High Light Extraction Efficiency Research Articles

Enhancing the light extraction efficiency of AlGaN LED with nanowire photonic crystal and graphene transparent electrode

Abstract The performance of AlGaN deep ultraviolet(UV) light emitting diodes(LEDs) has been limited by the extremely low light extraction efficiency(LEE), which is due to the severe light absorption by metal electrode and unique transverse magnetic polarization. Here, we propose an ordered nanowire array structure UV-LEDs with transparent graphene electrode. By using the finite-difference time domain simulation, the relationship between LEE and different nanowire structures (including ordered and disordered nanowires array) was studied. We demonstrate that ordered nanowire arrays are better than disordered nanowires in improving the LEE. A high LEE about 80% can, in principle, be expected from top surface with optimized nanowire radius and spacing. In addition, by studying the distribution of electric field intensity on the side and top of UV-LEDs with graphene electrode and Ni/Au electrode, we show that the graphene electrode can reduce the absorption of ultraviolet light and improve LEE more effectively than the conventional Ni/Au electrode. This proposed structure could be a promising candidate for high light extraction efficiency UV-LEDs.

Design rules for white light emitters with high light extraction efficiency.

The finite-difference time-domain method is employed to study the light extraction efficiency of white light emitters. The cone arrays designed on top of white light emitters eliminate the dependency of light extraction on the wavelength and the cavity thickness and that leads to significant enhancement in light extraction efficiency for whole visible light spectrum. The light extraction efficiency of 81% has been achieved. Most importantly, the high extraction efficiency is achieved for the whole visible spectrum from 400 nm to 700 nm. This work will provide guidelines for designing highly efficient white light emitters for general illumination and display purpose.

Angular color shift of micro-LED displays.

Sidewall emission of a micro-scale light emitting diode (micro-LED) improves the light extraction efficiency, but it causes mismatched angular distributions between AlGaInP-based red micro-LED and InGaN-based blue/green counterparts due to material difference. As a result, color shift of RGB micro-LED displays may become visually noticeable. To address this issue, we first analyze the angular distributions of RGB micro-LEDs and obtain good agreement between simulation and experiment. Next, we propose a device structure with top black matrix and taper angle in micro-LEDs, which greatly suppresses the color shift while keeping a reasonably high light extraction efficiency.

Improved Output Power Behavior of Ultraviolet GaN-Based Light-Emitting Diode By Using Agpdcu Reflector

GaN-based ultraviolet UVLEDs have attracted a great deal of interest because of their applications in water and air purification and solid-state lighting. These applications have stimulated the technological development of high-efficiency UVLEDs. On the one hand, UVLEDs exhibit relatively lower external quantum efficiencies (EQEs) than visible LEDs. In particular, high-power UVLEDs should have high light extraction and heat dissipation efficiencies. For the flip-chip (FC) LEDs, the fabrication of high-performance p-type reflector with low contact resistance and high reflectivity is reqyired. Silver (Ag) is a commonly adopted p-type reflector for near UV LEDs but suffers from thermal instability, namely, agglomerated when annealed in air above 300 °C. In addition, electromigration (EM) is one of the major failure mechanisms of electronic interconnect under high current density (> ~105 A/cm2) stressing in microelectronics devices. In this talk, we investigated the opto-electrical properties, and electromigration reliability of a Ag-Pd-Cu alloy (APC) reflector for near UV (385 nm)-LEDs as a function of temperature and compared with those of Ag only reflector. SEM results revealed that the Ag samples were less stable than the APC samples upon annealing. The contact resistivities of the Ag and APC contacts annealed at 500 °C were measured to be 2.59 × 10– 4 and 1.85 × 10–4 Wcm2, respectively. The Ga 2p core level for the APC sample shifted towards the lower binding-energies by 0.67 eV as compared to that of the as-deposited sample. The forward I-V properties of near UV LEDs fabricated with the Ag and APC reflectors were observe to be similar each other. Notably, the UV LEDs with the APC reflector produced 9.04% higher output at 100 mA than those with the Ag reflector. To investigate the device reliability, the EM behaviour of Ag and APC films were also characterised. The APC samples exhibited better EM resistance than the Ag samples. The APC samples yielded higher activation energies of the EM failures than the Ag sample. The better EM behaviour of the APC samples will be described and discussed on the basis of XRD pole figure and electron back scatter diffraction, phi scan, and inverse pole figures (IPFs) examination results.

