The enhancement of light-emitting efficiency using GaN-based multiple quantum well light-emitting diodes with nanopillar arrays

The quest for higher modulation speed and lower energy consumption has inevitably promoted the rapid development of semiconductor-based solid lighting devices in recent years. GaN-based light-emitting diodes （LEDs） have emerged as promising candidates for achieving high efficiency and high intensity, and have received increasing attention among many researchers in this field. In this paper, we use a self-assembled array-patterned mask to fabricate InGaN/GaN multi- quantum well （MQW） LEDs with the intention of enhancing the light-emitting efficiency. By utilizing inductively coupled plasma etching with a self-assembled Ni cluster as the mask, nanopillar arrays are formed on the surface of the InGaN/GaN MQWs. We then observe the structure of the nanopillars and find that the V-defects on the surface of the conventional structure and the negative effects of threading dislocation are effectively reduced. Simultaneously, we make a comparison of the photoluminescence （PL） spectrum between the conventional structure and the nanopillar arrays, achieved under an experimental set-up with an excitation wavelength of 325 mm. The analysis demonstrates that MQW-LEDs with nanopillar arrays achieve a PL intensity 2.7 times that of conventional LEDs. In response to the PL spectrum, some reasons are proposed for the enhancement in the light-emitting efficiency as follows： 1） the improvement in crystal quality, namely the reduction in V-defects; 2） the roughened surface effect on the expansion of the critical angle and the attenuated total reflection; and 3） the enhancement of the light-extraction efficiency due to forward scattering by surface plasmon polariton modes in Ni particles deposited above the p-type GaN layer at the top of the nanopillars.     

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.     

GaN/In_xGa_(1-x)N型最后一个量子势垒对发光二极管内量子效率的影响

GaN/In_xGa_(1-x)N型最后一个量子势垒结构能有效提高发光二极管(LED)器件内量子效率,缓解LED效率随输入电流增大而衰减的问题.本文综述了该结构及其结构变化——In组分梯度递增以及渐变、GaN/In_xGa_(1-x)N界面极化率改变等对改善LED器件性能的影响及优势,归纳总结了不同结构的GaN/In_xGa_(1-x)N型最后一个量子垒的工作机理,阐明极化反转是该结构提高LED性能的根本原因.在综述该结构发展的基础之上,通过APSYS仿真计算,进一步探索和深入分析了该结构中In_xGa_(1-x)N层的In组分及其厚度变化对LED内量子效率的影响.结果表明:In组分的增加有助于在GaN/In_xGa_(1-x)N界面产生更多的极化负电荷,增加GaN以及电子阻挡层处导带势垒高度,减少电子泄漏,从而提高LED的内量子效率;但GaN/In_xGa_(1-x)N型最后一个量子势垒中In_xGa_(1-x)N及GaN层厚度的变化由于会同时引起势垒高度和隧穿效应的改变,因而In_xGa_(1-x)N和GaN层的厚度存在一个最佳比值以实现最大化的减小漏电子,提高内量子效率.     

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.     

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     

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.     

Fabrication and optical properties of InGaN/GaN multiple quantum well light emitting diodes with amorphous BaTiO₃ ferroelectric film

BaTiO3(BTO) ferroelectric thin films are prepared by the sol-gel method.The fabrication and the optical properties of an InGaN/GaN multiple quantum well light emitting diode(LED) with amorphous BTO ferroelectric thin film are studied.The photoluminescence(PL) of the BTO ferroelectric film is attributed to the structure.The ferroelectric film which annealed at 673 K for 8 h has the better PL property.The peak width is about 30 nm from 580 nm to 610 nm,towards the yellow region.The mixed electroluminescence(EL) spectrum of InGaN/GaN multiple quantum well LED with 150-nm thick amorphous BTO ferroelectric thin film displays the blue-white light.The Commission Internationale De L’Eclairage(CIE) coordinate of EL is(0.2139,0.1627).EL wavelength and intensity depends on the composition,microstructure and thickness of the ferroelectric thin film.The transmittance of amorphous BTO thin film is about 93% at a wavelength of 450 nm-470 nm.This means the amorphous ferroelectric thin films can output more blue-ray and emission lights.In addition,the amorphous ferroelectric thin films can be directly fabricated without a binder and used at higher temperatures(200℃-400℃).It is very favourable to simplify the preparation process and reduce the heat dissipation requirements of an LED.This provides a new way to study LEDs.     

