INCREASING PUMPING POWER
Part 11 Part 11
PHOTOCONDUCTIVE
TNPUI-SERESPONSE AND EXCESS CARRIER LIFETIME
O F Cr-DOPED
GaAsChapter 6 GENERAL INTRODUCTION 6.0 I n t r o d u c t i o n
Since i t was f i r s t found t h a t h i g h r e s i s t i v i t y GaAs ( p % 10 R-cm) 8 c o u l d be c o n s i s t e n t l y prepared by doping i t w i t h chromium [I], semi- i n s u l a t i n g Cr:GaAs has been us..d i n a v a r i e t y o f a p p l i c a t i o n s . I t i s w i d e l y employed i n e p i t a x i a l growth as a h i g h r e s i s t i v i t y s u b s t r a t e and,
due t o i t s r e l a t i v e l y h i g h m o b i l i t y , i t i s used i n microwave devices above the x-band frequencies [Z]. Recently, i t was suggested as a mate- r i a l s u i t a b l e f o r f a s t photodetectors [33 and h i g h speed o p t o e l e c t r o n i c swi tches [4].
C r y s t a l s o f Cr:GaAs have been t h e s u b j e c t o f i n t e n s i v e i nves ti gations d u r i n g the l a s t f i f t e e n years. M a t e r i a l growth has been improved t o
y i e l d h i g h r e s i s t i v i t y s i n g l e c r y s t a l s . However, n o t o n l y i s t h e impur- i t y c o n c e n t r a t i o n d i f f e r e n t from one growth t o another, b u t t h e d i s t r i b u - t i o n i s n o t u n i f o r m w i t h i n t h e same i n g o t . T h i s e x p l a i n s the wide range o f r e s u l t s from p h o t o e l e c t r o n i c measurements performed on t h i s m a t e r i a l ([36-401 and many o t h e r s ) .
The s u b j e c t of P a r t I 1 o f t l ~ i s t h e s i s i s t h e photoconductive i m - p u l s e response and excess c a r r i e r l i f e t i m e o f C r : GaAs. The p h o t o c a r r i e r 1 i f e t i me i n C r : GaAs has been s t u d i e d by photoconducti v i t y measurements w i t h b o t h continuous and p u l s e d i 1 lutnination, Under steady s t a t e condi
-
t i o n s and w i t h photon energy n e a r o r below t h e a b s o r p t i o n edge, c a r r i e r l i f e t i m e s o f 250 psec [53] t o 0.1 psec 1591 have been measured. I n con- t r a s t , when i l l u m i n a t e d w i t h an u l t r a s h o r t l a s e r p u l s e ( % 30 psec) and
photon energy much h i g h e r t h a n t h e band gap energy, l i f e t i r e s s h o r t e r than 100 psec have been r e p o r t e d [3,4]. The s h o r t c a r r i e r l i f e t i m e i n Cr:GaAs was a t t r i b u t e d [ 4 ,531 t o r e c o m b i n a t i o n c e n t e r s formed n e a r t h e GaAs @id-gap by t h e C r dopant. However, p r i o r t o t h e work p r e s e n t e d i n t h i s t h e s i s , no a r l a l y t i c a l c a l c u l a t i o n s o f t h e process o f recombi na- t i o n t h r o u g h f l a w s i n Cr:GaAs have been p u b l i s h e d , and t h e i n c o n s i s t e n c y between t h e CW and t h e p u l s e d measured l i f e t i m e s has n o t been s o l v e d .
I n o u r experiment, t h e p h o t o c o n d u c t i v e impulse response o f C r : GaAs was s t u d i e d b y i r r a d i a t i n g t h e m a t e r i a l w i t h a c o n t i n u o u s t r a i n o f p i c o - secorld 1 i g h t p u l s e s w i t l ~ photon e n e r g above t h e hand-gap energy. The p u l s e s were g e n e r a t e d b y a mode l o c k e d CW dye l a s e r , d e s c r i b e d i n t h e f i r s t p a r t o f t h i s t h e s i s . D i s t o r t i o n o f t h e p h o t o c u r r e n t [iulses by d i s - p e r s i o n o r r e f l e c t i o n s were m i n i m i z e d so t h a t they d i d n o t a f f e c t t h e o b s e r v a t i o n . A p h o t o c o n d u c t i v e decay t i m e o f 1 ~ 6 7 psec was deduced f r o m t h e e x p e r i m e n t a l r e s u l t s , and was i n t e r p r e t e d as t h e r e s u l t o f b o t h b u l k and s u r f a c e r e c o m b i n a t i o n . I t was shown a n a l y t i c a l l y t h a t t h e 6 7 psec decay t i m e agreed w e l l w i t h t h e c a r r i e r l i f e t i m e measured under s t e a d y s t a t e c o n d i t i o n s .
