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 lowestΓ 8 candΓ 7 cconduction bands and the three highestΓ 9 v,Γ 7 + v, andΓ 7 − vvalence bands. We conclude that the corresponding three gaps between the valence bands and theΓ 8 cconduction band are quasi‐direct, while the ones involving the s ‐likeΓ 7 cconduction band are direct. The optical oscillator strengths are also calculated outside the Γ point. Their influence on the observability of excitons in absorption and emission spectra is investigated. The almost dipole‐forbidden transitions into the lowest p ‐likeΓ 8 cconduction 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.