EXAFS STUDY OF A QUITE RELAXED ZINC-BLENDE LATTICE : THE GaAsySb1-y ALLOY

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EXAFS STUDY OF A QUITE RELAXED

ZINC-BLENDE LATTICE : THE GaAsySb1-y ALLOY

A. Marbeuf, F. Karouta, H. Dexpert, P. Lagarde, A. Joullié

To cite this version:

A. Marbeuf, F. Karouta, H. Dexpert, P. Lagarde, A. Joullié. EXAFS STUDY OF A QUITE RE-

LAXED ZINC-BLENDE LATTICE : THE GaAsySb1-y ALLOY. Journal de Physique Colloques, 1986,

47 (C8), pp.C8-369-C8-373. �10.1051/jphyscol:1986872�. �jpa-00226194�

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JOURNAL DE PHYSIQUE

Colloque C8, supplgment au n o 12, Tome 47, dgcembre 1986

EXAFS STUDY OF A QUITE RELAXED ZINC-BLENDE LATTICE : THE GaAsySb,-, ALLOY

A. MARBEUF, F. KAROUTA*, H. DEXPERT", P. LAGARDE** and A. JOULLIE'

Laboratoire d e Physique des Solides, CNRS, 1 , Place A. Briand, F-92195 Meudon Principal Cedex, France

" ~ q u i p e de Microopto6lectronique de Montpellier, U A 392, USTL, F-34060 Montpellier Cedex, France

* LURE, Bstiment 209C, Universite Paris-Sud, F-91405 Orsay Cedex, France

RCsumC :Les spectres EXAFS apres les seuils K de Ga et As, et L I de Sb sur ~ ~ GaAsySby ont CtB enregistrgs. ~'analyse des spectres, dans le cas du seuil Gal pour des compositions situCes de part et d'autre de la lacune de miscibilite (0 6 y 4 .11,.9 6 y 4 l),conduit aux distances entre Ga et ses plus proches voisins (NN). La diztribution des distances cstion-anion est bimodale (d (Ga-As)=

2.46 2 .02 A, d (Ga-Sb) = 2.63 _+ .O1 A) et tres proche de la limite de PAULING- HUGGINS. Cette grande relaxation de la structure blende de zinc slexplique bien B la lumikre d'un modele de champ.deforces de valence (Keating),en cherchant pour quelle configuration les forces Clastiques agissant sur chaque atome s'annullent:

les deux sous-rgseaux apparaissent distordus,tandis qu'une distribution multimodale des distances entre seconds voisins (NNN) est prCdite,en accord avec llBlargissement du pic correspondant de la fonction de distribution radiale.

Abstract' : EXAFS measurements above the K-edge of Ga, As and the L ~ = ~ - e d g e of Sb in G a A ~ ~ s b l - ~ are performed. The analysis of data above the Ga K-edge,on both sides of the miscibility gap ( 0

<

y 4 .11, .9 ,( y

<

^l),leads to nearest-neighbor (NN) distances around Ga asom. The distribution of cation-anion distances is bimodal (d(Ga-As) = 2.46 r 02 A, d (Ga-Sb) = 2.63 f .O1 ) and very close to the PAULING-HUGGINS limit. This great relaxation of the zinc-blende structure is ex- plained by using a Valence-Force-Field model (Keating) and searching for the configuration at which the elastic forces acting upon every atom vanish : both sublattices are distorded and multimodal second-neighbor distribution of distance is predicted,in agreement with the broadening of the corresponding peak of the radial distribution (NMN distances).

1. Introduction.

Among the I11 V and I1 VI pseudobinary systems, is interesting because of its miscibility gap below peritectic transformation (t= 7 4 7 ' C ) which allows one to predict clustering tendency [I]. Further crystallographic information is nee- ded to understand this behaviour which may influence the carrier mobility of the alloy or limit device feasabilityL2-31. On the other hand, alloy description by the virtual crystal approximation (VCA) [4] is probably ,unadequate, as shown in other materials 15-71. However, a local structure study such as EXAFS analysis can-provide precise information on the GaAs Sb zinc blende lat'tice.

