RECONSTRUCTION OF ENERGY SPECTRUM OF ALLOYS Pbl-xSnxTe WITH AN INDIUM IMPURITY LEVEL UNDER THE COMBINED EFFECT OF MAGNETIC FIELD AND HYDROSTATIC PRESSURE

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RECONSTRUCTION OF ENERGY SPECTRUM OF

ALLOYS Pbl-xSnxTe WITH AN INDIUM IMPURITY

LEVEL UNDER THE COMBINED EFFECT OF

MAGNETIC FIELD AND HYDROSTATIC

PRESSURE

B. Akimov, N. Brandt, L. Ryabova, S. Chudinov

To cite this version:

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JOURNAL D E PHYSIQUE Colloque C6, suppl6ment au no 8, Tome 39, aotit 1978, page

~6-1079

RECONSTRUCTION O F ENERGY SPECTRUM OF ALLOYS Pbl-,SnXTe W I T H AN I N D I U M I M P U R I T Y L E V E L UNDER T H E COMBINED E F F E C T OF MAGNETIC F I E L D AND H Y D R O S T A T I C PRESSURE

B.A. Akimov, N.B. Brandt, L.I. Ryabova, and S.M. Chudinov

Low Temperature Laboratory, Physics Deparbent, Moscow S t a t e U n i v e r s i t y , U.S.S.R.

R6sumg.- On a montrC que dans les alliages Pbl-xSnxTe 0,22 s x

<

0,28 contenant 0,5 %

atomique d'In, le niveau des impuretds se trouve dans la bande interdite. On a observ6 les transitions rngtalz isolant, induites par la variation de la concentration X et de la pression. La transition est produite par le recouvrement de la bande de conductivitg ou de la bande de valence avec le niveau des impuretgs. On a montrd que dans les alliages Pbl-,SnXTe le facteur

g

des porteurs libres est

<

2 dans la rEgion du spectre direct et 2 2 dans celle du spectre inverse.

Abstract.- It has been found that In impurity level in alloys p b ~ - ~ S n ~ T e lies in the forbidden gap for the interval 0.22<x50.28 at 4.2 K.Insulator* metal transitions occuring as a result of change in composition and hydrostatic pressure leading to an overlapping of conduc:ion of valency band with impurity level have been observed. It is shown that rela- tions g 5 Land g22 are satisfied for direct and inverse spectrum respectively for the effective g-factor of carriers in the alloys Pbl-,SnxTe.

Oscillatory and galvanomagnetic effects have been investigated for alloys Pb SnxTe

1-X

(0.25xc0.3) with 0.5 % In impurity, over a wide range of temperature 12-300 K under magnetic fields of upto 60 kOe. Moreover, alloys with

X = 0.25, 0.26 have been investigated under pressures upto 20 kbar.

Upon increasing concentration of SnTe from X = 0.2 to = 0.25, the resistivitypof the samples of the alloy at 4.2 K increases by more than 7 orders of magnitude, passes through a maximum pmax 2 4x10~ ohm. cm for X = 0.25 after which

it decreases again by 7 orders of magnitude in the interval 0.25<x<0.3. The reciprocal of Hall coef- ficient in magnetic field of 10kOe decreases by 4 orders of magnitude in the interval 0.2<x<0.25 after which it changes sign and increases by more than 4 orders of magnitude in the interval 0.26: ~20.3. The sign of Hall mobility for ~ ~ 0 . 2 5 cor- responds to electrons and for x>0.258 to hole conductivity. Shubnikov-de-Haas oscillations at heliumtemperatures were observed at P = 0 for alloys with X = 0.20, 0.29, 0.30, and in the pressure interva1'4.5<P512.5 kbar for alloy with

X = 0.25. It is seen that the Fermi surface cross- section of electrons rises with increasing pressure, passes through a maximum at P = 8-9 kbar and then falls.

The temperature dependence of conductivity of the alloys Pbl-xSnxTe + 0.5 at.Z In in the

interval 0.22<x<0.27 has a semiconducting character

-

at P=O. In the temperature interval 12<T<60 K the

1

dependence has a linear part whose inclina- tion determines the minimum energy of activation

E O in the spectrum of the alloys.

