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Photoquenching effect and its consequence in p type gallium arsenide
~
Solid State Communications, V o l . 5 2 , N o . l l , pp.901-904, 1984. P r i n t e d in Great B r i t a i n .
PHOTOQUENCNING
EFFECT
AND
ITS
CONSEQUENCE
IN
0038-1098/84 $3.00 + .00 Pergamon Press Ltd.
p TYPE
GALLIUM
R. E c h e v e r r ~ a , A.B. V i c e n t and N.V. J o s h i C e n t r o de E s t u d i o s A v a n z a d o s en O p t i c a - F a c u l t a d Ciencias Universidad de Los Andes Merida Zona Postal 5101 Venezuela (RECEIVED
ON
MAY
26 th
1984
by
R.C.C.
ARSENIDE
de
LEITE)
In o r d e r to e x a m i n e the o r i g i n of the o c c a s i o n a l l y o b s e r v e d s h i f t in the p h o t o c o n d u c t i v i t y peak with respect to the b a n d gap v a l u e ; o p t i c a l and e l e c t r o - o p t i c a l investig a t i o n of a p type GaAs c r y s t a l was c a r r i e d out. T h e a b s o r p ! ion and p h o t o c h o n d u c t i v i t y s p e c t r a w e r e r e c o r d e d and by c o m p a r i n g b o t h s p e c t r a , a m o d e l b a s e d on the t r a n s i t i o n of e l e c t r o n s fron the o x y g e n " a d s o r b a t e s u r f a c e s t a t e s " is p r o p o s e d to e x p l a i n the s t r o n g p h o t o q u e n c h i n g o b s e r v e d on the h i g h e n e r g y side of the b a n d gap v a l u e . This also e x p l a i n s the o b s e r v e d shift.
is 240 c m 2 / v o l t s - s e c and the e t c h pit d e n s i t y , w h i c h is a m e a s u r e of d i s l o c a t ion c r o s s i n ~ the s u r f a c e , was r e l a t i v e l y low (5 x i03 cm-2) . O h m i c c o n t a c t s to p type GaAs w i t h Zn i m p u r i t y was f o u n d to be a m a j o r impe d i m e n t in r e c o r d i n g the p h o t o c o n d u c t i v i ~ y s p e c t r u m . T h e s e w e r e a c h i e v e d by m e l t i n g p e l l e t s of i n d i u m m e t a l in a r g o n a t m o s p h e r e . In o r d e r to o b t a i n a u n i f o r m c o n t a c t b e t w e e n the l i q u i d i n d i u m and the s e m i c o n d u c t o r , the f o r m e r was pressed w i t h t e f l o n 6. The o b t a i n e d c o n t a c t s w e r e o h m i c and the r e s i s t i v i t y was f o u n d to be 0 . 2 2 d % c m . The p h o t o c o n d u e t i v i t y was measured with conventional phase sensiti ve d e t e c t i o n technique with a chopping f r e q u e n c y of 30 Hz. The r a d i a t i o n i n t e n sity u s e d in this i n v e s t i g a t i o n was k e p t of the o r d e r of a few m i c r o - w a t t s throughout the s p e c t r a l range. The p h o t o c o n d u c t i v i t y spectrum record ed at 300 K is s h o w n in f i g u r e i. As mentioned earlier, the m o s t u n c o m m o n f e a t u r e s of the s p e c t r u m are not the d e t a i l s r e l a t e d w i t h it but the p o s i t i o n i t s e l f . It is n e c e s s a r y , therefore, to k n o w the p r e c i s e v a l u e of the b a n d gap w h i c h can be o b t a i n e d u s i n g the f o l l o w ing e q u a t i o n ,
G a l l i u m A r s e n i d e is an e s t a b l i s h e d d e v i c e m a t e r i a l of s i g n i f i c a n t commercial i m p o r t a n c e for its o p t o - e l e c t r o n i c and microwave a p p l i c a t l o n s I. B e c a u s e of h i g h photoresponse in the i n t r i n s i c r e g i o n z, opto-electrical properties of GaAs in its h i g h l y pure f o r m as w e l l as in d o p e d m a t e r i a l s have b e e n w i d e l y s t u d i e d . An earlier experimental i n v e s t i g a t l o n 3 shows that i m p u r i t y a t o m s c r e a t e d i s o r d e r in a p e r i o d i c p o t e n t i a l and, n a t u r a l l y , the b a n d s t r u c t u r e is p e r t u r b e d . This g i v e s rise to the band tailing effect and causes the s h i f t i n g of the o p t i c a l a b s o r p t i o n and photonconductivity s p e c t r a t o w a r d s the low e n e r g y side w i t h r e s p e c t to the unperturbed b a n d gap p o s l t i o n 3 , 4 . 