Comparison between blue lasers and light‐emitting diodes for future solid‐state lighting

Solid‐state lighting (SSL) is now the most efficient source of high color quality white light ever created. Nevertheless, the blue InGaN light‐emitting diodes (LEDs) that are the light engine of SSL still have significant performance limitations. Foremost among these is the decrease in efficiency at high input current densities widely known as “efficiency droop.” Efficiency droop limits input power densities, contrary to the desire to produce more photons per unit LED chip area and to make SSL more affordable. Pending a solution to efficiency droop, an alternative device could be a blue laser diode (LD). LDs, operated in stimulated emission, can have high efficiencies at much higher input power densities than LEDs can. In this article, LEDs and LDs for future SSL are explored by comparing: their current state‐of‐the‐art input‐power‐density‐dependent power‐conversion efficiencies; potential improvements both in their peak power‐conversion efficiencies and in the input power densities at which those efficiencies peak; and their economics for practical SSL.     

LED结温与光谱特性关系的测量

采用恒定驱动电流改变环境温度和恒定环境温度改变驱动电流两种方法分别对直径5 mm封装的AlGaInP型红光和黄光LED,InGaN型绿光和蓝光LED,以及InGaN蓝光+荧光粉的白光LED的结温与其光谱特性进行了测量,得到了不同条件下LED结温与光谱特性的关系.结果表明;AlGaInP LED的峰值波长与结温有良好线性关系,InGaN LED的峰值波长则与结温没有明显对应关系;但白光LED发射光谱的白、蓝功率比与结温有良好线性关系;对AlGaInP LED及蓝光激发的白光LED,通过光谱特性测量可快速、准确地确定光源系统中各LED的结温继而预测光源系统的有效寿命.     

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.     

Investigation of blue InGaN light-emitting diodes with p-AlGaN/InGaN superlattice interlayer

The blue InGaN light-emitting diodes (LEDs), employing a lattice-compensated p-AlGaN/InGaN superlattice (SL) interlayer to link the last quantum barrier and electron blocking layer (EBL), are proposed and investigated numerically. The simulation results indicate that the newly designed LEDs have better hole injection efficiency, lower electron leakage, and smaller electrostatic fields in the active region over the conventional LEDs mainly attributed to the mitigated polarization-induced downward band bending. Furthermore, the markedly improved output power and efficiency droop are also suggested when the conventional LEDs corresponding to experiment data are replaced by the newly designed LEDs.     

红、绿CdSe@ZnS量子点配比对三波段标准白光LED器件的影响

将一步法合成的具有梯度合金结构的红光、绿光CdSe@ZnS量子点与硅胶均匀混合后,作为光转换层涂覆到蓝色InGaN LED芯片上,制备了不含荧光材料的三波段白光LED器件。研究了峰值为650 nm和550 nm的高效率红、绿量子点在硅胶中的含量及配比对白光LED色坐标以及效率的影响。结果表明,当红、绿量子点配比为2：3时,可得到发射纯正白光的QDs-LED器件,色坐标为（0.322 8,0.335 9）、色温为5 725 K,功率效率为26.61 lm/W,显色指数为72.7。光谱中红、蓝、绿三色发光峰的半高宽分别为30,25,38 nm,表明器件具有很好的单色性和高色纯度。     

Effect of localized states on internal quantum efficiency of III‐nitride LEDs

A model is suggested accounting for effects of localized electron and hole states formed by composition fluctuations in the InGaN active region of a III‐nitride LED on non‐radiative carrier recombination at threading dislocations. The model enables explanation of the abnormal temperature dependence of internal quantum efficiency (IQE) of a green LED structure recently observed at low current densities. The theoretical predictions are in quantitative agreement with experiment in the temperature range between 200 K and 453 K. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Temperature‐dependent electroluminescence intensity in green and blue (In,Ga)N multiple‐quantum‐well diodes

The electroluminescence (EL) intensity has been investigated of green and blue (In,Ga)N multiple‐quantum‐well diodes grown on c ‐plane sapphire over a wide temperature range and as a function of current between 0.01 mA and 10 mA. The EL intensity of the green diode with p‐(Al,Ga)N electron blocking layer does not show low‐temperature quenching, especially at low injection levels, previously observed for the blue (In,Ga)N quantum‐well diodes. This finding rules out possi‐ bilities that the freeze‐out of holes at deep Mg acceptor levels and the failure of hole injections through the p‐(Al,Ga)N layer are directly responsible for the EL quenching at temperatures below 100 K. Variations of the EL efficiency with current level suggest that capture/escape efficiencies of injected carriers by the wells play an important role for the determination of EL external quantum efficiency. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Inhomogeneity of a highly efficient InGaN based blue LED studied by three‐dimensional atom probe tomography

