INTRINSIC AND EXTRINSIC EDGE LUMINESCENCE IN EPITAXIAL GAP

Cathodoluminescence spectra have been obtained from undoped and n-type gallium phosphide crystals at temperatures between 80°k and 300°k. The gallium phosphide was grown epitaxially by vapour transport on GaAs substrates, the n-type specimens being doped with either sulphur or tellurium. The crystals showed only weak edge luminescence, and this is interpreted as the radiative recombination of free excitons and free holes at neutral donor centres. The free-exciton recombination bands show a weak no-phonon component and other bands that give evidence of the expected phonon co-operation for indirect band to band transitions. The phonon energies deduced from these bands are in good agreement with those observed in the optical absorption edge and lattice vibration absorption spectra. The luminescence attributed to free-hole recombination at donor centres has enabled values for ED, the donor electron binding energy, to be obtained for the two impurities involved (sulphur, 105±5 mev; tellurium, 90±5 mev). The temperature dependence of the luminescence features is shown to be consistent with the proposed interpretation.