Silicon-on-insulator (SOI) presents a unique model system for exploring the stability of crystalline nanomaterials in metastable configurations. We show that the initial destabilization of ultrathin SOI is related to mechanical stress, in contrast to phenomena at later times driven by the energy of the ${\text{SiO}}_2/\text{Si}$ interface. Stepped rectangular truncated pyramids, with lateral dimensions of tens of nanometers, are formed on the outer Si layer of ultrathin (001)-oriented SOI during heating in ultrahigh vacuum. Pyramid edges are bounded by doubled atomic steps, with corners consisting of a complex series of single-layer steps. The shape of these nanopyramids represents a balance between stress-induced roughening and the elastic interaction between steps. SOI allows the precisely known energetics of silicon surfaces to be readily adapted to materials with nanoscale dimensions.

• Received 30 November 2009

DOI:https://doi.org/10.1103/PhysRevB.81.041302