RHEED INVESTIGATION OF GE SURFACE SEGREGATION DURING GAS SOURCE MBE OF SI/SI1-XGEX HETEROSTRUCTURES

Ge segregation during silicon gas source molecular beam epitaxy (Si-GSMBE) has been studied by in situ growth rate measurements using reflection-high-energy-electron diffraction (RHEED) intensity oscillations. Growth rate of Si on Si1-xGex gradually decreases to the Si homoepitaxial growth rate, which is attributed to Ge surface segregation at the growth interface. This segregation has been modelled using a mass balance equation and it has been found that the observed growth rate enhancement can be used as a direct measure of the Ge segregation. Using this novel in situ technique, concentration dependence of Ge segregation was studied, and it was found that the segregation decay curve is nonlinear, resulting in two segregation regimes dependent upon the Ge concentration, consistent with previous studies. Temperature dependence studies reveal that surface hydrogen, which is produced by the dissociation of Si2H6 and GeH4 on the surface during growth, may act as a growth controlling surfactant, and comparison with solid source growth results suggests that it significantly suppresses the Ge segregation, leading to a more precise control of the interface. Finally, the thermal stability of the segregated surfaces was examined. Growth interruption and annealing during Si overlayer growth on Si1-xGex resulted in a small increase in the surface Ge concentration, which may be ascribed to the outdiffusion effect of Ge from the near surface region.