Size effects in vapor-solid-solid Ge nanowire growth with a Ni-based catalyst

Abstract

We report a dramatic size effect on the morphology of Ge nanowires synthesized by low-temperature vapor-solid-solid (VSS) growth using a NiGe catalyst. Nanowires with diameter greater than 25 nm are 〈111〉-oriented, have a high density of grown-in defects, and exhibit frequent kinking. However, nanowires with diameter below 25 nm are straight, despite also having a substantial density of crystal defects. The latter wires grow preferentially in the 〈110〉 direction. The absence of kinking in the small nanowires coincides with the observation of a low-energy, epitaxial NiGe/Ge interface. Our results show that the occurrence of (1) this solid-solid epitaxial interface and (2) the low-energy sidewall facets of the 〈110〉 wire orientation strongly bias the Ni-Ge binary system toward kink-free nanowire growth in the VSS regime. We further demonstrate that kinking in larger nanowires occurs via multiple twinning events facilitated by the slow growth and anisotropic catalyst/wire interfaces typical of VSS growth. Such effects are expected in other VSS systems where a range of nanowire morphologies are observed.