Electronic structure and the glass transition in pnictide and chalcogenide semiconductor alloys. II. The intrinsic electronic midgap states

Abstract

We propose a structural model that treats in a unified fashion both the atomic motions and electronic excitations in quenched melts of pnictide and chalcogenide semiconductors. In Part I (submitted to J. Chem. Phys.), we argued these quenched melts represent aperiodic ppσ-networks that are highly stable and, at the same time, structurally degenerate. These networks are characterized by a continuous range of coordination. Here we present a systematic way to classify these types of coordination in terms of discrete coordination defects in a parent structure defined on a simple cubic lattice. We identify the lowest energy coordination defects with the intrinsic midgap electronic states in semiconductor glasses, which were argued earlier to cause many of the unique optoelectronic anomalies in these materials. In addition, these coordination defects are mobile and correspond to the transition state configurations during the activated transport above the glass transition. The presence of the coordination defects may account for the puzzling discrepancy between the kinetic and thermodynamic fragility in chalcogenides. Finally, the proposed model recovers as limiting cases several popular types of bonding patterns proposed earlier, including: valence-alternation pairs, hypervalent configurations, and homopolar bonds in heteropolar compounds.

Electronic structure and the glass transition in pnictide and chalcogenide semiconductor alloys. I. The formation of the ppσ-network

“…(In the eventual deformed structure, the chains are deformed and, likely, of rather limited length. 20 ) This observation allows one to extend the trends established by Papoian and Hoffmann in periodic crystals to aperiodic lattices. Since the symmetry can be restored partially or fully by high pressure, the argued view of glasses as loweredsymmetry versions of high-symmetry structures is supported by independent arguments on (negative) pressuredriven glass transition in covalently bonded materials.…”

“…Such a "defect" is analogous to a small region occupied by an interface between two distinct lattices, which incurs a significant free energy cost. The companion article 20 shows that a mechanism for a reversible formation of such defected configurations arises naturally in the present structural model.…”

“…We will return to this important point in the companion article. 20 The following picture of structure-formation in ppσ-networked materials thus emerges from the above considerations: These materials may be thought of as symmetry-broken versions of a simple cubic structure. The symmetry breaking is an interplay of several types of perturbation: (a) Peierls-instability proper for extended near-linear chains; (b) cooperative second-order JahnTeller distortion that results from mixing of the p-orbitals with other orbitals, mostly s, and from electronic interactions; (c) steric effects due to vacancies, if the latter must be present owing to stoichiometry, as in Fig.…”

“…This common view would imply excess enthalpies of the order eV per several atoms, while avoiding to address the mechanism of transport in the melt. In the present and companion article, 20 we test the hereby proposed microscopic picture in two relatively separate stages. The present article is devoted to the first stage, in which we argue that ppσ-networks can represent the quenched liquid and frozen glass forms of these substances in the first place.…”

“…Because such a shift incurs bond breaking, these alternative parent structures are separated by barriers. It is understood that since the parent structure is generally aperiodic, 20 Figs. 7-9 apply only to a small fragment of such an aperiodic structure.…”