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.     

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.     

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.     

Phonon–replica transitions in InGaN/GaN quantum well structures

A side-bump feature in a photoluminescence (PL) spectrum of an InGaN compound was widely observed. With reasonable fitting to PL spectra with three Gaussian distributions, the temperature variations of the peak positions, integrated PL intensities, and peak widths of the main and first side peaks of three InGaN/GaN multiple quantum well samples with different nominal indium contents are shown and interpreted. The existence of the side peaks is attributed to phonon–replica transitions. The variations of the peak position separations and the decreasing trends of the first side peak widths beyond certain temperatures in those samples were explained with the requirement of phonon momentum condition for phonon–replica transitions. In the sample with 25% nominal indium content, the phonon–replica transition could become stronger than the direct transition of localized states.     

垒层厚度对InGaN/GaN多量子阱电注入发光性能的影响及机理

研究了不同垒厚对InGaN/GaN多量子阱电注入发光性能的影响及机理。实验发现,当GaN垒层的厚度从6 nm增大到24 nm时,垒厚的样品发光强度更强,而且当注入电流增加时,适当增加垒厚,可以更显著增加发光强度。进一步结合发光峰位和光谱宽度的研究表明,由于应力和极化效应的存在,当垒层厚度在6~24 nm范围内时,适当增加垒层厚度不仅会使得能带的倾斜加剧,减少电子泄露,而且也会增加InGaN阱层的局域态深度,从而改善量子阱的发光性能。     

InGaN/GaN多量子阱LED载流子泄漏与温度关系研究

通过测量光电流,直接观察了InGaN/GaN量子阱中载流子的泄漏程度随温度升高的变化关系。当LED温度从300K升高到360K时,在相同的光照强度下,LED的光电流增大,说明在温度上升之后,载流子从量子阱中逃逸的数目更多,即载流子泄漏比例增大。同时,光电流的增大在激发密度较低的时候更为明显,而且光电流随温度的增加幅度与激发光子的能量有关。用量子阱-量子点复合模型能很好地解释所观察到的实验现象。实验结果直接证明,随着温度的升高,InGaN/GaN量子阱中的载流子泄漏将显著增加,而且在低激发密度下这一效应更为明显。温度升高导致的载流子泄漏增多是InGaN多量子阱LED发光效率随温度升高而降低的重要原因。     

Temperature dependent growth of InGaN/GaN single quantum well

InGaN/GaN single quantum well (SQW) structures under various InGaN growth temperatures have been grown by metal organic chemical vapor deposition (MOCVD), the surface morphologies and optical properties are investigated. The radius of the typical V-pits on the SQW surface is affected by the InGaN well-temperature, and the surface roughness decreased as the well-temperature reduced. Room-temperature photoluminescence (PL) and cathode luminescence (CL) shows the quantum well and quantum dot (QD)-like localized state light emission of the SQWs grown at 700 and 690 °C, respectively, whereas the samples grown at 670 and 650 °C present hybrid emission peaks. Excitation power dependent PL spectra indicates the QD-like localized state emission dominates at low excitation power and the quantum well emission starts to take over at high excitation power.     

InGaN/GaN多量子阱蓝光发光二极管老化过程中的光谱特性

系统地研究了小注入电流(＜4 mA)下InGaN/GaN多量子阱结构蓝光发光二极管的发光光谱特性在老化过程中的变化。对比老化前后的电致发光(EL)光谱,发现在注入电流1 mA下的峰值波长(peak wavelength)和半高宽(FWHM)随老化时间增加而减小,变化过程分两个阶段：前期(＜100 h)减小速度较快,而后逐渐变缓,呈现出与LEDs的发光光功率一致的变化规律,说明LEDs的等效极化电场在老化过程中减弱,这一变化和量子阱内缺陷的增加有明确的关系。通过电学特性测量发现同一结电压(V_j=1.8 V)下的结电容C_j和由交流小信号I—V方法计算得到的注入电流1 mA下的结电压V_j随老化时间增加而增大,明确了在同等小注入电流下量子阱内的载流子浓度随老化过程增加。分析表明在老化过程中InGaN/GaN 多量子阱结构蓝光发光二极管量子阱内的缺陷及其束缚的载流子数量增加,形成了增强的极化电场屏蔽效应,减弱的等效极化电场导致了量子阱的能带倾斜变小,带边辐射复合能量增大,能态密度增多,对应的发光过程的峰值波长变短(蓝移),半高宽变窄。     

