Color-tunable and Phosphor-free White-light Multi-layered Light-emitting Diodes

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Flip-chip InGaN light-emitting diodes with an integrated microlens array

The fabrication of hexagonally close-packed lens array on flip-chip bonded InGaN LED by nanosphere lithography is reported. A self-assembled monolayer of silica spheres with diameters of 1-μm serves as an etch mask to be transferred onto the sapphire face of the LED to form hemispherical lenses. Without degrading electrical characteristic, the light output power of lensed LED is increased by more than one-quart, compared with an unpatterned LED. The optical behavior of individual lenses and converging effect of lensed LED are by ray-tracing and confocal imaging. © 2013 The Japan Society of Applied Physics.published_or_final_versio

Density-tunable non-close-packed monolayer of silica nanospheres prepared by single-step freeze-drying

The formation of density-tunable non–close-packed (ncp) monolayers of silica nanospheres via freeze drying is demonstrated. Monolayers of silica spheres with diameters of 120, 490, and 990 nm are achieved through a single-step freeze dry process. The densities of the spheres can be tuned by adjusting either the volume fraction of the spheres or the settling time before freezing. Issues involving defect-formation and defect-reduction are investigated. The experimental data suggest that this freeze drying method represents an efficient and robust way for achieving ncp monolayers of nanoparticles.published_or_final_versio

Broadband emission from an ensemble of nano-pillars with multiple diameters

Conference 8641: Light-Emitting Diodes: Materials, Devices, and Applications for Solid State Lighting 17The Conference program's website is located at http://spie.org/x92791.xmlGenerating white light from monochromatic light sources is commonly achieved via one of two common methods: exciting fluorescence phosphors from a shorter wavelength LED, or mixing light from three or more LED chips, commonly known as RGB LEDs. Phosphor efficiency degrade over time, and have lifetimes shorter than the chip itself. RGB LEDs require turning on three or more p-n junctions and suffer from color mixing issues. We introduce a promising approach towards achieving phosphor-free white light emission, tapping on strain engineering and nanoscale processing. The proposed approach makes use of a long wavelength chip, which is …published_or_final_versio

Air-spaced GaN nanopillar photonic band gap structures patterned by nanosphere lithography

We report on the fabrication of ordered hexagonal arrays of air-spaced GaN nanopillars by nanosphere lithography. A self-assembled two-dimensional silica nanosphere mask was initially formed by spin-coating. Prior to pattern transfer to the GaN substrate, a silica-selective dry etch recipe was employed to reduce the dimensions of the nanospheres, without shifting their equilibrium positions. This process step was crucial to be formation of air-spaced hexagonal arrays of nanospheres, as opposed to closed-packed arrays normally achieved by nanosphere lithography. This pattern is then transferred to the wafer to form air-spaced nanopillars. By introducing air gaps between pillars, a photonic band gap (PBG) in the visible region can be opened up, which is usually nonexistent in closed-packed nanopillar arrays. The PBG structures were designed using the plane wave expansion algorithm for band structure computations. The existence and positions of band gaps have been verified through optical transmittance spectroscopy, which correlated well with predictions from simulations. From photoluminescence (PL) spectroscopy, a fourfold increase in PL intensity was observed and compared to an as-grown sample, demonstrating the effectiveness of well-designed self-assembled PBG structures for suppressing undesired optical guiding mode via PBG and for promoting light extraction. The effects of defects in the nanopillar array on the optical properties are also critically assessed. © 2011 American Institute of Physics.published_or_final_versio

InGaN light-emitting diodes with indium-tin-oxide photonic crystal current-spreading layer

Photonic crystal patterns on the indium tin oxide layer of an InGaN/GaN light-emitting diode are fabricated via nanosphere lithography in combination with dry etching. The silica spheres acting as an etch mask are self-assembled into a hexagonal closed-packed monolayer array. After etching, the photonic crystal (PhC) pattern is formed across the indium-tin-oxide (ITO) films so that the semiconductor layers are left intact and thus free of etch damages. Despite slight degradation to the electrical properties, the ITO-PhC light-emitting diodes (LEDs) exhibit enhancements of their optical emission power by as much as 64% over an as-grown LED. The optical performances and mechanisms of the photonic crystal LEDs are investigated with the aid of rigorous coupled wave analysis and finite-difference time-domain simulations. © 2011 American Institute of Physics.published_or_final_versio

Single-mode whispering gallery lasing from metal-clad GaN nanopillars

An ordered hexagonal closed-packed nanopillar array is fabricated on GaN. A metal coating is then applied to encapsulate the pillars for promoting optical confinement within the cylindrical cavity. Room-temperature lasing at 373 nm is observed under pulsed excitation, at a lasing threshold of 0.42 MW/cm 2. With pillar diameters of around 980 nm, the number of modes overlapping the emission spectrum is reduced, giving rise to single-mode whispering gallery stimulated emission. Finite-difference time-domain simulations are carried out for the prediction of resonant frequencies and electric field patterns corresponding to the resonant modes. © 2012 Optical Society of America.published_or_final_versio

Tunable clover-shaped GaN photonic bandgap structures patterned by dual-step nanosphere lithography

The fabrication of close-packed clover-shaped photonic crystal structure on GaN by dual-step nanosphere lithography is demonstrated. By shrinkage of spheres prior to pattern transfer, a non-closed-packed clover-shaped photonic bandgap (PBG) structure, as designed by modified 3D finite-difference time-domain simulation, is also realized. The PBG of the close-packed and non-close-packed clover-shaped structures is verified through optical transmission spectroscopy, found to agree well with simulated results. A threefold enhancement in photoluminescence (PL) intensity is observed from the optimized structure, when the PBG is tuned to overlap with the emission band of the InGaN/GaN multi-quantum wells. From time-resolved PL measurements, shortened decay lifetimes are observed. © 2012 American Institute of Physics.published_or_final_versio

The contribution of sidewall light extraction to efficiencies of polygonal light-emitting diodes shaped with laser micromachining

The light extraction efficiencies of InGaN/GaN blue light-emitting diodes (LEDs) of different geometries ranging from a triangle to a decagon have been simulated by ray-tracing. The conventional rectangular LED was found to be the most inefficient among the investigated polygons, and light extraction through the device sidewalls was the key factor. The results were experimentally verified by fabricating LEDs shaped into polygons by nanosecond-pulsed laser micromachining, which proved the simulated results. The mechanism of light extrac-tion in polygonal LEDs is discussed in detail. © 2010 American Institute of Physics.published_or_final_versio

Close-packed hemiellipsoid arrays: A photonic band gap structure patterned by nanosphere lithography

A self-assembled hexagonal close-packed hemiellipsoidal photonic crystal structure was fabricated on GaN material. An ordered monolayer silica nanosphere coating served as a hard mask in an inductively coupled plasma etching process. The shape of the arrayed hemiellipsoids can be controlled by adjusting the etch selectivities and durations according to the fabrication model. The existence of a photonic band gap is established through planar transmissivity measurement whereby a transmission dip centered at 440 nm was identified. A threefold enhancement in light extraction was achieved, as determined from the measured angular photoluminescence emission pattern. © 2009 American Institute of Physics.published_or_final_versio