Synthesis, Processing and Characterization of Silicon-based Templated Nanowires

Semiconductor and metallic nanowires have attracted substantial attention due to their wide variety of applications, ranging from nanoelectronics to energy storage devices. In particular, self-assembled silicon nanowires (SiNWs) may be an attractive alternative to conventionally processed planar silicon since SiNWs can potentially function as both the switch (i.e. transistor) and local interconnect (e.g. metal silicide nanowire) to form an inherently integrated nanoelectronic system. Also, hierarchical (branched) nanowire systems hold potential for catalysts or porous electrode applications for energy applications;Controlled synthesis and processing of solid-liquid-solid (SLS)-grown and vapor-liquid-solid (VLS)-grown nanowires as templates for the surface formation of metal silicide were studied to investigate fundamental aspects of nanowire growth and formation of conductive, core-shell, nanowires. Firstly, a previously unreported high temperature SLS silica nanowire growth mode was observed and investigated. The SLS nanowire nucleation and subsequent growth was uniquely promoted by---and coupled to---the formation of thermally-etched pyramidal pits in the Si substrate which formed during a high-temperature anneal phase prior to the onset of SLS nanowire formation. The silicon-oxide-mediated thermal pit formation process enhanced Si transport to Au-Si alloy droplets directly adjacent to the pyramidal pits. The catalytic nature of the pyramidal pits resulted in the observation of SLS nanowire 'blooms' at the pit locations. This newly observed process is termed 'thermal pit-assisted growth.';Secondly, conductive Si-based nanowires structures were processed, fabricated, and tested based on self-assembled Si nanowires synthesized via SLS and VLS processing to investigate formation of conductive core-shell nanostructures. Metallization utilized W and Ni silicidation. TEM-EDS results show that ALD W was conformally deposited on the surface of SiNWs. In contrast, evaporated Ni was asymmetrically deposited on the nanowire although the resultant silicide appeared symmetric. Conformal Ni deposition and silicidation were successfully performed via Ni PEALD. Metal silicide nanowires exhibited an improvement in electrical conductivity of eight orders of magnitude compared with that of as-grown silicon nanowires.;Lastly, a novel multi-walled carbon nanotube (MWNT) growth process was investigated based on carbon incorporation in a nickel catalyst layer deposited via PEALD on SiNWs and Si wafer substrates. For these films the carbon originated from the PEALD precursor ligand and MWNT growth occurred during post-PEALD annealing in the absence of a vapor-phase carbon feedstock. MWNT growth relied on the formation of nickel silicide at the PEALD Ni/Si interface which increased the local carbon concentration in the Ni film sufficiently to promote carbon saturation/precipitation at Ni catalyst grains and nucleate MWNT growth. Similar MWNT growth from annealed PEALD Ni films was not observed on SiO2-coated Si wafer substrates, consistent with the role of silicidation in the observed Ni-catalyzed MWNT growth on Si. This MWNT growth mode requires neither the catalytic decomposition of a gaseous hydrocarbon source nor the high temperature pyrolysis of metallocene materials and purposely avoids a catalyst diffusion barrier at the Si substrate, commonly used in MWNT growth processes on Si.