The InGaN based multiple quantum well (MQW) structure in a commercially available white light emitting diode (LED) was studied by transmission electron microscopy (TEM) and three‐dimensional atom probe tomography (APT). The average In mole fraction by three‐dimensional (3D) APT was found to be about 18% in the InGaN well which is consistent with the secondary ion mass spectrometry (SIMS) analysis. The In distribution in the InGaN well layer was analyzed by the iso curve mapping of 3D APT and found to be non‐uniform in the InGaN active layer. In clustering or In rich regions in the range of 2–3 nm size were found, in contrast to recent reports. Our results thus indicate that In clustering is essential for high‐brightness InGaN based LEDs. We have also observed a discontinuity in the range of 50–100. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigations on AlGaN‐Based Deep‐Ultraviolet Light‐Emitting Diodes With Si‐Doped Quantum Barriers of Different Doping Concentrations

In this work, we investigate the impact of Si doped AlGaN quantum barriers on the optical powers for [0001] oriented III‐nitride based deep‐ultraviolet light‐emitting diodes (DUV LEDs). The polarization‐induced electric field in the active region is screened as the result of Si‐doped quantum barriers, which gives rise to the improved spatial overlap between electron and hole wave functions. The polarization screening effect within the quantum wells is further proven by the observation of the blue shift for the wavelength. However, the hole distribution across the active region can be significantly retarded if the Si dosage in the quantum barriers is too high. Therefore, the improved radiative recombination within the active region can be realized provided that the Si dosage in the quantum barriers is moderately adjusted to guarantee both the better hole injection efficiency and the screened polarization effect in the multiple quantum wells.     

Excitation dependent quenching of luminescence in LED phosphors

Droop, the decrease of efficiency with increased power density, became a major topic with InGaN LEDs, after its introduction in 2007. This paper provides insight into droop in localized center luminescence phosphors, exemplified here by Eu²⁺ doped materials. This topic is of increasing importance, as high brightness blue LEDs have reached outputs >1 W/mm². The nonlinearities in phosphor quantum efficiency result in drive‐dependent color point shift and reduction of overall efficiency of phosphor converted white LEDs which utilize Eu²⁺ activated phosphors. The efficiency quenching can be traced back to two processes, well‐known in laser physics, excited state absorption or/and cross relaxation by Foerster/Dexter transfer. Both processes lead to reduction in phosphor efficiency, but they can be differentiated. Understanding the root cause of efficiency quenching opens ways to minimize the practical consequences. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

InGaN蓝光与CdTe纳米晶基白光LED

报道了倒装焊InGaN蓝光LED与黄光CdTe纳米晶的复合结构。利用蓝光作为CdTe纳米晶的激发源,通过光的下转换机制,将部分蓝光转化为黄光,复合发射出白光。室温下正向驱动电流为10mA时,发光色品坐标为x=0.29,y=0.30。实验表明,该复合结构白光LED的一大优点在于,复合光的色品坐标几乎不随正向驱动电流大小变化,颜色稳定。     

白光LED衰减的光谱分析

为了研究白光LED衰减的机理,通过试验跟踪并分析了采用YAG荧光粉、荧光粉晶片、RGB三合一方式封装的PLCC-4型白光LED,以及采用YAG荧光粉封装的大功率白光LED的发射光谱老化衰减曲线。试验在相同的环境下,对上述四种类型的白光LED进行了通电老化,同一类型白光LED老化电流及时间相同,老化完成后测试其光谱分布。通过分析光谱分布曲线的变化来研究白光中各色光的衰减情况,通过对比各色光的衰减情况来推断白光LED的衰减原因。分析表明白光LED的衰减主要是由蓝光LED的衰减及荧光粉的猝灭引起:采用YAG荧光粉、采用荧光粉晶片及RGBLED封装的白光LED衰减特性基本相同,白光的衰减主要是由蓝光的衰减引起;大功率白光LED与PLCC-4型白光LED衰减特性稍有不同,白光的衰减除了因蓝光的衰减外,还有荧光粉的衰减所引起的白光衰减,而蓝光的衰减所占比例至少不低于80%。通过上述分析可以进一步推断:在散热条件足够理想的情况下,白光LED的衰减主要由蓝光的衰减引起,而随着系统温度的提升,荧光粉的衰减将加剧白光LED的衰减。所得结果将为白光LED的应用及进一步对白光LED衰减原因的研究提供了参考。     

Effects of multiple interruptions with trimethylindium-treatment in the InGaN/GaN quantum well on green light emitting diodes

In this study, the influence of multiple interruptions with trimethylindium(TMIn)-treatment in InGaN/GaN multiple quantum wells(MQWs) on green light-emitting diode(LED) is investigated. A comparison of conventional LEDs with the one fabricated with our method shows that the latter has better optical properties. Photoluminescence(PL) full-width at half maximum(FWHM) is reduced, light output power is much higher and the blue shift of electroluminescence(EL) dominant wavelength becomes smaller with current increasing. These improvements should be attributed to the reduced interface roughness of MQW and more uniformity of indium distribution in MQWs by the interruptions with TMIn-treatment.     

