We present ellipsometry data of the dielectric function of wurtzite ZnO in a wide energy range ͑2.5-32 eV͒. The ordinary and extraordinary components show a strong anisotropy above 10 eV, a feature for which ZnO deviates from the other II-VI wurtzite compounds. With the aid of ab initio calculations, performed within many-body perturbation theory ͑MBPT͒ and within time-dependent density-functional theory ͑TDDFT͒, we analyze the origin of the measured optical structures. TDDFT, with the use of a static long-range exchangecorrelation kernel, proves to be a cheaper computational tool than MBPT to yield a good description of the whole spectrum. Theoretical results for the zinc-blende phase are also presented.
The ordinary complex dielectric function (DF) of Al x Ga 1-x N alloys with 0 ≤ x ≤ 0.53 is determined by fitting spectroscopic ellipsometry data from the infrared to the vacuum ultraviolet spectral region (0.74 eV ≤ E ≤ 9.8 eV). The dispersion of the real part of the DF below the band gap is found to be in excellent agreement with previously published data. The obtained band gap energies are verified by photoluminescence and photoreflectance spectroscopy. In the high-energy range, three critical points of the band structure are clearly resolved. By applying a third-derivative based DF line shape analysis, the corresponding transition energies are determined. Their compositional dependences can be described on the basis of small bowing parameters.
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