Qianqian Jiao

Resonant absorption and scattering suppression of localized surface plasmons in Ag particles on green LED

The metallic-structure dependent localized surface plasmons (LSPs) coupling behaviors with InGaN QWs in a green LED epitaxial wafer are investigated by optical transmission, scanning electron microscopy (SEM) and photoluminescence (PL) measurements. Ag nanoparticles (NPs) are formed by thermal annealing Ag layer on the green LED wafer. SEM images show that for higher annealing temperature and/or thicker deposited Ag layer, larger Ag NPs can be produced, leading to the redshift of absorption peaks in the transmission spectra. Time resolved PL (TRPL) measurements indicate when LSP-MQW coupling occurs, PL decay rate is greatly enhanced especially at the resonant wavelength 560 nm. However, the PL intensity is suppressed by 3.5 folds compared to the bare LED. The resonant absorption and PL suppression are simulated by three dimension finite-difference-time-domain (FDTD), which suggests that Ag particle with smaller size and lower height lead to the larger dissipation of LSP.

Study on the morphology and shape control of volcano-shaped patterned sapphire substrates fabricated by imprinting and wet etching

A volcano-shaped patterned sapphire substrate (VPSS) was fabricated by imprinting lithography and wet etching to enhance the light output of LED devices. A hexagonally arranged pattern with different crystal orientations was imprinted onto the sapphire substrate. As the etching time increased, the pattern with a crater in its center was changed from truncated triangular pyramids to truncated hexagonal pyramids with symmetrical sidewall facets. Small craters surrounded by three {108} facets appeared with 3-fold or 6-fold symmetry at the boundaries with neighboring pyramids. The mechanism of sapphire wet etching for VPSS synthesis was correlated to thermodynamics limits and the SiO2 mask pattern. The as-fabricated VPSS with slant angles of 34.3° and 69.9° was considered to enhance the internal quantum efficiency (IQE) and light extraction efficiency (LEE) of GaN-based LEDs.

Fabrication and Effects of Ag Nanoparticles Hexagonal Arrays in Green LEDs by Nanoimprint

In this letter, the Ag nanoparticles (NPs), which are located inside the hexagonal photonic crystals (PhCs) array holes, are successfully fabricated in green light-emitting diode (LED) by nanoimprint and lift-off techniques. The photoluminescence intensity of the green LED is increased by 4.5 folds compared with that of the bare LED due to the PhCs effect and the localized surface plasmon (LSP) multiple quantum wells coupling effect, which is further confirmed by the enhanced decay rate of LSP-functioned LED. In the simulation of 3-D finite difference time domain, it reveals that the morphology of Ag NP will affect the LSP resonant strength and the light scattering efficiency besides the periodic structure.

Study on Light Extraction from GaN-based Green Light-Emitting Diodes Using Anodic Aluminum Oxide Pattern and Nanoimprint Lithography.

An anodic aluminum oxide (AAO) patterned sapphire substrate, with the lattice constant of 520 ± 40 nm, pore dimension of 375 ± 50 nm, and height of 450 ± 25 nm was firstly used as a nanoimprint lithography (NIL) stamp and imprinted onto the surface of the green light-emitting diode (LED). A significant light extraction efficiency (LEE) was improved by 116% in comparison to that of the planar LED. A uniform broad protrusion in the central area and some sharp lobes were also obtained in the angular resolution photoluminescence (ARPL) for the AAO patterned LED. The mechanism of the enhancement was correlated to the fluctuations of the lattice constant and domain orientation of the AAO-pattern, which enabled the extraction of more guided modes from the LED device.

Silane controlled three dimensional GaN growth and recovery stages on a cone-shape nanoscale patterned sapphire substrate by MOCVD

Three dimensional (3D) growth induced by silane was performed on cone-shape nano-scale patterned sapphire substrates (NPSS) by metal organic chemical vapor deposition (MOCVD). The growth evolution for the silane controlled 3D growth process and the recovery stage were investigated by a series of growth interruptions. The GaN epilayers grown on the templates with different 3D growth conditions were characterized by X-ray diffraction (XRD), Raman scattering, and atomic force microscopy (AFM) measurements. The full width at half maximums (FWHMs) of the (002) and (102) reflections in the XRD rocking curves were 267 and 324 arcsec, respectively, for the sample on NPSS with 600 s of 3D growth. An extremely smooth surface was achieved with an average roughness of 0.10 nm over 3 × 3 μm2. All the above data were superior to those for the planar sample or the NPSS ones without the optimized 3D growth time. The silane addition caused effective 3D growth. The size, homogeneity, and faceted sidewalls of the islands by the 3D growth led to a high crystalline quality, much strain relaxation and a specular surface for the GaN epilayers.

The Coupling Behavior of Multiple Dipoles and Localized Surface Plasmons in Ag Nanoparticles Array

In this work, the coupling behavior of multiple dipoles and localized surface plasmons (LSPs) in Ag nanoparticle arrays is explored based on experimental results and 3D finite difference time domain (FDTD) simulations. The Ag nanoparticles (NPs) located inside the hexagonal photonic crystal (PhC) array holes are embedded in a green light-emitting diode (LED), which enhances emission efficiency significantly. In the simulation of the 3D FDTD, five spaced x-polarized dipoles are approximated as five quantum wells. The internal quantum efficiency (IQE) and light extraction efficiency (LEE) of the LSP-coupled LED are deduced respectively from the original IQE of the bare LED and the FDTD simulation results. Besides, the dynamic LSP-dipole coupling behavior is also explored considering the interaction of the five dipoles and their feedback effect to LSP, which lead to the magnification of the LSP-dipole coupling enhancement effect and the reduction of energy dissipation in Ag NPs.

Capability of GaN based micro-light emitting diodes operated at an injection level of kA/cm2

Different size InGaN/GaN based micro-LEDs (μLEDs) are fabricated. An extremely high injection level above 16 kA/cm2 is achieved for 10 μm-diameter LED. The lateral current density and carrier distributions of the μLEDs are simulated by APSYS software. Streak camera time resolved photoluminescence (TRPL) results show clear evidence that the band-gap renormalization (BGR) effect is weakened by strain relaxation in smaller size μLEDs. BGR affects the relaxation of free carriers on the conduction band bottom in multiple quantum wells (MQWs), and then indirectly affects the recombination rate of carriers. An energy band model based on BGR effect is made to explain the high-injection-level phenomenon for μLEDs.

Modification of far-field radiation pattern by shaping InGaN/GaN nanorods

In this work, we report on the fabrication of “golftee,” “castle,” and “pillar” shaped InGaN/GaN nanorod light-emitting diode (LED) arrays with a typical rod diameter of 200 nm based on nanoimprint lithography, dry etching, and wet etching. The photoluminescence (PL) integral intensities per active region area for “golftee,” “castle,” and “pillar” shaped nanorod samples were found to be 2.6, 1.9, and 2.2 times stronger than that of a conventional planar LED. Additionally, the far-field radiation patterns of the three different shaped nanorod samples were investigated based on angular resolved PL (ARPL) measurements. It was found that the sharp lobes appeared at certain angles in the ARPL curve of the “golftee” sample, while broad lobes were observed in the ARPL curves of the “castle” and “pillar” samples. Further analysis suggests that the shorter PL lifetime and smaller spectral width of the “golftee” sample were due to the coupling of photon modes with excitons, which also led to the observed high effic...