Intrinsic Absorption-Edge Spectrum of Gallium Phosphide

Abstract

The optical absorption edge has been measured at many temperatures between 1.6 and 300\ifmmode^\circ\else\textdegree\fi{}K in exceptionally perfect single crystals of gallium phosphide. The corresponding transitions are of the allowed indirect type and involve the creation of free excitons and electron-hole pairs. Absorption components associated with four different phonon energies have been resolved, one of which is apparently due to a two-phonon process. The energies of the three single phonons are 12.8\ifmmode\pm\else\textpm\fi{}0.5, 31.3\ifmmode\pm\else\textpm\fi{}0.5, and 46.5\ifmmode\pm\else\textpm\fi{}1.0 meV. These are thought to be the respective energies of the transverse-acoustic, longitudinal-acoustic, and transverse-optic branches of the lattice dispersion curves at the $〈100〉$-type boundaries of the reduced zone, although the longitudinal-acoustic energy is \ensuremath{\sim}10 meV larger than previous estimates for this phonon. Indirect absorption associated with the longitudinal optic phonon has not been resolved, apparently because these transitions are forbidden via the energetically favorable conduction-band minimum ${\ensuremath{\Gamma}}_{1}$. The internal binding energy of the indirect exciton is 10.0\ifmmode\pm\else\textpm\fi{}1.0 meV. Fine structure, probably associated with an excited state of the indirect exciton and also with anomalies in the combined densities of states for the exciton and the acoustical phonons, has been resolved in the low-temperature spectra. The indirect energy gap is 2.339\ifmmode\pm\else\textpm\fi{}0.002 eV at 1.6\ifmmode^\circ\else\textdegree\fi{}K and 2.259\ifmmode\pm\else\textpm\fi{}0.003 eV at 300\ifmmode^\circ\else\textdegree\fi{}K. These values are considerably larger than previous estimates, especially at 300\ifmmode^\circ\else\textdegree\fi{}K. The consequences of the larger energy gap in the quantitative analysis of donor-acceptor-pair fluorescence spectra and of the spectra of bound exitons in gallium phosphide are considered.