Inserting a low-temperature n-GaN underlying layer to separate nonradiative recombination centers improves the luminescence efficiency of blue InGaN/GaN LEDs

We have investigated the effects of nonradiative recombination centers (NRCs) on the device performance of InGaN/GaN multi-quantum-well (MQW) light-emitting diodes (LEDs) inserting low-temperature n-GaN (LT-GaN) underlying layers. Inserting an LT-GaN underlying layer prior to growing the MQWs is a successful means of separating the induced nonradiative recombination centers because a growth interrupt interface exists between the n-GaN template and the InGaN QW. We found that by introducing this technique would improve the external quantum efficiency of the as-grown conventional LEDs. The electroluminescence relative intensity of a blue LED incorporating a 70-nm-thick LT-GaN was 20.6% higher (at 20 mA current injection) than that of the corresponding as-grown blue LED in the best case.     

低温生长n型氮化镓插入层提高LED抗静电能力

在这篇论文里,我们通过在InGaN/GaN 多量子阱和n型氮化镓层中间插入一层低温生长的n型氮化镓显著提高了LED的抗静电能力。通过引入低温生长的氮化镓插入层使得LED抗击穿电压超过4000V的良品率从9.9%提升到74.7%。低温生长的氮化镓插入层作为后续生长的多量子阱的缓冲层,释放了量子阱中的应力并且改善了量子阱的界面质量。另外,我们证明了在氮气气氛下生长低温氮化镓插入层对于LED抗静电能力的改善要强于氢气气氛,同时也进一步证明低温插入层对量子阱中应力的释放有利于提高LED的抗静电能力。光电测试结果表明,在引入低温nGaN缓冲层后,LED的电学特性并没有衰退,并且LED的光输出功率提高了13.9%。     

Effects of the p-AlInGaN/GaN superlattices’structure on the performance of blue LEDs

The advantages of the p-AIInGaN/GaN superlattices＇ （SLs） structure as an electron blocking layer （EBL） for InGaN blue light-emitting diodes （LEDs） were studied by experiment and APSYS simulation. Elec- troluminescence （EL） measurement results show that the LEDs with the p-AllnGaN/GaN SLs＇ structure EBL ex- hibited better optical performance compared with the conventional A1GaN EBL due to the enhancement of hole concentration and hole carrier transport efficiency, and the confinement of electrons＇ overflow between multiple quantum-wells （MQWs） and EBL.     

Enhanced carrier confinement in AlInGaN-InGaN quantum wells in near ultraviolet light-emitting diodes

To increase carrier confinement, the GaN barrier layer was substituted with an AlInGaN quaternary barrier layer which was lattice-matched to GaN in the GaN-InGaN multiple quantum wells (MQWs). Photoluminescence (PL) and high-resolution X-ray diffraction measurements showed that the AlInGaN barrier layer has a higher bandgap energy than the originally used GaN barrier layer. The PL intensity of the five periods of AlInGaN-InGaN MQWs was increased by three times compared to that of InGaN-GaN MQWs. The electroluminescence (EL) emission peak of AlInGaN-InGaN MQWs ultraviolet light-emitting diode (UV LED) was blue-shifted, compared to a GaN-InGaN MQWs UV LED and the integrated EL intensity of the AlInGaN-InGaN MQWs UV LED increased linearly up to 100 mA. These results indicated that the AlInGaN-InGaN MQWs UV LED has a stronger carrier confinement than a GaN-InGaN MQWs UV LED due to the larger barrier height of the AlInGaN barrier layer compared to a GaN barrier layer.     

硅衬底GaN基LED外延生长的研究

采用在AlN缓冲层后原位沉积SiN掩膜层,然后横向外延生长GaN薄膜.通过该法在硅衬底上获得了1.7 μm无裂纹的GaN薄膜,并在此基础上外延生长出了GaN基发光二极管(LED)外延片,其外延片的总厚度约为1.9 μm.采用高分辨率双晶X-射线衍射(DCXRD)、原子力显微镜(AFM)测试分析.结果表明,GaN薄膜(0002)面的半峰全宽(FWHM)降低到403 arcsec,其表面平整度得到了很大的改善;InGaN/GaN多量子阱的界面较平整,结晶质量良好.光致发光谱表明,GaN基LED峰值波长为469.2 nm.     

