Group IV element (Si, Ge and α-Sn) superlattices - low temperature MBE

Si/Ge and Sn/Ge heterostructures and short-period superlattices are grown by a modified molecular beam epitaxy (MBE) technique on Si and Ge substrates. Low energy electron diffraction and Auger electron spectroscopy are used in order to optimize the growth conditions with respect to interface roughness, segregation and intermixing. Transmission electron micrographs reveal that the interface quality of short period superlattices is improved by substrate temperature modulation during deposition. For Si/Ge superlattices we use a temperature range between 250 and 400°C. However, due to the extreme tendency of Sn to segregate on the film surface, it is necessary to further decrease the substrate temperature during overgrowth of the Sn layers. The optimized growth conditions for high-quality Sn/Ge superlattices were found to be in the temperature range between 45 and 300°C. It is demonstrated that low temperatures, low growth rates, and temperature variations during deposition of the individual layers in MBE open the possibility to synthesize structures far beyond thermodynamic equilibrium conditions.