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.     

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     

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

A strain-compensated InGaN quantum well(QW) active region employing a tensile AlGaN barrier is analyzed.Its spectral stability and efficiency droop for a dual-blue light-emitting diode(LED) are improved compared with those of the conventional InGaN/GaN QW dual-blue LEDs based on a stacking structure of two In0.18Ga0.82N/GaN QWs and two In0.12Ga0.88N/GaN QWs on the same sapphire substrate.It is found that the optimal performance is achieved when the Al composition of the 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,which can provide a better carrier confinement and effectively reduce leakage current.     

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.     

量子阱结构对有机电致发光器件效率的影响

实验中共制备了五种有机量子阱结构电致发光器件,分别对这五种量子阱结构器件的电致发光特性进行了研究,分析了量子阱结构的周期数和势垒层的厚度对器件电学性能的影响.实验结果表明适当周期数的量子阱结构器件的亮度和电流效率比传统的三层结构器件的要大,主要原因是量子阱结构对电子和空穴的限制作用,这种限制作用提高了电子和空穴在发光层中形成激子和复合的概率,从而提高了发光的亮度和效率.当改变阱结构器件中势阱层的厚度时,也会对器件的亮度和效率产生影响,采用适当的势阱层厚度能够提高器件的亮度和效率. 关键词： 量子阱结构 电致发光 电流效率 光谱     

Fabrication and optical properties of InGaN/GaN multiple quantum well light emitting diodes with amorphous BaTiO3 ferroelectric film

BaTiO3 （BTO） ferroelectric thin films are prepared by the sol,el method. The fabrication and the optical properties of an InGaN/GaN multiple quantum well light emitting diode （LED） with amorphous BTO ferroelectric thin film are studied. The photolumineseence （PL） of the BTO ferroelectric film is attributed to the structure. The ferroeleetric film which annealed at 673 K for 8 h has the better PL property. The peak width is about 30 nm from 580 nm to 610 nm, towards the yellow region. The mixed electroluminescence （EL） spectrum of InGaN/GaN multiple quantum well LED with 150-nm thick amorphous BTO ferroelectric thin film displays the blue-white light. The Commission Internationale De L＇Eclairage （CIE） coordinate of EL is （0.2139, 0.1627）. EL wavelength and intensity depends on the composition, microstructure and thickness of the ferroelectric thin film. The transmittance of amorphous BTO thin film is about 93% at a wavelength of 450 nm-470 nm. This means the amorphous ferroelectrie thin films can output more blue-ray and emission lights. In addition, the amorphous ferroelectric thin films can be directly fabricated without a binder and used at higher temperatures （200 ℃-400 ℃）. It is very favourable to simplify the preparation process and reduce the heat dissipation requirements of an LED. This provides a new way to study LEDs.     

光致电化学法提高垂直结构发光二极管出光效率的研究

研究了在垂直结构发光二极管(VLED)器件中, 光致电化学法(PEC)刻蚀N极性n-GaN的速率受不同刻蚀条件(刻蚀浓度、刻蚀时间和光照强度)的影响. 并选择N极性n-GaN表面含有较理想六角金字塔结构(侧壁角为31°)的样品制成器件, 研究PEC刻蚀对VLED的欧姆接触和光电性能的影响. 结果表明, 与未粗化样品相比, PEC刻蚀后的样品接触电阻率明显降低, 形成更好的欧姆接触; 其电学特性有较好的改善, 光输出功率有明显提高, 在20 mA电流下光输出功率增强了86.1%. 对不同金字塔侧壁角度的光提取效率用时域有限差分法(FDTD)模拟, 结果显示光提取效率在侧壁角度为20°– 40°有显著提高, 在23.6° (GaN-空气界面的全反射角)时达到最大.     

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...     

