Performance improvement of AlGaN-based deep ultraviolet light-emitting diodes with double electron blocking layers

The AlGaN-based deep ultraviolet light-emitting diodes（LED） with double electron blocking layers（d-EBLs） on both sides of the active region are investigated theoretically. They possess many excellent performances compared with the conventional structure with only a single electron blocking layer, such as a higher recombination rate, improved light output power and internal quantum efficiency（IQE）. The reasons can be concluded as follows. On the one hand, the weakened electrostatic field within the quantum wells（QWs） enhances the electron–hole spatial overlap in QWs, and therefore increases the probability of radioactive recombination. On the other hand, the added n-AlGaN layer can not only prevent holes from overflowing into the n-side region but also act as another electron source, providing more electrons.

Realization of high-efficiency AlGaN deep ultraviolet light-emitting diodes with polarization-induced doping of the p-AlGaN hole injection layer

We investigate the polarization-induced doping in the gradient variation of Al composition in the pAl_(0.75)Ga_(0.25)N/Al_xGa_(1-x)N hole injection layer(HIL)for deep ultraviolet light-emitting diodes(DUV-LEDs)with an ultrathin p-GaN(4 nm)ohmic contact layer capable of emitting 277 nm.The experimental results show that the external quantum efficiency(EQE)and wall plug efficiency(WPE)of the structure graded from 0.75 to 0.55 in the HIL reach 5.49%and 5.04%,which are improved significantly by 182%and 209%,respectively,compared with the structure graded from 0.75 to 0.45,exhibiting a tremendous improvement.Both theoretical speculations and simulation results support that the larger the difference between 0.75 and x in the HIL,the higher the hole concentration that should be induced;thus,the DUV-LED has a higher internal quantum efficiency(IQE).Meanwhile,as the value of x decreases,the absorption of the DUV light emitted from the active region by the HIL is enhanced,reducing the light extraction efficiency(LEE).The IQE and LEE together affect the EQE performance of DUV-LEDs.To trade off the contradiction between the enhanced IQE and decreased LEE caused by the decrease in Al composition,the Al composition in the HIL was optimized through theoretical calculations and experiments.

Deep ultraviolet light-emitting diodes based on a well-ordered AlGaN nanorod array

The nanorod structure is an alternative scheme to develop high-efficiency deep ultraviolet light-emitting diodes(DUV LEDs). In this paper, we first report the electrically injected 274-nm AlGaN nanorod array DUV LEDs fabricated by the nanosphere lithography and dry-etching technique. Nanorod DUV LED devices with good electrical properties are successfully realized. Compared to planar DUV LEDs, nanorod DUV LEDs present>2.5 times improvement in light output power and external quantum efficiency. The internal quantum efficiency of nanorod LEDs increases by 1.2 times due to the transformation of carriers from the exciton to the free electron–hole, possibly driven by the interface state effect of the nanorod sidewall surface. In addition, the nanorod array significantly facilitates photons escaping from the interior of LEDs along the vertical direction, contributing to improved light extraction efficiency. A three-dimensional finite-different time-domain simulation is performed to analyze further in detail the TE-and TM-polarized photon extraction mechanisms of the nanostructure. Our results demonstrate the nanorod structure is a good candidate for high-efficiency DUV emitters.

Overcoming the fundamental light-extraction efficiency limitations of deep ultraviolet light-emitting diodes by utilizing transverse-magnetic-dominant emission

While the demand for deep ultraviolet(DUV)light sources is rapidly growing,the efficiency of current AlGaN-based DUV light-emitting diodes(LEDs)remains very low due to their fundamentally limited light-extraction efficiency(LEE),calling for a novel LEE-enhancing approach to deliver a real breakthrough.Here,we propose sidewall emission-enhanced(SEE)DUV LEDs having multiple light-emitting mesa stripes to utilize inherently strong transverse-magnetic polarized light from the AlGaN active region and three-dimensional reflectors between the stripes.The SEE DUV LEDs show much enhanced light output power with a strongly upward-directed emission due to the exposed sidewall of the active region and Al-coated selective-area-grown n-type GaN micro-reflectors.The devices also show reduced operating voltage due to better n-type ohmic contact formed on the regrown n-GaN stripes when compared with conventional LEDs.Accordingly,the proposed approach simultaneously improves optical and electrical properties.In addition,strategies to further enhance the LEE up to the theoretical optimum value and control emission directionality are discussed.