The magnitude of c for alloys with x=0.25 . falls for pressures p<pl = 4.5 kbar and rises for p>q = 4.5 kbar, for the alloy X = 0.26 p1 = 5.7, p2 = 8.7 kbar. In longitudinal magnetic field

+

H I

1

<loo;, the activation energy increases in the region of direct (p<pi) and decreases in the region of inverse (p>p.) spectra with increasing magnetic field. The point pi represents the gapless state in the spectrum of Pbl-xSnxTe. It is found

a~

that the quantity--0 is independent of E and

aH g

equal

*l

= 0.06 meV/kbar,

3

1

= -0.05 aH P < P ~ aH p'pi

meV/kbar for alloys with X = 0.25, 0.26.

On the basis of oscillations and galvanomagnetic measurements, the diagram of reconstruc-

tion of energy spectrum for alloys Pbl-xSnxTe with In impurity level /1,2/ is presented for va- rying alloy composition and under hydrostatic pressure. For X = 0.22 the In-level coincides with the bottom of the conduction band and for X = 0.28 with the top of the valency band. For alloy with

X = 0.25 the impurity level is situated within the forbidden gap. As the impurity level crosses the allowed bands, and insulatorfmetal transition takes place. In the insulating state the electri- cal conductivity of the alloy is mainly determined

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b y t h e i m p u r i t y l e v e l and i s c h a r a c t e r i s e d by e f f e c - t i v e m o b i l i t y v % 1 0 - ~ cm2/v.s. The c a r r i e r concen- t r a t i o n i n a l l o y e d bands f o r m o b i l i t y U % 105 cm2/ 1V.s i s determined o n l y by t h e r m a l e x c i t a t i o n . The 3 t r a n s i t i o n i n s u l a t o r + m e t a l a l s o t a k e s p l a c e under p r e s s u r e ( f i g u r e 1 ) . Moreover, t h e energy E In aeIn i n c r e e a s e s f o r a l l o y X = 0.25 a t t h e r a t e of -= ap = 1 meV/kbar and d e c r e a s e s f o r X = 0.26 a t t h e 2E1n mev r a t e of

-

= 0.8

-

a

P k b a r ' F i g . 1 : Diagram of r e c o n s t r u c t i o n of spectrum f o r a l l o y Pbo.7sSno.esTe + 0.5 a t . % I n under p r e s s u r e . It i s observed t h a t t h e s i g n of t h e d e r i - v a t i v e

%

changes a t t h e band i n v e r s i o n p o i n t and t h e q u a n t i t y

[%I

i s independent of E Thesere-

g'

s u l t s a r e i n agreement w i t h Dimmock's t h e o r y / 3 / . The a n a l a g o u s r e s u l t s have b e e n e a r l i e r o b t a i n e d

f o r undoped Pbl-xSnxSe a l l o y s 141. From t h e e x p e r i - m e n t a l c u r v e s

E

(H) t h e magnitude o f E (0) was mo- r e a c c u r a t e l y d e t e r m i n e d and t h e c u r v e s f o r e f f e c - t i v e & f a c t o r and s p e c t r o s c o p i c one were o b t a i n e d b o t h f o r d i r e c t (E >0) and i n v e r s e ( E CO) s p e c t r a

g g

( f i g u r e 2) i n Pbl-xSnxTe a l l o y s .

R e f e r e n c e s

/ I / Averkin, A.A., Kaidanov, V . I . , Melnik, R.B., F i z i k a Tekhnika Poluprovodnikov

5

(1971) 91.

121 Akimov, B.A., Wadhwa, R.S., Zlomanov, V.P., Ryabova, L . I . , Chudinov, S.M., F i z i k a Tekhnika Poluprovodnikov

11

(1 977) 1077.

131 Dimmock, J . O . , Phys. Semimet. and Narrow Gap Semicond. P r o c . I n t . Conf. D a l l a s , (1970) 319

(Pergamon P r e s s , N.Y.) 1971.

/ 4 / Calawa, A.R., Dimmock, J.O., Harman, T.C., M e l n g a i l i s , Phys. Rev. L e t t .

2

(1969) 7.