Temperature dependence of the optical phonon reflection band in GaP

TL;DR

This study investigates how temperature affects the energy, linewidth, and dielectric properties of optical phonons in GaP, revealing temperature-induced redshift and broadening due to anharmonic phonon interactions.

Contribution

It provides detailed temperature-dependent measurements of optical phonon parameters in GaP using Fourier transform infrared ellipsometry and the Lowndes-Gervais model, including two-phonon decay effects.

Findings

TO phonon broadening exceeds LO phonon broadening

Temperature causes redshift and broadening of phonons

Dielectric constant varies with temperature due to thermal expansion

Abstract

We explore the effect of temperatures between 80 and 720 K on the energy and linewidth of zone-center transverse (TO) and longitudinal (LO) optical phonons in bulk gallium phosphide (GaP) using Fourier transform infrared ellipsometry from 0.03 to 0.60 eV. We extract the optical phonon parameters of GaP by fitting the ellipsometric angles with the Lowndes-Gervais model, which applies two different broadening parameters to the TO and LO phonons. In GaP, the two-phonon density of states is larger for the decay of TO phonons than for LO phonons. Therefore, we observed a larger TO phonon broadening (compared to the LO phonon) and an asymmetric reststrahlen line shape. This would lead to a negative imaginary part of the dielectric function just above the LO phonon energy, but the addition of two-phonon absorption avoids this. We find a temperature dependent redshift and broadening of TO and LO phonons with increasing temperature due to thermal expansion and anharmonic phonon-phonon scattering, involving three and four phonon decay processes. We also investigate the temperature-dependence of the high-frequency dielectric constant. Its variation is explained by thermal expansion and the temperature dependence of the Penn gap.