6.1 O u t l i n e o f P a r t I 1
The p h o t o c o n d u c t i v i t y e x p e r i m e n t and i t s r e s u l t s a r e d e s c r i b e d i n Chapter 7. The e f f e c t s o f t h e e l e c t r i c c i r c u i t and t h e d e t e c t i o n system on t h e observed c u r r e n t irnpulse a r e d i s c c r s ~ e d and a n a l y z e d i n Chapter 8, and t h e p h o t o c o n d u c t i v e i m p u l s e response o f t h e samples i s deduced f r o m i t . I n Chapter 9, t h e p h o t o c a r r i e r l i f e t i m e o f Cr:GaAs i s d i s c u s s e d ,
based on t h e e l e c t r i c and chemical p r o p e r t i e s o f t h e m a t e r i a l . The
experimental results are compared t o analytical calculations which take i n t o account both deep impurity levels and sirrface s t a t e s .
Chapter 7 THE EXPERIMENT 7.0 I n t r o d u c t i o n
I n t h i s c h a p t e r o u r e x p e r i n ~ e n t a l set-up used t o s t u d y t h e
photoconducti ve impulse response o f Cr:GaAs w i 11 be described, and the observed p l i o t o c u r r e n t impulse w i l l be presented.
7.1 The E x ~ e r i m e n t a l Set-Ua
The e x p e r i m e n t a l set-up i s shown i n F i g u r e 7.1.1.
I t
resembles a t y p i c a l arrangement f o r photoconducti v i t y rneasurernen t s i n whi ch t h e i n v e s - t i g a t e d m a t e r i a l i s connected i n s e r i e s t o t h e e l e c t r i c c i r c u i t and t h e c u r r e n t i s measured as a f u n c t i o n o f m a t e r i a l i l l u m i n a t i o n . I n o u r exper- iment, the s e m i - i n s u l a t i n g ( 5 1 ) GaAs sample b r i d g e d a gap i n t h e conductive s t r i p o f a m i c r o s t r i p t r a n s m i s s i o n l i n e . The m i c r o s t r i p was c o a x i a l l y connected on one s i d e t o a dc power supply, and on t h e other, d i r e c t l y t o a sampling o s c i 1 loscope. The o p t i c a l picosecond pulses were generated by t h e p a s s i v e l y mode l o c k e d CW dye l a s e r described i n P a r t I of t h i st h e s i s . The pulses were o f wavelength h = 6100-61501, a d u r a t i o n o f l e s s than 2 ps, peak power o f about 1 kW, and a r e p e t i t i o n r a t e o f 4.4 x 10 pps. 5
Due t o t h e h i g h r e s i s t a n c e o f t h e s e m i - i n s u l a t i n g GaAs ($ 10 9 Q), o n l y a weak dark c u r r e n t flowed i n t h e c i r c u i t . When t h e o p t i c a l p u l s e was i n c i d e n t on t h e C r : GaAs, e l e c t r o n - h o l e pai
rs
were generated and t h e conduc- tance o f t h e gap increased. As a r e s u l t the l i n e was d-i scharged through i t and a c u r r e n t impulse was produced. Mhen t h e SI-GaAs was r e s t o r e d t o i t s h i g t ~ r e s i s t i v i t y s t a t e , t h e c u r r e n t r e t u r n e d t o i t s l a w l e v e l . T h i s causede l e c t r i c pulses t o propagate i n t h e l i n e a t the r e p e t i t i o n r a t e o f the o p t i c a l pulses. The c u r r e n t pulses were d e t e c t e d b y t h e sampling o s c i l - loscope. Because o f t h i s s w i t c h - l i ke o p e r a t ion, t h e term "swi t c h i n g u n i t " w i l l be used t o designate t h e element which c o n s i s t s o f t h e m i c r o - s t r i p l i n e w i t h t h e SI-GaAs a t i t s gap, and t h e c o a x i a l connectors a t - tached t o i t .
