (4.17) gives the magnitude of the p i e z o e l e c t r i c charge d e n s i t y below: py - * 0 £ k 2 (4.18) Using some t y p i c a l v a l u e s i n (4.18): 4>Q - 0.3 V o l t s 6 - 11.0e o k - 2»r/8 nm y i e l d s : py - 18 C/m3 Since t h i s charge moves at v g = 2860 m/s i n the x^ d i r e c t i o n , the e q u i v a l e n t c u r r e n t i s 5 . 1 x l 0 4 A/mz. Assuming an equal image c u r r e n t i n the channel p l a t e and a 400 pm channel width, the channel p l a t e c u r r e n t I C p i s c a l c u l a t e d as: I c p - 82 pA The v o l t a g e drop across the chromium window i s then 3 mV, much s m a l l e r than the « 300 mV SAW p o t e n t i a l . As a r e s u l t , 103 t h e a s y m m e t r i c c h a r g e t r a n s f e r b e h a v i o u r must have a n o t h e r c a u s e , n o t c u r r e n t l y u n d e r s t o o d by the a u t h o r . F i g u r e s 4.11a to 4.11c d e s c r i b e o u t p u t p u l s e r e s p o n s e as a f u n c t i o n o f s i g n a l l e v e l f o r e l e c t r i c a l c h a r g e i n j e c t i o n . As t h e i n p u t was i n c r e a s e d , c h a r g e s p i l l e d p r e d o m i n a n t l y i n t o t r a i l i n g w e l l s . 4.6 Charge T r a n s f e r E f f i c i e n c y The c h a r g e t r a n s f e r e f f i c i e n c y was e s t i m a t e d by two i n d e p e n d e n t t e c h n i q u e s a s s u m i n g a p r o p o r t i o n a l l o s s mechanism. In t h e f i r s t method, the f r e q u e n c y r e s p o n s e o f the ACT d e v i c e was m easured as RF b u r s t s o f i n c r e a s i n g f r e q u e n c y were a p p l i e d t o t h e d e v i c e i n p u t [ 8 3 ] . F i g u r e 4.12a and 4.12b i l l u s t r a t e d e v i c e r e s p o n s e t o RF b u r s t s o f i n c r e a s i n g f r e q u e n c y a p p l i e d to the d e v i c e . A m p l i t u d e r e s p o n s e was m e a s u r e d and the d a t a c u r v e f i t t e d to the t h e o r e t i c a l r e s p o n s e g i v e n by: | G ( f ) | = e x p ( - N t e t [ l - cos ( 2 7 r f / f 0 ) ] ) (4.19) where: e t = c h a r g e t r a n s f e r i n e f f i c i e n c y f = RF b u r s t f r e q u e n c y N t = number o f e q u i v a l e n t t r a n s f e r s £Q = c l o c k f r e q u e n c y U s i n g f 0 = 360.7 MHz, N t = 250, and et = 0.008, a f i t to t h e e x p e r i m e n t a l d a t a shown i n F i g u r e 4.13 was a c h i e v e d . The c h a r g e t r a n s f e r e f f i c i e n c y d e t e r m i n e d by t h i s method was 0.992. 104 I n p u t Ohmic V o l t a g e (-1 V o l t ) (a) 100 n s / d i v I n p u t Ohmic V o l t a g e (-1.15 V o l t ) (b) 100 n s / d i v i n I n p u t Ohmic V o l t a g e (-1.35 V o l t ) ( c ) 100 n s / d i v > V e r t i c a l : ACT o u t p u t c u r r e n t (« 4.1 / i A / d i v ) ( C o n t a c t i n j e c t i o n ) F i g u r e 4.11: P u l s e shape v a r i a t i o n w i t h i n j e c t i o n l e v e l 105 200 n s / d i v > (a) fBURST = 1 0 M H z 200 n s / d i v > (b) ^BURST = ^2 MHz Top t r a c e : I n p u t ohmic v o l t a g e (0.5 V / d i v ) Bottom T r a c e : O u t p u t ohmic c u r r e n t (~ 1.6 / i A / d i v ) C harge L o a d : 1 . 6 x l 0 6 e l e c t r o n s / c m ( f o r e a c h 8 pm F i g u r e 4.12: ACT RF b u r s t r e s p o n s e 106 F i g u r e 4.13: Charge t r a n s f e r e f f i c i e n c y r e s p o n s e method 107 u s i n g f r e q u e n c y An a l t e r n a t e t i m e domain method [84] i n v o l v e d a p p l i c a t i o n o f a l o n g d u r a t i o n p u l s e t o t h e i n p u t o f the d e v i c e and measurement the r i s e t i m e and f r a c t i o n o f c h a r g e l o s t f r o m t h e l e a d i n g edge o f the o u t p u t p u l s e . F o r s m a l l c h a r g e t r a n s f e r i n e f f i c i e n c y , the e x p r e s s i o n g i v e n below was u s e d [45] : c t = ( t r f 0 / N t ) ( Q L / Q t ) (4.20) where: t r = r i s e time o f the l e a d i n g edge o f t h e o u t p u t p u l s e (QL/Q t) = f r a c t i o n o f c h a r g e l o s t f r o m t h e l e a d i n g edge o f t h e o u t p u t p u l s e f g and N t a r e as p r e v i o u s l y d e f i n e d f o r t h e f r e q u e n c y r e s p o n s e method. C a l c u l a t i o n o f c h a r g e t r a n s f e r e f f i c i e n c y u s i n g t h i s t e c h n i q u e y i e l d e d a v a l u e o f 0.993. The e s t i m a t e s o f c h a r g e t r a n s f e r e f f i c i e n c y o b t a i n e d u s i n g t h e s e methods were l o w e r l i m i t s s i n c e t h e f r e q u e n c y r e s p o n s e o f t h e o u t p u t a m p l i f i e r w i l l have d e g r a d e d the o u t p u t p u l s e r i s e t i m e . B o t h r e s u l t s were c l o s e to t h e v a l u e o f 0.996 o b t a i n e d by H o s k i n s [45] on a s i m i l a r d e v i c e f o r a c h a r g e l o a d o f 3xl0^cm"^. E x p e r i m e n t s were n o t u n d e r t a k e n to d e t e r m i n e how c h a r g e t r a n s p o r t e f f i c i e n c y v a r i e d w i t h c h a r g e l o a d though t h e s e measurements have been r e p o r t e d by H o s k i n s [45] page 146. 108 4.7 Device L i n e a r i t y C h a r a c t e r i s t i c s and Input/Output Impedance L i n e a r i t y of the device output c u r r e n t as a f u n c t i o n of inp u t ohmic v o l t a g e and focussed o p t i c a l power are given i n F i g u r e s 4.14 and 4.15 f o r e l e c t r i c a l and o p t i c a l input s i g n a l s r e s p e c t i v e l y . The maximum output c u r r e n t measured was c o n s t r a i n e d by sprea d i n g of the output p u l s e at high i n p u t l e v e l s i n t o t r a i l i n g charge w e l l s f o r cont a c t i n j e c t i o n . The maximum l i g h t i n t e n s i t y a v a i l a b l e from the AlGaAs LED l i m i t e d the amount of charge which c o u l d be o p t i c a l l y induced. The l i n e a r i t y of the device f o r o p t i c a l i n p u t s i g n a l s was apparent from F i g u r e 4.15 over the range of o p t i c a l power where measurements were made. The sl o p e i n d i c a t e d an o p t i c a l r e s p o n s i v i t y of approximately 5 mA/W. The l i n e a r i t y c h a r a c t e r i s t i c f o r e l e c t r i c a l charge i n j e c t i o n was s i m i l a r to th a t measured by Hoskins [45]. An approximate v a l u e f o r the r e a l p a r t of the device i n p u t impedance i s given by t a k i n g the r e c i p r o c a l of the slope of the l i n e a r i t y c h a r a c t e r i s t i c f o r e l e c t r i c a l i n j e c t i o n . T h i s y i e l d e d a value of » 26 kO, s l i g h t l y higher than the 25 kO value r e p o r t e d by Hoskins. Although not p e r f e c t l y t r u e , the g e n e r a l l y symmetric geometry of the ACT inp u t and output would imply an output impedance of « 26 kO. 109 110 0-1 1 1 1 1— 0 20 40 60 80 Focussed O p t i c a l Power F i g u r e 4.15: ACT output l i n e a r i t y f o r o p t i c a l i n j e c t i o n 111 4.8 Quantum E f f i c i e n c