Heteroepitaxy of semiconductor-on-insulator structures: Si and Ge on CaF2/Si(111)

Abstract

Si and Ge layers have been grown on CaF2/Si(111) by molecular-beam epitaxy. Both Ge and Si grow as islands, and both the island size and the spacing between nucleation sites are considerably larger for Ge (∼300 nm) than for Si (∼100 nm). In addition, Ge and Si layers are found to be a mixture of type-A (aligned with the underlying CaF2) and type-B (rotated 180° about the surface normal with respect to the underlying CaF2) regions. The crystalline quality and surface morphology of the Ge layers are much better than those of the Si layers. This is thought to be due to the larger island size of the Ge deposits and to a greater ease of movement of the boundaries between type-A and type-B regions in Ge. Hall measurements show electron mobilities of up to 664 cm2/V s in Si layers and hole mobilities of 234 cm2/V s for Ge layers. Finally, the use of a GexSi1−x-Si superlattice, when grown on a GexSi1−x buffer layer which is lattice matched to the CaF2 at the growth temperature, is shown to improve Si heteroepitaxy.