Si Nanowires Grown via the Vapour–Liquid–Solid Reaction

Abstract

Si nanowires as thin as 10 nm have been grown using silane gas as the source gas in the Vapour–Liquid–Solid (VLS) reaction. This paper describes the nucleation, growth and oxidation of the wires and a technique developed to control the wire diameter and position. A very thin layer of Au is deposited onto a Si(111) surface at room temperature. Silane gas is then introduced into the chamber as the VLS Si source gas and the temperature is raised to 300 to 700 °C. Initially a catalytically active Au surface phase which coats the surface leads to the growth of a defective epitaxial Si layer. As Au/Si molten alloy balls nucleate and grow in size to approach the threshold size for VLS wire growth, which is determined by the Gibbs-Thomson effect, the epitaxial layer growth rate decreases and a transition to Si nanowire growth occurs. The morphology and width of the wires is strongly dependent on the growth temperature and pressure. At low pressure and high temperature relatively thick well-formed wires grow straight up from the substrate surface along the [111] direction. As the temperature is decreased and the pressure is increased thinner wires grow which tend to exhibit growth defects. The Si nanowires can be thinned down by oxidation. A light oxidation yields Si cores which are of the order of 5 nm in diameter. A method to control the position of the Si wires which exploits the difference in Au sticking coefficient on Si and SiO2 surfaces at elevated temperature (T > 540 °C) has recently been developed. Using thermally oxidized Si(111) as a starting substrate, circular holes ≈1.5 μm in diameter were etched through the SiO2 to expose the Si. The sample was heated to 700 °C and exposed to a flux of Au atoms. For low Au fluxes (≈1×1014 atoms cm—2 s—1) the Au sticking coefficient on the SiO2 surface is negligible at 700 °C but remains unity on the exposed Si surface in the holes. The liquid alloy which forms in the holes during Au deposition is able to agglomerate to form a single ball in each hole. The quantity of Au which contributes to each ball can be controlled because it is determined by the hole size and the total Au dose. When silane gas is then introduced into the chamber each alloy ball grows to form a Si wire of controlled diameter whose position is quasi-controlled by the initial patterning. No wire growth occurs on the SiO2 surface due to the absence of Au. This ability to control the size and position of the Si wires should make it possible to conduct optical and transport measurements on a single wire of known dimensions.