Optical constants of epitaxial AlGaN films and their temperature dependence

We have studied the dependence of the absorption edge and the refractive index of wurtzite AlxGa1-xN films on temperature and composition using transmission and photothermal deflection spectroscopy. The Al molar fraction of the AlxGa1-xN films grown by plasma induced molecular beam epitaxy was varied through the entire range of composition (0 less than or equal to x less than or equal to 1). We determined the absorption edges of AlxGa1-xN films and a bowing parameter of 1.3 +/- 0.2 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 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 0.05 +/- 0.01 and in an energy shift of the absorption edge of about 64 +/- 5 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. (C) 1997 American Institute of Physics.