Efficiency enhancement of ultraviolet light-emitting diodes with segmentally graded p-type AlGaN layer

AlGaN-based ultraviolet light-emitting diodes(UV-LEDs) have attracted considerable interest due to their wide range of application fields. However, they are still suffering from low light out power and unsatisfactory quantum efficiency.The utilization of polarization-doped technique by grading the Al content in p-type layer has demonstrated its effectiveness in improving LED performances by providing sufficiently high hole concentration. However, too large degree of grading through monotonously increasing the Al content causes strains in active regions, which constrains application of this technique, especially for short wavelength UV-LEDs. To further improve 340-nm UV-LED performances, segmentally graded Al content p-Al_xGa_(1-x)N has been proposed and investigated in this work. Numerical results show that the internal quantum efficiency and output power of proposed structures are improved due to the enhanced carrier concentrations and radiative recombination rate in multiple quantum wells, compared to those of the conventional UV-LED with a stationary Al content AlGaN electron blocking layer. Moreover, by adopting the segmentally graded p-Al_xGa_(1-x)N, band bending within the last quantum barrier/p-type layer interface is effectively eliminated.

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.

The Transport Mechanisms of Reverse Leakage Current in Ultraviolet Light-Emitting Diodes

The transport mechanisms of the reverse leakage current in the UV light-emitting diodes （380nm） are investi- gated by the temperature-dependent current-voltage measurement first. Three possible transport mechanisms, the space-limited-charge conduction, the variable-range hopping and the Poole-Frenkel emission, are proposed to explain the transport process of the reverse leakage current above 295 K, respectively. With the in-depth investigation, the former two transport mechanisms are excluded. It is found that the experimental data agree well with the Poole Frenkel emission model. Furthermore, the activation energies of the traps that cause the reverse leakage current are extracted, which are 0.05eV, 0.09eV, and 0.11 eV, respectively. This indicates that at least three types of trap states are located below the bottom of the conduction band in the depletion region of the UV LEDs.

Reduction of Electron Leakage in a Deep Ultraviolet Nitride Laser Diode with a Double-Tapered Electron Blocking Layer

A double-tapered AlGaN electron blocking layer (EBL) is proposed to apply in a deep ultraviolet semiconductor laser diode. Compared with the inverse double-tapered EBL, the laser with the double-tapered EBL shows a higher slope efficiency, which indicates that effective enhancement in the transportation of electrons and holes is achieved. Particularly, comparisons among the double-tapered EBL, the inverse double-tapered EBL, the singletapered EBL and the inverse single-tapered EBL show that the double-tapered EBL has the best performance in terms of current leakage.

Pterygium associated with light-emitting diode use:a case report

Background:Pterygium is a sun-related ocular surface disease secondary to ultraviolet(UV)radiation exposure.Outdoor occupational UV exposure is known to occur secondary to sun exposure.We present a unique case of pterygium associated with indoor occupational light-emitting diode(LED)exposure not previously described in the literature.Case Description:A mobile phone repairer presented with blurred vision and a superotemporal pterygium of his dominant left eye associated with a magnifying glass LED work lamp was diagnosed.This was excised routinely with conjunctival autografting to the defect.Histopathology confirmed benign pterygium and recovery was uncomplicated with resolution of blur.Conclusions:The development of pterygium in our patient may have arisen due to the LED lamp’s wavelengths possibly falling within the UV as well as the upper end of the visible light radiation spectrum.Given the increasing reliance on LED light sources in modern life,ocular conditions arising from exposure to these radiation sources may now need to be listed in the differential diagnoses of patients with pterygium.Appropriate UV protection counselling for these types of lights may also now need to be considered.

Structural Controls of Tetraphenylbenzene-based AIEgens for Non-doped Deep Blue Organic Light-emitting Diodes

Aggregation-induced emission(AIE)has emerged as a new concept,giving highly efficient solid-state photoluminescence.Particularly,AIE luminogens(AIEgens)with deep blue emission(400–450 nm)have displayed salient advantages for non-doped organic light-emitting diodes(OLEDs).However,deep blue emitters with Commission Internationale de L’Eclairage(CIE)coordinates less than 0.08 are still rare.In this review,we outline the latest achievements in the molecular guidelines based on the AIE core of tetraphenylbenzene(TPB)for developing efficient deep blue AIEgens.We provide insights into the construction of deep blue emitters with high horizontal orientation by regulating the length of the linear molecule.We also discuss the luminescence mechanisms of these AIEgens-based OLEDs by using the magnetic field effects measurements.Finally,a summary of the challenges and perspectives of deep blue AIEgens for non-doped OLEDs is also presented.

