THE LOCAL STRUCTURE IN THE MIXED CHALCOPYRITE CuGax In1-xSe2

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THE LOCAL STRUCTURE IN THE MIXED CHALCOPYRITE CuGax In1-xSe2

G. Antonioli, S. Bini, P. Lottici, C. Razzetti

To cite this version:

G. Antonioli, S. Bini, P. Lottici, C. Razzetti. THE LOCAL STRUCTURE IN THE MIXED CHAL- COPYRITE CuGax In1-xSe2. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-431-C8-434.

�10.1051/jphyscol:1986887�. �jpa-00226218�

THE LOCAL STRUCTURE IN THE MIXED CHALCOPYRITE C ~ G a ~ 1 n ~ _ ~ S e ~

G. ANTONIOLI, S. BINI, P.P. LOTTICI and C. RAZZETTI'

Dipartimento di Fisica de1l'~niversitii and GNSM-CNR, CISM-MPI, I-43100 Parma, Italy

'and MASPEC-CNR Institute, I-43100 Parma, Italy

Rdsumd - La structure de plus proches voisins dans l'alliage chalcopyrite pseudoternaire CuGaxInl-,Se2 a dtc? Qtudiee par EXAFS sur les seuils K de Cu, Ga et Se. Les distances de liaison en fonction de la composition ont dtd determinees et nous avons calculd l e parametre de distorsion u(x) spdcifiant la position de l'anion dans la maille uni taire. Nous avons trouvde une variation non-lineaire de u(x), causde par une variation de la distance Ga-Se. Ceci donne une contribution structurale A la ddpendance non-lindare avec la composition de la gap optique, en analogie avec l e comportement des al- liages pseudobinaires.

Abstract - T h e nearest neighbour structure in the pseudoter- nary chalcopyrite alloy CuGaxln,-, Se2 has been investigated by EXAFS measurements on Cu, Ga and S e K-edges. Bond lenghts have been determined a s a function of composition and the internal distortion parameter u(x), which specifies the anion position in the unit cell, has been calculated. W e have found a nonlinear variation of u(x), due to a relatively large variation of the Ga-Se bond lenght. This effect may give rise

to a structural contribution to the optical gap nonlinearity observed in this alloy, in analogy to what found in pseudobi- nary systems.

INTRODUCTION

ABC2 chalcopyrites crystallize in the tetragonal structure I42d which derives directly from the cubic zincblende lattice F43m. Each A or B cation is tetrahedrally coordinated to four C anions, whereas the anions are coordinated to two A and two B cations. T h e unit cell is tetragonally distorted with a distortion parameter 9=c/2a, a and c being the lattice constants. T h e anions are displaced from the ideal tetrahedral sites X by an amount bu=u-Y, u being an internal distor- tion parameter. This anisotropy is reflected in different interatomic distances R(A-C) and R(6-C) whose determination is of extreme impor- tance in structural and electronic studies.

CulnSe2-based solar cells and heterodiodas are among the most promi- sing candidates for large scale applications (I). A controlled tuning of the optical and lattice properties is desirable to improve perfor- mances: the system CuGaxIn,-, Se2 , which shows complete miscibili t y and a continuous variation of structural and electronic parameters, is therefore of great interest (2). Here w e report preliminary EXAFS results on different compositions of this system, with the main scope to obtain informations on its structural parameters.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986887

JOURNAL DE PHYSIQUE

EXPERIMENT AND DATA ANALYSIS

We synthesized by chemical vapour deposition five compositions belon- ging to the CuGa,Inl-,Se2 system (x=0.,0.3,0.5,0.7,1.). Samples were obtained a s single crystals ( 1 0 x 3 ~ 2

after one week reaction with a 50 "C temperature gradient around 750 "C. The crystals were then crushed and the powdered material w a s characterized by X-ray diffrac- tion. The lattice constants obtained are reported as a function of composition in T a b l e 1. Precisely calculated amounts of the same pow- ders were then deposited on Millipore membranes by filtering from methanol dispersion. The membranes were then fastened to Kapton tape and used for EXAFS measurements on the Cu,Ga and Se K-edges. The spetra were taken on the X-ray line of Adone at the PULS-INFN facili- t y in Frascati, Italy. T h e EXAFS x(k) spectra were Fourier transfor- med and back-transformed to obtain single shell contributions k%(k), amplitudes and phases. A s threshold energies w e choose the inflection points at the absorption edges. Ternary end members, by using known crystal parameters (31, were used as experimental models, except in the case of S e K-edge data where theoretical phases and amplitudes were employed (4). A least square fitting procedure was used to ob- tain distances and disorder parameters. We report in Figs.1-2 examples of the EXAFS Z (k), in Figs.3-4 of EXAFS radial distribution fun- ctions and in Figs.5-6 two examples of the fitting results.

