Xu et al. Nanoscale Research Letters 2011, 6:113 http://www.nanoscalereslett.com/content/6/1/113 N A N O E X P R E S S Open Access Synthesis of long group IV semiconductor nanowires by molecular beam epitaxy Tao Xu1, Julien Sulerzycki1, Jean Philippe Nys1, Gilles Patriarche2, Bruno Grandidier1*, Didier Stiévenard1 Abstract We report the growth of Si and Ge nanowires (NWs) on a Si(111) surface by molecular beam epitaxy. While Si NWs grow perpendicular to the surface, two types of growth axes are found for the Ge NWs. Structural studies of both types of NWs performed with electron microscopies reveal a marked difference between the roughnesses of their respective sidewalls. As the investigation of their length dependence on their diameter indicates that the growth of the NWs predominantly proceeds through the diffusion of adatoms from the substrate up along the sidewalls, difference in the sidewall roughness qualitatively explains the length variation measured between both types of NWs. The formation of atomically flat {111} sidewalls on the <110>-oriented Ge NWs accounts for a larger diffusion length. Introduction Semiconductor nanowires (NWs) consist of a solid rod with a diameter usually smaller than 100 nm and a length that can vary from the nanometer to the milli- meter-scale depending on the technique used to synthe- size the rods. Although, for the majority of the NWs, their growth is described by the vapor-liquid-solid mechanism, based on the catalytic effect of a metal seed particle, their length is mostly related to the way the chemical compounds are supplied. In chemical vapor deposition (CVD), a wide range of partial pressures for the reactive source gases can be used. As a result, Si NWs with a millimeter-scale length have been success- fully synthesized with a reasonable time [1]. Conversely, when elemental compounds are supplied instead of gas precursors, as it is the case for the growth of NWs in molecular beam epitaxy (MBE), ultra high vacuum (UHV) conditions are required. The pressure in the sys- tem is around 10-9 times smaller than in a CVD cham- ber and the NW length typically does not exceed a few micrometers [2,3]. As illustrated in Figure 1 for the MBE growth, the ratio between the exposed surface of a seed particle and the collection area between the seed particles is usually * Correspondence: bruno.grandidier@isen.fr 1Département ISEN, Institut d’Electronique, de Microélectronique et de Nanotechnologie, IEMN (CNRS, UMR 8520), 41 bd Vauban, 59046 Lille Cedex, France Full list of author information is available at the end of the article small. Due to the low pressure in the growth chamber, the direct impingement of elemental compounds onto the seed particle has a small probability to occur. There- fore, growth predominantly proceeds from the diffusion of elemental compounds that adsorb on the substrate between the seed particles. The adatoms reach the seed particles after diffusing on the substrate and the side- walls with different diffusion length coefficients, lS and lf, respectively. As surface diffusion is a rather slow pro- cess, it is only because the crystallization at the interface between the seed particles and the NW is high enough that NWs emerge from the film growth. Such mass transport mechanism yields a NW length that is inver- sely proportional to the NW diameter [3,4]. In contrast to MBE grown III-V semiconductor NWs that can reach micrometer-scale lengths, group IV semi- conductor NWs, which are generally grown with a <111> orientation, show a much smaller length [4,5], making their integration into devices more difficult. In a purpose to understand the physical mechanisms that prevent the fabrication of group IV semiconductor NWs with micrometer-scale lengths, we investigated the growth of Si and Ge NWs on a Si(111) surface with the MBE technique. In this article, we show that not only <111> but also <110>-oriented Ge NWs are grown on the Si(111) surface. Surprisingly, the length of the latter can reach a few micrometers. From a comparative study of differences in the structural morphology between © 2011 Xu et al; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.