Aluminium/aluminium oxide nanocomposites prepared using a filtered cathodic vacuum arc

Aluminium oxide based coatings were reactively deposited using a filtered cathodic vacuum arc and their microstructure and optical properties were determined using X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, energy filtered transmission electron microscopy and spectroscopic ellipsometry. Coatings with compositions ranging from near stoichiometric Al2O3 to aluminium rich aluminium oxide were produced using different oxygen process pressures. The aluminium rich films consisted of Al metal clusters in an aluminium oxide matrix and exhibited optical and electronic properties which depended on the Al metal content. The refractive index at a wavelength of 500 nm varied systematically from 1.75 to 1.9 as the Al content increased to approximately 50 at.%. Films with high Al content were also found to be more electrically conductive, consistent with the presence of metallic Al. A coating was also successfully prepared in which the refractive index was varied with thickness by varying the oxygen pressure during deposition. The optical absorption of this coating showed near zero reflectance at 450 nm which indicated that these gradient index coatings could be successfully used as solar selective coatings.