zyxwvutsrq zyxwv zyxwvut D. GOMEZ-VELA et al.: EXAFS Study of the Cu-As-Se Amorphous Alloys 303 phys. stat. sol. (b) 169, 303 (1992) Subject classification: 61.40 and 78.70; S8.16 Departamento Ingenieria Elictrica, Escuela Universitaria Politicnica de Cadiz, Universidad de Cadiz’) ( a ) , Departamen to Estructura y Propriedades de 10s Materiales, Facultad de Ciencias, Universidad de Cadiz’j (b), and Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Cientificas, Facultad de Ciencias (C-4), Universidad Autbnoma, Madrid3), and Laboratoire pour I’Utilization du Rayonnement Electromagnetique, Orsay ( c ) EXAFS Study of the Cu-As-Se Amorphous Alloys BY D. GOMEZ-VELA (a), L. ESQUIVIAS (b), and C. PRIETO (c) Some amorphous alloys of the chalcogenide Cu- As-Se family are studied by EXAFS spectroscopy. Results are presented, obtained for the two samples Cu,As,,Se,, and Cu,,As,,Se,,. The mean coordination number of the three different atoms and their distances to the first coordination sphere are determined in both the alloys. It is found that the copper atoms are fourfold and the arsenic ones are threefold coordinated in both the compounds and the coordination numbers for selenium are 3.8 and 2.2 in Cu,As,,Se,, and Cu,,As,,Se,,, respectively. zyxwvuts Les alliages amorphes des calcogenures de la famille Cu-As-Se ont ete etudies par la spectroscopie EXAFS. Dans ce travail, ils seront presentes les resultats obtenues pour deux Cchantillons avec les formules Cu,As,,Se,, et Cu,,As3,Se3,. Les coordinations moyennes des trois atomes difierents et ses distances Li leurs premieres sphkres de coordination ont kte ditermines pour les deux alliages. On a trouvk que les atomes du cuivre sont tetra-coordinis et ces de l’arsenic tri-coordints dans les deux ichantillons et les coordinations pour les atomes du selenium sont 3,8 et 2,2 pour le Cu,As,,Se,, et le Cu,,As3,Se3, respectifment. 1. Introduction Amorphous semiconductors have interesting properties, such as photoconduction, switching, Hall effect, etc. The presence of some uncontrolled impurities does not determine their characteristic properties. Chalcogenide glasses containing metal atoms form an interesting class of amorphous semiconductors. The family obtained by adding Cu to the As-Se glasses becomes interesting because the conductivity increases by Cu addition and the activation energy decreases. Those electronic properties are related with the glassy structure and the aim of the present work is to determine the distance and the coordination of each component. Experimentally the metal atoms appear as tetrahedrally coordinated [l, 21 and the general structural mode proposed by Liu and Taylor [3] gives a tetrahedral structure when the metal is added to the glasses. A lot of experimental works have been reported in the literature studying the Cu-As-Se family, where the alloy stoichiometry came from the addition of a small quantity of Cu to ’) E-11005 Cadiz, Spain. ’) Apdo. 40, Puerto Real, E-11510 Cadiz, Spain. 3, E-28049 Madrid, Spain. z 304 zyxwz zyxwvutsrq zyxwvu zyxwvut D. GOMEZ-VELA, L. ESQUIVIAS, and C. PRIETO some As-Se glasses, generally AsSe and As,Se, [4], whose stoichiometry may give also crystalline compounds. In this work, we are interested in the structural behavior of two alloys: the first one is composed of a small quantity of copper and a As/Se ratio far from 1 or 213 (corresponding to the AsSe and As,Se,), and the second one is composed of a big quantity of copper and the As/Se ratio equals 1. The extended X-ray absorption fine structure (EXAFS) spectroscopy is a technique very useful in order to determine the pair distances and the coordination number separately for the three components, it has been recently used for some other alloys of the same Cu-As-Se amorphous family [5]. The average coordination number can be calculated and it will be compared with that obtained from wide angle X-ray diffraction spectroscopy in order to construct a model via Monte-Carlo methods [6 to 81. On 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. Two samples were selected, Cu,As,,Se,, and Cu,,As3,Se,,, in order