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We have examined the formation of silicon nanowires grown by self-assembly from Si substrates with thin aluminum films. Postgrowth and in situ investigations using various Al deposition and annealing conditions suggest that nanowire growth takes place with a supercooled liquid droplet (i.e., the vapor-liquid-solid system), even though the growth temperatures are below the bulk Al/Si eutectic temperature. Wire morphology as a function of processing conditions is also described. It is shown that when Al environmental exposure is prevented before wire growth a wide process window for wire formation can be achieved. Under optimum growth conditions, it is possible to produce excellent crystal quality nanowires with rapid growth rates, high surface densities, low diameter dispersion, and controlled tapering. Photoelectron spectroscopy measurements indicate that the use of Al leads to active doping levels that depend on the growth temperature in as-grown nanowires and increase when annealed. We suggest that these structural and electronic properties will be relevant to photovoltaic and other applications, where the more common use of Au is believed to be detrimental to performance.

We visualize atomic level dynamics during Si nanowire growth using aberration corrected environmental transmission electron microscopy, and compare with lower pressure results from ultra-high vacuum microscopy. We discuss the importance of higher pressure observations for understanding growth mechanisms and describe protocols to minimize effects of the higher pressure background gas. [ABSTRACT FROM AUTHOR]

The formation of abrupt Si/Ge heterointerfaces in nanowires presents useful possibilities for bandgap engineering. We grow Si nanowires containing thick Ge layers and sub-1 nm thick Ge "quantum wells" and measure the interfacial strain fields using geometric phase analysis. Narrow Ge layers show radial compressive strains of several percent, while stress at the Si/Ge interface causes lattice rotation. High strains can be achieved in these heterostructures, but we show that they are unstable to interdiffusion. [ABSTRACT FROM AUTHOR]

We describe a new approach for achieving controlled spatial placement of VLS-grown nanowires that uses an oxygen-reactive seed material and an oxygen-containing mask. Oxygen-reactive seed materials are of great interest for electronic applications, yet they cannot be patterned using the approaches developed for noble metal seed materials such as Au. This new process, nanoscale chemical templating, takes advantage of the reactivity of the blanket seed layer by depositing it over a patterned oxide that reacts with the seed material to prevent nanowire growth in undesired locations. Here we demonstrate this technique using Al as the seed material and SiO2 as the mask, and we propose that this methodology will be applicable to other reactive metals that are of interest for nanowire growth. The method has other advantages over conventional patterning approaches for certain applications including reducing patterning steps, flexibility in lithographic techniques, and high growth yields. We demonstrate its application with standard and microsphere lithography. We show a high growth yield and fidelity, with no NWs between openings and a majority of openings occupied by a single vertical nanowire, and discuss the dependence of yield on parameters. [ABSTRACT FROM AUTHOR]

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010. [ABSTRACT FROM PUBLISHER]

Abstract: A method for growing heteroepitaxial Si films on sapphire was developed using a 6nm thin Al layer at substrate temperature of 600°C. Subsequently, the growth of Si nanowires was demonstrated on these films at 490°C without breaking vacuum. We characterized the properties of the Si films by Raman scattering, X-ray diffraction and transmission electron microscopy and show that the crystal quality and dopant control are promising for photovoltaic applications. [Copyright &y& Elsevier]