Light Extraction Of Light-emitting Diodes Research Articles

Optimization on the design of nano-patterned ZnS:Cu LED surface using FDTD simulation

Communication technology is one of the most important parts of human history and is fast developing. Visible Light Communication (VLC) can be categorized as a Light Fidelity (Li-Fi) communication that uses visible light through a Light Emitting Diode (LED) to transfer data and information. Even though LEDs are considered as very good light sources, they also have problems with effectiveness, which is influenced by their surface structures. The LEDs must also be able to focus the transmitted data and greatly reduce the signal-to-noise ratio. One of the problems encountered with LEDs is the presence of Total Internal Reflection (TIR) due to the large difference in refractive index between the LED and air, which causes photons to be trapped in the LED. Some trapped photons' energy may change into heat, thus reducing the LED's Light Extraction Efficiency (LEE). Using nanopatterns on the surface of LEDs is one way to reduce TIR on LEDs and enhance photon extraction from LEDs. Many parameters can influence the performance of nanopatterns on LEDs, so modelling efforts are needed before fabrication to save time and cost. Ansys Lumerical FDTD can be used to simulate and model the effects of nanopatterns on LED surfaces on LEE and beam focusing effect. In this study, simulations were carried out using Ansys Lumerical FDTD with variations in nanopattern shape parameters in the form of grating, blaze grating, triangular grating, and hemisphere. Apart from that, variations were also made to the height and width parameters of the grating to see the effect of these parameters on the efficiency of the LED. The FDTD simulation codes were validated using the Ansys database. The optimization results show that the most optimum shape is the grating nanopattern with a width of 216 nm and a height of 300 nm, which produces a peak wavelength of 480 nm in the far field pattern and has the highest increase in the LEE of 300 times in ±15° and 5500 times in ±5°. The huge spike in LEE enhancement at ±5° indicates that the nanopattern caused a focusing effect. The simulation results were compared using previous experimental data of Fabricated ZnS:Cu LED. It is shown that the simulation results are in line with the experimental data. The result shows that the simulation is very useful in designing the nano-patterned ZnS:Cu LED surface to achieve the best performance in the LEE and LED focusing effect for a specific application such as the VLC.

Design and fabrication of photonic crystal structures by single pulse laser interference lithography

Photonic crystal (PhC) structures formed by periodic surface nanostructuring have emerged as pivotal elements for controlling light-matter interactions. One important application is reducing losses due to the high surface reflectivity of semiconductor optoelectronic devices, such as enhancing light absorption in photovoltaic cells or improving light extraction in light-emitting diodes (LEDs). Although various methods for fabricating such structures have been documented, the utilization of single pulse laser interference lithography (LIL) using commercial photoresist and its subsequent effective use as an etch mask has not been previously reported. Rapid exposure of photoresists with single nanosecond pulses offers benefits for high throughput patterning and reduces the requirement for a stable optical platform. We have successfully employed single pulse LIL to fabricate antireflective PhC structures on GaAs substrates using a commercial photoresist. Exposure is performed with single 7 ns 355 nm pulses of relatively low energy (<10 mJ). High-quality nanohole arrays of pitch of approximately 365 nm are fabricated and depths up to 400 nm have been etched using inductively coupled plasma (ICP) through the exposed photoresist mask. Reflectivity analyses confirmed that these structures reduce the average reflectance of the GaAs to below 5 % across the 450 nm to 700 nm visible wavelength range. The fabrication of PhC structures using this approach has potential for low-cost wafer-level patterning to provide improved light extraction in LEDs and enhanced light trapping in solar cells.

Enhanced light extraction of light-emitting diodes with micro patterns by femtosecond laser micromachining for visible light communication.

A significant enhancement of light extraction of light-emitting diodes (LEDs) with micro patterns has been experimentally investigated. The micro patterns on the surface of a polymer layer are fabricated by a femtosecond laser Bessel beam for obtaining microhole arrays with large depth, resulting in the reduction of photon loss by total internal reflection (TIR) at the surface of the LED. The light output power of the LED is apparently increased by introducing the array patterns without influencing its current-voltage (I-V) characteristics. Moreover, the electroluminescence spectra of a multi-color LED and its angular radiation profiles with orthogonal and hexagonal patterns also have been explored. In addition, the optical field distributions of the micro patterns simulated by the finite difference time domain method have expressed the modulation effect of the array depth. Finally, the patterned LED as a transmitter is embedded in the visible light communication system for evaluating the transmission signal quality.

