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

We have used spectroscopic ellipsometry to measure the dielectric func tion epsilon(omega) of GaP from 10 to 640 K in the 1.6-5.6-eV photon-e nergy region. By performing a line-shape analysis of the observed stru ctures, the interband critical-point (CP) parameters (strength, thresh old energy, broadening, and excitonic phase angle) and their temperatu re dependence have been determined. Special emphasis is put on the E0' , E0' + DELTA0', and E2 CP's. We determine the spin-orbit splitting of GAMMA15c to be DELTA0'=160+/-10 meV. The observed decrease in energy of the CP's (after correction for the effect of thermal expansion) and the corresponding increase in broadening with increasing temperature agree reasonably well with results of a calculation that takes into ac count the Debye-Waller and self-energy terms of the deformation-potent ial-type electron-phonon interaction. New local empirical pseudopotent ial form factors were fitted to the available band-structure data and used in the electron-phonon calculations.