Optical constants of epitaxial AlGaN films and their temperature dependence

Abstract

We have studied the dependence of the absorption edge and the refractive index of wurtzite ${\mathrm{Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N}$ films on temperature and composition using transmission and photothermal deflection spectroscopy. The Al molar fraction of the ${\mathrm{Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N}$ films grown by plasma induced molecular beam epitaxy was varied through the entire range of composition $\text{(0⩽x⩽1).}$ We determined the absorption edges of ${\mathrm{Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N}$ films and a bowing parameter of $\text{1.3±0.2\hspace{0.17em}}\mathrm{eV}.$ The refractive index in the photon energy range between 1 and 5.5 eV and temperatures between 7 and 295 K was deduced from the interference fringes. The static refractive index $\text{n(0)}$ changed from 2.29 for GaN to 1.96 for AlN at room temperature. A variation of temperature from 295 to 7 K resulted in a decrease of refractive index (at photon energies close to the band gap) by $\text{0.05±0.01}$ and in an energy shift of the absorption edge of about $\text{64±5\hspace{0.17em}}\mathrm{meV}$ independent of the Al content of the films. Using the Kramers–Kronig dispersion relation and an approximation for the dispersion coefficient for photon energies near the band gap, the refractive index could be described as a function of photon energy, Al content, and temperature.