Growth of Boron Nanowires by Chemical Vapor Deposition
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1017-DD03-04-EE02-04
Growth of Boron Nanowires by Chemical Vapor Deposition Li Guo, and Raj N. Singh University of Cincinnati, Cincinnati, OH, 45221
ABSTRACT Motivated by the extensive research on carbon nanotubes (CNTs), boron and its related nano-structures have attracted increasing interests for potential applications in nanodevices and nanotechnologies due to their extraordinary properties. B-related nanostructures are successfully grown on various substrates in a CVD process. The boron nanowires have diameters around 50200 nanometers and lengths up to a few microns. The gas chemistry is monitored by the in-situ mass-spectroscopy, which helps to identify reactive species in the process. Modified vapor-solid growths as well as VLS growth mechanisms are proposed for the growth of these nanostructures. The role of the catalysts in the synthesis is also discussed. INTRODUCTION Boron, a group III element, identified by the electron deficiency in unoccupied p orbital and short covalent radius, tends to form very strong covalent bonds. It has a high melting point (Tm ~ 2300 ºC), low density (ρ = 2.35 g/cm-3), good resistance to corrosion, and one of the hard materials (Vicker Hardness Hv =3000) [1]. Boron is also a semiconducting element besides carbon. The band gap of β-rhombohedral B was measured experimentally around 1.3-1.5 eV [2]. Recent discovery of superconductivity in MgB2 [3] initiated a lot of interests in boron and its compounds as a non-copper oxide bulk superconductor. The elemental B has also been reported to transform from a nonmetal to a superconductor under high pressures [4]. These properties make boron potentially useful in thermo-electric devices, high temperature semiconductors, and also a potential candidate used as a reinforcing element for certain composite materials [5-6]. In recent years, boron nanowires (BNWs) have been successfully synthesized through different experimental routes, which can be broadly classified into several groups, such as radio frequency (RF) – magnetron sputtering [7], chemical vapor deposition (CVD) [8], laser ablation [9], thermal vapor transport process [10], and solid state chemical reaction [11]. BNW growth by the vapor-liquid-solid (VLS) mechanism was proposed [9], but no tip attachment of the catalyst particles was found at the end. In addition, this mechanism can not be applied to BNWs formed without any catalyst on various substrates [7-8]. So it is worthwhile to study the growth mechanism of boron nanowires. EXPERIMENTAL Boron nanowires have been grown on 20nm thick Ni thin film coated Si (111) substrates using diborane (B2H6, 5 vol% diluted in H2) as the gas precursor and nitrogen (N2) as the carrier
gas in a thermal CVD process. The experimental parameters were optimized at 20 torr, 900 ºC, 5 sccm diborane (B2H6), and 55 sccm nitrogen (N2) [12-14]. These nanostructures were characterized by ESEM (Environmental Scanning Electron Microscope), TEM (Transmission Electron Microscope), SAD (Selected Area Diffraction), Raman spectroscopy, and EDAX (Energy Disper
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