基于锥形超晶格p-AlInGaN层的AlGaN基深紫外发光二极管性能优化

为了解决AlGaN基深紫外(DUV)发光二极管(LED)中的严重电子溢出和低空穴注入的问题,本文提出了一种新型锥形超晶格p-AlInGaN层,它大幅改善了基于AlGaN的DUV LED的光电特性.与传统结构相比,所提出结构的输出功率提高了337.8%;同时它的内部量子效率(IQE)也高达96%,并且没有效率下降现象.仿真计算结果表明,锥形超晶格p-AlInGaN层的引入明显增加了多量子阱(MQWs)内载流子的浓度并降低了量子阱(QWs)内的电场,导致了更高的辐射复合率,为改善DUV LED的性能提供了一个有吸引力的解决方案.

Prognostics of radiation power degradation lifetime for ultraviolet light-emitting diodes using stochastic data-driven models

With their advantages of high efficiency,long lifetime,compact size and being free of mercury,ultraviolet light-emitting diodes(UV LEDs)are widely applied in disinfection and purification,photolithography,curing and biomedical devices.However,it is challenging to assess the reliability of UV LEDs based on the traditional life test or even the accelerated life test.In this paper,radiation power degradation modeling is proposed to estimate the lifetime of UV LEDs under both constant stress and step stress degradation tests.Stochastic data-driven predic-tions with both Gamma process and Wiener process methods are implemented,and the degradation mechanisms occurring under different aging conditions are also analyzed.The results show that,compared to least squares regression in the IESNA TM-21 industry standard recommended by the Illuminating Engineering Society of North America(IESNA),the proposed stochastic data-driven methods can predict the lifetime with high accuracy and narrow confidence intervals,which confirms that they provide more reliable information than the IESNA TM-21 standard with greater robustness.

基于分离多量子垒电子阻挡层的AlGaN基深紫外发光二极管

通过分离多量子垒电子阻挡层(EBL)结构,实现了AlGaN基DUV-LED器件性能的提升。由仿真结果可得,与传统的块状EBL相比,采用分离多量子垒结构的EBL可以获得更高的空穴浓度和辐射复合速率。这得益于EBL中间的夹层形成了空穴加速区,使得空穴在加速区获得能量,从而提高了空穴注入效率。另外,多量子势垒结构还能够通过提高电子势垒有效抑制电子泄漏,从而大幅度提升器件性能。综上所述,多量子垒电子阻挡层的引入可以显著提升AlGaN基DUV-LED器件的性能。

有源区掺杂的AlGaN基深紫外激光二极管性能优化

为了改善深紫外激光二极管的性能,本文提出了有源区量子势垒n掺杂、p掺杂和n-p掺杂三种结构.利用Crosslight软件,对原始结构和有源区掺杂的三种结构进行仿真研究,比较四种结构的P-I特性曲线、V-I特性曲线、载流子浓度、辐射复合速率和能带图.仿真结果表明,有源区量子势垒n-p掺杂结构的性能更优,其阈值电压和阈值电流分别为4.40V和23.8mA;辐射复合速率达到1.64×10^(28)cm^(-3)/s;同一注入电流下电光转换效率达到42.1%,比原始结构增加了3.9%;改善了深紫外激光二极管的工作性能.

Stable deep blue organic light emitting diodes with CIE of y<0.10 based on quinazoline and carbazole units

Achieving stable deep blue organic light emitting diodes(OLEDs)with narrow full width at half maximum(FWHM)and color gamut in the range of the commission International de L’Eclairage(CIE)ofy≤0.10 is still challenging in display and lighting applications.In this investigation,three donor-acceptor(D-A)deep-blue emitters were designed and synthesized via integrating asymmetric quinazoline(PQ)acceptor with weak donating carbazole(Cz)donor.The effect of the position and number of Cz group in PQunit are investigated,which is also first examples for syste matic research about the effect of different position of asymmetric PQ as acceptor on deep OLEDs.Their bandgaps of 3.12~3.19 eV and the singlet state energy levels of 3.12~3.19 eV were found to be sufficiently large to achieve deep blue light.As expected,these emitters-based OLEDs exhibit deep blue emission with the maximum wavelength≤450 nm and narrow FWHM≈60 nm.Especially,a CIE ofy=0.080 was achieved for 4 PQ-Cz-based OLED.Significantly,the deep blue electroluminescence(EL)spectra of these three emitters-based OLEDs are very stable and the corresponding CIE coordinates deviation(ΔCIE(x,y))can be negligible under the applied voltage ranging from 5 V to 9 V.

