The photomagnetic effect and photoconductivity in n-type GaAs

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ABSTRACTS OF P A P E R S IN SOVIET PHYSICS-SOLID STATE

constituted on the a v e r a g e 8 - 10oc. The t e m p e r a t u r e dependence of the absolute differential t h e r m o - e m f of the s a m p l e s studied is shown graphically. A c o m p a r i s o n of the e x p e r i m e n t a l data with t h e o r e t i c a l calculations was c a r r i e d out and the following e x p r e s s i o n for the differential t h e r m o - e m f of the n-type s e m i c o n d u c t o r was found: q~=-~l +

l :Zl

eT

co

K ~L ~,~fo,

,~ o ~21 ~'b--~

de

~ o e 1 ( ~fo~ ~ , de

where ~ is the kinetic energy, 1 is the mean free path of the electron, fo is-the u n p e r t u r b e d distribution function, ~ is the r e d u c e d F e r m i level. The computed c u r v e s which were in good a g r e e m e n t with the e x p e r i m e n t a l data were obtained by taking into account, for the t e m p e r a t u r e interval 100 to 400°K, the dominant s c a t t e r i n g by i m p u r i t y ions and the value of m n = 0.07; at high t e m p e r a t u r e s it was n e c e s s a r y to take into account the s t r o n g influence of the s c a t t e r i n g by the lattice vibrations, and in addition, to a s s u m e that the effective m a s s i n c r e a s e s to m n = 0.09. In these samples, as the author a s s u m e s , s c a t t e r i n g by polar vibrations plays an unimportant role, although in a p u r e r m a t e r i a l its contribution should be large. Thus, a value of the e l e c t r o n effective m a s s is obtained in this paper which is lower than that obtained by R. Newman for I~lP with a large i m p u r i t y content (5 x 1018 cm -°) ,

Bibliography: 4 titles

E F F E C T OF OXYGEN ON RECOMBINATION IN SILICON. V.N. Mordkovich (Vol. 6, No. 7, pp. 2177-2178). In the p r e v i o u s work of the author the effect of heat t r e a t m e n t at 430oc on the conductivity in g e r m a n i u m was studied, and it was shown that the heating of c r y s t a l s leads to the a p p e a r a n c e of e n e r g y levels in the forbidden gap of silicon a s s o c i a t e d with the f o r m a t i o n of s i l i c o n - o x y g e n complexes. In this paper it is shown that the d e e p - l y i n g levels g e n e r a t e d in the t r e a t m e n t add a substantial contribution to the r e c o m b i n a tion p r o c e s s . The lifetime was d e t e r m i n e d in a wide r a n g e of t e m p e r a t u r e s by the methods of decaying photoconductivity and of s t e a d y - s t a t e photoconductivity. The initial silicon s a m p l e s

v

p o s s e s s e d p - t y p e conductivity, and their r e s i s tivity was r o u g h l y 20 fi cm, while the oxygen c o n c e n t r a t i o n was equal to (5-6) x 1017 c m - 3 . The t e m p e r a t u r e dependence obtained for the lifetime in such s a m p l e s shows that for t e m p e r a t u r e s around r o o m t e m p e r a t u r e , the lifetime is constant. On elevation of the t e m p e r a t u r e , it initially i n c r e a s e s ; then the r a t e of i n c r e a s e slows down and the lifetime begins to d e c r e a s e rapidly. This effect is o b s e r v e d up to a t e m p e r a t u r e of about 150°C, after which the lifetime again i n c r e a s e s . A c o m p a r i s o n of the lifetime t e m p e r a t u r e dependence obtained here, with the r e s u l t s of the above mentioned work by the author shows a c o r r e l a t i o n between the r e c o m b i n a t i o n p r o c e s s and the a c c e p t o r center, E v + 0, 35 eV, the existence of which is a s s o c i a t e d with the p r e s e n c e of oxygen in the silicon. Thus, a m i n i mum in the lifetime is o b s e r v e d at the same t e m p e r a t u r e at which the F e r m i level c r o s s e s the E v + 0.35 eV trapping l e v e l . The c o n c e n t r a t i o n of these c e n t e r s i n c r e a s e s after heating the c r y s tals at 430oc. Concurrently, both the magnitude of the lifetime at r o o m t e m p e r a t u r e , and in p a r ticular its m i n i m u m value at roughly 150°C dec r e a s e . Heating s a m p l e s of p - t y p e conductivity at 430°C after some time leads to change in the conductivity type. This is caused by the t h e r m a l f o r m a t i o n of donor s i l i c o n - o x y g e n complexes containing four a t o m s of oxygen. The depth at which the c o r r e s p o n d i n g level lies o c c u r s a c c o r d i n g to our p r e v i o u s work, at E c - 0.16 eV. It was e s tablished that there is a s t r o n g a g g r e g a t i o n of the " t h e r m a l " donors in n-type s a m p l e s when they are m e a s u r e d at t e m p e r a t u r e s below r o o m t e m p e r a ture. It is r e m a r k e d that the ionization of " t h e r m a l " d o n o r s and a change in the magnitude of a g g r e g a t i o n o c c u r in the same t e m p e r a t u r e int e r v a l . The dependence of the amount of a g g r e g a tion on the t e m p e r a t u r e of h e a t - t r e a t m e n t is in good a g r e e m e n t with the model of s i l i c o n - o x y g e n complex f o r m a t i o n p r o p o s e d by K a i s e r et al. The a s s o c i a t i o n of the a g g r e g a t e d c e n t e r s with the donor level at E c - 0.16 eV, and the dependence of the extent of a g g r e g a t i o n on the t e m p e r a ture of h e a t - t r e a t m e n t a r e also confirmed by control e x p e r i m e n t s on n-type silicon alloyed with phosphorous. Bibliography:

