RADIAL DISTRIBUTION FUNCTION STUDIES OF GLASSY GERMANIUM-SILVER-CHALCOGEN ALLOYS

Transcription

1 RADIAL DISTRIBUTION FUNCTION STUDIES OF GLASSY GERMANIUM-SILVER-CHALCOGEN ALLOYS L. Bourne, S. Rowland, A. Bienenstock To cite this version: L. Bourne, S. Rowland, A. Bienenstock. RADIAL DISTRIBUTION FUNCTION STUDIES OF GLASSY GERMANIUM-SILVER-CHALCOGEN ALLOYS. Journal de Physique Colloques, 1981, 42 (C4), pp.c4-951-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1981 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque C4, suppl6ment nu n070, Tome 42, octobre 7981 page C4-951 RADIAL DISTRIBUTION FUNCTION STUDIES OF GLASSY GERMANIUM-SILVER- CHALCOGEN ALLOYS a L.C. Bourne, S.C. Rowland and A. Bienenstock Ccpartnent of Physics, Andrews University, Berrien Springs Michigan 49104, b.s.a. * Pepartment of 1.lateriaZs Science, Stunford University, Stanf~rd, California S4.305, 1l.S.A. Abstract.-X-ray diffraction radial distribution function (rdf) studies have been performed on 15 different alloys in the glassy Ge Ag,X systems, where X is S or Se. Assumption of fourfold ~e~cograinaxion and no S-S bonds in the sulfur alloys leads to direct determination of Ag coordination numbers in the range 3 to 4. The further assumption that the S and Se materials have the same near neighbor coordination numbers leads to the determination of chalcogen coordination numbers which increase with y to approximately 4. The resulting coordination numbers are, on the avcrage, consistent with the 8-6 rule. The Ag-X distances are closer to the covalent than to the ionic distances and are consistent with those in the crystalline compound Ag GeX 8 6' Introduction.- 'Je report here the results of x-ray diffraction radial distribution function (rdf) studies of 15 alloys in the glassy Ge Ag,X,-x- systems, where X is S or Se. To our knowledge, this work constitutes thg fir5t rzpurt of glasses in the system constaining Se. These systems were chosen for structural studies for a number of reasons. The first follows from the work of Liang et al. (1) and Hunter et al. (2) on the Cu-As,Se system in which it was found that monovalent 3 Cu is a fourfold coordinated nstwork former, rather than being a network modifier, so that the average chalcogen coordination number increases with increas ing Cu concentration. In network modification, the opposite would take place. kle were anxious, therefvrei to see whether this was a more general phenomenon with the addition of noble metals to amorphous scmiconductors. In addition, some of the glasses in the systems studied here had the same compositions as those which must be obtained in the photodoping of GeS2 and GeSeZ by Ag. Consequently, thcse rdf studies are the first of a series of structural studies aimed at understanding the photodoping process. Similarly, we hope that these structural studies will yield more understanding of thc high diffusion rates of the noble metals in amorphous semiconductors and C:le large conductivity increases which their additions cause. Finally, these particular systems have large glass-forming regions and Ag has a sufficiently large atomic number so that it is possible to see trends with composition. Experimental.- The glassy samples used in this study were prepared through quenches from the melt, and tt~ei r densities determined, in the mqnners described by Rowland et al. (3) In the sulfur system, glasses were chosen from the glass-forming region defined by Feltz and Thieme (4). To our knowledge, this region i-n the selenium system has not yet been determined, so that this work serves to indicate some portions of it. Two different x-ray diffractometer systems were used for acquisition of the Cntensity data. One was a Picker system previously described by us. The other was a Norelco systerp with additional slits provided to achieve complete Article published online by EDP Sciences and available at

