The optical characteristics of single and multilayer III-V nitride thin film systems deposited by RF magnetron sputtering onto different substrates have measured and analyzed. Spectroscopic Ellipsometry measurements of amorphous (Al, Ga, In) N single layers are carried out at two angles of incidence, 70° and 75°, over the wavelength range 300 – 1400 nm. The measured ellipsometric data are fitted to models consisting of air / a-AlN / c-Si (111), air / roughness / a-GaN/ c-Si (111), and air / roughness / a-InN / c-Si (111). The roughness is modeled using the Bruggeman effective medium approximation, assuming 50 % a-GaN (or a-InN) and 50 % voids. The optical constants and the thicknesses of a-AlN are obtained by analysis of the measured ellipsometric spectra through the Cauchy–Urbach model whereas the optical constants and the thicknesses of a-(Ga, In) N are determined using the Tauc-Lorentz model. Analysis of the absorption coefficient of a-AlN (in the range 200 – 1400 nm) and a-GaN show the optical bandgap to be 5.82 ± 0.05 and 3.38 ± 0.05 eV, respectively. While the analysis of the absorption coefficient of a-InN shows the optical bandgap energy to be 1.68 ± 0.071 eV. These values are confirmed using different optical methods such as spectrophotometry, photoluminescence and polarized absorptivity.
From the angle dependence of the p-polarized reflectivity we deduce Brewster and principal angles of the single layers. Measurement of the polarized optical properties reveals a high transmissivity (70 % – 95 %) and low absorptivity (< 18 %) for all three thin films in the visible and near infrared regions. X - ray diffraction analysis verifies the amorphous nature of the (Al, Ga, In) N films.
This work also reports the successful growth of sputtered bilayer and multilayer antireflection coatings on quartz using alternating layers of AlN and InN. The designed layers are optimized at 500 nm and 50° – 70° incidence for their performance as a longwave-pass filter and coatings for solar cells with high efficiency by varying the number of periods of AlN / InN multilayer systems. The XRD patterns of these systems reveal a polycrystalline structure with a maximum diffraction peak at 31.2° (InN (002)).