7 titles

THE PHOTOMAGNETIC E F F E C T AND PHOTOCONDUCTIVITY IN n - T Y P E Ga.As. N.M. V o r o n kova and D.N. Naslyedov (Vol. 6, No. 7, pp. 21962197). The change in s t e a d y - s t a t e effects on

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ABSTRACTS OF PAPERS IN SOVIET PHYSICS-SOLID STATE

l o w e r i n g the t e m p e r a t u r e f r o m r o o m t e m p e r a t u r e to 80°K w a s i n v e s t i g a t e d . The t e m p e r a t u r e d e p e n d e n c e w a s studied by illuminating the s a m p l e with s t r o n g l y a b s o r b e d light of )~ = 600 to 800 m~. T h i s r e g i o n of the s p e c t r u m was s e p a r a t e d out by m e a n s of two g l a s s f i l t e r s . Photoconductivity w a s o b s e r v e d in all s a m o l e s with.a c a r r i e r conc e n t r a t i o n l e s s than 1017 cm -3. The c h a r a c t e r i s t i c b e h a v i o r of the photoconductivity in the t e m p e r a t u r e i n t e r v a l investigated, which is exhibited in g r a p h s , p e r m i t s a n u m b e r of i n f e r e n c e s to be drawn. The p h o t o m a g n e t i c effect is o b s e r v e d in s a m p l e s with c u r r e n t c a r r i e r c o n c e n t r a t i o n s of 1015cm-3 and l e s s . F o r low r e s i s t a n c e single c r y s t a l s the p h o t o m a g n e t i c e f f e c t d e c r e a s e s v e r y r a p i d l y on l o w e r i n g the t e m p e r a t u r e and thus it could be studied only in a s h o r t t e m p e r a t u r e int e r v a l . In high r e s i s t a n c e s a m p l e s , the photom a g n e t i c e f f e c t a l s o d e c r e a s e s on l o w e r i n g of the t e m p e r a t u r e . It is noted, however, that the b e h a v i o r of the photoconductivity and the photom a g n e t i c effect d i f f e r s f o r high and low r e s i s t a n c e s a m p l e s . F o r the f o r m e r both e f f e c t s v a r y with t e m p e r a t u r e identically, while f o r the l a t t e r , on l o w e r i n g the t e m p e r a t u r e , the photoconductivity i n c r e a s e s , w h e r e a s the p h o t o m a g n e t i c effect fal ls off. The i n v e s t i g a t i o n of the influence of light i n t e n s i t y on the e f f e c t s studied, w a s a l s o c a r r i e d out in this e x p e r i m e n t . It was found that the p h o t o c o n d u c t i v i t y and the p h o t o m a g n e t i c effect i n c r e a s e l i n e a r l y with a v a r i a t i o n in int e n s i t y of the incident r a d i a t i o n f r o m 1014 to 1016 q u a n t a / c m 2 s e c . On v a r y i n g the intensity of the m a g n e t i c field up to 10 koe, the p h o t o m a g n e t i c s h o r t - c i r c u i t c u r r e n t w a s a l s o found to v a r y l i n e a r l y . In addition, the s p e c t r a l dependence of the photoconductivity w a s d e t e r m i n e d at r o o m and liquid n i t r o g e n t e m p e r a t u r e s , and f o r a n u m b e r of i n t e r m e d i a t e t e m p e r a t u r e s a s well. A c h a r a c t e r i s t i c p r o p e r t y of the s p e c t r a l dependence of the p h o t o c o n d u c t i v i t y is the a b s e n c e of a sudden drop in s e n s i t i v i t y f o r w a v e l e n g t h s c o r r e s p o n d i n g to the l i m i t of the f u n d a m e n t a l a b s o r p t i o n band. The l o n g - w a v e l e n g t h drop which begins in the r e g i o n 800 to 930 m~ at r o o m t e m p e r a t u r e e x tends to 1.4 m~ f o r low r e s i s t a n c e and to 2.2 m~ f o r high r e s i s t a n c e c r y s t a l s . In addition to the f u n d a m e n t a l m a x i m u m , a second m a x i m u m is o b s e r v e d in the s h o r t w a v e l e n g t h r e g i o n of the s p e c t r u m c o r r e s p o n d i n g to the a c t i v a t i o n e n e r g y of 1.94 eV. The p o s i t i o n of this m a x i m u m does not depend on the method of t r e a t m e n t of the

