The group of III-V semiconductors is emerging as highly attractive materials for a wide range of applications, particularly the gallium nitride family of alloys. Undoubtedly, the development of nitride-based light-emitting diodes (LEDs) and laser diodes (LDs) represented a quantum leap in the advancement of optoelectronics. The timely arrival of InGaN blue LEDs enable full-color mixing with existing red and green LEDs based on AlInGaP and GaP alloys respectively, promoting the progress of solid-state lighting and displays, while the blue-violet LDs has revolutionized high-density optical data storage in the form of blu-ray. Extensive research efforts have been conducted on producing energy-efficient and highly reliable LEDs and LDs in the past decade. Amongst potential strategies, nanotechnology promises to offer significant boosts to device performances. Nano-structure on a scale of wavelength of light exhibits prominent effects on the propagation behavior of photons. However, the formation of well-defined nanostructure relies heavily on processing techniques. Although e-beam lithography enables precise direct-writing of nano-patterns, high equipment cost and time-consuming processes make mass production impractical. On the other hand, the technique of nanosphere lithography (NSL) as adopted in the work reported in this thesis is a practical alternative approach. Uniform spheres acting as etch masks are capable of self-assembling into hexagonal close-packed arrays. The resultant nanopillar array serves as the photonic crystals (PhCs) extracting guided light while the individual pillars may act as circular cavities supporting the whispering gallery (WG) mode. Due to total internal reflections at the GaN-ambient interface with high refractive index contrast, low extraction efficiency is one of the major bottlenecks for LEDs. To assist with light extraction, the LED surfaces are textured via NSL to form PhC structures. The feature dimensions of the resultant patterns are scalable according to the diameter of nanospheres used. Such ordered closedpacked arrays are capable of promoting light extraction via the dispersion and diffraction properties of PhCs. To extend the functionality of sphere-patterned arrays, a dimension-adjusting procedure is developed to realize photonic bandgap (PBG) structures. Finite spacing between individual spheres is introduced, resulting in air-spaced nano-pillar PhCs structure with a wavelength-tunable PBG. Distinguished from typical PhCs in the form of air-holes or pillars, a clovershaped structure with a wide PBG is fabricated by dual-step NSL. The PBG structures have been exploited for suppressing lateral wave-guiding and possibly redirecting a significant proportion of trapped photons for extraction. Among various lasing mechanisms in nitride-based material, the whispering gallery mode based on cylindrical resonators is a promising candidate with attractive properties of intrinsically high Q factor, low lasing threshold and simple fabrication process. Since multiple-modes lasing from larger micro-resonators patterned by conventional photolithographic method has limited applications, smaller disk-shaped cavities patterned by NSL open up opportunities to realize short-wavelength single-mode resonators. By employing a modified NSL process, photo-pumped blue lasing modes has been demonstrated from nanoring arrays, with low threshold of ~10 mJ/cm2 and a high Q factor of ~5000. Single-mode UV lasing at 373 nm has also been observed from metal-clad pillar structures. The lasing mechanisms are all verified through finite-difference time domain simulations. / published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

The irreversible trend of replacing the conventional incandescence light bulbs and fluorescent tubes with white light emitting diodes (LEDs) aims to use less energy for lighting. Plenty of the commercially available white LEDs are made from blue LED chips with few-micron-thick gallium nitride (GaN) grown on several hundred micron thick transparent sapphire substrates, followed by coating of yellow phosphor powder on top of the chips for converting the emitted blue light to white light. Not only does such approach give the white LEDs a high colour temperature, but also introduces conversion loss from the phosphor powder. The former issue makes users feel unpleasant for living while the latter wastes energy. Therefore, a new version of phosphor-free multilayered vertically-stacked colour-tunable LED structure is proposed in this thesis such that it allows users to regulate the colour temperature of light source according to their preference. Simultaneously, the device replaces light conversion agents with direct light generation. The fabrication of the proposed device involved the use of backside laser micromachining of trenches on the substrates of the upper layers of basic colour LED chips at a size just enough to fit the wire-bonded wire of lower layer LED chips inside. With equal-sized basic colour LED chips tightly packed together, colour homogeneity of the proposed device is enhanced and thus provides the proposed device the capability to substitute the conventional RGB LED devices with basic colour LED chips separately aligned. To improve the internal quantum efficiency and light extraction of nitride-based LEDs, thin film photonic crystal LED is proposed. Light and heat trapping sapphire substrate is removed by laser lift-off (LLO), forming GaN thin film on an electrically conductive opaque substrate with better heat conductivity than sapphire. By proper etching, N-dopped GaN layer can be exposed, resulting in the formation of vertical LED. Compared with conventional lateral LEDs with sapphire substrate, carrier path of vertical LED is greatly reduced and hence achieving lower internal resistance. To further boost light extraction, the device top surface is patterned with nanopillars by nanosphere lithography. A monolayer of closely-packed silica nanospheres is patterned on the N-GaN surface by spin coating. It acts as a mask for etching the nanopillars which bandfold lights from diffracted modes to radiative modes located above the light line for extraction. A typical laser LLO process results in thin films with undopped gallium nitride (U-GaN) surface or N-GaN (after etching) faces up. If P-side up is necessary, the GaN layers are first required to attach to a temporary substrate for LLO and then the LLO exposed surface is adhered to the real substrate before temporary substrate is detached. This method is proposed to relieve the issue of light channeling inside the sapphire substrate of full colour LED micro-display panel fabricated on a single GaN on Sapphire wafer. With the elimination of sapphire, “parasitic” blue emissions from the area surrounding pixels are reduced which in turns improved the observable effects from the microspheres jet-printed on the top surface of the panel. / published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

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