The o s c i l 1 oscope used i n t h e experiments was T e k t r o n i x 790417S11- 7 T l l w i t h a sampling head S-4 which had a nominal r i s e t i m e of 25 psec.
This o s c i l l o s c o p e had t o be t r i g g e r e d about 90 nsec p r i o r t o d e t e c t i o n . Since a j i t t e r o f more than % I 0 ps would a f f e c t t h e observation, i t was b e s t t o t r i g g e r w i t h t h e same o p t i c a l p u l s e which was b e i n g d e t e c t e d ( t h e p u l s e t r a i n was n o t p e r f e c t l y p e r i o d i c , as e x p l a i n e d i n Chapter 2, so t h a t t r i g g e r i n g by a previous p u l s e would n o t p r e v e n t a j i t t e r ) . I n S e c t i o n 2.6 t h e a c o u s t o o p t i c dumper was described. I t d e f l e c t e d a por- t i o n o f each i n t e r n a l o p t i c a l p u l s e forward, and another p o r t i o n back- ward. The backward p u l s e was used t o t r i g g e r t h e sampling o s c i l l o s c o p e , whi 1e t h e f o r w a r d def 1ected p u l s e passed through a 90 nsec o p t i c a l delay
l i n e ( i n f r e e a i r ) b e f o r e b e i n g focused on t h e sample by a lens of focal l e n g t h f = 10 cm. I t was p o s s i b l e t o change t h e l i g h t s p o t s i z e and t o scan i t across t h e gap, Generally, t h e spot diameter was b i g enough t o i l l u m i n a t e t h e whole gap.
7.2 The Various Types o f S w i t c h i n g
--
U n i t sSeveral methods were employed t o connect the Cr:GaAs sample t o the c i r c u i t. The b e s t temporal r e s o l u t i o n was achieved when t h e semi-
i n s u l a t o r was assembled i n a t r a n s m i s s i o n l i n e , b r i d g i n q a gap i n i t s conductive s t r i p . This package was r e f e r r e d t o i n t h e previous s e c t i o n as t h e " s w i t c h i n g u n i t " (SU). To s t u d y t h e rtependence o f t h e c u r r e n t impulse on t h e e l e c t r i c a l c i r c u i t, s e v e r a l types o f s w i t c h i n g u n i t s have been s t u d i e d . O f each type, s e v e r a l u n i t s w i t h d i f f e r e n t parameters and v a r i a t i o n s i n design have been tested. There have been t h r e e general types o f s w i t c h i n g u n i t s w i t h f a s t response. Two a r e based on t h e m i c r o - s t r i p l i n e and w i 11 be r e f e r r e d t o as types SUA and SUB, w h i l e the t h i r d
i s based on a c o a x i a l t r a n s m i s s i o n l i n e , and w i l l be r e f e r r e d t o as type SUC. For purposes o f bookkeeping, t h e swi t c h f ng u n i t s a r e numbered, f o r i n s t a n c e , SUA-23, SUB-10, e t c . The t h r e e types o f SU's w i l l now he de- s c r i b e d :
( a ) S w i t c h i n g u n i t o f t h e SUA type
The SI-GaAs was used h e r e as t h e d i e l e c t r i c f i l l i n g o f t h e niicro- s t r i p l i n e , as shown i n F i g u r e 7,z.l. T y p i c a l l y , t h e wafers were o f dimension 3 x 1 x0.036 cm, An Au-Ge conductive s t r i p w i t h a gap was pre- pared as w i l l be d e s c r i b e d i n S e c t i o n 7.3. I t s w i d t h was determined so t h a t t h e c h a r a c t e r i s t i c l i n e impedance was 50R, i .e., matched t o t h e i n p u t impedance o f t i l e sampling o s c i I l o s c o p e . The w a f e r was then placed on a copper b l o c k which, i n a d d i t i o n t o s u p p o r t i n g t h e SI-GaAs s u b s t r a t e , served as t h e b o t t o m c o n d u c t i v e p l a n e r e q u i r e d t o complete t h e m i c r o s t r i p s t r u c t u r e . M i n i a t u r e c o a x i a l connectors designed t o o p e r a t e i n t h e range o f dc-18 GHz w i t h maximum v o l t a g e standing-wave r a t i o (VSWR)" o f 1.25 [5a]
*
VSWR 5
I+/PJ
where p i s the r e f l e c t i o n c o e f f i c i e n t . 1-
l p lF i g . 7.2.1 Switching u n i t o f SUA type. The copper block
i s
3 x 1 . 9 x 1 . 3 cm. The gap 4s 400
urn
(typically,lOOvm
gap was used). More d e t a i l s can be found i n the t e x t .
