ELECTRON TRANSFER IN Hg1-xCdxTe-CdTe HETEROSTRUCTURES

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ELECTRON TRANSFER IN Hg1-xCdxTe-CdTe HETEROSTRUCTURES

G. Boebinger, J. Vieren, Y. Guldner, M. Voos, J. Faurie

To cite this version:

G. Boebinger, J. Vieren, Y. Guldner, M. Voos, J. Faurie. ELECTRON TRANSFER IN Hg1-xCdxTe- CdTe HETEROSTRUCTURES. Journal de Physique Colloques, 1987, 48 (C5), pp.C5-337-C5-340.

�10.1051/jphyscol:1987572�. �jpa-00226776�

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JOURNAL DE PHYSIQUE

Colloque C5, supplement

au

noll, Tome 48, novembre 1987

ELECTRON TRANSFER IN Hg,-,Cd,Te-CdTe HETEROSTRUCTURES

G.S. BOEBINGER, J.P. VIEREN, Y. GULDNER, M. VOOS and J.P. FAURIE*

Groupe de Physique des Solides de 1'Ecole Normale ~ u p 6 r i e u r e ( l ) , 24, Rue Lhomond, F-75231 Paris Cedex 05, France

'~epartment of Physics, University of Illinois at Chicago, Chicago, IL 60680, U.S.A.

Des experiences de magn&to-absorption

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1,6K dans des h&t&rojonctions HgCdTe-CdTe montrent qu'un gaz dl&lectrons bidimensionnel est forme' l'interface du cste' HgCdTe. La masse effective des electrons des deux sous-bandes peuplges est mesurge et llorigine du transfert est discute'e.

Far infrared magneto-absorption experiments performed at 1.6K in HgCdTe-CdTe heterojunctions show that a two-dimensional electron gas is formed in the HgCdTe layer at the HgCdTe-CdTe interface. The electron effective masses of the two populated subbands is obtained and compared to previous theoretical calculations. The electron transfer across the

interface involves deep traps in the CdTe layers.

We report magneto-optical experiments at low temperatures performed on a two-dimensional electron gas in Hgl-xCdXTe-CdTe heterojunctions grown by molecular beam epitaxy (MBE)

.

With this growth technique, the interfaces are better. defined than in the metal-oxide-semiconductor (MOS) structures which have been used until now to study two-dimensional electron gases in 11-VI compounds [I]. The samples (Sl, S2) used here were grown by

MBE [21 on GaAs substrates with the following growth sequence: for sample S1, a (100) CdTe buffer layer with thickness dl

-

2.02pm, a Hgo 8Cd0 2Te layer with thickness d2=1.02p, and a CdTe cap layer with thickness d3=300A:

for sample 52, a (100) CdTe buffer layer (dl=2.06p), a HgTe sandwich layer with thickness dSw=100A, a Hgo 8Cd0 2Te layer (d2=0.89pm), and a CdTe cap layer (d3-300A). The far-infrared magneto-absorption experiments reported here were done at 1.6K using as an infrared source a molecular laser pumped by a C02 laser.

Typical transmission spectra obtained at different infrared wavelengths are given in Figure 1 for sample S1 and 8 = 0', where 8 is the angle between the direction of the magnetic field and the normal to the layer plane. Two transmission minima can be clearly seen. As shown in Figure 2, the magnetic field positions of the minima depend on 8, and varies roughly as (cod)-', indicating that we are dealing with a two-dimensional System.

Figure 3 gives as a function of B the infrared photon energy, E l corresponding to the transmission minima detected for 8=0' (Fig.1). The observed optical transitions extrapolate to an energy E-0 at B=O. Taking

(')~aboratoire associe au C.N.R.S.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987572

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JOURNAL DE PHYSIQUE

into account the n-type nature of the investigated structures, as obtained from Hall experiments, these transitions are attributed to two-dimensional electron cyclotron resonance transitions. The weak feature appearing in Fig.2 at B-0.5T for 8=45' may correspond to bulk cyclotron resonance in the HgCdTe layer, with an electron effective mass m*=O. 0055 mo. Quantum Hall effect measurements at 4.2K yield a two-dimensional electron density, nS, a few times 10"crn'~, and a simple analysis of the width of these cyclotron lines yields an electron mobility ~ - 5 x 1 0 ~ c m ~ ~ - ' s - ' .

