Progress and prospects of GaN-based LEDs using nanostructures

Progress with GaN-based light emitting diodes （LEDs） that incorporate nanostructures is reviewed, especially the re- cent achievements in our research group. Nano-patterned sapphire substrates have been used to grow an A1N template layer for deep-ultraviolet （DUV） LEDs. One efficient surface nano-texturing technology, hemisphere-cones-hybrid nanostruc- tures, was employed to enhance the extraction efficiency of InGaN flip-chip LEDs. Hexagonal nanopyramid GaN-based LEDs have been fabricated and show electrically driven color modification and phosphor-free white light emission because of the linearly increased quantum well width and indium incorporation from the shell to the core. Based on the nanostruc- tures, we have also fabricated surface plasmon-enhanced nanoporous GaN-based green LEDs using AAO membrane as a mask. Benefitting from the strong lateral SP coupling as well as good electrical protection by a passivation layer, the EL intensity of an SP-enhanced nanoporous LED was significantly enhanced by 380%. Furthermore, nanostructures have been used for the growth of GaN LEDs on amorphous substrates, the fabrication of stretchable LEDs, and for increasing the 3-dB modulation bandwidth for visible light communication.

Fabrication of GaN-based LEDs with 22° undercut sidewalls by inductively coupled plasma reactive ion etching

We use a simple and controllable method to fabricate GaN-based light-emitting diodes （LEDs） with 22° undercut sidewalls by the successful implementation of the inductively coupled plasma reactive ion etching （ICP-RIE）. Our exper- iment results show that the output powers of the LEDs with 22° undercut sidewalls are 34.8 rnW under a 20-mA current injection, 6.75% higher than 32.6 mW, the output powers of the conventional LEDs under the same current injection.

GaN-based LEDs for light communication

Rapid improvement in the efficiency of GaN-based LEDs not only speed up its applications for general illumination, but offer the possibilities for data transmission. This review is to provide an overview of current progresses of GaN-based LEDs for light communications. The modulation bandwidth of GaN-based LEDs has been first improved by optimizing the LED epilayer structures and the modulation bandwidth of 73 MHz was achieved at the driving current density of 40 A/cm2 by changing the multi-quantum well structures. After that, in order to increase the current density tolerance, different parallel flip-chip micro-LED arrays were fabricated. With a high injected current density of ~7900 A/cm2, a maximum modulation bandwidth of ~227 MHz was obtained with optical power greater than 30 mW. Besides the increase of carrier concentrations, the radiative recombination coefficient B was also enhanced by modifying the photon surrounding environment based on some novel nanostructures such as resonant cavity, surface plasmon, and photonic crystals. The optical 3 dB modulation bandwidth of GaN-based nanostructure LEDs with Ag nanoparticles was enhanced by 2 times compared with GaN-based nanostructure LEDs without Ag nanoparticles.Our results demonstrate that using the QW-SP coupling can effectively help to enhance the carrier spontaneous emission rate and also increase the modulation bandwidth for LEDs, especially for LEDs with high intrinsic IQE. In addition, we discuss the progress of the faster color conversion stimulated by GaN-based LEDs.

Effect of AlGaN interlayer on luminous efficiency and reliability of GaN-based green LEDs on silicon substrate

The effect of AlGaN interlayer in quantum barrier on the electroluminescence characteristics of GaN-based green light emitting diodes(LEDs)grown on silicon substrate was investigated.The results show that AlGaN interlayer is beneficial to improve the luminous efficiency of LED devices and restrain the phase separation of In GaN.The former is ascribed to the inserted AlGaN layers can play a key role in determining the carrier distribution and screening dislocations in the active region,and the latter is attributed to the increased compressive stress in the quantum well.However,when the electrical stress aging tests were performed at a current density of 100 A/cm^2,LED devices with AlGaN interlayers are more likely to induce the generation/proliferation of defects in the active region under the effect of electrical stress,resulting in the reduced light output power at low current density.