y f o r O p t i c a l S i g n a l I n j e c t i o n The ACT device quantum e f f i c i e n c y f o r near - i n f r a r e d i n c i d e n t l i g h t was d e f i n e d by r e l a t i n g the number of e l e c t r o n s d e t e c t e d at the device output to the number of photons i n c i d e n t on the input window. Using the o p t i c a l l i n e a r i t y graph ( F i g u r e 4.15) output c u r r e n t f o r an i n c i d e n t f o c u s s e d o p t i c a l power of 7 0 fiM was 3.6 nk. The output c u r r e n t of 3.6 i m p l i e d a net g e n e r a t i o n r a t e of 2.27x10*-* e l e c t r o n s / s e c o n d . Using t h i s i n f o r m a t i o n and the LED peak wavelength of em i s s i o n of 730 nm, the quantum e f f i c i e n c y was c a l c u l a t e d u s i n g : I J - ghc/ A P P 0 P T (4.21) where: g - net g e n e r a t i o n r a t e h — Planck's constant c - speed of l i g h t i n vacuum A p - wavelength of LED l i g h t P Q P X ~ o p t i c a l power i n c i d e n t on Cr window The v a l u e o b t a i n e d was rj — 0.0 9 . To determine the device " i n t r i n s i c " quantum e f f i c i e n c y , the o p t i c a l l o s s and r e f l e c t i o n due to the chromium windows was c a l c u l a t e d u s i n g the a n a l y s i s p resented by Schneider [85]. To perform the a n a l y s i s , complex r e f r a c t i v e i n d i c e s of GaAs, Cr, and A i r were r e q u i r e d . Values used were: N - 1 f o r a i r ; N - 1.5-J2.0 f o r chromium [86] and N - 3 . 7 4 - j O . l l l f o r GaAs [61]. The chromium l a y e r t h i c k n e s s was 25 nm. O p t i c a l experiments used a F u j i t s u AlGaAs LED ( P a r t No. FED07 3K1WA) 112 w i t h peak emission wavelength of 730 nm and s p e c t r a l h a l f w i d t h of 25 nm. For the c a l c u l a t i o n , the l i g h t was assumed monochromatic. The c a l c u l a t e d t r a n s m i t t a n c e and r e f l e c t a n c e were 0.37 and 0.46 r e s p e c t i v e l y . D e t a i l s of the c a l c u l a t i o n are given i n Appendix B. Hence, with only » 37% of i n c i d e n t o p t i c a l power r e a c h i n g the GaAs, the r e c a l c u l a t e d quantum e f f i c i e n c y became 0.24. U n f o r t u n a t e l y , the accuracy of t h i s r e s u l t i s debatable s i n c e the o p t i c a l c onstants quoted i n the l i t e r a t u r e f o r chromium va r y w i d e l y . The r e f r a c t i v e index f o r chromium i s s e n s i t i v e t o : r a t e of d e p o s i t i o n of the f i l m ; s u b s t r a t e temperature d u r i n g d e p o s i t i o n ; vacuum p r e s s u r e ; t h i c k n e s s of the f i l m and the frequency of l i g h t [87, 88]. The v a l u e used i n the a n a l y s i s was f o r a chromium f i l m evaporated under s i m i l a r e xperimental c o n d i t i o n s as the p r e s e n t work. The o p t i c a l c onstants f o r chromium were measured at a wavelength of 546 nm. There i s evidence t h a t the o p t i c a l c o n s t a n t s f o r chromium f i l m s i n c r e a s e w i t h frequency i n the 500 nm to 1000 nm wavelength range [88], The magnitude of t h i s i n c r e a s e i s of the order of 10% between 546 nm and 730 nm. Use of l a r g e r v a l u e s f o r the r e f r a c t i v e index of chromium improves the c a l c u l a t e d quantum e f f i c i e n c y . 113 5.0 CONCLUSION An ACT d e v i c e was d e s i g n e d and f a b r i c a t e d . I n t e r f a c e c i r c u i t r y and t e s t f i x t u r e s were c o n s t r u c t e d a l l o w i n g e l e c t r i c a l and o p t i c a l c h a r g e i n j e c t i o n e x p e r i m e n t s to be u n d e r t a k e n . R e s u l t s o b t a i n e d have e s t a b l i s h e d an o p t i c a l c h a r g e i n j e c t i o n t e c h n i q u e o f p o s s i b l e use i n f u t u r e f r o n t s i d e i l l u m i n a t e d ACT i m a g i n g d e v i c e s . E l e c t r o n i n j e c t i o n was a c h i e v e d u s i n g an ohmic c o n t a c t as d e s c r i b e d by H o s k i n s e t a l . [9, 45, 63] . E x p e r i m e n t a l r e s u l t s were i n g e n e r a l agreement w i t h t h e p r e v i o u s work. In p a r t i c u l a r , the ACT c h a r g e t r a n s f e r e f f i c i e n c y f o r s m a l l SAW d u t y r a t i o s (~ 5%) was i n e x c e s s o f 0.992 f o r a c h a r g e l o a d o f 1.6x10^ e l e c t r o n s / c m . T h i s compares t o a v a l u e o f 0.996 q u o t e d by H o s k i n s e t a l . f o r a c h a r g e l o a d o f 4x10^ e l e c t r o n s / c m and SAW p o t e n t i a l o f ~ 0.32 V. The p u l s e shape was f o u n d to be a s t r o n g f u n c t i o n o f : d e v i c e b i a s i n g ; SAW p o t e n t i a l ; i n j e c t i o n l e v e l and the RF d u t y r a t i o o f the t r a n s d u c e r . Minimum p u l s e r i s e and f a l l t i m e s were 10 ns , s i m i l a r t o r e s u l t s o b t a i n e d by H o s k i n s [45] t h o u g h s m a l l e r c h a r g e l o a d s were a c h i e v e d w i t h the p r e s e n t d e v i c e . O p t i c a l c h a r g e i n j e c t i o n t h r o u g h t h i n s e m i - t r a n s p a r e n t windows on the s u r f a c e o f the ACT was d e m o n s t r a t e d f o r n e a r -i n f r a r e d l i g h t (Ap = 730 nm). The i n j e c t i o n p r o c e s s i n v o l v e d t h e c r e a t i o n and s e p a r a t i o n o f e l e c t r o n - h o l e - p a i r s , w i t h s u b s e q u e n t t r a n s p o r t o f the e l e c t r o n s i n p o t e n t i a l w e l l s d e f i n e d by t h e SAW. The p r e s e n c e o f h o l e s i n the c h a n n e l and t h e d ynamics o f t h e i r r e m o v a l r e p r e s e n t e d a p h y s i c a l 114 s i t u a t i o n fundamentally d i f f e r e n t from the case of contact I n j e c t i o n r e p o r t e d i n the p a s t . The net quantum e f f i c i e n c y of the device b u i l t f o r t h i s work was = 9 %. C a l c u l a t i o n s of the r e f l e c t i o n and l o s s due to t h i n chromium windows on a GaAs s u b s t r a t e y i e l d e d an " i n t r i n s i c " quantum e f f i c i e n c y i n excess of 24%. A p r e c i s e d e t e r m i n a t i o n was not p o s s i b l e due to the wide range of o p t i c a l c o nstants quoted i n the l i t e r a t u r e f o r chromium. O p t i c a l p u l s e responses had r i s e and f a l l times of = 20 ns. A p o r t i o n of the r i s e and f a l l times was d i r e c t l y a t t r i b u t e d to the response of the AlGaAs LED used to provide the near - i n f r a r e d l i g h t and leakage of the f o c u s s e d o p t i c a l s i g n a l i n t o the gap between the channel p l a t e and guard r i n g . Since the o p t i c a l p e n e t r a t i o n depth at - 730 nm was approximately the same as the 2.2 pm t h i c k e p i t a x i a l l a y e r , v ery few e l e c t r o n - h o l e - p a i r s were c r e a t e d i n the s u b s t r a t e . Hence, " t a i l s " were not v i s i b l e on the o p t i c a l l y induced