Detailed studies on sputter-deposited Hf and HfC thin films for solar energy devices

Thin films of hafnium and hafnium carbide were deposited by pulsed magnetron sputtering of a Hf cathode in an atmosphere of argon and acetylene (C2H2) at room temperature. Different flow rates of C2H2 gas were injected into the chamber to form distinct HfCx compounds and to analyze changes in their structural, chemical, morphological and optical properties in the UV-VIS-NIR regions. The analyzed samples are flat, with an average roughness of less than 1 nm. The refractive index of HfCx films shows a weak dependence on the chosen C2H2 flow rates, whereas the extinction coefficient decreases significantly with increasing gas flow rate, making the use of this material attractive in many applications. The accurate knowledge of the complex refractive index of this material is essential to fill the gap in the current literature and to accurately design optical structures for energy conversion purposes, such as selective solar absorbers (SSAs), selective emitters (SE), dielectric mirrors (DM). We have designed and optimized SSAs based on HfCx, Si3N4, and SiO2 that maximize the solar-to-thermal energy conversion efficiency in high vacuum solar thermal panels.