Direct Transition and Exciton Effects in the Photoconductivity of Gallium Phosphide

Photoconductivity spectra between 0.6 and 3.3 eV of vapor-grown, high- and low-resistivity, GaP whiskers and of high-resistivity floating-zone-refined crystals of GaP were measured as a function of temperature. Structure seen in the spectra of high-resistivity whiskers and interpreted as direct exciton formation (with subsequent dissociation) permitted a determination of the energy gap at k=0 and its temperature variation. It could be fitted by $E_G=2.895\mathrm{}\pm 0.004-(1.17\pm 0.01)\times {10}^{-6\mathrm{}}{T}^2$ in eV for temperature below 300°K. This structure was seen as a result of an anomalously large photoconductive response in the direct transition region, which was attributed to a mobility increase from optical excitation similar to that seen previously in InP, CdSe, and GaAs. In low-resistivity, sulfur-doped whiskers photoconductivity arising from absorption in bound exciton states associated with neutral sulfur donors was observed. The photoconductive lifetime at 296°K in high-resistivity whiskers was found to be 3×${10}^{-9\mathrm{}}$ sec.