output p u l s e as might have been expected from e l e c t r o n t r a p p i n g e f f e c t s i n the semi - i n s u l a t i n g s u b s t r a t e . The pulse response shape and magnitude over the f o u r semi-transparent windows was w e l l p r e s e r v e d . ACT output c u r r e n t f o l l o w e d a l i n e a r r e l a t i o n s h i p with r e s p e c t to f o c u s s e d o p t i c a l power on the s e m i -transparent windows over the o p t i c a l power range measured. S e v e r a l key i s s u e s need to be addressed i n the design of an ACT imaging device based on the i n j e c t i o n technique d e s c r i b e d . The problem of low e x t e r n a l quantum e f f i c i e n c y 115 c o u l d p r e s u m a b l y be overcome u s i n g a more s o p h i s t i c a t e d semi-t r a n s p a r e n t c h a n n e l p l a t e e l e c t r o d e , k e e p i n g th e a c o u s t i c p r o p e r t i e s o f t h e c h o s e n m a t e r i a l i n mind. SAW s c a t t e r i n g , d i f f r a c t i o n , and beam s t e e r i n g would p l a y a p r o m i n e n t r o l e i n d e t e r m i n i n g t h e maximum p r a c t i c a l i m a g i n g a r e a . More work i s n e e d e d t o e s t a b l i s h the b e s t e p i t a x i a l w a f e r growth t e c h n i q u e w h i c h w i l l p r o v i d e t h e l o w e s t SAW a t t e n u a t i o n . Once l a u n c h e d , the SAW p o t e n t i a l w e l l s c a n n o t be s l o w e d down o r s t o p p e d . A method i s t h e r e f o r e r e q u i r e d to i n t e g r a t e enough c h a r g e b e f o r e the a c o u s t i c p i x e l has moved one w a v e l e n g t h . To p r e v e n t d i s t o r t i o n o r s m e a r i n g o f the image, the c h a r g e w e l l s w o u l d need to be s h i e l d e d f r o m a d d i t i o n a l r a d i a t i o n u n t i l c l o c k e d o u t o f the d e v i c e . L a r g e a m p l i t u d e RF p u l s e s (> 24 dBm) were r e q u i r e d to l a u n c h a SAW o f s u f f i c i e n t wave p o t e n t i a l to d r i v e the c h a r g e t r a n s p o r t p r o c e s s . To a v o i d breakdown o f t h e t r a n s d u c e r , c o n t i n u o u s wave o p e r a t i o n was n o t u s e d . More work i s r e q u i r e d to a c h i e v e CW o p e r a t i o n . One p o s s i b l e i d e a f o r improvement w o u l d be t o use a more e f f i c i e n t IDT. A s p l i t -f i n g e r d e s i g n a p p e a r s p r o m i s i n g b a s e d on i t s l o w - r e f l e c t i v i t y p r o p e r t i e s . Use o f a r e f l e c t i v e g r a t i n g t o r e d i r e c t the SAW e n e r g y p r o p a g a t i n g away from the c h a r g e t r a n s p o r t c h a n n e l w o u l d improve the IDT g e n e r a t i o n e f f i c i e n c y by a p p r o x i m a t e l y 3 dB. A s s u m i n g , t h e r e f o r e , t h a t the t o t a l IDT c o n v e r s i o n l o s s c o u l d be r e d u c e d f r om ~ 7 dB to « 4 dB, a 1 V o l t SAW wave p o t e n t i a l w o u l d r e q u i r e an i n p u t RF d r i v e s i g n a l a m p l i t u d e o f 116 ~ 29 dBm f o r a c h a n n e l w i d t h o f 600 pm. The l e v e l o f t h i s s i g n a l p r e s e n t s a c h a l l e n g i n g t h e r m a l d i s s i p a t i o n p r o b l e m f o r the d e v i c e o p e r a t e d i n CW mode. ACT d e v i c e s have been o p e r a t e d w i t h c o n t i n u o u s wave d r i v e s i g n a l s up t o + 33 dBm [55] . D e t a i l s o f the t h e r m a l d e s i g n u s e d to a c h i e v e t h i s d r i v e l e v e l have n o t been p u b l i s h e d . Compared to an ACT d e v i c e , a f o u r - p h a s e CCD w i t h 1 V c l o c k v o l t a g e s has s m a l l e r RF power r e q u i r e m e n t s . To g e n e r a t e a m u l t i - p h a s e c l o c k s i g n a l f o r t h e CCD, one might use a 180 d e g r e e h y b r i d to s p l i t a s i n u s o i d a l s o u r c e i n t o two e q u a l power components f o l l o w e d by two 90 d e g r e e h y b r i d s to g e n e r a t e f o u r p h a s e s . A s s u m i n g a l o s s o f 1 dB f o r each h y b r i d , a p p r o x i m a t e l y 18 dBm o f d r i v e power w o u l d be r e q u i r e d f o r t h i s CCD i n a 50 ohm s y s t e m . C l e a r l y , t h e much h i g h e r RF d r i v e m a g n i t u d e r e q u i r e m e n t s f o r t h e ACT and a s s o c i a t e d t h e r m a l d i s s i p a t i o n p r o b l e m s must be b a l a n c e d a g a i n s t t h e i n t r i n s i c s i m p l i c i t y o f i t s d r i v e c i r c u i t r y . A n a r r o w b a n d RF a m p l i f i e r c a n be u s e d to p r o v i d e a s i n g l e - p h a s e c l o c k s i g n a l f o r the ACT. The CCD w i l l r e q u i r e a d d i t i o n a l c i r c u i t r y f o r the m u l t i - p h a s e d r i v e s i g n a l . B e t t e r c h a r g e t r a n s f e r e f f i c i e n c y i s a c h i e v e d f o r CCD d e v i c e s when t h e y a r e d r i v e n w i t h " s q u a r e " c l o c k s i g n a l s s i n c e t h i s i n c r e a s e s the f r i n g i n g f i e l d s u n d e r t h e g a t e s . O b t a i n i n g s u c h c l o c k waveforms a t VHF o r UHF f r e q u e n c i e s f u r t h e r i n c r e a s e s the c o m p l e x i t y o f the CCD d r i v e c i r c u i t r y o v e r t h a t r e q u i r e d by t h e ACT. The wide c h a n n e l w i d t h (400 yum) o f t h e d e v i c e b u i l t f o r 117 t h i s r e s e a r c h made i t p o s s i b l e f o r the r e l a t i v e l y low SAW p o t e n t i a l o f « 0.3 V o l t s to c a r r y enough c h a r g e p e r w e l l f o r s i m p l e d e t e c t i o n o f t r a n s p o r t e d c h a r g e a t the o u t p u t . As p r e v i o u s l y d i s c u s s e d , wave p o t e n t i a l s i n e x c e s s o f 0.26 V p r o v i d e a p p r o x i m a t e l y l i n e a r i n c r e a s e s i n t h e c h a r g e c a p a c i t y . Hence, an ACT d e v i c e w i t h a 10 /zm c h a n n e l w i d t h and 1.6 V wave p o t e n t i a l c o u l d a c h i e v e t h e same maximum c h a r g e c a p a c i t y as a d e v i c e w i t h a 400 pm c h a n n e l w i d t h and 0.3 V wave p o t e n t i a l . T h i s w o u l d be an i m p o r t a n t c o n s i d e r a t i o n i n the d e s i g n o f a t w o - d i m e n s i o n a l i m a g i n g a r r a y i n w h i c h n a r r o w c h a n n e l w i d t h s a r e d e s i r e d f o r r e s o l u t i o n i n the t r a n s v e r s e d i r e c t i o n . F u t u r e e x p e r i m e n t s w i l l be r e q u i r e d to e s t a b l i s h the f e a s i b i l i t y o f ACT as a d e v i c e c a p a b l e o f r e s o l v i n g i o n i z i n g r a d i a t i o n . D e t e c t i o n o f the s m a l l number o f i n j e c t e d e l e c t r o n s c r e a t e d by a s i n g l e n u c l e a r e v e n t w i l l i n v o l v e r e d e s i g n o f the o u t p u t c i r c u i t r y t o m i n i m i z e n o i s e . F o r t u n a t e l y , t y p i c a l c h a r g e l o a d s w i l l be s m a l l . T h e r e f o r e , t h e r e q u i r e m e n t f o r a s o p h i s t i c a t e d t r a n s d u c e r t o g e n e r a t e a l a r g e a m p l i t u d e SAW i s r e d u c e d . D e v i c e d e g r a d a t i o n due to l o n g - t e r m r a d i a t i o n e x p o s u r e and t r a n s i e n t r a d i a t i o n e f f e c t s d i s c u s s e d b r i e f l y i n c h a p t e r 1 w i l l be o f c o n s i d e r a b l e i m p o r t a n c e i n t h i s a p p l i c a t i o n . 