The structure of the alloy may be approximately described by a compo- sition dependent u(x), by defining an effective group 111-anion di- stance R(X-Se) a s the weighted average of R(Ga-Se) and R(1n-Se):

Fig.1 -X(k) for CuGaSe2 (edge Ga)

.2

rdf

Fig.2 -%(k) for (edge Ga)

.2

rdf

.1

0.

1. 1.8

2.6

1. 1.8 2.6 3.4 42

R (A) R (A)

Fig.3 - RDF for CuInSe, Fig.4 - RDF for C ~ G a ~ ~ ~ 1 n ~ ~ , S e ,

( edge Cu) . ^{(edge Cu)}

T h e first neighbours distances and the distortion parameters obtained are shown in Table 1. T h e R(Cu-Se) distances change linearly between the end members whereas R(1n-Se) is nearly constant with composi- tion. As to R(Ga-Se) w e find a larger variation of nonlinear charac- ter. This accounts for the nonlinear behaviour of u(x) a s defined by Eq.(l) and shown in F i g . 7 . For comparison, in the Table 1 are also shown the structural parameters a s obtained by the CTB (conservation of tetrahedral bonds) approximation, which essentially assumes that the cation-anion distances are given by the sum of tetrahedral Pau- ling radii (5).

.---.---.

I CuGa,In,-, Se2 1 x=O.O x=0.3 ~ 1 0 . 5 x=0.7 x=l.O

I---I--- I

I

I a( x 1 5.78 5.73 5.70 5.66 5.61 I

I T ( x ) 1 1 . 0 0 2 0.996 0.991 0.989 0.980 I

I R(Cu-Se) 1 2.425 2.42 2.42 2.42 2.414 I I R( Ga-Se) I - ^{2,.43 2.455 2.44 2.417} - ^I

I R( In-Se) 1 2.595 2.60 2.60 2.60 I

I U(X) 1 0.2244 0.2311 0.2337 0.2400 0.2495 I

I I I

1 CTB a(x) 1 5 . 8 5 2 5.798 5.767 5.729 5.681 I

I CTB u(x) 1 0.2367 0.2446 0.2500 0.2554 0.2636 1

Table 1 - First and second row: Experimental a(x) and ~ ( x ) lattice parameters. Third to fifth: first neighbour distances as determined by EXAFS. Sixth: internal deformation parameter u(x) a s calculated by EXAFS and X rays data. Seventh and eighth: predictions of the CTB model. All distances and lattice parameters a(x) are in A.

COMMENTS

In AEC2 chalcopyrites the bandgaps are substantially smaller than in

their binary analogs. Detailed studies of the electronic structure

(5) have shown that a large part of this anomaly results from a

structural contribution due to unequal R(A-C) and R(B-C) bond

lenghts, i.e. to du-u-4=0, an effect which is present also in pseu-

dobinary alloys ( 6 ) . Additional contributions of "chemicaln origin,

due to p-d hybridizations and to different cation electronegativities,

are present even if R[A-C)=R(B-C) .

JOURNAL DE PHYSIQUE

As to pseudoternary chalcopyrite alloys, whereas the anion-mixed pounds like C U I ~ S ~ , S ~ ~ ~ ~ - , , ( 7 ) show a linear change of the optical band gap with composition, the cation cross substitution generally produces an optical gap bowing. I n CuGaxInl-, S e 2 , Bodnar et al. ( 8 ) found a nonlinear change of Eg between the end member values, which is represented in Fig.7. In CuInSe,, according to band structure cal- culations ( 5 1 , a decrease i n the distortion parameter u corresponds to a decrease of the band gap. I n CuGa,Inl-, Se2 alloy, a linear chan- ge of u between the values observed i n the end members should produ- ce a linear change in Eg(x). T h e nonlinear change of ulx) mirrors the band gap bowing observed (Fig.7) i n this alloy. This structural con- tribution to the optical gap nonlinearity could be as important as disorder or chemical effects, due to nonlinear change with composi- tion i n the p-d hybridization of the uppermost valence band. Anion mixing, on the other hand, produces small structural effects, because the anion radii have little influence on u(x) and then on the depen- dence of the optical gap on composition (5). This could be supported by our EXAFS results on mixed defect chalcopyrites ( 9 ) , where the bond distances seem to present only m a l l changes wi th composi tion and therefore the internal distortion parameters should change li- nearly.

Fig.7 - Composition depen- 1.6 dence of the opti-

cal band gap ( 8 ) (small circles,

Cl*

left scale) and -

of the internal -

distort ion parame- x ter u(x) as given

cn by Eq.1 (large * ^1.2

circles, right scale) i n CuGaxInl-, Se?.

T h e dashed l ~ n e 1 . : f connects the end

member values.

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