Inversion of absorption anisotropy and bowing of crystal field splitting in wurtzite MgZnO

The anisotropic optical properties of wurtzite Mg_xZn_1−xO thin films $(0≤x≤0.45)$ grown on m-plane ZnO substrates by plasma assisted molecular beam epitaxy are studied using spectroscopic ellipsometry at room temperature. The data analysis provides the dielectric functions for electric field polarizations perpendicular and parallel to the optical axis. The splitting between the absorption edges of the two polarization directions decreases between x = 0 and x = 0.24, while an inverted absorption anisotropy is found at higher Mg content, indicating a sign change of the crystal field splitting $Δcr$ as for the spin orbit parameter. The characteristic energies such as exciton binding energies and band gaps are determined from the analysis of the imaginary parts of the dielectric functions. In particular, these data reveal a bowing parameter of $b=−283 meV$ for describing the compositional dependence of the crystal field splitting and indicate $Δcr=−327 meV$ for wurtzite MgO. The inverted valence band ordering of ZnO (⁠$Γ7-Γ9-Γ7$⁠) is found to be preserved with increasing Mg content, while the optical selection rules interchange.