118 REFERENCES 1. R. D. B e l l e m and W.C. J e n k i n s , " R a d i a t i o n E f f e c t s on GaAs Charge C o u p l e d D e v i c e s w i t h H i g h R e s i s t i v i t y Gate S t r u c t u r e s , " IEEE T r a n s . N u c l e a r S c i e n c e , v o l . NS-33, no. 4, pp. 1084-1089, Aug. 1986. 2. M. Simons and E.E. K i n g , " L o n g - t e r m R a d i a t i o n T r a n s i e n t s i n GaAs F E T s , " IEEE T r a n s . N u c l e a r S c i e n c e , v o l . NS-26, no. 6, pp. 5080-5086, Dec. 1979. 3. C. Baack, G. E l z e , and G. W a i f , "GaAs M.E.S.F.E.T. : A H i g h - S p e e d O p t i c a l D e t e c t o r , " E l e c t r o n i c s L e t t e r s , v o l . 13, no. 7, p. 193, Mar. 1977. 4. I. Deyhimy, R.C. Eden, R.J. A n d e r s o n , and J . S . H a r r i s , J r . , "A 500-MHz GaAs Charge - C o u p l e d D e v i c e , " A p p l . Phys. L e t t . , v o l . 36, no. 2, pp. 151-153, J a n . 1980. 5. Y.Z. L i u , R.J. A n d e r s o n , I. Deyhimy, and R.A. M i l a n o , "AlGaAs/GaAs H e t e r o j u n c t i o n CCD Imager," G a l l i u m A r s e n i d e and R e l a t e d Compounds [ C o n f . S e r . - I n s t . Phys. No. 5 6 ] , pp. 393-402, 1981. 6. R. S a h a i , R.L. P i e r s o n , J r . , R.J. A n d e r s o n , E.H. M a r t i n , E.A. S o v e r o , and J.A. H i g g o n s , "GaAs CCD's w i t h T r a n s p a r e n t (ITO) G a t e s f o r I maging and O p t i c a l S i g n a l P r o c e s s i n g , " I E E E E l e c t r o n D e v i c e L e t t e r s , v o l . EDL-4, no. 12, pp. 463-464, Dec. 1983. 7. S. A d a c h i , "GaAs, A l A s , and A l ^ G a ^ _ Y . A S : M a t e r i a l P a r a m e t e r s f o r use i n R e s e a r c h and D e v i c e A p p l i c a t i o n s , " J . A p p l . Phys., v o l . 58, no. 3, pp. R1-R29, Aug. 1985. 8. J . S a p r i e l , J.C. M i c h e l , J.C. T o l e d a n o , R. V a c h e r , J . K e r v a r e c , and A. R e g r e n y , " L i g h t S c a t t e r i n g from V i b r a t i o n a l Modes i n GaAs - G a ^ . X A 1 J J A S S u p e r l a t t i c e s and R e l a t e d A l l o y s , " P h y s i c a l Review B, v o l . 28, no. 4, pp. 2007-2016, Aug. 1983. 9. M.J. H o s k i n s , H. Morkoc, and B . J . H u n s i n g e r , "Charge T r a n s p o r t by S u r f a c e A c o u s t i c Waves i n GaAs," A p p l . Phys. L e t t . , v o l . 41, no. 4, pp. 332-334, Aug. 1982. 10. K.A. I n g e b r i g t s e n , " L i n e a r and N o n l i n e a r A t t e n u a t i o n o f A c o u s t i c S u r f a c e Waves i n a P i e z o e l e c t r i c C o a t e d w i t h a S e m i c o n d u c t i n g F i l m , " J . A p p l . Phys., v o l . 41, no. 2, pp. 454-459, Feb. 1970. 11. N.I. Meyer and M.H. J o r g e n s e n , " A c o u s t o e l e c t r i c E f f e c t s i n P i e z o e l e c t r i c S e m i c o n d u c t o r s w i t h main Emphasis on CdS and ZnO," F e s t k o r p e r p r o b l e m e X. pp. 21-124, 1970. 119 12. N.A. P a p a n i c o l a o u and H.C. L i n , "Charge T r a n s f e r i n S i l i c o n w i t h S u r f a c e A c o u s t i c Waves," 1976 U l t r a s o n i c s Symp. P r o c , IEEE C a t . #76CH1120-5SU, pp. 201-204, 1976. 13. K. T s u b o u c h i , T. H i g u c h i , M. Nagao , and N. M i k o s h i b a , "Charge T r a n s f e r by S u r f a c e Waves on M o n o l i t h i c MIS S t r u c t u r e , " 1978 U l t r a s o n i c s Symp. P r o c . , I E E E C a t . #78CH1344-1SU, pp. 20-24, 1978. 14. N.A. P a p a n i c o l a o u , H.C. L i n , and H.F. Benz , "A M o n o l i t h i c S u r f a c e A c o u s t i c Wave Charge T r a n s f e r D e v i c e and i t s A p p l i c a t i o n s , " O p t i c a l E n g i n e e r i n g , v o l . 19, no. 4, pp. 587-589, J u l y - A u g . 1980. 15. F. L. A u g u s t i n e , R.J. S c h w a r t z , and R.L. Gunshor, " M o d e l i n g o f Charge T r a n s f e r by S u r f a c e A c o u s t i c Waves i n a M o n o l i t h i c M e t a l / Z n O / S i 0 2 / S i System," IEEE T r a n s . E l e c t r o n D e v i c e s , v o l . ED-29, no. 12, pp. 1876-1883, Dec. 1982. 16. S. D. Gaalema, R.J. S c h w a r t z , and R.L. Gunshor, " A c o u s t i c S u r f a c e Wave I n t e r a c t i o n Charge - C o u p l e d D e v i c e , " A p p l . Phys. L e t t . , v o l . 29, no. 2, pp. 82-83, J u l y 1976. 17. D. L. Smythe, R.W. R a l s t o n , B.E. B u r k e , and E. S t e r n , "An A c o u s t o e l e c t r i c SAW/CCD D e v i c e , " 1978 U l t r a s o n i c s Symp. P r o c , IEEE C a t . #78CH1344 -1SU, pp. 16 - 19, 1978. 18. I . Deyhimy, J. S . H a r r i s , R.C. Eden, D.D. E d w a l l , S . J . A n d e r s o n , and L.O. B u b u l a c , "GaAs Charge-Coup1ed D e v i c e s , " A p p l . Phys. L e t t . , v o l . 32, no. 6, pp. 383-385, March 1978. 19. M.D. C l a r k , C L . A n d e r s o n , R. A. J u l l e n s , and G.S. Kamath, " P l a n a r S e a l e d - C h a n n e l G a l l i u m A r s e n i d e S c h o t t k y - B a r r i e r Charge - C o u p l e d D e v i c e s , " IEEE T r a n s . E l e c t r o n D e v i c e s , v o l . ED-27, no. 6, pp. 1183-1188, June 1980. 20. I . Deyhimy, R.C. Eden, and J . S . H a r r i s , J r . , "GaAs and R e l a t e d H e t e r o j u n c t i o n Charge - C o u p l e d D e v i c e s , " IEEE T r a n s . E l e c t r o n D e v i c e s , v o l . ED-27, no. 6, pp. 1172-1180, June 1980. 21. U. A b l a s s m e i e r , W. K e l l n e r , H. H e r b s t , and H. Kniepkamp, " T h r e e - P h a s e GaAs S c h o t t k y - B a r r i e r CCD O p e r a t e d up to 100-MHz C l o c k F r e q u e n c y , " IEEE T r a n s . E l e c t r o n D e v i c e s , v o l . ED-27, no. 6, pp. 1181-1183, June 1980. 22. I. Deyhimy, R.J. A n d e r s o n , R.C. Eden, and J . S . H a r r i s , J r . , "Charge - Coup l e d D e v i c e s i n G a l l i u m A r s e n i d e , " I EE P r o c , v o l . 1 2 7 , p t . I , no.5, pp. 278 - 286, O c t . 1980. 120 23. I. Deyhimy, W.A. H i l l , and R.J. A n d e r s o n , " C o n t i n u o u s l y C l o c k e d 1 GHz GaAs CCD," IEEE E l e c t r o n D e v i c e L e t t e r s , v o l . EDL-2, no. 3, pp. 70-72, March 1981. 24. M.J. Cohen, "GaAs Charge C o u p l e d D e v i c e s f o r H i g h Speed S i g n a l P r o c e s s i n g A p p l i c a t i o n s , " 1981 I n t e r n a t i o n a l E l e c t r o n D e v i c e s M e e t i n g T e c h n i c a l D i g e s t , IEEE C a t . #81CHl708-7, pp. 622-625, 1981. 25. J.A. H i g g i n s , R.A. M i l a n o , E.A. S o v e r o , and R. S a h a i , " R e s i s t i v e Gate GaAs Charge C o u p l e d D e v i c e s , " 1982 IEEE GaAs IC Symposium, IEEE C a t . #82CH1764-0, pp. 49-52, 1982. 