Determination of AlxGa1-xN refractive indexes at low and room temperature, in the 300-600 nm range, for the optimisation of GaN-based microcavities

AlxGa1-xN alloys are deposited on (111)Si substrate by molecular beam epitaxy. The Al composition is deduced from energy dispersive X-ray spectroscopy and photoluminescence experiments. The refractive index n and the extinction coefficient k, at room temperature, in the 300-600 nm range, are extracted from a two-stage procedure based on spectroscopic ellipsometry and reflectivity measurements. In the transparent region, the refractive index is determined as a function of wavelength, using a Sellmeier law. The accurate knowledge of the later is essential for the fabrication of optimised GaN-based microcavities. For this purpose, two simulations of microcavities with nitride-based distributed Bragg reflectors are presented from the refractive indexes determined in this work at 5 K. The first one is constituted by a (lambda/2) GaN cavity embedded between two Al0.19Ga0.81N/Al0.675Ga0.325N distributed Bragg reflectors. In the second one, a lower aluminium composition (0.47) is used in place of (0.675) in order to reduce the strain effects due to the difference between the lattice parameters.