3 C4-952 JOURNAL DE PHYSIQUE elimination of XB radiation. One sample, Ge Ag ose 52 was run with both systems. The first neighbor peak area was the Picker system and on the Norelco system. Hcnce, we do not believe that there are significant systematic differences bctwecn the data obtained on the two systems. This result gives some idea of the reproducibility of rdf areas obtained under different conditions. It does not, however, indicate the absolute error since this could be affected by unknown errors in the atomic scattering factor curves used in the data reductions. The x-ray diffraction data were analyzed to obtain radial distribution functions as described by Betts et al. (5), with one significant difference. The computer program has been modified to take into account the fact that there is frequently a small constant background counting rate in the diffraction measurements. This constant background rate was determined and subtracted using the method proposed by Fuoss (6). In analyzing the rdf's, areas under overlapping peaks were determined ~ising a computer program developed to fit a series of Gaussian peaks to an appropriate curve. Following the suggestion of Waser and Schomaker (7) that Xo(R) sllould be symmetric for isolatcd peaks, we assumed Ro(R) to be a sum of overlapring Gaussian peaks. The program PRAXIS is used to determine the set of parameters for a set of such Gaussian peaks which best fit the data according to the method of least squares. Areas qnd peak positions are then obtained for the separated peaks in the function 4nR2p(R). The rdf peak corresponding to the first coordination shell could be fit by a single Gaussian for each sample in this study. For tliose with Ag content in excess of 30 at X, an additional small peak could be fit <lose to 3g. This is consistent with bond lengths for some of the Ag-S bonds found in crystalline Ge AgSS (8). 6 Results and Couc1usions.- Tables 1 and 2 list the results of these studies for the sulfur and selenium systems, respectively. As xe and others have noted repeatedly, it is impossible to obtain unique structural interpretations of the x-ray radial distribution functions for complex polyatomic glasses in which the relevant covalent or ionic radii are approximately equal or differ by just a few tenths of an angstrom. To progress in the analysis, we will make the simplifying assumption that the only types of near neighbor bonds present in the glasses are germanium-chalcogen, germanium-germanium and silver-chalcogen. We will also assume that the Ge is always fourfold coordinated. Under these assumptions, the area of the first neighbor peak in the rdf is given by A = 2qGeaxzGezx + qge(l-a)x~ 2 + G 2q~gYz~gz~' (1) Here, qj.and Z. arc the near neighbor coordination number and atomic number of the species 3, a id the fraction of Ge atoms which are bonded to chalcogens while the fraction of Ge bonded to Ge is given by (l-a). A particularly interesting feature of the sulfur system is that Z = 2ZS, so that the calculated areas are independent of the variable a. As a resuff. the silver coordination nurpber can be determined directly from the composition and the observed first neighbor peak area through the relation The resulting q based on the assumed q = 4 are shown in Table 1. It is evident that they.rangeaefrom approximately 3 to? and increase with increasing y. These low coordination numbers may be taken as prima facie evidence that the silver-chalcogen bonding is covalent and the silver is acting like a network former rather than a network modifier. Further evidence for this vi~ypoiqt is obtained from the observed near neighbor distances. The sum of the Ag -X ionic radii are and for S and Se, respectively. In these materials, the addition of Ag

