New Type of Charged Defect in Amorphous Chalcogenides

We report on density-functional-based tight-binding 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, $[{\mathrm{A}\mathrm{s}}_4{]}^{-}$-$[{\mathrm{S}}_3{]}^{+}$, consisting of a fourfold coordinated arsenic site in a seesaw configuration and a threefold coordinated sulfur site in a near-planar trigonal configuration, was found in several models. The valence-alternation pairs $[{\mathrm{S}}_3{]}^{+}$-$\mathrm{S}_1{}^{-}$ are converted into $[{\mathrm{A}\mathrm{s}}_4{]}^{-}$-$[{\mathrm{S}}_3{]}^{+}$ 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 photodarkening in these materials.

Figure 1

Planar trigonal, $[{\mathrm{S}}_3{]}^{+}$ (marked by letter $A$), and seesaw, $[{\mathrm{A}\mathrm{s}}_4{]}^{-}$ (marked by letter $E$), configurations in (a) a fragment of model 2(III) [the dangling bonds show where the displayed configuration connects to the rest (not shown) of the network], and (b), (c) charged isolated clusters (the dangling bonds are terminated with hydrogen atoms). The shading of the As atoms is darker than that of the S atoms. Bond lengths are (Å) as follows: (a) $AB=AD=2.42$, $AC=2.39$, $EF=2.83$, $EG=2.54$, $EH=2.35$, $EI=2.33$; (b) $AB=2.34$, $AC=AD=2.33$; (c) $EF=2.55$, $EG=2.46$, $EH=2.30$, $EI=2.32$.

Figure 2

An ${\mathrm{A}\mathrm{s}}_4{\mathrm{S}}_{10}{\mathrm{H}}_8$ cluster containing both defect centers $[{\mathrm{A}\mathrm{s}}_4{]}^{-}$ (marked by letter $A$) and $[{\mathrm{S}}_3{]}^{+}$ (marked by letter $B$). The shading of the atoms is the same as in Fig. 1. The black solid lines signify elongated bonds. (a) Optimized ground-state geometry. Bond lengths are (Å) as follows: $AC=3.00$, $AD=2.32$, and $BE=2.4$. (b) Isosurfaces corresponding to the value of 0.025 of electron density in the HOMO [darker surface, left side, (red online)] and LUMO [lighter surface, right side (cyan online)] states for the structure shown in (a). (c) Optimized geometry in the first singlet excited state. Bond lengths are (Å) as follows: $AC=2.43$, $AD=2.44$, and $BE=2.82$. (d) Same as (b), but for the structure shown in (c).