Aging mechanism of GaN-based yellow LEDs with V-pits

GaN-based yellow light-emitting diodes(LEDs) on Si substrates are aged at a direct current density of 50 A/cm^2 for500 h. After the aging process, it can be found that the LEDs have a stable electrical property but their light output power is decayed by 4.01% at 35 A/cm^2. Additionally, the aging mechanism of GaN-based yellow LED is analyzed. It is found that the decay of light output power may be attributed to the following two reasons: one is the increase of Shockley–Rrad–Hall recombination and the other is the change of the transport path of holes via V-pits after aging, which may induce the radiative recombination current to decrease. In this paper, not only the aging mechanism of GaN-based yellow LED is investigated, but also a new possible research direction in LED aging is given.

Analysis of stress-induced inhomogeneous electroluminescence in GaN-based green LEDs grown on mesh-patterned Si(111)substrates with n-type AlGaN layer

Inhomogeneous electroluminescence(EL)of InGaN green LEDs grown on mesh-patterned Si(111)substrate had been investigated.Sample with n-AlGaN inserted between the pre-strained layers and the first quantum well showed the inhomogeneous EL in the low current density range.Near-field EL emission intensity distribution images depicted that inhomogeneity in the form of premature turn-on at the periphery of the LED chip,results in stronger emission intensity at the edges.This premature turn-on effect significantly reduces the luminous efficacy and higher ideality factor value due to locally current crowding effect.Raman measurement and fluorescence microscopy results indicated that the partially relaxed in-plane stress at the edge of the window region acts as a parasitic diode with a smaller energy band gap,which is a source of edge emission.Numerical simulations showd that the tilted triangular n-AlGaN functions like a forward-biased Schottky diode,which not only impedes carrier transport,but also contributes a certain ideality factor.

Fabrication and properties of high quality InGaN-based LEDs with highly reflective nanoporous GaN mirrors

Distributed Bragg reflectors (DBRs) are essential components for the development of optoelectronic devices.In this paper, we first report the use of the nanoporous GaN (NP-GaN) DBR as a template for regrowth of InGaN-based light-emitting diodes (LEDs). The wafer-scale NP-GaN DBR, which is fabricated by electrochemical etching in a neutral solution, has a smooth surface, high reflectivity (>99.5%), and wide spectral stop band width (>70 nm). The chemical composition of the regrown LED thin film is similar to that of the reference LED, but the photoluminescence (PL) lifetime, PL intensity, and electroluminescence intensity of the LED with the DBR are enhanced several times compared to those of the reference LED. The intensity enhancement is attributed to the light reflection effect of the NP-GaN DBR and improved crystalline quality as a result of the etching scheme, whereas the enhancement of PL lifetime is attributable to the latter.

不同波长LEDs光源对苦荞芽菜生长、生物活性物积累及抗氧化活性的影响

为探究不同波长LEDs光源对苦荞芽菜生长的影响,本研究以黑暗(D)作对照,采用单色红(R)、蓝(B)、绿(G)光源培养苦荞芽菜,收获后分别测定其形态指标、生物活性物积累量、类黄酮合成关键基因表达量及抗氧化活性指标,结果表明:B处理的苦荞芽菜下胚轴呈深红色,伸长生长受到强烈抑制,花青素、总黄酮及总酚积累量均最高,分别是D的7.18、2.96和2.49倍;R处理的下胚轴呈浅粉色,上述3种生物活性物积累量分别是对照的2.56、1.68和1.40倍;G处理的下胚轴伸长与R相似,外观呈透明白色,生物活性物积累量与对照无显著差异。B和R均强烈诱导类黄酮生物合成通路上关键结构基因表达,显著增加苦荞芽菜下胚轴抗氧化活性。此外,各单色光处理均未显著影响苦荞芽菜可食用部位的鲜重。由此可见,红、蓝LEDs光源可显著影响苦荞芽菜形态生长,并增加其生物活性物积累及抗氧化能力,以蓝光最佳。

The Efficiency Droop of InGaN-Based Green LEDs with Different Superlattice Growth Temperatures on Si Substrates via Temperature-Dependent Electroluminescence

