Comparison of nitride-based dual-wavelength light- emitting diodes with an InAlN electron-blocking layer and with p-type doped barriers

The advantages of nitride-based dual-wavelength light-emitting diodes (LEDs) with an InAlN electron blocking layer (EBL) are studied. The emission spectra,carrier concentration in the quantum wells (QWs),energy band and internal quantum efficiency (IQE) are investigated. The simulation results indicate that an LED with an InAlN EBL performs better over a conventional LED with an AlGaN EBL and an LED with p-type-doped QW barriers. All of the advantages are due to the enhancement of carrier confinement and the lower electron leakage current. The simulation results also show that the efficiency droop is markedly improved and the luminous intensity is greatly enhanced when an InAlN EBL is used.     

Effects of Mg doping in the quantum barriers on the efficiency droop of GaN based light emitting diodes

The effects of Mg doping in the quantum barriers(QBs) on the efficiency droop of GaN based light emitting diodes(LEDs) were investigated through a duel wavelength method. Barrier Mg doping would lead to the enhanced hole transportation and reduced polarization field in the quantum wells(QWs), both may reduce the efficiency droop. However,heavy Mg doping in the QBs would strongly deteriorate the crystal quality of the QWs grown after the doped QB. When increasing the injection current, the carriers would escape from the QWs between n-GaN and the doped QB and recombine non-radiatively in the QWs grown after the doped QB, leading to a serious efficiency droop.     

新型电子阻挡层结构对蓝光InGaN发光二极管性能的提高

分别对3种不种电子阻挡层的蓝光AlGaN LED进行数值模拟研究。3种阻挡层结构分别为传统AlGaN电子阻挡层,AlGaN-GaN-AlGaN电子阻挡层和Al组分渐变的AlGaN-GaN-AlGaN电子阻挡层。此外对这对三种器件的活性区的载流子浓度、能带图、静电场和内量子效率进行比较和分析。研究结果表明,相较于传统AlGaN和AlGaN-GaN-AlGaN两种电子阻挡层的LED,具有Al组分渐变的AlGaN-GaN-AlGaN电子阻挡层结构的LED具有较高的空穴注入效率、较低的电子外溢现象和较小的静电场(活性区)。同时,具有Al组分渐变的AlGaN-GaN-AlGaN电子阻挡层结构的LED的efficiency droop现象也得到一定的缓解。     

2018年5期 电子期刊

研究了不同Mn/Pb量比的Mn掺杂CsPbCl₃（Mn：CsPbCl₃）钙钛矿量子点的发光性质。Mn/Pb的量比增加引起的Mn²⁺发光峰的红移,被认为是来源于高浓度Mn²⁺掺杂下的Mn²⁺-Mn²⁺对。进一步研究了Mn：CsPbCl₃量子点的发光效率与Mn/Pb的量比之间的关系,发现随着量比达到5：1时,其发光效率明显下降。这种发光效率下降是由于Mn掺杂浓度引起的发光猝灭。Mn：CsPbCl₃量子点的变温发光光谱证实,随着温度的升高,Mn离子发光峰蓝移,线宽加宽,但其发光强度明显增加。     

Improvement of characteristics of InGaN light-emitting diode by using a staggered AlGaN electron-blocking layer

The optical and physical properties of InGaN light-emitting diode (LED) with a specific design of staggered AlGaN electron-blocking layer (EBL) are investigated numerically in detail. The electrostatic field distribution, energy band, carrier concentration, electroluminescence (EL) intensity, internal quantum efficiency (IQE), and the output power are simulated. The results reveal that this specific design has a remarkable improvement of optical performance compared with the design of conventional LED. The lower electron leakage current, higher hole injection efficiency, and consequently mitigated efficiency droop are achieved. The significant decrease of electrostatic field at the interface between the last barrier and the EBL of LED could be one of the main reasons for these improvements.     

