Growth and characterisation of Ge Nanowires by chemical vapour deposition

Abstract

One-dimensional Ge/Si nanostructures have attracted much attention because of their potential applications in the design of novel electronic (Fan et al, 2006; Xiang et al, 2006; Liang et al, 2007; Hu et al, 2007), photonic (Holmes et al, 2000), battery(Chan et al 2008a; Chan et al 2008b) and sensing devices (Cui et al, 2001). For examples, due to their high mobility of electrons and holes, Ge nanowires show their promising application in highspeed field-effect transistor (Xiang et al, 2006; Liang et al, 2007). And, Ge nanowires are potentially useful for high-speed quantum computing due to the long decoherence time because of their predominance of spin-zero (Hu et al, 2007; Tyryshkin et al, 2006) nuclei and the advantage of a large excitonic Bohr radius in Ge (24.3 nm) allowing for quantum confinement to be observed in relatively large structures (Sun et al, 2007; Maeda et al, 1991) and at high temperatures. In order to realize these applications, controllable and high quality nanowires growths are important. Much attention has been paid to the growth of Ge nanowires(Lauhon et al, 2002; Greytak et al 2004; Kodambaka et al 2007).In most cases, the nanowires are grown in chemical vapour deposition (CVD) method visa vapor liquid solid (VLS) mechanism (He et al, 2006; Kamins et al, 2004; Fuhrmann et al, 2005; Allen et al,2008; Hannon et al, 2006), in which the formation of Ge-Au eutectic plays the most important role in the synthesis of nanowires. It has been clarified that the VLS mechanism dominates the axial growth of Ge nanowires. However, the radial growth of nanowires is easily ignored and there are only few papers to talk about it. And, the impacts of the surface conditions on the VLS (vapor-liquid-solid) synthesis of Ge nanowires are rarely discussed. Properly understanding it could help us engineer the growth of nanowires. In this chapter, the growth of Ge nanowires via CVD method will be discussed in detail. Vapor-solid-solid (VSS) growth mechanism is supposed to dominate the nanowire growth in a radial direction, which contributes to an increase in the diameter of nanowire. After the Au catalysts on the tip are consumed over a long growth time, the nanowire with a rough surface will be grown due to the selective VSS radial growth. The impacts of surface condition on the growth of Ge nanowires on Si (100) substrate will also be observed. On the SiO2-terminated Si substrate, high-density Ge nanowires can easily be grown. However, it is more complex for the growth of Ge nanowires on H-terminated Si substrate. Due to Si