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Exciton effect in the electroreflectance of zinc selenide single crystal
Volume 31A, number 5
]PHYSICS LETTERS
EXCITON EFFECT IN THE OF ZINC SELENIDE
9 March 1970
ELECTROREFLECTANCE SINGLE CRYSTAL
S. F U J I W A R A , K. H A T T O R I and M. F U K A I Central Research Laboratory, Matsushita Electric Industrial Company, Ltd., Kadoma, Osaka, Japan Received 19 January 1970
The electroreflectance spectrum of ZnSe has been measured for photon energies in the neighborhood of the fundamental absorption edge at the temperatures between 4.2°K and 300OK. The new signal, which is observed in the low energy side of the fundamental absorption edge at 300°K, is attributed to exciton effect, not impurity effect. The negative peak position of the field-induced absorption change associated with e x citon shifts slightly to low energy with increasing electric field.
Recently, anomalous electroreflectance peaks h a v e b e e n o b s e r v e d at e n e r g i e s l e s s than the fund a m e n t a i a b s o r p t i o n e d g e on G a A s [1], InP [2]. In G a A s , t h i s p e a k i s e x p l a i n e d a s the s i g n a l a s s o c i a t e d with the i m p u r i t y - t o - b a n d t r a n s i t i o n by S e r a p h i n et al. [1], M. C a r d o n a et al. [2], E. W i l l i a m s et al. [3] etc. In ZnSe, we h a v e obs e r v e d n e w s h a r p e l e c t r o r e f l e c t a n c e s i g n a l at l o w e r e n e r g y than the f u n d a m e n t a l a b s o r b t i o n e d g e at r o o m t e m p e r a t u r e s u p p e r i m p o s e d on the b a n d - t o - b a n d t r a n s i t i o n s i g n a l , a s shown in fig. 1 (A). In t h i s note, it i s shown that t h i s s i g n a l i s due to e x c i t o n on the b a s i s of b o t h the e l e c t r i c f i e l d and t e m p e r a t u r e d e p e n d e n c e of the s i g n a l . T h o u g h the f r e e e x c i t o n of ZnSe h a s b e e n obs e r v e d by r e f l e c t a n c e [4,5], and p h o t o l u m i n e s c e n c e [6] at low t e m p e r a t u r e , it i s the f i r s t t i m e that it w a s d i s t e n c t l y o b s e r v e d at r o o m t e m p e r ature. The ZnSe c r y s t a l s [7] u s e d f o r t h i s i n v e s t i g a t i o n w e r e g r o w n by d i r e c t f u s i o n u n d e r h i g h i n e r t g a s p r e s s u r e , and f i r e d in e i t h e r m o l t e n z i n c o r doping m a t e r i a l s (Ga, In of AI) - z i n c a l l o y s to r e d u c e the r e s i s t i v i t y . In o r d e r to o b t a i n the t e m p e r a t u r e d e p e n d e n c e of s i g n a l , the s u r f a c e barrier layer technique was used. Fig. 1 (B) s h o w s the e l e c t r o r e f l e c t a n c e d a t a at 4 . 2 ° K f o r the undoped ZnSe w i t h the c a r r i e r c o n c e n t r a t i o n of 5 × 1015 c m - 3 at 7 7 ° K and the electroabsorption curve which has been obtained by K r a m e r s - K r o n i n g a n a l y s i s of the e l e c t r o r e f l e c t a n c e data. T h e e l e c t r o r e f l e c t a n c e A R / R at 8 0 ° K i s s i m i l a r to the d a t u m o b s e r v e d at 7 7 ° K by F o r m a n and C a r d o n a [8] e x c e p t that n = 1 e x citon l e v e l s h i f t s to h i g h e r e n e r g y by 15 m e V and 258
(B)
Frsec - 8
4.2 ° k.
( E ,~ = 1.8 x I 0 4 V/cm --5xtO 3
m ~ o ~
E <] n=2
,
-OIq
-O.L
a=~(A)
........ n=l
(e
I~
<]
V) rr.c-
17
"-/:z_':j
30~
-- -5 x 103
- I x 104
-15x I0 4
Fig. 1. (A) Electroreflectance spectrum of undoped ZnSe at room temperature (. . . . ) and electroabsorption obtain ed by K-K analysis from its electroreflectance ( ). Average e l e c t r i c field (E) was estimated from surface capacitance. (B) Electroreflectance spectra (. . . . ) of undoped ZnSe at 4.2°K and electroabsorption ( ) obtained from its K-K analysis. The arrow, n = l indicates the position of ground state of exciton and the arrow n= 2 is the position of excited state of exciton.