Vol. 2, No.9

s u r f a c e and does not change with a d e c r e a s e in temperature. Bibliography: 9 titles DETERMINATION OF THE YIELD OF R E COMBINATION RADIATION ASSOCIATED WITH DISLOCATIONS IN GERMANIUM. A . A . Gippius, V.S. Vavilov and V.S. Konoplev (Vol. 6, No. 7, pp. 2200-2202). By the quantum yield of i m p u r i t y r e c o m bination r a d i a t i o n the a u t h o r s r e f e r to the r a t i o of the n u m b e r of quanta of r e c o m b i n a t i o n r a d i a tion to the total n u m b e r of a c t s of r e c o m b i n a t i o n at a given type of c e n t e r s . In o r d e r to obtain a sufficiently intense volume excitation, the s a m ple w a s i r r a d i a t e d by a b e a m of ~1 m e V e l e c t r o n s in a Van de G r a a f f e l e c t r o s t a t i c g e n e r a t o r . A lead sulfide photoconductive cell was used a s a r e c e i v e r , c a l i b r a t e d b e f o r e h a n d by use of a " b l a c k body". S a m p l e s of n-type g e r m a n i u m w e r e u s e d in the m e a s u r e m e n t s having an e l e c t r o n c o n c e n t r a t i o n of n o ~ 5 × 1014 cm -3 and a d e n s i t y of d i s l o c a t i o n s Nd - 105 cm -2 which w e r e i n t r o d u c e d by t h e r m a l shock, i . e . , by a s h a r p change in t e m p e r a t u r e d u r i n g the p r o c e s s of c r y s t a l growth. The following r e s u l t s w e r e obtained. The r e c o m b i n a t i o n r a d i a t i o n at the d i s locations in g e r m a n i u m is c h a r a c t e r i z e d by a v e r y s m a l l yield (Ynuant N 10-4 at 80OK and 10 -6 at 130OK). ~uch a s m a l l value of Yquant c o n f i r m s the h y p o t h e s i s as to the m e c h a n i s m of r a d i a t i o n r e c o m b i n a t i o n at d i s l o c a t i o n s and a s to the nature of the " e m i t t e r s " a s s o c i a t e d with the d i s l o c a t i o n s . The o v e r w h e l m i n g n u m b e r of a c t s of r e c o m b i n a t i o n at d i s l o c a t i o n s o c c u r without r a d i a t i o n . The r a d i a t i o n l e s s r e c o m b i n a t i o n channel at d i s l o c a t i o n s has a v e r y high e f f i c i e n c y (p = 6 x 10-6 cm at 130OK), which p o s s i b l y is a s s o c i a t e d with the e x i s t e n c e of an e n e r g y s p e c t r u m in the d i s l o c a t i o n s . It is noted that the r a d i a t i o n r e c o m b i n a t i o n can take p l a c e not only along the whole length of the d i s l o c a t i o n but on s e p a r a t e s e c t i o n s of it, constituting the " e m i t t e r s " which a p p a r e n t l y a r e c o m p o s e d of v a r i o u s d i s a r r a n g e m e n t s in the s t r u c t u r e of the d i s l o c a t i o n s . The quantum yield of r e c o m b i n a t i o n r a d i a t i o n in this c a s e is n a t u r a l l y s m a l l . Bibliography: 6 titles