were a t t a c h e d t o t h e copper b l o c k . T h e i r i n n e r conductors, which were 0.010" rods, were s o l d e r e d t o the conductive s t r i p on t h e SI-GaAs wafer.
The impedance o f t h e s w i t c h i n g u n i t was t e s t e d by a T e k t r o n i x time domain r e f l e c t o m e t e r w i t h 25 ps r i s e t i m e (% 5 mm r e s o l u t i o n ) , and was found t o be 50G w i t h i n t h e impedance r e s o l u t i o n o f the TDR (see S e c t i o n 8.1). The gap w i d t h s used i n t h e SUA s w i t c h i n g u n i t s were 70, 100, and 400 vm.
( b ) S w i t c h i n g u n i t o f t h e SUB t y p e
I n t h i s t y p e o f s w i t c h i n g u n i t , shown i n F i g u r e 7.2.2, t h e SI-GaAs was assembled i n a rnicrostr,ip whish had been f a b r i c a t e d from a commerci a1 copper c l a d s u b s t r a t e w i t h a d i e l e c t r i c f i 11 i ng o f p o l y s t y r e n e
[ 6 ] . The d i e l e c t r i c c o n s t a n t o f t h e p o l y s t y r e n e s u b s t r a t e was 2.54 a t 10 GHz, i t s d i s s i p a t i o n f a c t o r 0.005, and i t s t h i c k n e s s 7 9 4 c 100 pm [ 6 ] . F o r a c h a r a c t e r i s t i c impedance o f 50p, i t was found e m p i r i c a l l y t h a t t h e s t r i p w i d t h a s s o c i a t e d w i t h a d i e l e c t r i c thickness o f 794 pm s h o u l d be 0.188 cm (see S e c t i o n 8.1). The copper m i c r o s t r i p was prepared p h o t o l i t h o - g r a p h i c a l l y . P o s i t i v e p h o t o r e s i s t , S h i p l e y 13505 [7] was s p i n coated on one s i d e whi l e t h e o t h e r was coated w i t h a Q - t i p . A f t e r conventi onal exposure t o UV l i g h t through t h e a p p r o p r i a t e mask, and development o f t h e photore- s i s t ( u s i ng Shi p l e y MF312 [7]), t h e exposed copper was etched by a h o t aqueous s o l u t i o n o f f e r r i c c h l o r i d e .
The
t h e rernaini ng p h o t o r e s i s t was r e - moved from b o t h s i d e s . To accommodate t h e SI-GaAs sample, t h e copper s t r i p was f a b r i c a t e d w i t h a gap o f 1.5 mm. The l o c a t i o n o f t h i s gap, r e l a t i v e t ot h e d i s t a n c e f r o m the end connectors, v a r i e d from one s w i t c h i n g u n i t t o another. The samples themselves had pads o f Au-Ge, Au-Zn, o r C r as contacts. The pads, which had been prepared p h o t o l i t h o g r a p h i c a l l y , were
n COPPER
/ Cr: Ga As SAMPLE
( b )
F ? a .
7.2.2 A s s w i t c h i n q u n i t o f type SUB: ( a ) a cross s e c t i o n along the c e n t e r o f the m i c r o s t r i p , ( b ) a s i d e view.F i q . 7 . 2 . 3 Assembly c o n f i a u r a t i o n s of t h e Cr:GaAs sample i n s w i t c h i n g u n i t SUB: ( a ) The exposed, ( b ) the covered, and ( c ) t h e b u r i e d sample.
