Low Temperature Gas Phase Synthesis of Germanium Nanowires
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Low Temperature Gas Phase Synthesis of Germanium Nanowires Sanjay Mathur*, Hao Shen and Ulf Werner Institute of New Materials D-66041 Saarbruecken, Germany ABSTRACT Single crystal Ge nanowires (NWs) were obtained in high yield by gas phase decomposition of germanium di-cyclopentadienylide ([Ge(C5H5)2]), at 325 °C on iron substrates. Highresolution electron microscopy (SEM/TEM) showed Ge NWs to be uniform in terms of diameter (20 nm) and length (> 25 µm). The wire growth is selective and appears to be governed by a GeFe alloy epilayer formed by the reaction between Ge clusters and iron substrate, during the initial stages of the CVD process. The supersaturation of Ge-Fe solid-solution with respect to Ge content induces the spontaneous formation of single crystal germanium nuclei that act as templates for the nanowire growth. X-ray and electron diffraction revealed the NWs to be single crystals of cubic germanium with a preferred growth direction [11-2]. The proposed base-growth model on Fe substrate is supported by TEM, EDX and XPS studies. INTRODUCTION Non-carbon one-dimensional (1D) nanostructures such as nanotubes (NTs) and nanowires (NWs) are receiving increasing attention due to the finite size effects and their possible applications in designing nanodevices [1-4]. Semiconductor materials such as Ge and Si display interesting electronic, optical and mechanical properties due to the quantization of electronic states in confined (nano)geometries [5]. Further, Germanium is an important semiconductor with high carrier mobility and a narrow band gap and owing to the larger excitonic Bohr radius in Ge (24.3 nm), the quantum size effects are expected to be more prominent when compared to silicon (4.9 nm). Although much effects have been made to obtain Ge nanowires, the successful growth of 1D Ge nanostructures is limited to a few reports [6-12]. Lieber and Morales have employed laser ablation (ca. 800 oC)for synthesizing Ge nanowires [8]. Yang et al. have used vapor transport reaction at rather high temperatures (900-1100 oC) to grow random and aligned Ge nanowires by in situ synthesis and disproportionation of GeI2 on Au islands [10]. Hanrath et al. have used a solvothermal method to synthesize Ge nanowires by decomposing mixed Au-Ge precursors under high temperature and pressure conditions (300-400 oC, 100 atm) [11]. The chemical vapour deposition of Ge nanowires has recently been achieved by the reduction of tetrahydrogermane (GeH4) through H2 addition at 275 oC [12]. The control of GeH4 decomposition through H2 is inevitable in order to obtain the high quality Ge nanowires. One-dimensional structures can be generated from the gas phase synthesis by well-known Vapor-Liquid-Solid (VLS) [13] growth process where a molten catalyst is used to promote axial instead of radial growth. Chemical vapour deposition (CVD) is an effective technique to produce single or core-shell type nanowires, because it allows an easy regulation of the precursor feedstock, in the vapor phase. The VLS type metal nanoparticle-catalyze
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