"Inverted hut" structure of Si-Ge nanocrystals studied by extended x-ray absorption fine structure method

Local structure around Ge in Si/Ge superlattices containing the "inverted hut" nanocrystals has been investigated by using the extended x-ray absorption fine structure (EXAFS) technique. In contrast to the usual nanometer-sized Ge "hut clusters" commonly grown on top of Si layers using the conventional Stranski-Krastanow self-organized growth mode, SiGe-alloy nanocrystals can be formed beneath the Ge wetting layer and grown into the Si layer in Si/Ge superlattices prepared in a low-temperature molecular beam epitaxy growth mode, and exhibit inverted hut nanocrystal structures regularly spaced along the Si/Ge interface. The EXAFS results obtained with varying Ge wetting layer thickness provide a direct evidence that intermixing of Ge and Si atoms takes place in a zone of about 1-3 monolayers on each side of the Si/Ge interface. The intermixing of constituent atoms allows a mechanism other than the usual formation of misfit dislocations to release the strain energy resulted from lattice mismatch between Si and Ge at the interface.