to have a large difference in the Cu concentration and to find the difference in their glass structure. -1.01 zyxwvutsrqp zyxwvut zyx I 77600 I I I I f900 I 72200 I I I I 72500 0.6 - 0.4 - C - 0.2 - - 0 - 7 1 I I I I I I I I Fig. 1. Room temperature EXAFS spectra of the Cu,As,,Se,, alloy taken at a) Cu K-edge, b) As K-edge, and c) Sc K-edge 305 EXAFS Study of the Cu-As-Se Amorphous Alloys 2. Experimental z Amorphous bulk materials were prepared by the usual melt-quench technique, the constituents Cu, As, and Se (4N purity) were weighed and sealed in an evacuated and inert quartz ampoule, which was then heated at 950 "C for 4 h. During the melt process the tube was rotated in order to intermix the constituent to ensure homogenization of the melt [9]. EXAFS experiments were carried out on the EXAFS-111 beamline at D.C.I. storage ring (Orsay) with an electron beam energy of 1.85 GeV and an average current of 250 mA. Data were collected with a fixed exit monochromator using two flat Si(311)crystals in transmission mode, detection was made by using two ion chambers with air as fill gas. The fast EXAFS acquisition operative mode [lo] was used to collect at least ten spectra in order to accumulate them and to have a good enough signal-to-noise ratio. Energy resolution was estimated to be about 2 eV by the Cu foil 3d near edge feature. The energy calibration was monitored using the Cu foil sample, and was set as 8991 eV at the first maximum above the edge. Samples were prepared by powdering, sieving, and the selecting small particles less than 10 pm in size by floating the powder. The particles were then deposed on tape. About two layers of tape were used to fabricate samples with Apx of 0.1 to 1.0 and a total px less than 1.5, zyxwvutsr zyx z zyxwvutsrq zyxwvutsrqponml zyxwvutsrqponm -7.01 I 7 6W I 1 I 17900 I I I 12200 I I I 12500 0 12500 1.BW ?3700 - 73100 energy (e 1) I Fig. 2. Room temperature EXAFS spectra of the Cu,,As,,Se,, alloy taken at a) Cu K-edge, b) As K-edge, and c) Se K-edge z 306 zyxw zyxwvutsr zyxw D. G~MEZ-VELA, L. ESQUIVIAS, and C. PRIETO 3. Analysis of the Results Fig. 1 and 2 show the absorption spectra at the K-edges of the three components of the sample Cu,As,,Se,, and Cu,,As,,Se,,, respectively. Measurements have been performed on the same thickness of each sample. A classical procedure has been used to analyze the EXAFS spectrum: Above the edge, the signal background were removed by a multi-iteration curve smoothing procedure. The analysis of the EXAFS signal to get the position of the neighbors around the absorber atom has been carried out using the well-known EXAFS expression [ll] zyxwvut zyxwvu zyxwvut =--z ~(k) N. exp (- 2k2a,2)exp (- T j R j / k&(k) ) sin [2kRj kR3 J j + qnj(k)]. (1) This expression describes the EXAFS oscillations for a Gaussian distribution of neighbors around the central atom, in the single scattering theory and in the plane-wave approximation. k is the wave vector of the photoelectron, which is related to the electron mass (me) and the threshold energy (E,) by k = [2;- L ( E - E,) , 0 0.1 0.2 0.3 0.4 distonce / n m ) 0.5 zy Fig. 3. Fourier transform of the k 3 ~ ( k ) weighted EXAFS signal of the Cu,As2$e6, alloy at the Cu edge (-), As edge (- - -), and Se edge (._.........) zyxwv , 06 zy zyxwvu zyxw zyxwvuts z 307 EXAFS Study of the Cu-As-Se Amorphous Alloys N j is the average coordination number for the Gaussian distribution of distances centered at the R j value, ojis the Debye-Waller contribution, cpj(k)= 26(k) + yj(k)the phase shift, 6(k) and yj(k)being the central and backscattering atom phase shifts, respectively, f j f k ) is the amplitude of the backscattering atoms, and T j is related with the mean free path of the photoelectron. Fig. 3 and 4 show the Fourier transform of the k 3 ~ ( kweighted ) signals for the two alloys, a Hanning window being used within an interval from 30 to 120 nm- '. These transforms are related to the radial distribution function surrounding the three component atoms. The information of the first coordination spheres is given in the mean peaks centered about 0.2 nm and these are