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

Improved extraction efficiency of CsPbBr3 perovskite light-emitting diodes due to anodic aluminum oxide nanopore structure

In this work, we investigate the improvement in the performance of a CsPbBr₃ perovskite light-emitting diode (PeLED) due to an anodic aluminum oxide (AAO) nanopore structure. The AAO structure in the CsPbBr₃ PeLED structure can improve the light extraction efficiency of CsPbBr₃ PeLEDs in two ways: the emission light in the side direction being redirected to the normal direction due to the light scattering effect caused by aluminum oxide nanopores and the effective emission area as a result of the rough surface of the AAO structure. The peak luminance, current efficiency, and external quantum efficiency (EQE) were 11,460 cd/m², 2.03 cd/A, and 0.69% at a bias of 6.0 V, respectively. For comparison, the luminance, current efficiency, and EQE values of CsPbBr₃ PeLEDs with the AAO structure using 50 V of pore-expanding voltage demonstrated improvements of 282%, 190%, and 1280%, respectively, over CsPbBr₃ PeLEDs without the AAO structure.

Enhanced light extraction efficiency of an LED package by a surface-mounted amorphous photonic structure

In this study, we propose a low-cost, simple and feasible post-processing approach to improve the light extraction efficiency (LEE) of LED packages. Amorphous photonic structures (APSs) with only short-range order are fabricated from anodic aluminum oxide (AAO) and transferred to intermediate polymer stamp (IPS) by nanoimprint technology. The IPS with APSs is directly mounted onto the surface of an LED package, where the LEE is achieved as 94.6%. The scanning electron microscope (SEM) images of AAO templates and imprinted IPS are analyzed by radial distribution function and diameter histogram. The far-field patterns of APS-mounted LED packages are measured in electroluminescence (EL). The three-dimensional finite-difference time-domain (3D-FDTD) calculations of transmittance of APSs confirm that they improve the light extraction above the critical angle. Two-dimensional Fourier power spectra from SEM images of APSs are also calculated. The LEE enhancement is attributed to that the APSs have short-range order on a length scale comparable to emission wavelength of LED. We provide novel multistage simulations in a simplified FDTD model for the LED package. Finally, we discuss the influence of the morphology of APSs on the LEE of the APS mounted LEDs.

Light extraction from quantum dot light emitting diodes by multiscale nanostructures

Improving the light extraction efficiency by introducing optical-functional structures outside of quantum dot light emitting diodes (QLEDs) for further enhancing the external quantum efficiency (EQE) is essential for their application in display and lighting industries. Although the efficiency of QLEDs has been optimized by controlling the synthesis of the quantum dots, the low outcoupling efficiency is indeed unresolved because of total internal reflections, waveguides and metal surface absorptions within the device. Here, we utilize multiscale nanostructures attached to the outer surface of the glass substrate to extract the trapped light from the emitting layers of QLEDs. The result indicates that both the EQE and luminance are improved from 12.29% to 17.94% and 122 400 cd m^-2 to 178 700 cd m^-2, respectively. The maximum EQE and current efficiency improve to 21.3% and 88.3 cd A^-1, respectively, which are the best performances among reported green QLEDs with light outcoupling nanostructures. The improved performance is ascribed to the elimination of total internal reflection by multiscale nanostructures attached to the outer surface of the QLEDs. Additionally, the simulation results of the finite-difference time domain (FDTD) also demonstrate that the light trapping effect is reduced by the multiscale nanostructures. The design of novel light outcoupling nanostructures for further improving the efficiency of QLEDs can promote their application in display and lighting industries.

Strain modulated nanostructure patterned AlGaN-based deep ultraviolet multiple-quantum-wells for polarization control and light extraction efficiency enhancement

AlGaN-based deep ultraviolet (DUV) multiple-quantum-wells (MQWs) incorporating strain-modulated nanostructures are proposed, demonstrating enhanced degree of polarization (DOP) and improved light extraction efficiency (LEE). The influence of Al composition and bi-axial strains on the optical behaviors of the DUV-MQWs were carefully examined. Compared with planar DUV-MQWs, strain-modulated nanostructure patterned MQWs show three times higher photoluminescence and increased DOP from -0.43 to -0.16. Moreover, nanostructure patterned DUV-MQWs under compressive strains further illustrate higher DOP and LEE values than those under tensile strains due to more efficient diffraction of the guided modes of the transverse electric (TE) polarized light. Our work demonstrates, for the first time, that a combination of compressive in-plane strain and surface nanostructure show unambiguous advantages in facilitating TE mode emission, thus have great promises in the design and optimization of highly efficient polarized DUV optoelectronic devices.

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.