The exciton-longitudinal-optical-phonon coupling in InGaN/GaN single quantum wells with various cap layer thicknesses

This paper studies the exciton-longitudinal-optical-phonon coupling in InGaN/GaN single quantum wells with various cap layer thicknesses by low temperature photoluminescence (PL) measurements.With increasing cap layer thickness,the PL peak energy shifts to lower energy and the coupling strength between the exciton and longitudinal-optical (LO) phonon,described by Huang-Rhys factor,increases remarkably due to an enhancement of the internal electric field.With increasing excitation intensity,the zero-phonon peak shows a blueshift and the Huang-Rhys factor decreases.These results reveal that there is a large built-in electric field in the well layer and the exciton-LO-phonon coupling is strongly affected by the thickness of the cap layer.     

Surface optical phonon modes in ternary aligned crystalline InGaN/GaN multi‐quantum‐well nanopillar arrays

The optical properties of focused ion beam‐engraved, perfectly aligned and spatially controlled multi‐quantum‐well (MQW) InGaN/GaN nanopillars were investigated. Crystalline MQW nanopillars with a diameter of 30–95 nm and high aspect ratios (7:1–16:1) showed a maximum of three‐fold enhancement in emission intensity per unit active area. A light‐emitting contour map of Taiwan is drawn with a nanopillar structure to demonstrate the site control of the technique adopted in the present study. Raman scattering studies were used to characterize the newly created surfaces. Unknown peaks in GaN and InGaN nanostructures are identified for surface optical (SO) phonon modes with proper assignments of wave vectors using multiple excitations, and the SO mode for the ternary phase is reported for the first time. Copyright © 2009 John Wiley & Sons, Ltd.     

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.     

In含量对InGaN/GaN LED光电性能的影响

运用软件模拟和理论计算的方法分析了In含量对发光二极管光电性能的影响,研究了In含量与光谱功率密度、量子阱中载流子的浓度、辐射速率、发光功率等之间的关系。分析结果表明:电子泄漏与能带填充是影响光电性能的主要原因。当In含量较低时,随着电流密度增大(<8 kA/cm2),光谱发生蓝移程度相对较小,但电流密度太大(>8 kA/cm2)会造成电子泄漏,发光功率降低;而当In含量较高时,随着电流密度增大,光谱发生蓝移程度相对较大,但在电流密度较大时,会获得较高的发光功率。因此,为了使InGaN/GaN发光二极管获得最大量子效率与发光效率,应该根据电流密度的大小(8 kA/cm2)来选择In含量的高低。     

垒掺In提高InGaN/GaN多量子阱发光特性

利用金属有机物化学气相淀积技术在蓝宝石衬底上生长InGaN/GaN多量子阱结构.对多量子阱垒层掺In和非掺In进行了比较研究，结果表明，垒掺In 的样品界面质量变差，但明显增加了光致发光谱的峰值强度和积分强度，带边峰与黄光峰强度之比增大，降低了表面粗糙度.利用这两种结构制备了相应的发光二极管(LED)样品.通过电荧光测量可知，垒掺In的LED比非掺In的LED有较高的发光强度和相对均匀的波长，这主要是由于垒掺In后降低了阱与垒之间晶格失配的应力，从而降低了极化电场，提高了辐射复合效率. 关键词： InGaN/GaN多量子阱 X射线双晶衍射 原子力显微镜 光致发光     

