Investigation of structural characteristics of III-V semiconductor nanowires grown by molecular beam epitaxy

Abstract

One-dimensional nanowires made of III-V semiconductors have attracted significant research interest in the recent decades due to their distinct physical and chemical properties that can potentially lead to a wide range of applications in nanoelectronics and optoelectronics. As a key class of III-V semiconductor nanowires, InAs nanowires have attracted special attention due to their narrow bandgap, relatively high electron mobility, and small electron effective mass, which made them a promising candidate for the applications in future optical and high-frequency electronic devices. One of the challenges in realizing these unique III-V nanowire properties in nanowire-based devices is to integrate nanowires on the nano-devices or chips with well-organized arrangement. To solve this issue, the epitaxial nanowire growth provides the uniqueness that well aligned nanowires can be grown on the chosen substrates, and by selecting substrates with particular orientations, specifically orientated nanowires can be grown. Au-assisted nanowire growth is one of the most common methods to grow epitaxial III-V nanowires via the vapor-liquid-solid (VLS) mechanism or vapor-solid-solid (VSS) mechanism. For III-V nanowires, one of the coherent problems is that the stacking faults and/or twin defects can easily be introduced in both wurtzite or zinc-blende structured nanowires due to the small energy differences between wurtzite and zinc-blende stacking sequences of their dense planes. For III-V nanowires to be practically useful, it is of significant importance to minimize the lattice defects in nanowires, and to control their crystal phase and structural quality. In this regard, in this thesis, we investigated the growth parameters on the growth behaviour, crystal phase and structural quality of III-V nanowires, particularly InAs and GaAs nanowires. By carefully tuning the growth parameters, we have successfully achieved the controlled growth of InAs and GaAs nanowires with defect-free wurtzite structure and/or zinc-blende structure. Meanwhile, although InAs nanowires have been grown by different growth techniques, such as metalorganic chemical vapor deposition (MOCVD) or chemical beam expitaxy (CBE), very few studies have been devoted to the growth of InAs nanowires in molecular beam epitaxy (MBE). In this thesis, we employed the MBE system to grow Au-catalysed InAs nanowires on the GaAs{111}B substrate. Our extensive experiments have determined the relatively narrow V/III ratio window for the growth of InAs nanowires in MBE. Furthermore, by designing a two-V/III-ratio procedure, the pure defect-free zinc-blende structured InAs nanowires were achieved.