Efficiency droop alleviation in blue light emitting diodes using the InGaN/GaN triangular-shaped quantum well

The InGaN/GaN blue light emitting diode(LED) is numerically investigated using a triangular-shaped quantum well model,which involves analysis on its energy band,carrier concentration,overlap of electron and hole wave functions,radiative recombination rate,and internal quantum efficiency.The simulation results reveal that the InGaN/GaN blue light emitting diode with triangular quantum wells exhibits a higher radiative recombination rate than the conventional light emitting diode with rectangular quantum wells due to the enhanced overlap of electron and hole wave functions(above 90%) under the polarization field.Consequently,the efficiency droop is only 18% in the light emitting diode with triangular-shaped quantum wells,which is three times lower than that in a conventional LED.     

Controlling the defect density to improve the output power of InGaN/GaN-based vertical light-emitting diodes by using substrates patterned with SiO2 lenses

In this study, we investigated the effect of SiO₂ lenses on the output power of InGaN/GaN-based vertical light-emitting diodes (VLEDs; wavelength = 445 nm) and compared the results to those of reference VLEDs without the SiO₂ lenses (planar samples). Arrays of SiO₂ lenses (pitch = 3 μm, width = 2.5 μm, height = 1.0 μm) were formed on c-plane sapphire substrates. The external quantum efficiency (EQE) of the packaged VLEDs with planar and patterned substrates was characterised. At 5 mA, the EQE of the patterned samples was 150% higher than that of the planar samples. A patterned, N-polar, n-GaN sample contained far fewer nanopipes (approximately 2.2 × 10⁵ cm^?2) than a planar n-GaN sample (approximately 2.4 × 10⁶ cm^?2). Furthermore, the patterned samples contained far fewer threading dislocations (approximately 1.0 × 10⁸ cm^?2) than the planar samples (approximately 5.0 × 10⁸ cm^?2). Scanning electron microscopy (SEM) images showed that the photoelectrochemical (PEC)-etched patterned samples contained cones that were 150% larger than that of the PEC-etched planar samples. In addition, SEM images, cathode luminescence measurements and finite-difference time-domain simulations were used to characterise the improved light output of the patterned samples.     

Droop improvement in blue InGaN light-emitting diodes with GaN/InGaN superlattice barriers

GaN/InGaN superlattice barriers are used in InGaN-based light-emitting diodes （LEDs）. The electrostatic field in the quantum wells, electron hole wavefunction overlap, carrier concentration, spontaneous emission spectrum, light-current performance curve, and internal quantum efficiency are numerically investigated using the APSYS simulation software. It is found that the structure with GaN/InGaN superlattice barriers shows improved light output power, and lower current leakage and efficiency droop. According to our numerical simulation and analysis, these improvements in the electrical and optical characteristics are mainly attributed to the alleviation of the electrostatic field in the active region.     

功率型发光二极管的研究与应用进展

文章首先对功率型发光二极管的起源和发展作了回顾和简要的叙述．然后以固体光源照明为目标，给出了几种可见光功率发光二极管芯片和封装的典型结构，并且对它们各自的特点进行了比较．最后指出了功率发光二极管作为固体光源取代真空灯泡用于照明在未来的五至十年内将成为现实．     

Efficiency enhancement of InGaN based blue light emitting diodes with InGaN/GaN multilayer barriers

The advantages of InGaN based light-emitting diodes with InGaN/GaN multilayer barriers are studied.It is found that the structure with InGaN/GaN multilayer barriers shows improved light output power,lower current leakage,and less efficiency droop over its conventional InGaN/GaN counterparts.Based on the numerical simulation and analysis,these improvements on the electrical and the optical characteristics are mainly attributed to the alleviation of the electrostatic field in the quantum wells(QWs) when the InGaN/GaN multilayer barriers are used.     

Effects of reflector-induced interferences on light extraction of InGaN/GaN vertical light emitting diodes

We provide a large F-P cavity model to analyze the effects of reflector-induced interferences on light extraction of InGaN/GaN vertical light emitting diodes (VLEDs). It shows that the distance (d) between the active region and the metal reflector has a significant influence on extraction efficiency due to interferences. The maximum in extraction efficiency corresponding to the optimal d is about three times the neighboring minimum. The reflector of different metals is considered in this model and the results show that the optimal d and the value of the maximum in the extraction efficiency are directly related to the type of metal, which can be attributed to varied reflection phase shift and reflectivity on different metals, respectively.     