26. E.A. S o v e r o , R. S a h a i , W.A. H i l l , and J.A. H i g g i n s , "Microwave F r e q u e n c y GaAs Charge - C o u p l e d D e v i c e s , " 1984 I E E E GaAs IC Symposium, IEEE C a t . #84CH2065-1, pp. 101-104, 1984. 27. M.J. H o s k i n s , M.J. Brophy, J.M. D a l l e s a s s e , M.J. M i l l e r , and J.W. P e t e r s o n , " A c o u s t i c Charge T r a n s p o r t P r i n c i p l e s and P e r f o r m a n c e , " 4 0 t h A n n u a l F r e q u e n c y C o n t r o l Symposium, IEEE C a t . #86CH2330-9, pp. 285-291, 1986. 28. R.C. W i l l i a m s o n , "Problems E n c o u n t e r e d i n H i g h - F r e q u e n c y S u r f a c e - W a v e D e v i c e s , " 1974 U l t r a s o n i c s Symp. P r o c . , I E E E C a t . #74CH0896-1SU, pp. 321-328, 1974. 29. M.J. H o s k i n s and B . J . H u n s i n g e r , " M o n o l i t h i c GaAs A c o u s t i c Charge T r a n s p o r t D e v i c e s , " 1982 U l t r a s o n i c s Symp. P r o c , IEEE C a t . #82CH1823-4, pp. 456-460, 1982. 30. C.E. Warren, " N o n d e s t r u c t i v e S e n s i n g i n B u r i e d C h a n n e l T r a v e l l i n g Wave Charge T r a n s f e r D e v i c e s , " C o o r d i n a t e d S c i e n c e L a b o r a t o r y R e p o r t R-1051, U n i v e r s i t y o f I l l i n o i s , U r b a n a , I l l i n o i s , S e p t . 1985. 31. F.W. V o l t m e r , E.P. Ippen, R.M. W h i t e , T.C. Lim, and G.W. F a r n e l l , "Measured and C a l c u l a t e d S u r f a c e Wave V e l o c i t i e s , " P r o c . IEEE, v o l . 56, no. 9, pp. 1634-1635, S e p t . 1968. 32. J . J . C a m p b e l l and W.R. J o n e s , " P r o p a g a t i o n o f P i e z o e l e c t r i c S u r f a c e Waves on C u b i c and H e x a g o n a l C r y s t a l s , " J . A p p l . Phys., v o l . 41, no. 7, pp. 2796-2801, June 1970. 33. M. Bruun, S. L u d v i k , and C.F. Quate , " F i e l d E f f e c t T r a n s i s t o r s on E p i t a x i a l GaAs as T r a n s d u c e r s f o r A c o u s t i c S u r f a c e Waves," A p p l . P hys. L e t t . , v o l . 18, no. 4, pp. 118-120, Feb. 1971. 121 34. J.M. Deacon and J . Heighway, " A c o u s t i c - Surface-Wave P r o p a g a t i o n on G a l l i u m A r s e n i d e , " E l e c t r o n i c s L e t t e r s , v o l . 8, no. 1, pp. 6-7, J a n . 1972. 35. A.O.W. Spie r m a n n , " S t u d i e s o f A c o u s t i c S u r f a c e Waves i n E p i t a x i a l G a l l i u m A r s e n i d e , " P h y s i c s L a b o r a t o r y I I I , T e c h n i c a l U n i v e r s i t y o f Denmark, S e p t . 1974. 36. D. P e n u n u r i and K.M. L a k i n , "Leaky S u r f a c e Wave P r o p a g a t i o n on S i , GaAs, GaP, A I 2 O 3 and Q u a r t z , " 1975 U l t r a s o n i c s Symp. P r o c , IEEE C a t . #75CH0994-4SU, pp. 478-483, 1975. 37. M. Feldmann, J . H e n a f f , and M-A. K i r o v , " SAW and SSBW P r o p a g a t i o n i n G a l l i u m - A r s e n i d e , " 1981 U l t r a s o n i c s Symp. P r o c , IEEE C a t . #81CH1689 - 9, pp. 264-267, 19.81. 38. R.T. Webster and P.H. C a r r , " R a y l e i g h Waves on G a l l i u m A r s e n i d e , " i n E.A. Ash and E.G.S. P a i g e , E d s . , R a y l e i g h Wave T h e o r y and A p p l i c a t i o n s . S p r i n g e r - V e r l a g , New Y o r k , pp. 122-130, 1985. 39. W.D. Hunt, R.L. M i l l e r , and B . J . H u n s i n g e r , "Slowness S u r f a c e Measurements f o r Z e r o and F i v e Degree [ 1 0 0 ] - c u t GaAs," J . A p p l . Phys., v o l . 60, no. 10, pp. 3532 - 3538, Nov. 1986. 40. V.M. R i s t i c , P r i n c i p l e s o f A c o u s t i c D e v i c e s . J o h n W i l e y and Sons, T o r o n t o , p.248, 1983. 41. F.S. H i c k e r n e l l , " Z i n c - O x i d e T h i n - F i l m S urface-Wave T r a n s d u c e r s , " P r o c . I E E E , v o l . 64, no. 5, pp. 631-635, May 1976. 42. C-C. T s e n g and R.M. Whit e , " P r o p a g a t i o n o f P i e z o e l e c t r i c and E l a s t i c S u r f a c e Waves on t h e B a s a l P l a n e o f H e x a g o n a l P i e z o e l e c t r i c C r y s t a l s , " J . A p p l . Phys., v o l . 38, no. 11, pp. 4274-4280, O c t . 1967. 43. J . J . C a m p b e l l and W.R. J o n e s , "A Method f o r E s t i m a t i n g O p t i m a l C r y s t a l C u t s and P r o p a g a t i o n D i r e c t i o n s f o r E x c i t a t i o n o f P i e z o e l e c t r i c S u r f a c e Waves," IEEE T r a n s . S o n i c s and U l t r a s o n i c s , v o l . SU-15, no. 4, pp. 209-217, O c t . 1968. 44. S. D a t t a and B . J . H u n s i n g e r , " A n a l y s i s o f S u r f a c e Waves u s i n g O r t h o g o n a l F u n c t i o n s , " J . A p p l . Phys., v o l . 49, no. 2, pp. 475-479, Feb. 1978. 45. M.J. H o s k i n s , " A c o u s t i c Charge T r a n s p o r t i n G a l l i u m - A r s e n i d e , " Ph.D. D i s s e r t a t i o n , U n i v e r s i t y o f I l l i n o i s , U r b a n a , I l l i n o i s , 1983. 122 46. T.L. Szabo and A . J . S l o b o d n i k , J r . , "The E f f e c t o f D i f f r a c t i o n on the D e s i g n o f A c o u s t i c S u r f a c e Wave D e v i c e s , " IEEE T r a n s . S o n i c s and U l t r a s o n i c s , v o l . SU-20, no. 3, pp. 240-251, J u l y 1973. 47. G.W. F a r n e l l , " E l a s t i c S u r f a c e Waves" i n H. Matthews, Ed., S u r f a c e Wave F i l t e r s D e s i g n . C o n s t r u c t i o n , and Use. J o h n W i l e y and Sons, T o r o n t o , pp.1-53, 1977. 48. A . J . S l o b o d n i k , J r . , " S u r f a c e A c o u s t i c Waves and SAW M a t e r i a l s , " P r o c . I E E E , v o l . 64, no. 5, pp. 581-595, May 1976. 49. C. K o c o t and C.A. S t o l t e , " B a c k g a t i n g i n GaAs MESFET's," IEE E T r a n s . E l e c t r o n D e v i c e s , v o l . ED-29, no. 7, pp. 1059-1064, J u l y 1982. 50. R.F. M i l s o m , M. Redwood, and N.H.C. R e i l l y , "The I n t e r d i g i t a l T r a n s d u c e r " , i n H. Matthews, Ed., S u r f a c e Wave F i l t e r s D e s i g n . C o n s t r u c t i o n , and Use. J o h n W i l e y and Sons, T o r o n t o , pp. 55-108, 1977. 51. S. Takada, H. Hayakawa, T. I s h i g u r o , and N. M i k o s h i b a , " D e p l e t i o n - L a y e r T r a n s d u c t i o n o f S u r f a c e Waves on P i e z o e l e c t r i c S e m i c o n d u c t o r , " J a p a n e s e J . A p p l . Phys., v o l . 11, no. 1, pp. 24-30, J a n . 1972. 52. E.G. Bogus, M.J. H o s k i n s , and B . J . H u n s i n g e r , " C a n c e l i n g S u r f a c e A c o u s t i c Wave R e f l e c t i o n s f r o m M e t a l l i c S t r i p s on GaAs u s i n g a B i m e t a l Geometry," IE E E T r a n s . S o n i c s and U l t r a s o n i c s , v o l . SU-30, no. 5, pp. 317-320, S e p t . 1983. 53. J . J . G a g n e p a i n , " R a y l e i g h Wave R e s o n a t o r s and O s c i l l a t o r s , " i n E.A. Ash and E.G.S. P a i g e , Eds., R a y l e i g h - W a v e T h e o r y and A p p l i c a t i o n . S p r i n g e r - V e r l a g , New Y o r k , pp. 151-172, 1985. 