Ge surface segregation during Si1−xGex(0 1 1) gas-source molecular beam epitaxy from Si2H6 and Ge2H6

Ge surface segregation during Si1−xGex gas-source molecular beam epitaxy on Si(0 1 1) was investigated using in situ D2 temperature programmed desorption (TPD). The Si1−xGex(0 1 1) layers, x=0–0.20, were grown from Si2H6/Ge2H6 mixtures at temperatures Ts ranging from 475°C to 750°C. Immediately following film growth, the samples were quenched and exposed to atomic D at 250°C until saturation coverage. TPD spectra from Si1−xGex(0 1 1) consist of five second-order peaks due to D2 desorption from, in order of decreasing temperature, Si rest-atom and adatom monodeuterides, Si dideuteride, and Ge rest-atom and adatom monodeuteride phases. Temperature-dependent Ge surface coverages θGe, determined from the TPD results, increase sharply with x ranging, at Ts=550°C, from 0.27 ML with x=0.04 to 0.74 ML for layers with x=0.20. For a given film composition, θGe decreases with decreasing Ts due to the corresponding increase in the fraction fSi,H of Si surface atoms terminated with H. From these data, we find that the Ge segregation enthalpy ΔHs for Si1−xGex(0 1 1) varies from −0.18 eV at Ts=750°C (fSi,H<0.003) to −0.09 eV at Ts=475°C (fSi,H=0.22).