Pair Spectra and "Edge" Emission in Gallium Phosphide

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Abstract
The photoluminescence of GaP crystals grown from gallium solution has been observed at 20°K and below. The spectra contain many sharp lines. Some of these lines arise from excitons bound to point defects, but most are caused by the radiative recombination of electrons and holes trapped at donors and acceptors. The many lines arise from the many possible separations between the donors and acceptors. Five different types of spectra have been observed as a result of doping the crystals. The spectra may be analyzed to give the type of geometrical arrangement of donors and acceptors, and any particular line may be assigned to a particular pair separation. Considerable information may be deduced about the chemical centers involved; thus, the "pure" crystals are believed to contain S-Si pairs which are responsible for the fluorescence. Other pair spectra identified involve Te-Si, Se-Si, Zn-S, and Cd-S pairs. A great deal of the fluorescence occurs as broad emission. This so-called "edge" emission is observed to change with the line spectra as different impurities are incorporated in the crystals. Consequently, the broad emission is ascribed to recombination occurring at widely separated, nonisolated pairs. This conclusion is supported by observations of the change of the fluorescence with exciting intensity, and of the time decay of the fluorescence.

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