Anisotropy of the momentum matrix element, dichroism, and conduction-band dispersion relation of wurtzite semiconductors

By comparison of calculated imaginary parts of the ordinary and extraordinary dielectric functions of wurtzite semiconductors near the band gap with experimental results, we demonstrate that the interband matrix elements E-P(parallel to) and E-P(perpendicular to) of the momentum operator parallel and perpendicular to the optic axis are different. E-P(parallel to) exceeds E-P(perpendicular to) and their ratio increases along the series CdSe, CdS, ZnO. The u parameters of GaN, InN, and AlN suggest that the E-P(parallel to)/E-P(perpendicular to) ratio should increase along this series as well. We also determined the conduction-band dispersion relation and nonparabolicity, as well as the effective mass, as a function of electron concentration and wave vector for GaN, ZnO, CdS, and CdSe. In addition, optical response due to transitions into exciton-phonon complexes was observed and analyzed. We conclude that up to about ten phonons may participate in such absorption processes in ZnO.