UV reflectivity of GaN: Theory and experiment

Abstract

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 reHectivity 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 mufBn-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 e2(w) 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.

VUV‐ellipsometry on GaN: Probing conduction band properties by core level excitations

“…The experimental analysis of electronic transitions, which is mandatory for testing and validating the theoretical efforts, is also much more demanding than that of the above mentioned "classical" semicon-ductors [7,9,10]. On the one hand, the stable crystal structure of the III-nitrides under normal pressure and temperature is the hexagonal wurtzite lattice.…”

“…As compared to the "classical" semiconductors which usually crystallize in the cubic zincblend or diamond structures the symmetry lowering related to the wurtzite phase yields a more complex set of material properties. On the other hand, material specific properties such as the characteristic electronic interband transitions, and the concomitant interband critical points, fall into the vacuum-ultraviolet (VUV) spectral range which is not accessible for usual optical spectroscopy techniques [7,[9][10][11]. Modulation spectroscopy, reflection spectroscopy and spectral ellipsometry are established experimental techniques for analysing electronic interband critical points [4,12,13].…”

Identification of van Hove singularities in the GaN dielectric function: a comparison of the cubic and hexagonal phase

“…The assignment considers the individual dipole transition probabilities depending on the crystal symmetry and the geometry of the measurement. Furthermore, a detailed theoretical band-toband analysis of dielectric function features, published by Lambrecht et al [1], was considered. In conclusion, we suggest a new labeling of absorption structures as used in classical III -V materials like GaAs, which reflects the origin of transition structures from specific points in the respective Brillouin zones.…”

Phase stability, electronic structure, and optical properties of indium oxide polytypes

“…rigid shift of all the CB states, so that the optical spectra shall also be shifted accordingly. 56,65,66,80 The search of literature shows that among In 2 O 3 -I and -II experimental data is available for the latter in Ref. 48, where reflectivity and transmittance spectra were measured by spectrophotometry at room temperature.…”