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.     

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.     

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     

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.     

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.     

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.     

Properties of InGaN/GaN quantum wells and blue light emitting diodes

We have reported the effects of growth interruption time on the optical and structural properties of high indium content In_xGa_1−xN/GaN (x>0.2) multilayer quantum wells (QWs). The InGaN/GaN QWs were grown on c-plane sapphire by metal organic chemical vapor deposition. The interruption was carried out by closing the group-III metal organic sources before and after the growth of the InGaN QW layers. The transmission electron microscopy (TEM) images show that with increasing interruption time, the quantum-dot-like region and well thickness decreases due to indium reevaporation or the thermal etching effect. As a result the photoluminescence (PL) peak position was blue-shifted and the intensity was reduced. The sizes and number of V-defects did not differ with the interruption time. The interruption time is not directly related to the formation of defects. The V-defect originates at threading dislocations and inversion domain boundaries due to higher misfit strain. Temperature dependent PL spectra support the results of TEM measurements. Also, the electroluminescence spectra of light-emitting diode show that dominant mechanism in InGaN/GaN QWs is a localized effect in the quantum-dot-like regions.     

InGaN green light emitting diodes with deposited nanoparticles

We grew an InGaN/GaN-based light-emitting diode (LED) wafer by metal–organic chemical vapor deposition (MOCVD), fabricated devices by optical lithography, and successfully deposited ellipsoidal Ag nano-particles by way of e-beam lithography on top. The diodes exhibited good device performance, in which we expected an enhancement of the radiated intensity by the simulations and emission measurements. The obtained results showed the feasibility of plasmon-assisted LED emission enhancement.     

InGaN/GaN量子阱垒层和阱层厚度对GaN基激光器性能的影响及机理

采用LASTIP软件研究了InGaN/GaN(In组分为15%)量子阱垒层和阱层厚度对GaN基蓝紫光激光器性能的影响及机理. 模拟计算结果表明, 当阱层太薄或太厚时, GaN基激光器的阈值电流增加、输出功率下降, 最优的阱层厚度为4.0 nm左右; 当阱层厚度太薄时, 载流子很容易泄漏, 而当阱层厚度太厚时, 极化效应导致发光效率降低, 研究还发现, 与垒层厚度为7 nm 相比, 垒层厚度为15 nm时激光器的阈值电流更低、输出功率更高, 因此适当地增加垒层厚度能显著抑制载流子泄漏, 从而改善激光器性能.     

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     

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     

Enhanced performances of InGaN/GaN-based blue light-emitting diode with InGaN/AlInGaN superlattice electron blocking layer

InGaN/AlInGaN superlattice(SL) is designed as the electron blocking layer(EBL) of an InGaN/GaN-based lightemitting diode(LED). The energy band structure, polarization field at the last-GaN-barrier/EBL interface, carrier concentration, radiative recombination rate, electron leakage, internal quantum efficiency(IQE), current–voltage(I–V) performance curve, light output–current(L–I) characteristic, and spontaneous emission spectrum are systematically numerically investigated using APSYS simulation software. It is found that the fabricated LED with InGaN/AlInGaN SL EBL exhibits higher light output power, low forward voltage, and low current leakage compared with those of its counterparts.Meanwhile, the efficiency droop can be effectively mitigated. These improvements are mainly attributed to the higher hole injection efficiency and better electron confinement when InGaN/AlInGaN SL EBL is used.     

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.     

Enhanced performances of InGaN/GaN-based blue light-emitting diode with InGaN/AIInGaN superlattice electron blocking layer

InGaN/AIlnGaN superlattice （SL） is designed as the electron blocking layer （EBL） of an InGaN/GaN-based light- emitting diode （LED）. The energy band structure, polarization field at the last-GaN-barrier/EBL interface, carrier concen- tration, radiative recombination rate, electron leakage, internal quantum efficiency （IQE）, current-voltage （l-V） perfor- mance curve, light output-current （L-l） characteristic, and spontaneous emission spectrum are systematically numerically investigated using APSYS simulation software. It is found that the fabricated LED with InGaN/AIInGaN SL EBL exhibits higher light output power, low forward voltage, and low current leakage compared with those of its counterparts. Meanwhile, the efficiency droop can be effectively mitigated. These improvements are mainly attributed to the higher hole injection efficiency and better electron confinement when InGaN/AIlnGaN SL EBL is used.     

A GaN AlGaN InGaN last quantum barrier in an InGaN/GaN multiple-quantum-well blue LED

The advantages of a GaN–AlGaN–InGaN last quantum barrier(LQB) in an InGaN-based blue light-emitting diode are analyzed via numerical simulation. We found an improved light output power, lower current leakage, higher recombination rate, and less efficiency droop compared with conventional GaN LQBs. These improvements in the electrical and optical characteristics are attributed mainly to the specially designed GaN–AlGaN–InGaN LQB, which enhances electron confinement and improves hole injection efficiency.     

