Effect of overgrowth on shape, composition, and strain of SiGe islands on Si(001)

We present a method and results based on x-ray scattering capable of resolving the shape and strain distribution in buried islands, as well as their vertical composition gradient. As an example, results are presented obtained for a single layer of SiGe dome-shaped islands capped by a 160-nm Si layer. For a growth temperature of 700 degreesC, a significant decrease of the average Ge content from about x=0.78 before overgrowth to about x=0.37 is found. The diameter of the islands increases from 110 to about 180 nm, their height shrinks from about 13 nm to 6 nm. This significant change of the island shape and content is accompanied by a pronounced change of their average in-plane lattice constant. The strain status of the overgrown flat islands is close to that of an embedded SiGe quantum well, i.e., with respect to the relaxation status of the uncapped islands a considerable strain redistribution takes place.