Xingxing Fu

GaN-based light-emitting diodes with photonic crystals structures fabricated by porous anodic alumina template

In this paper, we propose and demonstrate a convenient and flexible approach for preparation large-area of photonic crystals (PhCs) structures on the GaN-based LED chip. The highly-ordered porous anodic alumina (AAO) with pitch of wavelength scale was adopted as a selective dry etching mask for PhCs-pattern transfer. The PhCs with different pore depths were simultaneously formed on the entire surfaces of GaN-based LED chip including ITO, GaN surrounding contacts and the sidewall of the mesa by one-step reactive ion etching (RIE). The light output power improvement of PhCs-based GaN LED was achieved as high as 94% compared to that of the conventional GaN-based LED.

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.

Light transmission from the large-area highly ordered epoxy conical pillar arrays and application to GaN-based light emitting diodes

To improve the light transmission from the surface, we report a facile and cost-effective approach for the formation of wavelength-scale conical pillar arrays on a large surface area of epoxy resin. The highly ordered epoxy conical pillar arrays with a pitch of about 460 nm and height of about 800 nm have been successfully fabricated by the technique of shape-controlled anodization of Al foil followed by hot embossing. By replicating the tapered pore arrays onto a transparent semi-cylindrical epoxy structure, the incident angular resolved light transmission of the epoxy conical pillar arrays has been obtained. The integrated transmission of conical pillar arrays as high as 62.2% has been achieved which is confirmed to be 223% and 11.3% higher than that of planar epoxy and the cylindrical pillar arrays, respectively. It is reasonable to consider the wavelength-scale conical pillar array as a particular multilayer consisting of a series of two-dimensional photonic crystals with gradually increasing filling factor towards the surface. It can therefore be treated as a multilayer with continuously reducing refractive index towards the air. The conical and cylindrical pillar arrays of epoxy have been directly employed as the encapsulant of a GaN based flip-chip LED. Compared to the LED encapsulated by planar epoxy, the enhancement of light extraction from the LED covered with conical and cylindrical pillar arrays have been demonstrated to be 46.8% and 34.9%, respectively.

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 of top ITO nano-gratings on GaN LEDs

This study investigates the effect of nano-scale ITO cone gratings on the light extraction efficiency of GaN LEDs using FDTD analysis. First, we show the existence of a standing wave interference pattern in the region between the MQW layer and the LED surface for the non-grating cases. Then we use this knowledge to determine an ideal ITO material thickness above the MQW region to maximize light extraction at the LED surface. Optimizing the ITO layer thickness allowed us to improve the light extraction efficiency by 22% between the best and worst non-grating cases. We also found that when nano-scale gratings were implemented onto an optimal ITO layer thickness, they improved light extraction up to 27% over the no grating case and up to 30% over the no grating, no ITO LED. When a worst case ITO layer thickness is used, the gratings are shown to improve light extraction efficiency by up to 48% over the no grating case, and up to 70% over the no grating, no ITO LED.

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.

Light extraction improvement of GaN LEDs using nano-scale top transmission gratings

In this paper, we use a Finite-Difference Time-Domain GaN LED model to study constant wave (CW) average power of extracted light. The structure simulated comprises of a 200nm-thick p-GaN substrate, 50nm-thick MQW, 400nm-thick n-GaN substrate, and a 200nm n-GaN two-dimensional Photonic Crystal(2PhC) grating. We focus on optimizing three design parameters: grating period (A), grating height (d), and fill factor (FF). In the primary set of simulations, we fix the fill factor at 50% and simulate ten different grating periods (100 to 1000nm in steps of 100nm) and four different grating heights (50 to 200nm in steps of 50nm), and calculate the average power output of the device. The results from these simulations show that for both conical and cylindrical gratings, the maxmium light extraction improvement occurs when A =100nm. In the second set of simulations, we maintain a constant grating period A = 100nm and sweep the fill factor from 25 to 75%. The results of these simulations show that the fill factor affects clyindrical and conical gratings differently. As a whole, we see that the nano-structure grating mostly depends on period, but also depends on height and fill factor. The grating structure improves light extraction in some cases, but not all.

Simulation of nanoscale ITO top grating of GaN LED

Today’s advanced technology allows engineers to fabricate GaN LEDs with various heights, widths, shapes, and materials. Total internal reflection is a key factor in GaN LED design, because all light that is created inside the LED is lost unless it approaches the chip to air interface at an angle less than 23.58° with respect to the normal. The narrow range of angles at which light can successfully escape the chip is a result of the large difference in refractive indices between GaN and air. Adding a layer of ITO to the GaN reduces the difference in refractive indices between steps and increases the critical angle to 28.4°. Transmitting from ITO into epoxy reduces this difference in refractive indices again, bringing the critical angle to 47.9°. Because a higher critical angle should allow more light to escape the LED, we focus on enhancing light extraction efficiency of GaN LEDs that utilize an ITO to epoxy interface using FDTD simulations. The simulation results show us that increasing the critical angle to 47.9° improves light extraction by 40%, proving that the critical angle does play a significant role in light extraction. From this initial result, we then compare light extraction efficiencies of ITO and GaN gratings over varied grating periods, and show that adding an Ag reflection layer improves overall efficiency. Finally, we show that the light extraction for LEDs utilizing an Ag reflection layer is highly dependent on the sapphire substrate thickness.

Transmission efficiency study of grating layer location of a GaN nano-grated structure

This is a study regarding nano-grating structure location of GaN LEDs. A 2D model of a nano-grating GaN LED is developed and used for simulations. The analyses focus on enhancement of light extraction efficiency (LEE) through the surface grating location optimization in relation to the Multi-quantum well (MQW) source region. By studying the effects of source location on the total light transmission across the GaN surface-air interface, we find that, as the location of the grating surface increases, or in other words the distance between the grating surface and the source increases, there is a general improvement in the enhancement of the transmission. However, compared to the non-grating case, some locations can improve the LEE by up to 133%, while the other locations may reduce LEE. We find that optimizing location increases invariance of transmission to the surface grating period.