Energy-Dependent Capture Cross Sections and the Photoluminescence Excitation Spectra of Gallium Phosphide above the Threshold for Intrinsic Interband Absorption

Oscillatory structure connected with intrinsic absorption processes involving the ${\Gamma }_{15}\to X_1$ indirect energy gap and an enhanced response near the ${\Gamma }_{15}\to {\Gamma }_1$ direct energy gap have been observed in the luminescence excitation spectra of excitons bound to nitrogen and bismuth isoelectronic substituents in gallium phosphide. The oscillation period indicates that hot photogenerated indirect excitons rapidly lose energy by the cascade emission of low-wave-number longitudinal optical phonons without undergoing dissociation. The oscillatory structure appears in the luminescence excitation spectra of these centers, although it cannot be seen in absorption, because excitons of low kinetic energy are preferentially captured by the isoelectronic impurities. Thus the impurity center induces an energy-dependent capture cross section because of the presence of a shallow bound state. A lifetime effect is the only one of several mechanisms suggested for the oscillatory structure observed in interband photoconductivity excitation spectra of several direct-gap semiconductors which can be relevant in luminescence excitation spectra. Just below the ${\Gamma }_{15}\to {\Gamma }_1$ direct energy gap, near the onset of the second region of anomalous luminescence response, indirect absorption is predominantly due to transitions which produce low kinetic-energy holes. It is believed that these absorption transitions preferentially contribute to the luminescence excitation spectra because the probability of forming the excited states of these impurity complexes, particularly for nitrogen, is much greater for cold holes. Excitation spectra of shallow donor-acceptor pair luminescence in gallium phosphide do not show either of these selective excitation effects. The shape of the spectra can be accounted for solely from a consideration of the intrinsic absorption spectrum, including processes involving the creation of free-electron hole pairs as well as free excitons, and surface recombination processes. The latter processes depend in detail upon the particular crystal and, as previously reported by Gershenzon and Mikulyak, upon which of the principal $〈111〉$ faces of solution-grown platelets is illuminated. The ${\Gamma }_{15}\to {\Gamma }_1$ direct exciton transition appears as a well-defined dip in the low-temperature excitation spectra of pair luminescence.