Infrared Lattice Absorption of GaP

Abstract

Transmission and reflectivity measurements on GaP in the wavelength region 1 to 40\ensuremath{\mu} have been analyzed to obtain information on the lattice vibrations. The fundamental reflection band has been fitted by dispersion theory and the strength, width, and resonance wavelength (27.3\ensuremath{\mu}) determined. The dielectric constant is found to be 8.5\ifmmode\pm\else\textpm\fi{}0.2 in the range $1<\ensuremath{\lambda}<12\ensuremath{\mu}$ and 10.2 for $\ensuremath{\lambda}>40\ensuremath{\mu}$. The background absorption in $n$-type samples due to free carriers was reduced by copper diffusion to make possible accurate transmission measurements of the lattice combination bands. An assignment scheme using five frequencies is proposed to explain the combination bands. The temperature dependence of the bands is consistent with theory for two-phonon processes. One band is observed which appears to be due to molecular vibrations of an impurity not yet identified. From the integrated absorption it is concluded that the principal mechanism for the combination bands is the anharmonic potential energy.