Non-equilibrium Interface and Surface Dynamics

Elementary Processes in Nanowire Growth

Jerry Tersoff

IBM Research

Semiconductor nanowires are commonly grown by the vapor-liquid-solid (VLS) process. A liquid catalyst sitting on the end of the wire captures materials from a vapor precursor. This material is deposited onto the end of the wire, allowing steady state growth of wires less than 10 nm in diameter. Under typical growth conditions, uniform nanowires grow side by side with a zoo of unintended morphologies. One common example is kinking of a wire from one growth direction into another. Another is growth of lateral wires, formed by crawling of the catalyst along the surface. Most applications require suppression of such unintended growth modes. On the other hand, by controlling kinking and crawling it is possible to grow novel device structures. For either purpose, it is important to understand the underlying mechanisms that lead to these complex growth morphologies. Here we show that the complexity of nanowire growth can be understood as arising from the interplay of just three simple elements: facet dynamics (i.e. growth of the facet by liquid-phase epitaxy from the catalyst); droplet statics (i.e. ordinary wetting); and the formation of new facets at the trijunction. We incorporate these processes in a simple model for VLS growth. Our models addresses fully faceted nanowires, such as are seen in experiment. Using this model, simulations of nanowire growth exhibit many of the same phenomena observed in experiment. We present several examples, including: (1) kinking from one direction to another under an external perturbation; (2) a competition between growth of free-standing nanowires and wires growing along the surface by catalyst crawling; and (3) the occurrence of different shapes for the wire-catalyst interface under different growth conditions.