The effects of thermal treatment under argon–oxygen ambient on the microstructure and optical properties of DC sputtered Cr thin films for selective solar absorbers applications

Abstract

This work investigated the effect of thermal treatment of as-deposited chromium (Cr) thin films in a vacuum and argon/oxygen ambient for effective deposition of spectral selective solar absorber multilayer films. The DC sputtered Cr thin films were thermally exposed at different temperatures in a controlled argon/oxygen environment. Both as-deposited films and those thermally treated in vacuum and in oxygen–argon ambient revealed a weak XRD diffraction peak at 2θ≈44.3° corresponding to (110) plane of BCC structure of Cr metal. Raman analysis revealed one peak at 846 cm−1 corresponding to Cr–O vibration bonds for films thermally treated at 300 °C in a vacuum and 150 °C and 300 °C in argon–oxygen ambient. As determined by AFM and FE-SEM, surface roughness decreased with temperature increase for films thermally treated in a vacuum. Besides, a mixed trend in the evolution of surface roughness was observed for films thermally treated in argon/oxygen ambient. The average spectral transmittance for samples thermally treated in a vacuum decreased with an increase in temperature; however, the samples thermally treated in an argon–oxygen environment exhibited an increasing trend in average spectral transmittance with the increase in temperature. The results clearly show that post-deposition processing at elevated temperatures, particularly in argon–oxygen ambient, influences the structural and optical properties of prior-deposited Cr thin films. This should be considered when designing and depositing multi-layered dielectric/Cr spectral selective solar absorber films.