Improved Light Extraction Efficiency Research Articles

Improvement of light extraction efficiency in AlGaInP-based vertical miniaturized-light-emitting diodes via surface texturing.

AlGaInP-based light-emitting diodes (LEDs) suffer from a low external quantum efficiency (EQE), which is mainly restrained by the poor light extraction efficiency. Here, we demonstrate AlGaInP-based vertical miniaturized-LEDs (mini-LEDs) with a porous n-AlGaInP surface using a wet etching process to boost light extraction. We investigated the effects of etching time on the surface morphology of the porous n-AlGaInP surface. We found that as the etching time is prolonged, the density of pores increases initially and decreases subsequently. In comparison with the vertical mini-LED with a smooth n-AlGaInP surface, the vertical mini-LEDs with the porous n-AlGaInP surface reveal improvement in light output power and EQE, meanwhile, without the deterioration of electrical performance. The highest improvement of 38.9% in EQE measured at 20 mA is observed from the vertical mini-LED with the maximum density of the pores. Utilizing a three-dimensional finite-difference time-domain method, we reveal the underlying mechanisms of improved performance, which are associated with suppressed total internal reflection and efficient light scattering effect of the pores.

Significant reduction in sidewall damage related external quantum efficiency (EQE) drop in red InGaN microLEDs (∼625 nm at 1 A cm−2) with device sizes down to 3 μm

Ultra-small (10 μm) InGaN-based red microLEDs (625 nm at 1 A cm−2) are necessary for modern displays. However, an increase in surface-area-to-volume ratio with a decrease in the micro-LED size resulting in higher surface recombination causes a drop in efficiency with device size. In this letter, we demonstrate microLEDs from 60 μm down to 3 μm with significantly reduced sidewall-related efficiency reduction using a two-step passivation technique using Al2O3 and Si3N4. The peak on-wafer EQE changes from 0.21% to 0.35% as the device size reduces from 60 to 3 μm, possibly due to improved light extraction efficiency for smaller mesa-widths.

Improved light extraction efficiency of AlGaN DUV light emitting diodes using Al/MgF2-based highly reflective film

AlGaN DUV light emitting diodes (DUV-LEDs) (275 nm emission) are safe, eco-friendly and smart alternatives for inactivating viruses and bacteria. However, DUV-LEDs suffer from the main bottleneck of low external quantum efficiencies, which are strongly associated with the low light extraction efficiency caused by the strong optical polarisation of Al-rich AlGaN. Optical simulation results show that the luminous intensity of DUV-LEDs was increased by 10% owing to the synergistic effect of the sidewalls and substrates. The optical power of DUV-LEDs was increased to 16.8%, the far-field pattern was expanded to 130 degrees and the emission intensity was more focused on the central region, proving that the highly reflective sidewalls and substrates could re-direct the sideways-travelling photons for extraction. Moreover, we also investigated the reflective mechanism of Al/MgF2 layers. Optimizing the refractive index distribution of Al/MgF2 layers could change the electric field intensity and improve the reflectivity. At the same time, the temperature of the sample after coating was significantly reduced by 6.83%. Thermal radiation benefits and the high stability of bonding interfaces are the main reasons to reduce the temperature of DUV-LEDs after Al/MgF2 coating. The present strategy is proposed from the point of view of chip fabrication, which is cost-effective and able to be manufactured at a large scale.

Core-Shell Nanorods as Ultraviolet Light-Emitting Diodes.

