Electroreflectance of CdSnP2

PHYSICS

Volume 33A, number 1

LETTERS

ELECTROREFLECTANCE

OF

J. KAVALIAUSKAS, Institute

of Semiconductor

Physics,

N. A. GORYUNOVA, A.F.

Yoffe Physico-technical

21 September 1970

CdSnP2

A. SHILEIKA

AC. Sci. Lithuanian

SSR,

Vilnius,

USSR

E. I. LEONCV and V. M. ORLOV

Institute,

AC. Sci.

USSR,

Leningrad

K-21,

USSR

Received 7 July 1970

The electroreflectance spectra of CdSnP2 have been investigated at ‘77 and 300cK in the spectral region 1 to 4 eV. The energies of some optical transitions and separations between the valence bands (at F) have been determined.

CdSnP2 belongs to the family of diamond-like semiconducting ternary compounds of the AIIBIVCKI type [ 1,2] and is the closest electron analog of InP. The laser action [3,4], high nonlinear dielectric constant [ 11, photoconductivity and photovoltage [4,5] indicate the various applications of this compound. We have investigated the electroreflectance (ER) spectra of n-type CdSnP2 at 300 and 77OK. The samples used in this study were of the dry sandwich type (CdSnPa-CaFa-Ni). Optical and electronic equipment employed was described elsewhere [S]. The ER spectra obtained by electrolyte technique were similar to those of the dry sandwich

Fig. 1. The ER spectra of n-type CdSnP2 in the vicinity of the absorption edge at 300 and 77°K. (a) Vd, L 0, ,

I’-,, = 0.8 V, (b) V& = -0.8

V,

V,,

= 0.3 v.

Table 1 The energy (in eV) of the ER peaks of n-type CdSnP2. 77%

3oooK

Eil)

1;227

1.164

EA2)

1.316

1.245

E(3)

1.396

1.326

Er)

2.693

2.608

Ef)

2.826

2.733

Ey)

3.012

2.926

Ei4)

3.100

3.004

Ei5)

3.63

3.81

E(6) 1

3.81

3.73

0

samples. The sign of ARLR is that observed when the electric field on the reflecting surface is increased. Fig. l(a) shows an ER spectrum of CdSnP2 near the absorption edge at 3000K. We suggest that three negative peaks marked by arrows correspond to the different optical transitions at F point. These transitions occur from the three spin-orbit and crystal field splitted valence bands (FS, F7, Fe) to the conduction band (F7) [2,7,8]. The ER structures due to these three different optical transitions are distinctly separated at ‘77oK in a low electric field (a small voltage applied), as is seen from fig. l(b). When the electric field increases the line shape ap-

49

Volume 33A, n u m b e r 1

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PHYSICS

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LETTERS

21 September 1970

ly. At 3 0 0 ° K t h e y a r e a p p r o x i m a t e l y e q u a l (81 m e V ) . T h e s e v a l u e s s h o w t h a t t h e s p l i t t i n g s of t h e v a l e n c e b a n d a t F p o i n t c a u s e d b y t h e c r y s t a l f i e l d a n d s p i n - o r b i t i n t e r a c t i o n a r e of t h e s a m e o r d e r of m a g n i t u d e . T h e r e l a t i v e l o c a t i o n of t h e v a l e n c e b a n d s c a n b e d e t e r m i n e d f r o m t h e m e a s u r e m e n t s of t h e E R w i t h p o l a r i z e d l i g h t . In t h e s p e c t r a l r e g i o n 2.4 t o 3.9 eV we h a v e o b s e r v e d a c o m p l i c a t e d s t r u c t u r e of t h e E R s p e c t r a (fig. 2). W e s u g g e s t t h a t t h e o b s e r v e d E R s i g n a l s c a n b e e x p l a i n e d a s a r e s u l t a n t of s i x d i f f e r e n t o p t i c a l t r a n s i t i o n s . T h e p o s i t i o n s of t h e E R p e a k s a r e l i s t e d i n t a b l e 1. T h e c o m p l e x i ty of t h e E R s p e c t r u m in a c o m p a r i s o n w i t h t h a t of I n P in t h i s s p e c t r a l r e g i o n i s c a u s e d by t h e l a r g e r n u m b e r of z o n e s [7,8] d u e to t h e l o w e r s y m m e t r y of t h e t e r n a r y c o m p o u n d s .

:

36

eNeeGr(ov) l ~ c f g~'G~nc c s

Fig. 2. The ER s p e c t r a of n-type CdSn]? 2 in the energy region 2.4 to 3.9 eV at 300 and 77°K. (a) Vdc = 0, Vao = 0.8 V, (b) Vdc = 0, Vac = 1.2 V. p r o a c h e s to t h a t o b s e r v e d a t 300OK. T h e p o s i t i o n s of t h e E R p e a k s a r e l i s t e d i n t a b l e 1. It i s s e e n , t h a t t h e v a l u e s of t h e e n e r g y g a p a t 300 a n d 77°K a r e in a good a g r e e m e n t w i t h t h o s e 1.16 a n d 1.22 eV d e t e r m i n e d f r o m t h e o p t i c a l a b s o r p tion, emission and photoconductivity studies [2-4]. The separations between the valence bands v 1 - v 2 a n d v 2 - v 3 d e t e r m i n e d f r o m the E R s p e c t r a at 7 7 ° K a r e 89 M e V a n d 80 m e V , r e s p e c t i v e -

50

[1] N. A. Goryunova, ]?roe. IXth Intern. Conf. Phys. Semicond., Moscow, I968 (Nauka, Leningrad) V. 2 (1968) p. 119S. [2] N. A. Goryunova, A.S. ]?oplavnoi, Yu. I. ]?olygalov and V. A. Chaldyshev, Phys. Stat. Sol. 39 (1970) 9. [3] F. M. Berkovskii et al., Fiz. Tekh. Poluprov. 2 (1968) 1218. [4] J . L . Shay, R. F. Leheny, E. Buehler and J. Wernick, Appl. Phys. L e t t e r s 16 (1970) 357. [5] G.]?.Shpenikov, Izv. Akad. Nauk SSSR, Neorgan. Materialy 5 (1969) 1143. [6] A. Irzhikevichius, J. Kavaliauskas and A. Shileika, Liet. Fiz. Rinkinys 9 (1969) 535. [7] V.A. Chaldyshev, V.N. P r o k r o v s k i i , Izv. Vys. Ucheb. Zaved. SSSR, Fiz. 2 (1960) 173. [8] V. A. Chaldyshev, G . F . K a r a v a e v , Izv. V y s Ucheb. Zaved. SSSR, Fiz. 5 (1963) 103.