n = 2 e x c i t o n l i n e i s m a s k e d by the b r o a d e r n : 1 e x c i t o n line. But the s i g n a l s a r e s e p a r a t e d at 4 . 2 ° K and the e n e r g y d i f f e r e n c e b e t w e e n n = 1 and n = 2 e x c i t o n l i n e s i s in good a g r e e m e n t f o r r e f l e c -
Volume 31A, number 5
PHYSICS LETTERS
tance o b s e r v e d by Hite et al. [5]. Exciton energy of 2.809 eV which we o b s e r v e d at 4.2°K i s also different from 2.803 eV obtained by Y. S. P a r k et al. [4]. T h i s difference of exciton e n e r g y may be due to the p r e s e n c e of sulpher i m p u r i t y , which was detected in ZnSe single c r y s t a l by f l u o r e s cent X - r a y a n a l y s i s . The e n e r g y of the dominant negative was independent of e l e c t r i c field in weak field r e g i o n (/~0 << E B) and slightly d e c r e a s e d with f u r t h e r i n c r e a s i n g e l e c t r i c field. (0 = (e2E2/2 m*) 113, EB, m* and E a r e binding e n e r g y of exciton, r e duced m a s s of exciton and e l e c t r i c field r e s p e c tively. ) The magnitude of this shift is 2 meV at the applied voltage of about 200 V. T h i s shift of the signal can be explained by the f i e l d - i n d u c e d change of a b s o r p t i o n obtained from the t h e o r e t i cal calculation for exciton in R a l p h ' s paper [9], in opposition to E n d e r l e i n ' s r e s u l t s [10]. The same shift has been o b s e r v e d in CdS [11]. The change of a b s o r p t i o n coefficient, As for n = l exciton was in p r o p o r t i o n to the applied voltage in the r e g i o n of weak field. With i n c r e a s i n g e l e c t r i c field, the amplitude of As b e c o m e s independent of the e l e c t r i c field at about 160 V (~ 7 x 104 V/cm). T h i s field s t r e n g t h c o r r e sponds to the binding e n e r g y of exciton in ZnSe which is e x t i m a t e d to be 22.9 meV u s i n g the r e duced m a s s 0.15 m o [5] and the d i e l e c t r i c constant, 9.1 [12]. The line width of the n--1 exciton signal does not exhibit the b r o a d e n i n g expected for an M otype b a n d - t o - b a n d a b s o r p t i o n and is n e a r l y independent of e l e c t r i c field. The o b s e r v e d line width of 2 meV at 80°K is n e a r l y equal with the value obtained by F o r m a n et al. [8] at 77°K. The t e m -
9 March 1970
p e r a t u r e dependence of the amplitude of this peak i s v e r y l a r g e in c o n t r a s t with that obtained for the b a n d - t o - b a n d absorption. The signal a s s o c i a t e d with an L0 p h o n o n - a s s i s t e d direct exciton obs e r v e d by Ikeda et al. [13], could not o b s e r v e d in the range of 4.2°K to 300OK. T h i s dominant negative peak extends to the new signal o b s e r v e d at 300°K. F u r t h e r m o r e , the e n e r g y of this signal i s independent of the kinds of doped i m p u r i t i e s . We would like to thank O. Eguchi for having supplied u s ZnSe c r y s t a l s .
References 1. B.O. Seraphin, J. Appl. Phys. 37 (1966) 721. 2. M. Cardona, K. L. Shaklee and F. H. Pollack, Phys. Rev. 154 (1967) 696. 3. E.W. Williams and V. Retm, Phys. Rev. 172 (1968) 798. 4. Y.S. Park and C. McConn, Phys. Letters 26A (1967) 483. 5. G. E. Hite, D.T.F. Marple, M. Aven and B. Segall, Phys. Rev. 156 (1967) 850. 6. Y. S, Park and J. R. Schneider, Phys. Rev. Letters 21 (1968) 798. 7. Y. Tsujimoto, Y. Onodera and M. Fukai, Japan J. Appl. Phys. 5 (1966) 636. 8. R.A. Forman and M. Cardona, II-VI Semiconducting compounds, ed. D. G. Thomas (Benjamin Inc., New York, 1967) p. 100. 9. H.I. Ralph, J. Phys. C1 (1968) 378. i0. R. Enderlein, Phys. Stat. Sol. 26 (1968) 509. U . H. Lange and E. Gutsche, Phys. Stat. Sol. 32 (1969) 293. 12. D. Berllncourt, H. Taffe and L. R. Shiozawa, Phys. Rev. 129 (1963) 1009. 13. K. Ikeda, Y. Hamakawa, H. Komiya and S. Ibuki, Phys. Letters 28A (1969) 646.