54. T.W. B r i s t o l , W.R. J o n e s , P.B. Snow, and W.R. Smith , " A p p l i c a t i o n s o f Double E l e c t r o d e s i n A c o u s t i c S u r f a c e Wave D e v i c e D e s i g n , " 1972 U l t r a s o n i c s Symp. P r o c . , I E E E C a t . #72CH0708-SU, 1972. 55. M.J. H o s k i n s , P r i v a t e C o mmunication, 1986. 56. R i s t i c , P r i n c i p l e s o f A c o u s t i c D e v i c e s , p. 255. 57. G. W e i n r e i c h , " U l t r a s o n i c A t t e n u a t i o n by F r e e C a r r i e r s i n Germanium," P h y s i c a l Review, v o l . 107, no. 1, pp. 317-318, J u l y 1957. 123 M. H o s k i n s , F. F l i e g e l , S. Mahon, S. D a t t a , and B . J . H u n s i n g e r , " E l e c t r o n T r a n s p o r t by L i n e A c o u s t i c Waves i n G a As:Cr," 1980 U l t r a s o n i c s Symp. P r o c . , IEEE C a t . #80CH1602-2, pp. 142-147, 1980. R . J . S t r a i n , " P r o p e r t i e s o f Charge - C o u p l e d D e v i c e , " IEEE v o l . ED-19, no. 10, pp. 1119 an I d e a l i z e d T r a v e 1 i n g - W a v e T r a n s . E l e c t r o n D e v i c e s , -1130, O c t . 1972. S.M. Sze, P h y s i c s o f S e m i c o n d u c t o r D e v i c e s . S e c ond E d i t i o n , J o h n W i l e y and Sons, T o r o n t o , 1981. D.E. A s p n e s , " T a b l e o f O p t i c a l F u n c t i o n s o f I n t r i n s i c GaAs: R e f r a c t i v e Index and A b s o r p t i o n C o e f f i c i e n t v s . E n e r g y (0-155 e V ) , " i n P r o p e r t i e s o f G a l l i u m A r s e n i d e . EMIS D a t a r e v i e w s S e r i e s No.2, INSPEC, New Y o r k , 1986. M.J. H o s k i n s and B . J . H u n s i n g e r , " S i m p l e T h e o r y o f B u r i e d C h a n n e l A c o u s t i c Charge T r a n s p o r t i n GaAs," J . A p p l . Phys., v o l . 55, no. 2, pp. 413-426, J a n . 1984. M.J. H o s k i n s , E.G. Bogus, and B . J . H u n s i n g e r , " E x p e r i m e n t a l P e r f o r m a n c e o f the B u r i e d - C h a n n e l A c o u s t i c Charge T r a n s p o r t D e v i c e , " IEEE E l e c t r o n D e v i c e L e t t e r s , v o l . EDL-4, no. 11, pp. 396-399, Nov. 1983. R.L. Van T u y l and C.A. L i e c h t i , " H i g h - S p e e d I n t e g r a t e d L o g i c w i t h GaAs MESFET's," IEEE J . S o l i d - S t a t e C i r c u i t s , v o l . SC-9, no. 5, pp. 269-276, O c t . 1974. D.C. D'Avanzo, " P r o t o n I s o l a t i o n f o r GaAs I n t e g r a t e d C i r c u i t s , " IEEE T r a n s . E l e c t r o n D e v i c e s , v o l . ED-29, no. 7, pp. 1051-1059, J u l y 1982. H.B. Pagge and B.M. Kemlage, " D o p i n g B e h a v i o u r o f S i l i c o n i n Vapour-Grown I I I - V E p i t a x i a l F i l m s , " J . C r y s t a l Growth, v o l . 31, pp. 183-189, 1975. A.Y. Cho, "Growth o f I I I - V S e m i c o n d u c t o r s by M o l e c u l a r Beam E p i t a x y and t h e i r P r o p e r t i e s , " T h i n S o l i d F i l m s , v o l . 100, no. 4, pp. 291-317, Feb. 1983. T. I t o , M. S h i n o h a r a , and Y. Imamura, " O r i g i n o f S u r f a c e D e f e c t s on M o l e c u l a r Beam E p i t a x i a l l y Grown GaAs," J a p a n e s e J . A p p l . Phys., v o l . 23, no. 8, pp. L524-L526, Aug. 1984. A . J . S l o b o d n i k , "GaAs A c o u s t i c - S u r f a c e - W a v e P r o p a g a t i o n L o s s e s a t 1GHz," E l e c t r o n i c s L e t t e r s , v o l . 8, no. 12, pp. 307-309, June 1982. 124 70. B. Tuck, G. A. Adegboyega, P.R. J a y , and M.J. C a r d w e l l , " O u t - D i f f u s i o n o f Chromium from GaAs S u b s t r a t e s , " G a l l i u m A r s e n i d e and R e l a t e d Compounds, [ I n s t . Phys.-C o n f . S e r . , No. 4 5 ] , pp. 114-124, 1979. 71. H.M. Cox and J.V. D i L o r e n z o , " C h a r a c t e r i s t i c s o f an A s C l 3 / G a / H 2 Two-Bubbler GaAs CVD System f o r MESFET A p p l i c a t i o n s , " G a l l i u m A r s e n i d e and R e l a t e d Compounds, [ I n s t . Phys. C o n f . - S e r . , No. 3 3 b ] , pp. 11-22, 1977. 72. P.D. Dapkus, H.M. M a n a s e v i t , and K.L. Hess, " H i g h P u r i t y GaAs P r e p a r e d f r o m T r i m e t h y l g a l l i u m and A r s i n e , " J . C r y s t a l Growth, v o l . 55, pp. 10-23, 1981. 73. R.E. W i l l i a m s , G a l l i u m A r s e n i d e P r o c e s s i n g T e c h n i q u e s . A r t e c h House I n c . , Dedham, Ma., pp. 232-241, 1984. 74. I . Ames, F.M. d ' H e u r l e , and R.E. Horstmann, " R e d u c t i o n o f E l e c t r o m i g r a t i o n i n Aluminum F i l m s by Copper D o p i n g , " IBM J . Res. D e v e l o p . , v o l . 14, no. 4, pp. 461-463, J u l y 1970. 75. E. L e v i n e and J . K i t c h e r , " E l e c t r o m i g r a t i o n I n d u c e d Damage and S t r u c t u r e Change i n C r - A l / C u and A l / C u I n t e r c o n n e c t i o n L i n e s , " 1984 I n t e r n a t i o n a l R e l a b i l i t y P h y s i c s Symposium, IEEE C a t . #84CH1990-1, pp. 242-249, 1984 . 76. W i l l i a m s , G a l l i u m A r s e n i d e P r o c e s s i n g T e c h n i q u e s , p . 286 . 77. H.I. S m i t h , F . J . B a c h n e r , and N. Efremow, "A H i g h - Y i e l d P h o t o l i t h o g r a p h i c T e c h n i q u e f o r S u r f a c e Wave D e v i c e s , " J . E l e c t r o c h e m . S o c , v o l . 118, no. 5, pp. 821-825, May 1971. 78. M. H a t z a k i s , B . J . C a n a v e l l o , and J.M. Shaw, " S i n g l e - S t e p O p t i c a l L i f t - O f f P r o c e s s , " IBM J . Res. D e v e l o p . , v o l . 24, no. 4, pp. 452-460, J u l y 1980. 79. T.H. M i e r s , " S c h o t t k y C o n t a c t F a b r i c a t i o n f o r GaAs MESFET's," J . E l e c t r o c h e m . S o c , v o l . 129, no. 8, pp. 1795-1799, Aug. 1982. 80. G.K. Reeves and H.B. H a r r i s o n , " O b t a i n i n g the S p e c i f i c C o n t a c t R e s i s t a n c e from T r a n s m i s s i o n L i n e Model Measurements," IEEE E l e c t r o n D e v i c e L e t t e r s , v o l . EDL-3, no. 5, pp. 111-113, May 1982. 81. J.A. C o p e l a n d , "A T e c h n i q u e f o r D i r e c t l y P l o t t i n g the I n v e r s e D o p i n g P r o f i l e o f S e m i c o n d u c t o r W a f e r s , " IEEE T r a n s . E l e c t r o n D e v i c e s , v o l . ED-16, no. 5, pp. 445-449, May 1969. 125 82. M.J. H o s k i n s and B . J . H u n s i n g e r , " O p t i c a l P r o b i n g o f HF G u i d e d Wave S u r f a c e D i s p l a c e m e n t s w i t h A r b i t r a r y P r o f i l e , " IEEE T r a n s . S o n i c s and U l t r a s o n i c s , v o l . SU-27, no. 3, pp. 103-111, May 1980. 83. M.F. T o m p s e t t and E . J . Zimany, J r . , "Use o f Charge - Coup l e d D e v i c e s f o r D e l a y i n g A n a l o g S i g n a l s , " I E E E J . S o l i d - S t a t e C i r c u i t s , v o l . SC-8, no. 2, pp. 151-157, A p r i l 1973. 