Self-trapping of single and paired electrons in Ge₂Se₃

Abstract

We report the theoretical prediction of single and paired electron self-trapping in Ge(2)Se(3). In finite atomic cluster, density functional calculations, we show that excess single electrons in Ge(2)Se(3) are strongly localized around single germanium dimers. We also find that two electrons prefer to trap around the same germanium dimer, rupturing a neighboring Ge-Se bond. Localization is less robust in periodic, density functional calculations. While paired electron self-trapping is present, as shown by wavefunction localization around a distorted Ge-Ge dimer, single-electron trapping is not. This discrepancy appears to depend only on the boundary conditions and not on the exchange-correlation potential or basis set. For single- and paired-electron trapping, we report the adiabatic barriers to motion and we estimate hopping rates and freeze-in temperatures. For the single trapped electron, we also predict the (73)Ge and (77)Se hyperfine coupling constants.

Electron Self-trapping in Ge2 Se3 and its Role in Ag and Sn Incorporation

“…In fact these features of the PDOS are strikingly similar to those of the single STE in the cluster calculations (see Figure 3 (a) in Ref. [5].) Taken together, the atomic relaxations and the electronic structure of the neutral interstial silver atom imply that in this bulk crystalline model, an isolated silver atom autoionizes, becoming Ag +1 , and the excess electron self-traps around a germanium pair.…”

“…We have shown that STE's are bound to either Ge-Ge or Sn-Ge dimers. This should persist in non-crystalline material because these dimers form the bottom of the conduction band [5]. We have also shown that both Sn and Ag autoionize upon entering Ge 2 Se 3 .…”

“…Figure 4 of Ref. [5], we showed that the wave function of the single STE is well localized on the moiety shown in Figure 3a. For paired STE's, Figure 3b, the distortion is more dramatic.…”

“…Details of the construction of the clusters are given in Ref. [5]. We used these in the study of self-trapping, although, as shown below, in periodic calculations single-electron selftrapping is evident in the presence of defects, and paired self-trapping is always present.…”

“…To this end, we report a set of electronic structure calculations, based on density functional theory (DFT), on a crystalline model of Ge 2 Se 3 with one or more Ag and/or Sn atoms. We incorporate recent results showing that electrons self-trap in this model system [5]. This is appropriate because in other Ge x Se (100-x) compounds, negatively charged defects have been invoked for the motion of the silver diffusion front in photodoping experiments [6,7].…”