MBE source for germanium–carbon co-evaporation

Carbon (C) doping is useful for suppression of outdiffusion from boron in SiGe-heterobipolartransistors. The typical doping range is 1019–1020 cm−3 which means a Ge–C doping ratio of 100–1000. We present a MBE source for coevaporation with a flux ratio Ge–C of roughly 1000. This source consists of a specially designed carbon crucible with molten Ge which is thermally treated for a long time at 1000 °C. At the operation temperature (1300 °C) both the Ge-melt and the dissolved C evaporate. The Ge-flux is controlled by a quadrupole mass spectrometer with closed loop feedback. Ge and C profiles in different SiGe structures were measured by secondary ion mass spectrometry. A codoping of C–Ge with a 1/1000 doping level ratio was proven. The main problem with long term operation of the cell is connected to the build up of a carbon skin on the Ge melt.