Silicon carbon is used as a diffusion barrier (18,108) to germanium so that a silicon layer (20,110) can be subsequently formed without being contaminated with germanium. This is useful in separating silicon layers (20, 110) from silicon germanium layers (16,106) in situations in which both silicon and silicon germanium are desired to be present on the same semiconductor device (10) such as for providing different materials for optimizing carrier mobility between N and P channel transistors (27) and for a raised source/drain (134,136) of silicon in the case of a silicon germanium body.