Giant reduction of the random lasing threshold in CH3NH3PbBr3 perovskite thin films by using a patterned sapphire substrate

Hybrid organic-inorganic metal halide perovskites are currently arousing enthusiasm and stimulating huge activity across several fields of optoelectronics due to their outstanding properties. In this study, we present the incoherent random lasing (RL) emissions from CH3NH3PbBr3 perovskite thin films on both planar fluorine-doped tin oxide (FTO) substrates and patterned sapphire substrates (PSSs). A detailed examination of the spectral evolution indicates that inelastic exciton-exciton scattering called P-emission is the most plausible mechanism accounting for the lasing emissions. The RL threshold of the perovskite films on PSSs is found to be effectively reduced by more than one order of magnitude from 2.55 to 0.15 μJ per pulse compared to that on FTO substrates. The giant threshold reduction is ascribed to the enhanced random scattering of light and the photon recycling induced by the multireflection processes at the perovskite/PSS interface, which increases the likelihood that the inoperative random rays will re-enter the possible optical loops formed among the perovskite particles, resulting in considerable optical resonance enhancement. The simulation results reveal that the light extraction efficiency on the top facet of the perovskites is significantly increased by approximately 155% by utilizing the PSS instead of the FTO substrate. Moreover, the first direct experimental observation of the multireflection phenomenon of light, as well as the dynamic processes of photon propagation in the composite PSS structure, is presented by Kerr-gate-based time-resolved photoluminescence. Our results provide an effective strategy to achieve high-performance perovskite random lasers and novel light-emitting devices for speckle-free full-field imaging and solid-state lighting applications, by introducing ingeniously designed periodic nano-/microscale optical structures.

Effect of strain relaxation on performance of InGaN/GaN green LEDs grown on 4-inch sapphire substrate with sputtered AlN nucleation layer

Here we demonstrate high-brightness InGaN/GaN green light emitting diodes (LEDs) with in-situ low-temperature GaN (LT-GaN) nucleation layer (NL) and ex-situ sputtered AlN NL on 4-inch patterned sapphire substrate. Compared to green LEDs on LT-GaN (19 nm)/sapphire template, green LEDs on sputtered AlN (19 nm)/template has better crystal quality while larger in-plane compressive strain. As a result, the external quantum efficiency (EQE) of green LEDs on sputtered AlN (19 nm)/sapphire template is lower than that of green LEDs on LT-GaN (19 nm)/sapphire template due to strain-induced quantum-confined Stark effect (QCSE). We show that the in-plane compressive strain of green LEDs on sputtered AlN/sapphire templates can be manipulated by changing thickness of the sputtered AlN NL. As the thickness of sputtered AlN NL changes from 19 nm to 40 nm, the green LED on sputtered AlN (33 nm)/sapphire template exhibits the lowest in-plane compressive stress and the highest EQE. At 20 A/cm², the EQE of 526 nm green LEDs on sputtered AlN (33 nm)/sapphire template is 36.4%, about 6.1% larger than that of the green LED on LT-GaN (19 nm)/sapphire template. Our experimental data suggest that high-efficiency green LEDs can be realized by growing InGaN/GaN multiple quantum wells (MQWs) on sputtered AlN/sapphire template with reduced in-plane compressive strain and improved crystal quality.

Nickel stamp origination from generic SU-8 nanostructure arrays patterned with improved thermal development and reshaping

In this study, we show that rapid, reliable, and scalable custom-input colour patterning and eye-readable data storage can be achieved through high-throughput nanoimprinting-exposure-thermal-treatment (NETT) and thermal development and reshaping (TDR) techniques. The main impediment for commercial realization of high-resolution metasurfaces using NETT and TDR is the cost and speed of stamp origination as well as the quality and durability of the fabricated stamp. In order to accelerate the patterning process, lower the fabrication costs, and obtain patterns with high-resolution, we introduce and optimize a new method for origination of durable Ni stamps by electroplating on an SU-8 master fabricated according to custom-input colour patterns via NETT and TDR. In these processes, laser exposure is used to locally activate the generic RGB pixels fabricated on SU-8 via thermal nanoimprint lithography (NIL), according to the custom design. Upon TDR treatment, the exposed regions crosslink while the unexposed areas flatten. TDR is optimized to find the fastest processing condition that results in minimum nanocone height reduction and maximum diffraction efficiency. AFM results show that the TDR-processed nanocones in all red, green, and blue subpixels witness minimal shrinkage in comparison with the corresponding as-imprinted RGB pixels. Among three different sets of direct baking and ramping temperature TDR experiments, direct 55 °C-10 min TDR is found to be the optimal recipe. As a proof-of-concept, the originated stamp was employed to replicate colour images on PET and glass substrates using UV-thermal NIL. The reproduced colour image, photographed at pre-defined lighting and viewing angles, bears vivid diffractive colours with different RGB ratios that are in good match with the custom-input image. Furthermore, the red, green, and blue diffraction peaks from the TDR-55 °C-baked sample exhibit either trivial or no distinguishable difference as compared to the corresponding peaks in the as-imprinted sample.

