Shape evolution of Ge domes on Si (0 0 1) during Si capping

Three-dimensional, coherently strained Ge/Si (001) islands were overgrown with thin Si layers and their shape evolution was studied by scanning tunneling microscopy. The Si cap, necessary for exploiting the clusters as self-assembled quantum dots, intermixes with the Ge layer leading the dome-shaped islands to transform first into {105} faceted pyramids and finally into stepped mounds with steps parallel to the 〈110〉 directions. The observed morphological transitions can be understood in a general picture in which the shape of an island mainly depends on its volume and composition.