DASteps and 2D Islands of Double Layer Height in the SiGe(001) System

The surfaces of step graded, partially relaxed ${\mathrm{Si}}_{1-x}{\mathrm{Ge}}_x/\mathrm{Si}\left(001\right)$ buffers were studied by scanning tunneling microscopy. The surface slips along $〈110〉$ forming the crosshatch pattern, consisting of bunches of $D_A$ steps of double layer height. The $D_A$ steps are present in regions of large surface gradients close to the slips, as well as in planar regions between the slips. These regions are also characterized by the appearance of 2D islands of double layer height. The observations can be explained by assuming the strain due to the misfit dislocations to be locally anisotropic. Anisotropic misfit strain and efficient strain relaxation by the ( $2\times 8$) Ge reconstruction were identified as the main factors causing the unusual step structure.