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For development and integration of Si nanowires into nanoelectronic devices, an understanding of Ni silicide formation in electrical contacts to Si nanowires is necessary. Here, we examine the kinetics of Ni silicide phase formation. For Si nanowires with [111] growth directions, NiSi2 is the only phase to form in the temperature range 400–550 °C, and the NiSi2 growth exhibits linear kinetics from 400 to 500 °C with an activation energy of 0.76 ± 0.10 eV. In the case of Si nanowires with [112] growth directions, growth of the θ-Ni2Si phase in contact with the Si nanowire occurs with parabolic kinetics over the temperature range 400–550 °C, and an activation energy of 1.45 ± 0.07 eV/atom is extracted. Differences in the growth rates for Ni silicide phases with different SiNW growth directions implies that for simultaneous preparation of SiNW devices with Ni silicide contacts, SiNWs with the same growth direction are necessary.

Tunnel field-effect transistors were fabricated from axially doped silicon nanowire p-n junctions grown via the vapor-liquid-solid method. Following dry thermal oxidation to form a gate dielectric shell, the nanowires have a p-n-n(+) doping profile with an abrupt n-n(+) junction, which was revealed by scanning capacitance microscopy. The lightly doped n-segment can be inverted to p(+) by modulating the top gate bias, thus forming an abrupt gated p(+)-n(+) junction. A band-to-band tunneling current flows through the electrostatically doped p(+)-n(+) junction when it is reverse biased. Current-voltage measurements performed from 375 down to 4.2 K show two different regimes of tunneling current at high and low temperatures, indicating that there are both direct band-to-band and trap-assisted tunneling paths.

The synthesis of group IV semiconductor nanowires (Si, Ge and Si1-xGex) by metal-mediated growth techniques is of interest for a variety of device applications including wrap-gate field-effect transistors, chemical and biological sensor and photovoltaics. These devices typically require the fabrication of complex nanowire structures in which the doping and/or composition is controlled along the wire axis or radius. In a chemical vapor deposition (CVD) environment, the growth of wire structures is promoted by the presence of metal particles which act as preferential sites for precursor adsorption and wire growth via a vapor-liquid-solid (VLS) mechanism. In VLS growth, the metal particle forms a liquid alloy with the semiconductor material and wire growth occurs upon supersaturation via crystallization of the solid phase from the melt. Vaporsolid (VS) deposition on the sidewalls of the nanowire can occur simultaneously with VLS growth resulting in the formation of structures with tapered diameters. Consequently, it is important to understand the impact of precursor chemistry and growth conditions on both VLS and VS growth in order to control the structure and composition of SixGe1-x nanowires.

Recent interest in low-power electronics has sparked considerable interested in gate-controlled tunneling-based transistors (TFETs), which have demonstrated inverse subthreshold slopes (S) better than the MOSFET limit of 60 mV/dec.1 While the natural progression of these devices to nanoscale dimensions promises improved performance23, there is a lack of experimental data regarding the physics of tunneling at reduced dimensions. Here we present a TFET fabricated from an individual axially-doped p+-n-n+ Si nanowire in a device layout that enables the study of tunneling physics as the wire dimensions are scaled to the 1D transport regime.

Nickel germanide contacts are expected to play an important role in Ge-based electronics similar to that of their nickel silicide counterparts in Si devices. Here we have studied the solid state reaction between Ni contact pads and Ge nanowires. We observe the formation of axial nickel germanide segments after annealing at temperatures as low as 300 °C for 2 min. The nickel germanide segments are polycrystalline, without an epitaxial relationship to the Ge nanowire, in contrast to observations of epitaxial nickel silicide formation from Si nanowires. The crystal structure of the nickel germanide phase is consistent with the Ni2In prototype structure. Annealing above 400 °C results in fracture in the nickel germanide segment; however, nickel germanide segments as long as 1.7 μm can be formed by annealing at 400 °C for 5 min.

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