InGaN-based green light-emitting diodes （LEDs） with different growth temperatures of superlattice grown on Si （111） substrates are investigated by temperature-dependent electroluminescence between 100 K and 350K. It is observed that with the decrease of the growth temperature of the superlattice from 895℃ to 855℃, the forward voltage decreases, especially at low temperature. We presume that this is due to the existence of the larger average size of V-shaped pits, which is determined by secondary ion mass spectrometer measurements. Meanwhile, the sample with higher growth temperature of superlattice shows a severer efficiency droop at cryogenic temperatures （about 100 K-150 K）. Electron overflow into p-GaN is considered to be the cause of such phenomena, which is relevant to the poorer hole injection into multiple quantum wells and the more reduced effective active volume in the active region.

CsPbBr_(3)@MIL-53纳米复合荧光粉的合成、性能及其白光LEDs应用

全无机钙钛矿(CsPbX3,X=Cl,Br,I)纳米晶因其卓越的光电性能被广泛应用于光电子器件领域,但稳定性问题仍然是制约其商业化发展的主要因素之一。基于此,本研究以提高CsPbBr_(3)纳米晶的稳定性和固态发光性能为研究目标,选用具有优异疏水性能的多孔MIL-53(Al)金属有机框架(MOFs)作为封装基质,通过热注射工艺在MIL-53(Al)孔道内原位限域生长CsPbBr_(3)纳米晶,成功制备了优异发光性能和稳定性的CsPbBr_(3)@MIL-53纳米复合荧光粉。MIL-53通过包含的苯环和有机配体与CsPbBr_(3)纳米晶螯合,将其稳固地锚定在孔道内,既保护了CsPbBr_(3)纳米晶免受外界环境的影响,又有效防止了纳米晶之间的聚集,从而避免了固态荧光猝灭。此外,MIL-53中的COO-官能团与CsPbBr_(3)纳米晶表面未配对的Pb2+结合,钝化了其表面的缺陷,抑制了载流子的非辐射复合。MIL-53包含的苯环及有机长链又赋予了纳米复合荧光粉出色的疏水性能。这些因素的协同作用显著提升了CsPbBr_(3)@MIL-53纳米复合荧光粉的光学性能和水稳定性,其荧光量子产率(Photoluminescence Quantum Yield,PLQY)为75.4%,是固态CsPbBr_(3)纳米晶粉体(33.2%)的2.3倍。将CsPbBr_(3)@MIL-53纳米复合荧光粉完全浸泡在水中10h,其荧光强度仍能维持初始值的75.6%。最后,将绿光发射的CsPbBr_(3)@MIL-53纳米复合荧光粉应用于白光发光二极管(LightEmittingDiodes,LEDs)器件,实现了126%NTSC和85%Rec.2020的宽色域覆盖面积,表明其在显示器件领域具有优异的应用前景。

31.38 Gb/s GaN-based LED array visible light communication system enhanced with V-pit and sidewall quantum well structure

Although the 5G wireless network has made significant advances,it is not enough to accommodate the rapidly rising requirement for broader bandwidth in post-5G and 6G eras.As a result,emerging technologies in higher frequencies including visible light communication(VLC),are becoming a hot topic.In particular,LED-based VLC is foreseen as a key enabler for achieving data rates at the Tb/s level in indoor scenarios using multi-color LED arrays with wavelength division multiplexing(WDM)technology.This paper proposes an optimized multi-color LED array chip for high-speed VLC systems.Its long-wavelength GaN-based LED units are remarkably enhanced by V-pit structure in their efficiency,especially in the“yellow gap”region,and it achieves significant improvement in data rate compared with earlier research.This work investigates the V-pit structure and tries to provide insight by introducing a new equivalent circuit model,which provides an explanation of the simulation and experiment results.In the final test using a laboratory communication system,the data rates of eight channels from short to long wavelength are 3.91 Gb/s,3.77 Gb/s,3.67 Gb/s,4.40 Gb/s,3.78 Gb/s,3.18 Gb/s,4.31 Gb/s,and 4.35 Gb/s(31.38 Gb/s in total),with advanced digital signal processing(DSP)techniques including digital equalization technique and bit-power loading discrete multitone(DMT)modulation format.