InGaN/GaN多量子阱蓝色发光二极管的实验与模拟分析

分别采用量子阱模型和量子点模型对蓝色InGaN/GaN多量子阱发光二极管电学和光学特性进行模拟,并和实验测量结果进行了比对,结果发现,量子点模型的引入,很好地解决了I-V和电致发光二方面的实验与理论模型间符合程度不好的问题.同时,在I-V曲线特性模拟中发现,在量子点理论模型的基础上,只有考虑到载流子的非平衡量子传输效应,才能得到和实验相接近的I-V曲线,揭示着在InGaN/GaN 多量子阱发光二极管电输运特性中,载流子的非 关键词： InGaN/GaN 发光二极管 数值模拟 量子点模型     

In组分渐变提高InGaN/GaN多量子阱发光二极管发光性能

利用金属有机物化学气相沉积系统在蓝宝石衬底上通过有源层的变温生长,得到In组分渐变的量子阱结构,从而获得具有三角形能带结构的InGaN/GaN多量子阱发光二极管(LED)(简称三角形量子阱结构LED).变温光致发光谱结果表明,相对于传统具有方形能带结构的量子阱LED(简称方形量子阱结构LED),三角形量子阱结构有效提高了量子阱中电子和空穴波函数的空间交叠,从而增加了LED的内量子效率;电致发光谱结果表明,三角形量子阱结构LED器件与传统结构LED器件相比,明显改善了发光峰值波长随着电流的蓝移现象.通过以上     

GaN基蓝光发光二极管正向电压温度特性研究

对GaN基蓝光发光二极管（LED）正向电压温度特性进行了研究,发现在温度较高时,正向电压随温度的变化系数逐渐减小,直至出现拐点,正向电压随温度的变化系数由负数变为正数.此时若继续升高温度,则正向电压随温度升高迅速增加,并常常伴随有器件失效的现象发生.在小电流情况下,这种现象不很明显,随着电流的增加,现象表现得越来越明显,拐点出现的温度也越来越低,而且温度超过拐点之后,正向电压值增加得更快.通过与相同封装的另一组器件测试结果对比,排除了封装材料玻璃转换温度的影响.分析认为,这一现象的出现是由器件等效串联电阻 关键词： 发光二极管 氮化镓 正向电压 温度系数     

Si(110)和Si(111)衬底上制备InGaN/GaN蓝光发光二极管

分别在Si(110)和Si(111)衬底上制备了In Ga N/Ga N多量子阱结构蓝光发光二极管(LED)器件.利用高分辨X射线衍射、原子力显微镜、室温拉曼光谱和变温光致发光谱对生长的LED结构进行了结构表征.结果表明,相对于Si(111)上生长LED样品,Si(110)上生长的LED结构晶体质量较好,样品中存在较小的张应力,具有较高的内量子效率.对制备的LED芯片进行光电特性分析测试表明,两种衬底上制备的LED芯片等效串联电阻相差不大,在大电流注入下内量子效率下降较小;但是,相比于Si(111)上制备LED芯片,Si(110)上LED芯片具有较小的开启电压和更优异的发光特性.对LED器件电致发光(EL)发光峰随驱动电流的变化研究发现,由于Si(110)衬底上LED结构中阱层和垒层存在较小的应力/应变而在器件中产生较弱的量子限制斯塔克效应,致使Si(110)上LED芯片EL发光峰随驱动电流的蓝移量更小.     

大功率GaN基发光二极管的电流扩展效应及电极结构优化研究

定性分析了GaN基LED的电流扩展效应,发现电流密度和电流横向扩展的有效长度对电流均匀扩展有很大影响.基于此,对GaN基大功率LED提出了优化的电极结构,以减缓电流拥挤效应,降低器件串联电阻.通过用红外热像仪测量器件表面的温度分布,发现具有优化的环形插指电极结构的GaN基大功率LED表面温度分布比较均匀,证明芯片接触处电流扩展均匀,局部电流密度降低,减小了焦耳热的产生,增强了器件的可靠性. 关键词： 氮化镓 发光二极管 电流扩展 电极结构优化     

大功率GaN基发光二极管的电流扩展效应及电极结构优化研究

定性分析了GaN基LED的电流扩展效应，发现电流密度和电流横向扩展的有效长度对电流均匀扩展有很大影响.基于此，对GaN基大功率LED提出了优化的电极结构，以减缓电流拥挤效应，降低器件串联电阻.通过用红外热像仪测量器件表面的温度分布，发现具有优化的环形插指电极结构的GaN基大功率LED表面温度分布比较均匀，证明芯片接触处电流扩展均匀，局部电流密度降低，减小了焦耳热的产生，增强了器件的可靠性.     

Air-gap embedding GaN template for enhanced emission from light-emitting diodes

Selective growth by metal-organic chemical vapor deposition (MOCVD), and electrochemical etching of a heavily Si-doped GaN (n⁺-GaN) interlayer were employed to obtain air-gaps embedded in a u-GaN layer. As confirmed by Raman spectroscopy, the introduction of an n⁺-GaN, which was later etched to obtain air-gaps, also enhanced the strain-compliance of GaN epilayer on sapphire substrate. An enhanced electroluminescence emission was observed from the light-emitting diodes (LEDs) fabricated on the air-gap embedding template. Using theoretical LED simulation, it was discerned that the increase in optical emission from the LED was caused predominantly by the redirection of photons at GaN/air-gap interface. Finite-difference time domain (FDTD) simulation method was employed to understand the mechanism of optical emission enhancement and its spatial variation over the LED surface.     

