Optical properties and band structure of highly doped gallium nitride

Spectroscopic ellipsometry is applied for determining the ordinary and extraordinary dielectric functions of wurtzite GaN for different free-electron concentrations. The analysis of the dielectric functions in the infrared spectral range yields the frequencies of the coupled longitudinal plasmonphonon modes. From the related plasma frequencies the anisotropy of the effective electron mass is determined. The data in the ultraviolet range indicate unambiguously that the L-valley of the conduction band for undoped material is located at least ≈ 2 eV above the conduction band minimum at the Γ-point. In contrast to the behavior at the Γ-point, the band structure at the L-valley does not undergo a renormalization with increasing free-electron density.

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