Light Output Improvement of InGaN-Based Light-Emitting Diodes by Microchannel Structure

We propose a microchannel structure with deep holes to enhance the extraction efficiency of InGaN-based light-emitting diodes (LEDs). Two different depth microchannel LEDs are examined experimentally. One has air holes penetrated through the active layer and the other does not. It is found that the light extraction efficiency in the LED with the penetrated air holes is significant h larger than that in the LED without penetrating holes. The reason can be attributed to the microchannel waveguide behaviors. In comparison to the conventional LEDs, the light output power of our fabricated LEDs with and without penetrating holes improved by 43.5% and 5.1%, respectively.     

Improved Light Extraction of Nitride-Based Flip-Chip Light-Emitting Diodes Via Sapphire Shaping and Texturing

A novel flip-chip structure of GaN-sapphire light-emitting diodes (LEDs) was developed to improve the external quantum efficiency, where the sapphire substrate was textured and shaped with beveled sidewalls using a wet etching technique. The forward voltage of the conventional flip-chip and shaped flip-chip GaN LEDs were 2.84 and 2.85 V at 20 mA, respectively. This indicates that the GaN LED was not destroyed during the sapphire wet etching process. It was found that the output power increased from 9.3 to 14.2 mW, corresponding to about 52% increases in the external quantum efficiency. The results agree well with the simulation data that the shaped flip-chip GaN LED can provide better light extraction efficiency than that of the conventional flip-chip sample     

Improved Light Output of GaN-Based Light-Emitting Diodes by Using AgNi Reflective Contacts

We investigated the light output power of blue light-emitting diodes (LEDs) fabricated with AgNi contacts as a function of the Ni content. Annealing the AgNi contacts at 400°C in air significantly improved their electrical characteristics. The AgNi samples with 10.0 wt.% Ni showed reflectance of 80.9% at 460 nm, whereas the Ag-only contacts gave 71.1%. After annealing at 400°C, the AgNi contacts exhibited better thermal stability than did the Ag-only contacts. Their current–voltage relationships showed that blue LEDs fabricated with Ag-only contacts gave a forward voltage of 3.33 V at 20 mA, whereas those fabricated with AgNi contacts with 10.0 wt.% Ni produced 3.03 V. LEDs fabricated with the AgNi contacts exhibited output power higher by 5.9% to 19.1% than those with Ag-only contacts. Based on scanning electron microscopy and x-ray photoemission spectroscopy results, the improved thermal and electrical behaviors are described and discussed.     

Nitride-Based Near-Ultraviolet Mesh MQW Light-Emitting Diodes

We have demonstrated nitride-based near-ultraviolet mesh multiquantum-well (MQW) light-emitting diodes (LEDs) by etching through the MQW active region. With 20-mA injection current, it was found that forward voltages were 3.29, 3.31, and 3.38 V while output powers were 7.5, 9.0, and 11.3 mW for the planar indium-tin-oxide (ITO) LED, mesh ITO LED, and mesh MQW LED, respectively. The larger LED output power is attributed to the increased light extraction efficiency.     

Improved Light Output Power of GaN-Based Light-Emitting Diodes Using Double Photonic Quasi-Crystal Patterns

The enhancement of light extraction from GaN-based light-emitting diodes (LEDs) with a double 12-fold photonic quasi-crystal (PQC) structure using nanoimprint lithography is presented. At a driving current of 20 mA on a transistor-outline-can package, the light output power of an LED with a nanohole patterned sapphire substrate (NHPSS) and an LED with a double PQC structure are enhanced by 34% and 61%, compared with the conventional LED. In addition, the higher output power of the LED with the double PQC structure is due to better reflectance on NHPSS and higher scattering effect on p-GaN surface using a 12-fold PQC structure pattern. These results provide promising potential to increase the output powers of commercial light-emitting devices.      

Thermal Study of High-Power Nitride-Based Flip-Chip Light-Emitting Diodes

This paper presents a chip-level thermal study of high-power nitride-based flip-chip (FC) light-emitting diodes (LEDs). In order to understand the thermal performance of the high-power FC LEDs thoroughly, a quantitative parametric analysis of the thermal dependence on the chip contact area, bump configuration, and bump defects was performed by finite-elementmodel (FEM) numerical simulation and thermal infrared (IR) microscopy testing, respectively. FEM numerical simulation results proved that the optimized bump configuration design was essential to get a uniform temperature distribution in the active layer and improve the thermal performance of the FC LED. IR microscopy testing results recognized that bump defects formed in the LED chip solder processing would lead to surface hot spots around the vicinity of these bump defects, particularly under high-current working conditions. In addition, a light-emitting dark zone was also observed in the optical field for FC LEDs with bump defects. In summary, optimized LED FC bump configuration design and good bonding quality in the chip bonding process are proved to be critical for improving the thermal performance and extending the operating longevity of high-power FC LEDs.      

