The size and diffusion effect of gold on silicon nanowire sidewall faceting

Single crystalline silicon nanowires (SiNWs) were grown using a gold (Au)-catalyzed vapor-liquid-solid (VLS) approach. In this study, we examine the influence of the size of Au catalyst droplets on the size of SiNWs and discuss the effect of Au diffusion and surface passivation on SiNW sidewall faceting and roughening in the VLS process. To simultaneously cover a variety of sizes of SiNWs on the same substrate, 2-nm-thick Au film was used on Si (111) substrate as a catalyst, since it is known that Au thin film-based seed offers relatively less control of the NW size, due to the randomness of the film breakup at reaction temperature. We then found that the grown SiNWs have two main types of surface morphologies on the sidewall of the nanowires (NWs). One type had a small and coarse surface morphology with no Au-Si droplets at the top of the NWs. The other type had a long and smooth surface and still had Au-Si droplets at the end. The fact that resulting SiNWs have two main different surface morphologies can be explained by three reasons: the size difference of Au-Si droplets, Au diffusion to the surface of the SiNWs, and surface passivation on the SiNW surface. Furthermore, we proposed a combined VLS and vapor-solid (VS) growth model to describe the creation of the facets and roughening of the sidewall of the NWs. Our results point toward the exciting possibility that the proposed well-controlled and coarse-surfaced SiNWs could be promising thermoelectric materials with low thermal conductivity, since the thermal conductivity is greatly reduced at the rough surface due to the enhancement of phonon scattering.