S I = Cr:GaAs sample, q = s o l d pads ( c o n t a c t s ) , c = copper s t r i p , qp = qround plane, d = d i e l e c t r i c f i l l l n q , and s = s o l d e r o r conductive epoxy.
separated by 100 pm from each o t h e r , and were s o l d e r e d t o t h e copper 1 i ne b r i d g i n g t h e gap i n t h e conducti ve s t r i p . The t h r e e s t r u c t u r e s shown i n F i g u r e 7.2.3 y i e l d e d s i n ~ i l a r r e s u l t s . Therefore, the s w i t c h i n g u n i t s o f t y p e SUB r e p o r t e d i n what f o 1 lows are, w i t h one exception, o f t h e ex- posed type ( F i g u r e 7.2.3a). The mi cros tri p was supported b y a copper o r l u c i t e b l o c k t o which m i n i a t u r e c o a x i a l connectors [5b] were attached. The
s p e c i f i c a t i o n s o f these connectors were s i m i l a r t o those used i n t h e SUA s w i t c h i n g u n i t s . To ensure good c o n t a c t between t h e connectors and t h e ground plane o f t h e m i c r o s t r i p , t h e ends o f t h e l o w e r copper sheet were f o l d e d over t h e copper b l o c k (see F i g u r e 7.2.2b).
( c ) S w i t c h i n g u n i t o f the SUC t y p e
This s w i t c h i n g u n i t f o l l o w e d t h e design o f a f a s t photodiode package l 8 1
.
I t s exploded diagram i s shown i n F i g u r e 7.2.4. The Cr:GaAss w i t c h i n g element was connected a t one s i d e o f t h e gap t o t h e t i p o f a m i n i a t u r e c o a x i a l connector [5c] and a t t h e o t h e r s i d e t o the package [9], 7.3 Sample P r e p a r a t i o n
Wafers o f s e m i - i n s u l a t i n g C r : GaAs were purchased from C r y s t a l Speci a1 ti es
,
I n c .f
101 o r r e c e i ved from the Naval Research L a b o r a t o r i e s[Ill. The wafers had been c u t a l o n g t h e (100) c r y s t a l l o g r a p h i c p l a n e t o a thickness o f about 400
vm
and were mechanically p o l i s h e d . Contacts o f Au-Ge, Au-Zn, o r C r were evaporated on t h e samples. Immediately b e f o r e e v a p o r a t i o n t h e w a f e r had been cleaved t o s i z e and cleaned thoroughly b y acetone, methanol, isopropanol, and HCI o r H2S04 D u r i n g t h e evaporation, which was performed a t 5 x 1 0 - ~ t o r r , l a y e r s o f t h e c o n t a c t m a t e r i a1 2.7000a were deposited. The conductive s t r i p s , i n the case o f SUA, and t h eAluminum Brass
Inner conductor
-
-AluminumTeflon
M i niot ure connector
F i g . 7.2.4 S w i t c h i n g
u n i t
o ftype
SUC.c o n d u c t i v e pads, i n t h e case o f SOB and SUC, were p r e p a r e d p h o t o l i t h o - g r a p h i c a l l y , u s i n g p o s i t i v c p h o t o r e s i s t ( S h i p l e y 13505 [7]) and metal
-
f r e e d e v e l o p e r ( S h i p l e y MF-312 [ 7 ] ) .
lidhen
Au-Ge o r Au-Zn were used as c o n t a c t s , t h e unwanted g o l d was e t c h e d away by a s o l u t i o n o f K I : 12: H20 = 275: 15:250. When C r was used, t h e s t r i p s were p r e p a r e d b y a l i f t - o f f method. The c o n t a c t s were a l l o y e d i n a h y d r o g e n i c environment. The Au-Ge c o n t a c t s were a l l o y e d a t 410°C and t h e Au-Zn a t 500°C, t y p i c a l l y f o r 15 seconds, a1 though l o n g e r a l l o y i n g p e r i o d s were t e s t e d too. The C r con- t a c t s were n o t a l l oyed.A f t e r b e i n g a l l o y e d , t h e Au-Ge showed r e l a t i v e l y h i g h r e s i s t a n c e about 10n p e r rnm l e n g t h (corresponds t o 2 x 1 0 - ~ ~ - c m , w h i c h i s 100 times t h e r e s i s t i v i t y o f p u r e g o l d ) . I t s t h i c k n e s s was measured b y a Sloan Dektak i n s t r u m e n t t o be ~ 7 0 0 0 1 . T h i s i s about 1-1/2 times t h e p e n e t r a t i o n depth o f p u r e g o l d , so t h a t t h e r e l a t i v e l y h i g h r e s i s t i v i t y was a t t r i b u t e d t o t h e i m p u r i t y o f t h e Au-Ge. Consequently, a n o t h e r l a y e r o f p u r e g o l d (99%) was e v a p o r a t e d on t o p o f t h e Au-Ge, i m m e d i a t e l y a f t e r i t s evapora- t i o n . The r e s i s t a n c e o f t h e Au-Ge/Au s t r i p t h e n dropped t o l e s s t h a n 2R/mm.
