Ordering of self-assembled Si0.55Ge0.45 islands on vicinal Si(001) substrates

The slightly vicinal Si(001) surface is intrinsically unstable against kinetic-step bunching during homoepitaxial growth. The height and period of the step bunches depend on miscut angle, growth temperature, growth rate and epilayer thickness. Using substrates with 4 degrees miscut along [110] we optimised our MBE-grown Si-buffers to show one-dimensional step bunches with a typical spacing of 100 nm, and a corrugation height of 4 nm. These Si buffers were capped by 50 A of Si0.55Ge0.45, which was grown at various temperatures. Only at very low temperatures around 350 degrees C, the Si0.55Ge0.45 film replicates the underlying ripples of the Si-buffer in a conformal manner. At moderate growth temperatures the stress in the top-layer leads to the formation of {105}-faceted ridges at the ripple flanks perpendicular to the main structure. This marks the transition from conformal Si/SiGe epilayer growth to strain-driven three-dimensional (3D) island growth for temperatures above 600 degrees C. The experiments clearly demonstrate that a reduction of the step-bunch spacing to a value approaching the average spacing of the Si0.55Ge0.45 islands under the chosen growth conditions couples the two otherwise independent mechanisms of kinetic (homoepitaxial) step bunching and of strain-induced 3D island growth. This way, long-range ordering of self-organised SiGe dots is achieved that is entirely based on self-organisation phenomena. (c) 2005 Elsevier B.V. All rights reserved.