Thermal quenching processes in the low temperature photoluminescence of excitons bound to nitrogen pairs in GaP

We have measured the photoluminescent efficiency of excitons bound to isolated nitrogen, and to nitrogen pairs of different separations, in GaP. Excitons bound to a particular center were excited selectively with a tunable dye laser, and the temperature dependence of the emission intensity measured between 10 and 100 K. Detailed balance considerations show that the efficiency at 10 K is near 100%. Luminescence from excitons bound by less than 40 meV is found to be thermally quenched by the escape of a free exciton. For excitons bound by 40 meV or more, we find that luminescence is quenched by the escape of the hole. In the case of excitons bound to nearest-neighbor pairs (binding energy 143 meV) the electron remains attached to the pair, forming a charged acceptorlike center with a lifetime long enough for quasiequilibrium to be established with the valence band. This leads to a superlinear dependence of emission intensity on exciting power.