Alternatives to thick MBE-grown relaxed SiGe buffers

We have investigated several growth concepts for strain relieved SiGe buffe rs as basis for high frequency transistors. Modulation doped quantum wells (MODQWs) were realized by molecular beam epitaxy (MBE) on top of thick grad ed buffers prepared by MBE, ultra-high vacuum chemical vapor deposition (UH VCVD) and low-energy plasma-enhanced CVD (LEPECVD). Additionally, thin buff ers including a specific layer grown at low temperature (LT) were realized entirely by MBE. The overgrown thick CVD samples show comparable transport properties and thermal stabilities to those on thick graded MBE buffers. Mo bilities of up to 90 000 cm(2)/V s have been measured at 30 K. Thin LT-MBE structures show slightly worse properties but are superior to conventional constant composition buffers. (C) 2000 Elsevier Science S.A. All rights res erved.