Existing barriers to efficient deep ultraviolet (UV) light-emitting diodes (LEDs) may be reduced or overcome by moving away from conventional planar growth and toward three-dimensional nanostructuring. Nanorods have the potential for enhanced doping, reduced dislocation densities, improved light extraction efficiency, and quantum wells free from the quantum-confined Stark effect. Here, we demonstrate a hybrid top-down/bottom-up approach to creating highly uniform AlGaN core-shell nanorods on sapphire repeatable on wafer scales. Our GaN-free design avoids self-absorption of the quantum well emission while preserving electrical functionality. The effective junctions formed by doping of both the n-type cores and p-type caps were studied using nanoprobing experiments, where we find low turn-on voltages, strongly rectifying behaviors and significant electron-beam-induced currents. Time-resolved cathodoluminescence measurements find short carrier liftetimes consistent with reduced polarization fields. Our results show nanostructuring to be a promising route to deep-UV-emitting LEDs, achievable using commercially compatible methods.

Inverse Tapered AlGaN Micropillar and Nanowire LEDs for Improved Light Extraction Efficiency at 270 nm

Deep-ultraviolet (DUV) light emitting diodes (LEDs) show great potential in a wide range of applications, but suffer from poor external quantum efficiencies (EQEs) when compared to visible LEDs, due to their exceptionally poor light extraction efficiency (η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">EXT</sub> ). One successful method of improving the η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">EXT</sub> of DUV LEDs is the use of micropillar and nanowire arrays. Our previous works have reported the development of an “inverse taper” profile in AlGaN micropillars during wet etching in heated hydroxyl-based chemistries. Here, we study the effects of inverse tapering on the η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">EXT</sub> of AlGaN microstructures and nanostructures at 270 nm using finite-difference time-domain simulations, in accordance with our experimental results. Results show that η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">EXT</sub> can be increased from ∼45% to more than 95% for TM-polarized emission in microstructures with diameters of 1 μm and heights >1 μm, and from ∼35% to more than 85% for TE-polarized emission by an inverse taper angle of 5 °. Results for nanostructures also indicate significant η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">EXT</sub> improvements through tuning of the inverse taper angle. These findings, along with our demonstration of inverse tapered high Al-content AlGaN micro and nanostructures, could enable the development of record-high EQE DUV LEDs based on arrays of high aspect ratio structures with high η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">EXT</sub> .

Improvement of the Emission Intensity of GaN-Based Micro-Light Emitting Diodes by a Suspended Structure

Herein, suspended GaN-based micro-light emitting diodes (LEDs) are ingeniously proposed and fabricated, showing a substantially enhanced light emission compared to conventional micro-LEDs on the sapphire substrate. The suspended architecture is prepared by transferring epitaxial layers to micrometal pillars on the copper plate after removing the original sapphire substrate. The photoluminescence intensity of the suspended micro-LED exhibits 150% higher than that of the normal device, and the electroluminescence intensity is increased by 114% in the current injection range of 0–10 mA. The enhancement of the output intensity benefits from the partially relaxed strain of the epitaxial film and the resultant reduction of the quantum confined Stark effect in the InGaN quantum well active region, as well as the improved light extraction efficiency due to the larger light-escaping area and less optical absorption and trapping, which are unambiguously verified by Raman spectroscopy and ray-tracing simulations. This study provides a new promising route to design and fabricate highly efficient micro-LEDs for practical applications.

Light extraction enhancement of light-emitting diodes by nanopillar hierarchical structures

Patterning of p-GaN and/or indium-tin-oxide (ITO) surfaces is investigated to improve light extraction efficiency for GaN-based light-emitting diodes (LEDs). A nanopillar-patterned p-GaN layer, a nanopillar-patterned ITO contact layer, and dual nanopillar-patterned p-GaN and ITO layers are fabricated. When compared with conventional LEDs, LEDs with the nanopillar-patterned p-GaN, nanopillar-patterned ITO, and dual nanopillar-patterned p-GaN and ITO layers showed light output enhancement by 50%, 90%, and 120%, respectively. The light extraction enhancement mechanism of these nanopillar-patterned nanostructures is investigated via the 3D finite-difference time-domain method. The results show that the LED with the dual nanopillar-patterned p-GaN and ITO layers offers the best light extraction efficiency, which could stimulate further efforts to increase the escape light cone effectively and overcome total internal reflection. This methodology for synchronous nanopatterning of two light-emitting interfaces is believed to provide a major light extraction enhancement.

