Films with spatially graded refractive indices can be formed by a variety of techniques to produce high efficiency coatings. Cermet films with graded compositions are useful as selective surfaces and their absorptivity can be enhanced by coating with a microscopically textured surface to provide solar absorptivities approaching 0.99. High temperature stable Mo-Al20 and Pt-Al203 have been formed with this enhancement. Microscopically textured but chemically homogeneous materials can also be used for simple and effective selective surfaces. Semiconductors have intrinsic selectivity in addition to that induced by the structure. The use of reactive ion etching and selective chemical etching to produce the desired microstructure of submicron sized cones or columns in semiconductor and other materials is described.
H. G. Craighead,
"Highly Absorbing Coatings Using Graded Refractive Indices And Textured Surfaces", Proc. SPIE 0401, Thin Film Technologies I, (28 November 1983); doi: 10.1117/12.935538; https://doi.org/10.1117/12.935538