Strain relaxation and ordering in SiGe layers grown on (100), (111), and (110) Si surfaces by molecular-beam epitaxy

Si0.5Ge0.5/Si superlattices and thick Si 0.5Ge0.5 layers grown on (100), (111), and (110) Si surfaces by molecular-beam epitaxy (MBE) exhibit different growth morphologies and defect structures. The best morphology is achieved on (100) surfaces at low temperatures (∼400 °C), while thin and defect-free SiGe layers grown at higher temperatures (∼600 °C) tend to exhibit undulated surfaces due to the mismatch strain. Strained SiGe layers grown on (111) and (110) surfaces are much more susceptible to twin formation. SiGe layers grown on (100) surfaces at low temperatures exhibit a long-range order along the 〈111〉 directions. Our results indicate that such ordering occurs only in thick and relaxed SiGe layers but not in thin SiGe layers strained in a SiGe/Si superlattice structure. No ordering was observed in SiGe layers grown on (111) and (110) surfaces.