Preferential carbon etching by hydrogen inside hexagonal voids of 6H-SiC(0001)

6H-SiC(0001) samples have been etched in a hot-wall chemical vapor deposition reactor at a hydrogen pressure of 13 mbar at $1800 °C.$ The surface morphology and elemental composition have been studied by scanning electron microscopy and micro-Auger analysis. Stoichiometric etching of SiC with equal atomic concentrations of Si and C is found on the flat sections of the surface, but in hexagonal voids of the SiC samples, a selective removal of C, leading to a pure Si surface at the bottom of the voids, is observed. Fast gas diffusion is the main transport mechanism for etching of the flat surface, while Knudsen diffusion becomes important inside the voids. It is proposed that the lower diffusion constant of reaction products containing Si compared to those containing C, leads to a preferential removal of C and a Si enrichment inside the voids.