Kinetic pathway in Stranski-Krastanov growth of Ge on Si(001).

The transition from 2D to 3D growth of Ge on Si(001) has been investigated with scanning tunneling microscopy. A metastable 3D cluster phase with well-defined structure and shape is found. The clusters have a {105} facet structure. Results suggest that these clusters define the kinetic path for formation of ``macroscopic'' Ge islands.