Concurrent Inhibition and Redistribution of Spontaneous Emission from All Inorganic Perovskite Photonic Crystals

All inorganic cesium lead halide perovskite semiconductors exhibit great potential for nanolasers, light-emitting diodes, and solar cells, because of their unique properties including low threshold, high quantum efficiency and low cost. However, the high material refractive index of perovskite semiconductors hinders light extraction efficiency for photonic and illumination applications. In this paper, we demonstrate high light extraction efficiency achieved in CsPbBr2.75I0.25 two-dimensional photonic crystals. The perovskite photonic crystals exhibit both emission rate inhibition and light energy redistribution simultaneously. We observed a 7.9-fold reduction of spontaneous emission rate with a slower decay in CsPbBr2.75I0.25 photonic crystals, because of the photonic bandgap effect (PBG). We also observed a 23.5-fold PL emission enhancement, as a result of light energy redistribution from 2D guided modes to vertical direction in perovskite photonic crystals thin films, indicating a high intrinsic light e...

Efficient metal halide perovskite light-emitting diodes with significantly improved light extraction on nanophotonic substrates

Metal halide perovskite has emerged as a promising material for light-emitting diodes. In the past, the performance of devices has been improved mainly by optimizing the active and charge injection layers. However, the large refractive index difference among different materials limits the overall light extraction. Herein, we fabricate efficient methylammonium lead bromide light-emitting diodes on nanophotonic substrates with an optimal device external quantum efficiency of 17.5% which is around twice of the record for the planar device based on this material system. Furthermore, optical modelling shows that a high light extraction efficiency of 73.6% can be achieved as a result of a two-step light extraction process involving nanodome light couplers and nanowire optical antennas on the nanophotonic substrate. These results suggest that utilization of nanophotonic structures can be an effective approach to achieve high performance perovskite light-emitting diodes.

Improvement in Light Output of Ultraviolet Light-Emitting Diodes with Patterned Double-Layer ITO by Laser Direct Writing.

A patterned double-layer indium-tin oxide (ITO), including the first unpatterned ITO layer serving as current spreading and the second patterned ITO layer serving as light extracting, was applied to obtain uniform current spreading and high light extraction efficiency (LEE) of GaN-based ultraviolet (UV) light-emitting diodes (LEDs). Periodic pinhole patterns were formed on the second ITO layer by laser direct writing to increase the LEE of UV LED. Effects of interval of pinhole patterns on optical and electrical properties of UV LED with patterned double-layer ITO were studied by numerical simulations and experimental investigations. Due to scattering out of waveguided light trapped inside the GaN film, LEE of UV LED with patterned double-layer ITO was improved as compared to UV LED with planar double-layer ITO. As interval of pinhole patterns decreased, the light output power (LOP) of UV LED with patterned double-layer ITO increased. In addition, UV LED with patterned double-layer ITO exhibited a slight degradation of current spreading as compared to the UV LED with a planar double-layer ITO. The forward voltage of UV LED with patterned double-layer ITO increased as the interval of pinhole patterns decreased.

Enhancement of light extraction efficiency in the GaN-based light-emitting diodes by selective growth of ZnO nanorods

We demonstrate high light extraction efficiency of GaN-based light-emitting diodes (LEDs) using the selective growth of ZnO nanorods (NRs). The chemical wet-etched hexagonal patterns of ZnO transparent conductive film were used to achieve the selective growth of ZnO NRs, which could be grown at the non-etched hexagonal pattern regions due to the wet-etching effect. The optical transmittance of GaN-based LEDs was decreased by forming additional different types of ZnO transparent conductive films, such as the hexagonal ZnO patterns and the ZnO NRs selectively grown on hexagonal ZnO patterns. However, the light output power of GaN-based LEDs increased on decreasing the optical transmittance by employing a hexagonal ZnO transparent conductive film and selectively grown ZnO NRs. Based on these results, we suggest that the combined structure of the surface hexagonal patterns and the selectively grown ZnO NRs decreases the optical transmittance, but is more effective in improving the light extraction efficiency in the GaN-based LEDs.

