Investigation of InGaN green light-emitting diodes with chirped multiple quantum well structures

The effect of using chirped multiple quantum-well (MQW) structures in InGaN green light-emitting diodes (LEDs) is numerically investigated. An active structure, which is with both thick QWs with low indium composition on the p-side and thin QWs with high indium composition next to the n-region, is presented in this study. The thickness and indium composition in each single QW is specifically tuned to emit the same green emission spectrum. Comparing with conventional active structure design of green LEDs, which is using uniform MQWs, the output power is increased by 27% at 20 mA, and by 15% at 100 mA current injections. This improvement is mainly attributed to the enhanced efficiency of carrier injection into QWs and the improved capability of carrier transport.     

载流子分布对GaN基LED频率特性的影响

分别在直流偏置和交流偏置下,对大功率GaN基LED的电学和光学特性进行了研究。结果显示,通过改变靠近p型层的量子垒（也就是最后一个量子垒）中的In组分可以调控有源区中的载流子分布。有源区内积累的电子会引起负电容效应。而通过降低有源区量子垒的势垒高度,可以改善LED中载流子传输特性,并实现载流子复合速率及通信调制带宽20%的提高。这个工作将有助于理解GaN基LED中载流子分布对频率特性的影响,并为设计适用于可见光通信的大功率高速LED奠定基础。     

Spectral and emission characteristics of LED and its application to LED-based sun-photometry

In recent years, light-emitting diodes (LEDs) have found applications in fields like space science instrumentation in addition to its use in illumination. Sun photometry is one of the techniques for measuring aerosol optical depth. Photometers generally consisting of an interference filter and a photo-detector, measures the intensity of radiation in the wavelength band of the interference filter. LED alone replaces both the interference filter and the detector and works as a spectrally selective photo-detector. The spectral response (extinction spectra) of LED is required to calculate the aerosol optical depth. In general, it is assumed that the emission spectra and spectral response should be same. It has been found experimentally that the emission spectra and spectral response are different. The peak wavelength at which the maximum emission occurs is found to be higher than the peak of the spectral response curve. The FWHM of both are also found to be different. A typical example of Langley plot obtained from the LED-based Sun photometer is shown and optical depth obtained with this is compared with the conventional Sun photometer.     

发光二极管在差分吸收光谱系统中的应用研究

研究了新型发光二极管(LEDs)作为主动差分吸收光谱技术(DOAS)光源的可行性及其应用.分析了LEDs发光特性、谱的形状、光谱范围和谱的稳定性.结果表明LEDs作为主动DOAS光源是可行的,只是当温度不恒定时,LEDs光谱中的法布里-珀罗标准具效应将影响DOAS精确反演,若把其结构提取参与拟合可以很好地去除其影响.并成功地利用LEDs-DOAS系统监测了大气NO₂的浓度,与基于高压氙弧灯为光源DOAS系统测量结果的相关性达到0.99以上.当光程为0.7km时,检测限为1.1×10< 关键词： 发光二极管(LEDs) 差分吸收光谱技术(DOAS) 可行性     

Portable optical water-and-oil analyzer based on a mid-IR (1.6–2.4 μm) optron consisting of an LED array and a wideband photodiode

An optical method for measuring the water and oil content using mid-IR (1.6–2.4 μm) LEDs and a wideband photodiode is suggested for the first time. This method is developed based on the absorption spectra of pure water, dewatered oil, and water—oil emulsions (cut oil) with different content of water and uses 10 types of LEDs in the spectral range 1.6–2.4 μm. It is shown that pure water heavily absorbs the LED radiation in the spectral range 1.85–2.05 μm, oil absorbs in the range 1.67–1.87 μm, and the LED radiation with a maximum at 2.20 μm is equally weakly absorbed by water and oil. An optical cell of the water-and-oil analyzer is designed on the basis of a three-element diode array with radiation maxima at 1.65 (detection of oil), 1.94 (detection of water), and 2.2 μm (reference signal) wideband photodiode covering the spectral range 1.3–2.4 μm. A calibration curve is derived that represents the dependence of the water concentration in oil on the amplitude of the reduced signal obtained by processing three signals from the LEDs. This optical method of measuring the water content in oil underlies a portable analyzer making possible online measurements directly in an oil well.     

