Tight-Binding Theory of Phase-Change Materials

Abstract

A simple theory of Ge, Sb, and Te crystals and glasses, based upon atomic valence orbitals with coupling given by universal parameters, is outlined. It is seen how dangling hybrids from Ge atoms can behave as negative U centers. In contrast, in Sb the nonbonding s orbitals play essentially no role, except to produce three-fold coordination and 90° bond angles. Te also has nonbonding s states, but also nonbonding p states causing two-fold coordination. These nonbonding p states form the top of the valence band, rather than the bond orbitals which formed the valence-band in Ge. We argue against lumping these totally different behaviors together as “lone pairs”. In the glass, bonds can form between all atom types, but bonds between different elements are energetically favored. Further, when Sb or Te atoms are involved, bonds can switch between neighbors with small energy change, in a manner suggested by Kastner, Adler and Fritsche. It is seen how to estimate the energy for these various different geometric configurations.