Composition and temperature dependent optical properties of AlxGa1-xN alloy by spectroscopic ellipsometry

Abstract

A series of AlxGa1-xN/AlN/Sapphire films with x=0.35–0.75 and different thickness of epi-layer were prepared by metalorganic chemical vapor deposition (MOCVD). Spectroscopic ellipsometry (SE) was used to study the temperature-dependent refractive indices and optical bandgaps of the AlxGa1-xN films ranging from 300 to 823K. Parametric semiconductor (PSEMI) models were used to describe the dielectric functions of AlGaN/AlN layers. The fitting results of refractive index, energy bandgap, thickness and surface roughness at 300K are in good agreement with photoluminescence (PL), scanning electron microscopy (SEM) measurements and the existing literature. Our finding indicates that the crystal quality of the samples with x=0.47 and 0.60 are better than those with x=0.35 and 0.75. As the temperature rises, the increasing of refractive index for the low Al content AlxGa1-xN layers is stronger than that of high Al content in the transparent region, and the reduction of bandgap with high Al content is larger than that of low Al content. For all the samples (x=0.35–0.75), an analytical expression for temperature-dependent refractive index in the wavelength range of 195–1650nm was obtained using the Sellmeier law, and the quantitative analysis of the SE-derived temperature-dependent bandgap was conducted by using the Bose-Einstein equation.