Effect of mismatch strain on Stranski-Krastanow transition in epitaxial Ge_xSi_1-x/Si(0 0 1) gas-source growth

Mechanisms of strain-relief during epitaxial growth of Ge_0.6Si_0.4/Si(0 0 1) alloy at 500°C have been investigated using in situ scanning tunneling microscopy. The reduction of mismatch strain due to reduced Ge content of the epilayer (2.6% relative to 4.2% in Ge/Si(0 0 1)) has a profound effect not only on the final film morphology, but seem to alter the entire sequence of intermediate surface morphologies which, under these conditions, is dominated by layer-mounding rather than by faceting. Low-angle facets (approx. 6°), different from the 11°-{5 0 1} facets in the case of pure Ge/Si(0 0 1), appear only at the final stages of growth. Understanding of roughening transitions in strained-layer growth is essential for controlling the cluster size and geometry for applications in quantum dot devices.