III-Nitride light-emitting diodes incorporating nanosphere-patterned nanophotonic structures

Abstract

Nanophotonic structures such as photonic crystals (PhCs) and plasmonic structures have unique capabilities of being able to control and manipulate the propagation of light. The incorporation of two-dimensional (2D) PhCs into GaN-based light-emitting diodes (LEDs) has also been demonstrated to effectively promote light extraction efficiency. Such ordered periodic nanostructures, with the ability of manipulating spontaneous emission, can be extremely useful for extracting guided modes to air, thus enlarging the escape cone. With a well-defined periodic arrangement and with sufficiently large refractive index contrast between GaN and ambient, a photonic bandgap (PBG) may be established, which forbids the propagation of light within a specific range of frequencies dependent on the dimension and pitch of the array. The PBG can thus be exploited for suppressing lateral wave-guiding and possibly redirect a significant proportion of trapped photons for extraction, overcoming one of the major limitations of nitride LEDs. Confinement effects associated with the PBG may also be exploited for nitride laser structures. Traditionally, the formation of two-dimensional PBG structure relies heavily on high-precision direct-write electron-beam lithography but low-throughput and high equipment cost makes this impractical for large-scale fabrication. Nanosphere lithography (NSL) has emerged as a practical approach for patterning large-area ordered periodic nanostructures. NSL overcomes resolution issues arising from diffraction limit in optical lithography and even beam size limitations in e-beam lithography. Jet-printing of large-area ordered nanosphere arrays is demonstrated in our group. The formation of a wide range of GaN nanophotonic structures by NSL, including the closed-packed nano-pillars, air-spaced nanopillars, nano-tips, nano-rings, hemispherical and clover structures is demonstrated. A photonic bandgap can be established, and in some cases, optical-pumped lasing observed.