We present a catalyst enhanced etch process with high etch rates for amorphous Si based alloys (e.g. alpha-Si, alpha-Si:C, alpha-Si:P, alpha-SiCP) and low etch rates for crystalline Si (e.g. c-Si, c-Si:C, c-Si:P, c-SiCP) with etch rate ratios up to similar to 200. The addition of a suitable surface catalyst such as Ge (e.g. from GeH4) during HCl based etch processes increases both, the etch rate of amorphous Si alloys and the etch rate selectivity against c-Si alloys. The Ge source dynamically forms a SiGe surface layer during the etch process. Ge penetrates fast into alpha-Si through diffusion, forming an alpha-SiGe film with high [Ge] concentration. Ge diffusion into c-Si however is very limited; a rather slow surface-sub-surface exchange reaction (segregation) causes a penetration depth of only one monolayer. Repeated cycles of a selective chemical vapor etch process following a non-selective deposition process enable effective selective epitaxial growth. (C) 2011 Elsevier B.V. All rights reserved.