基于钠硼铝硅酸盐玻璃的PbSe量子点红外单模光纤激光

根据实验制备的钠硼铝硅酸盐PbSe量子点玻璃及其透射电子显微镜(TEM)图、吸收谱和发射谱,计算机数值模拟了以PbSe量子点作为激活增益介质的红外单模光纤激光。应用遗传算法,通过数值求解粒子数速率方程和激光谐振腔振荡方程,优化计算了量子点光纤激光器(QDFL)的最佳抽运波长、光纤长度、掺杂浓度及出射镜反射率。结果表明:饱和抽运功率为2 W,在1676nm激光波长处,QDFL最大输出功率可达1.36 W,抽运效率达68%。与通常的掺稀土离子(Yb3+、Er3+)的光纤激光器相比,QDFL具有抽运效率高、激励阈值低、掺杂密度可调、光纤饱和长度短等特点。由于量子点辐射波长的尺寸依赖特性,容易形成多波长激射或波长可调的新型激光器。     

红光量子点掺杂PVK体系的发光特性研究

无热处理制备了红光CdSe/ZnS量子点掺杂PVK的ITO/PVK:QDs/Alq3/Al结构电致发光器件.测试器件的发光光谱和电学特性等,研究了掺杂浓度(质量分数)对体系发光特性的影响,将非掺杂与掺杂体系做了比较,提出了优化掺杂体系的一些可行方案.量子点掺杂浓度较低时,主要为Alq3的发光;掺杂浓度为20%时,Alq3的发光得到抑制,红光发射最佳;继续增大掺杂浓度,QDs发光峰发生微弱红移,器件性能变差.与非掺杂体系相比,掺杂浓度合适的PVK:QDs体系大大提高了器件的稳定性.     

Performance improvement of blue light-emitting diodes with an AlInN/GaN superlattice electron-blocking layer

The characteristics of a blue light-emitting diode (LED) with an AlInN/GaN superlattice (SL) electron-blocking layer (EBL) are analyzed numerically. The carrier concentrations in the quantum wells, energy band diagrams, electrostatic fields, and internal quantum efficiency are investigated. The results suggest that the LED with an AlInN/GaN SL EBL has better hole injection efficiency, lower electron leakage, and smaller electrostatic fields in the active region than the LED with a conventional rectangular AlGaN EBL or a AlGaN/ GaN SL EBL. The results also indicate that the efficiency droop is markedly improved when an AlInN/GaN SL EBL is used.     

Ba/Mg 比值对(BaxMg)2/(x+1)Al10O17∶Eu2+晶体结构和发光特性的影响

采用溶液燃烧法在600℃成功合成了(BaxMg)2/(x 1)Al10O17∶Eu2 (0.6≤x≤1.2)蓝色荧光粉,着重研究了基质阳离子Ba/Mg比值的变化对其晶体结构及发光特性的影响。结果表明,合成的产物为纯相,且随Ba/Mg比值的增加,样品的晶格参量逐渐增加;当Ba/Mg比增加时,发射光谱的强度明显增强,至Ba/Mg为0.9时达到最大值,然后随Ba/Mg比继续增大,发射光谱的强度反而下降;Ba/Mg比值减少,导致基质的晶场增强和电子云膨胀效应的发生,致使Eu2 发射主峰向长波方向移动。Eu2 的掺杂浓度对样品的发光性能有显著的影响,随Eu2 浓度增大,发光中心增多,Eu2 离子间相互作用增强,能量传递加快,发光强度逐渐增大,并达到一个最大值。此后,随Eu2 的浓度进一步增加,Eu2 之间的能量传递速率将超过发射速率,呈现浓度猝灭特性。     

Improvement of characteristics of an InGaN light-emitting diode by using a staggered AlGaN electron-blocking layer

The optical and physical properties of an InGaN light-emitting diode (LED) with a specific design of a staggered AlGaN electron-blocking layer (EBL) are investigated numerically in detail. The electrostatic field distribution, energy band, carrier concentration, electroluminescence (EL) intensity, internal quantum efficiency (IQE), and the output power are simulated. The results reveal that this specific design has a remarkable improvement in optical performance compared with the design of a conventional LED. The lower electron leakage current, higher hole injection efficiency, and consequently mitigated efficiency droop are achieved. The significant decrease of electrostatic field at the interface between the last barrier and the EBL of the LED could be one of the main reasons for these improvements.     