7.4 E x p e r i m e n t a l R e s u l t s
Two t y p i c a l o s c i l l o s c o p e d i s p l a y s o f t h e response o f t h e e l e c t r i c c i r c u i t t o picosecond 1 ig h t i l l u m i n a t i a n o f t h e Cr:GaAs a r e shown i n F i g u r e 7.4.1. The r i s e t i m e (10-90%) i n b o t h i s about 25 ps, and t h e f a l l t i m e ( l / e o f t h e peak) i s about 75 ps. The " k i n k " a t t h e decay i s a t about 7/10 o f t h e peak.
( b )
F i g . 7.4.1 T y p i c a l o s c i l l o s c o p e d i s p l a y s o f t h e s w i t c h i n g u n i t response
0
t o a picosecond p u l s e i l l u m i n a t i o n ( ~ = 6 1 0 0 A ) of t h e Cr:GaAs.
Fourteen switching units are l i s t e d in Table
I .Various samples of Cr: GaAs have been investigated, "Standard" mans t h e regular Cr: GaAs as purchased from Crystal S p e c i a l t i e s ,
I n c .[lo], while "high doped" in- di cates samples with higher Cr concentration than "standard" ClOb].
More d e t a i l s on t h i s c l a s s i f i c a t i o n will be given i n Chapter
9. "NRL"i s a sample received from the Naval Research Laboratories [ l l ] .
The transmission i s defined here as the r a t i o between the peak voltage displayed on the osc~illoscope to the applied dc voltage. lrlhen marked by "low," the transmission i s below 0.1%. Notice, though, t h a t in studying the photoconductive impulse response the e f f o r t s were channeled tokqard improving the temporal resolution rather than increasl'nq
thetrans- mission. The transmission depends on the level of e x c i t a t i o n , the quality
o f
the contacts,
a n dthe gap length.
The "kink" shown iro Figure 7.4.lb has been observed i n the impulse response of the switching units which are marked by
( x ) .The extinction of the switching unit was measured t o
bea t l e a s t
5 7
1 :
10 (dark dc current:peak photocurrent),
andca1 culated t o
be1: 10 a t
theexcitation levels used in the experiment (see Section
7,1).T a b l e 1
1 / e Fa1
1
Swi t s h S t r i pCr
: GaAs Contact.Gar)
T r a n s - TimeNumber Width Sainp 1e M a t e r i a l ( r t ) m i s s i o n ( p s e c ) Kink SUA-14
SUB-14a SUA- 18 SUA-23 SUA-25 SUB -1 SU6-2 SUB-3 SUB -4 SUB-5 S LIB
-
6SUB-8 d )
coaxi a1
s t a n d a r d s t a n d a r d s t a n d a r d s , t a n d a r d
N RL h i g h doped s t a n d a r d s t a n d a r d s t a n d a r d s t a n d a r d h i g h doped s t a n d a r d s t a n d a r d s t a n d a r d
Au-Zn a
1
ALI
- Ge
a1 era)
Au-Zn a
1
Au
-
Zn a1
Au- Zn a
1
Au- Zn a
1
~ u - ~ n ~
low 150
2% 120
1
% 1001 ow 75
l o w 100 1 % 75
0.3% 75
0.1% 70
a ) ~ o l d e r e d to t h e c o p p e r s t r i p by an i n d a l l o y s o l d e r Indium: 90%, S i l v e r : 10% C12-J.
b ) ~ o n n e c t e d to t h e c o p p e r s t r i p by g o l d epoxy
" ~ o n n e c t e d t o
the
s w i t c h i n g u n i t by s i l v e r epoxyd i ~ a m e a s SUB-5 b u t o f t h e b u r i e d s w i t c h i n g e l e m e n t t y p e ( s e e F i g u r e 7 . 2 . 3 ) .
