Improvement in the Light Output Power of GaN-Based Light-Emitting Diodes by Natural-Cluster Silicon Dioxide Nanoparticles as the Current-Blocking Layer

In this study, the fabrication and characterization of InGaN–GaN multiple-quantum-well light-emitting diodes (LEDs) with silicon dioxide ($hbox{SiO}_{2}$) nanoparticles as the current-blocking layer (CBL) are described. The performance was improved by introducing $hbox{SiO}_{2}$ nanoparticles, not deposited by commonly used plasma-enhanced chemical vapor deposition, as the CBL beneath the p-pad. The injected current was forced to spread outside instead of flowing directly downward. At 20 mA, the light output power of the LED with a CBL was increased 15.7% as compared to that of the conventional LED. The forward voltage of the LED with the $hbox{SiO}_{2}$ nanoparticle CBL was 3.34 V at 20 mA, which was slightly higher than that of the conventional LED (3.29 V). The increase in the light output power can be attributed to the injection of additional current into the light-emitting active layer of the LED by the $hbox{SiO}_{2}$ nanoparticles CBL and thus a reduction in optical absorption at the p-pad.      

Enhanced output power of near-ultraviolet InGaN-GaN LEDs grown on patterned sapphire substrates

Near-ultraviolet nitride-based light-emitting diodes (LEDs) with peak emission wavelengths around 410 nm were fabricated onto c-face patterned sapphire substrates (PSS). It was found that the electroluminescence intensity of the PSS LED shown 63% larger than that of the conventional LED. For a typical lamp-form PSS LED operating at a forward current of 20 mA, the output power and external quantum efficiency were estimated to be 10.4 mW and 14.1%, respectively. The improvement in the light intensity could be attributed to the decrease of threading dislocations and the increase of light extraction efficiency in the horizontal direction using a PSS.     

Extraction Efficiency Enhancement of GaN-Based Light-Emitting Diodes by Microhole Array and Roughened Surface Oxide

The light-output power of GaN-based light-emitting diodes (LEDs) was enhanced by microhole array pattern and roughened $hbox{GaO}_{x}$ film grown on the exposed surface. The $hbox{GaO}_{x}$ film was grown by photoelectrochemical (PEC) oxidation via $hbox{H}_{2}hbox{O}$ and formed a naturally rough oxide surface and $hbox{GaO}_{x}/hbox{GaN}$ interface. Compared with that of conventional broad-area LEDs, the output power of the microhole array LED and the surface-oxidized microhole array LED increased by 1.38 and 1.82 times at 20-mA forward current, respectively. The results show that the microhole array pattern with the roughened surface oxide method could significantly enhance light extraction efficiency and be a candidate for manufacturing high-efficient low-cost GaN-based LEDs.      

GaN-Based LEDs With GaN $mu$-Pillars Around Mesa, Patterned Substrate, and Reflector Under Pads

Nitride-based light-emitting diodes (LEDs) with textured sidewall, GaN $mu$-pillars around mesa region, patterned sapphire substrate (PSS), and highly reflective Ag–Cr–Au electrode pads were fabricated using the conventional lithography method (labeled as experimental LEDs). When a 20-mA injection current was applied, forward voltages were 3.18 and 3.4 V for the conventional and experimental LEDs, respectively. The high 20-mA $V_{f}$ of LEDs with Ag–Cr–Au electrode pads could be attributed to the fact that the specific contact resistance of $hbox{n}^{+}$-GaN–Ag–Cr–Au is slightly higher than that of the $hbox{n}^{+}$ -GaN–Cr–Au contact. It was found that we could achieve much stronger LED output power with textured sidewalls, GaN $mu$-pillars around mesa region, PSS, and highly reflective Ag–Cr–Au electrode pads. It was also found that we could enhance LED output power by more than 80% compared with the conventional LEDs.      