硅衬底InGaN/GaN基蓝光发光二极管droop效应的研究

InGaN/GaN基阱垒结构LED当注入的电流密度较大时, LED的量子效率随注入电流密度增大而下降, 即droop效应.本文在Si (111)衬底上生长了 InGaN/GaN 基蓝光多量子阱结构的LED,通过将实验测量的光电性能曲线与利用ABC模型模拟的结果进行对比, 探讨了droop效应的成因.结果显示:温度下降会阻碍电流扩展和降低空穴浓度, 电子在阱中分布会越来越不平衡,阱中局部区域中因填充了势能越来越高的电子而溢出阱外, 从而使droop效应随着温度的降低在更小的电流密度下出现且更为严重, 不同温度下实验值与俄歇复合模型模拟的结果在高注入时趋势相反.这此结果表明,引起 droop效应的主因不是俄歇非辐射复合而是电子溢出,电子溢出的本质原因是载流子在阱中分布不均衡.     

Study on GaN-based light emitting diode with InGaN/GaN/InGaN multi-layer barrier

The current study investigates GaN-based light-emitting diodes (LEDs) with InGaN/GaN/InGaN multi-layer barrier (MLB). Simulation results show that GaN-based LEDs with MLB have better performance than conventional GaN-based LEDs with only one GaN barrier because of the enhancement in hole injection into the quantum well and decrease in electron leakage current.     

Bottom-up approaches to microLEDs emitting red,green and blue light based on GaN nanowires and relaxed InGaN platelets

Miniaturization of light-emitting diodes (LEDs) with sizes down to a few micrometers has become a hot topic in both academia and industry due to their attractive applications on self-emissive displays for high-definition televisions, augmented/mixed realities and head-up displays, and also on optogenetics, high-speed light communication, etc. The conventional top-down technology uses dry etching to define the LED size, leading to damage to the LED side walls. Since sizes of microLEDs approach the carrier diffusion length, the damaged side walls play an important role, reducing microLED performance significantly from that of large area LEDs. In this paper, we review our efforts on realization of microLEDs by direct bottom-up growth, based on selective area metal-organic vapor phase epitaxy. The individual LEDs based on either GaN nanowires or InGaN platelets are smaller than 1 μ in our approach. Such nano-LEDs can be used as building blocks in arrays to assemble microLEDs with different sizes, avoiding the side wall damage by dry etching encountered for the top-down approach. The technology of InGaN platelets is especially interesting since InGaN quantum wells emitting red, green and blue light can be grown on such platelets with a low-level of strain by changing the indium content in the InGaN platelets. This technology is therefore very attractive for highly efficient microLEDs of three primary colors for displays.     

Auger carrier leakage in III‐nitride quantum‐well light emitting diodes

Auger induced leakage is shown to be a contributing factor for the internal quantum efficiency (IQE) droop in III‐nitride quantum‐well light emitting diodes (LEDs). The mechanism is based on leakage current from carrier spill‐out of the well originating from energy transfer during Auger recombination. Adding this leakage reduces the Auger coefficient by 50% when compared to a standard Auger model with cubic density dependence. As reference, experimental data of a green quantum‐well LED are taken. Direct leakage due to non‐ideal carrier capture and re‐emission out of the well affects the IQE at current densities much larger than the maximum IQE point. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Performance improvement of InGaN blue light-emitting diodes with several kinds of electron-blocking layers

The performance of InGaN blue light-emitting diodes(LEDs) with different kinds of electron-blocking layers is investigated numerically.We compare the simulated emission spectra,electron and hole concentrations,energy band diagrams,electrostatic fields,and internal quantum efficiencies of the LEDs.The LED using AlGaN with gradually increasing Al content from 0% to 20% as the electron-blocking layer(EBL) has a strong spectrum intensity,mitigates efficiency droop,and possesses higher output power compared with the LEDs with the other three types of EBLs.These advantages could be because of the lower electron leakage current and more effective hole injection.The optical performance of the specifically designed LED is also improved in the case of large injection current.     

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     

Optical properties of ultra-thin InN layer embedded in InGaN matrix for light emitters

We theoretically investigate the optical properties of an ultra-thin InN layer embedded in InGaN matrix for light emitters. The peak emission wavelength extends from ultraviolet (374 nm) to green (536 nm) with InN quantum well thickness increasing from 1 monolayer to 2 monolayers, while the overlap of electron-hole wave function remains at a high level (larger than 90%). Increase of In content in InGaN matrix provides a better approach to longer wavelength emission, which only reduces the spontaneous emission rate slightly compared with the case of increasing In content of the conventional InGaN quantum well. Also, the transparency carrier density derived from gain spectrum is of the same order as that in the conventional blue laser diode. Our study provides skillful design on the development of novel structure InN-based light emitting diodes as well as laser diodes.