Phonon Assisted Indirect Transition, Impurity-bound Excitons and D-A Pair Spectra in GaP

Motivated by the current interest in MBE-grown quantum well structures involving GaP layers and substrates, we have studied the piezo-modulated transmission and photoluminescence spectra of GaP. The piezo-modulated transmission spectrum of the purest GaP at 8 K yielded distinct signatures of excitonic transitions accompanied by the emission of TA(X), TO(X) and LA(X) phonons. In substrates with S and N impurities, bound-exciton signatures were manifested in both modulated transmission and photoluminescence spectra. The photoluminescence spectra display no-phonon signatures associated with excitons bound to S and N as well as phonon satellites. A self-consistent assignment of features in photoluminescence and modulation spectra yielded 13.2, 31.6 and 45.3 meV for TA(X), TO(X) and LA(X) phonons, respectively, and a free exciton energy of 2.330 eV. The well-known striking D-A pair spectra observed in some of the specimens are associated with the simultaneous presence of S donors and Mg acceptors. Supported by NSF (DMR 94-00415).