Subwavelength-scale nanorods implemented hexagonal pyramids structure as efficient light-extraction in Light-emitting diodes

Subwavelength-scale nanorods were implemented on the hexagonal pyramid of photochemically etched light-emitting diodes (LEDs) to improve light extraction efficiency (LEE). Sequential processes of Ag deposition and inductively coupled plasma etching successfully produce nanorods on both locally unetched flat surface and sidewall of hexagonal pyramids. The subwavelength-scale structures on flat surface offer gradually changed refractive index, and the structures on side wall of hexagonal pyramid reduce backward reflection, thereby enhancing further enhancement of the light extraction efficiency. Consequently, the nanorods implemented LED shows a remarkable enhancement in the light output power by 14% compared with that of the photochemically etched LEDs which is known to exhibit the highest light output power. Theoretical calculations using a rigorous coupled wave analysis method reveal that the subwavelength-scale nanorods are very effective in the elimination of TIR as well as backward reflections, thereby further enhancing LEE of the LEDs.

Enhancing GaN LED Efficiency through Nano-Gratings and Standing Wave Analysis.

Based on our recent work, this paper reviews our theoretical study on gallium nitride (GaN) light-emitting-diode (LED). The focus of the paper is to improve LED light extraction efficiency through various nano-grating designs. The gratings can be designed at different locations, such as at the top, the middle, and the bottom, on the LED. They also can be made of different materials. In this study, we first present a GaN LED error-grating simulation model. Second, nano Indium Tin Oxide (ITO) top gratings are studied and compared with conventional LED (CLED) using standing wave analysis. Third, we present results related to a patterned sapphire substrate (PSS), SiO2 Nanorod array (NR), and Ag bottom reflection layer. Finally, we investigate the nano-top ITO grating performance over different wavelengths to validate our design simulation, which focusing on a single wavelength of 460 nm.

(Invited) The Effects of Zinc-Tin-Oxide Nanosphere Monolayer on Improvement of Light Extraction in Light-Emitting Diodes

Over the last two decades, III-nitride-based light-emitting diodes (LEDs) have been widely used for our daily life, such as illumination, various backlight units for full-color displays, and automotive headlamps. In spite of the versatile advantages of GaN-LEDs, relatively low external quantum efficiency is still a problem to expand their application fields. For the viable solution to unravel pending issues in solid state light-emitting research fields, many efforts have been conducted. In this study, we report on the applications of zinc-tin-oxide (ZTO) nanoparticles and monolayer as a light extraction layer for GaN-based LEDs. The ZTO layers formed on top of an LED epi-structure with a variable Sn-ratio, which was deposited by the spin coating method. The transmission spectra of the ZTO layers with Zn to Sn ratios of 1:1 (ZTO-I) and 1:5 (ZTO-II) exhibited optical transmittances of 98% and 88% in the visible region, respectively. The electroluminescence (EL) and light power-current-voltage (L-I-V) measurements show that double ZTO layers consisting of various Zn to Sn ratios led to enhanced light extraction from blue LEDs. Finally, we will discuss the effects of ZTO monolayer transferred onto the p-GaN surface on the external quantum efficiency of LEDs. The ZTO monolayers were prepared by using the ZTO nanospheres synthesized by precipitation solution method. The transferred ZTO nanosphere monolayers on top surface of LED are very effective way to improve the light extraction efficiency.

Improving light extraction in light-emitting diodes using zinc-tin-oxide layers

In this study, we report on the synthesis and applications of zinc-tin-oxide (ZTO) as a light extraction layer for GaN-based light-emitting diodes (LED). The ZTO layers formed on top of an LED epi-structure with a variable Sn-ratio, which was deposited by the spin coating method. The transmission spectra of the ZTO layers with Zn to Sn ratios of 1:1 (ZTO-I) and 1:5 (ZTO-II) exhibited optical transmittances of 98% and 88% in the visible region, respectively. The electroluminescence (EL) and light power-current-voltage (L-I-V) measurements show that double ZTO layers consisting of various Zn to Sn ratios led to enhanced light extraction from blue LEDs. The improvement of light extraction in the LEDs can be attributed to the gradually reduced refractive index of the ZTO layers, which enlarges the photon escape cone and minimizes the Fresnel refraction loss at the interface between air and GaN.

