Silicon nanostructures via selective surface chemical reactions under ultrahigh vacuum

Silicon nanostructures have been successfully formed by nitridation and selective oxygen etching of a silicon surface under ultrahigh vacuum. Silicon nitride islands were prepared by two different nitridation methods: (1) a nitrogen ion dose at room temperature followed by high-temperature annealing and (2) nitrogen gas exposure at high temperature. Scanning tunneling microscope analysis on the surface reacted with nitrogen ions revealed that silicon nitride islands of 6-15 nm in diameter could be formed by annealing the surface at 980 °C and that the island size did not increase after further annealing. In the case of the nitrogen gas exposure, the silicon nitride islands could be obtained at relatively lower temperatures and they became larger after prolonged annealing. In both cases, since the silicon nitride islands were strongly resistive to oxygen etching, oxygen exposure at high temperature led to the formation of silicon nanostructures by etching away the bare silicon region. Topologies of the nanostructures obtained via the two different pathways were compared and are discussed on the basis of the bulk diffusion mechanism of the nitrogen species.