Control of surface morphology through variation of growth rate in SiGe/Si(100) epitaxial films: Nucleation of "quantum fortresses"

The surface morphology of Si_0.7Ge_0.3 films grown at 550°C by molecular-beam epitaxy is found to be highly controllable through changes in growth rate. A growth rate of 0.9 Å/s results in a surface morphology that begins as shallow pyramidal pits, which then become decorated by ordered quadruplets of islands that surround the edges of the pits. This "quantum fortress" structure represents a symmetry with potential application to quantum cellular automata geometries. A higher growth rate of 3 Å/s produces similar results. However, when the growth rate is reduced to 0.15 Å/s, the surface morphology that develops instead consists of elongated ridges.