Effect of nanometer sized Ni dot/Ag/Pt reflection metal of InGaN light emitting diodes on conversion efficiency of yellow phosphor

Nanometer sized Ni-dot/Ag/Pt metal coating films with different Ni-dot sizes were incorporated in InGaN based blue light emitting diodes as p-type reflection metal to enhance the extraction efficiency of blue light. For the reflectivity change from 84% to 93.7%, the optical output power of blue light before phosphor coating and the luminous flux of white light after phosphor deposition have improved by 49.3% and 58.2% at 350 mA, respectively. For the presented white light devices, the blue and yellow luminescence respectively has been improved relatively up to 1.52 and 1.67 times. The improved extraction efficiency of yellow light from the phosphor integrated blue light emitting diode is thought to be attributed to the reduction of round trip loss of blue and yellow light inside the device due to the enhanced reflectivity of nanometer sized Ni/Ag/Pt p-type reflection metal.     

Analysis of localization effect in blue-violet light emitting InGaN/GaN multiple quantum wells with different well widths

Four blue-violet light emitting InGaN/GaN multiple quantum well(MQW) structures with different well widths are grown by metal–organic chemical vapor deposition. The carrier localization effect in these samples is investigated mainly by temperature-dependent photoluminescence measurements. It is found that the localization effect is enhanced as the well width increases from 1.8 nm to 3.6 nm in our experiments. The temperature induced PL peak blueshift and linewidth variation increase with increasing well width, implying that a greater amplitude of potential fluctuation as well as more localization states exist in wider wells. In addition, it is noted that the broadening of the PL spectra always occurs mainly on the low-energy side of the PL spectra due to the temperature-induced band-gap shrinkage, while in the case of the widest well, a large extension of the spectral curve also occurs in the high energy sides due to the existence of more shallow localized centers.     

Performance enhancement of an InGaN light-emitting diode with an AIGaN/InGaN superlattice electron-blocking layer

The efficiency enhancement of an InGaN light-emitting diode （LED） with an A1GaN/InGaN superlattice （SL） electron-blocking layer （EBL） is studied numerically, which involves the light-current performance curve, internal quan- tum efficiency electrostatic field band wavefunction, energy band diagram carrier concentration, electron current density, and radiative recombination rate. The simulation results indicate that the LED with an A1GaN/InGaN SL EBL has better optical performance than the LED with a conventional rectangular A1GaN EBL or a normal A1GaN/GaN SL EBL because of the appropriately modified energy band diagram, which is favorable ibr the injection of holes and confinement of elec- trons. Additionally, the efficiency droop of the LED with an AIGaN/InGaN SL EBL is markedly improved by reducing the polarization field in the active region.     

Structure optimisation of short-wavelength ridge-waveguide InGaN/GaN diode lasers

Room-temperature (RT) continuous-wave (CW) operation of the 405-nm ridge-waveguide (RW) InGaN/GaN quantum-well diode lasers equipped with the n-type GaN substrate and two contacts on both sides of the structure has been investigated with the aid of the comprehensive self-consistent simulation model. As expected, the mounting configuration (p-side up or down) has been found to have a crucial impact on the diode laser performance. For the RT CW threshold operation of the otherwise identical diode laser, the p-side up RW laser exhibits as high as nearly 68°C maximal active-region temperature increase whereas an analogous increase for the p-side down laser was equal to only 24°C. Our simulation reveals that the lowest room-temperature lasing threshold may be expected for relatively narrow and deep ridges. For the structure under consideration, the lowest threshold current density of 5.75 kA/cm² has been determined for the 2.2-μm ridge width and the 400-nm etching depth. Then, the active-region temperature increase was as low as only 24 K over RT. For wider 5-μm ridge, this increase is twice higher. An impact of etching depth is more essential for narrower ridges. Quite high values (between 120 and 140 K) of the characteristic parameter T₀ convince very good thermal properties of the above laser.     

Dual-blue light-emitting diode based on strain-compensated InGaN-AlGaN/GaN quantum wells

Strain-compensated InGaN quantum well (QW) active region employing tensile AlGaN barrier is analyzed. Its spectral stability and efficiency droop for dual-blue light-emitting diode (LED) are improved compared with those of the conventional InGaN/GaN QW dual-blue LED based on stacking structure of two In_0.18Ga_0.82N/GaN QWs and two In_0.12Ga_0.88N/GaN QWs on the same sapphire substrate. It is found that the optimal performance is achieved when the Al composition of strain-compensated AlGaN layer is 0.12 in blue QW and 0.21 in blue-violet QW. The improvement performance can be attributed to the strain-compensated InGaN-AlGaN/GaN QW that can provide a better carrier confinement and effectively reduce leakage current.