Synthesis and characterisation of wide-bandgap semiconductor nanostructures towards opto-electronic applications

Semiconductor structures offer unique properties at the nanoscale. Mastering the growth of semiconductor crystals and fabrication processes is crucial towards developing better opto-electronic devices. Morphological variations at nanoscale have a direct effect on available electronics states, impacting fundamental electronic properties and photonics capabilities of the material. This thesis focuses on one-dimensional Gallium Nitride (GaN) nanowires, Graphene Quantum Dots (GQDs) and two-dimensional hexagonal-Boron Nitride (hBN). First, synthesis of semiconductor nanostructures with a focus on nanoparticle mediated growth of one-dimensional Gallium Nitride crystal is developed. Catalyst-assisted growth of GaN nanowires using a Low Pressure Chemical Vapour Deposition (LPCVD) system is shown. The choice of growth catalyst influences directly the GaN crystal and the morphology of nanowires along with growth parameters such as temperature, vacuum pressure, precursor gases or time. An uncommon nanostructure of GaN in the shape of pine trees is introduced. Next, promising emission from GaN nanowires and hBN monolayers in the visible range is reported. Defects in LPCVD-grown GaN nanowires show yellow and green luminescences. Besides, femto-second pulsed laser is used, on defect-free nanowires grown by molecular beam epitaxy, to intentionally induce optically-active defects. Above a laser threshold power the irradiated GaN nanowires melt, higher power sublimates the crystal. Irradiated GaN is characterised as Wurtzite crystal and show emission in the visible range, additionally to the band-to-band UV luminescence. Similar experiment on hBN monolayers show defect-related visible emission near the irradiated areas. Contrary to GaN nanowires, where a wide emission is observed, hBN defects show strong and sharp emission. Point defects created in hBN - monolayers and flakes - are shown to be single photons emitters. Finally, fabrication of efficient micro- and nano-scale emitters is of crucial importance for optical nano-devices. In conventional semiconductors, such as Si or GaAs, fabrication processes can involve expensive and time consuming electron beam lithography and etching. Using femto-second pulsed laser, efficient and cost-effective GaN and hBN-based localised micro and nanometer-size light emitters can be fabricated using optically active defect states. Coupling these nano-emitters with surface plasmons of another nanostructure can improve the emission rate as shown by coupling GQDs to single Bi2Te3 nanoplates. From synthesis by chemical vapour deposition methods to structural and optical characterisation, this thesis will show how emission wavelength can be modulated and localised micro-/nano-emitters be fabricated before coupling them with surface plasmons to improve the emission rate for more efficient and bright emitters. Doctor of Philosophy