TEMPERATURE-DEPENDENCE OF THE DIELECTRIC FUNCTION AND THE INTERBAND CRITICAL-POINT PARAMETERS OF GAP

We have used a rotating analyser type of spectroscopic ellipsometer to measure the dielectric function epsilon(omega) of GaP from 10 to 640 K in the 1. 6 - 5.6 eV photon energy region. By performing a line shap e analysis of the observed structures in d2epsilon/domega2 with the As pnes equation, the interband critical-point (CP) parameters (strength, threshold energy, broadening, and excitonic phase angle) and their te mperature dependence have been determined. Special emphasis is put on the E0', E0'+DELTA0', and E2 CPs. We determine the spin-orbit splittin g DELTA0' of GAMMA15c to be 160 +/- 10 meV. The observed decrease in e nergy of the CPs (after correction for the effect of thermal expansion ) and the corresponding increase in broadening with increasing tempera ture agree reasonably well with results of a calculation which takes i nto account the Debye-Waller and self-energy terms of the deformation potential type electron-phonon interaction. These calculations are bas ed on an empirical pseudopotential band structure and a phonon shell m odel fitted to neutron scattering data and evaluate the strength of th e deformation potential interaction using perturbation theory (rigid p seudoion method).