0 p p o s i t e e f f e c t , n a m e l y , the s h i f t i n g of the a b s o r p t i o n edge t o w a r d s high e n e r g y side is also o b s e r v e d in case of a p type degenerate semiconductor and the d e t a i l s of such b e h a v i o u r has b e e n e x t e n s i v e l y d i s c u s s e d e a r l i e r 3. In a d d i t i o n to this, in o c c a s i o n s , the p h o t o c o n d u c t i v i t y spectrum shows a remarkable peak slightly b e l o w the b a n d gap 5, i n d i c a t i n g that there is a lack of one to one c o r r e s p o n dance with absorption spectrum. Such d a t a is v e r y rare and h e n c e c o m p l e t e s t u d y for the o r i g i n of the p e c u l i a r b e h a v i o u r has not b e e n c a r r i e d out so far. The p u r p o s e of the p r e s e n t i n v e s t i g a t i o n , therefore, is to s t u d y e x p e r i m e n t a l l y the p h o t o conductivity s p e c t r u m in a s i m i l a r s y s t e m and analyze the d a t a in o r d e r to understand the c a u s e for the r e p o r t e d b e h a v i o u r . For this, m o n o c r y s t a l s of p - t y p e GaAs: Zn w e r e f o u n d to be m o r e s u i t a b l e . High purity crystals were obtained from MCP E l e c t r o n i c Materials, U.K.; t h ~ 7 c m carrier concentration w a s 1.2 x i0 The c r y s t a l s w e r e cut and p o l i s h e d in a u s u a l way. The hole m o b i l i t y at 300 K
E = E (0) g g
e T2 T+B
(I)
w h e r e E (T) is the b a n d gap at T K e l v i n s , E (0) i§ the b a n d gap v a l u e at 0 K and e g and B are c o n s t a n t s . For GaAs the v a l u e s of O and B are k n o w n to be 8 . 8 7 1 x i0 and 572 r e s p e c t i v e l y 3. U s i n g the a b o v e m e n t i o n e d v a l u e s , the b a n d gap at 300 K is f o u n d to be 1.43 eV. It can be seen f r o m Fig. i that the
901
902
PHOTOQUENCHING EFFECT IN p TYPE GALLIUM ARSENIDE
1240 120 "~
Wavelength (nm) 1033 886 '
'
775 120
~
(b) 90
90
60 !
6O
Q.
O
,
O
I
12
i
i
14
L
~0
16
Energy (eV) Figure 1 - Comparative study between a b s o r p t i o n and p h o t o c o n d u c t i v i t y properties of G a A s : Z n + at 300 K. a) P h o t o c o n d u c t i v i t y spectrum b) A b s o r p t i o n spectrum B o t h are p r e s e n t e d in r e l a t i v e units.
photoconductivity spectrum shows a peak at 1.28 eV, i.e. it is s h i f t e d on the low e n e r g y side by 150 meV. It is w e l l k n o w n that the p h o t o c o n d u c t i v i t y p e a k is not a l w a y s e x p e c t e d p r e c i s e l y at the band gap v a l u e , but the m a g n i t u d e of the observed difference s u g g e s t s a n o t h e r ori gin. R e d f i e l d et al 5 have r e p o r t e d simi Y far b e h a v i o u r in G a A s , w i t h Si i m p u r i t y , h o w e v e r , in t h e i r case the p e a k was s h i f t e d t o w a r d s the low e n e r g y side by o n l y 40 meV. T h e o r i g i n for this behaviour, unfortunately, is not d i s c u s s e d . The u n c o m m o n f e a t u r e o b s e r v e d in the present investigation is a r e m a r k a b l e photoquenching on the h i g h e n e r g y side of the o b s e r v e d peak. In o r d e r to u n d e r s t a n d the a b o v e m e n t i o n e d e f f e c t s , the o b s e r v e d d a t a w i l l be a n a l y z e d in d e t a i l . On the low e n e r g y side of the p h o t o conductivity p e a k , there is a s h o u l d e r l o c a t e d at 1.18 eV. To c o n f i r m the p o s i t ion of this s h o u l d e r and to i d e n t i f y the transitions originated in the p h o t o c o n d u c t i v i t y p r o c e s s , the a b s o r p t i o n d a t a is u s u a l l y a g r e a t a s s e t and, t h e r e f o r e , s u c h s t u d y for the same s a m p l e was c a r r i e d out. The recorded absorption s p e c t r u m of the s a m p l e is a l s o s h o w n in Fig. I for a comparison purpose. The a b s o r p t i o n s p e c t r u m does c o n f i r m a w e l l m a r k e d p e a k at 1.18 eV. Such a t r a n s i t i o n was r e p o r t e d e a r l i e r in a photoluminescence s t u d y at low t e m p e r a t u re in a Zn d o p e d GaAs c r y s t a l 7 However, the o r i g i n for the e n e r g y s t a t e s r e s p o n s i b l e for this t r a n s i t i o n was not identified, and, as far as we know, it has not b e e n r e p o r t e d in any s u b s e q u e n t ~iterature.
Vol. 52, No.