Enhancing radiative energy transfer through thermal extraction

Abstract Thermal radiation plays an increasingly important role in many emerging energy technologies, such as thermophotovoltaics, passive radiative cooling and wearable cooling clothes [1]. One of the fundamental constraints in thermal radiation is the Stefan-Boltzmann law, which limits the maximum power of far-field radiation to P0 = σT4S, where σ is the Boltzmann constant, S and T are the area and the temperature of the emitter, respectively (Fig. 1a). In order to overcome this limit, it has been shown that near-field radiations could have an energy density that is orders of magnitude greater than the Stefan-Boltzmann law [2-7]. Unfortunately, such near-field radiation transfer is spatially confined and cannot carry radiative heat to the far field. Recently, a new concept of thermal extraction was proposed [8] to enhance far-field thermal emission, which, conceptually, operates on a principle similar to oil immersion lenses and light extraction in light-emitting diodes using solid immersion lens to increase light output [62].Thermal extraction allows a blackbody to radiate more energy to the far field than the apparent limit of the Stefan-Boltzmann law without breaking the second law of thermodynamics.Thermal extraction works by using a specially designed thermal extractor to convert and guide the near-field energy to the far field, as shown in Fig. 1b. The same blackbody as shown in Fig. 1a is placed closely below the thermal extractor with a spacing smaller than the thermal wavelength. The near-field coupling transfers radiative energy with a density greater than σT4. The thermal extractor, made from transparent and high-index or structured materials, does not emit or absorb any radiation. It transforms the near-field energy and sends it toward the far field. As a result, the total amount of far-field radiative heat dissipated by the same blackbody is greatly enhanced above SσT4, where S is the area of the emitter. This paper will review the progress in thermal extraction. It is organized as follows. In Section 1, we will discuss the theory of thermal extraction [8]. In Section 2, we review an experimental implementation based on natural materials as the thermal extractor [8]. Lastly, in Section 3, we review the experiment that uses structured metamaterials as thermal extractors to enhance optical density of states and far-field emission [9].

Surface Antireflection and Light Extraction Properties of GaN Microdomes

GaN microdomes were fabricated and measured as both an antireflection surface and a light extraction enhancement structure. The combination of self-assembled micro/nanosphere lithography and reactive ion etching process was used to fabricate GaN microdomes with different aspect ratios. ${\rm SiO}_{2} $ microspheres with diameters of 1000 and 500 nm, deposited on top of the GaN substrate using a dip-coating method, serve as the mask for the formation of GaN microdomes. The GaN microdome shapes and sizes were determined through control of the plasma etching conditions. The antireflection properties of the GaN microdomes with different sizes and shapes were characterized. Two different mechanisms were proposed to explain the surface reflection properties of incidence wavelength below and above GaN band gap, respectively. The trend shows that the surface reflection is reduced with the increase in the aspect ratio of the GaN microdomes for incidence wavelength above the band gap. For incidence wavelength below the band gap, the trend is totally different. Studies indicate that the microdomes are applicable not only as antireflection structures in solar cells but for enhancing light extraction in light-emitting diodes as well.

Core–shell GaN–ZnO moth-eye nanostructure arrays grown on a-SiO2/Si (1 1 1) as a basis for improved InGaN-based photovoltaics and LEDs

Self-forming, vertically-aligned, ZnO moth-eye-like nanoarrays were grown by catalyst-free pulsed laser deposition on a-SiO2/Si (111) substrates. X-Ray Diffraction (XRD) and Cathodoluminescence (CL) studies indicated that nanostructures were highly c-axis oriented wurtzite ZnO with strong near band edge emission. The nanostructures were used as templates for the growth of non-polar GaN by metal organic vapor phase epitaxy. XRD, scanning electron microscopy, energy dispersive X-ray microanalysis and CL revealed ZnO encapsulated with GaN, without evidence of ZnO back-etching. XRD showed compressive epitaxial strain in the GaN, which is conducive to stabilization of the higher indium contents required for more efficient green light emitting diode (LED) and photovoltaic (PV) operation. Angular-dependent specular reflection measurements showed a relative reflectance of less than 1% over the wavelength range of 400–720nm at all angles up to 60°. The superior black-body performance of this moth-eye-like structure would boost LED light extraction and PV anti-reflection performance compared with existing planar or nanowire LED and PV morphologies. The enhancement in core conductivity, provided by the ZnO, would also improve current distribution and increase the effective junction area compared with nanowire devices based solely on GaN.