Fabrication of Mesa Shaped InGaN-Based Light-Emitting Diodes Through a Photoelectrochemical Process

InGaN-based light-emitting diodes (LEDs) were fabricated to have a higher light extraction through the photoelectrochemical (PEC) mesa shaping process. After the PEC selective wet oxidation and wet etching processes, stable and controllable crystallographic etching planes were formed as p-type GaN {} planes and n-type GaN {} planes included at an angle of 27 deg. The ever-present cone-shaped structure of a PEC-treated LED has a larger light scattering area and higher light extraction cones on the mesa sidewall, as analyzed by microphotoluminescence and light output power measurement. This cone-shaped-sidewall LED has a higher light output power and a larger divergence angle compared with a conventional LED measured in an LED chip form.     

Optical and Structural Properties of InGaN–AlGaN Ultraviolet Light-Emitting Diodes

We fabricated and investigated the performances of InGaN–AlGaN ultraviolet (UV) light-emitting diodes (LEDs) emitting at 380 nm. The output power of a conventional LED, a patterned sapphire substrate LED (PSS LED), and a PSS flip-chip LED (PSS FCLED) were about 0.94, 1.86, and 5.18 mW, respectively, at a forward injection current of 20 mA. These results indicate that the light output–powers of the PSS LED and PSS FCLED were enhanced as much as 97% and 451% compared to the conventional LED. Subsequent optical simulations confirm the remarkable enhancements in optical power of the PSS FCLED at UV wavelengths.      

Nitride-Based Asymmetric Two-Step Light-Emitting Diode With In$_{0.08}$ Ga$_{0.92}$ N Shallow Step

A nitride-based asymmetric two-step light-emitting diode (LED) with $hbox{In}_{0.08} hbox{Ga}_{0.92}hbox{N}$ shallow step was proposed and fabricated. It was found that the low indium content $hbox{In}_{0.08} hbox{Ga}_{0.92}hbox{N}$ layer can significantly enhance phase separation and/or inhomogeneous indium distribution in the active $hbox{In}_{0.27}hbox{Ga}_{0.73}hbox{N}$ layer. It was also found that we can enhance LED output power by a factor of 2.27 by simply inserting an $hbox{In}_{0.08} hbox{Ga}_{0.92}hbox{N}$ shallow step.      

Efficiency improvement of near-ultraviolet InGaN LEDs using patterned sapphire substrates

The use of conventional and patterned sapphire substrates (PSSs) to fabricate InGaN-based near-ultraviolet (410 nm) light-emitting diodes (LEDs) was demonstrated. The PSS was prepared using a periodic hole pattern (diameter: 3 /spl mu/m; spacing: 3 /spl mu/m) on the (0001) sapphire with different etching depths. From transmission-electron-microscopy and etch-pit-density studies, the PSS with an optimum pattern depth (D/sub h/=1.5 /spl mu/m) was confirmed to be an efficient way to reduce the thread dislocations in the GaN microstructure. It was found that the output power increased from 8.6 to 10.4 mW, corresponding to about 29% increases in the external quantum efficiency. However, the internal quantum efficiency (@ 20 mA) was about 36% and 38% for the conventional and PSS LEDs, respectively. The achieved improvement of the output power is not only due to the improvement of the internal quantum efficiency upon decreasing the dislocation density, but also due to the enhancement of the extraction efficiency using the PSS. Finally, better long-time reliability of the PSS LED performance was observed.     

GaN-Based Light-Emitting Diode Prepared on Nano-Inverted Pyramid GaN Template

Using self-aligned $hbox{SiO}_{2}$ nano-spheres as an etching mask, the authors demonstrated the formation of a GaN-based nano-inverted pyramid (NIP) structure. It was found that crystal quality of the GaN epilayer prepared on an NIP/GaN template was significantly better than that prepared with conventional low-temperature GaN nucleation layer. With the NIP structure, it was found that 20-mA light-emitting-diode (LED) output power can be enhanced by 32%, as compared with the conventional LED.      