Y 1-Y

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986872

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C8-370 JOURNAL D E PHYSIQUE

2. Sample p r e p a r a t i o n and a n a l y s i s t e c h n i c s .

Bulk m a t e r i a l s were Bridgman grown by c o o l i n g Ga-rich melt (xGa=.50, t = 800

P -

1150°C). Homogeneous i n g o t s were s e l e c t e d by microprobe a n a l y s i s of X-Ray d i f f r a c - t i o n measurements of t h e l a t t i c e parameter a?y) [8]. Samples were t h e n powdered ( c r y s t a l l i l e s i z e s m a l l e r t h a n 15bm).

GaSb and GaAs were taken a s s t r u c t u r a l s t a n d a r d s f o r e x t r a c t i n g t h e b a c k s c a t t e - ring-amplitudes andphases u s e d i n t h e EXAFS a n a l y s i s . Absorption s p e c t r a around Ga

(As)K-edge and Sb LIII-edge were performed a t LURE (ORSAY, FRANCE) u s i n g e i t h e r a double Si(311) c r y s t a l o r a Si(400) channel c u t monochromator. The D C I s t o r a g e r i n g energy was 1.72eV and t h e e l e c t r o n c u r r e n t w a s t y p i c a l l y 180 mA.

The EXAFS modulation f u n c t i o n ~ ( k ) was e x t r a c t e d from t h e background w i t h a Victoreen f i t of t h e pre-edge, followed by a f i t of t h e a t o m i c l i k e smooth background (*). The k ~ ( k ) 3 d a t a a r e Fourier-transformed t o r e a l space and t h e c o n t r i b u - t i o n of t h e f i r s t s h e l l (NN atoms) o r t h e second s h e l l (NNN atoms) i s b a c k t r a n s f o r - med

.

3 . R e s u l t s .

The r a d i a l d i s t r i b u t i o n f u n c t i o n s F ( R ) around t h e Ga o r t h e As K-edge a r e s u i t a b l e f o r a n a l y s i s , b u t n o t t h e ones around t h e Sb L I I r e d g e because of t h e poor r e s o l u t i o n due t o t h e s m a l l energy range a c c e s s i b l e between L I11 and L ~ ~ - edge : AE = 250 eV. F i g . 1 shows F(R) s p e c t r a around t h e Ga K-edge f o r y = . I 1 and y =.go. The f i r s t peak corresponding t o Ga-V d i s t a n c e s (V= A s , Sb) was f i l t e -

red i n t h e range 1.80

-

2.75 A and Fou-

I

^A

I

rier-backtransformed b e f o r e f i t t i n g . From

3 0 . 0 1 . 0 2 . 0 3 . 0 4 . 0 5 . 0

t h e adjustment of t h e r e s u l t i n g k ~ ( k ) up

0 . R

thl

t o a maximum of 16 A

-

^I and u s i n g GaAs

and GaSb a s r e f e r e n c e compounds, t h e NN Fig.1-Radial d i s t r i b u t i o n f u n c t i o n of Ga d i s t a n c e s have been d e r i v e d . As shown by K-edge s p e c t r a of GaAs S b l y . F i g . 2 a n d F i g . 3 , t h e f i t r e q u i r e s t w o anion Y s u b s h e l l s around Ga atoms: t h u s , t h e d i s -

t r i b u t i o n of dNN i s bimodal (d Ga-As #

-

dGa-sb) with a r e l a x a t i o n parameter of t h e l a t t i c e E = [dGa-sb-dGa-AA/

[(a>aSb

-

a:aAs) n / 4

]

which does n o t v a r y w i t h y ( E exp ^Q- 0 . 9 0 ) .

Fig.2-Expsrirnental k 3 ~ ( k ) (...) f i t t e d

I V

-

_with₁a n i o n s u b s h e l l (--) f o r t h e composition y = 0.90.

R cA -l)

*

A l l computer c a l c u l a t i o n s (Analysis & Mode1)have been made a t CIRCE (ORSAY,FRANCE).

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Fig.3-Experimental k 3 ~ ( k ) (...) fitted with 2 anion subshells (-1 for the composition y = 0.90.

3. 5 . 7 . 9 . ii. i 3 ,

Fitting results with fixed As and Sb atom numbers are listed in Table I.

Table I

-

Parameters used in the fit (o 2 are Debye-Waller factors, AE is the edge shift, is the reliability factor).

