Strain relaxation by alloying effects in Ge islands grown on Si(001)

Transmission electron microscopy is used to study the morphology and the composition profile of ``pure'' Ge islands grown at high temperature on Si(001) by molecular beam epitaxy. An alloying process, involving mass transport from the substrate to the islands during the island growth, was identified. It was found that, as a result of Si mass transport to the Ge islands, the island/substrate interface moves towards the substrate, and trenches form on the substrate surface around the islands. Reduction of the misfit strain at the island/substrate interface is the driving force for this process.