Enhanced Luminance Efficiency of Wafer-Bonded InGaN–GaN LEDs With Double-Side Textured Surfaces and Omnidirectional Reflectors

A p-side-up GaN-based light-emitting diode (LED) on a silicon substrate was designed and fabricated using a combination of omnidirectional reflector (ODR) and double-side textured surface (both p-GaN and undoped-GaN) structures via surface-roughening, laser lift-off (LLO) and wafer-bonding technologies. The reflectivity of the designed ODR can reach 99.1% at a wavelength of 460 nm. The textured surface of top p-GaN was achieved under low temperature (LT) conditions using metalorganic chemical vapor deposition. It was found that the GaN LED with an extra 200-nm-thick LT p-GaN layer exhibits a 50% enhancement in luminance intensity. The luminance efficiency of double-side roughened silicon–ODR–GaN LED with a small chip size of 250 $mu {hbox {m}} times {hbox {500}}~mu$m can be improved from 23.2% to 28.2% at an injection current of 20 mA. For the case of 1 mm $times$ 1 mm in chip size, the saturation behavior of the light output power is not observed when an injection current increased from 20 to 350 mA, where the luminance efficiency at 20 mA can reach 28.9%, demonstrating an enhancement by 46%, as compared with that of the conventional GaN–sapphire LEDs. These enhanced results can be attributed to higher reflectivity from the ODR and multiple chances of light emitted from the active region to escape, as well as a centralizing effect of light along the vertical direction.      

Effect of Residual Stress and Sidewall Emission of InGaN-Based LED by Varying Sapphire Substrate Thickness

The effect of residual stress and the sidewall emission in InGaN–GaN films with different thickness of sapphire substrate were investigated. The peak wavelength of electroluminescence was blue-shifted as thinning the sapphire substrate, because the wafer bowing-induced mechanical stress alters the piezoelectric field in the InGaN–GaN multiple quantum-well active region of the light-emitting diode (LED). A sideview LED with 170-$muhbox{m}$ -thick sapphire exhibited the highest output power of 14.04 mW at a forward current of 20 mA, improved by 7% compared to that with 80-$muhbox{m}$-thick sapphire. The maximum output power can be obtained by considering both the photon escaping probability from the edges of the sapphire and the photon absorption probability in the sapphire as well as the residual mechanical stress induced by the wafer bowing.      

Asymmetric λ/4-shifted InGaAsP/InP DFB lasers

1.5 μm asymmetriclambda/4-shifted InGaAsP/InP DFB lasers, in which thelambda/4-shift position was moved from the center of the DFB region toward the front side, were made in order to obtain higher output power with high single-longitudinal-mode (SLM) yield. Statistical measurements revealed that it was effective for the increase of the differential quantum efficiency from the front facet without a remarkable decrease of the SLM yield to move thelambda/4-shift position to the front facet by 10-15 percent of the total DFB length. The output efficiencies of the diodes with AR coatings on the window structure were almost coincident to those expected from theoretical calculations. The main to submode ratioP_{0}/P_{1}in the rear spectra of the asymmetric devices noticeably decreased with increasing asymmetry. From an experimental point of view, a criterion for stable SLM operation, for example,P_{0}/P_{1} gsim 1.5in the rear spectrum atI = 0.9 Ithfor thelambda/4-shift positionl_{f} : l_{r} = 35 : 65, is presented.     

高亮度AIGalnP红光发光二极管

对用于提高AlGaInP红光发光二极管(LED)出光效率的分布布拉格反射镜(DBR)和增透膜进行了分析,用金属有机物化学气相沉积(MOCVD)生长了包含DBR和增透膜的LED,在20mA注入电流下,LED的峰值波长为623nm,光强达到200mcd,输出光功率为2.14mW。与常规的LED相比,光强和输出光功率有很大的提高。     

高亮度AlGaInP红光发光二极管

对用于提高AlGaInP红光发光二极管(LED)出光效率的分布布拉格反射镜(DBR)和增透膜进行了分析,用金属有机物化学气相沉积(MOCVD)生长了包含DBR和增透膜的LED,在20 mA注入电流下,LED的峰值波长为623 nm,光强达到200 mcd,输出光功率为2.14 mW.与常规的LED相比,光强和输出光功率有很大的提高.     

