A new type of charged defect in amorphous chalcogenides

TL;DR

This paper uses DFTB simulations to identify a novel charged defect pair in amorphous arsenic sulfide, revealing a potential mechanism for photo-darkening related to structural transformations under electronic excitation.

Contribution

The study discovers a new charged defect pair in amorphous arsenic sulfide and links its formation to electronic excitation and optical property changes.

Findings

Identification of a new defect pair [As4]--[S3]+ in amorphous arsenic sulfide.

Structural transformation under excitation reduces the band gap.

Potential explanation for photo-darkening in chalcogenide glasses.

Abstract

We report on density-functional-based tight-binding (DFTB) simulations of a series of amorphous arsenic sulfide models. In addition to the charged coordination defects previously proposed to exist in chalcogenide glasses, a novel defect pair, [As4]--[S3]+, consisting of a four-fold coordinated arsenic site in a seesaw configuration and a three-fold coordinated sulfur site in a planar trigonal configuration, was found in several models. The valence-alternation pairs S3+-S1- are converted into [As4]--[S3]+ pairs under HOMO-to-LUMO electronic excitation. This structural transformation is accompanied by a decrease in the size of the HOMO-LUMO band gap, which suggests that such transformations could contribute to photo-darkening in these materials.