Laser lift-off technique and the re-utilization of GaN-based LED films grown on sapphire substrate

A thin GaN LED film, grown on 2-inch-diameter sapphire substrates, is separated by laser lift-off. Atom force microscopy （AFM） and the double-crystal X-ray diffraction （XRD） have been employed to characterize the performance of Gan before and after the lift-off process. It is demonstrated that the separation and transfer processes do not alter the crystal quality of the GaN films obviously. InGaN/GaN multi-quantum-wells （MQW＇s） structure is grown on the separated sapphire substrate later and is compared with that grown on the conventional substmte under the same condition by using PL and XRD spectrum.

UVC-LEDs辐照对芒果保鲜期的影响

为了探究以UVC-LEDs为光源的紫外辐照技术在芒果贮运保鲜中的应用性能,设计了一款便携式食品杀菌保鲜装置。使用该装置对“台农一号”芒果进行紫外线杀菌,并比较不同剂量紫外辐照对芒果感官特征的影响。结果表明,UV-C处理可以推迟芒果病斑出现的时间,其中,161 mJ/cm^(2)剂量的紫外线辐照可以将芒果保鲜期延长3~4 d。以UVC-LEDs为光源的便携式食品杀菌保鲜装置对芒果的保鲜效果较好,可为今后食品杀菌保鲜产品的应用开发提供依据。

Optimization of a GaN-based irregular multiple quantum well structure for a dichromatic white LED

GaN-based irregular multiple quantum well （IMQW） structures assembled two different types of QWs emitting complementary wavelengths for dichromatic white light-emitting diodes （LEDs） are optimized in order to obtain near white light emissions. The hole distributions and spontaneous emission spectra of the IMQW structures are analysed in detail by fully considering the effects of strain, well-coupling, valence band-mixing and polarization effect through employing a newly developed theoretical model from the k. p theory. Several structure parameters such as well material component, well width, layout of the wells and the thickness of harrier between different types of QWs are employed to analyse how these parameters together with the polarization effect influence the electronic and the optical properties of IMQW structure. Numerical results show that uniform hole distributions in different types of QWs are obtained when the number of the QWs emitting blue light is two, the number of the QWs emitting yellow light is one and the barrier between different types of QWs is 8nm in thickness. The near white light emission is realized using GaN-based IMQW structure with appropriate design parameters and injection level.

Effect of Light Emitting Diodes (LEDs) on Growth, Mineral Composition, and Nutritional Value of Wheat & Lentil Sprouts

Sprouts are ready-to-eat and are recognized worldwide as functional components of the human diet.Recent advances in innovative agricultural techniques could enable an increase in the production of healthy food.The use of light-emitting diode(LED)in indoor agricultural production could alter the biological feedback loop,increasing the functional benefits of plant foods such as wheat and lentil sprouts and promoting the bioavailability of nutrients.The effects of white(W),red(R),and blue(B)light were investigated on the growth parameters and nutritional value of wheat and lentil sprouts.In the laboratory,seeds were sown under three different LED treat-ments:white,red,and blue light,while normal incandescent light served as a control.Percentage seed germina-tion improved by 18.34%and 12.67%for wheat and 18.34%and 12.67%for lentil sprouts under LED treatments R and B,respectively.An increase in total soluble protein and sugar by 33.4%and 9.23%in wheat and by 31.5%and 5.87%in lentils was observed under the R LED treatment.Vitamin C concentrations in wheat and lentils were significantly increased by R LED compared to all other treatments.Other parameters,including potassium and sodium concentrations,were significantly increased under red and blue light compared to the control;white light,on the other hand,significantly decreased all these parameters.According to the experimental data,red and blue LED light could be beneficial in the production of functional wheat and lentil sprouts with high nutrient concentrations.