II
It is c l e a r f r o m Fig. 1 that t h e r e is lack of one to one c o r r e s p o n d a n c e b e t w e e n the a b s o r p t i o n and the p h o t o c o n d u c t i v i t y s p e c t r a . The p h o t o c o n d u c t l v i t y p e a k l o c a t e d at 1.28 eV is not v i s i b l e in the a b s o r p t i o n s p e c t r u m , not even as a structure; i n s t e a d , it shows a m a x i m u m v a l u e at a b o u t 1.45 eV, c o r r e s p o n d i n g to the band gap t r a n s i t i o n . The e x p e r i m e n t a l o b s e r v e d band gap v a l u e is s l i g h t l y lower and this can be e a s i l y u n d e r s t o o d by considering the high d e n s i t y of the a c c e p t o r s t a t e s , w h i c h c a u s e s band tailing. As m e n t i o n e d e a r l i e r , the l o c a t i o n of the m a i n p h o t o c o n d u c t i v l t y peak lies b e l o w the band gap v a l u e and, a c c o r d i n g to our k n o w l e d g e , such b e h a v i o u r has b e e n r e p o r t e d only in p type m a t e r i a l and ther e f o r e , t h e p r e s e n t r e s u l t s are a n a l y z e d t a k i n g this into a c c o u n t . Generally, the r e d u c t i o n of the p h o t o r e s p o n s e on the h i g h e n e r g y side of the band gap is a s c r i b e d to the h a r m f u l s u r f a ce e f f e c t s ( p a r t i c u l a r l y high r e c o m b i n a t ~ ion rate, r a d i a t i v e and non r a d i a t i v e tran s l t i o n s , e t c . ) ; w h i c h are i m p o r t a n t near the s u r f a c e w h e r e the h i g h e r e n e r g y r a d i a t ion is a b s o r b e d . The o b s e r v e d s h a r p dip on the high e n e r g y side s u g g e s t s that it is n e c e s s a r y to get m o r e i n f o r m a t i o n a b o u t the p o s i t i o n of the s u r f a c e s t a t e s , particularly w i t h r e s p e c t to the top of the v a l e n c e band, and to e x a m i n e the role of these s t a t e s in the p h o t o c o n d u c t i o n process. In p type s e m i c o n d u c t o r s , photoconduction takes p l a c e t h r o u g h the h o l e s and the r e d u c t i o n in the p h o t o c u r r e n t i m p l i e s that the d e n s i t y of the h o l e s is reduced considerably; a s s u m i n g that the variation in the m o b i l i t y is not n o t i c e a ble. This is p o s s i b l e only if the r a d i a t ion of e n e r g y 1.37 eV (905 nm) e j e c t s e l e c t r o n s f r o m a deep v a l e n c e level to the top of the v a l e n c e b a n d , c a u s i n g rec o m b i n a t i o n w i t h the h o l e s . P h o t o q u e n c h ing due to the a b o v e p r o c e s s has k e e n r e p o r t e d very r e c e n t l y in C u G a T e 2 v. U n d e r these c i r c u n s t a n c e s , the o b s e r v e d peak in photoconductivity (1.28 eV) is not a p e a k in the true sense, e v e n t h o u g h it looks like it b e c a u s e of the c o n s e q u e n c e of the photoquenching on the high e n e r g y side w h i c h does not p e r m i t to f o l l o w the same trend as the a b s o r p t i o n spectrum. In order to test this p o s s i b i l i t y , the deep s t a t e s of the v a l e n c e band s h o u l d be e x a m i n e d . One of the p o w e r f u l t e c h n i q u e s to s t u d y deep e n e r g y s t a t e s and v a l e n c e band s t r u c t u r e is s u r f a c e s e n s i t i v e p h o t o e m i s sion s p e c t r o s c o p y . Recently, experimental s t u d y for GaAs has b e e n c a r r i e d out u s i n g synchrotron radiation9; it has b e e n found out that core levels of Ga and As are l o c a t e d at a b o u t 20 eV and 41 eV r e s p e c t l vely. S u r f a c e t r e a t m e n t and a d s o r p t i o n of o x y g e n a t o m s do s h i f t the core levels, h o w e v e r , they are s i t u a t e d too deep to take part in the o p t i c a l a b s o r p t i o n process, particulary, in the o b s e r v e d e n e r g y r e g i o n , it has b e e n e x p e r i m e n t a l l y o b s e r v e d that the a d s o r b e d o x y g e n takes
Vol. 52, No.
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PHOTOQUENCHING EFFECT IN p TYPE GALLIUM ARSENIDE
away e l e c t r o n s p r e f e r e n t i a l l y f r o m As atoms 9 (atoms f r o m e l e m e n t s of c o l u m n V in Ill - V c o m p o u n d s ) and this c r e a t e s adsorbate surface states. Naturally, this is an e n t i r e l y n e w s i t u a t i o n as far as d i s t r i b u t i o n of the e n e r g y s t a t e s in the v a l e n c e b a n d are c o n c e r n e d , and this d e m a n d s d e t a i l e d study. D e p e n d i n g u p o n the level of the surface oxidation, c l e a v a g e p l a n e and other surface conditions, the fermi level m a y stay p i n n e d or r e m a i n free 9. If it is p i n n e d , s h a r p e l e c t r o n d i s t r i b u t i o n c u r v e s are o b s e r v e d b e l o w the top of the v a l e n c e band. It m a y also h a p p e n that s u r f a c e r e s o n a n c e s t a t e s a p p e a r in the r e g i o n b e t w e e n I and 2 eV. It is not p o s s i b l e to e s t i m a t e the p r e c i s e p o s i t i o n of the e l e c t r o n d i s t r i b u t i o n c u r v e s (EDC) s i n c e it d e p e n d s r e m a r k a b l y on the s u r f a c e c o n d i t i o n . Earlier experimental e v i d e n c e of the e x i s t a n c e of EDC c o u l d have a d i r e c t consequence on the s t r o n g p h o t o q u e n c h ing o b s e r v e d in p type GaAs. The o b s e r v ed d i s c r e p a n c y can be u n d e r s t o o d w i t h the help of f i g u r e 2.
