Diffusion and surface segregation in thin SiGe/Si layers studied by scanning transmission electron microscopy

A multiple quantum well structure consisting of SiGe/Si layers of nominal widths of 1, 2, 5 and 10 nm and Ge compositions of 10, 20, 30 and 40 at% grown by low pressure chemical vapour epitaxy has been studied in cross-section by scanning transmission electron microscopy, Quantitative high-angle annular dark field imaging from as-grown and two annealed specimens gave the activation energy for bulk diffusion to be 2.3+/-0.3 eV. This is less than half the value typically obtained for Si:Ge dilute alloys and also significantly lower than the value for Ge diffusion in polycrystalline bulk SiGe. The diffusivities at the growth temperature of 610 degrees C are between 10(-23) and 10(-22) m(2)s(-1) and the diffusion lengths are <1 nm. Hence the interface widths of 4-6 nm experimentally observed must mainly be due to Ge segregation during growth. The standard 2-state-exchange model for surface segregation using lattice site exchange probabilities in agreement with surface sensitive in-situ techniques, however, fails to reproduce these interface widths.