Performance enhancement of an InGaN light-emitting diode with an AlGaN/InGaN superlattice electron-blocking layer

The efficiency enhancement of an InGaN light-emitting diode(LED) with an AlGaN/InGaN superlattice(SL)electron-blocking layer(EBL) is studied numerically,which involves the light-current performance curve,internal quantum efficiency electrostatic field band wavefunction,energy band diagram carrier concentration,electron current density,and radiative recombination rate.The simulation results indicate that the LED with an AlGaN/InGaN SL EBL has better optical performance than the LED with a conventional rectangular AlGaN EBL or a normal AlGaN/GaN SL EBL because of the appropriately modified energy band diagram,which is favorable for the injection of holes and confinement of electrons.Additionally,the efficiency droop of the LED with an AlGaN/InGaN SL EBL is markedly improved by reducing the polarization field in the active region.     

Enhanced performance of AlGaN-based ultraviolet light-emitting diodes with linearly graded AlGaN inserting layer in electron blocking layer

The conventional stationary Al content Al GaN electron blocking layer(EBL) in ultraviolet light-emitting diode(UV LED) is optimized by employing a linearly graded Al Ga N inserting layer which is 2.0 nm Al_(0.3) Ga_(0.7) N/5.0 nm Alx Ga_(1-x) N/8.0 nm Al_(0.3) Ga_(0.7) N with decreasing value of x. The results indicate that the internal quantum efficiency is significantly improved and the efficiency droop is mitigated by using the proposed structure. These improvements are attributed to the increase of the effective barrier height for electrons and the reduction of the effective barrier height for holes,which result in an increased hole injection efficiency and a decreased electron leakage into the p-type region. In addition,the linearly graded AlGaN inserting layer can generate more holes in EBL due to the polarization-induced hole doping and a tunneling effect probably occurs to enhance the hole transportation to the active regions, which will be beneficial to the radiative recombination.     

较高掺杂浓度下CdSe/ZnS量子点光纤光致荧光光谱

制备了一种较高浓度掺杂的CdSe/ZnS量子点掺杂光纤.测量了不同掺杂浓度和不同光纤长度下的量子点光纤光致荧光光谱,得剑了荧光峰值增益最大时的量子点掺杂浓度和光纤长度.与低浓度掺杂光纤相比,较高掺杂浓度光纤中的荧光峰值光强明显提高.荧光峰值光强随光纤长度的变化在短距离内(L<1 cm)急剧上升,之后缓慢均匀下降.波长473 nm激励光强随光纤}乏度的变化呈指数形式衰减,消光系数为0.26～1.02 cm-1.在给定激励光强和激励波长的条件下,光纤中可达到最大荧光辐射的晕子点总数为一恒量.光纤中的荧光峰值波长存在红移,红移大小约8～15 nm,红移量与掺杂浓度以及光纤长度有关.这些实验结果可为今后量子点光纤放大器的研制提供参考.     

反对称n-AlGaN层对GaN基双蓝光波长发光二极管性能的影响

采用数值分析方法对在InGaN/GaN混合多量子阱活性层和n-GaN之间引入n-AlGaN层的GaN基双蓝光波长发光二极管进行模拟分析.结果发现,与传统的具有p-AlGaN电子阻挡层的双蓝光波长发光二极管相比,这种反对称n-AlGaN层能有效改善电子和空穴在混合多量子阱活性层中的分布均匀性及减少电子溢出,实现电子空穴在各个量子阱中的平衡辐射,从而减弱了双蓝光波长发光二极管的效率衰减.此外,通过改变Al组分可以提高双蓝光波长发光二极管发射光谱的稳定性:当Al组分为0.16时,双蓝光波长发光二极管的光谱在小电流下比较稳定,而Al组分为0.12时,光谱在大电流下比较稳定.     

利用梯度掺杂获得高量子效率的GaAs光电阴极

获得高量子效率且稳定性良好的阴极一直是近年来发展GaAs光电阴极的重要方向。对晶面为(100),掺杂Be,厚度为1μm分子束外延生长的反射式GaAs发射层,设计了一种从体内到表面掺杂浓度由高到低分布的新型梯度掺杂结构。掺杂浓度的范围从1×1019cm-3到1×1018cm-3,并利用(Cs,O)激活技术制备了GaAs光电阴极。光谱响应测试曲线显示,与传统均匀掺杂的GaAs光电阴极相比,梯度掺杂的GaAs光电阴极的量子效率在整个波段都有提高,积分灵敏度可达1580μA/lm,且具有更好的稳定性。讨论了这种新型GaAs光电阴极获得更高量子效率的内在机理。该设计结构是现实可行的,且具有很大发展潜力,它为国内发展高性能GaAs光电阴极提供了一条重要途径。     