Enhancing performance of GaN-based LDs by using GaN/InGaN asymmetric lower waveguide layers

The effects of GaN/InGaN asymmetric lower waveguide (LWG) layers on photoelectrical properties of InGaN multiple quantum well laser diodes (LDs) with an emission wavelength of around 416 nm are theoretically investigated by tuning the thickness and the indium content of InGaN insertion layer (InGaN-IL) between the GaN lower waveguide layer and the quantum wells, which is achieved with the Crosslight Device Simulation Software (PIC3D, Crosslight Software Inc.). The optimal thickness and the indium content of the InGaN-IL in lower waveguide layers are found to be 300 nm and 4%, respectively. The thickness of InGaN-IL predominantly affects the output power and the optical field distribution in comparison with the indium content, and the highest output power is achieved to be 1.25 times that of the reference structure (symmetric GaN waveguide), which is attributed to the reduced optical absorption loss as well as the concentrated optical field nearby quantum wells. Furthermore, when the thickness and indium content of InGaN-IL both reach a higher level, the performance of asymmetric quantum wells LDs will be weakened rapidly due to the obvious decrease of optical confinement factor (OCF) related to the concentrated optical field in the lower waveguide.     

多光谱可见光通信信道串扰分析

在可见光通信领域, 通过波分复用技术可以增加信道个数, 从而提高系统通信容量. 然而发光二极管(LED)的辐射光谱具有一定线宽, 当信道个数增加, 信道间隔将变小, 尽管有滤光片的通道选择, 但LED的辐射光谱会出现重叠从而产生信道串扰. 本文基于LED光谱重叠现象分析了多光谱波分复用可见光通信系统的信道串扰问题. 首先结合LED的物理机制和实际LED的光谱形状对其光谱进行建模; 然后根据光谱重叠现象和可见光通信信道推导出信道串扰公式; 最后利用不同中心波长的LED在两通道可见光通信系统中验证了信道串扰公式的正确性. 仿真和实验结果表明, 当两信道的信道间隔大于28 nm时, 两信道之间的信道串扰不超过-13.6 dB. 对多光谱波分复用可见光通信系统的信道串扰分析对未来可见光通信增加信道数量有一定指导作用.     

Performance improvement of InGaN blue light-emitting diodes with several kinds of electron-blocking layers

The performance of InGaN blue light-emitting diodes(LEDs) with different kinds of electron-blocking layers is investigated numerically.We compare the simulated emission spectra,electron and hole concentrations,energy band diagrams,electrostatic fields,and internal quantum efficiencies of the LEDs.The LED using AlGaN with gradually increasing Al content from 0% to 20% as the electron-blocking layer(EBL) has a strong spectrum intensity,mitigates efficiency droop,and possesses higher output power compared with the LEDs with the other three types of EBLs.These advantages could be because of the lower electron leakage current and more effective hole injection.The optical performance of the specifically designed LED is also improved in the case of large injection current.     

Confocal microscopy as a tool for the study of the emission characteristics of high-power LEDs

We report on an experimental set-up based on a confocal principle in order to acquire the light-intensity distribution (XZ and XY optical sections) of high-power LEDs. To be able to record the emission characteristics of millimeter-sized LEDs and to carry out the measurements with high precision the set-up consists of a moving stage and stationary rather than scanning optics, along with a lock-in amplifier in combination with a photodiode as a detection unit. The optical sections recorded provide valuable information on the light-intensity distribution and the light propagation both within transparent substrates (in case of flip-chip LEDs) as well as in the ambient of the LEDs. In order to evaluate the accuracy of the measurement technique, the impact of the numerical aperture of the objective lens on the shape of the optical sections recorded was tested for a set of different objective lenses. The method reported provides new opportunities for a direct determination not only of the amount but also the directionality of the light extraction from LEDs that are processed in order to improve the light-extraction efficiency.     

Progress and prospects of GaN-based LEDs using nanostructures

Progress with GaN-based light emitting diodes(LEDs) that incorporate nanostructures is reviewed,especially the recent achievements in our research group.Nano-patterned sapphire substrates have been used to grow an Al N template layer for deep-ultraviolet(DUV) LEDs.One efficient surface nano-texturing technology,hemisphere-cones-hybrid nanostructures,was employed to enhance the extraction efficiency of In GaN 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 nanostructures,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 the3-d B modulation bandwidth for visible light communication.     

