Fabrication and Characterisation of III-Nitride based Nanostructure Devices using Nanosphere Lithography Techniques

In this work, fabrication and characterisation of nanostructure devices has been performed on InGaN/GaN multiple quantum wells (MQW) grown on either c-plane sapphire or (111) silicon substrates. A cost effective nanosphere lithography technique has been employed for the fabrication of a number of nano structures such as nanorod arrays, nanoholes arrays; and single micro-disk lasers. Photonic crystal structures based on nanohole arrays have been designed and then fabricated on InGaN/GaN MQWs with an emission wavelength of 500 nm grown on c-plane sapphire by means of a nanosphere lithography technique, demonstrating a clear photonic crystal effect. Significant suppression of spontaneous emission has been observed when the emission is within the photonic bandgap. Angular dependent measurements show a change in the far-field pattern when the emission lies outside the photonic bandgap compared with the emission which lies inside the photonic bandgap. A coherent nanocavity a two-dimensional (2D) periodic array of nanodisks, was designed and fabricated on an InGaN/GaN MQW structure with an emission wavelength at 510 nm, leading to a significant enhancement in the internal quantum efficiency (IQE) as a result of enhanced spontaneous emission rate. Finite-difference time-domain (FDTD) analysis has performed for the structure design. The coherent nanocavity effect has been confirmed using means of time-resolved photoluminescence measurements, showing a clear enhancement in spontaneous emission rate. Finally, an improvement in IQE of 88 times at 510 nm has been achieved. Optically pumped green lasing has been achieved with thresholds as low as 1 kW/cm2, using an InGaN/GaN based micro-disk with an undercut structure on silicon substrates. The micro-disks with a diameter of around 1 μm were fabricated by means of a combination of a cost-effective silica micro-sphere approach, dry-etching and subsequent a wet-etching. The combination of these techniques both minimises the roughness of the sidewalls of the micro-disks and also produces excellent circular geometry. Utilizing this fabrication process, lasing has been achieved at room temperature under optical pumping from a continuous-wave laser diode. Time–resolved micro-photoluminescence (PL) and confocal PL measurements have been performed in order to further confirm the lasing action in whispering gallery modes and also investigate the excitonic recombination dynamics of the lasing