High-Performance Green Flip-Chip LEDs with Double-Layer Electrode and Hybrid Reflector

High-performance green flip-chip light-emitting diode (FC-LED) was realized by combining double-layer electrode and highly reflective hybrid reflector. The introduction of double-layer electrode improved uniformity of current spreading over the entire active region. The highly reflective hybrid reflector, composed of SiO2 total internal reflection (TIR) layer and Nb2O5/SiO2 distributed Bragg reflector (DBR), offer higher reflectance and wider bandwidth compared to conventional DBR. Numerical simulation and experimental results indicated that the green FC-LED has advantages of uniform current spreading and high light extraction efficiency (LEE) compared to green top-emitting LED (TE-LED). Due to uniform current spreading, the forward voltage of the green FC-LED is 0.28 V lower than that of the TE-LED at 20 mA. Compared to TE-LED, the light emission intensity of the green FC-LED at 20 mA is enhanced by 18% due to the higher reflectance of hybrid reflector and increased utilization ratio of active region area caused by double-layer electrode structure.

Design of uniformly collimating light-emitting diode lenses of multiple freeform and Fresnel surfaces with different inter-surface mapping orders.

In this work, we propose a design method of composite freeform collimating lenses that integrates freeform reflection, refraction, and Fresnel surfaces to realize highly collimated and uniformed light output in a single lens element. The algorithm is designed considering a consistent combination of different surfaces, so the light rays reaching the Fresnel surface through refraction and reflection should generate the same luminous intensity. The Fresnel rings can be designed in two different ways, depending on the sequence of light rays propagating between the reflection and the Fresnel surfaces. The lighting effects of the lenses are analyzed in a 3D simulation, and the light output of both schemes reaches a high level of collimation and uniformity, agreeing well with design expectations. The obtained lenses have the advantages of a compact volume, high light extraction efficiency, and variable geometry by changing the combination form of the freeform surfaces.

A Vision toward Ultimate Optical Out-Coupling for Organic Light-Emitting Diode Displays: 3D Pixel Configuration.

Despite stringent power consumption requirements in many applications, over years organic light‐emitting diode (OLED) displays still suffer unsatisfactory energy efficiency due to poor light extraction. Approaches have been reported for OLED light out‐coupling, but they in general are not applicable for OLED displays due to difficulties in display image quality and fabrication complexity and compatibility. Thus to date, an effective and feasible light extraction technique that can boost efficiencies and yet keep image quality is still lacking and remains a great challenge. Here, a highly effective and scalable extraction‐enhancing OLED display pixel structure is proposed based on embedding the OLED inside a three‐dimensional reflective concave structure covered with a patterned high‐index filler. It can couple as much internal emission as possible into the filler region and then redirect otherwise confined light for out‐coupling. Comprehensive multi‐scale optical simulation validates that ultimately high light extraction efficiency approaching ≈80% and excellent viewing characteristics are simultaneously achievable with optimized structures using highly transparent top electrodes. This scheme is scalable and wavelength insensitive, and generally applicable to all red, green, and blue pixels in high‐resolution full‐color displays. Results of this work are believed to shed light on the development of future generations of advanced OLED displays.

A facile method for bright, colour-tunable light-emitting diodes based on Ga-doped ZnO nanorods

Bottom-up fabrication of nanowire-based devices is highly attractive for oxide photonic devices because of high light extraction efficiency; however, unsatisfactory electrical injection into ZnO and poor carrier transport properties of nanowires severely limit their practical applications. Here, we demonstrate that ZnO nanorods doped with Ga donors by in situ dopant incorporation during vapour–solid growth exhibit superior optoelectronic properties that exceed those currently synthesised by chemical vapour deposition, and accordingly can be electrically integrated into Si-based photonic devices. Significantly, the doping method was found to improve the nanorod quality by decreasing the concentration of point defects. Light-emitting diodes (LEDs) fabricated from the Ga-doped ZnO nanorod/p-Si heterojunction display bright and colour-tunable electroluminescence (EL). These nanorod LEDs possess a dramatically enhanced performance and an order of magnitude higher EL compared with equivalent devices fabricated with undoped nanorods. These results point to an effective route for large-scale fabrication of conductive, single-crystalline ZnO nanorods for photonic and optoelectronic applications.

