Identification of shape transitions in coherent Ge/Si islands using transmission electron microscopy

As a consequence of strain relaxation, Ge coherent islands on Si(001) substrates evolve to different shapes as islands grow. By measuring the size and the strain simultaneously in a large population of individual islands using two simple and robust plan-view transmission electron microscopy-based techniques, we can identify island shapes easily because island shape is a function of strain. We briefly introduce the mechanisms of these two techniques. We then show that there is a metastable shape of Ge islands involved in the shape transition between pyramids and domes. The strain relaxation changes discontinuously as islands grow from pyramids to the metastable form and then finally to domes indicating that the shape transition between pyramids and domes is first order. We also show that the shape of this metastable island is a truncated dome and the faceted planes are {103}.