Electronic structure and the glass transition in pnictide and chalcogenide semiconductor alloys. Part I: The formation of the $pp\sigma$-network

TL;DR

This paper explores the formation of $pp\sigma$-networks in chalcogen and pnictogen semiconductor glasses, linking their structure to electronic midgap states and anomalies, and proposing a model for their stability and degeneracy.

Contribution

It introduces a structural model explaining the formation and stability of $pp\sigma$-networks in vitreous semiconductors, connecting network structure to electronic properties.

Findings

$pp\sigma$-networks are stable despite many weak bonds.

Midgap electronic states arise from $pp\sigma$-network formation.

Networks are symmetry-broken, distorted high-symmetry structures.

Abstract

Semiconductor glasses exhibit many unique optical and electronic anomalies. We have put forth a semi-phenomenological scenario (J. Chem. Phys. 132, 044508 (2010)) in which several of these anomalies arise from deep midgap electronic states residing on high-strain regions intrinsic to the activated transport above the glass transition. Here we demonstrate at the molecular level how this scenario is realized in an important class of semiconductor glasses, namely chalcogen and pnictogen containing alloys. Both the glass itself and the intrinsic electronic midgap states emerge as a result of the formation of a network composed of $\sigma$-bonded atomic $p$-orbitals that are only weakly hybridized. Despite a large number of weak bonds, these $pp\sigma$-networks are stable with respect to competing types of bonding, while exhibiting a high degree of structural degeneracy. The stability is rationalized with the help of a hereby proposed structural model, by which $pp\sigma$-networks are symmetry-broken and distorted versions of a high symmetry structure. The latter structure exhibits exact octahedral coordination and is fully covalently-bonded. The present approach provides a microscopic route to a fully consistent description of the electronic and structural excitations in vitreous semiconductors.