Formation of inverse cones in crystalline silicon by selective etching of amorphous regions resulting from epitaxial breakdown

We report on a process to form micron-scale inverse cones in crystalline silicon without any masking steps using a selective, low temperature (<175 C) plasma process. The selectivity of the process originates from the use of a H2 plasma that preferentially etches away amorphous cones, formed as a result of epitaxial breakdown, leaving the surrounding crystalline material behind. Efficient etching is realized by pre-coating the reactor walls with hydrogenated amorphous silicon oxide (a-SiO x :H) to prevent any chemical transport of silicon towards the substrate. The etch depth of the amorphous cones is linked directly to the time of exposure to plasma. When densely packed, such structures exhibit useful optical properties, such as specular reflectance as low as 2% (with the sample appearing visibly black in the high density and fully etched regions) and iridescence (in regions of partial etching).