Linear Cascade GaN-Based Green Light-Emitting Diodes With Invariant High-Speed/Power Performance Under High-Temperature Operation

We demonstrate the performance of linear cascade green light-emitting diode (LED) arrays suited for use in plastic optical fiber (POF) communications in automobiles or harsh environments. With this three-LED array, driven by the constant voltage bias of an in-car battery output (12 V) we obtain high-speed ($sim$100-Mb/s eye-opening), high-coupling power (0.9 mW), and a very small variation of coupled power versus temperature [$-$ 0.12%$^{circ}hbox{C}^{-1}$ at room temperature (RT)] for the whole measured temperature range (i.e., RT to 120 $^{circ}$ C). Even under high bias current (100 mA) operation, our device can sustain a clear 150-Mb/s eye-opening from RT to 120 $^{circ}$C. The static and dynamic measurement results indicate that the speed and power performance of this device are less sensitive to variations in ambient temperature than are those of the red resonant-cavity LEDs utilized for POF communication.      

AlGaN/GaN HEMT High Power Densities on $hbox{SiC/} hbox{SiO}_{2}$/poly-SiC Substrates

In this letter, successful operation at 10 GHz of T-gate HEMTs on epitaxial structures grown by metal–organic chemical vapor deposition (MOCVD) or MBE on composite substrates is demonstrated. The used device fabrication process is very similar to the process used on monocrystalline SiC substrate. High power density was measured on both epimaterials at 10 GHz. The best value is an output power density of 5.06 W/mm associated to a power-added efficiency (PAE) of 34.7% and a linear gain of 11.8 dB at $V_{rm DS} = hbox{30} hbox{V}$ for the components based on MOCVD-grown material. The output power density is 3.58 W/mm with a maximum PAE of 25% and a linear gain around 15 dB at $V_{rm DS} = hbox{40} hbox{V}$ for the MBE-grown material.      

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     

Active Region Cascading for Improved Performance in InAs–GaSb Superlattice LEDs

Cascading of active regions in InAs–GaSb superlattice light-emitting diodes (LEDs) grown by molecular beam epitaxy is demonstrated as an effective means of increasing optical emission. Devices were fabricated into $120times 120 mu{hbox {m}}^{2}$ mesas to demonstrate suitability for high resolution projection systems. Devices with 1, 4, 8, and 16 stages were designed for midwave infrared emission at 3.8 $mu{hbox {m}}$ operating at 77 K, and quasi-continuous-wave output powers in excess of 900 $mu{rm W}$ from a 16-stage LED have been demonstrated. External quantum efficiency is shown to improve substantially with cascading, approaching 10% for a 16-stage device.      

宽反射角DBR红光发光二极管

对用于提高AlGaInP红光发光二极管出光效率的传统DBR进行了分析,用MOCVD生长了包含对垂直入射光反射的DBR和对斜入射光反射的DBR复合在一起的红光LED,在20 mA注入电流下,LED的峰值波长为630 nm,轴向光强达到137 mcd,输出光功率为2.32 mW.与常规的LED相比,光强和输出光功率有很大的提高.     

Methods of Increasing Luminous Efficiency of Phosphor-Converted LED Realized by Conformal Phosphor Coating

White light output was realized through combining a cerium-doped yttrium aluminum garnet (YAG:${hbox{Ce}}^{3+}$ ) phosphor with a gallium nitride (GaN)-based blue light-emitting diode (LED). In order to coat a phosphor layer of high quality on a GaN chip, the slurry coating technique was applied and investigated because of the coating efficiency and uniformity. The phosphor layer of conformal structure was realized on the LED chip with the self-exposure method. The influences of ammonium dichromate (ADC) as photosensitizer of photoresist in the slurry are discussed. Two methods are brought forward to reduce the absorption of Cr ions introduced by ADC. And experimental results indicate that these two methods are feasible to increase the luminous efficiency of white LEDs.