ABSTRACT
The group of III–V semiconductors is emerging as highly attractive materials for a wide range of applications, particularly the gallium nitride (GaN) family of alloys. The development of nitride-based light-emitting diodes (LEDs) represented a quantum leap in the advancement of optoelectronics. The timely arrival of InGaN blue LEDs enables 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. Due to total internal reflection at the GaN–ambience interface with a high refractive index contrast, low extraction efficiency is one of the major bottlenecks for LEDs. Extensive research efforts have been conducted on producing energy-efficient and highly reliable LEDs in the past decade. Among potential strategies, nanotechnology promises to offer significant boosts to device performance. Nanostructure 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 electron beam lithography enables precise direct writing of nanopatterns, 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 works reported in this chapter is a practical alternative approach. Uniform spheres acting as etch masks are capable of self-assembling into hexagonal closed-packed arrays. The resultant nanopillar array serves as the photonic crystals (PhCs) extracting guided light. The feature dimensions 394of the resultant patterns are scalable according to the diameter of nanospheres used. Such ordered closed-packed arrays are capable of promoting light extraction via the dispersion and diffraction properties of weak PhCs. To extend the functionality of sphere-patterned arrays, a dimension-adjusting procedure is developed to realize strong PhC structures. Finite spacing between individual spheres is introduced, resulting in strong air-spaced nanopillar PhCs structure with a wavelength-tunable photonic bandgap (PBG). Distinguished from typical PhCs in the form of air holes or pillars, a clover-shaped 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. Additionally, the PhC is able to enhance light extraction in the visible and in the infrared simultaneously, so the devices emit more light while radiating more heat, a feature that is beneficial to overall efficiencies of the emitters.