4 Table 1 - rdf Data for Ge Ag S xy-l-x-y _y_ 1st Peak Area Position %A ~ ~ cIOO ~ ~ ~ ~ l ~ ~0.3.l ~0.3 Table 2 - rdf Data for Ce Ag A _y_ 1st Peak Area Position a.36.l L z ~ y introduces into the radial distribution function a near neighbor peak component in the region of , which is more typical of covalent bonding. Thus, both the areas and the distances support the picture of coualent silver bonding with coordination numbers in the range of three to four. This type of bonding is the predominant banding of silver in the compounds Ag GeX6 (8). It is more difficult to obtain coordination numbers for the Se syste.? *because Eq. (2) does not hold for this system. 'To proceed further, we will make further simplitying assumptions and discuss the consequences of the resulting parameters. In particular, for any values of X and y, we assume that the values of the parameters q and a have the same values for the selenium glass as they do for the sul%: ~ S ; S? X That is, we are assuming that the glasses of the same types of corppositions are essentially isostructural. (We do not believe that these assumptions are strictly valid, but do think that the trends they bring forth are likely to have validity.) With this assumption, it is readily shown that the parameter a is given by the equation Results for the selenium system are presented in Table 2, together with chalcogen coordination numbers obtained from the expression The results for q in Table 2 indicate that the chalcogen coordination numbers X. increase steadily with increasing Ag concentration. Such a trend could be rationalized through the same types of models put forth by Hunter et al. (2) to explain the almost identical types of results in the CU-As2Se3 system. That is, the Ag may be viewed as having a formal valence of -3 or -4 when covalently bonded by 4 and 3 chalcogens, respectively. These electrons would be tnansferred formally from the chalcogen atoms which, in turn, would have positive formal valences and would, therefore, increase their coordination numbers correspondingly in keeping with the 8-N rule, where N is the formal number of valence electrons. This is just the opposite of what happens in normal network modification in which the modifying metal atom gives up electrons to the oxygen or chalcogen and reduces the coordination number. If this picture is correct, one would anticipate that the composition average of the coordination numbers of the elements in the sample would be equal to eight minus the composition average of the number of valence electrons of the atoms

5 C4-954 JOURNAL DE PHYSIQUE in the sample, as we find within experimental error. Consequently, it appears as if the assumptions presented above yield a consistent picture of the structures of these glasses even though that picture is by no means unique. In that picture, the silver is covalently coordinated with low coordination numbers of the order of three or four. The chalcogen coordination numbers increase correspondingly to maintain a balance of formal charge and perfect pairing of electrons. Such a picture is not one which leads naturally to states in the gap : Instead, one would expect that the valence band is broadened to take into account these bonding patterns. We note, as well, that this coordination scheme is consistent with those in the crystals AggGeX6. In the sulfur compound, for example, the Ge is surrounded by an irregular teirahedron of S atoms with interatomic distagces which range from to A. There are no S-S bonds shorter than 3.5 A. There are 8 inequivalent Ag sitzs. Each Ag is surrounded by 4 S atoms, but at distances which range from 2.5 to 3.5 A. These distances, however, fall into :WO groups, those which are less than and those which are greater than 2.94 A. The average coordination number based on those in the first group is 3.125, but two of the eight Ag atoms are fourfold coordinated, five are threefold coordinated and one is twofold coordinated within this definition : thus, the silver coordination is quite variable. Acknowledgements.- We express appreciation to P. Barnett of the Whirlpool Corporation Research and Engineering Center for assistance in data acquisition on their x-ray diffractometer and to the University of Notre Dame Geological Laboratory for use of their Berman density balance. This work was supported in part by the National Science Foundation throueh Grant No. DMR AOI and the central facilities of the Stanford University Center for Haterials Research. References.- 1. LIANG, K.S., BIENENSTOCK, A. and BATES, C. : Phys. Rev. B15 (1 974) HUNTER, S.H., BIENENSTOCK, A. and HAYES, T.M. : Proc. Seventh Internat. Conf. on Amorphous and Liquid Semiconductors, Ed. Spear W.E. (1977) ROWLAND, S.C., RITLAND, F., HAFERBURNS, D. and BIENENSTOCK, A. : Ibid., p FELTZ, A. and THIEME, C. : Z. Chem. 14 (1974) BETTS, F., BIENENSTOCK, A., KEATING, D.T. and DENEUFVILLE,J. : J. Non-Cryst. Solids 7 (1972) FUOSS, P. : Ph.D. Thesis, Stanford University (1980), unpublished 7. WASER, J. and SCHOMAKER, V. : Rev. Mod. Phys. 25 (1953) EULENBERGER, G. : Monatsh. Chem. 108 (1977) 901