Influence of deposition power and thickness on the microstructure and optical properties of DC-sputtered Al_xO_y for selective solar absorber applications

The alumina films are reported to differ in their structure, optical properties and electrical properties depending on the basic atomic structures and the preparation technique. Al_xO_y thin films in this study were deposited on glass substrates through DC reactive sputtering. The XRD results revealed that the films were amorphous; however, the composition of the films, as analysed by EDX and Raman spectroscopy, depicted the presence of O and Al. The surface morphology analysed by scanning electron microscopy revealed uniform grains with a size ranging between 169 nm and 101 nm subject to power and thickness. The films exhibited optical transmittance above 75% and the corresponding reflectance below 30% which is suitable for multilayer selective absorber depositions. The films’ optical constants (n and k), determined through the fitting of experimental transmittance and reflectance into SCOUT software confirmed the dielectric nature of the films. Besides, the refractive index of the films increased with the increase in thickness and decreased with the increase in power. The results from this study clearly show that the optical properties can be engineered using power and thickness.