Impact of structural short-range order and tetrahedral germanium on electronic and dielectric properties in phase-change materials

The optoelectronic properties of prototypal Telluride amorphous phase-change materials (GeTe and Ge2Sb2Te5) are investigated from ab initio molecular dynamics simulations. Local tetrahedral germanium geometries are identified from topological angular constraint counting and this permits to relate exactly their contribution to targeted properties. The analysis of our computation reveals that the dominant population of tetrahedral Ge contributes to the tail of the valence and conduction band but with an increased electronic localization for the latter, whereas residual (essentially octahedral) geometries induce an overall constant localization at E>E-F except close to the Fermi gap, where p electrons are largely delocalized. The detailed calculation of the atomic dipoles in the amorphous state indicates that tetrahedral Ge leads to lower momenta, especially in a-GeTe, and corresponding Ge-based correlations with Wannier centers also indicate the dual nature of the local geometries. These features which drive optical and dielectric contrast exemplify the unique properties of phase-change materials, and represent an obvious breakdown of the well-known Zachariasen rule stating that the short-range order is similar in crystals and glasses.