Refractive index study of AlxGa1−xN films grown on sapphire substrates

Abstract

A prism coupling method was used to measure the ordinary (no) and extraordinary (ne) refractive indices of AlxGa1−xN films, grown by hydride vapor phase epitaxy (HVPE) and metalorganic chemical vapor deposition (MOCVD) on sapphire, at several discrete wavelengths from 442 nm to 1064 nm. In addition, spectroscopic transmittance and reflectance, correlated with the prism coupling results, were used to measure no as a continuous function of wavelength between the band gap of each sample (255 nm to 364 nm, depending on Al fraction) and 2500 nm. The Al mole fractions (x), determined by energy dispersive x-ray spectroscopy (EDS), were x=0.144, 0.234, 0.279, 0.363, 0.593, and 0.657 for the HVPE-grown samples, and x=0.000, 0.419, 0.507, 0.618, 0.660, and 0.666 for the MOCVD-grown samples. The maximum standard uncertainty in the EDS-determined value of x was ±0.02. The maximum standard uncertainty in the refractive indices measured by prism coupling was ±0.005 and a one-Sellmeier-term equation was adequate to fit the wavelength dependence of ne from 442 nm to 1064 nm. Due to the spectral proximity of the absorption edge, the wavelength dependence of no measured by spectroscopic transmittance/reflectance (correlated with the prism-coupling results), from the band gap of each sample to 2500 nm, was fit with a two-Sellmeier-term equation.