27 - Strong two-band electron self-trapping, state hybridization effects and related pressure-induced phenomena in semiconductors

Summary

Abstract

A review is presented of a recent theory of strong two-band electron selftrapping in a semiconductor, for which hybridization of the related electron state with the band states is essential and gives rise to new features of both electron and atomic dynamics. Pressure-induced phenomena in such materials predicted in the theory are discussed.

Introduction

As demonstrated in many works (see, e.g. [1–9]), the type of self-trapping of quasi-particles, e.g., electrons or holes in semiconductors, depends on properties of the materials. Whatever the origin of self-trapping, in most papers [1–4, 9] generally important contributions come from states of a single energy band, e.g. of the conduction band for electrons. Hence, single-band self-trapping has largely been considered in most works, which holds true, insofar as the characteristic self-trapping energy W_ST (< 0) is substantially less in magnitude than the interband, or mobility, gap width E⁽⁰⁾_g, W_ST|<<E⁽⁰⁾_g. However, there are realistic semiconducting materials in which two-band self-trapping occurs, in the sense that contributions from states of both conduction and valence bands are important and hybridization of the states in the gap gives rise to new effects [5–8]. For instance, a single-band self-trapping energy for a single electron in a harmonic atomic lattice W_ST= W₁≃ – W∂Q²_d/(2ka²₀) may be comparable in magnitude to E⁽⁰⁾_g/2, as self-trapping occurs at a soft ‘defect’ exhibiting a small effective atomic spring constant k<<k₀, for typical values Q_d≈3–5 eV, E⁽⁰⁾_g ≈ 1–3 eV, and K₀ ≈ 30–50 eV Ų (hole self-trapping can be treated in a similar way, with trivial substitutions of conduction band states for valence band ones and vice versa).