In situ transmission electron microscopy observations of the formation of self-assembled Ge islands on Si.
The in situ transmission electron microscope allows us to visualise processes occurring at surfaces and interfaces in real time and is therefore capable of providing detailed, quantitative information about reaction mechanisms. We have used a UHV TEM equipped with in situ growth capabilities to study the process of chemical vapour deposition of Ge on Si(100), with particular emphasis on the formation of self-assembled, nanosize Ge islands. Video-rate image acquisition enables us to track the development of individual islands from nucleation onwards and to observe the introduction of dislocations as the strained islands relax. For islands less than 80 nm in diameter, which are coherently strained, we observe an interesting coarsening process during growth. This coarsening results in a bimodal distribution of island sizes at certain times and a narrow size distribution at later times. We explain the phenomenon by a model in which coarsening occurs among a population of islands for which the equilibrium island shape depends on the size. Numerical simulations of coarsening in the presence of a shape transition are in good agreement with experiment. As the islands grow larger, dislocations form and we observe rapid shape changes associated with dislocation introduction. These changes can also be understood by considering a strain-dependent island shape. The insight that these results provide into the understanding of island growth and evolution can be used to develop arrays of uniformly sized islands ("quantum dots") for a variety of potential applications.