Sharp Sb doping profiles have been achieved in Ge by low temperature molecular beam epitaxy on Ge(001) substrates. The strong tendency of Sb to segregate on the growing Ge surface has been overcome by large substrate temperature modulations during growth. In this way homogeneous doping profiles with a free carrier concentration up to 1.5 × 1020 cm−3 and δ-doped Ge layers with a sheet carrier density of 3 × 1013 cm−2 and a doping profile width of less than 6 nm have been achieved. Selective doping of a Si3Ge3/Ge(001) heterostructure leads to a two-dimensional electron gas in the strained Si3Ge3 superlattice quantum well. We have characterized such systems by Hall and Shubnikov-de Haas measurements and found low temperature electron mobilities up to 6800 cm2/V s and sheet carrier concentrations of 2 × 1012 cm−2.