Thus, it is quite reasonable to consider that a two-dimensional electron gas occurs in this HgCdTe-CdTe heterojunction. We now seek to establish that: (i) the charge transfer occurs at least in part from the wide gap CdTe deep traps to the narrow gap HgCdTe layer, and (ii) the observed two-dimensional electron gas occurs at the heterojunction interface and not at 'the surface of the sample.

Figure 4 gives schematically the variation of the

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conduction and

T8

valence band-edges in sample S1 in the z direction perpendicular to the plane of the layers, taking into account electron transfer from the CdTe to the Hgo gCdO 2Te layer. The band gaps of CdTe and Hgo aCdO 2Te at 1.6K are 1.6eV and 60meV, respectively. The valence band offset between these two materials is not known, but it should be roughly similar to the HgTe-CdTe of £set

.

Unfortunately, the offset in HgTe-CdTe heterostructures is not yet firmly established, since reported values range from 40 to 350mev [3-51.

In any case', it is substantially smaller than 0.8eV, half the band gap of CdTe. In addition, the Fermi level, EF, in the CdTe layer is approximately located at the CdTe mid-gap, as shown by XPS measurements and transport measurements on CdTe epilayers grown under similar MBE conditions. This leads to the situation shown in Fig. 4 and, as a result, we believe that the two-dimensional electron gas is at least partly due to electron transfer from CdTe mid-gap deep levels to the HgCdTe layer, even if bulk HgCdTe electrons contribute to the kwo-dimensional layer.

Turning to point (ii), the data shown in Fig. 3 imply that two electron subbands, with energies E l and E2, are populated. The corresponding electron effective masses are ml=0.016 m0 and m2=0.007 mo for B-IT. The value of m, is comparable to that obtained recently (-0.015 mo) in an inversion layer occurring in a (p-type) Hgo aCdO 2Te MOS structure with a comparable electron density 161. Detailed agreement is not expected because the shape of the quantum well in the MOS structure is necessarily different from that in sample S1, where one is dealing with an accumulation layer.

In sample S2, the situation is somewhat more complicated because of the inclusion of a thin HgTe sandwich layer. The experimental data from S2 suggest the existence of a two-dimensional electron gas with two populated electron subbands, as in S1. However, in 52, due to the thinness of the HgTe sandwich layer, the two-dimensional electron gas is partially located in both the HgTe and Hgo 8Cd0 2Te layers, a new situation in semiconductor hetero junctions. In S2, the experimental electron effective masses are

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m1=0.023 mo and m2=0.013 mo a t B - I T w i t h v a l u e f o r nS s i m i l a r t o t h a t i n S1.

A s a c o n s e q u e n c e of t h e HgTe c o n d u c t i o n band mass ( - 0 . 0 3 m o ) , m l and m2 a r e l a r g e r i n S2 t h a n i n S1 b u t , n e v e r t h e l e s s , s m a l l e r t h a n t h e HgTe b u l k mass.

T h i s i s c o n s i s t e n t w i t h t h e t w o - d i m e n s i o n a l e l e c t r o n g a s b e i n g l o c a t e d a t t h e h e t e r o j u n c t i o n i n t e r f a c e i n t h e s e s a m p l e s . F i n a l l y , t h e e x p e r i m e n t a l r e s u l t s a r e u n a f f e c t e d b y c h e m i c a l e t c h i n g ( u s i n g bromine m e t h a n o l ) o f t h e e n t i r e CdTe c a p l a y e r and up t o one h a l f of t h e HgO8Cdo2Te e p i l a y e r . Thus, we b e l i e v e t h a t t h e o b s e r v e d two-dimensional e l e c t r o n g a s i s l o c a t e d a t t h e h e t e r o j u n c t i o n i n t e r f a c e .

U n f o r t u n a t e l y , due t o l a r g e d i f f e r e n c e s i n nS, it i s n o t p o s s i b l e t o make a s i g n i f i c a n t comparison between o u r r e s u l t s a n d t h o s e p r e v i o u s l y r e p o r t e d [ 7 - 9 1 . A l s o , p r e v i o u s t h e o r e t i c a l r e s u l t s c o r r e s p o n d t o l a r g e r v a l u e s o f nS [ l o ] . However, u n p u b l i s h e d c a l c u l a t i o n s [ I l l f o r nS-3x101'cm~2 y i e l d two o c c u p i e d s u b b a n d s w i t h m l = 0 . 0 1 5 mo a n d m2=0. 010 mo i n t h e c a s e o f MOS Hgo 8Cd0 2Te s t r u c t u r e s w i t h N A -N D- - 1 0 ' ~ c r n - ~ . These v a l u e s a r e c o n s i s t e n t w i t h t h e r e s u l t s o b t a i n e d i n sample S1. I n any c a s e , p r e c i s e c a l c u l a t i o n s w i l l b e c o m p l i c a t e d b y t h e r e s o n a n c e between t h e i n t e r f a c e w a v e f u n c t i o n and t h e b u l k ' Hgo 8Cd0 2Te v a l e n c e b a n d [ 1 , 1 0 ] . I n t h e c a s e of s a m p l e S2, f u r t h e r c o m p l i c a t i o n r e s u l t s from t h e p r e s e n c e of t h e HgTe sandwich l a y e r .

