Study of catalyzed growth of germanium nanowires [electronic resource]
Keywords: Materials Science and Engineering.

Semiconductor nanowires are of great interest for application in nanoelectronics, nanophotonics, sensors and energy technologies. Particular attention has focused on Si and Ge nanowires because of their compatibility with Si integrated circuit technology. The great majority of literature studies of Ge nanowire growth have used Au as a catalyst for "bottom-up" synthesis of deposited wires. In most cases, growth occurs by the vapor-liquid-solid (VLS) mechanism. Gold has been a popular choice as a catalyst in part because it forms a eutectic liquid with Ge at temperatures below 400 °C, permitting generally high-quality and spatially-controlled crystal growth at very low temperatures. However, concerns exist about possible Au contamination of VLS-grown nanowire devices and, in particular, semiconductor processing facilities used to fabricate them. Gold is a fast diffuser in diamond cubic crystals and produces trap levels deep in the band gap of both Ge and Si, making it a significant cross-contamination hazard in semiconductor fabrication. Moreover, VLS is not well suited to synthesis of nanowires with abrupt grown-in p-n junctions or semiconductor heterostructures, which are interesting for many device applications. This has provided additional motivation to investigate the growth of semiconductor nanowires via the vapor-solid-solid (VSS) mechanism using alternative catalysts to Au. VSS nanowires can be grown at reduced temperatures compared to VLS, as the catalyst is not molten. Literature reports indicate that the morphology of Ge nanowires grown using alternative catalysts via VSS is not as easily controlled as that of VLS grown Ge nanowires with Au as a catalyst; mixtures of straight, twisted, and defective nanowires are reported for different catalysts and growth conditions. A size-dependent wire morphology transition from straight to tortuous nanowires in VSS growth of Ge nanowires using a Ni-based catalyst is discussed here. The catalyst phase was identified as orthorhombic NiGe, which is reported to be a state-of-the-art contact material in Si-compatible semiconductor devices. Using detailed transmission electron microscopy analysis of the nanowire and catalyst morphology and composition, the role of sidewall and catalyst/nanowire interface energetics as well as crystal defects in dictating the observed wire diameter effect on VSS growth morphology was analyzed. Development of optimized processes for bottom-up synthesis of these nanowires requires a more quantitative understanding of the VSS wire growth mechanism. Therefore, the kinetics of VSS nanowire growth, probing the rate-limiting step for various growth conditions was investigated. The effect of growth parameters such as growth temperature and precursor partial pressure on the nanowire growth rate was studied in order to gain an insight into the growth kinetics. Two different growth regimes were observed for VSS grown Ge nanowires at different temperature ranges. At higher temperatures (345-375 ºC), the diffusion or mass transport of germane precursor to the catalyst surface was found to be rate limiting. At lower temperatures (300-345 ºC) either the surface reaction or incorporation of Ge at growth step could be rate limiting.