Enhanced light out-coupling efficiency of quantum dot light emitting diodes by nanoimprint lithography

Extracting light from quantum dot light emitting diodes (QLEDs) by applying optical-functional nanostructures inside and outside the devices is essential for their commercial application in illumination and displays. In this paper, we demonstrate the highly effective extraction of waveguided light from the active region of QLEDs by embedding internal grating patterns fabricated using a nanoimprint lithography technique. The grating couples out waveguide mode power into the substrate without changing the device's electrical properties, resulting in an increase in both the external quantum efficiency and luminous efficiency for a green QLED from 11.13% to 13.45%, and 29 010 cd m-2 to 44 150 cd m-2, respectively. The observed improvement can be ascribed to the elimination of the waveguide mode by the grating nanostructures introduced in the device. Furthermore, the finite-difference time-domain (FDTD) simulation also demonstrated that the power loss due to the waveguide mode was reversed. The results indicate that internal nano-scattering pattern structures are attractive for enhancing the out-coupling efficiency of QLEDs.

Light extraction efficiency enhancement of flip-chip blue light-emitting diodes by anodic aluminum oxide

We produced an anodic aluminum oxide (AAO) structure with periodic nanopores on the surface of flip-chip blue light-emitting diodes (FC-BLEDs). The nanopores had diameters ranging from 73 to 85 nm and were separated by distances ranging from approximately 10 to 15 nm. The light extraction efficiency enhancement of the FC-BLEDs subjected to different durations of the second pore-widening process was approximately 1.6-2.9%. The efficiency enhancement may be attributed to the following mechanism: periodic nanopores on the surface of FC-BLEDs reduce the critical angle of total reflection and effective energy transfer from a light emitter into a surface plasmon mode produced by AAO.

Morphologies and optical and electrical properties of InGaN/GaN micro-square array light-emitting diode chips

InGaN/GaN micro-square array light-emitting diode (LED) chips (micro-chips) have been prepared via the focused ion beam (FIB) etching technique, which can not only reduce ohmic contact degradation but also control the aspect ratio precisely in three-dimensional (3D) structure LED (3D-LED) device fabrication. The effects of FIB beam current and micro-square array depth on morphologies and optical and electrical properties of the micro-chips have been studied. Our results show that sidewall surface morphology and optical and electrical properties of the micro-chips degrade with increased beam current. After potassium hydroxide etching with different times, an optimal current-voltage and luminescence performance can be obtained. Combining the results of cathodoluminescence mappings and light output-current characteristics, the light extraction efficiency of the micro-chips is reduced as FIB etch depth increases. The mechanisms of micro-square depth on light extraction have been revealed by 3D finite difference time domain.

Numerical and experimental investigation of GaN-based flip-chip light-emitting diodes with highly reflective Ag/TiW and ITO/DBR Ohmic contacts

We demonstrate two types of GaN-based flip-chip light-emitting diodes (FCLEDs) with highly reflective Ag/TiW and indium-tin oxide (ITO)/distributed Bragg reflector (DBR) p-type Ohmic contacts. We show that a direct Ohmic contact to p-GaN layer using pure Ag is obtained when annealed at 600°C in N₂ ambient. A TiW diffusion barrier layer covered onto Ag is used to suppress the agglomeration of Ag and thus maintain high reflectance of Ag during high temperature annealing process. We develop a strip-shaped SiO₂ current blocking layer beneath the ITO/DBR to alleviate current crowding occurring in FCLED with ITO/DBR. Owing to negligibly small spreading resistance of Ag, however, our combined numerical and experimental results show that the FCLED with Ag/TiW has a more favorable current spreading uniformity in comparison to the FCLED with ITO/DBR. As a result, the light output power of FCLED with Ag/TiW is 7.5% higher than that of FCLED with ITO/DBR at 350 mA. The maximum output power of the FCLED with Ag/TiW obtained at 305.6 A/cm² is 29.3% larger than that of the FCLED with ITO/DBR obtained at 278.9 A/cm². The improvement appears to be due to the enhanced current spreading and higher optical reflectance provided by the Ag/TiW.

Tunable Magneto-Optical Kerr Effects of Nanoporous Thin Films

Magnetoplasmonics, combining magnetic and plasmonic functions, has attracted increasing attention owing to its unique magnetic and optical properties in various nano-architectures. In this work, Ag, CoFeB and ITO layers are fabricated on anodic aluminum oxide (AAO) porous films to form hybrid multi-layered nanoporous thin films by magnetron sputtering deposition process. The designed nanostructure supports localized surface plasmon resonance (LSPR) and tunable magneto-optical (MO) activity, namely, the sign inversion, which can be controlled by AAO porous film geometry (pore diameter and inter-pore spacing) flexibly. The physical mechanism of this special MO phenomena is further analyzed and discussed by the correlation of Kerr rotation and electronic oscillations controlled by the surface plasmon resonance that is related to the nanoporous structure.