1.43 eV
< F i g u r e 2 - E n e r g y s c h e m e to i n t e r p r e t the photoquenching e f f e c t at 905 nm. T h e b a n d t a i l i n g p r o d u c e d by z i n c i m p u r i t i e s as w e l l as the d e e p v a l e n c e s t a t e s p r o d u c e d by the a d s o r b e d o x y g e n a t o m s are shown.
903
It is e x p e c t e d that the p h o t o c o n d u c t i v i t y s h o u l d f o l l o w the same t r e n d as the a b s o r p t i o n . H o w e v e r a f t e r 1.3 eV, the a b s o r p t i o n coefficient i n c r e a s e s as e x p e c t e d w h i l e the p h o t o c u r r e n t shows quenching. Electrons f r o m the f i e l d s u r f a c e s t a t e s are e j e c t e d to the top of the v a l e n c e b a n d w h e r e e l e c t r o n hole r e c o m b i n a t i o n takes p l a c e . T h i s produces a photoquenching effect just b e f o r e the b a n d gap. N a t u r a l l y , this does not p e r m i t p h o t o c u r r e n t to f o l l o w the a b s o r p t i o n c u r v e and t h e r e f o r e , the r e g i o n n e a r 1.28 eV a p p e a r s as a peak. Differences between absorption and photoconductivity m a x i m a n a t u r a l l y depend on the p o s i t i o n of the s u r f a c e s t a t e s originated from adsorbed oxygen, which in turn d e p e n d s on the s u r f a c e and i t s preparation, and v a r i e s f r o m s a m p l e to s a m p l e . A d i f f e r e n t ~ a g n i t u d e of s h i f t (40 meV) as r e p o r t e d J e a r l i e r is, t h e r e fore, u n d e r s t a n d a b l e . An e x p e r i m e n t a l confirmation for the e x i s t a n c e of the s u r f a c e s t a t e s in the investigated s a m p l e s c o u l d be o b t a i n e d , in p r i n c i p l e , by o b s e r v i n g thetransitions f r o m t h e s e s t a t e s to the b o t t o m of the c o n d u c t i o n b a n d (~ 2.8 eV). S u c h a t t e m p , t h e r e f o r e , w a s m a d e and the c o r r e s p o n d ing a b s o r p t i o n s p e c t r u m did s h o w a h u m p at 2.8 eV. U n f o r t u n a t e l y , b e c a u s e of the accidental coincidence b e t w e e n this v a l u e and the E. (% 3- %1) gap v a l u e in GaAs 3, it is no~ p o s s i b l e to i d e n t i f y the o r i g i n u n i q u e l y and h e n c e , d i r e c t confirmation c o u l d not be o b t a i n e d . It is, t h e r e f o r e , n e c e s s a r y to rely on earlier experimental observations. It can be seen then, that t h e r e exist a large concentration of l e v e l s (EDC) b e l o w the top of the v a l e n c e b a n d , p r o b a b l y c a u s e d by a d s o r p t i o n of o x y g e n ; and are s i t u a t e d a p r o x i m a t e l y at 1.37 eV b e l o w it. E l e c t r o n s f r o m t h e s e levels are e ~ e c t e d w i t h r a d i a t i o n of 905 nm w a v e l e n g t h and r e c o m b i n e w i t h the e x i s t i n g holes on the top of the v a l e n c e hand, h e n c e , q u e n c h e s the p h o t o c o n d u c t i v i t y just b e f o r e it r e a c h e s its m a x i m u m . The r e l e v a n c e of the a b o v e c i t e d m o d e l is that it does not only e x p l a i n the s t r o n g p h o t o q u e n c h i n g b e l o w the b a n d gap but also e x p l a i n s the a n o m a l o u s p o s i ! ion of the p h o t o c o n d u c t i v i t y peak.
Acknowledgements: A u t h o r s are t h a n k f u l to Dr. R. C a s a n o v a for v a l u a b l e d i s c u s s i o n . T h a n k s are due to C . D . C . H . (U.L.A. M e r i d a ) and C O N I C I T , V e n e z u e l a , for f i n a n c i a l assistance.
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PHOTOQUENCHING EFFECT IN p TYPE GALLIUM ARSENIDE
Vol. 52, No. I I
REFERENCES
i) Boissy M.C., Diguet D., Hallais J. and Schemali C., Gallium Arsenide and Related Compounds; Ed. Hilsum C., Institute of Physics, series 33a, (1976). 2) A m b r o z i a k A., S e m i c o n d u c t o r P h o t o e l e c t r i c Devices, lliffe Books Ltd., London (1968). 3) Pankove J.l., Optical Process in Semiconductors, Dover Pmblication, (1971). 4) Pankove J.l., J. Appl. Phys., 39, 5368, (1968). 5) Redfield D. and Wittke J.P., Proc. of the third Intern. Conf. on Photoconductivity, p. 29, Ed. Pell, Pergamon Press, (1971). 6) Linden K., PhD. Thesis, R.C.A. Laboratory, (1964). 7) Hwang C.I.,J. Appl. Phys., 39, 4307, (1967); Ibid , 4313, (1967); 8) 3oshi N.Y. and E c h e v e r r l a R., Solid State Comm., 47, 251, (1983). 9) Spicer W.E., Lindau I., Gregory P.E., Garner C.M., Pianetta P. and Chye P.W., J. of Vacuum Science & Technology, 13, 780, (1976).