I n summary, we have d e m o n s t r a t e d t h a t a two-dimensional e l e c t r o n g a s c a n occur i n Hgl - X C d Te-CdTe h e t e r o j u n c t i o n s . The e l e c t r o n transfer i n v o l v e s d e e p c e n t e r s i n t h e g a p o f t h e CdTe l a y e r , c o r r e s p o n d i n g t o a new c h a r g e t r a n s f e r mechanism.

We w i s h t o t h a n k F . Koch a n d J . C . T h u i l l i e r f o r h e l p f u l d i s c u s s i o n s . T h i s work was s u p p o r t e d b y t h e D i r e c t i o n d e s R e c h e r c h e s , E t u d e s e t T e c h n i q u e s . GSB i s s u p p o r t e d by a NATO P o s t d o c t o r a l F e l l o w s h i p .

I I I I

I

0 2 4 6 8

B (Tesla 1

F i g u r e 1: T y p i c a l t r a n s m i s s i o n s p e c t r a a s a f u n c t i o n o f t h e m a g n e t i c f i e l d , B, o b t a i n e d f o r d i f f e r e n t i n f r a r e d w a v e l e n g t h s i n sample S1 a t 1.6K f o r 8 = 0 ' .

F i g u r e 2: T r a n s m i s s i o n s p e c t r a v e r s u s B o b t a i n e d i n s a m p l e S1 f o r a n i n f r a r e d w a v e l e n g t h h = 1 1 8 p f o r two v a l u e s o f 8 .

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C5-340 J O U R N A L D E PHYSIQUE

-

@roo

^-

T = 1.6K

I

0 2 4 6

B ( Tesla

F i g u r e 3 : P o s i t i o n o f t h e t r a n s m i s s i o n minima for s a m p l e S 1 a s a function o f f a r - i n f r a r e d p h o t o n e n e r g y , E , a n d m a g n e t i c f i e l d , B ( o p e n c i r c l e s ) . The s o l i d l i n e s a r e o n l y g u i d e s t o t h e e y e .

F i g u r e 4 : S c h e m a t i c v a r i a t i o n o f t h e c o n d u c t i o n a n d v a l e n c e b a n d s i n t h e z d i r e c t i o n f o r s a m p l e S1.

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1656 ( 1 9 8 6 ) .

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907 ( 1 9 8 3 ) .

4 . S . P . Kowalczyk, J . T . Cheung, E.A. K r a u t a n d R.W. G r a n t , P h y s . Rev. L e t t . S, 1605 ( 1 9 8 6 ) .

5 . T r a n Minh Duc, C. Hsu, a n d J . P . ~ a u r i e , P h y s . Rev. L e t t .

s,

1127 ( 1 9 8 7 ) .

6 . R. W o l l r a b a n d F . Koch, p r i v a t e communication.

7 . J . S c h o l z , F . Koch, J . Z i e g l e r a n d H . M a i e r , S o l i d S t a t e Commun.

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665 ( 1 9 8 3 ) .

8 . J . S i n g l e t o n , F . N a s i r a n d R . J . N i c h o l a s , S u r f a c e S c i e n c e 170, 409 ( 1 9 8 6 ) .

9 . F . Koch, i n S p r i n g e r S e r i e s i n S o l i d S t a t e S c i e n c e s , V o l . 53, e d i t e d b y G . B a u e r , e t a l . , p . 20, 1 9 8 4 .

1 0 . Y . T a k a d a , K. A r a i a n d Y . Uemura, i n P h y s i c s o f Narrow Gap Semi- c o n d u c t o r s , L e c t u r e N o t e s ixn P h y s i c s IS?,., e d i t e d b y E. G o r n i k ( S p r i n g e r , H e i d e l b e r g , 1 9 8 2 ) , p . 1 0 1 .

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