Enhancement of organic light-emitting diode light extraction by texturing PDMS layers

We have used different methods to produce a texture on the surface of polydimethylsiloxane layers. The layers were produced by spin coating on templates such as sandblasted and lithographed glass and sandpaper. The most striking result was obtained on layers templated on sandblasted glass surfaces, where an increase in luminous efficacy of 36% was observed.

Investigation on bandgap, diffraction, interference, and refraction effects of photonic crystal structure in GaN/InGaN LEDs for light extraction.

In this paper, we have made a clear differentiation among bandgap, diffraction, interference, and refraction effects in photonic crystal structures (PhCs). For observing bandgap, diffraction, and refraction effects, PhCs are considered on the top p-GaN surface of light emitting diodes (LEDs), whereas for interference effect, hole type PhCs are considered to be embedded within n-GaN layer of LED. From analysis, it is observed that at a particular lattice periodicity, for which bandgap lies within the wavelength of interest shows a significant light extraction due to inhibition of guided mode. Beyond a certain periodicity, diffraction effect starts dominating and light extraction improves further. The interference effect is observed in embedded photonic crystal LEDs, where depth of etching supports constructive interference of outward light waves. We have also shed light on refraction effects exhibited by the PhCs and whether negative refraction properties of PhCs may be useful in case of LED light extraction.

Enhanced light extraction of light-emitting diodes via nano-honeycomb photonic crystals

We present an effective light extraction scheme for GaN-based multi-quantum-well (MQW) light emitting diodes with periodic SiO2 nano-honeycomb arrays fabricated by natural lithography and reactive ion etching. The nano-honeycombs significantly boost light output by providing additional light extraction channels, not only guiding the internal modes into air but also alleviating the sever droop effect at high input power. At the driving current of 400mA, light output power through the nano-honeycombs is 77.8% higher than that of the bare device. In addition, the output power is particularly enhanced at the diffraction angle around 65°, which is attributed to the intensive first order diffraction on the honeycombs. Simulations based on finite-difference time-domain method are also carried out to reveal the field distribution across device interfaces.

Design of Wide-Bottomed Patterned Sapphire Substrates for Performance Improvement of GaN-Based Light-Emitting Diodes

We propose patterned sapphire substrates (PSSs) with wide-bottomed cone-shaped patterns to enhance the light extraction efficiency of GaN-based light-emitting diodes (LEDs). Simulation reveals that PSS with wide-bottomed cone-shaped patterns is of higher yield in LED light extraction efficiency. The subsequent fabrication of and epitaxial growth of LED structure on the proposed wide-bottomed PSS verify the simulation results experimentally. It is demonstrated that wide-bottomed patterns with narrow distance between patterns can effectively release stress, avoid the decline in crystal quality and improve light extraction efficiency. By comparison with the popular commercially available PSS, LED grown on the proposed PSS whose wide-bottomed patterns are of 3.91 μm in width, 2.37 μm in height and 0.09 μm in distance is proven to be of as high as 16.7% improvement in luminous efficacy and 33.8% drop in dislocation density, which shows the promising future for its application in the highly efficient LEDs.

Room-temperature larger-scale highly ordered nanorod imprints of ZnO film

Room-temperature large-scale highly ordered nanorod-patterned ZnO films directly integrated on III-nitride light-emitting diodes (LEDs) are proposed and demonstrated via low-cost modified nanoimprinting, avoiding a high-temperature process. with a 600 nm pitch on top of a critical 200 nm thick Imprinting ZnO nanorods of 200 nm in diameter and 200 nm in height continuous ZnO wetting layer, the light output power of the resulting integrated ZnO-nanorod-film/semi-transparent metal/GaN/InGaN LED shows a two-fold enhancement (100% light extraction efficiency improvement) at the injection current of 150 mA, in comparison with the conventional LED without the imprint film. The increased optical output is well explained by the enhanced light scattering and outcoupling of the ZnO-rod structures along with the wetting film, as verified by the numerical simulations. The wetting layer is found to be essential for better impedance matching. The current-voltage characteristics and electroluminescence measurements confirm that there is no noticeable change in the electrical or spectral properties of the final LEDs after ZnO-nanorod film integration. These results suggest that the low-cost high-quality large-scale ZnO-nanorod imprints hold great promise for superior LED light extraction.

