The electronic structure of amorphous silica: A numerical study

TL;DR

This study computationally investigates how disorder affects the electronic properties of amorphous silica, revealing insights into the density of states, localization, and optical absorption related to structural disorder.

Contribution

It introduces a numerical approach using tight-binding Hamiltonian to analyze the impact of disorder on amorphous silica's electronic structure, linking it to experimental observations.

Findings

Disorder influences the density of states and localization properties.

Eigenstates are localized by potential energy fluctuations within a mobility gap.

The mismatch between photoconductivity threshold and optical absorption is explained by localized eigenstates.

Abstract

We present a computational study of the electronic properties of amorphous SiO2. The ionic configurations used are the ones generated by an earlier molecular dynamics simulations in which the system was cooled with different cooling rates from the liquid state to a glass, thus giving access to glass-like configurations with different degrees of disorder [Phys. Rev. B 54, 15808 (1996)]. The electronic structure is described by a tight-binding Hamiltonian. We study the influence of the degree of disorder on the density of states, the localization properties, the optical absorption, the nature of defects within the mobility gap, and on the fluctuations of the Madelung potential, where the disorder manifests itself most prominently. The experimentally observed mismatch between a photoconductivity threshold of 9 eV and the onset of the optical absorption around 7 eV is interpreted by the picture of eigenstates localized by potential energy fluctuations in a mobility gap of approximately 9 eV and a density of states that exhibits valence and conduction band tails which are, even in the absence of defects, deeply located within the former band gap.