铟网位置对无极灯253.7 nm共振谱线的影响

无极灯是一种基于高频电磁感应和无极气体放电的新型电光源,由Hg 253.7 nm共振谱线激发荧光粉进而发出可见光。通过原子发射光谱分析,实验研究了铟网位置对Hg 253.7 nm共振谱线的影响规律。研究发现,Hg 253.7 nm共振谱线的相对强度,当铟网位于耦合线圈两端时最强,位于耦合线圈中部时较弱,远离耦合线圈时最弱。并推断,存在一个最佳的铟网位置,对应最高的发光效率。结合Maxwell 3D有限元仿真,从气体放电角度对实验结果进行了定性分析,对无极灯模型设计与参数优化具有指导意义。     

Efficiency Improvement in Polymer Light‐Emitting Diodes by “Far‐Field” Effect of Gold Nanoparticles

The “far‐field” effect of metal nanoparticles (NPs), when chromophores localized nearby metal NPs (typically the distance >λ/10), is an important optical effect to enhance emission in photoluminescence. The far‐field effect originates mainly from the interaction between origin emission and mirror‐reflected emission, resulting in the increased irradiative rate of chromophores on the mirror‐type substrate. Here, the far‐field effect is used to improve emission efficiency of polymer light‐emitting diodes (PLEDs). A universal performance improvement is achieved for the full visible light (red, green, blue) PLEDs, utilizing gold (Au) NPs to modify the indium tin oxide (ITO) substrates; this is shown by experimental and theoretical simulation to mainly come from the far‐field effect. The optimized distance, between the NPs and chromophores with visible light emission ranging from 400 to 700 nm, is 80–120 nm. Thus the scope of the far‐field may overlap the light‐emitting profile very well to enhance the efficiency of optoelectronic devices. The 30–40% enhancement is obtained for different color‐emitting materials through distance optimization. The far‐field effect is demonstrated to enhance device performance for materials in the full‐visible spectral range, which extends the optoelectric applications of Au NPs.     

Large-scale SiO2 photonic crystal for high efficiency GaN LEDs by nanospherical-lens lithography

Wafer-scale SiO2 photonic crystal （PhC） patterns （SiO2 air-hole PhC, SiO2-pillar PhC） on indium tin oxide （ITO） layer of GaN-based light-emitting diode （LED） are fabricated via novel nanospherical-lens lithography. Nanoscale polystyrene spheres are self-assembled into a hexagonal closed-packed monolayer array acting as convex lens for expo- sure using conventional lithography instrument. The light output power is enhanced by as great as 40.5% and 61% over those of as-grown LEDs, for SiO2-hole PhC and SiO2-pillar PhC LEDs, respectively. No degradation to LED electrical properties is found due to the fact that SiO2 PhC structures are fabricated on ITO current spreading electrode. For SiO2- pillar PhC LEDs, which have the largest light output power in all LEDs, no dry etching, which would introduce etching damage, was involved. Our method is demonstrated to be a simple, low cost, and high-yield technique for fabricating the PhC LEDs. Furthermore, the finite difference time domain simulation is also performed to further reveal the emission characteristics of LEDs with PhC structures.     

Tunable photoluminescence and electroluminescence of size-controlled silicon nanocrystals in nanocrystalline-Si/SiO2 superlattices

We present photoluminescence and electroluminescence of silicon nanocrystals deposited by plasma-enhanced chemical vapor deposition (PECVD) using nanocrystalline silicon/silicon dioxide (nc-Si/SiO₂) superlattice approach. This approach allows us to tune the nanocrystal emission wavelength by varying the thickness of the Si layers. We fabricate light emitting devices (LEDs) with transparent indium tin oxide (ITO) contacts using these superlattice materials. The current-voltage characteristics of the LEDs are measured and compared to Frenkel-Poole and Fowler-Nordheim models for conduction. The EL properties of the superlattice material are studied, and tuning, similar to that of the PL spectra, is shown for the EL spectra. Finally, we observe the output power and calculate the quantum efficiency and power conversion efficiency for each of the devices.     