the EXAFS contribution which will be considered. Data analysis has been made by comparison of filtered data with the EXAFS contribution calculated by (1)in the k and R spaces, after this interactive process a standard minimization procedure has been applied in order to obtain the parameters better fitting to the experimental data. Physically, each atom may be surrounded by the three different types of atoms, in order to reduce the unknown parameter in (1) we do not distinguish between As and Se atoms, that is, we have taken the same amplitude and backscattering functions for As and Se. Their difference of one unit in the atomic number justify this similarity. In every case, the zyxwvutsrqponm zyxwvuts Fig. 4. Fourier transform of the k 3 ~ ( k ) weighted EXAFS signal of the Cu,,As,,Se,, alloy at the Cu edge (-), As edge (- - -), and Se edge (.____......) 0 0.I 0.2 0.3 0.4 distance (nm) 0.5 0.6 zyxw 308 D. GOMEZ-VELA, L. ESQUIVIAS, and C. PRIETO amplitude and phase functions of (1) were taken as those reported by McKale et al. [12] and the backscattering function for the undistinguishable (As-Se) atoms as the average values given for the As and Se atoms. The reference crystalline compounds As,Se, (c) and AsSe,Ag (c) have been fitted to the McKale phase and amplitude functions with the distance and coordination number as known values and the other parameters have been fitted. Two different distances were taken in order to fit the As K-edge EXAFS signal for As,Se, (c) and only one for AsSe,Ag (c) it is given by the crystal structure, it must be noted that the rj parameter (which is related with the photoelectron mean free path) hardly changes from one compound to the other as the conductivity does. In our analysis, the Debye-Waller factors ( 0 ; ) were taken, in a zyxw zyx z ti L zyx zyxwvu zyxwvutsr -3 -0.7 zyxwvutsrqponmlkjihgfedcba zyxwvu E a" 2 c 0.05 g o . I 4 -0.05 -0.15 zyx zyxwvutsrqp -0.75 L I 0 0..I I 02 distance l n m ) I 0.3 0. c Fig. 5. Distance space comparison between the calculated )-( and the experimental filtered data the modulus and the imaginary part of the Fourier transform of the three absorption edges of the Cu,As,,Se,, alloy. a) Cu K-edge, b) As K-edge, and c) Se K-edge ( 0 )of zyxwvutsrqp zyxwv zyxwvu zy zyxwvu zyx zyxwvutsr 309 EXAFS Study of the Cu-As-Se Amorphous Alloys first approximation, for both amorphous samples the same as those obtained for the crystalline As,Se,, and the Tjparameters the same as those corresponding to AsSe,Ag (c). Fittings have been carried out both in k and R spaces. The values shown in Table 1 are obtained by minimization of the &-parameterwhich gives the deviation of the calculated EXAFS signal from the experimental data. Because of the same experimental error for all the spectrum points we have defined this merit parameter as: G = (l/N) (data, - model,)’. 1 i Fig. 5 and 6 show the comparison between the experimental filtered data and the calculated ones in the distance space for the two samples at the three edges. The pictures of the fittings are of similar quality as in the k space but for the sake of brevity they are not given. Table 1 summarizes the best fitting parameters for the two amorphous samples and for crystalline , zyxwvu , - 0.15 0 0.I 0.2 0.3 distance (nm) 0 - zyxw Fig. 6. Distance space comparison between the calculated )-( and the experimental filtered data the modulus and the imaginary part of the Fourier transform of the three absorption edges of the Cu,,As,,Se,, alloy. a) Cu K-edge, b) As K-edge, and c) Se K-edge ( 0 )of 21 physica (b) 169/2 zyxw zyxw zyx 310 D. GOMEZ-VELA, L. ESQUIVIAS, and C. PRIETO Table 1 Set of values obtained as the best fit of the EXAFS signals of crystalline (labeled with (c)) and amorphous alloys where the experiments have been performed at the three absorption K-edges (labeled with [Cu], [As], and [Se], respectively). N number of neighbors, R distance, E , energy threeshold, Debye-Waller factor, and A& energy shift introduced in order to compensate the errors in E , between the actual energy thresholds and the corresponding ones for the gas atoms where the