Abrupt "delta-like" doping in Si and SiGe films by UHV-CVD

A structure and method of forming an abrupt doping profile is described incorporating a substrate, a first epitaxial layer of Ge less than the critical thickness having a P or As concentration greater than 5x10 atoms/cc, and a second epitaxial layer having a change in concentration in its first 40 Å from the first layer of greater than 1x10 P atoms/cc. Alternatively, a layer of SiGe having a Ge content greater than 0.5 may be selectively amorphized and recrystalized with respect to other layers in a layered structure. The invention overcomes the problem of forming abrupt phosphorus profiles in Si and SiGe layers or films in semiconductor structures such as CMOS, MODFET'S, and HBT's.