Solid State Communications, V o l . 5 2 , N o . l l , pp.901-904, 1984. P r i n t e d in Great B r i t a i n .
PHOTOQUENCNING
EFFECT
AND
ITS
CONSEQUENCE
IN
0038-1098/84 $3.00 + .00 Pergamon Press Ltd.
p TYPE
GALLIUM
R. E c h e v e r r ~ a , A.B. V i c e n t and N.V. J o s h i C e n t r o de E s t u d i o s A v a n z a d o s en O p t i c a - F a c u l t a d Ciencias Universidad de Los Andes Merida Zona Postal 5101 Venezuela (RECEIVED
ON
MAY
26 th
1984
by
R.C.C.
ARSENIDE
de
LEITE)
In o r d e r to e x a m i n e the o r i g i n of the o c c a s i o n a l l y o b s e r v e d s h i f t in the p h o t o c o n d u c t i v i t y peak with respect to the b a n d gap v a l u e ; o p t i c a l and e l e c t r o - o p t i c a l investig a t i o n of a p type GaAs c r y s t a l was c a r r i e d out. T h e a b s o r p ! ion and p h o t o c h o n d u c t i v i t y s p e c t r a w e r e r e c o r d e d and by c o m p a r i n g b o t h s p e c t r a , a m o d e l b a s e d on the t r a n s i t i o n of e l e c t r o n s fron the o x y g e n " a d s o r b a t e s u r f a c e s t a t e s " is p r o p o s e d to e x p l a i n the s t r o n g p h o t o q u e n c h i n g o b s e r v e d on the h i g h e n e r g y side of the b a n d gap v a l u e . This also e x p l a i n s the o b s e r v e d shift.
is 240 c m 2 / v o l t s - s e c and the e t c h pit d e n s i t y , w h i c h is a m e a s u r e of d i s l o c a t ion c r o s s i n ~ the s u r f a c e , was r e l a t i v e l y low (5 x i03 cm-2) . O h m i c c o n t a c t s to p type GaAs w i t h Zn i m p u r i t y was f o u n d to be a m a j o r impe d i m e n t in r e c o r d i n g the p h o t o c o n d u c t i v i ~ y s p e c t r u m . T h e s e w e r e a c h i e v e d by m e l t i n g p e l l e t s of i n d i u m m e t a l in a r g o n a t m o s p h e r e . In o r d e r to o b t a i n a u n i f o r m c o n t a c t b e t w e e n the l i q u i d i n d i u m and the s e m i c o n d u c t o r , the f o r m e r was pressed w i t h t e f l o n 6. The o b t a i n e d c o n t a c t s w e r e o h m i c and the r e s i s t i v i t y was f o u n d to be 0 . 2 2 d % c m . The p h o t o c o n d u e t i v i t y was measured with conventional phase sensiti ve d e t e c t i o n technique with a chopping f r e q u e n c y of 30 Hz. The r a d i a t i o n i n t e n sity u s e d in this i n v e s t i g a t i o n was k e p t of the o r d e r of a few m i c r o - w a t t s throughout the s p e c t r a l range. The p h o t o c o n d u c t i v i t y spectrum record ed at 300 K is s h o w n in f i g u r e i. As mentioned earlier, the m o s t u n c o m m o n f e a t u r e s of the s p e c t r u m are not the d e t a i l s r e l a t e d w i t h it but the p o s i t i o n i t s e l f . It is n e c e s s a r y , therefore, to k n o w the p r e c i s e v a l u e of the b a n d gap w h i c h can be o b t a i n e d u s i n g the f o l l o w ing e q u a t i o n ,
G a l l i u m A r s e n i d e is an e s t a b l i s h e d d e v i c e m a t e r i a l of s i g n i f i c a n t commercial i m p o r t a n c e for its o p t o - e l e c t r o n i c and microwave a p p l i c a t l o n s I. B e c a u s e of h i g h photoresponse in the i n t r i n s i c r e g i o n z, opto-electrical properties of GaAs in its h i g h l y pure f o r m as w e l l as in d o p e d m a t e r i a l s have b e e n w i d e l y s t u d i e d . An earlier experimental i n v e s t i g a t l o n 3 shows that i m p u r i t y a t o m s c r e a t e d i s o r d e r in a p e r i o d i c p o t e n t i a l and, n a t u r a l l y , the b a n d s t r u c t u r e is p e r t u r b e d . This g i v e s rise to the band tailing effect and causes the s h i f t i n g of the o p t i c a l a b s o r p t i o n and photonconductivity s p e c t r a t o w a r d s the low e n e r g y side w i t h r e s p e c t to the unperturbed b a n d gap p o s l t i o n 3 , 4 . 