UV reflectivity of GaN: Theory and experiment

A synchrotron radiation and theoretical study of the UV reflectivity of wurtzite GaN films in the range 0–30 eV is presented. First-principles calculations of the reflectivity and related optical constants are reported and used to analyze the experimental results. The calculations are performed assuming the local-density approximation and using a muffin-tin orbital basis set. The calculated peak positions are found to be in good agreement with the experiment to within a few tenths of an eV except for a constant shift of 0.98 eV which simultaneously corrects the minimum band gap. This constant upward shift of the conduction bands of the Kohn-Sham eigenvalue band structure is in good agreement with a recently calculated self-energy gap correction of 0.95 eV. A discrepancy in absolute intensities increasing with increasing energy is found between theory and experiment, the origin of which is discussed. The imaginary part of the dielectric function ${\mathrm{\varepsilon }}_2$(ω) is decomposed into its most important interband contributions and the relation between the features in the optical response functions and the band structure is described. The transitions from the upper N 2p-like valence bands occur in the range 3.5–15 eV. The transitions from the rather localized Ga 3d and N 2s bands whose energy is expected to need additional self-energy corrections are presented separately. The spectrum of the transitions from Ga 3d is closely related to the Ga 4p-like density of states in the conduction band.