Electronic structure and the glass transition in pnictide and chalcogenide semiconductor alloys. Part II: The intrinsic electronic midgap states

摘要

We propose a structural model that treats in a unified fashion both theatomic motions and electronic excitations in quenched melts of pnictide andchalcogenide semiconductors. In Part I (submitted to J. Chem. Phys.), we arguedthese quenched melts represent aperiodic $pp\sigma$-networks that are highlystable and, at the same time, structurally degenerate. These networks arecharacterized by a continuous range of coordination. Here we present asystematic way to classify these types of coordination in terms of discretecoordination defects in a parent structure defined on a simple cubic lattice.We identify the lowest energy coordination defects with the intrinsic midgapelectronic states in semiconductor glasses, which were argued earlier to causemany of the unique optoelectronic anomalies in these materials. In addition,these coordination defects are mobile and correspond to the transition stateconfigurations during the activated transport above the glass transition. Thepresence of the coordination defects may account for the puzzling discrepancybetween the kinetic and thermodynamic fragility in chalcogenides. Finally, theproposed model recovers as limiting cases several popular types of bondingpatterns proposed earlier, including: valence-alternation pairs, hypervalentconfigurations, and homopolar bonds in heteropolar compounds.