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

In this thesis, the carrier localization effect, the quantum confinement Stark effect (QCSE) and nonlinear optical properties of the as-grown InGaN/GaN multiple-quantum-wells (MQWs) structure and nanopillars with diameters of 100 nm and 160 nm and height of 700 nm have been investigated with linear and nonlinear photoluminescence (PL) techniques, In order to investigate the carrier localization effect and QCSE, “S-shaped” temperature dependent PL peak positions of the samples are quantitatively simulated and analyzed with the localized-state ensemble luminescence model. It is found that after nanotexturing both the carrier localization effect and QCSE become weakened. Moreover, the smaller the pillars the weaker the two effects will be. In addition, the nanotexturing introduced the new radiative recombination pathways of carriers are confirmed on the sidewalls of the nanopillars with cathodoluminescence (CL) spectrum and panchromatic CL image. Two-photon absorption (TPA) induced PL spectra of the three samples are measured to investigate the nonlinear optical properties. A peculiar excitation-power dependence, say I~P1.53, of the PL intensity is revealed. It was proposed that a mixed excitation mechanism, namely two-step successive one-photon absorption occurring in the InGaN well layers and one-step two-photon absorption mainly taking place in the GaN barrier and buffer layers, to interpret the observed phenomenon. Besides, the steady-state energetic distribution of carriers excited via this mixed excitation mechanism is very different from that of carriers via one-step one-photon excitation. In contrast with the case of one-photon PL in the samples, the influence of carrier localization effect becomes weaker in the TPA PL of the two nanopillars. / published_or_final_version / Physics / Master / Master of Philosophy

Optical properties of graphene/GaN hybrid structure were investigated by using a variety of optical spectroscopy techniques including low-temperature photoluminescence (PL) spectroscopy, time-resolved PL (TRPL) spectroscopy, confocal scanning micro-Raman spectroscopy. Single-layer graphene grown by chemical vapor deposition was transferred to GaN epilayer surface, which is verified by the Raman spectrum with a sharp characteristic peak at ~2690 cm-1and a homogeneous Raman image. Three main band-edge emissions including the free exciton A transition (denoted as FXA), the donor bound exciton transition (denoted as DX) and the third peak (denoted as Ix) were well resolved in the PL spectra of the hybrid structure as well as the as-grown GaN epilayer at low temperatures. Interestingly, the FXA transition and Ix line of the GaN epilayer were found to be dramatically altered by the top graphene layer while the DX is almost unaffected. The intensity of Ix line substantially drops after the transfer of graphene layer on GaN, indicating surface defect nature of the Ix line. More interestingly, an unpredictable dip structure develops in the FXA peak when the temperature is beyond 50 K. Similar spectral structure change also occurred in the emission of free exciton B (referred as FXB)with higher transition energy .A free exciton dissociation and electron transfer model was proposed to explain the “dip effect”. More supporting evidence to the model was found in the time-resolved PL spectra of the hybrid structure and the control sample. The results showed the significant influence of graphene monolayer on the fundamental optical properties of GaN. / published_or_final_version / Physics / Master / Master of Philosophy

published_or_final_version / abstract / toc / Physics / Doctoral / Doctor of Philosophy

No description available.

No description available.

Thesis (M.S.)--Ohio University, March, 2000. / Title from PDF t.p.

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2008. / Includes bibliographical references. Also available in electronic version.

Thesis (Ph. D.)--University of Hong Kong, 2009. / Includes bibliographical references (leaves 89-96) Also available in print.

Thesis (M. Phil.)--University of Hong Kong, 2008. / Includes bibliographical references (leaf 78-85) Also available in print.