Size-controllable nanopyramids photonic crystal selectively grown on p-GaN for enhanced light-extraction of light-emitting diodes

Size-controllable p-GaN hexagonal nanopyramids (HnPs)-photonic crystal (PhC) structures were selectively grown on flat p-GaN layer for the elimination of total internal reflection of light-emitting diodes (LEDs). The LEDs with HnPs-PhC of 46.3% bottom fill factor (PhC lattice constant is 730 nm) showed an improved light output power by 99.9% at forward current of 350 mA compared to the reference LEDs with flat p-GaN layer. We confirmed the effect of HnPs-PhC with different bottom fill factors and the effect of nanopyramid-shaped and nanocolumn-shaped PhC on the light-extraction of LEDs was also investigated by using three-dimensional finite-difference time-domain simulations.

Enhancing LED Light Extraction by Optimizing Cavity and Waveguide Modes in Grating Structures

An optimization of light extraction efficiency (LEE) from light-emitting diodes (LEDs) is provided. The optimization is achieved when both the cavity and waveguide modes are in their best conditions and their emitted waves interfere constructively in the grating-assisted structures. In other words, high LEE appears when the structural parameters results in good cavity modes in both planar and grating structures, and also when the propagation direction of the fundamental waveguide mode is significantly converted to the direction normal to the grating surface by the gratings. In this paper, we use two simple equations, one for cavity modes and the other for waveguide modes, to predict the loci of the optimal structural parameters of gratings. The prediction is verified with the rigorous coupled wave analysis (RCWA). We systematically present an investigation of the various structural effects of gratings on the LEE of the thin film vertical gallium-nitride (GaN) based LEDs. The results show that ultra-high LEE can be achieved when the equivalent cavity thickness is good for fundamental cavity mode, the source is positioned around antinodes for both cavity and waveguide modes, there is significant diffraction, and the fundamental waveguide mode is largely converted to the normal direction to the air, and the emitted waves from fundamental cavity and waveguide modes interfere constructively.

Surface-Roughened Light-Emitting Diodes: An Accurate Model

Surface roughening is frequently employed to increase light extraction from light-emitting diodes (LEDs), especially in the important case of III-Nitride LEDs. We explore the physics governing this scheme. We introduce a numerical model, based on solving Maxwell's equations, to accurately describe scattering by a roughened semiconductor interface. This model reveals the complex angular dependence of the scattering properties. We then couple this approach to an LED light extraction model and predict how surface roughness impacts light extraction. We focus on two important cases, thin-film LEDs and volumetric LEDs. We show that optical losses in the LED dictate light extraction, and that volumetric LEDs offer an opportunity for ultimate efficiency.

Improved AlGaInP vertical emitting light-emitting diodes using direct printing

In this study, we fabricated a high-brightness AlGaInP light-emitting diode (LED) using the direct printing technique and dry etching. In general, wet etching is used for surface roughening to improve the light extraction of AlGaInP red LEDs. However, a structure fabricated by wet etching has limited height and shows a tiled cone shape after the etching process due to the AlGaInP crystal structure. These limitations reduce the light extraction of the LED. As a result, we fabricated a perfectly cone-shaped pattern with high aspect ratio using direct printing by etching to maximize the LED light extraction efficiency. Compared to the red LED with a wet-etched structure, the patterning enhanced the light output power by 12% without electrical degradation. This enhanced light output power was maintained even after the packaging process.

An 83% enhancement in the external quantum efficiency of ultraviolet flip-chip light-emitting diodes with the incorporation of a self-textured oxide mask

We demonstrate here a 380-nm ultraviolet InGaN flip-chip (FC) light-emitting diode (LED) with self-textured oxide mask (STOM-FCLED) structures fabricated in a large-area (1125 × 1125 μm2) FC configuration. An 83% enhancement in the external quantum efficiency was achieved for the STOMFCLEDs when compared with FCLEDs without the STOM structure operating at an injection current of 350 mA. For STOM-FCLEDs operating at an injection current of 1000 mA, a light output of approximately 400 mW was obtained. These results could be attributed to the introduction of the STOM structure, which not only reduces the density of threading dislocation but also intensifies the LED light extraction.