GeO x and SiO x nanowires grown via the active oxidation of Ge and Si substrates
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Yi Yang Department of Electronic Materials Engineering, Research School of Physics and Engineering, Australian National University, Canberra ACT 0200, Australia; and School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
Taehyun Kim and Robert G. Elliman Department of Electronic Materials Engineering, Research School of Physics and Engineering, Australian National University, Canberra ACT 0200, Australia (Received 27 January 2011; accepted 27 April 2011)
In this study, we show that the volatile monoxide species generated during the active oxidation of Ge and Si substrates can be utilized in the presence of Au catalytic nanoparticles to nucleate and grow GeOx and SiOx nanowires. A simple thermodynamic model is developed to ascertain the critical O2 partial pressure as a function of temperature required for the active oxidation of Ge and Si substrates and is experimentally verified. The ideal conditions for uniform nanowire growth across the substrate are shown to be primarily dependent on the O2 partial pressure, the annealing temperature and thicknesses of the surface oxide, and deposited Au. The role of a metastable surface oxide separating the active oxidation and NW nucleation processes is also discussed. I. INTRODUCTION
Recently, metal and semiconducting oxide nanowires (NWs) have received much attention as they offer a number of unique surface and microstructural properties that can be exploited for a wide range of catalytic, electronic, and optical applications; see, for example, reviews by Barth et al.,1 Wang,2 and Rao et al.3 These NWs can be grown successfully via a number of techniques, but these often involve the use of complex processing chambers and/or the use of toxic precursor gases and are therefore costly to implement. Many pure and compounded refractory metals and semiconductors react with O2 gas to produce a volatile oxide vapor at temperatures below the melting point of the solid material. For semiconductors, volatilization of the substrate typically occurs at very low O2 partial pressures and is often referred to as active oxidation, resulting in a decomposition of the substrate surface and the formation of etch pits. Significantly, these oxide vapors can be used as the gas phase source of reactants for the growth of NWs via the vapor–liquid–solid (VLS) growth mechanism. In this case, specialized external vapor sources are not required because the NW vapor precursor is directly produced at the surface of the material where the NWs are to be grown. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.150 2240
J. Mater. Res., Vol. 26, No. 17, Sep 14, 2011
http://journals.cambridge.org
Downloaded: 14 Jul 2014
The growth of self-catalyzed metallic oxide NWs, including Fe2O3, CuO, and ZnO, via active oxidation type processes was first reported in the 1950s, but the theory of oxidation failed to explain the observed NW growth.4 At about the same time, Weiss and Weiss5 reported the discovery of a new fibrous
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