Interfacial abruptness in Si/SiGe heteroepitaxy grown by ultrahigh vacuum chemical vapor deposition

High-quality Si/Si_1-xGe_x strained-layer superlattices have been grown in the temperature range 525°C-550°C by ultrahigh vacuum chemical vapor deposition. We employed X-ray reflection and high-resolution double-crystal X-ray diffraction measurements to study the dimensional and compositional variations in a Si/Si_1-xGe_x superlattice. From the X-ray reflection results, the interfacial roughness of Si layers on SiGe, and SiGe layers on Si, is 0.1 nm for growth at 525°C and 0.2 nm for growth at 550°C. A simple model for estimating the interfacial abruptness of Si/Si_1-xGe_x heterojunctions is proposed. In this model, a transition region with a linearly graded Ge composition is assumed to exist at both Si/Si_1-xGe_x interfaces. The Ge composition x of a Si/Si_1-xGe_x superlattice is found to increase with the growth time at a constant gas phase composition. This phenomenon can be explained by this model, and the thickness of the transition region and the transition time can be extracted from these fitting results. The transition thicknesses are found to be about 0.3-0.7 nm for growth at 525°C and 1-1.5 nm for growth at 550°C.