Straining nanomembranes via highly mismatched heteroepitaxial growth: InAs islands on compliant Si substrates.

Abstract

Freestanding, edge-supported silicon nanomembranes are defined by selective underetching of patterned silicon-on-insulator substrates. The membranes are afterward introduced into a molecular beam epitaxy chamber and overgrown with InAs, resulting in the formation of InAs islands on flat areas and at the top of the Si nanomembranes. A detailed analysis of sample morphology, island structure, and strain is carried out. Scanning electron microscopy shows that the membrane stays intact during overgrowth. Atomic force microscopy reveals a lower island density on top of the freestanding membranes, denoting a modified wetting or diffusivity in these areas. An observed bending of the membrane indicates a strain transfer from the InAs islands to the compliant substrate. X-ray diffraction and finite-element modeling indicate a nonuniform strain state of the island ensemble grown on the freestanding membrane. A simulation of the bending of the nanomembranes indicates that the islands at the center of the freestanding area are highly strained, whereas islands on the border tend to be fully relaxed. Finally, continuum elasticity calculations suggest that for a sufficiently thin membrane InAs could transfer enough strain to the membrane to allow coherent epitaxial growth, something not possible on bulk substrates.