INVESTIGATION OF SI-GE HETEROSTRUCTURES BY X-RAY REFLECTOMETRY

Grazing-incidence x-ray reflectometry has been applied to the investigation of Si-Ge heterostructures grown by molecular beam epitaxy. Si1-xGex/Si strained-layer superlattices with period length 20-50 nm and Ge fraction 0.2 < x < 0.5 were first examined. X-ray reflectometry proved to be more sensitive to superlattice imperfections than double-crystal diffraction. Modelling of the reflectivity profiles has allowed us to estimate the thickness fluctuations and interfacial sharpness of these structures. Discrete, continuous and random thickness variations could easily be distinguished from the line shape of the reflectivity curve. Typically, period-length fluctuations of the order of 1-2% and an interface roughness of 2-3 monolayers were found in these structures. Pseudomorphic (Si(m)Ge(n))p atomic layer superlattices consisting of alternating monolayers of pure Si and Ge were also investigated. Good fits of the experimental reflectivity curves were obtained by introducing an interfacial roughness of 0.5-1 monolayer. Structures grown on (100) Ge had better interfaces than equivalent structures grown on (100) Si. The sharpness of the interfaces deteriorated with increasing period length, consistent with a transition to a non-planar growth mode. Annealing studies were also performed on (Si(m)Ge(n))p superlattices. The investigation revealed that interface mixing existed in all the samples in the as-grown condition and that appreciable interdiffusion occurred even after moderate annealing treatments (e.g. 700-degrees-C for 20 s), suggesting that strain may enhance interfacial diffusion.