be omitted from the layer structure because either of these layers can produce the required band offset to provide confinement of the holes in strained si 1 v ge v layer 430 in fig6 p type modulation doping is incorporated within bottom si 1 z ge z buffer layer 420 in this case bottom si 1 z ge z buffer layer 420 is divided into p type supply layer 460 and substantially undoped spacer layer 470 the thickness of undoped spacer layer 470 must be at least 0 5 nm to ensure that the high mobility is maintained in si 1 v ge v quantum well 430 similar to layer structures in fig2 5 the layer structure in fig6 may also include modulation doping in either bottom si 1 z ge z buffer layer 420 or top si 1 m ge m buffer layer 440 or both a buried insulating layer under si 1 x ge x buffer layer 410 and or a si 1 y ge y interposer layer between bottom si 1 z ge z buffer layer 420 and si 1 x ge x layer 410 the possible range of thickness for si 1 z ge z buffer layer 420 is from.