We have studied the deposition of GexSi1−x layers on (100) Si substrates by gas source molecular beam epitaxy (GSMBE) using disilane and germane.
The investigation of RHEED intensity oscillations during growth reveals the well known rate enhancement obtained when adding a small amount of germane to the disilane flux. However, when exposing a previously deposited Ge layer to a pure disilane flux the growth rate during the first few monolayers remains at an enhanced value but returns to its homoepitaxial value after about 10 to 15 monolayers. This behaviour was observed under a variety of growth conditions. It is in marked contrast to the experience obtained in conventional Si/Ge MBE and suggests a catalytic effect of the particular surface present during GSMBE growth. We propose that this effect is caused by the surface segregation of Ge species and leads to a smear-out of the Ge profile in the layer.