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259
]PHYSICS LETTERS
EXCITON EFFECT IN THE OF ZINC SELENIDE
9 March 1970
ELECTROREFLECTANCE SINGLE CRYSTAL
S. F U J I W A R A , K. H A T T O R I and M. F U K A I Central Research Laboratory, Matsushita Electric Industrial Company, Ltd., Kadoma, Osaka, Japan Received 19 January 1970
The electroreflectance spectrum of ZnSe has been measured for photon energies in the neighborhood of the fundamental absorption edge at the temperatures between 4.2°K and 300OK. The new signal, which is observed in the low energy side of the fundamental absorption edge at 300°K, is attributed to exciton effect, not impurity effect. The negative peak position of the field-induced absorption change associated with e x citon shifts slightly to low energy with increasing electric field.
Recently, anomalous electroreflectance peaks h a v e b e e n o b s e r v e d at e n e r g i e s l e s s than the fund a m e n t a i a b s o r p t i o n e d g e on G a A s [1], InP [2]. In G a A s , t h i s p e a k i s e x p l a i n e d a s the s i g n a l a s s o c i a t e d with the i m p u r i t y - t o - b a n d t r a n s i t i o n by S e r a p h i n et al. [1], M. C a r d o n a et al. [2], E. W i l l i a m s et al. [3] etc. In ZnSe, we h a v e obs e r v e d n e w s h a r p e l e c t r o r e f l e c t a n c e s i g n a l at l o w e r e n e r g y than the f u n d a m e n t a l a b s o r b t i o n e d g e at r o o m t e m p e r a t u r e s u p p e r i m p o s e d on the b a n d - t o - b a n d t r a n s i t i o n s i g n a l , a s shown in fig. 1 (A). In t h i s note, it i s shown that t h i s s i g n a l i s due to e x c i t o n on the b a s i s of b o t h the e l e c t r i c f i e l d and t e m p e r a t u r e d e p e n d e n c e of the s i g n a l . T h o u g h the f r e e e x c i t o n of ZnSe h a s b e e n obs e r v e d by r e f l e c t a n c e [4,5], and p h o t o l u m i n e s c e n c e [6] at low t e m p e r a t u r e , it i s the f i r s t t i m e that it w a s d i s t e n c t l y o b s e r v e d at r o o m t e m p e r ature. The ZnSe c r y s t a l s [7] u s e d f o r t h i s i n v e s t i g a t i o n w e r e g r o w n by d i r e c t f u s i o n u n d e r h i g h i n e r t g a s p r e s s u r e , and f i r e d in e i t h e r m o l t e n z i n c o r doping m a t e r i a l s (Ga, In of AI) - z i n c a l l o y s to r e d u c e the r e s i s t i v i t y . In o r d e r to o b t a i n the t e m p e r a t u r e d e p e n d e n c e of s i g n a l , the s u r f a c e barrier layer technique was used. Fig. 1 (B) s h o w s the e l e c t r o r e f l e c t a n c e d a t a at 4 . 2 ° K f o r the undoped ZnSe w i t h the c a r r i e r c o n c e n t r a t i o n of 5 × 1015 c m - 3 at 7 7 ° K and the electroabsorption curve which has been obtained by K r a m e r s - K r o n i n g a n a l y s i s of the e l e c t r o r e f l e c t a n c e data. T h e e l e c t r o r e f l e c t a n c e A R / R at 8 0 ° K i s s i m i l a r to the d a t u m o b s e r v e d at 7 7 ° K by F o r m a n and C a r d o n a [8] e x c e p t that n = 1 e x citon l e v e l s h i f t s to h i g h e r e n e r g y by 15 m e V and 258
(B)
Frsec - 8
4.2 ° k.
( E ,~ = 1.8 x I 0 4 V/cm --5xtO 3
m ~ o ~
E <] n=2
,
-OIq
-O.L
a=~(A)
........ n=l
(e
I~
<]
V) rr.c-
17
"-/:z_':j
30~
-- -5 x 103
- I x 104
-15x I0 4
Fig. 1. (A) Electroreflectance spectrum of undoped ZnSe at room temperature (. . . . ) and electroabsorption obtain ed by K-K analysis from its electroreflectance ( ). Average e l e c t r i c field (E) was estimated from surface capacitance. (B) Electroreflectance spectra (. . . . ) of undoped ZnSe at 4.2°K and electroabsorption ( ) obtained from its K-K analysis. The arrow, n = l indicates the position of ground state of exciton and the arrow n= 2 is the position of excited state of exciton.