Phosphor-Free GaN-Based Transverse Junction White-Light Light-Emitting Diodes With Regrown n-Type Regions

In this study, we demonstrate a GaN-based phosphor-free white-light light-emitting diode (LED), which is composed of GaN-based dual-wavelength (blue and yellow-green) multiple-quantum-wells (MQWs) and a transverse p-n junction. The device was realized by the regrowth of n-type GaN layers on the sidewall of p-type GaN and undoped MQWs. The problems related to the bias-dependent shape of the electroluminescence spectra that occur in traditional phosphor-free white-light LEDs (with vertical p-n junctions) are greatly minimized. The current-voltage performance of our device is comparable to that of the commercially available phosphor white-light LEDs. In addition, the dynamic measurement results indicate that we can attain a much higher modulation bandwidth (22 versus 3 MHz) with this device than with the currently available commercial ones.     

Effects of growth interruption on the properties of InGaN/GaN MQWs grown by MOCVD

These applications normally employ an InGaN/GaN mul- tiple quantum wells (MQWs) as the active layer. But the growth of high quality MQWs is very difficult. Firstly, the growth condition is strict due to the high volatility of InN and the low thermal pyrol…     

生长停顿改善MOCVD生长InGaN/GaN多量子阱的特性

利用金属有机物化学气相淀积(MOCVD)生长了InGaN/GaN多量子阱(MQWs)结构,研究了生长停顿对InGaN/GaN MQWs特性的影响.结果表明,采用生长停顿,可以改善MQWs界面质量,提高MQWs的光致发光(PL)与电致发光(EL)强度;但生长停顿的时间过长,阱的厚度会变薄,界面质量变差,不仅In组分变低,富In的发光中心减少,而且会引入杂质,致使EL强度下降.     

InGaN绿光LED中p型GaN层的优化设计

InGaN绿光LED的量子阱结构具有较高的In含量,InN与GaN之间较大的晶格失配度使得该结构的稳定性下降.由于量子阱结构生长完成之后的p型GaN的生长温度要远高于量子阱结构生长温度,因此,p型GaN的生长过程对多量子阱质量有重大影响.论文探讨了p型GaN的生长温度与厚度对绿光LED的材料结构及器件性能的影响.研究发现,p-GaN过高的生长温度和过大的厚度都能加剧多量子阱结构中In组分的波动,使得发光峰宽化,同时降低绿光量子阱的发光效率.论文据此提出了优化的p型GaN生长温度与厚度,探讨了量子阱保护层对InGaN绿光LED性能的影响,该结构有利于增强绿光LED发光波长的稳定性.     

High Color Rendering Index Hybrid III‐Nitride/Nanocrystals White Light‐Emitting Diodes

An excellent hybrid III‐nitride/nanocrystal nanohole light‐emitting diode (h‐LED) has been developed utilizing nonradiative resonant energy transfer (NRET) between violet/blue emitting InGaN/GaN multiple quantum wells (MQWs) and various wavelength emitting nanocrystals (NCs) as color‐conversion mediums. InGaN/GaN MQWs are fabricated into nanoholes by soft nanoimprint lithography to minimize the separation between MQWs and NCs. A significant reduction in the decay lifetime of excitons in the MQWs of the hybrid structure has been observed as a result of the NRET from the nitride emitter to NCs. The NRET efficiency of the hybrid structures is obtained from the decay curves, as high as 80%. Moreover, a modified Förster formulation has exhibited that the exciton coupling distance in the hybrid structures is less than the Förster's radius, demonstrating a strong coupling between MQWs and NCs. Finally, based on a systemic optimization for white emission indexes, a series of hybrid ternary complementary color h‐LEDs have been demonstrated with a high color rendering index, up to 82, covering the white light emission at different correlated color temperatures ranging from 2629 to 6636 K, corresponding to warm white, natural white, and cold white.     