Fabrication and Characterization of a GaN-Based 320×256 Micro-LED Array

Design, fabrication and characterizations of GaN-based blue micro light emitting diode （LED） arrays are reported. The GaN micro-LED array consists of 320×256 pixels with a pitch size of 30μm. Each pixel is 25×25μm^2 in size, which is designed for backside emission and high density flip-chip packaging. The selected LED pixels being tested exhibit good uniformity in terms of turn-on voltage and reverse leakage current. The efficiency droop behavior and reliability behavior under high forward current stress are also studied. The micro-LED pixel shows improved reliability, which is likely caused by enhanced heat dissipation.

钙钛矿量子点色转换Micro-LEDs:稳定性与图案化研究进展

微型发光二极管(Micro light-emitting diode,Micro-LED)显示具有优异的显示性能和光电性质,被称为“下一代”终极显示技术。为了满足近眼显示需求,Micro-LED需要进一步微缩与集成化。随着微纳级图案化技术的不断革新,荧光色转换层法表现出低制造成本等显著优势,相较于三色芯片法,更适合应用于对色域、分辨率有更高要求的虚拟/增强现实显示应用。钙钛矿量子点是最有前景的荧光色转换材料,然而自身晶格固有的不稳定性和外界环境因素刺激共同导致的结构降解是一大问题。另外,如何制备与Micro-LED芯片阵列相匹配的微米级荧光阵列图案是至关重要的。为此,本文首先讲述了造成钙钛矿量子点结构不稳定性的原因,其次,总结了配体交换、离子掺杂、表面包覆和化学交联等方案在提升钙钛矿量子点稳定性方面的应用,最后,总结了光刻技术和喷墨打印技术在制备高分辨率钙钛矿量子点荧光阵列的最新研究进展。

Simulation Study on Optical Properties of GaN-based Blue LED

The optical properties of GaN-based blue light-emitting diodes(LEDs)are extremely important to study as these LEDs are utilized in a great many industries due to their excellent qualities,including high brightness,high energy efficiency,low energy consumption,and rapid reaction time.In this paper,Silvaco TCAD simulation software is used to do two-dimensional modeling and simulation of a GaN-based blue single quantum well vertical structure LED,with an emphasis on varied forward voltages,In components in InGaN,and quantum well thickness.The volt-ampere characteristic curve is compared and evaluated,as well as the energy band structure,carrier concentration,radiation recombination efficiency,electroluminescence spectrum,and internal current density distribution.The results show that when the forward voltage is 3.5V and the thickness of the quantum well is constant,the luminescence spectrum will show a red shift with the increase of the In content in the quantum well,and the luminescence spectrum will also show a red shift when the thickness of the quantum well is increased.However,when the quantum well thickness and In component are kept constant,the luminescence spectrum appears a red shift with increasing forward voltage.

基于粉浆法提高功率型白光LEDs的提取效率(英文)

讨论了基于水溶性感光胶的荧光粉的涂层技术,利用粉浆法,在蓝光LED芯片的表面获得了荧光粉平面涂层。证实了得到的白光LED的光提取效率与粉浆中各成份的浓度和相应的比例有关,并在实验的基础上,采用了一些切实切可行的措施来增加器件的光提取效率。得到的白光LED在700 mA加速老化168 h后,其光能量为初始时的95.95%。

提高基于粉浆法的功率型白光LEDs发光效率的研究

基于水溶性感光胶的自光LEDs平面涂层技术，在蓝光LED芯片表面上得到了可控的荧光粉层．采用降低粉浆中ADC的浓度和提高荧光粉的含量两种措施，减少Cr^3＋在433．6和620nm两处吸收对器件出光效率的影响；新配制的粉浆在暗室中静置3～5h，既可以提高器件的出光效率同时又避免了暗反应带来的影响；在蓝光LED表面上得到粉层后，再涂覆硅胶层，由于硅胶的折射率与粉层的更接近，不但使出光色调偏向蓝光区域而且有更多光子出射，光通量由未加硅胶层时的44．8～59lm提高到了79．4～84．9lm．