Absorption and Luminescence of Excitons at Neutral Donors in Gallium Phosphide

Abstract

Absorption and photoluminescence spectra of excitons weakly bound to sulphur, selenium, and tellurium donors in gallium phosphide have been studied at 25\ifmmode^\circ\else\textdegree\fi{}K and below. Relatively weak satellite photoluminescence lines have been discovered. These photoluminescence satellites are phonon replicas, and some of the corresponding absorption satellites have been observed. Comparison with the intrinsic absorption-edge spectrum shows that some of these are the momentum-conserving (MC) phonons in the indirect transition. The more accurate estimates of these phonon energies provided from the bound exciton spectra are, (TA) 13.1\ifmmode\pm\else\textpm\fi{}0.1 meV, (LA) 31.5\ifmmode\pm\else\textpm\fi{}0.1 meV, (TO) 45.3\ifmmode\pm\else\textpm\fi{}0.1 meV (sulphur spectrum). Replicas associated with the zone-center optical phonons, of energy 45.4\ifmmode\pm\else\textpm\fi{}0.1 meV (TO) and 50.1\ifmmode\pm\else\textpm\fi{}0.1 meV (LO), are also prominent. Luminescence spectra associated with the relatively heavy donors selenium and tellurium also contain prominent 23- and 47-meV phonon replicas which do not appear in absorption. These bands are apparently associated with "in-band resonance" local modes occurring at possible regions of low density of thelattice modes. The relative intensity of adjacent Mc (LA) replicas is increased when the local modes are prominent. Apart from this apparent interference with the local modes, the intensities of the optical and acoustical (MC) phonon replicas vary together between spectra involving different donors, as do the intensities of the no-phonon line and the zone-center replicas. The binding energy of the exciton to the donor does not vary as expected with the ionization energy of the donor or with the strength of the coupling to the momentum-conserving phonons. An absorption satellite 40.6\ifmmode\pm\else\textpm\fi{}0.2 meV above the no-phonon lines for all three donors may also be a phonon replica, but is anomalously strong in the absorption spectra. Additional absorption satellites are apparently associated with excited states of the exciton-neutral-donor complex. The intensities of the satellites relative to the principal no-phonon line vary more than the relative transition energies between these three group-VI donors. The spectral positions of these excited states and the activation energy for thermal quenching of the luminescence intensity (29\ifmmode\pm\else\textpm\fi{}1.5 meV) suggest the liberation of free electrons and holes rather than free excitons from these complexes.