Dielectric Function of Nitride Semiconductors: Recent Experimental Results

A review on the experimental determination of the dielectric function for hexagonal nitride semiconductors is presented. The peculiarities of nitride samples such as surface roughness and extended interface layers alter in comparison to an ideal film spectroscopic ellipsometry or reflectance spectra in a characteristic manner. It requires the application of multi-layer models for data analysis in order to determine reliable dielectric functions. Results of such an analysis for GaN covering a broad spectral range are given. Below the band gap, both ordinary and extraordinary components of the dielectric function tensor are determined for GaN as well as for AlN. The dielectric functions of MBE-grown InN characterised by a band gap of around 0.75 eV and a sputtered film exhibiting an absorption edge of around 1.9 eV are compared with results of first-principles calculations. Good agreement between theory and experiment is only found for the MBE-grown material providing further evidence that InN is a "narrow" band gap semiconductor. Finally, photocurrent measurements of a GaN Schottky-diode reveal the influence of electric fields on the shape of the excitonic absorption edge. The interpretation is supported by results of dielectric function calculations.