The characteristics ofnitride-based blue light-emitting diodes （LEDs） with A1GaN composition-graded barriers are analyzed numerically. The carrier concentrations in the quantum wells （QWs）, the energy band di- agrams, the electrostatic fields, and the light output power are investigated by APSYS software. The simulation results show that the LED with AlGaN composition-graded barriers has a better performance than its AlGaN/InGaN counterpart owing to the increase of hole injection and the enhancement of electron confinement. The simulation results also suggest that the output power is enhanced significantly and the efficiency droop is markedly improved when the AIGaN barriers are replaced by AlGaN composition-graded barriers.
A simple method is proposed to design free- form surface for Lambertian extended source. In this method, it can take advantage of the designing method for point source via substituting each incident ray with a dynamically calculated equivalent ray. For each facet on the freeform surface, the equivalent ray emits from the energy weighted average-emitting-position for the corre- sponding incident beam, and redirects into the direction which is determined by a source-to-target mapping. The results of the designing examples show that the light distributions＇ uniformities can be improved by this method, e.g., even the improvement of 59% can be achieved.
Kun WANGYanjun HANHongtao LIYi LUOZhibiao HAOLai WANGChangzheng SUNBing XIONGJian WANG
Wafer-scale SiO2 photonic crystal （PhC） patterns （SiO2 air-hole PhC, SiO2-pillar PhC） on indium tin oxide （ITO） layer of GaN-based light-emitting diode （LED） are fabricated via novel nanospherical-lens lithography. Nanoscale polystyrene spheres are self-assembled into a hexagonal closed-packed monolayer array acting as convex lens for expo- sure using conventional lithography instrument. The light output power is enhanced by as great as 40.5% and 61% over those of as-grown LEDs, for SiO2-hole PhC and SiO2-pillar PhC LEDs, respectively. No degradation to LED electrical properties is found due to the fact that SiO2 PhC structures are fabricated on ITO current spreading electrode. For SiO2- pillar PhC LEDs, which have the largest light output power in all LEDs, no dry etching, which would introduce etching damage, was involved. Our method is demonstrated to be a simple, low cost, and high-yield technique for fabricating the PhC LEDs. Furthermore, the finite difference time domain simulation is also performed to further reveal the emission characteristics of LEDs with PhC structures.
We report a new method for the fabrication of two-dimensional photonic crystal （PhC） hole arrays to improve the light extraction of GaN-based light-emitting diodes （LEDs）. The PhC structures were realized using nanospherical-lens photolithography and the selective-area epitaxy method, which ensured the electrical properties of the LEDs through leaving the p-GaN damage-free. At a current of 350 mA, the light output power of LEDs with PhC hole arrays of 450 nm and 600 nm in diameter with the same lattice period of 900 nm were enhanced by 49.3% and 72.2%, respectively, compared to LEDs without a PhC. Furthermore, the LEDs with PhC hole structures showed an obviously smaller divergent angle compared with conventional LEDs, which is consistent with the results of finite-difference time-domain simulation.
In this study, the effect of double superlattices on GaN-based blue light-emitting diodes(LEDs) is analyzed numerically. One of the superlattices is composed of InGaN/GaN, which is designed before the multiple quantum wells(MQWs). The other one is AlInGaN/AlGaN, which is inserted between the last QB(quantum barriers) and p-GaN. The crucial characteristics of double superlattices LEDs structure, including the energy band diagrams, carrier concentrations in the active region, light output power, internal quantum efficiency, respectively,were analyzed in detail. The simulation results suggest that compared with the conventional AlGaN electronblocking layer(EBL) LED, the LED with double superlattices has better performance due to the enhancement of electron confinement and the increase of hole injection. The double superlattices can make it easier for the carriers tunneling to the MQWs, especially for the holes. Furthermore, the LED with the double superlattices can effectively suppress the electron overflow out of multiple quantum wells simultaneously. From the result, we argue that output power is enhanced dramatically, and the efficiency droop is substantially mitigated when the double superlattices are used.
Ga N基白光LED不断向高功率、高性能和长寿命方向发展,Ga N基白光LED的热性能成为在使用中的重要参数。采用瞬态热测量方式,对工作在不同的温度和驱动电流的Ga N基发光二极管的热性能进行了研究。通过比较光、电特性发现LED芯片对器件热阻影响很小,但同时管芯附着层对LED器件的热特性起着非常重要的作用。此外,热电阻随着电流和环境温度的升高而增加,主要归因于管芯附着层声子或粒子的平均自由程减小。
The light extraction enhancement of freestanding GaN-based flip-chip light-emitting diodes （FS- FCLEDs） using two-step roughening methods is investigated. The output power of LEDs fabricated by using one-step and two-step roughening methods are compared. The results indicate that two-step roughening meth- ods show more potential for light extraction. Compared with flat FS-FCLEDs, the output power of FS-FCLEDs with a nanotextured hemisphere surface shows an enhancement of 90.7%.