Chapter 8
DEDUCTION OF THE PHOTOCONDUCTIVE IMPULSE RESPONSE OF Cr:GaAs FROM THE EXPERIMENTAL RESULTS 8.0 I n t r o d u c t i o n
I n Chapter 7 t h e r e s u l t s o f t h e Cr:GaAs p h o t o c o n d u c t i v i t y e x p e r i - ment have been presented, The observed impulse ( F i g u r e 7.4.9) does n o t e x a c t l y correspond t o t h e photoconductive impulse response o f t h e
Cr:GaAs sample which we w i s h t o i n v e s t i g a t e . I t i s , r a t h e r , a convolu- t i o n o f t h e c u r r e n t p u l s e which r e s u l t s from t h e change i n t h e gap con- d u c t i v i t y a f t e r t h e picosecond i l l u m i n a t i o n and t h e impulse response of t h e o s c i l l o s c o p e . To deduce t h e temporal b e h a v i o r o f t h e Cr:GaAs con- d u c t i v i t y f r o m t h e observed impulse, we must f i r s t ensure t h a t t h e c u r r e n t pulses a r e n o t d i s t o r t e d w h i l e p r o p a g a t i n g a l o n g t h e t r a n s m i s s i o n 1 i n e f r o m t h e gap i n t h e s w i t c h i n g u n i t t o t h e o s c i l l i s c o p e . Such d i s t o r t i o n can be a r e s u l t o f r e f l e c t i o n s f r o m impedance d i s c o n t i n u i t i e s and d i s p e r - s i v e broadening o f t h e pulses.
We s t a r t t h i s c h a p t e r by showing t h a t r e f l e c t i o n s and d i s p e r s i v e e f f e c t s have been minimized and do n o t a f f e c t the p r o f i l e o f t h e d e t e c t e d c u r r e n t pulses. F o l l o w i n g t h a t , the o s c i l l o s c o p e d i s p l a y i s deconvolved and t h e p h o t o c u r r e n t pulses a r e analyzed so as t o determine t h e photo- c o n d u c t i v e impulse response o f o u r C r : GaAs samples. A t the end o f the
chapter, t h e observed photocor~derctance o f t h e gap i s compared w i t h i t s t h e o r e t i c a l value, c a l c u l a t e d from b u l k c o n d u c t i v i ly parameters o f C r : GaAs
.
8.1 Impedance Matching o f a M i c r o s t r i p
--
L i n eThe t r a n s m i s s i o n l i n e chosen f o r t h e t r a n s i e n t photoconducti u i t y experiments was the m i c r c s t r i p l i n e , A r i i i c r o s t r i p c o n s i s t s c f a s t r i p conductor and a ground plane, separated by a d i e l e c t r i c medium, as shown i n F i g u r e 8.1.1. I n the f i g u r e , E i s the d i e l e c t r i c c o n s t a n t o f the sub- s t r a t e , h i s the s u b s t r a t e thickness, w i s t h e w i d t h o f t h e s t r i p con- d u c t o r , and t i s i t s thickness. It i s r e l a t i v e l y easy t o f a b r i c a t e , as t h e s u b s t r a t e can be wide and o n l y t h e upper s u r f a c e r e q u i res speci a1 photo1 it h o g r a p h i c ( o r o t i l e r ) treatmen t. Other types o f t r a n s m i s s i o n l i n e which m i g h t have been used a r e t h e s t r i p l i n e ( F i g u r e 8.1.2) and t h e co-
a x i a l l i n e . They a r e more d i f f i c r ~ l t t s f a b r i c a t e as we need access f o r the l i g h t t o i l l u m i n a t e t h e sample a t t h e l i n e gap, w h i l e no s i g n i f i c a n t advantage i s gai ned by u s i n g them. Thei r i s 0 1 a t i o n f r o m n e i g h b o r i n g c i r c u i t s i s b e t t e r than i n a m i c r o s t r i p, b u t t h i s i s
n o t
an i m p o r t a n t f a c t o r i n the p r e s e n t experiment.F i e l d l i n e s o f a m i c r o s t r i g a r e shown i n F i g u r e 8.1.3. A1 though n o t 811 of them a r e e n t i r e l y c o n t a i n e d w i t h i n t h e s u b s t r a t e , and t h e r e - f o r e t h e p r o p a g a t i n g modes a r e n o t p u r e l y TEM, quasi-TEM modes
a r e considered i n a n a l y z i n g