High Light Extraction Efficiency of Deep Ultraviolet LEDs Enhanced Using Nanolens Arrays

To improve the light extraction efficiency (LEE) of AlGaN-based deep ultraviolet LEDs (DUV-LEDs) by simple and effective method, this has greatly attracted ever-growing attention in DUV-LEDs field. Here, nanolens arrays (NLAs) fabricated by nanophotolithography and wet-etched technique is proposed to improve the LEE of DUV-LEDs, and its theoretical feasibility is verified by the Monte Carlo Ray-Trace method and finite element analysis which show an obvious improvement of light and electric field distribution benefit from NLAs structure. By controlling the time of wet etching, the effect of the uniform patterns of NLAs with adjustable morphology on the LEE of DUV-LEDs is systematically studied. Compared with the sandwiched flat lens, experimental results show that the obviously enhancements of the light output power are achieved by using the proposed NLAs to be 13.0%, 21.1%, 24.7%, 13.0%, and 11.5%, respectively, corresponding to their radius of 325, 340, 350, 360, and 400 nm, under the driving current of 300 mA. Therefore, the highest light output power of DUV-LEDs is used the optimized NLAs with radius of 350 nm, and its emission angle also shows the largest improvement of ~14°, suggesting the best LEE.

31.3: Low Optical Crosstalk Micro‐LED Micro‐Display with Semi‐Sphere Micro‐Lens for Light Collimation

A 400×240 monochromic micro‐LED micro‐display based on GaN vertical‐LED structure is presented. The micro‐display has the pixel pitch of 30 μm and the size of micro‐LEDs is only 8 μm. It is realized by bonding the GaN epi with the pre‐defined anode pad array to the specific pixel driver on the IC backplane, followed by laser lift‐off and ICP etch process to form isolated micro‐LED pixel array. The electrical connection of n‐type GaN on micro‐LEDs is done by a layer of metal‐grid which serves as a common cathode. Comparing with conventional lateral micro‐LED structure, the vertical‐LED architecture used in our micro‐LED micro‐display is more advantageous of achieving higher device performance such as better current injection, higher light extraction efficiency, low light crosstalk and is more capable of producing compact micro‐LED arrays. In addition, wafer‐level processed semi‐sphere micro‐lens array is also integrated into the micro‐display with each micro‐lens coupled to a single micro‐LED pixel. The micro‐lenses can effectively collimate divergent light emitted from micro‐LEDs leading to a significant improvement of projection brightness when used in a projection light engine. The self‐emissive micro‐LED micro‐display shows its great potentials for projectors and many other applications.

Influence of grating parameters on the performance of a high-power blue external-cavity semiconductor laser.

The influence of grating parameters, including the diffraction efficiency and ruling density, on the performance of high-power blue external-cavity (EC) semiconductor lasers has been studied. We find that for the Littrow-type EC laser with a zeroth-order diffracted beam as a coupling output, the high zeroth-order diffraction efficiency benefits the high external light extraction efficiency, and the high first-order diffraction efficiency contributes to the low threshold current and the wide wavelength tuning bandwidth. By using a grating with a zeroth-order diffraction efficiency of 76% and a first-order diffraction efficiency of 11.5%, the external light extraction efficiency can reach up to 80% with an output power of more than 1200mW and a tuning bandwidth of 3.6nm. When a grating with a first-order diffraction efficiency of 58.5% and a zeroth-order diffraction efficiency of 28.6% is used, the threshold current is reduced by 46% at the central Fabry-Perot resonance wavelength. Moreover, a tuning bandwidth of 8.3nm ranging from 442.2nm to 450.5nm with an output power of more than 500mW is achieved. Besides, we also find that the high ruling density is helpful in obtaining narrow lasing linewidth, especially for the EC laser system injected with low current. By using a grating with a ruling density of 2400grooves/mm, the spectral linewidth of the EC laser can be narrowed to be 0.02nm with an operating current of 300mA. As the injection current is increased, the lasing linewidth is little affected by the ruling density of the grating due to the broadened feedback spectrum by the flattening of gain spectrum. The results above shed light on the improvement of the performance of EC lasers by selecting appropriate parameters for the grating.