Characterization of quaternary AlInGaN epilayers and polarization-reduced InGaN/AlInGaN MQW grown by MOCVD

We have demonstrated the growth of quaternary AIlnGaN compounds at different growth temperatures and pressures with metalorganic chemical vapor deposition (MOCVD). The optical properties of the samples have been investigated by photoluminescence (PL) at different temperatures. The results show that the sample grown at higher temperature (850℃) exhibits the best optical quality for its sharp band edge luminescence and weak yellow luminescence. The AlInGaN exhibited three-dimensional (3D) growth mode at higher pressure. The band edge emission almost disappeared. With the optimization of AlInGaN growth parameters, we replaced the traditional barrier in InGaN/GaN multiple quantum wells (MQWs) with AlInGaN barriers. The peak wavelength for the InGaN/AlInGaN-MQW based light emitting diodes (LEDs) was very stable at various injection current levels because of the polarization-matched InGaN/AlInGaN MQWs.     

预应变InGaN/GaN多量子阱发光特性调控研究

为了减弱InGaN/GaN量子阱内的压电极化场,在蓝紫光InGaN/GaN多量子阱激光器结构中采用了预应变InGaN插入层,通过变温电致发光和高分辨X射线衍射测量研究了预应变插入层对量子阱晶体质量和发光特性的影响。实验结果显示,常温下有预应变层的量子阱电致发光谱积分强度显著提高。模拟计算进一步表明,预应变层对量子阱内压电极化场有调制效果,有利于量子阱中的应力弛豫,可以有效减弱量子限制斯塔克效应,有助于提高量子阱的发光效率。     

Luminescence properties of blue and green dual wavelength InGaN/GaN multi-quantum well light-emitting diode

Blue and green dual wavelength InGaN/GaN multi-quantum well (MQW) light-emitting diode (LED) has wide applications in full color display, monolithic white LED and solid state lighting, etc. Blue and green dual wavelength LEDs, which consist of InGaN strain-reduction layer, green InGaN/GaN MQW and blue InGaN/ GaN MQW, were grown by metal-organic chemical vapor deposition (MOCVD), and the luminescence properties of dual wavelength LEDs with different well arrangements were studied by photoluminescence and electrolumines-cence. The experimental results indicated that well position played an important role on the luminescence evolvement from photoluminescence to electroluminescence.     

Structural and optical investigation of InGaN/GaN multiple quantum well structures with various indium compositions

We have studied the influence of indium (In) composition on the structural and optical properties of In_x Ga_1−xN/GaN multiple quantum wells (MQWs) with In compositions of more than 25% by means of high-resolution x-ray diffraction (HRXRD), photoluminescence (PL), and transmission electron microscopy (TEM). With increasing the In composition, structural quality deterioration is observed from the broadening of the full width athalf maximum of the HRXRD superlattice peak, the broad multiple emission peaks oflow temperature PL, and the increase of defect density in GaN capping layers and InGaN/GaN MQWs. V-defects, dislocations, and two types of tetragonal shape defects are observed within the MQW with 33% In composition by high resolution TEM. In addition, we found that V-defects result in different growth rates of the GaN barriers according to the degree of the bending of InGaN well layers, which changes the period thickness of the superlattice and might be the source of the multiple emission peaks observed in the In_xGa_1−xN/GaN MQWs with high in compositions.     

垒层Si掺杂对多量子阱InGaN绿光LED性能的影响

InGaN系绿光LED的量子阱结构具有较高的In含量,InN与GaN之间较大的晶格失配度使得绿光器件的量子限制Stark效应更显著。对内建电场的屏蔽可以有效提高载流子的辐射复合效率。论文探讨了绿光多量子阱中垒层的Si掺杂对绿光器件性能的影响。研究发现,多量子阱中垒层适度Si掺杂(3.4×1016 cm-3)可以改善多量子阱结构界面质量和In组分波动,在外加正向电流的作用下更大程度地屏蔽极化电场;同时,还能够增强电流的横向扩展性,提高活化区的有效发光面积。然而,多量子阱中垒层的过度Si掺杂对于绿光LED器件的性能带来诸多的负面影响,比如加剧阱垒晶格失配、漏电途径明显增加等,致使器件光效大幅度降低。     

纳米柱InGaN/GaN多量子阱的干法刻蚀制备技术

纳米柱GaN基多量子阱(MQW)拥有量子尺寸效应以及应变释放等特性,对于提高GaN基发光二极管(LED)的发光效率具有重要意义.采用快速热退火(RTA)形成的自组装Ni纳米颗粒作为刻蚀掩膜,利用电感耦合等离子体反应离子刻蚀(ICP-RIE)制备纳米柱InGaN/GaN MQW.通过改变RTA温度发现在800℃以上才能有效形成Ni纳米颗粒掩膜.不同的ICP和射频(RF)功率条件下制备的纳米柱MQW光致发光强度相比于相同结构的平面MQW会发生显著变化.通过优化ICP-RIE的刻蚀条件,可以获得发光强度显著提高的纳米柱MQW结构.同时,纳米柱MQW中压电极化场的减弱会形成光致发光峰位蓝移.     

