Effects of H coverage on Ge segregation during Si_1-xGe_x gas-source molecular beam epitaxy

The effects of H coverage θ_H on Ge segregation during Si_1-xGe_x gas-source molecular beam epitaxy (GS-MBE) were investigated using D₂ temperature programmed desorption (TPD). Si_1-xGe_x films with x = 0.01-0.30 were grown from Si₂H₆/Ge₂H₆ mixtures at T_s = 450-800°C, held at the growth temperature for 30 s, cooled to <200°C, and then exposed to atomic deuterium until saturation coverage. D₂ TPD spectra were fit using four peaks corresponding, in order of decreasing activation energy, to desorption from Si monodeuteride, Ge-Si mixed-dimer monodeuterides, Si dideuteride, and Ge monodeuteride. Steady-state Ge surface coverages were determined from the TPD data as a function of T_s and X. In contrast to solid-source MBE film grown in this temperature regime, Ge segregation during GS-MBE decreases with decreasing T_s due to the increasing H coverage. The results were well described by a model accounting for the Si/Ge site exchange and θ_H. The Ge segregation enthalpy varies from -0.28 eV at T_s≥800°C, where the steady-state hydrogen coverage θ_H approaches zero, to -0.10 eV at T_s≤450°C, where θ_H is nearly saturated.