0 p p o s i t e e f f e c t , n a m e l y , the s h i f t i n g of the a b s o r p t i o n edge t o w a r d s high e n e r g y side is also o b s e r v e d in case of a p type degenerate semiconductor and the d e t a i l s of such b e h a v i o u r has b e e n e x t e n s i v e l y d i s c u s s e d e a r l i e r 3. In a d d i t i o n to this, in o c c a s i o n s , the p h o t o c o n d u c t i v i t y spectrum shows a remarkable peak slightly b e l o w the b a n d gap 5, i n d i c a t i n g that there is a lack of one to one c o r r e s p o n dance with absorption spectrum. Such d a t a is v e r y rare and h e n c e c o m p l e t e s t u d y for the o r i g i n of the p e c u l i a r b e h a v i o u r has not b e e n c a r r i e d out so far. The p u r p o s e of the p r e s e n t i n v e s t i g a t i o n , therefore, is to s t u d y e x p e r i m e n t a l l y the p h o t o conductivity s p e c t r u m in a s i m i l a r s y s t e m and analyze the d a t a in o r d e r to understand the c a u s e for the r e p o r t e d b e h a v i o u r . For this, m o n o c r y s t a l s of p - t y p e GaAs: Zn w e r e f o u n d to be m o r e s u i t a b l e . High purity crystals were obtained from MCP E l e c t r o n i c Materials, U.K.; t h ~ 7 c m carrier concentration w a s 1.2 x i0 The c r y s t a l s w e r e cut and p o l i s h e d in a u s u a l way. The hole m o b i l i t y at 300 K
E = E (0) g g
e T2 T+B
(I)
w h e r e E (T) is the b a n d gap at T K e l v i n s , E (0) i§ the b a n d gap v a l u e at 0 K and e g and B are c o n s t a n t s . For GaAs the v a l u e s of O and B are k n o w n to be 8 . 8 7 1 x i0 and 572 r e s p e c t i v e l y 3. U s i n g the a b o v e m e n t i o n e d v a l u e s , the b a n d gap at 300 K is f o u n d to be 1.43 eV. It can be seen f r o m Fig. i that the
901
902
PHOTOQUENCHING EFFECT IN p TYPE GALLIUM ARSENIDE
1240 120 "~
Wavelength (nm) 1033 886 '
'
775 120
~
(b) 90
90
60 !
6O
Q.
O
,
O
I
12
i
i
14
L
~0
16
Energy (eV) Figure 1 - Comparative study between a b s o r p t i o n and p h o t o c o n d u c t i v i t y properties of G a A s : Z n + at 300 K. a) P h o t o c o n d u c t i v i t y spectrum b) A b s o r p t i o n spectrum B o t h are p r e s e n t e d in r e l a t i v e units.
photoconductivity spectrum shows a peak at 1.28 eV, i.e. it is s h i f t e d on the low e n e r g y side by 150 meV. It is w e l l k n o w n that the p h o t o c o n d u c t i v i t y p e a k is not a l w a y s e x p e c t e d p r e c i s e l y at the band gap v a l u e , but the m a g n i t u d e of the observed difference s u g g e s t s a n o t h e r ori gin. R e d f i e l d et al 5 have r e p o r t e d simi Y far b e h a v i o u r in G a A s , w i t h Si i m p u r i t y , h o w e v e r , in t h e i r case the p e a k was s h i f t e d t o w a r d s the low e n e r g y side by o n l y 40 meV. T h e o r i g i n for this behaviour, unfortunately, is not d i s c u s s e d . The u n c o m m o n f e a t u r e o b s e r v e d in the present investigation is a r e m a r k a b l e photoquenching on the h i g h e n e r g y side of the o b s e r v e d peak. In o r d e r to u n d e r s t a n d the a b o v e m e n t i o n e d e f f e c t s , the o b s e r v e d d a t a w i l l be a n a l y z e d in d e t a i l . On the low e n e r g y side of the p h o t o conductivity p e a k , there is a s h o u l d e r l o c a t e d at 1.18 eV. To c o n f i r m the p o s i t ion of this s h o u l d e r and to i d e n t i f y the transitions originated in the p h o t o c o n d u c t i v i t y p r o c e s s , the a b s o r p t i o n d a t a is u s u a l l y a g r e a t a s s e t and, t h e r e f o r e , s u c h s t u d y for the same s a m p l e was c a r r i e d out. The recorded absorption s p e c t r u m of the s a m p l e is a l s o s h o w n in Fig. I for a comparison purpose. The a b s o r p t i o n s p e c t r u m does c o n f i r m a w e l l m a r k e d p e a k at 1.18 eV. Such a t r a n s i t i o n was r e p o r t e d e a r l i e r in a photoluminescence s t u d y at low t e m p e r a t u re in a Zn d o p e d GaAs c r y s t a l 7 However, the o r i g i n for the e n e r g y s t a t e s r e s p o n s i b l e for this t r a n s i t i o n was not identified, and, as far as we know, it has not b e e n r e p o r t e d in any s u b s e q u e n t ~iterature.
Vol. 52, No.
