Defects in Se, As2S3, and GeS2 glasses: Ab initio analyses of related clusters

Abstract

Dangling bonds in Se, As2S3, and GeS2 glasses and wrong bonds in the compounds have been comparatively studied using an ab initio molecular-orbital software. Defect energies and the densities in the chalcogenide glasses are estimated from the total energy Et of small clusters such as H-nSe-H, As3S4H, and Ge4S4H. Properties of corner- and edge-shared connections are also explored for the As-S and Ge-S systems. The three kinds of clusters show for a meta-stable energy ΔEt, relative to the total energy Et of normal bonding structures, a magnitude order of ΔEt(IVAP) < ΔEt(2D0) < ΔEt(D+/D−). Based on this result, we discuss why only GeS2 exhibits unpaired electron spins. The analysis also manifests that ΔEts of clusters containing wrong bonds (As-As, Ge-Ge, and S-S) are comparable with those of ΔEt(IVAP). Regarding electronic levels, the three glasses seem to possess marked gap states having different origins.