theoretical phase and amplitude functions were calculated 3.0 2 1 2 2.41 2.42 2.42 0.08 0.08 0.08 0.9 0.9 0.9 13.0 3 2.41 0.105 2.5 8 976.0 1 1857.6 10.0 2.0 12646.0 6.0 3.96 0.11 2.86 0.44 1.81 2.43 2.46 2.40 2.35 2.39 0.103 0.062 0.064 0.07 0.066 2.0 2.0 2.0 2.0 2.0 0.29 3.70 0.66 2.33 1.93 1.90 2.50 2.39 2.41 2.415 2.36 2.42 0.08 0.109 0.06 0.075 0.087 0.069 2.0 2.0 2.0 2.0 2.0 2.0 As-Se As-Se Se-As 1 1857.6 12643.7 As-Se 11857.6 Cu-(AsSe) AS-CU As- (AsSe) Se-Cu Se -(As%) 5.0 zyxwvut cu-cu Cu-(AsSe) AS-CU As-(AsSe) Se-Cu Se - (AsSe) 8977.0 2.0 11 857.0 0 12646.0 5.0 As,Se, and AsSe,Ag. Distances are similar in all alloys as in the crystalline compounds, it can be noted that the obtained coordinations are compatible with the stoichiometry of the samples and this may be seen as a self-consistency because there was no a priori supposition on the coordination number. On the other hand, a good agreement is obtained by comparison of the mean coordination number and distance with those obtained from the radial distribution function (RDF)in both samples [8,13]. The mean coordination number obtained directly from diffraction RDF measurements is the area below the first peak of A,,, = 2.54 and 3.58 at. units for Cu&s&66 and Cu,,As,,Se,,, respectively, to compare with the values obtained by EXAFS which are A,,,,, = 2.51 and 3.6, respectively. The next step will be to use the Monte-Carlo calculation in order to establish a model compatible with the RDF measurements and the EXAFS experiments where the distinction between As and Se as backscatterer atoms will be made. This procedure will be reported for those samples shortly [13, 141. zyxwvu 4. Conclusions In this work we have studied two samples of the Cu-As-Se amorphous system with a big difference in the copper concentration. Our EXAFS results give a coordination which remains contant for the Cu and As atoms and which is variable from 2 to 4 for the Se ones. This fact seems to indicate that the Se atoms may change continuously their coordination number up to 4 when Cu is added in the interval where it is possible to obtain amorphous alloys in the system Cu-As-Se. zy zyxw zyxwvut EXAFS Study of the Cu-As-Se Amorphous Alloys 311 Acknowledgements Special thanks must be given to V. Mastelaro and to H. Dexpert for the reference compound spectra and for helpful discussion and encouragement during this work. We acknowledge the Spanish Ministry of Education for supporting the stay of C. Prieto at Orsay and to the L.U.R.E. for the support of travelling costs. On the other hand, we thank the staff in charge of D.C.I. machine for the allocation of the beam time. References zy K. S. LIANG,A. BIENENSTOCK, and C. W. BATES,Phys. Rev. B 10, 1528 (1974). S. NEOV,M. BAEVA, I. GERASSIMOVA, and M. NIKIFOROVA, phys. stat. sol. (a) 57, 795 (1980). J. Z. LUI and P. C. TAYLOR, Solid State Commun. 70, 81 (1989). Z. M. SALEH,G. A. WILLIAMS, and P. C. TAYLOR, Phys. Rev. B 40, 10557 (1989). [ 5 ] C. Y. YANG,M. A. PAESLER, and D. E. SAYERS, Phys. Rev. B 36, 980, 9160 (1987); B 39, 10342 (1 989). [6] L. ESQUIVIAS and F. SANZ,J. non-crystall. Solids 70, 221 (1985). [7] N. DE LA ROSA-FOX, L. ESQUIVIAS, P. VILLARES, and R. JIMENEZ-GARAY, Phys. Rev. B 33, 4094 (1986). [8] D. GOMEZ-VELA, L. ESQUIVIAS, and C. PRIETO,Actas de la 3a Reunion Nacional de Ciencia de Materiales, Ed. I. CARRIZOSA, J. A. ODRIOZOLA et al., ICMS-CSIC, Sevilla 1990 (p. 100). [9] A. E. OWEN,Electronic and Structural Properties of Amorphous Semiconductors, Ed. P. G. LE COMBER and J. MORT,Academic Press, London 1973. [lo] C. PRIETO,P. LAGARDE, H. DEXPERT, V. BRIOIS,F. VILLAIN, and M. VERDAGUER, Measur. Sci. and Tech., in the press. [I I] B. K. TEO,EXAFS: Basic Principles and Data Analysis, Inorganic Chemistry Concepts, Vol. 9, Springer-Verlag, Berlin 1986. [12] A. G. MCKALE,B. W. VEAL,A. P. PAULIKAS, S. K. CHAN,and G. S. KNAPP,J. Amer. Chem. SOC. 110, 3763 (1988). [13] D. GOMEZ-VELA, L. ESQUIVIAS, and C. PRIETO,J. non-crystall. Solids, in the press. [I41 D. GOMEZ-VELA, L. ESQUIVIAS, and C. PRIETO,to be published. [l] [2] [3] [4] zyxwvutsr zyxw zyx (Received August 29, 1991) 21*