The growth and doping of Si and SiGe layers on Si substrates were investigated by molecular beam epitaxy (Si-MBE). Growth took place at temperatures below the standard Si-MBE process regime (<550 degrees C). The lowest epitaxial temperature achieved was 140 degrees C. Results on matrix growth, metastability of strained-layer SiGe, and doping are given, and the structure and characteristics of two hetero-devices (MODFET and resonant tunneling element) are examined.<<ETX>>