具有三角形InGaN/GaN多量子阱的高内量子效率的蓝光LED(英文)

对InGaN量子阱LED的内量子效率进行了优化研究。分别对发光光谱、量子阱中的载流子浓度、能带分布、静电场和内量子效应进行了理论分析。对具有不同量子阱数量的InGaN/GaN LED进行了理论数值比对研究。研究结果表明,对于传统结构的LED而言,2个量子阱的结构相对于5个和7个量子阱具有更好的光学性能。同时还研究了具有三角形量子阱结构的LED,研究结果显示,三角形多量子阱结构具有较高的电致发光强度、更高的内量子效率和更好的发光效率,所有的优点都归因于较高的电子-空穴波函数重叠率和低的Stark效应所产生的较高的载流子输入效率和复合发光效率。     

LED结温与光谱特性关系的测量

采用恒定驱动电流改变环境温度和恒定环境温度改变驱动电流两种方法分别对直径5 mm封装的AlGaInP型红光和黄光LED,InGaN型绿光和蓝光LED,以及InGaN蓝光+荧光粉的白光LED的结温与其光谱特性进行了测量,得到了不同条件下LED结温与光谱特性的关系.结果表明;AlGaInP LED的峰值波长与结温有良好线性关系,InGaN LED的峰值波长则与结温没有明显对应关系;但白光LED发射光谱的白、蓝功率比与结温有良好线性关系;对AlGaInP LED及蓝光激发的白光LED,通过光谱特性测量可快速、准确地确定光源系统中各LED的结温继而预测光源系统的有效寿命.     

Blue-violet luminescence double peak of In-doped films prepared by radio frequency sputtering

ZnO films doped with different contents of indium were prepared by radio frequency sputtering technique. The structural, optical and emission properties of the films were characterized at room temperature using XRD, XPS, UV-vis-NIR and PL techniques. Results showed that the indium was successfully incorporated into the c-axis preferred orientated ZnO films, and the In-doped ZnO films are of over 80% optical transparency in the visible range. Furthermore, a double peak of blue-violet emission with a constant energy interval (∼0.17 eV) was observed in the PL spectra of the samples with area ratio of indium chips to the Zn target larger than 2.0%. The blue peak comes from the electron transition from the Zn_i level to the top of the valence band and the violet peak from the In_Zn donor level to the V_Zn level, respectively.     

Optimization towards reduction of efficiency droop in blue GaN/InGaN based light emitting diodes

Light emitting diodes (LEDs) based on GaN/InGaN material suffer from efficiency droop at high current injection levels. We propose multiple quantum well (MQW) GaN/InGaN LEDs by optimizing the barrier thickness and high–low–high indium composition to reduce the efficiency droop. The simulation results reflect a significant improvement in the efficiency droop by using barrier width of 10 nm and high–low–high indium composition in MQW LED.     

Influence of electron-beam irradiation in SEM on the cathodoluminescence and electron-beam-induced current in InGaN/GaN light-emitting diodes with a buried active region

The influence of irradiation in a scanning electron microscope on the optical properties inherent to light-emitting diodes (LEDs) with multiple InGaN/GaN quantum wells, assembled by means of the flip-chip mounting technique, has been investigated via the cathodoluminescence (CL) and electron-beam-induced current methods. It is demonstrated that the action of an electron beam qualitatively varies both these LEDs and structures with a thin upper GaN layer only at large beam energies. It has been revealed that irradiation not only leads to changes in the spectrum and intensity of CL but also decreases the energy corresponding to the excitation of emission associated with quantum wells. A similar effect is also observed in structures whose external quantum efficiency has been decreased several times due to long-running tests performed at an injection current density of 35 A/cm² and a temperature of 100°C.     

Photoluminescence properties of blue and green multiple InGaN/GaN quantum wells

The photoluminescence(PL) properties of blue multiple InGaN/GaN quantum well(BMQW) and green multiple InGaN/GaN quantum well(GMQW) formed on a single sapphire substrate are investigated. The results indicate that the peak energy of GMQW-related emission(PG) exhibits more significant "S-shaped" dependence on temperature than that of BMQW-related emission(PB), and the excitation power-dependent carrier-scattering effect is observed only in the PG emission; the excitation power-dependent total blue-shift(narrowing) of peak position(line-width) for the PGemission is more significant than that for the PBemission; the GMQW shows a lower internal quantum efficiency than the BMQW. All of these results can be attributed to the fact that the GMQW has higher indium content than the BMQW due to its lower growth temperature and late growth, and the higher indium content in the GMQW induces a more significant compositional fluctuation, a stronger quantum confined Stark effect, and more non-radiative centers.