Forbidden Band-Edge Excitons of Wurtzite-GaP: A Theoretical View

By means of an approximate quasiparticle description we study the electronic structure of wurtzite-GaP in more detail for the two lowest Gamma 8c and Gamma 7c conduction bands and the three highest Gamma 9v, Gamma 7+v, and Gamma 7-v valence bands. We conclude that the corresponding three gaps between the valence bands and the Gamma 8c conduction band are quasi-direct, while the ones involving the s-like Gamma 7c conduction band are direct. The optical oscillator strengths are also calculated outside the Gamma point. Their influence on the observability of excitons in absorption and emission spectra is investigated. The almost dipole-forbidden transitions into the lowest p-like Gamma 8c conduction band become substantial for finite wave vectors perpendicular to the c-axis. Therefore, the finite extent of the exciton envelope functions gives rise to non-vanishing transition strengths, which explain the intense photoluminescence observed experimentally by forbidden p-type excitons.