PHYSICAL REVIEW B VOLUME 48, NUMBER 14 1 OCTOBER 1993-II Modeling of the spatial structure by means of wide-angle x-ray scattering and extended x-ray-absorption fine structure for the CusAs26Se« and Cu26As37Se37 semiconductor alloys D. Gomez-Vela Departamento de Ingenieria Electrica y Electronica, Escuela Universitaria Politecnica de Cadiz, Universidad de Cadiz, 11003 Cadiz, Spain L. Esquivias Departamento de Estructura y Propiedades de los Materiales, Facultad de Ciencias, Universidad de Cadiz, Apartado 40, Puerto Real (Cadiz), Spain C. Prieto Instituto de Ciencia de Materiales de Madrid (sede B), Consej o Superior de Investigaciones CientiPcas, Universidad Autonoma (C 4), 2804-9 Madrid, Spain and Laboratoire pour l'Utilisation du Rayonnement Electromagnets'que, 91405 Orsay Cedex, France (Received 6 April 1992; revised manuscript received 28 June 1993) Short-range orders of two amorphous alloys of the chalcogenide Cu-As-Se family have been studied by extended x-ray-absorption fine structure (EXAFS) and wide-angle x-ray scattering (WAXS). Up to now, the construction of spatial models by a modified metropolis Monte Carlo simulation technique carried out by us was solely based on the information gained in the WAXS experiments. In this work, we also take into account the information extracted from the spectroscopic EXAFS experiments to get more- realistic spatial models. In this way, the average coordination numbers of the three different atoms and their distances to the first coordination sphere have been determined in Cu, As, 6Se«and Cu26As37Se37. The extraction of the inverse Fourier transforms to the generated atomic distribution function leads us to a simulated reduced interference function si(s ), in good agreement with its experimental counterpart. I. INTRODUCTION During the last few years amorphous semiconductors have been used in the manufacture of solar cells, bat- teries, and phototransistors' as well as in some steps of the technological processing of very-large-scale integra- tion (VLSI) microelectronic circuits. Recently the chal- cogenide family (CdS„Se, „) appears to play an impor- tant role in the synthesis of materials with nonlinear opti- cal properties made of a ceramic matrix and the disper- sion within of a chalcogenide phase. In this way, the structural study of these compounds has gained addition- al interest, as electronic properties are related to the glassy structure. The addition of copper to amorphous As-Se alloy sys- tems has been studied by many authors ' and there result significant modifications of its properties, including the observation of a glass transition, network connectivity, and electrical conductivity. The last one strongly de- pends on short-range order (SRO), and one of the main purposes of this work is to determine the distances and pair coordinations of each component. Experimentally, the metal atoms appear to be tetrahedrally coordinat- ed, ' and the general structural model proposed by Liu and Taylor gives a tetrahedral structure when the metal is added to the glasses. The particular spatial models proposed in this work also satisfy this feature. Wide-angle x-ray scattering (WAXS) is a mature tech- nique. A complete data analysis begins by obtaining the radial distribution function (RDF) that carries the global average structural parameters concerned with the first coordination shell, such as, the average distance between atoms pairs and the coordination number. This is related to the position and area of the first peak. Extended x-ray-absorption fine structure (EXAFS) is a very useful technique to determine separately the coordi- nation number for the three components. In this way the EXAFS experiments have been performed at the three absorption K edges corresponding to the Cu, As, and Se elements at room temperature. The availability of EXAFS spectroscopy has led to the determination of a set of parameters describing the SRO that also satisfies WAXS experiments, which has challenged us to con- struct dimensional models that include this information. This means that EXAFS could act as a way of testing and refining our previous Metropolis Monte Carlo (MC) simulation technique as a second local probe. The pur- pose is to reach more realistic and accurate models from a physical point of view. II. EXPERIMENTAL Selected samples were named 1. and H referring to their low or high copper concentration. Case I, represents a deviation in the As/Se ratio of the widely studied composition Cu„-(As2Se3), „, while the other presents a higher Cu concentration and an As/Se ratio equal to 1. Both compositions are included in the amorphization region (see Borisova ). 0163-1829/93/48{14)/10110{8)/$06. 00 48 10 110 1993 The American Physical Society