0 -2 -2

Y N As dGa-As(A) NSb dGabsb(A) "Ga-AS (10-2 i2) (10-2 i2) AEo(eV)

"

^Ga-Sb

R(%)

Similar analysis of the As K-edge for As-rich composition alloys (y t0.90) leads to NN distances d = 2.447 in excellent agreement with the above results.

As-Ga

The 12 NNN atoFs around Ga atoms are Ga and contribute to the peaks of F(R) between 3.20 and 4.70 A. The shape of these peaks shows the presence of two main groups of dGa-Ga. Due to the different Ga-As and Ga-Sb bondlengthes, it can be predicted that the Ga-Ga distances with an As atom between Ga atoms (dGa-(As)-Ga ) will be shorter than the Ga-Ga distances with a Sb atom (d Ga-(Sb)-Ga). In the simulation,am- plitude and backscattering phase are extracted from the second shell of GaAs for Ga- As-Ga and from the second shell of GaSb for Ga-Sb-Ga. A bimodal behaviour of the dGa-Ga distribution is still found as follows for y = 0.90 :

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JOURNAL DE PHYSIQUE

These cation-cation distances are rather close to that in constituent binaries GaAs and GaSb. All these results indicate a quite relaxation of the blende structure.

4. Structural model of the G ~ A S ~ S ~ _ , - ~ lattice.

Because of the existence of the NN and NNN distances bimodal distribution, the lattice distorsion occurs in both fcc sublattices. In order to understand how zinc blende structure accomodates such distorsions, the Valence-Force-Field model (VFF) in the ~eating's scheme [9] is used. In this approach, the deformation energy U of each unit cell is expressed in terms of the variations of the scalar quantities

-c - " I

Here s and s' refer to the two atoms of the ucit cell while i,j refer to the 4 bonds connecting these two atoms with thrse at the vertices of each tetrahedron. U contains only two elastic constants :ai (aGaAs or a [101) which describes the cen-

GaSb

tral-part of the potential (bond-stretching) and 6 ij (6GaAsEl~], pGasbDo] and ⁼ J B ~ ~) ~corresponding to the non-central forces . ~ ~ ~ ~ ~ ' ( bond-bending); ro is the bond distance in the undistorded crystal (r = a0+@/4). The computer simulation of the alloy lattice with an anion random distribution is carried out by searching fwr the configuration at which the elastic forces acting upon each atom vanish. In order to minimize the influence of the surface atoms, the cluster contains 1000 atoms. Atomic radial distributions P(d) around Ga, As or Sb are then obtained. In the case ofP(d) around Ga, the bimodal distribution of dNN and dNNN is obtained as a resultofagreat relaxation in both sublattices (Fig. 4). Good agreement between experimental and

Fig.4- Calculated.atomic radial distri- Fig.5-Calculated cation-anion distances bution around Ga for G ~ A S . ~ ~ S ~ . , ~ (-) in GaAs Sb (.measured

Y 1-Y values, X Vegard's law).

calculated dNN is shown on Fig.5, but with a calculated relaxation parameter

(scale=

0.85) smaller than the experimental one (E = 0.90) and a slight variation versusy exp

Of d ~ a - ~ s and d Ga-Sb. The bimodal NNN distribution corresponds to the broadening of

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t h e c o r r e s p o n d i n g p e a k s i n F ( R ) . The roo-t mea6"square d e v i a t i o n of t h e t e t r a h e d r o n a n g l e i s l e s s t h a n 1.40' around t h e 109.41" mean v a l u e .

I f a non-random d i s t r i b u t i o n o f a n i o n s i s t a k e n i n t o a c c o u n t , c a l c u l a t i o n s y i e l d s i m i l a r r e s u l t s . Thus, c l e a r e v i d e n c e of c l u s t e r i n g tendency c o u l d be made by u s i n g o t h e r t e c h n i c s , such a s X-Ray d i f f u s e s c a t t e r i n g .

R e f e r e n c e s

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(1984) 31 1 .

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(1984) 2295.

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and PODGORNY M., J . C r y s t . Growth, 1_1 (1985) 205.

[8] MAN1 H., Thitse d e 3&me c y c l e , U n i v e r s i t e d e M o n t p e l l i e r (1984), FRANCE

pJ KEATING P.N., Physj- Rev.,

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