Electronic structure and the glass transition in pnictide and chalcogenide semiconductor alloys. I. The formation of the ppσ-network

Abstract

Semiconductor glasses exhibit many unique optical and electronic anomalies. We have put forth a semi-phenomenological scenario (J. Chem. Phys. 132, 044508 (2010)) in which several of these anomalies arise from deep midgap electronic states residing on high-strain regions intrinsic to the activated transport above the glass transition. Here we demonstrate at the molecular level how this scenario is realized in an important class of semiconductor glasses, namely chalcogen and pnictogen containing alloys. Both the glass itself and the intrinsic electronic midgap states emerge as a result of the formation of a network composed of σ-bonded atomic p-orbitals that are only weakly hybridized. Despite a large number of weak bonds, these ppσ-networks are stable with respect to competing types of bonding, while exhibiting a high degree of structural degeneracy. The stability is rationalized with the help of a hereby proposed structural model, by which ppσ-networks are symmetry-broken and distorted versions of a high symmetry structure. The latter structure exhibits exact octahedral coordination and is fully covalently-bonded. The present approach provides a microscopic route to a fully consistent description of the electronic and structural excitations in vitreous semiconductors.

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

“…These conditions are satisfied in many amorphous chalcogenides and pnictides. 1 The number of the intrinsic midgap states is about 2 per interface, implying a concentration of about 2/ξ 3 , where ξ is the cooperativity length from Eq. (5).…”

“…5(b) of Ref. 1 ), place pnictogens and chalcogens in the rock salt fashion (as in Fig. 6(b) or 7 of Ref.…”

“…1 presents a systematic way to classify amorphous structures and the associated electronic-structure peculiarities in vitreous ppσ-networks. Without claiming complete generality, we will consider the following specific type of parent structures, which are largely based on Burdett and MacLarnan's model of (the crystalline) black phosphorous and rhombohedral arsenic.…”

“…It is the intimate relation of the amorphous lattice to its covalently bonded parent structure that allowed us to rationalize, for the first time to our knowledge, two seemingly contradicting features of a bulk glass, i.e., its relative stability and structural degeneracy. 1 Yet the argued presence of the degeneracy of the ppσ-network does not, by itself, guarantee that the network can be realized as an equilibrated supercooled liquid or a quenched glass: For instance, elemental arsenic can be made into an amorphous film but does not vitrify readily. To be a liquid, the network should contain a large equilibrium concentration of structural motifs corresponding to the transition states for activated transport in quenched melts.…”

“…In the preceding article, 1 we presented a chemical bonding theory for an important class of pnictogen-and chalcogen-containing quenched melts and glasses. These materials exhibit many unique electronic and optical anomalies 2 not found in crystals, and also are of great interest in applications such as information storage and processing.…”