无序光子晶体提高GaN基蓝光发光二极管光提取效率的研究

亚波长尺度光子晶体结构可有效提升发光二极管(LED)的光提取效率(LEE),然而在制造过程中会存在缺陷或无序.利用时域有限差分法对理想方形光子晶体结构进行了优化,在此基础上对三种无序光子晶体结构进行了仿真,研究了光子晶体结构参数的无序变化对GaN基蓝光LED LEE的影响.结果表明,光子晶体空气孔位置和半径的无序变化使优化的80 nm光子晶体LED的LEE下降,而可使非优化的60nm光子晶体LED的LEE增加;当光子晶体空气孔位置和半径的无序变化量从0到士20 nm之间变化时,LEE最大会产生53.8％的浮动;光子晶体刻蚀深度的无序变化对LEE影响较小,一般可以忽略,研究结果为高性能蓝光光子晶体LED的设计制作提供了重要的理论参考.     

Tuning a nano-pillar array for enhancing the photoluminescence extraction efficiency of GaN-based light-emitting diodes

We demonstrate the fabrication of hexagonal nano-pillar arrays at the surface of GaN-based light-emitting diodes (LEDs) by nanosphere lithography. By varying the oxygen plasma etching time, we could tune the size and shape of the pillar. The nano-pillar has a truncated cone shape. The nano-pillar array serves as a gradual effective refractive index matcher, which reduces the reflection and increases light cone. It is found that the patterned surface absorbs more pumping light. To compare extraction efficiencies of LEDs, it is necessary to normalize the photoluminescence power spectrum with total absorption rate under fixed pumping power, then we could obtain the correct enhancement factor of the photoluminescence extraction efficiency and optimized structure. The highest enhancement factor of the extraction efficiency is 10.6.     

Temperature-dependent efficiency droop behaviors of GaN-based green light-emitting diodes

The efficiency droop behaviors of GaN-based green light-emitting diodes (LEDs) are studied as a function of temperature from 300 K to 480 K. The overall quantum efficiency of the green LEDs is found to degrade as temperature increases, which is mainly caused by activation of new non-radiative recombination centers within the LED active layer. Meanwhile, the external quantum efficiency of the green LEDs starts to decrease at low injection current level (<1 A/cm2 ) with a temperature-insensitive peak-efficiency-current. In contrast, the peak-efficiency-current of a control GaN-based blue LED shows continuous up-shift at higher temperatures. Around the onset point of efficiency droop, the electroluminescence spectra of the green LEDs also exhibit a monotonic blue-shift of peak energy and a reduction of full width at half maximum as injection current increases. Carrier delocalization is believed to play an important role in causing the efficiency droop in GaN-based green LEDs.     

Internal quantum efficiency of GaN-based light-emitting diodes grown on silicon substrates determined from rate equation analyses

We present a convenient and reliable method for determining the internal quantum efficiency (IQE) in GaN-based blue light-emitting diodes (LEDs) grown on Si(111) substrates based on the carrier rate equation model. By using the peak point of the efficiency curve in photoluminescence (PL) measurements as the parameter of the rate equation analysis, the IQE can be unambiguously determined without any pre-assumed parameters. The theoretical IQE model is used to fit the measured PL efficiency curves and the IQE of LED samples are determined. The maximum IQE of the LED sample grown on the Si substrate was obtained to be 0.74, which is found to agree well with the results obtained by conventional temperature-dependent PL measurements.     

Enhancing light extraction of GaN-based blue light-emitting diodes by a tuned nanopillar array

Surface patterning of p-GaN to improve the light extraction efficiency of GaN-based blue light-emitting diodes(LEDs) has been investigated. Periodic nanopillar arrays on p-GaN have been fabricated by polystyrene(PS) nanosphere lithography; the diameter of the nanopillars can be tuned to optimize the electrical and optical properties of the LEDs. The electroluminescence intensity of the nanopillar-patterned LEDs is better than that of conventional LEDs; the greatest enhancement increased the intensity by a factor of 1.41 at a 20 mA injection current. The enhancements can be explained by a model of bilayer film on a GaN substrate. This method may serve as a practical approach to improve the efficiency of light extraction from LEDs.