n = 2 e x c i t o n l i n e i s m a s k e d by the b r o a d e r n : 1 e x c i t o n line. But the s i g n a l s a r e s e p a r a t e d at 4 . 2 ° K and the e n e r g y d i f f e r e n c e b e t w e e n n = 1 and n = 2 e x c i t o n l i n e s i s in good a g r e e m e n t f o r r e f l e c -
Volume 31A, number 5
PHYSICS LETTERS
tance o b s e r v e d by Hite et al. [5]. Exciton energy of 2.809 eV which we o b s e r v e d at 4.2°K i s also different from 2.803 eV obtained by Y. S. P a r k et al. [4]. T h i s difference of exciton e n e r g y may be due to the p r e s e n c e of sulpher i m p u r i t y , which was detected in ZnSe single c r y s t a l by f l u o r e s cent X - r a y a n a l y s i s . The e n e r g y of the dominant negative was independent of e l e c t r i c field in weak field r e g i o n (/~0 << E B) and slightly d e c r e a s e d with f u r t h e r i n c r e a s i n g e l e c t r i c field. (0 = (e2E2/2 m*) 113, EB, m* and E a r e binding e n e r g y of exciton, r e duced m a s s of exciton and e l e c t r i c field r e s p e c tively. ) The magnitude of this shift is 2 meV at the applied voltage of about 200 V. T h i s shift of the signal can be explained by the f i e l d - i n d u c e d change of a b s o r p t i o n obtained from the t h e o r e t i cal calculation for exciton in R a l p h ' s paper [9], in opposition to E n d e r l e i n ' s r e s u l t s [10]. The same shift has been o b s e r v e d in CdS [11]. The change of a b s o r p t i o n coefficient, As for n = l exciton was in p r o p o r t i o n to the applied voltage in the r e g i o n of weak field. With i n c r e a s i n g e l e c t r i c field, the amplitude of As b e c o m e s independent of the e l e c t r i c field at about 160 V (~ 7 x 104 V/cm). T h i s field s t r e n g t h c o r r e sponds to the binding e n e r g y of exciton in ZnSe which is e x t i m a t e d to be 22.9 meV u s i n g the r e duced m a s s 0.15 m o [5] and the d i e l e c t r i c constant, 9.1 [12]. The line width of the n--1 exciton signal does not exhibit the b r o a d e n i n g expected for an M otype b a n d - t o - b a n d a b s o r p t i o n and is n e a r l y independent of e l e c t r i c field. The o b s e r v e d line width of 2 meV at 80°K is n e a r l y equal with the value obtained by F o r m a n et al. [8] at 77°K. The t e m -
9 March 1970
p e r a t u r e dependence of the amplitude of this peak i s v e r y l a r g e in c o n t r a s t with that obtained for the b a n d - t o - b a n d absorption. The signal a s s o c i a t e d with an L0 p h o n o n - a s s i s t e d direct exciton obs e r v e d by Ikeda et al. [13], could not o b s e r v e d in the range of 4.2°K to 300OK. T h i s dominant negative peak extends to the new signal o b s e r v e d at 300°K. F u r t h e r m o r e , the e n e r g y of this signal i s independent of the kinds of doped i m p u r i t i e s . We would like to thank O. Eguchi for having supplied u s ZnSe c r y s t a l s .
References 1. B.O. Seraphin, J. Appl. Phys. 37 (1966) 721. 2. M. Cardona, K. L. Shaklee and F. H. Pollack, Phys. Rev. 154 (1967) 696. 3. E.W. Williams and V. Retm, Phys. Rev. 172 (1968) 798. 4. Y.S. Park and C. McConn, Phys. Letters 26A (1967) 483. 5. G. E. Hite, D.T.F. Marple, M. Aven and B. Segall, Phys. Rev. 156 (1967) 850. 6. Y. S, Park and J. R. Schneider, Phys. Rev. Letters 21 (1968) 798. 7. Y. Tsujimoto, Y. Onodera and M. Fukai, Japan J. Appl. Phys. 5 (1966) 636. 8. R.A. Forman and M. Cardona, II-VI Semiconducting compounds, ed. D. G. Thomas (Benjamin Inc., New York, 1967) p. 100. 9. H.I. Ralph, J. Phys. C1 (1968) 378. i0. R. Enderlein, Phys. Stat. Sol. 26 (1968) 509. U . H. Lange and E. Gutsche, Phys. Stat. Sol. 32 (1969) 293. 12. D. Berllncourt, H. Taffe and L. R. Shiozawa, Phys. Rev. 129 (1963) 1009. 13. K. Ikeda, Y. Hamakawa, H. Komiya and S. Ibuki, Phys. Letters 28A (1969) 646.
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259