Nitride-Based Thin-Film Light-Emitting Diodes With Photonic Quasi-Crystal Surface

In this letter, the nitride-based thin-film light-emitting diodes (TFLEDs) with eight-fold photonic quasi-crystal (PQC) surfaces are proposed and demonstrated by a combination of wafer bonding, laser lift-off, and electron-beam lithography processes. By adopting a PQC surface, the light–output power (at 350 mA) of the PQC-TFLEDs exhibits 140% output power enhancement as compared with that of TFLEDs without a PQC surface.      

Further Enhancement of Nitride-Based Near-Ultraviolet Vertical-Injection Light-Emitting Diodes by Adopting a Roughened Mesh-Surface

In this letter, the nitride-based near-ultraviolet (NUV) vertical-injection light-emitting diodes (VLEDs) with roughened mesh-surface are proposed and demonstrated by a combination of pattern sapphire substrate, wafer bonding, laser lift-off, and chemical wet etching processes. With the help of adopting a roughened mesh-surface, the light-output power (at 350 mA) of the NUV-VLEDs could be further enhanced about 20% as compared with that of the conventional NUV-VLED.     

Matrix-Addressable Micropixellated InGaN Light-Emitting Diodes With Uniform Emission and Increased Light Output

Micropixellated InGaN light-emitting diodes (micro- LEDs) have a wide number of potential applications in areas including microdisplays, fluorescence-based assays and microscopy, and cell micromanipulation. Here, we present fabrication and performance details of matrix-addressable micro-LED devices which show significant improvements over their earlier counterparts. Devices with 64 x 64 micropixel elements, each of them having a 16-mum-diameter emission aperture on a 50-mum pitch, have been fabricated at blue (470 nm), green (510 nm), and UV (370 nm) wavelengths, respectively. Importantly, we have adopted a scheme of running n-metal tracks adjacent to each n-GaN mesa, so that resistance variation between the devices is reduced to below 8%, in contrast to the earlier fivefold resistance variation encountered. We have also made improvements to the spreading-layer formation scheme, resulting in significant increases in output power per element, improved current handling, and reduced turn-on voltages. These devices have been combined with a computer- driven programmable driver interface operating in constant- current mode, and representative microdisplay outputs are presented.     

Enhanced Light Output in Roughened GaN-Based Light-Emitting Diodes Using Electrodeless Photoelectrochemical Etching

We have demonstrated enhanced output power from roughened GaN-based light-emitting diodes (LEDs) by using electrodeless photoelectrochemical etching with a chopped source (ELPEC-CS etching). It was found that the 20-mA output power of the ELPEC-CS treated LED (with roughened surfaces on the top p-type and bottom n-type GaN surface as well as the mesa sidewall) was 1.41 and 2.57 times as high as those LEDs with a roughened p-type GaN surface and a conventional surface, respectively. The light output pattern of the ELPEC-CS treated LED was five times greater than the conventional LED at 0deg which was caused by the roughened GaN surface that improved the light extraction efficiency of the LED     

Light Output Improvement of Oxide-Textured InGaN-Based Light-Emitting Diodes by Bias-Assisted Photoelectrochemical Oxidation With Imprint Technique

The improvement of light output power in InGaN–GaN multiple quantum-well (MQW) light-emitting diodes (LEDs) with oxide textural films are observed. The oxide textural films are grown by alternating current bias-assisted photoelectrochemical oxidation with an imprint technique. At an injection current of 20 mA, the light output power of the oxide-textured InGaN–GaN MQW LEDs is 22% higher than that of the conventional LEDs. Relatively low values in leakage current for the oxide-textured are also observed, as compared to the conventional LEDs. This enhancement in the oxide-textured LEDs performance is attributed to the increase in external quantum efficiency by the nanoscale roughness of the convex oxide films and to the passivation in possible leakage paths of the devices.      

Light Output Enhancement of InGaN Light-Emitting Diodes Grown on Masklessly Etched Sapphire Substrates

A maskless wet-etching method is used to prepare patterned sapphire substrates for enhancing the output power of InGaN light-emitting diodes (LEDs). Blue LEDs grown on the patterned sapphire substrates exhibit an output power of 24.9 mW, which is 19.4% higher than that of the devices grown on flat substrates. The uniformity of the optical and electrical properties of LEDs across a 2-in wafer is slightly improved as well.      

