Influence of copper on the electronic properties of amorphous chalcogenides

Abstract

We have studied the influence of alloying copper with amorphous arsenic sulfide on the electronic properties of this material. In our computer-generated models, copper is found in two-fold nearlinear and four-fold square-planar configurations, which apparently correspond to Cu(I) and Cu(II) oxidation states. The number of overcoordinated atoms, both arsenic and sulfur, grows with increasing concentration of copper. Overcoordinated sulfur is found in trigonal planar configuration, and overcoordinated (four-fold) arsenic is in tetrahedral configuration. Addition of copper suppresses the localization of lone-pair electrons on chalcogen atoms, and localized states at the top of the valence band are due to Cu 3d orbitals. Evidently, these additional Cu states, which are positioned at the same energies as the states due to There has been a significant amount of experimental research on the role of copper in amorphous chalcogenide alloys. It is generally believed that copper is found in tetrahedral configuration and is bonded exclusively to chalcogen atoms [1,2]. However, as noted in Ref.[1], the interpretation of experimental radial distributions of ternary systems is necessarily ambiguous. More recent experimental results [3] indicate that copper also bonds to arsenic as well as to chalcogen atoms. It was observed [4] that photodarkening (red shift of the optical absorption edge under illumination) disappears in Cu x (As 0.4 Z 0.6 ) 1−x alloys, starting from x = 1% for Z=S and x = 5% for Z=Se. Two possible interpretations of this observation, given in Ref. [5], are either that the photodarkening is masked by some electronic states due to Cu atoms, or the addition of copper interferes with correlations of lone-pair orbitals on a scale greater than that of a nearest-neighbor distance. Alternatively, the Cu-related states at the band edges may be efficient nonradiative centers which provide an alternative channel to that resulting in photostructural bond rearrangement of the chalcogenide network. The observation that photoinduced phenomena, other than photodarkening, remain intact after the addition of copper [6] favors the first interpretation.It is a natural next step in the investigation of these materials to obtain the first detailed description of their electronic and structural properties by means of computer simulation. This is the topic of this paper.Following Refs. [7,8], we have employed a densityfunctional-based tight-binding (DFTB) method [9,10], which has allowed us to create realistic models of arsenic sulfide (As 2 S 3 ). The DFTB method allows one to improve upon the standard tight-binding approximation by including a so-called self-consistent charge (SCC) correction [11], derived from density-functional theory (DFT), to the total energy. The procedure for generating the tabulated data sets for pairwise interatomic interactions is outlined in Ref. [12]. The S-S [12], As-As and As-S [13], and Cu-Cu, Cu-S and Cu-As [14] tables, and information on their creation, are available from the authors. The followin...

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“…5 is actually a prediction. This is an interesting contrast to the work of Simdyankin and coworkers, 38 who show that for low concentrations of Cu, the gap decreases with addition of Cu in AsS and AsSe glasses. Care is needed in comparing these results since the hosts and transition metals are different, and our models have far higher metal content.…”

Atomic structure and bonding properties in amorphous Cu (As2S3)1− by ab initio molecular-dynamics simulations

“…Apart from our present work, there is a very recent work performed by Symdyankin et al [24]. In that study it is suggested from semi-empirical tight-binding calculations that the [As 4 ] À -[S 3 ] + defect pairs are responsible for masking photodarkening in Cu doped chalcogenides.…”

Effects of Cu alloying on the structural and electronic properties in amorphousCux(As0.4S0.6)1−Shimojo1, Aniya2 2006Phys. Rev. B2000View full textAdd to dashboardCite

“…First, the size of the system could be a problem, considering the fact that the number of atoms used in the previous studies based on tight binding schemes 17,18,20,22 is about 200. Since the edge length of our supercell is about 14 Å, we are unable to investigate the spatial correlation beyond half the edge length ϳ7 Å.…”

“…[16][17][18][19][20][21][22][23][24] Li and Drabold have carried out molecular-dynamics ͑MD͒ simulations of amorphous As 2 Se 3 based on a nonorthogonal tight binding scheme. 17 They investigated the local vibrational modes and the electronic density of states, as well as the structure of atoms and showed that their calculated results are consistent with experiments.…”

“…More recently, semiempirical tight-binding calculations have been carried out for amorphous As 2 S 3 by Elliott and co-workers. [20][21][22] They suggested that there exist defect pairs, ͓As 4 ͔ − -͓S 3 ͔ + , in addition to the charged coordination defects, and that such defect pairs are responsible for photodarkening under illumination. They also studied the influence of alloying Cu on the electronic properties and discussed the mechanism of the suppression of photodarkening in Cu-doped chalcogenides.…”