One-dimensional nanoscale materials play a key role in nanotechnology, as well as provide model systems to explore new properties. Semiconductor nanowires grown by the vapor-liquid-solid (VLS) process are a natural choice to explore electronic and optoelectronic properties of 1D systems because of their high crystalline quality and because their diameters and length can be controlled during synthesis in a predictable manner. In VLS growth gaseous reactants drive nanowire growth from liquid metal catalyst nanodots, the size of which defines the diameter of the nanowires.[ [4] However despite numerous studies there is still only limited understanding of the basic mechanisms that control nanowire nucleation and the resulting orientation dependence. [5][6][7][8][9] In the present work we introduce a plasma during silane-hydrogen VLS nanowire growth on Si (100) The ability to carry out growth at low temperatures is important for device integration and is essential for maintaining the integrity of nanoscale structures. For thermally activated, low-pressure chemical vapor deposition (LPCVD) with silane-hydrogen gas mixtures, VLS growth temperatures of 450°C or higher are typically required for reasonably rapid growth rates and for high quality single crystal nanowires. However, the addition of low power rf (13.56 MHz) plasma increases the concentration of more reactive Si containing gas species and the incorporation of Si into the Au liquid eutectic catalyst. This effect in turn promotes higher growth rates. One advantage of growing nanowires at low temperatures is that the mobility and size coarsening of the Au seeds is minimized, promoting the growth of smaller diameter nanowires.Up to now there have been only a few studies reported for plasma enhanced growth of nanowires. Hofmann et al.[10] reported Au catalyzed Si nanowire growth on amorphous SiO 2 substrates below 400°C by LPCVD with silane under rf plasma stimulation and found significant enhancements in the growth rate. Sharma and Sunkara [11] demonstrated Si nanowire growth from molten Ga in the presence of microwave plasmas of silane and hydrogen gases. We are interested in quantitatively comparing nanowire nucleation, growth rates, and orientations under thermal and plasma stimulated conditions for seeded growth from crystalline substrates. In our work Si nanowires were grown in an LPCVD reactor on Au seeded Si (100) surfaces from hydrogen-diluted silane gases under thermal conditions, both with and without a 2.5W rf plasma. Growth temperatures were varied from 300 to 430°C. Generally, the overall nanowire density is significantly greater under plasma compared to thermal growth conditions.
The authors report the use of in situ optical reflectometry to determine the incubation time for the onset of growth, mean growth rate, and average length of Si nanowires during chemical vapor deposition vapor-liquid-solid synthesis. Results for the constructive and destructive interferences of 635nm linearly polarized laser light scattering from growing nanowire layers are compared to simulations. This real time optical reflectance approach is shown to quantitatively determine nanowire growth rates as well as reveal a pressure dependence for the time to nucleate nanowire growth.
A maskless process for the directed assembly of Ni contacts to Si nanowires on prepatterned electrodes is reported. Microarrays of thin Au∕Cr electrodes were lithographically formed on oxidized Si substrates followed by electric-field assisted alignment of Si nanowires between the electrodes. The nanowire ends were then embedded in Ni by selective electrodeposition over the prepatterned electrodes. Annealing to 300°C provided good electrical contacts for transport through the doped nanowires. This approach provides a parallel, maskless method to establish metal contacts to the nanowires without the need of high resolution electron beam lithography for electrical and mechanical applications.
An ex situ proximity technique is demonstrated for the electrical doping of silicon nanowires with spin on dopant (SOD) used as the boron source. The technique is based on solid-state diffusion and is comprised of two stages: predeposition and drive in. During predeposition, a predetermined amount of boron is introduced into the near surface region of the nanowires by holding the SOD source in close proximity to the nanowires. The boron concentration in the nanowires is controlled by the appropriate selection of predeposition temperature and time, with 800 and 950 °C and 5–10 min used in the present studies. The boron is then diffused further into the nanowires during the drive-in stage. The doped nanowires were characterized using scanning electron microscopy, secondary ion mass spectrometry, transmission electron microscopy, and four-probe electrical transport measurements. The high temperatures employed in this doping process do not result in any observable damage to these 120–180 nm diameter nanowires and good control over the dopant concentration in the range from 1018 to 1020 cm−3 is obtained. This ex situ doping technique provides a useful alternative to the methods currently available for electrical doping of nanowires, which are predominantly in situ techniques.
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