Spontaneous transition from epitaxially constrained to equilibrium Ge nanocrystals on silicon-on-insulator (1 0 0)

Epitaxial constraints define the stress-driven self-assembly of faceted nanocrystals and in particular one of their key characteristics––their shape. Here we identify a technologically relevant system, Ge islands grown on ultrathin silicon-on-insulator (SOI) substrate in which nanocrystals, whose shape is initially defined by epitaxial constraints, spontaneously overcome those constraints and transform to their equilibrium shape. Ge nanocrystals on ultrathin SOI form initially as huts and then transform into domes, similar to the sequence of epitaxially constrained shapes they assume on bulk Si(1 0 0). While the sequence on bulk Si ends here, we observe further dramatic morphological changes on ultrathin SOI: a spontaneous transformation to equilibrium-shaped Ge nanocrystals.