The optical properties of sputtered composite of Al–AlN

Optical constants n and k have been determined for aluminum–aluminum nitride composite in a reactive gas flow from zero to a flow giving stoichiometric nitride. A method using a dielectric function model was applied for each component in the Bruggeman equation to fit the experimental reflectance and transmittance spectra in the wavelength range of 0.3–2.5 μm. The refractive index of aluminum–aluminum nitride showed a maximum as the reactive gas increased. The extinction coefficient, on the other hand, decreased with increased reactive gas flow. The determined optical properties were then used to design a multilayer selective surface for solar thermal applications. Two absorbing layers with a graded feature plus an anti-reflection layer resulted in a high calculated solar absorptance of 0.97. The optimized metal content corresponds to the composite around the maximum refractive index. A solar absorptance of 0.96 was reached for experimentally prepared surfaces according to the theoretical prediction.