The photoluminescence of GaP crystals doped with bismuth, the isoelectronic impurity, has been measured at temperatures between 2K and 77K. The spectra contain many sharp lines at lower temperatures. These lines arise from the donor-Bi pair recombinations and from the recombinations of excitons bound to Bi-traps. The local-mode phonon (26.0±0.4 meV) and the zone-center phonon (49±1 meV) associate with these transitions. The spectral shapes of two series of zone-center phonon sidebands, which dominate the whole spectra, have been analyzed by the configuration coordinate model. The phonon-coupling parameter S for the spectra due to Bi-traps is fairly large in comparison with that for the shallow donor-acceptor pair spectra. This corresponds to the fact that the impurity-lattice interaction is strong for the Bi-isoelectronic trap, since holes or excitons are tightly bound to this center by the short range potential.
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Tajima et al. (1976) studied this question.