A Novel Low Temperature Synthesis Method for Semiconductor Nanowires
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A Novel Low Temperature Synthesis Method for Semiconductor Nanowires Shashank Sharma, Mahendra K. Sunkara*, and Raul Miranda Department of Chemical Engineering, University of Louisville, Louisville, KY 40292, USA Guoda Lian and Elizabeth C. Dickey Department of Chemical and Materials Engineering, University of Kentucky, Lexington, KY 40506, USA * Corresponding author: [email protected] ABSTRACT We present a novel synthesis technique to grow bulk quantities of semiconductor nanowires at temperatures less than 500 °C. Gallium is used as the liquid medium in a mechanism similar to vapor-liquid-solid (VLS). We demonstrated this low temperature technique with silicon and carbon nanowires. Gallium exhibits extremely low solubility for several elemental semiconductors. This property enables nucleation and growth of nanometer scale wires from large sized gallium droplets (>1 µm) eliminating the need for creation of quantum sized metal droplets. INTRODUCTION Nanotubes and nanowires possess interesting properties due to quantum confinement, low dimensionality and surface effects.1-6 Discovery and the subsequent realization of useful properties of carbon nanotubes7 rekindled the interest of scientists worldwide to synthesize nanotubes and nanowires of different materials. In this regard, new chemical processes for bulk synthesis that can provide control on size, orientation and crystallinity are in high demand. So far, the techniques relied on nanometer scale sized catalyst metal droplets to direct the synthesis of nanowires and nanotubes.8-19 All these techniques typically require a minimum temperature of 500 ºC. In addition, the creation of quantum-sized droplets is a non-trivial process. In this regard, we recently reported a novel low temperature VLS method that does not require nanometer scale droplets for synthesis of nanowires at temperatures lower than 500 ºC.20 In this technique, we used low miscible, low melting and non-catalytic metals such as gallium to synthesize nanowires using microwave plasma mediated gas phase chemistry. In this paper, we discuss further details of this synthesis technique and report bulk synthesis of carbon nanowires. EXPERIMENTAL DETAILS For silicon nanowire growth experiments, we exposed a clean p-type (100) Si substrate covered with droplets of gallium to a microwave nitrogen plasma containing various percentages of hydrogen in a ASTEX 5210 microwave plasma reactor. The growth experiments were performed for 4-9 hrs at microwave powers of 600-1000 W, 30-50 torr, H2/N2 ranging from 0.25 to 1.2%. The temperature of the substrate was measured using an optical pyrometer to be approximately 440 ºC for microwave power and pressure of 700 W and 30 torr respectively. At the end of the experiments, a grayish mass was visibly observed on top of the gallium droplet and on the silicon substrate, which was identified as a blanket of fibers in SEM. For carbon Y1.6.1
nanowires growth, a pyrolytic boron nitride substrate covered with droplets of gallium was exposed to a hydrogen-diluted methane pla
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