II
It is c l e a r f r o m Fig. 1 that t h e r e is lack of one to one c o r r e s p o n d a n c e b e t w e e n the a b s o r p t i o n and the p h o t o c o n d u c t i v i t y s p e c t r a . The p h o t o c o n d u c t l v i t y p e a k l o c a t e d at 1.28 eV is not v i s i b l e in the a b s o r p t i o n s p e c t r u m , not even as a structure; i n s t e a d , it shows a m a x i m u m v a l u e at a b o u t 1.45 eV, c o r r e s p o n d i n g to the band gap t r a n s i t i o n . The e x p e r i m e n t a l o b s e r v e d band gap v a l u e is s l i g h t l y lower and this can be e a s i l y u n d e r s t o o d by considering the high d e n s i t y of the a c c e p t o r s t a t e s , w h i c h c a u s e s band tailing. As m e n t i o n e d e a r l i e r , the l o c a t i o n of the m a i n p h o t o c o n d u c t i v l t y peak lies b e l o w the band gap v a l u e and, a c c o r d i n g to our k n o w l e d g e , such b e h a v i o u r has b e e n r e p o r t e d only in p type m a t e r i a l and ther e f o r e , t h e p r e s e n t r e s u l t s are a n a l y z e d t a k i n g this into a c c o u n t . Generally, the r e d u c t i o n of the p h o t o r e s p o n s e on the h i g h e n e r g y side of the band gap is a s c r i b e d to the h a r m f u l s u r f a ce e f f e c t s ( p a r t i c u l a r l y high r e c o m b i n a t ~ ion rate, r a d i a t i v e and non r a d i a t i v e tran s l t i o n s , e t c . ) ; w h i c h are i m p o r t a n t near the s u r f a c e w h e r e the h i g h e r e n e r g y r a d i a t ion is a b s o r b e d . The o b s e r v e d s h a r p dip on the high e n e r g y side s u g g e s t s that it is n e c e s s a r y to get m o r e i n f o r m a t i o n a b o u t the p o s i t i o n of the s u r f a c e s t a t e s , particularly w i t h r e s p e c t to the top of the v a l e n c e band, and to e x a m i n e the role of these s t a t e s in the p h o t o c o n d u c t i o n process. In p type s e m i c o n d u c t o r s , photoconduction takes p l a c e t h r o u g h the h o l e s and the r e d u c t i o n in the p h o t o c u r r e n t i m p l i e s that the d e n s i t y of the h o l e s is reduced considerably; a s s u m i n g that the variation in the m o b i l i t y is not n o t i c e a ble. This is p o s s i b l e only if the r a d i a t ion of e n e r g y 1.37 eV (905 nm) e j e c t s e l e c t r o n s f r o m a deep v a l e n c e level to the top of the v a l e n c e b a n d , c a u s i n g rec o m b i n a t i o n w i t h the h o l e s . P h o t o q u e n c h ing due to the a b o v e p r o c e s s has k e e n r e p o r t e d very r e c e n t l y in C u G a T e 2 v. U n d e r these c i r c u n s t a n c e s , the o b s e r v e d peak in photoconductivity (1.28 eV) is not a p e a k in the true sense, e v e n t h o u g h it looks like it b e c a u s e of the c o n s e q u e n c e of the photoquenching on the high e n e r g y side w h i c h does not p e r m i t to f o l l o w the same trend as the a b s o r p t i o n spectrum. In order to test this p o s s i b i l i t y , the deep s t a t e s of the v a l e n c e band s h o u l d be e x a m i n e d . One of the p o w e r f u l t e c h n i q u e s to s t u d y deep e n e r g y s t a t e s and v a l e n c e band s t r u c t u r e is s u r f a c e s e n s i t i v e p h o t o e m i s sion s p e c t r o s c o p y . Recently, experimental s t u d y for GaAs has b e e n c a r r i e d out u s i n g synchrotron radiation9; it has b e e n found out that core levels of Ga and As are l o c a t e d at a b o u t 20 eV and 41 eV r e s p e c t l vely. S u r f a c e t r e a t m e n t and a d s o r p t i o n of o x y g e n a t o m s do s h i f t the core levels, h o w e v e r , they are s i t u a t e d too deep to take part in the o p t i c a l a b s o r p t i o n process, particulary, in the o b s e r v e d e n e r g y r e g i o n , it has b e e n e x p e r i m e n t a l l y o b s e r v e d that the a d s o r b e d o x y g e n takes
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PHOTOQUENCHING EFFECT IN p TYPE GALLIUM ARSENIDE
away e l e c t r o n s p r e f e r e n t i a l l y f r o m As atoms 9 (atoms f r o m e l e m e n t s of c o l u m n V in Ill - V c o m p o u n d s ) and this c r e a t e s adsorbate surface states. Naturally, this is an e n t i r e l y n e w s i t u a t i o n as far as d i s t r i b u t i o n of the e n e r g y s t a t e s in the v a l e n c e b a n d are c o n c e r n e d , and this d e m a n d s d e t a i l e d study. D e p e n d i n g u p o n the level of the surface oxidation, c l e a v a g e p l a n e and other surface conditions, the fermi level m a y stay p i n n e d or r e m a i n free 9. If it is p i n n e d , s h a r p e l e c t r o n d i s t r i b u t i o n c u r v e s are o b s e r v e d b e l o w the top of the v a l e n c e band. It m a y also h a p p e n that s u r f a c e r e s o n a n c e s t a t e s a p p e a r in the r e g i o n b e t w e e n I and 2 eV. It is not p o s s i b l e to e s t i m a t e the p r e c i s e p o s i t i o n of the e l e c t r o n d i s t r i b u t i o n c u r v e s (EDC) s i n c e it d e p e n d s r e m a r k a b l y on the s u r f a c e c o n d i t i o n . Earlier experimental e v i d e n c e of the e x i s t a n c e of EDC c o u l d have a d i r e c t consequence on the s t r o n g p h o t o q u e n c h ing o b s e r v e d in p type GaAs. The o b s e r v ed d i s c r e p a n c y can be u n d e r s t o o d w i t h the help of f i g u r e 2.