Size dependent characteristics of AlGaN-based deep ultraviolet micro-light-emitting-diodes

AlGaN-based deep ultraviolet (DUV) micro-light-emitting diodes (μLEDs) with emission wavelengths between 277 and 304 nm with mesa dimensions down to 20 μm were fabricated. Their size-dependent electrical and optical characteristics were analyzed. At 20 A cm−2, the external quantum efficiency (EQE) increased from 2.0% to 2.3% mainly due to the improved light extraction efficiency; the forward voltage was 7.6 V in 20 μm sized μLEDs in comparison to 9.1 V in 300 μm LEDs due to better current spreading in the smaller devices. The peak EQEs of the 20 μm μLEDs were 2.5% and 4.0% for 277 and 304 nm, among the highest reported for DUV μLEDs.

Directional high-efficiency nanowire LEDs with reduced angular color shift for AR and VR displays

The emission wavelength of InGaN/GaN dot-in-wire LED can be tuned by modifying the nanowire diameter, but it causes mismatched angular distributions between blue, green, and red nanowires because of the excitation of different waveguide modes. Besides, the far-field radiation patterns and light extraction efficiency are typically calculated by center dipoles, which fails to provide accurate results. To address these issues, we first compare the simulation results between central dipole and dipole cloud with experimental data. Next, we calculate and analyze the display metrics for full-color nanowire LEDs by 3D dipole cloud. Finally, we achieve unnoticeable angular color shift within ±20° viewing cone for augmented reality (AR) and virtual reality (VR) displays with an improved light extraction efficiency.

Using Modified-Intake Plasma-Enhanced Metal-Organic Chemical Vapor Deposition System to Grow Gallium Doped Zinc Oxide.

We have used a modified-intake plasma-enhanced metal–organic chemical vapor deposition (MIPEMOCVD) system to fabricate gallium-doped zinc oxide (GZO) thin films with varied Ga content. The MIPEMOCVD system contains a modified intake system of a mixed tank and a spraying terminal to deliver the metal–organic (MO) precursors and a radio-frequency (RF) system parallel to the substrate normal, which can achieve a uniform distribution of organic precursors in the reaction chamber and reduce the bombardment damage. We examined the substitute and interstitial mechanisms of Ga atoms in zinc oxide (ZnO) matrix in MIPEMOCVD-grown GZO thin films through crystalline analyses and Hall measurements. The optimal Ga content of MIPEMOCVD-grown GZO thin film is 3.01 at%, which shows the highest conductivity and transmittance. Finally, the optimal MIPEMOCVD-grown GZO thin film was applied to n-ZnO/p-GaN LED as a window layer. As compared with the indium–tin–oxide (ITO) window layer, the n-ZnO/p-GaN LED with the MIPEMOCVD-grown GZO window layer of the rougher surface and higher transmittance at near UV range exhibits an enhanced light output power owing to the improved light extraction efficiency (LEE).

Suppressing surface plasmon losses to improve the efficiency of blue organic light-emitting diodes using the plasmonic quasi-bandgap phenomenon

It is a persistent problem in organic light-emitting diode (OLED) display devices that the efficiency of blue-light source materials is lower than that of green- or red-light source materials. To address this problem, numerous studies have investigated blue-light-emitting materials. However, ensuring the reliability of the blue-light-emitting materials has been difficult in most studies. In this study, electrodes using asymmetric dielectric/metal/dielectric structures with plasmonic quasi-bandgap characteristics were developed to achieve a highly efficient blue fluorescent OLED. The electrodes were applied in a microcavity OLED and a transparent OLED. Using the developed electrode in fabricated OLED devices not only maximized the cavity resonance effect and transparency, but also preserved the advantage of the work function of the metal in terms of electrical properties, with high device stability. The approach also minimized losses caused by surface plasmon polaritons, which is a blind spot in the optical aspect of metal electrodes, resulting in improved light extraction efficiency.

