Electron-Phonon Interaction in Tetrahedral Semiconductors

TL;DR

This paper investigates how electron-phonon interactions influence the optical properties and energy gaps of tetrahedral semiconductors, combining theoretical calculations with experimental data, and explores implications for superconductivity in doped carbon.

Contribution

It provides state-of-the-art calculations of dielectric spectra considering electron-phonon effects and compares them with experiments, highlighting isotope and temperature influences on energy gaps.

Findings

Electron-phonon interactions significantly affect energy gaps in tetrahedral semiconductors.

Isotopic mass increases generally raise the energy gaps, except in specific cases like Cu and Ag compounds.

Temperature dependence of optical parameters can be modeled with Einstein oscillators, with T4 law at low temperatures.

Abstract

Effects of electron-phonon interactions on the band structure can be experimentally investigated in detail by measuring the temperature dependence of energy gaps or critical points (van Hove singularities) of the optical excitation spectra. These studies have been complemented in recent years by observing the dependence of such spectra on isotopic mass whenever different stable isotopes of a given atom are available at affordable prices. In crystals composed of different atoms, the effect of the vibration of each separate atom can thus be investigated by isotopic substitution. Because of the zero-point vibrations, such effects are present even at zero temperature (T = 0). In this paper we discuss state-of-the-art calculations of the dielectric function spectra and compare them with experimental results, with emphasis on the differences introduced by the electron-phonon interaction. The temperature dependence of various optical parameters will be described by means of one or two (in a few cases three) Einstein oscillators, except at the lowest temperatures where the T4 law (contrary to the Varshini T2 result) will be shown to apply. Increasing an isotopic mass increases the energy gaps, except in the case of monovalent Cu (e.g., CuCl) and possibly Ag (e.g., AgGaS2). It will be shown that the gaps of tetrahedral materials containing an element of the first row of the periodic table (C,N,O) are strongly affected by the electron-phonon interaction. It will be conjectured that this effect is related to the superconductivity recently observed in heavily boron-doped carbon.