Infrared Absorption inMg2Ge

Abstract

Optical-absorption measurements on $n$- and $p$-type ${\mathrm{Mg}}_2$Ge single crystals were made at wavelengths between 1.5 and 25 μ at temperatures between 4.2 and 300°K. The absorption edge in $p$-type crystals arises from allowed indirect transitions. The energy gap is 0.570 eV at 0°K and the phonon involved has an energy of 0.025 eV. At higher temperatures the temperature dependence of the energy gap is -1.8×${10}^{-4\mathrm{}}$ eV/deg. At longer wavelengths $p$-type crystals exhibit absorption bands interpreted as being caused by transitions between three valence bands at the center of the Brillouin zone as in germanium. The spin-orbit splitting of these bands is 0.20 eV and the ratio of heavy-hole mass to light-hole mass is 6±1. The $n$-type samples have an absorption band at 0.58 eV (0°K) of uncertain origin, and ${\lambda }^{2.5}$ free-carrier absorption, indicating optical-mode scattering.