84. R.W. B r o d e r s e n , D.D. Buss, and A l F. T a s c h , J r . , " E x p e r i m e n t a l C h a r a c t e r i z a t i o n o f T r a n s f e r E f f i c i e n c y i n C h a r g e - C o u p l e d D e v i c e s , " I E E E T r a n s . E l e c t r o n D e v i c e s , v o l . ED-22, no. 2, pp. 40-46, Feb. 1975. 85. M.V. S c h n e i d e r , " S c h o t t k y B a r r i e r P h o t o d i o d e s w i t h A n t i r e f l e c t i o n C o a t i n g , " B e l l System T e c h n i c a l J o u r n a l , v o l . 45, no. 9, pp. 1611-1638, Nov. 1966. 86. G. H e n d e r s o n and C. Weaver, " O p t i c a l P r o p e r t i e s o f E v a p o r a t e d F i l m s o f Chromium and C o p p e r " , J . O p t i c a l Soc. o f A m e r i c a , v o l . 56, no. 11, pp. 1551-1559, Nov. 1966. 87. K.B. Scow and R.E. Thun, "A S t u d y o f E v a p o r a t e d Chromium F i l m s , " 1962 T r a n s . N i n t h N a t i o n a l Vacuum Symp., A m e r i c a n Vacuum S o c i e t y , pp. 151-155, 1962. 88. E. I d c z a k , " O p t i c a l P r o p e r t i e s o f Au-Cr and C r - A g Double L a y e r s i n 0 . 4 - l u W a v e l e n g t h Range," i n E. Hahn, Ed., P r o c . I I C o l l o q u i u m on T h i n F i l m s , Vandenhoeck and R u p r e c h t i n G o t t i n g e n , Hungary, pp. 241-248, 1968. 89. J . F . Nye, P h y s i c a l p r o p e r t i e s o f c r y s t a l s T h e i r  r e p r e s e n t a t i o n by t e n s o r s and m a t r i c e s . 1985 e d i t i o n , C l a r e n d o n P r e s s , O x f o r d , 1985. 90. B.A. A u l d , A c o u s t i c F i e l d s and Waves i n S o l i d s . V o l . I, J o h n W i l e y and Sons, T o r o n t o , pp.79,275, 1973. 126 APPENDIX A: C a l c u l a t i o n of the Q u a s i s t a t i c P o t e n t i a l f o r  <110> Propagating SAW on (100) cut GaAs Q u a s i s t a t i c p o t e n t i a l and displacement components have been c a l c u l a t e d f o r <110> p r o p a g a t i n g SAW on (100) cut GaAs u s i n g the method of Tseng and White [42] and Campbell and Jones [43]. The method w i l l be o u t l i n e d below. Relevant tensor equations are given, assuming E i n s t e i n summation over a l l s u b s c r i p t s , and o r i e n t a t i o n shown i n F i g u r e 2.1: E l a s t i c - w a v e equation: d T i j P m d 2 u i dxj d t 2 ( A . l ) where: p m - mass d e n s i t y T j j — s t r e s s tensor u^ - p a r t i c l e displacement i n the i ' t h d i r e c t i o n Xj - space c o o r d i n a t e t - time v a r i a b l e S t r a i n - D i s p l a c e m e n t R e l a t i o n s h i p : 1 j"duk du^" S k i " " + (A-2) 2 l d x x d x k where: S k i - s t r a i n tensor Q u a s i s t a t i c P o t e n t i a l : - d ^ w E k (A.3) d x k where: E k - e l e c t r i c f i e l d ^ w — q u a s i s t a t i c p o t e n t i a l 127 C o n s t i t u t i v e R e l a t i o n s : c i j k l s k l " e k i j E k D i = e i k l s k l + € i k E k (A.4) (A.5) where: c i j k l = s t i f f n e s s c o e f f i c i e n t s e ^ k = p e r m i t t i v i t y c o e f f i c i e n t s e i j k = p i e z o e l e c t r i c c o e f f i c i e n t s = e l e c t r i c d i s p l a c e m e n t Gauss' Law (no u n c o m p e n s a t e d c h a r g e ) : dDL = 0 (A.6) dx^ S u b s t i t u t i o n o f (A.2) and (A.3) i n t o (A.4) and (A.5) y i e l d s = c ? j k l 1 2 d u k d u x + dx- d x k d<f> w + e d x k (A.7) D i e i k l 1 2 dui dxi dui dxi d^w d x k (A. 8) S u b s t i t u t i o n o f (A.7) and (A.8) i n t o ( A . l ) and (A.6) g i v e s c ? . I j k l d 2 U k d 2 u x d x ^ d x j d x k d x j + e d 2^w dx kdx- 'm d 2 u , dt' (A.9) : i k l d 2 u k d 2 u i 2 dx^dx^ d x k d x ^ 6 ? Ik d 2*w d x k d x ^ = 0 (A.10) S i n c e : c ^ j k l = c J J l k (A.11) and: e i j k = e i k j (A.12) e q u a t i o n s (A.9) and (A.10) can be s i m p l i f i e d : I j k l d 2 u x d x k d x j d ^ w d z U 4 + e i j k = P m (A.13) d x k d x j dt' 128 d 2 u 1 i k l 4* d 2 K = o (A.14) A p p l y i n g a b b r e v i a t e d s u b s c r i p t r u l e s [89] e q u a t i o n s (A.13) and (A. 14) c a n be w r i t t e n f o r the c u b i c GaAs c r y s t a l c l a s s a f t e r m o d i f i c a t i o n o f the s t i f f n e s s and p i e z o e l e c t r i c c o n s t a n t s t o a p p l y a l o n g the <110> p r o p a g a t i o n d i r e c t i o n . The m o d i f i c a t i o n i n v o l v e s a r o t a t i o n o f t h e x^ and x 2 axes by 45 d e g r e e s . The m o d i f i e d c o n s t a n t s a r e g i v e n below ( p r i m e d ) i n terms o f the u n r o t a t e d v a l u e s ( u n p r i m e d ) [ 9 0 ] : C ] = c i ! = c l 2 ~ •66 c i i c l 2 c 12 0 0 o • c i l c 12 0 0 0 c 1 2 c 1 2 c 11 o 0 0 0 0 0 c 4 4 0 0 0 0 0 0 c 4 4 0 0 0 0 0 0 c66. ( c l l + c 1 2 + 2 c 4 4 ) 2 ( c n + c 1 2 - 2 c 4 4 ) 2 ( c n - c12> (A.15a) (A.15b) (A.15c) (A.15d) [e'] = 0 0 0 0 0 0 e 1 4 - e 1 4 0 0 e - e 1 4 0 0 0 14 0 0 (A.16) 129 Using equations (A.15) and (A.16), equations (A.13) and (A.14) are r e w r i t t e n i n a b b r e v i a t e d s u b s c r i p t n o t a t i o n where f o r the R a y l e i g h wave s o l u t i o n , a l l terms involving spatial derivatives with respect to have been dropped: o i l d 2 u x dxj + c 5 5 d 2 u x dx2, + <c i 3 + c 5 5 ) d 2 u 3 Pm d 2 u x dt' dx^dx3 (A.17) + 2 e i 5 * 2 K dx^dx3 C66 d 2 u 2 dx£ + c 4 4 d 2 u 2 dx^ 'm d 2 u 2 dt' (A.18) ( c i 3 + c $ 5 ) d 2 u L ^55 d 2 u 3 d 2 u 3 ~ Pin dx^dx3 dxj (A.19) + c 3 3 d 2 u 3 dx§ + e i 5 dt' d^w dxj 2 e i 5 d 2 u x dx^dx3 + e i 5 d 2 u 3 dxf d 2 * w d**w dxj dx2, (A.20) Assuming a p a r t i a l wave s o l u t i o n of the form: • u l ' ui " u2 u 2 u 3 U 3 • w^ . V exp(-akx3) exp(jkx^ - jwt) X3>0 (A.21) 4>w - V 0 e x p ( k x 3 ) e x p ( j k x ! - jwt) x 3<0 where: w - r a d i a n frequency of SAW 130 (A.22) k — w/vg i s the -acoustic wavenumber (A. 23) and: vg — SAW v e l o c i t y S u b s t i t u t i o n o f (A.21) i n t o (A.17)-(A.20) g i v e s a matrix e q u a t i o n i n v o l v i n g U^, U2, U 3 , and V. (c$ 5a2/v§ - c i l / v s + ' m > (cJU^/vg* - C66/ V5 +Pm> j ( c i 3 + c ^ 5 ) f f 0 J 2 e i 5 a j (ci3+c$ 5 ) a / v ( 0 ( c 3 3 a 2 . c 5 5 +/>mvS> - • i s j2ei 5 a / v g 0 -«l5 e( l -a 2 ) , U l U 2 u 3 (A.24) - 0 The U £ s o l u t i o n i s decoupled from the s a g i t t a l plane s o l u t i o n i n v o l v i n g the and u 3 displacement components. The R a y l e i g h wave i s of i n t e r e s t , i n v o l v i n g the co n s t a n t s , U 3, and V. Denoting the matrix c o e f f i c i e n t s i n (A.24) that i n v o l v e , U 3, and V as A, we can w r i t e : U l U 3 V where: - 0 (A.25a) - C i l / V s + P m > i ( c i 3 + c 5 5 W v S J 2 e i 5 a / v g i l / v i + P mj ( c i . 