高光提取效率倒装发光二极管的设计与优化

高光提取效率倒装发光二极管的设计与优化

Molecular Beam Epitaxial Growth and Device Characterization of AlGaN Nanowire Ultraviolet-B Light-Emitting Diodes

We report on the design and fabrication of high performance AlxGa1−xN nanowire ultraviolet (UV) light-emitting diodes (LEDs) on silicon substrate by molecular beam epitaxy. The emission wavelength and surface morphology of nanowires can be controlled by varying the growth parameters that include substrate temperatures and/or Aluminum/Gallium flux ratios. The devices exhibit excellent current-voltage characteristics with relatively low resistance. Such nanowire LEDs generate strong emission in the UV-B band tuning from 290 nm to 330 nm. The electroluminescence spectra show virtually invariant blue-shift under injection current from 50 mA to 400 mA, suggesting the presence of a negligible quantum-confined Stark effect. Moreover, we have shown that, the AlGaN nanowire LEDs using periodic structures, can achieve high light extraction efficiency of ~ 89% and 92% for emissions at 290nm and 320nm, respectively. The randomly arranged nanowire 290 nm UV LEDs exhibit light extraction efficiency of ~ 56% which is higher compared to current AlGaN based thin-film UV LEDs.

Surface microstructure parameter optimisation for enhancing light extraction efficiency of LED

Surface structure modification is an effective way to enhance light extraction of LED, which is limited by total internal reflection on the interfacial surface. The periodic surface structure manufactured by micromachining was proved to be a feasible way to enhance the light extraction efficiency (LEE) of LEDs. In order to obtain the highest LEE, need to optimise the structural parameters. In this paper, a simplified ray tracing optical model was built to find out a high performance microstructure parameters by simulation. Then, a mathematical model was built to further optimise the optical simulation results, obtained the optimal microstructure parameters for LEE. The performance of optimal microstructure was also studied by optical simulation.

High wall-plug efficiency blue III-nitride LEDs designed for low current density operation.

Commercial LEDs for solid-state lighting are often designed for operation at current densities in the droop regime (~35 A/cm2) to minimize costly chip area; however, many benefits can be realized by operating at low current density (J ≈1 - 5 A/cm2). Along with mitigation of droop losses and reduction of the operating voltage, low J operation of LEDs opens the design space for high light extraction efficiency (LEE). This work presents detailed ray tracing simulations of an LED design for low J operation with LEE ≈94%. The design is realized experimentally resulting in a peak wall-plug efficiency of 78.1% occurring at 3.45 A/cm2 and producing an output power of 7.2 mW for a 0.1 mm2 emitting area. At this operation point, the photon voltage Vp=hνq exceeds the forward voltage (V), corresponding to a Vp/V = 103%.

Method for enhancing the favored transverse-electric-polarized emission of an AlGaN deep-ultraviolet quantum well.

An AlGaN quantum well (QW) structure of a deep-ultraviolet (UV) light-emitting diode (LED) needs to be well designed for controlling its band structure such that the heavy-hole (HH) band edge becomes lower than the split-off (SO) band edge and hence the transverse-electric (TE) polarization dominates the emission for achieving a higher light extraction efficiency. Here, we report the discovery of un-intentionally formed high-Al AlGaN nano-layers right above and below such a QW and their effects on the QW for changing the relative energy levels of the HH and SO bands. The comparison between the results of simulation study and polarization-resolved photoluminescence measurement confirms that the high-Al layers (HALs) represent the key to the observation of the dominating TE-polarized emission. By applying a stress onto a sample along its c-axis to produce a tensile strain in the c-plane for counteracting the HAL effects in changing the band structure, we can further understand the effectiveness of the HALs. The formation of the HALs is attributed to the hydrogen back-etching of Ga atoms during the temperature transition from quantum barrier growth into QW growth and vice versa. The Al filling in the etched vacancies results in the formation of an HAL. This discovery brings us with a simple method for enhancing the favored TE-polarized emission in an AlGaN deep-UV QW LED.