Epitaxial growth of III-V semiconductors on two-dimensional layered material substrates
Creators: name: Gopakumar Saraswathyvilasam, Aswani
Publisher: name: The Australian National University
Year: 2024

description: van der Waals epitaxy offers several advantages over conventional epitaxy. One significant advantage is that the absence of surface dangling bonds significantly relaxes the requirement for lattice matching between the grown layer and the substrate. As a result, it provides an ideal platform for growing single crystalline materials with reduced defect densities. Additionally, the weak van der Waals bonding allows for easy detachment of the grown layer from the substrate, enabling the fabrication of flexible devices. This thesis investigates the experimental and theoretical approaches to achieve a comprehensive understanding of the growth process, focusing on the synthesis of GaAs nanowires (NWs) and InAs NWs, as well as thin film structures, on 2D atomic layered materials. Overcoming challenges associated with lattice and thermal mismatch, this study utilises van der Waals epitaxy to grow III-V materials on unconventional substrates. The grown nanostructure/thin films and interface between the as-grown III-V materials and the van der Waals substrate is thoroughly examined, encompassing detailed characterisations of morphology, crystal structure, optical properties, and electrical properties. Notably, this research achieves a high vertical yield of GaAs nanowires (NWs) and sets a record for synthesizing GaAs quantum wires with a diameter as small as 5 nm. Additionally, the research attains a high yield of InAs NWs and successfully demonstrates uniform polycrystalline thin film structures over a large area on the h-BN surface. These achievements represent significant advancements in the field of heteroepitaxial growth. Successful synthesis of vertical GaAs NWs on synthetic mica substrates using Au-catalysed vapor-liquid-solid growth with metalorganic chemical vapor deposition is achieved. The influence of growth parameters and pre-flow on the NW yield is investigated, and the resulting NWs exhibit high optical quality with emission at 1.43 eV, corresponding to GaAs band edge emission. The bonding between GaAs NWs and mica is found to follow a physisorption-type bonding, enabling easy detachment from the 2D layered mica substrate. Furthermore, the synthesis and characterisation of GaAs quantum wires with dimensions as small as 5 nm in diameter are accomplished. The quantum wires exhibit excellent crystal morphology, pure zinc-blende structure, and enhanced negative stress with decreasing wire diameter. Extensive analysis of the electronic structure, surface properties, and bandgap characteristics demonstrates the increased confinement effects and altered electronic properties in GaAs quantum wires below the exciton Bohr radius. Nanoscale simulations are performed to examine the impact of quantum confinement on optical transition energies in both freestanding quantum wires and those embedded within an AlGaAs passivation layer. The simulation results highlight a more pronounced variation in the bandgap of freestanding quantum wires. Additionally, the growth of InAs NWs and thin films on h-BN/SiO2/Si van der Waals substrates without catalysts or surface modifications is systematically studied. The influence of growth parameters on the properties of the NWs and thin films is investigated, and a predominantly mixed wurtzite and zinc blende phase is observed. Uniform coverage of polycrystalline InAs thin films on h-BN/SiO2/Si, combined with room temperature photoluminescence and relatively high Hall mobility, demonstrates the potential for large-area, low-temperature growth of III-V thin films directly on van der Waals substrates using metalorganic chemical vapor deposition technique.