Shape and Size Distribution of Molecular Beam Epitaxy Grown Self-Assembled Ge Islands on Si (001) Substrates

We have performed a series of annealing experiments with Ge islands on Si (001) grown by molecular beam epitaxy, in order to clarify issues related to island stability and coarsening. The shape and size distribution of nanoislands as a function of annealing time at a temperature of 650 degrees C have been studied. Optical phonons from Raman spectra have been used as efficient probes to study the evolution of Si1-xGex islands. Both the alloy composition and residual strain in the islands have been determined from the phonon frequencies and Raman intensities. The experimental results are in good agreement with the strain relaxation estimated using X-ray rocking curves. The results indicate that the shape and size distribution of Ge islands are controlled via structural and compositional changes through strain relaxation by the periodic creation and extinction of tiny nanocrystals.