Nanostructuring for nitride light-emitting diodes and optical cavities (2013)

﻿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.&#13;\nDue 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.&#13;\nAmong 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.