Carrier transport via V-shaped pits in InGaN/GaN MQW solar cells

Carrier transport via the V-shaped pits(V-pits) in InGaN/GaN multiple-quantum-well(MQW) solar cells is numerically investigated. By simulations, it is found that the V-pits can act as effective escape paths for the photo-generated carriers. Due to the thin barrier thickness and low indium composition of the MQW on V-pit sidewall, the carriers entered the sidewall QWs can easily escape and contribute to the photocurrent. This forms a parallel escape route for the carries generated in the flat quantum wells. As the barrier thickness of the flat MQW increases, more carriers would transport via the V-pits. Furthermore, it is found that the V-pits may reduce the recombination losses of carriers due to their screening effect to the dislocations. These discoveries are not only helpful for understanding the carrier transport mechanism in the InGaN/GaN MQW, but also important in design of the structure of solar cells.     

具有超晶格应力调制结构的绿光InGaN/GaN多量子阱的发光特性

研究了具有InGaN/GaN超晶格(SL)插入结构的绿光InGaN/GaN多量子阱(MQW)的发光特性。结构测试表明,SL插入结构并没有引起MQW中平均In组份的增加,而是改变了In组份的分布,形成了高In组份的量子点和低In组份量子阱。其电致发光(EL)谱和光致发光(PL)谱均出现了双发光峰。我们认为这两个 峰分别来自于量子点和量子阱,且存在着载流子从阱向点转移的输运机制。最后变温PL积分强度的Arrhenius 拟合表明,SL插入结构并没有在MQW中引入新的缺陷,使其发光效率下降。     

硅衬底生长的InGaN/GaN多层量子阱中δ型硅掺杂n-GaN层对载流子复合过程的调节作用

为了解决在单晶硅衬底上生长的InGaN/GaN多层量子阱发光二极管器件发光效率显著降低的问题,使用周期性δ型Si掺杂的GaN取代Si均匀掺杂的GaN作为n型层释放多层界面间的张应力。采用稳态荧光谱及时间分辨荧光谱测量,提取并分析了使用该方案前后的多层量子阱中辐射/非辐射复合速率随温度（10~300 K）的变化规律。实验结果表明引入δ-Si掺杂的n-GaN层后,非辐射复合平均激活能由（18±3）meV升高到（38±10）meV,对应非辐射复合速率随温度升高而上升的趋势变缓,室温下非辐射复合速率下降,体系中与阱宽涨落有关的浅能级复合中心浓度减小,PL峰位由531 nm左右红移至579 nm左右,样品PL效率随温度的衰减受到抑制。使用周期性δ型Si掺杂的GaN取代Si均匀掺杂的GaN作为生长在Si衬底上的InGaN/GaN多层量子阱LED器件n型层,由于应力释放,降低了多层量子阱与n-GaN界面、InGaN/GaN界面的缺陷密度,使得器件性能得到了改善。     

Two‐photon absorption induced anti‐Stokes emission in single InGaN/GAN quantum‐dot‐like objects

We observed crossed transitions and anti‐Stokes emissions in single quantum‐dot‐like objects embedded in the active layer of InGaN/GaN quantum disks by two‐photon absorption techniques. We proposed a phenomenological model based on the interplay between Auger effect and crossed transitions to explain the origin of anti‐Stokes emissions and the preferential excitation of 0D objects at the expense of their surroundings. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

InGaN/GaN多量子阱热退火的拉曼光谱和荧光光谱

通过拉曼光谱及荧光光谱测量研究了采用低压金属有机化学沉积(MOCVD)方法生长的InGaN／GaN多量子阱高温快速热退火处理对量子阱光学性质的影响。观测到退火后InGaN／GaN量子阱的拉曼光谱E2,A1(LO)模式的峰位置出现了红移,而且该振动峰的半高宽也有微小变化。温度升高退火效果更明显。退火使量子阱内应力部分消除,同时In,Ga原子扩散出现相分离使拉曼谱表现出变化。在常温和低温下的光荧光谱表明,退火处理的量子阱发光主峰都出现了红移;而且低温退火出现红移,退火温度升高相对低温退火出现蓝移;同时在低温荧光光谱里看到经过退火处理后原发光峰中主峰旁边弱的峰消失了。讨论了退火对多量子阱光学性质的影响。     

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.