A Localized Exciton Bound to Cadmium and Oxygen in Gallium Phosphide

In the past several years there has been an intensive effort to understand the origin of the red luminescence in gallium phosphide. Practical considerations have played an important part in stimulating this effort. Carefully constructed gallium phosphide diodes emit red light with efficiences of greater than one per cent at room temperature.1 Further improvement of these devices would be facilitated by a detailed understanding of the red luminescence. Gershenzon, Trumbore,2,3 and Nelson4 have attributed the red luminescence to recombinations between electrons trapped on O donors and holes trapped on distant Cd (or Zn) acceptors. We have found that part of the low temperature red luminescence is not due to the recombination of distant donor-acceptor pairs, but to the decay of a bound exciton. In this paper we present a detailed study of the optical properties of this exciton.