Role of short-period superlattice buffers for the growth of Si0.75Ge0.25 alloy layers on Si(0 0 1) substrates

The short-period superlattices (SLs) consisting of (Si14/Ge1)20, (Si11/Ge1)20, (Si8/Ge1)30, (Si11/Ge2)30, (Si9/Ge2)30, (Si8/Ge2)40 and (Si7/Ge2)40 were grown step-by-step on Si(0 0 1) substrates at 500°C by molecular beam epitaxy. Residual strain, surface roughness and dislocations of Si0.75Ge0.25 alloy layers with a thickness of 2000 Å grown on the SLs have been characterized by X-ray diffraction (XRD), atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (XTEM), respectively, as a function of step number of SLs. By inserting the first step (Si14/Ge1)20 SL, a drastic decrease to about 10 Å in root-mean-squared (RMS) surface roughness is observed compared to that of about 40 Å of the sample grown without SLs, although the residual strain is almost the same. A noticeable change in RMS roughness is also observed when Ge is deposited as 2-monolayers mode from 1-monolayer mode. Residual strain in one-step and seven-step SL buffer samples are found to be about −0.38 and −0.15%, respectively. XTEM images verify that at various steps SL buffers help to relax the strain of the upper alloy layer by introducing dislocations in the buffer layers or deflecting them into the substrate. Furthermore, the alloy layers grown at 500°C using one-step SLs which are grown at different temperatures from 300 to 550°C are also studied. At 300°C, the alloy layer becomes about relaxed leaving behind only −0.08% residual strain (relaxation rate 90%) with small RMS roughness of about 10 Å.