Elastic Energies of Coherent Germanium Islands on Silicon

Pinholes, Dislocations and Strain Relaxation in InGaN

We analyse by means of transmission electron microscopy (TEM) and atomic force microscopy (AFM) the strain relaxation mechanisms in InGaN layers on GaN as dependent on the In content. At the experimentally given thickness of 100 nm, the layers remain coherently strained, up to an In concentration of 14 %. We show that part of the strain is reduced elastically by formation of hexagonally facetted pinholes. First misfit dislocations are observed to form at pinholes that reach the InGaN/GaN interface. We discuss these results in the framework of the Matthews-Blakeslee model for the critical thickness considering the Peierls force for glide of threading dislocations in the different slip systems of the wurtzite lattice.

Thermal stability of strained nanowires

Stranski-Krastanow strained islands undergo a shape anisotropy transition as they grow in size, finally evolving toward nanowires. This effect has been explained until now via simple energetic models that neglect thermodynamics. We investigate theoretically the stability of strained nanowires under thermal fluctuations of the long side. We find phase transitions from nanowires back to nanoislands as the temperature is increased and as the height of the nanostructure is raised or lowered, and we predict regions of phase coexistence. Our results are general, but explain recent data on the growth of erbium silicide on a vicinal Si surface.