Isoelectronic Traps Due to Nitrogen in Gallium Phosphide

Several sharp lines are seen in absorption and fluorescence in crystals of gallium phosphide containing nitrogen as an isolectronic substituent for phosphorus. These lines fall within about 0.15 eV of the band-gap energy. The most prominent line, the $A$ line, is due to a hole and an electron bound to an isolated nitrogen atom. A deeper series of lines, the $\mathrm{NN}$ lines, are due to pairs of nitrogen atoms, each line corresponding to a particular internuclear pair separation. This follows from the observation that the $\mathrm{NN}$-center concentrations vary as the square of the $A$-center concentration. The Zeeman patterns of the lines and several other pieces of evidence indicate that the deepest $\mathrm{NN}$ line corresponds to closest-neighbor nitrogen pairs, the second deepest to second-neighbor pairs, and so on. Vibrational sidebands can be seen in fluorescence included among which are local-mode frequencies. The substitution of ${\mathrm{N}}^{15}$ for ${\mathrm{N}}^{14}$ caused these local modes to shift in exact accord with theoretical expectations. A brief qualitative discussion of the nature of isoelectronic traps in semiconductors is presented.