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

TL;DR

This paper introduces a structural model for quenched melts of pnictide and chalcogenide semiconductors, linking atomic coordination defects to intrinsic electronic midgap states and explaining various optoelectronic anomalies.

Contribution

It presents a unified classification of atomic coordination defects and their role in electronic states, bridging structural and electronic properties in semiconductor glasses.

Findings

Coordination defects correspond to intrinsic midgap electronic states.

Defects are mobile and relate to activated transport above the glass transition.

The model explains discrepancies between kinetic and thermodynamic fragility.

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 $pp\sigma$-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.