Direct Synthesis of Silicon Nanowires using Silane and Molten Gallium
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Direct Synthesis of Silicon Nanowires using Silane and Molten Gallium Shashank Sharma, and Mahendra K. Sunkara* Department of Chemical Engineering, University of Louisville, Louisville, KY 40292, USA Elizabeth C. Dickey Department of Materials Science and Engineering, Pennsylvania State University, University Park, PA 16802 * Corresponding author: [email protected] ABSTRACT We report for the first time, bulk synthesis of single crystalline silicon nanowires using molten gallium pools and an activated vapor phase containing silane. The resulting silicon nanowires were single crystalline with growth direction. Nanowires contained an unexpectedly thin, non-uniform oxide sheath determined using high-resolution Transmission Electron Microscopy (TEM). Nanowires were tens of nanometers in diameter and tens to hundreds of microns long. The use of activated gas phase chemistry containing solute of interest over molten metal pools of low-solubility eutectics such as gallium offer a viable route to generate nanowire systems containing abrupt compositional hetero-interfaces. INTRODUCTION Traditional techniques for nanowire synthesis utilize nanometer scale clusters of transition metals to confine growth in one dimension. 1-5 In such methods, one nanowire can be grown from one metal cluster. Thus, in order to synthesize diameter monodispersed nanowires using traditional techniques, nanosized metal clusters will have to be assembled on a solid substrate. Regardless of significant advances in creating monodispersed metal clusters on solid substrates, there exists an inherent limitation in controlling the absolute nanowire diameter and diameter distribution. 6 In this regard, we recently reported a bulk nanowire synthesis technique in which multiple, nanometer scale nuclei form in a large gallium pool that grow as nanowires. 7-9 We pointed out that crops of nanowires could be synthesized using large pools of low melting metals unlike traditional transition metal based techniques. In our previous report with synthesis of silicon nanowires using molten gallium and a plasma mediated gas phase chemistry, 7 silyl radicals were generated in-situ via etching of solid silicon by atomic hydrogen. Here, we report bulk synthesis of silicon nanowires using silane directly in the gas phase over molten gallium pool. We also investigated the addition of nitrogen in silane plasmas for silicon nitride nanowire growth and addition of oxygen in silane plasma for silicon oxide nanowire growth.
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EXPERIMENTAL DETAILS The Nanowire synthesis experiments were carried out in a microwave plasma reactor (ASTEX 5010) using 2% SiH4/H2 in the feed stream. Quartz substrates covered with a thin film of gallium were exposed to microwave plasma containing a range of feed dilution fractions in hydrogen. The substrate temperature was measured using an infrared pyrometer (a Raytek Model RAYMA2SCCF) to be approximately 550 °C for 700 W microwave power, 40 Torr total pressure, and 50 sccm of 2% SiH4/H2. Upon exposure to the plasma, the molten galli
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