Infrared Absorption inp-Type GaP

Abstract

Infrared absorption measurements have been performed on Zn-doped GaP at 300 and 90 \ifmmode^\circ\else\textdegree\fi{}K. The absorption observed at 300 \ifmmode^\circ\else\textdegree\fi{}K between 1 and 12 \ensuremath{\mu} is attributed to a combination of intraband free-carrier absorption (FCA) and direct interband absorption arising from transitions between the ${\ensuremath{\Gamma}}_{8}$ and ${\ensuremath{\Gamma}}_{7}$ (splitoff) valence bands. The absorption at 90 \ifmmode^\circ\else\textdegree\fi{}K is identified as photoionization of holes from the Zn acceptor levels to the ${\ensuremath{\Gamma}}_{8}$ and ${\ensuremath{\Gamma}}_{7}$ valence bands. Because of the small spin-orbit splitting (82 meV) and strong nonparabolicity of the valence bands in GaP, both the interband and intraband absorption are qualitatively different from similar absorption in other III-V compounds. The interband transitions give rise to extremely broad overlapping absorption bands while the FCA exhibits an unusually large magnitude. We propose a model for FCA in $p\ensuremath{-}\mathrm{GaP}$ which includes the effect of virtual intermediate states in the light-hole and splitoff valence bands. It is shown, on the basis of this model, that FCA in $p\ensuremath{-}\mathrm{GaP}$ should have a wavelength dependence which depends on the scattering mechanism in the usual way, but whose magnitude is enhanced by the additional intermediate states. In agreement with transport measurements, we find that holes are scattered principally by acoustic and nonpolar optical phonons.