Edge-emitting electroluminescence polarization investigation of InGaN/GaN light-emitting diodes grown by metal-organic chemical vapor deposition on sapphire (0001)

The edge-emitting electroluminescence (FL) state of polarization of blue and green InGaN/GaN light-emitting diodes (LEDs) grown in EMCORE’s commercial reactors was studied and compared to theoretical evaluations. Blue (∼475 nm) LEDs exhibit strong EL polarization, up to a 3:1 distinction ratio. Green (∼530 nm) LEDs exhibit smaller ratios of about 1.5:1. Theoretical evaluations for similar InGaN/GaN superlattices predicted a 3:1 ratio between light polarized perpendicular (E⊥c) and light polarized parallel (E‖c) to the c axis. For the blue LEDs, a quantum well-like behavior is suggested because the E⊥c mode dominates the E‖c mode 3:1. In contrast, for the green LEDs, a mixed quantum well (QW)-quantum dot (QD) behavior is proposed, as the ratio of E⊥c to E‖c modes drops to 1.5:1. The EL polarization fringes were also observed, and their occurrence may be attributed to a symmetric waveguide-like behavior of the InGaN/GaN LED structure. A large 40%/50% drop in the surface root mean square (RMS) from atomic force microscopy (AFM) scans on blue/green LEDs with and without EL fringes points out that better surfaces were achieved for the samples exhibiting fringing. At the same time, a 25%/10% increase in the blue/green LED photoluminescence (PL) intensity signal was found for samples displaying EL interference fringes, indicating superior material quality and improved LED structures.     

Improvement of photon extraction efficiency of InGaN LEDs utilizing textured ZnO layer deposited by electrospray deposition

Improvement of photon extraction efficiency of InGaN LEDs has been achieved by deposition of textured ZnO layers on InGaN LED bare chips utilizing electrospray deposition. The electrical and spectral properties of the InGaN LED remain unchanged after the deposition of ZnO, while the InGaN LED with textured ZnO layer exhibits a wider far-field angular distribution. The optical power of the InGaN LED capped with ZnO layer is ∼30% higher than that without ZnO, which is due to increased photon escape probability as a result of the increased surface roughness.     

InGaN/GaN多量子阱纳米线发光二极管制备及研究

用SiO2纳米图形层作为模板在以蓝宝石为衬底的n-GaN单晶层上制备了InGaN/GaN多量子阱纳米线,并成功实现了其发光二极管器件(LED).场发射扫描电子显微镜(FESEM)的测量结果表明,InGaN/GaN多量子阱纳米线具有光滑的表面形貌和三角形的剖面结构.室温下阴极射线荧光谱(CL)的测试发现了位于461 nm...     

InGaN/GaN和InGaN/AlGaN多量子阱中应变对光致发光特性的影响

对蓝宝石衬底上的InGaN/GaN和InGaN/AlGaN多量子阱结构和经激光剥离去除衬底的InGaN/GaN和InGaN/AlGaN多量子阱结构薄膜样品,进行了光致发光谱、高分辨XRD和喇曼光谱测量.PL测量结果表明,相对于带有蓝宝石衬底的样品,InGaN/GaN多量子阱薄膜样品的PL谱峰值波长发生较小的蓝移,而InGaN/AlGaN多量子阱薄膜样品的PL谱峰值波长发生明显的红移;喇曼光谱的结果表明,激光剥离前后E2模的峰值从569.1减少到567.5cm-1.这说明激光剥离去除衬底使得外延层整体的压应力得到部分释放,但InGaN/GaN与InGaN/AlGaN多量子阱结构中阱层InGaN的应力发生了不同的变化.XRD的结果证实了这一结论.