Significantly boosted external quantum efficiency of AlGaN-based DUV-LED utilizing thermal annealed Ni/Al reflective electrodes

Improvement of light extraction efficiency of deep ultraviolet light-emitting-diodes (DUV-LEDs) was obtained by adopting a Ni/Al reflective electrode. Optical reflectivity up to 78.1% at 277 nm was achieved. The enhanced reflectivity after thermal annealing was ascribed to the formation of Ni clusters embedded inside the Al matrix as revealed by the correlation between the surface morphology and chemical stoichiometry. The peak external quantum efficiency of a 277 nm DUV-LED incorporating annealed Ni/Al reflective electrodes reaches 3.03%, which is 44% higher than that with conventional Ni/Au p-electrodes, revealing itself as a promising candidate in the realization of high-efficiency DUV emitters.

Light extraction efficiency improvement through double sided periodic photonic structures for deep UV light emitting diodes

Polarization-dependent light extraction efficiency (LEE) of AlGaN based flip-chip deep ultraviolet light emitting diode (peak emission at 280 nm) with embedded periodic photonic structure (PPS), surface PPS and double-sided PPS configuration have been studied by utilizing three-dimensional finite-difference time-domain (3D FDTD) method. Among the three considered configurations, double-sided PPS has been found to possess the maximum improvement in LEE, followed by embedded PPS configuration. The surface PPS offers the least LEE improvement among the three configurations with reference to the conventional deep-UV LED structure. The respective improvements for TE, TM and overall LEE have been found to be around 128.37%, 59.18% and 99% for the double-sided PPS as compared to the conventional deep-UV LED.

Improved light extraction efficiency of AlGaN deep-ultraviolet light emitting diodes combining Ag-nanodots/Al reflective electrode with highly transparent p-type layer.

Enhancement of light extraction efficiency (LEE) of AlGaN-based deep-ultraviolet (DUV) light emitting diodes (LEDs) has been attempted by adopting Ag-nanodots/Al reflective electrodes on a highly transparent complex p-type layer. By thinning the p-GaN to several nm, highly DUV transparent p-type layer is achieved, making it meaningful for the application of reflective electrodes composed of Ag-nanodots and Al film to allow most light emitted upward to be reflected back to the sapphire side. By this approach, the maximum light output power and external quantum efficiency of the DUV-LEDs with optimized Ag nanodots/Al electrodes are severally increased by 52% and 58%, respectively, compared to those with traditional Ni/Au electrodes when the current is below 200 mA.

Studies on high refractive index amorphous TiO2 thin film for possible improvement of light extraction efficiency in organic light emitting diodes

Studies on low-cost sol–gel spin-coated TiO2 thin film with high refractive index (HRI), which may be used as an intermediate layer to enhance the light extraction efficiency (LEE) of a typical bottom-emitting organic light emitting diode (OLED), are reported. The TiO2 solution is prepared using titanium tetra iso-propoxide, acetic acid, and ethanol. Spin coating method is used to deposit TiO2 thin films on glass substrate. Different optical characterizations of as-deposited and annealed (150°C, 300°C, and 450°C) TiO2 thin films on glass substrate are done, from which different properties of the film are derived. Ellipsometric measurement shows shrinkage in the thickness of the as-deposited TiO2 films after annealing at different temperatures. X-ray diffraction reveals amorphous TiO2 formation for all the samples. The RI of the coated film increases with the increase in annealing temperatures. Its value at 633 nm wavelength for the as-deposited film is found to be 2.1, which is quite high and it is seen that this RI can be further increased to 2.78 by annealing the samples at 450°C. This value is comparatively high compared to several other reported values of other researchers. The as-deposited sample reveals highest porosity, which further decreases with rise in annealing temperature. Our calculation of LEE for a typical OLED with the intermediate layer of this HRI as-deposited TiO2 film shows improvement of the LEE. However for annealed films, the experimentally obtained thicknesses are not adequate for this improvement, but it is shown that by increasing the film thickness, further improvement of LEE is possible.