On the polarization self-screening effect in multiple quantum wells for nitride-based near ultraviolet light-emitting diodes

The tilted energy band in the multiple quantum wells(MQWs) arising from the polarization effect causes the quantum confined Stark effect(QCSE) for [0001] oriented III-nitride-based near ultraviolet light-emitting diodes(NUV LEDs). Here, we prove that the polarization effect in the MQWs for NUV LEDs can be self-screened once the polarization-induced bulk charges are employed by using the alloy-gradient In_xGa_(1-x)N quantum barriers. The numerical calculations demonstrate that the electric field in the quantum wells becomes weak and thereby flattens the energy band in the quantum wells, which accordingly increases the spatial overlap for the electron-hole wave functions. The polarization self-screening effect is further proven by observing the blueshift for the peak emission wavelength in the calculated and the measured emission spectra. Our results also indicate that for NUV LEDs with a small conduction band offset between the quantum well and the quantum barrier,the electron injection efficiency for the proposed structure becomes low. Therefore, we suggest doping the proposed quantum barrier structures with Mg dopants.     

Numerical study of the advantages of ultraviolet light-emitting diodes with a single step quantum well as the electron blocking layer

The advantages of ultraviolet light-emitting diodes with a single step quantum well used as electron blocking layer (EBL) are studied numerically. The energy band diagrams, hole concentrations, electrostatic field near the EBL, current–voltage curve and internal quantum efficiency (IQE) are investigated by using the Crosslight simulation programs. The simulation results show that the structure with a single step quantum well has better performance over the conventional one, which can be attributed to the mitigated band bending near the EBL due to the change of electrostatic field by using a step well. Therefore, the efficiency of hole injection is improved, with which both the IQE and the total lighting power are increased.     

Optimizing GaN photocathode structure for higher quantum efficiency

To improve the quantum efficiency of GaN photocathode, we optimized the photocathode's structure in three aspects. We use AlN replacing GaN as the buffer layer, which can act as potential barrier to reflect electrons back to surface. The optimal thickness of emission layer is calculated as 162.5 nm, and considering the graded doping profile, we optimized the thickness as 180 nm. Three built-in electric fields are introduced by Mg graded dope, and the intensities of the high fields are calculated to give the quantitive results of their influence. After surface cleaning and activation, quantum efficiency of the optimized sample was increased and the highest value of 56% was achieved at 240 nm. More quantum efficiency enhancement is possible by further optimizing the photocathode structure.     

Performance improvement of blue InGaN light-emitting diodes with a specially designed n-AlGaN hole blocking layer

Blue InGaN light-emitting diodes (LEDs) with a conventional electron blocking layer (EBL), a common n-AlGaN hole blocking layer (HBL), and an n-AlGaN HBL with gradual Al composition are investigated numerically, which involves analyses of the carrier concentration in the active region, energy band diagram, electrostatic field, and internal quantum efficiency (IQE). The results indicate that LEDs with an n-AlGaN HBL with gradual Al composition exhibit better hole injection efficiency, lower electron leakage, and a smaller electrostatic field in the active region than LEDs with a conventional p-AlGaN EBL or a common n-AlGaN HBL. Meanwhile, the efficiency droop is alleviated when an n-AlGaN HBL with gradual Al composition is used.     

Effect of doping and annealing on the physical properties of ZnO:Mg nanoparticles

Well-dispersed undoped and Mg-doped ZnO nanoparticles with different doping concentrations at various annealing temperatures are synthesized using basic chemical solution method without any capping agent. To understand the effect of Mg doping and heat treatment on the structure and optical response of the prepared nanoparticles, the samples are characterized using X-ray diffraction (XRD), energy-dispersive X-ray (EDX), UV–Vis optical absorption, photoluminescence (PL), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) measurements. The UV–Vis absorbance and PL emission show a blue shift with increasing Mg doping concentration with respect to bulk value. UV–Vis spectroscopy is also used to calculate the band-gap energy of nanoparticles. X-ray diffraction results clearly show that the Mg-doped nanoparticles have hexagonal phase similar to ZnO nanoparticles. TEM image as well as XRD study confirm the estimated average size of the samples to be between 6 and 12 nm. Furthermore, it is seen that there was an increase in the grain size of the particles when the annealing temperature is increased.