3 + C 5 5 ) < 7 J 2 e i 5 a < c$3",- c55 +P mvg) *15 - • i 5 e(l-<^) J (A.25b) 131 F o r a n o n - t r i v i a l s o l u t i o n , d e t [A] = 0. When the d e t e r m i n a n t a l e q u a t i o n i s expanded, a c u b i c i n r e s u l t s . F o r s o l u t i o n s t h a t d e c a y i n t o the c r y s t a l , t h e r o o t s w i t h p o s i t i v e r e a l p a r t s a r e c h o s e n . The t h r e e r o o t s a r e d e n o t e d <jy, y = l , 2 , 3 . F o r each CTv , y = l , 2 , 3 t h e r e w i l l be an e i g e n v e c t o r n o r m a l i z e d to U]_ t h a t s a t i s f i e s : [A] = 0 1,2,3 (A.26a) where: a1 = U 3 / U x (A.26b) b x = V / U x (A.26c) The s o l u t i o n o f i n t e r e s t c a n be e x p r e s s e d i n terms o f the e i g e n v e c t o r components ( a y , b y ) and w e i g h t i n g f a c t o r s (Wy) t h a t a r e d e t e r m i n e d by the b o u n d a r y c o n d i t i o n s . The form o f th e s o l u t i o n s a r e as below: F o r x 3 > 0: U]_ = £ {Wyexp ( - < 7 y k x 3 ) e x p ( j k x ^ - j w t ) } y-1 (A.27) U3 = £ {ayWyexp(-CTykx 3) e x p ( j k x ^ - j u t ) y - i (A.28) <£w = £ { b y W y e x p ( - c r y k x 3 ) exp(jkx;L - j w t ) } (A.29) y - i F o r x 3 < 0: w^ = V 0 e x p ( k x 3 ) e x p ( j k x x - j w t ) (A.30) 132 The s o l u t i o n f o r a s h o r t e d s u r f a c e i s o f p r i m a r y i n t e r e s t f o r ACT o p e r a t i o n s i n c e the c h a n n e l p l a t e on t h e s u r f a c e o f the d e v i c e p r o v i d e s t h i s c o n d i t i o n . The b o u n d a r y c o n d i t i o n s a t an e l e c t r i c a l l y s h o r t e d s u r f a c e a r e g i v e n by: ^ w ( x 3 = 0) = 0 T 3 1 ( x 3 - 0 ) - 0 T 3 3 ( x 3 = 0) = 0 From (A.31) and (A.29) (A.31) (A.32) (A.33) S b yW y = 0 y-1 From ( A . 2 ) , ( A . 3 ) , and (A.4) (A.34) T 3 1 = c 5 5 d u 3 du^ + dx^ d x 3 + M l dx-and T 3 3 = c 3 1 du^ dx-+ c 3 3 du-dx-(A.36) (A.35) S u b s t i t u t i n g ( A . 2 7 ) - ( A . 2 9 ) i n t o (A.35) and (A.36) and s e t t i n g t h e s e e x p r e s s i o n s e q u a l to z e r o i n a c c o r d a n c e w i t h e q u a t i o n s ( A . 3 2 ) - ( A . 3 3 ) y i e l d s : 2 ( W y [ c 5 5 ( j a y + a y ) + j e 3 1 b y ] } = 0 y - i (A.37) 2 { W y [ j c 3 1 + c $ 3 a Y < 7 Y ] } = 0 (A.38) E q u a t i o n s ( A . 3 4 ) , ( A . 3 7 ) , and (A.38) t o g e t h e r f o r m a m a t r i x e q u a t i o n f o r W\ 133 *1 b 2 b 3 (A.39) c 5 5 ( J a l + a l ) + j e 3 1 b 1 c 5 5 (J a 2 + t 7 2 ) + j e 3 1 b 2 c 5 5 (J a 3 + t 7 3 > + j e 3 1 b 3 w2 - 0 + J c l 3 c 3 3 a 2 a 2 + j c i 3 c 3 3 a 3 a 3 + j c i 3 J . w 3 . E q u a t i o n (A.39) i s o f the form: [D] W2 = 0 (A.40) I w 3 J F o r a n o n - t r i v i a l s o l u t i o n : d e t [ D ] = 0 (A.41) The method o f s o l u t i o n f o l l o w s the s equence below: 1) Assume a v a l u e o f vg 2) C a l c u l a t e < 7 Y s u c h t h a t d e t [ A ] = 0 u s i n g (A.24) 3) C a l c u l a t e a y and by from (A.26) 4) Check i f d e t [ D ] = 0. I f n o t , change vg and r e p e a t s t e p s 1) to 3 ) . Once the SAW v e l o c i t y has been o b t a i n e d by t h i s method, the d i s p l a c e m e n t components and q u a s i s t a t i c p o t e n t i a l can be c a l c u l a t e d f r o m ( A . 2 7 ) - ( A . 2 9 ) a f t e r s o l v i n g f o r W2 and W3 i n terms o f an a r b i t r a r y c o n s t a n t W^  u s i n g (A. 3 9 ) . F o r the n u m e r i c a l c a l c u l a t i o n , W^  = 1 was u s e d . D e n o r m a l i z a t i o n f o r a r b i t r a r y a c o u s t i c power t a k e s p l a c e a t a l a t e r s t a g e i n the c a l c u l a t i o n . The a c o u s t i c power f l o w p e r u n i t a r e a i s g i v e n by P o y n t i n g ' s theorem f o r a c o u s t i c waves u s i n g t h e q u a s i s t a t i c a p p r o x i m a t i o n : 134 P a v g - 1/2 Ret-v'T + * w(ju>D)*] where: v^ -dt (A.42) (A.43) i s the p a r t i c l e v e l o c i t y In the d i r e c t i o n , the power flow i s g i v e n by: P x l - -1/2 w R e [ j ( u } T n + u $ T 3 1 + * W D ? ) ] (A.44) Noting t h a t and: du^ d u 3 d ^ w T l l ~ c i l + c i 3 + e31 dx^ dx 3 dx3 D l ~ e l 5 du^ du3 dX3 dx^ d<f> w dx^ (A.45) (A.46) A f t e r s u b s t i t u t i o n of (A.27)-(A.29), (A.35), (A.45), and (A.46) i n t o (A.44) and i n t e g r a t i o n from X3 - 0 to X 3 - «o y i e l d s the e x p r e s s i o n f o r the a c o u s t i c power flow i n u n i t s of Power per u n i t A c o u s t i c Beamwidth: P A I - Re S — — | W y | ^ ( j c i 1 + c i 3 a y a y + e ^ 1 a y b y-1 4Re(a y>  J J J J J +ay{ JC5 5 a y+c^ 5a y}+b y{ej[ 5a y+j eb*} ) (A.47) 135 APPENDIX B: T r a n s m i s s i o n o f L i g h t t h r o u g h Chromium onto GaAs To e v a l u a t e the r e f l e c t i o n and l o s s due to the chromium "windows" on the ACT, t h e method p r e s e n t e d by S c h n e i d e r [85] has b een u s e d . F i g u r e B . l shows the s u b s t r a t e c o n f i g u r a t i o n u n d e r a n a l y s i s . I n c i d e n t l i g h t on t h e chromium window i s p a r t i a l l y r e f l e c t e d (component r 2 ) and p a r t i a l l y t r a n s m i t t e d t o t h e GaAs (component t 2 ) . G e n e r a l i z e d i m m i t t a n c e s f o r each ( n o n - m a g n e t i c ) l a y e r a r e g i v e n by: g k = N k c o s 7 k ( B . l ) k = 0,1,2 f o r a "p" (TE) wave cos 7 k g k (B.2) k = 0,1,2 f o r an " s " (TM) wave To s i m p l i f y t h e a n a l y s i s , the c a s e o f n o r m a l i n c i d e n c e w i l l be c o n s i d e r e d . I n a c t u a l f a c t , t h e l e n s f o c u s s y s t e m u s e d to p r o v i d e the l i g h t f o r the o p t i c a l e x p e r i m e n t s gave 00<\IQ\<80 . W i t h n o r m a l i n c i d e n c e ( 7 Q = 1\ = 7 2 = u ) > a n d t h e e x p r e s s i o n s f o r t r a n s m i t t a n c e and r e f l e c t a n c e a r e i d e n t i c a l f o r TE and TM i n c i d e n t waves. Hence, o n l y the TE s o l u t i o n w i l l be c o n s i d e r e d . P e r t i n a n t e q u a t i o n s u s e d i n the a n a l y s i s a r e g i v e n below: Phase c o n s t a n t f o r chromium f i l m : j 2 7 r N i c o s 7 i PI - (B.3) A P where: A D = f r e e s p a c e o p t i c a l w a v e l e n g t h 136 d = 25 nm Inc i d e n t L i g h t Medium 2: P A i r d N 2 = 1 Medium l : Chromium >2 ! I i2 Medium 0: GaAs N 0 = n 0 " j k 0 0 ^ F i g u r e B . l : O p t i c a l model o f t h i n chromium window on GaAs u s e d i n c a l c u l a t i o n o f quantum e f f i c i e n c y 137 R e f l e c t i o n c o e f f i c i e n t f o r air-chromium i n t e r f a c e ( g 2 " g l ) ( g l + g o ) e x P ( ^ l d ) + ( g 2 + S l ) ( g l - g o ) e x P ( _ ^ l d ) r 2 (B.4) ( g 2 + g l ) < g l + g o ) e x P ^ l d ) + (g2"gl> (gl-go) e xP("A»l d) R e f l e c t a n c e of air-chromium i n t e r f a c e R - r 2 r 2 (B.5) T r a n s m i s s i o n c o e f f i c i e n t f o r chromium-GaAs i n t e r f a c e : 4 g l g 2 t 2 - (B .6) (gl+g2> ( g l + g o ) e x P ^ l d > + <g2 _gl) (gl-gO> e xP<-Pl d> Transmittance of chromium-GaAs i n t e r f a c e : I go I T t 2 t 5 (B.7) F r a c t i o n of i n c i d e n t o p t i c a l power l o s t i n the chromium f i l m : L C r - 1 - R - T (B.8) 138 

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
http://iiif.library.ubc.ca/presentation/dsp.831.1-0097092/manifest

Comment

Related Items