Two-color light-emitting diodes with polarization-sensitive high extraction efficiency based on graphene

In the present study, graphene photonic crystals are employed to enhance the light extraction efficiency(LEE) of two-color, red and blue, light-emitting diode(LED). The transmission characteristics of one-dimensional(1D) Fibonacci graphene photonic crystal LED(FGPC-LED) are investigated by using the transfer matrix method and the scaling study is presented. We analyzed the influence of period, thickness, and permittivity in the structure to enhance the LEE. The transmission spectrum of 1D FGPC has been optimized in detail. In addition, the effects of the angle of incidence and the state of polarization are investigated. As the main result, we found the optimum values of relevant parameters to enhance the extraction of red and blue light from an LED as well as provide perfect omnidirectional and high peak transmission filters for the TE and TM modes.     

3D simulation of InGaN/GaN micro-ring light-emitting diodes

We employed the APSYS software to perform 3D electrical and ray-tracing simulations on micro-ring light-emitting diodes (LEDs) to verify previous experimental findings that they have higher extraction efficiency than micro-disk and broad area LEDs. 3D ray-tracing indicates the importance of inter-ring optical interactions. Furthermore we found that the higher light extraction efficiency is at the expense of reduced internal quantum efficiency (IQE) as injection current is increased.     

Improving performances of ITO/GaP contact on AlGaInP light-emitting diodes

In this paper AlGaInP light emitting diodes with different types of electrodes:Au/Zn/Au-ITO Au/Ti-ITO Au/Ge/Ni-ITO and Au-ITO are fabricated. The photoelectricity properties of those LEDs are studied. The results show that the Au/Zn/Au electrode greatly improves the performance of LEDs compared with the other electrodes. Because the Au/Zn/Au electrode not only forms a good Ohmic contact with indium tin oxide (ITO), but also reduces the specific contact resistances between ITO and GaP, which are 1.273×10-6 ·cm2 and 1.743×10-3 ·cm2 between Au/Zn/Au-ITO and ITO-GaP respectively. Furthermore, the textured Zn/Au-ITO/Zn electrode is designed to improve the performances of LEDs, reduce the forward-voltage of the LED from 1.93 to 1.88 V, and increase the luminous intensity of the LEDs from 126 to 134 mcd when driven at 20 mA.     

Recent progress in single chip white light-emitting diodes with the InGaN underlying layer

Tremendous progress has been achieved in white light-emitting diodes (LEDs). To further improve the quality of white light and simplify the fabrication process, a single chip white-light LED with the InGaN underlying layer (UL) was studied and fabricated. The turn-on voltage of this type of LED was 2.7 V, and the spectrum at a forward bias current of 20 mA was comprised of blue (443 nm) and yellow (563 nm) lights. The intensity ratio of blue to yellow light was almost constant with the in- creasing injectio...     

Use of graphene for forming Al-based p-type reflectors for near ultraviolet InGaN/AlGaN-based light-emitting diode

We demonstrated the improved performance of near UV (365 nm) InGaN/AlGaN-based LEDs using highly reflective Al-based p-type reflectors with graphene sheets as a diffusion barrier. The use of graphene sheets did not degrade the reflectance of ITO/Al contacts, viz. ∼81% at 365 nm. The ITO/graphene/Al contacts annealed at 300 °C exhibited better ohmic behavior with a specific contact resistance of 1.5 × 10⁻³ Ωcm² than the ITO/Al contact (with 9.5 × 10⁻³ Ωcm²). Near UV LEDs fabricated with the ITO/graphene/Al contact annealed at 300 °C showed a 7.2% higher light output (at 0.1 W) than LEDs with the ITO/Al reflector annealed at 300 °C. The SIMS results exhibited that, unlike the ITO/graphene/Al, the ITO/Al contacts undergo a significant indiffusion of Al atoms toward the GaN after annealing. Furthermore, both Ga and Mg atoms were also more extensively outdiffused in the ITO/Al contacts after annealing. On the basis of the SIMS and electrical results, the possible explanations for the annealing-induced degradation of the ITO/Al contacts are described and discussed.     

The influence of A1GaN/GaN superlattices as electron blocking layers on the performance of blue InGaN light-emitting diodes

P-A1GaN/P-GaN superlattices are investigated in blue InGaN light-emitting diodes as electron blocking layers. The simulation results show that efficiency droop is markedly improved due to two reasons： （i） enhanced hole concentration and hole carrier transport efficiency in A1GaN/GaN superlattices, and （ii） enhanced blocking of electron overflow between multiple quantum-wells and A1CaN/GaN superlattices.