阳极氧化铝工艺用于提高LED的出光效率

使用纳米尺度的多孔阳极氧化铝(anodic aluminum oxide,AAO)作为刻蚀掩膜,刻蚀氧化铟锡(indium-tin oxide,ITO),形成纳米图形化表面,对于发光二极管的出光效率有明显的提升作用。AAO纳米掩膜的制备已广为报道,是纳电子学研究中常用的模板之一,工艺简单易行、可控性好。使用电感耦合反应离子刻蚀方法成功将纳米多孔结构转移到ITO上,形成ITO纳米结构。纳米图形化结构的引入使得器件有效减小了内部的全反射,在电压没有大幅提高,注入电流350 mA时,光学输出提高了7%。纳米尺度粗化结构LED与传统结构LED对比,提升了器件的外量子效率。     

Lattice Constant Effect of Photonic Crystals on the Light Output of Blue Light-Emitting Diodes

Photonic crystal (PC) slabs with triangular lattice constants of 230, 345, 460, and 690 nm have been fabricated onto InGaN–GaN multiquantum-well light-emitting diodes with a wavelength of 461 nm. For the shallow nanoholes in the depth range of 10–38.5 nm, the current–voltage characteristics were not changed but the electroluminescence intensities were enhanced by a factor of 3.5. The light output was most enhanced with the lattice constant corresponding to the wavelength due to the PC diffraction with constructive interference but was suppressed with the lattice constant of the half wavelength by forming a photonic bandgap.      

Improved Light Extraction Efficiency in AlGaInP Light-Emitting Diodes by Applying a Periodic Texture on the Surface

In this letter, AlGaInP-GaP-based light-emitting diodes (LEDs) were fabricated with an Si substrate and an SiO₂-ITO-Ag omni-directional reflector using a metal-to-metal bonding technique. To enhance light extraction efficiency, a periodic texture was applied to the (Al_0.5Ga_0.5)_0.5In_0.5P surface layer of AlGaInP-Si LEDs by photolithography and a wet etching process. The exterior of the etched texture consists of a series of bowl-shaped recesses. With a 350-mA current injection, the typical output power of the AlGaInP-Si LEDs with and without the textured surface (LED-I and LED-II, respectively) were measured at approximately 118 and 81 mW, respectively, when the LED chips were bonded on the TO 46 without resin encapsulation. The enhancement of output power in LED-I can be attributed to a multitude of bowl-shaped notches on the surface, which resulted in a reduction of the reabsorption probability of the photons due to the fact that the photon path length in LED-I is shorter than in LED-II before the photons escape into the free space.     

大功率GaN基LED的提取效率

影响GaN基LED效率的主要因素是内量子效率和提取效率. 蓝光GaN基的LED内量子效率可达70%以上， 紫外GaN基LED可达80%，进一步改善的空间较小. 而传统大面积结构GaN基LED由于全反射和吸收等原因，外提取效率只有百分之几，提高空间很大. 本文从几何和物理光学角度分析了影响GaN基LED外提取效率的因素，针对全反射、吸收、横向光波导等问题总结了现有的各种提高GaN基LED提取效率的手段及其优缺点.     

大功率GaN基LED的提取效率

影响GaN基LED效率的主要因素是内量子效率和提取效率.蓝光GaN基的LED内量子效率可达70%以上,紫外GaN基LED可达80%,进一步改善的空间较小.而传统大面积结构GaN基LED由于全反射和吸收等原因,外提取效率只有百分之几,提高空间很大.本文从几何和物理光学角度分析了影响GaN基LED外提取效率的因素,针对全反射、吸收、横向光波导等问题总结了现有的各种提高GaN基LED提取效率的手段及其优缺点.     

大功率GaN基LED的提取效率

影响GaN基LED效率的主要因素是内量子效率和提取效率.蓝光GaN基的LED内量子效率可达70%以上,紫外GaN基LED可达80%,进一步改善的空间较小.而传统大面积结构GaN基LED由于全反射和吸收等原因,外提取效率只有百分之几,提高空间很大.本文从几何和物理光学角度分析了影响GaN基LED外提取效率的因素,针对全反射、吸收、横向光波导等问题总结了现有的各种提高GaN基LED提取效率的手段及其优缺点.     

Enhanced Light Extraction From Triangular GaN-Based Light-Emitting Diodes

This study investigated the characteristics of a triangular light-emitting diode (LED) and compared it to a standard quadrangular LED. The total radiant flux from the packaged triangular LED increased by 48% and 24% at input currents of 20 and 100 mA, respectively, compared to that of a quadrangular LED which was grown on patterned sapphire substrate. In light far-field beam distribution, the light extraction in the horizontal direction of the LED was much higher than that of the quadrangular LED due to the enhancement of light emission from the side walls of the triangular LED.