Optical characteristics of amorphous III-V nitride thin films

The optical constants and polarized optical properties of amorphous III-V nitride thin films, a(Al, Ga, In) N, deposited by RF magnetron sputtering onto crystalline silicon, c-Si, (111) and glass substrates have been investigated over the wavelength range 300 - 1400 nm. The optical constants of a-AlN were obtained by analysis of the measured ellipsometric spectra through the Cauchy-Urbach model while the optical constants of a-(In, Ga) N were determined using the Tauc-Lorentz model. Analysis of the absorption coefficient of a-AlN (in the range 200-1400 nm) and a-GaN (in the range 300 - 1400 nm) show the optical bandgap to be 5.9 +/- 0.05 and 3.44 +/- 0.05 eV. The absorption coefficient of a-InN (in the range 300-1400 nn) as a function of photon energy shows the absorption edge to be about 1.74 +/- 0.05 eV. From the angle dependence of the p-polarized reflectivity we deduced Brewster and principal angles of these films. Measurement of the polarized optical properties revealed a high transmissivity (70 % - 95 %) and low absorptivity (< 18 %) for all three thin films in the visible and near infrared regions.