超晶格电子阻挡层周期数对AlGaN基深紫外发光二极管性能的影响

在AlGaN基深紫外发光二极管(DUV-LEDs)中设计了具有不同周期数的超晶格电子阻挡层(SL-EBL)结构,研究了SL-EBL周期数对DUV-LEDs发光效率、I-V特性、可靠性及有源区载流子复合机制的影响.研究结果表明,随着SL-EBL的周期数增加,DUV-LEDs的光输出功率(LOP)、外量子效率(EQE)和电光转换效率(WPE)均呈先上升后下降的趋势,同时泄漏电流减小,可靠性提升.当周期数为7时(厚度为28 nm),DUV-LEDs裸芯的EQE和WPE均达到最大值,在7.5 mA注入电流下分别为3.5%和3.2%.能带模拟结果证明了增加SL-EBL周期数可以有效提升电子势垒高度,而几乎不改变空穴势垒高度.然而,当SL-EBL超过一定厚度时,抑制了空穴向有源区的注入,导致EQE和WPE随SL-EBL周期数变化出现拐点.研究了SL-EBL周期数对DUV-LEDs载流子复合机制的影响,发现增加SL-EBL周期数可以有效地降低有源区内载流子非辐射复合.

不规则H形量子势垒增强AlGaN基深紫外发光二极管性能

针对AlGaN基多量子阱中有效的平衡载流子注入问题,研究了有源区势垒层中Al组分调制形成的非规则H形量子势垒对AlGaN基深紫外发光二极管(LED)器件性能的影响及载流子的输运行为。研究发现,与多量子阱中常用的单Al组分势垒相比,加入Al组分较高的双尖峰势垒可以有效地提高内量子效率和光输出功率。进一步研究表明,电子在有源区因凸起的尖峰势垒而得到了有效的阻挡,减少了电子的泄露,而空穴获得更多的动能从而穿过较高的势垒进入有源区。因此,采用非对称H形量子势垒的深紫外LED器件中载流子输运实现了较好的平衡,量子阱中的载流子复合速率远高于普通的深紫外发光二极管。

利用倒梯矩形电子阻挡层降低深紫外激光二极管的电子泄露

为了有效降低深紫外激光二极管(DUV-LD)在有源区的电子泄露,提出了一种新颖的具有倒梯矩形的电子阻挡层(EBL)结构.通过使用Crosslight软件将矩形,梯矩形和倒梯矩形三种不同的结构进行仿真研究,比较三种结构器件的能带图、辐射复合率、电子空穴浓度、P-I以及V-I特性等,得出倒梯矩形EBL更能有效地抑制电子的泄露,从而改善了器件的光学和电学性能.

量子阱层和垒层具有不同Al组分的270/290/330nm AlGaN基深紫外LED光电性能

为了研究AlGaN量子阱层和垒层中Al组分不同对AlGaN基深紫外发光二极管(LED)光电性能的影响,本文利用MOCVD生长、光刻和干法刻蚀工艺制备了AlGaN量子阱层和垒层具有不同Al组分的270/290/330nm深紫外LED,通过实验和数值模拟计算方法发现,量子阱层和垒层中具有低Al组分紫外LED的AlGaN材料具有较低的位错密度、较高的光输出功率和外量子效率。通过电流-电压(I-V)曲线拟合出的较大的理想因子(>3.5)和能带结构图表明,AlGaN深紫外LED的电流产生是隧穿机制占据主导作用,这是因为高Al组分AlGaN量子阱中强极化场造成了有源层区域较大的能带弯曲和电势降。

氮极性AlGaN基隧道结深紫外LED

针对AlGaN(Aluminum Gallium Nitride)基深紫外LED(Light Emitting Diode)发光效率低、工作偏压较大的问题,设计了一种带有n^(+)-GaN/Al_(0.4)Ga_(0.6)N/p^(+)-GaN隧道结的氮极性AlGaN基深紫外LED器件结构。该结构由n-Al_(0.65)Ga_(0.35)N电子提供层、 Al_(0.65)Ga_(0.35)N/Al_(0.5)Ga_(0.5)N多量子阱、组分渐变p-Al_(x)Ga_(1-x)N和n^(+)-GaN/Al_(0.4)Ga_(0.6)N/p^(+)-GaN隧道结构成。研究结果表明,相比于无隧道结的参考LED,隧道结LED具有更高的内量子效率和光输出功率,同时其具有更低的开启电压。隧道结LED光电特性的改善,归因于隧道结的引入提升了LED的空穴注入效率,提高了LED器件的电流扩展能力。通过模拟软件对半导体器件载流子输运、光电特性的模拟,有助于加深对半导体器件物理特性的理解。若在“半导体器件物理”学习课程加入对半导体器件模拟软件的学习,能有效提升学生对半导体器件物理知识的理解和探索。