Wafer-Level Monolithic Integration of Vertical Micro-LEDs on Glass

In an earlier study micro-LED micro display (<; 1 inch) on silicon CMOS backplane was demonstrated for augmented reality (AR) applications. Here we report the feasibility of wafer-level monolithic integration of micro-LEDs on glass substrate/backplane. Such issues as the cracking of GaN epitaxial layer, the deviation of alignment, and the peeling of insulator are discussed. SU-8 is proposed as the insulator material in vertical micro-LEDs, resulting into improved light extraction efficiency and allowing for the reduced light crosstalk between sub pixels if the integrated reflective mirrors are used. A directly driven micro-LED parallel array with a resolution of 320 × 720 with individual LED size ranging from 5 μm to 28 μm is demonstrated. It is believed that this monolithic technology on glass will play an important role in future high performance and low cost wearable and/or phone displays.

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.

Improved Light Extraction Efficiency of GaN Based Blue Light Emitting Diode Using Nano-Scaled Patterned Sapphire Substrate

In this study, two different conical-shape nano-patterns were fabricated on the 2-inch diameter sapphire wafer in order to improve the efficiency of light-emitting diodes. The conical-shape nano-patterns were fabricated on the sapphire wafer using a nanoimprint technique and dry etching method. A blue LED structure was grown on the nanoscale-patterned sapphire substrates. The photoluminescence and electroluminescence were measured to confirm the effectiveness of the nano-scale patterns. An improvement in the luminescence efficiency was observed in case that nano-patterned sapphire substrate was used; a 1.44 times-higher photoluminescence intensity and a 1.5 times-higher electroluminescence intensity were observed, compared to those of the light-emitting diodes structure grown on a conventional flat sapphire wafer.

Two-step optimization for a pattern array to improve light extraction efficiency and uniformity in light guide plates.

The pattern array distribution determines the primary optical performance in light guide plates (LGPs). This paper presents a two-step optimization method for the pattern array to improve the efficiency and uniformity of the backlight module. For the first step, we analyze the illuminance distribution based on the extended LED light source, and an initial circular pattern array is designed with the pattern density defined by a ninth-order polynomial function. For the second step, the pattern modes are finely optimized to search the optimal pattern array. This method is demonstrated through both simulation and experiment. From experimental verification, the LGPs are manufactured through silk-screen printing technology. The results show that square and regular hexagon pattern optimization can improve the efficiency and uniformity by 19.73% and 6.37%, respectively, when compared to the initial mode. Our approach will have broad prospects for pattern array design.

Deep ultraviolet light-emitting diodes based on a well-ordered AlGaN nanorod array

The nanorod structure is an alternative scheme to develop high-efficiency deep ultraviolet light-emitting diodes (DUV LEDs). In this paper, we first report the electrically injected 274-nm AlGaN nanorod array DUV LEDs fabricated by the nanosphere lithography and dry-etching technique. Nanorod DUV LED devices with good electrical properties are successfully realized. Compared to planar DUV LEDs, nanorod DUV LEDs present >2.5 times improvement in light output power and external quantum efficiency. The internal quantum efficiency of nanorod LEDs increases by 1.2 times due to the transformation of carriers from the exciton to the free electron–hole, possibly driven by the interface state effect of the nanorod sidewall surface. In addition, the nanorod array significantly facilitates photons escaping from the interior of LEDs along the vertical direction, contributing to improved light extraction efficiency. A three-dimensional finite-different time-domain simulation is performed to analyze further in detail the TE- and TM-polarized photon extraction mechanisms of the nanostructure. Our results demonstrate the nanorod structure is a good candidate for high-efficiency DUV emitters.