1.43 eV
< F i g u r e 2 - E n e r g y s c h e m e to i n t e r p r e t the photoquenching e f f e c t at 905 nm. T h e b a n d t a i l i n g p r o d u c e d by z i n c i m p u r i t i e s as w e l l as the d e e p v a l e n c e s t a t e s p r o d u c e d by the a d s o r b e d o x y g e n a t o m s are shown.
903
It is e x p e c t e d that the p h o t o c o n d u c t i v i t y s h o u l d f o l l o w the same t r e n d as the a b s o r p t i o n . H o w e v e r a f t e r 1.3 eV, the a b s o r p t i o n coefficient i n c r e a s e s as e x p e c t e d w h i l e the p h o t o c u r r e n t shows quenching. Electrons f r o m the f i e l d s u r f a c e s t a t e s are e j e c t e d to the top of the v a l e n c e b a n d w h e r e e l e c t r o n hole r e c o m b i n a t i o n takes p l a c e . T h i s produces a photoquenching effect just b e f o r e the b a n d gap. N a t u r a l l y , this does not p e r m i t p h o t o c u r r e n t to f o l l o w the a b s o r p t i o n c u r v e and t h e r e f o r e , the r e g i o n n e a r 1.28 eV a p p e a r s as a peak. Differences between absorption and photoconductivity m a x i m a n a t u r a l l y depend on the p o s i t i o n of the s u r f a c e s t a t e s originated from adsorbed oxygen, which in turn d e p e n d s on the s u r f a c e and i t s preparation, and v a r i e s f r o m s a m p l e to s a m p l e . A d i f f e r e n t ~ a g n i t u d e of s h i f t (40 meV) as r e p o r t e d J e a r l i e r is, t h e r e fore, u n d e r s t a n d a b l e . An e x p e r i m e n t a l confirmation for the e x i s t a n c e of the s u r f a c e s t a t e s in the investigated s a m p l e s c o u l d be o b t a i n e d , in p r i n c i p l e , by o b s e r v i n g thetransitions f r o m t h e s e s t a t e s to the b o t t o m of the c o n d u c t i o n b a n d (~ 2.8 eV). S u c h a t t e m p , t h e r e f o r e , w a s m a d e and the c o r r e s p o n d ing a b s o r p t i o n s p e c t r u m did s h o w a h u m p at 2.8 eV. U n f o r t u n a t e l y , b e c a u s e of the accidental coincidence b e t w e e n this v a l u e and the E. (% 3- %1) gap v a l u e in GaAs 3, it is no~ p o s s i b l e to i d e n t i f y the o r i g i n u n i q u e l y and h e n c e , d i r e c t confirmation c o u l d not be o b t a i n e d . It is, t h e r e f o r e , n e c e s s a r y to rely on earlier experimental observations. It can be seen then, that t h e r e exist a large concentration of l e v e l s (EDC) b e l o w the top of the v a l e n c e b a n d , p r o b a b l y c a u s e d by a d s o r p t i o n of o x y g e n ; and are s i t u a t e d a p r o x i m a t e l y at 1.37 eV b e l o w it. E l e c t r o n s f r o m t h e s e levels are e ~ e c t e d w i t h r a d i a t i o n of 905 nm w a v e l e n g t h and r e c o m b i n e w i t h the e x i s t i n g holes on the top of the v a l e n c e hand, h e n c e , q u e n c h e s the p h o t o c o n d u c t i v i t y just b e f o r e it r e a c h e s its m a x i m u m . The r e l e v a n c e of the a b o v e c i t e d m o d e l is that it does not only e x p l a i n the s t r o n g p h o t o q u e n c h i n g b e l o w the b a n d gap but also e x p l a i n s the a n o m a l o u s p o s i ! ion of the p h o t o c o n d u c t i v i t y peak.
Acknowledgements: A u t h o r s are t h a n k f u l to Dr. R. C a s a n o v a for v a l u a b l e d i s c u s s i o n . T h a n k s are due to C . D . C . H . (U.L.A. M e r i d a ) and C O N I C I T , V e n e z u e l a , for f i n a n c i a l assistance.
904
PHOTOQUENCHING EFFECT IN p TYPE GALLIUM ARSENIDE
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REFERENCES
i) Boissy M.C., Diguet D., Hallais J. and Schemali C., Gallium Arsenide and Related Compounds; Ed. Hilsum C., Institute of Physics, series 33a, (1976). 2) A m b r o z i a k A., S e m i c o n d u c t o r P h o t o e l e c t r i c Devices, lliffe Books Ltd., London (1968). 3) Pankove J.l., Optical Process in Semiconductors, Dover Pmblication, (1971). 4) Pankove J.l., J. Appl. Phys., 39, 5368, (1968). 5) Redfield D. and Wittke J.P., Proc. of the third Intern. Conf. on Photoconductivity, p. 29, Ed. Pell, Pergamon Press, (1971). 6) Linden K., PhD. Thesis, R.C.A. Laboratory, (1964). 7) Hwang C.I.,J. Appl. Phys., 39, 4307, (1967); Ibid , 4313, (1967); 8) 3oshi N.Y. and E c h e v e r r l a R., Solid State Comm., 47, 251, (1983). 9) Spicer W.E., Lindau I., Gregory P.E., Garner C.M., Pianetta P. and Chye P.W., J. of Vacuum Science & Technology, 13, 780, (1976).