AlGaN基深紫外发光二极管空穴注入效率的提高途径 下载: 2248次封面文章
1 引言
近年来,基于AlGaN材料的深紫外发光二极管(DUV LED),在杀菌消毒、水质净化、医学治疗以及生化探测等诸多领域具有广阔的应用前景[1-3],其发展得到了人们广泛的关注与重视。然而,相较于基于InGaN材料的蓝光发光二极管(blue LED),DUV LED器件的性能不尽人意,外量子效率(EQE)普遍低于10%[4-7],严重制约了DUV LED进一步的普及与应用。
造成DUV LED器件EQE表现不佳的原因有很多。研究结果表明,AlN材料的晶格场劈裂能不同于GaN材料,这种差异使得高AlN组分的AlGaN材料具有独特的价带分布顺序[8-11]。即在布里渊区中心点位置,能量最低的价带为晶格场劈裂带(CH),而不再是重空穴带(HH)。因此,随着AlN组分的增加,基于AlGaN材料的DUV LED的发光将由TE(Transverse Electric)模式偏振光转向TM(Transverse Magnetic)模式偏振光,这在极大程度上降低了表面出光器件的光提取效率(LEE)[12-13]。而且随着AlGaN量子阱中AlN组分的增加,TM偏振光所占的比例增大[8-9],DUV LED的LEE进一步降低。为此,研究人员对衬底、量子垒和量子阱进行选择优化,通过调节量子阱层所受到的压应力来增强TE模式偏振光,从而提高DUV LED器件的LEE[12,14-22]。同时,研究结果表明采用表面微腔光子晶体或设计一种截头圆锥形微结构阵列式钝化层(SiN
2 DUV LED器件结构组成
3 提高DUV LED空穴注入效率的若干途径
3.1 介电调控隧穿结
图 2. 计算结果。(a) AlxGa1-xN层的相对介电常数和不同AlN组分之间的关系;(b)平衡态时,器件A1与器件A2的隧穿结区电场强度对比图(插图是隧穿结区电场峰值强度与不同极化水平之间的关系图)。图片引自文献[ 57],已获得Wiley的版权许可
Fig. 2. Simulation results. (a) Relative dielectric constant of AlxGa1-xN layer versus AlN composition; (b) electric fields in tunneling regions for devices A1 and A2 at equilibrium (Inset shows peak field intensity versus polarization level). Reproduced from Ref. [57] with permission of Wiley
此外,关于不同隧穿结对DUV LED空穴注入效率的影响,本文也作了相关的初步研究。器件A3为不具有隧穿结的参考器件,而器件A4,A5和A6分别对应着传统隧穿结、极化隧穿结和介电调控隧穿结。由
图 3. 计算结果。(a)注入电流为35 mA 时,器件A4,A5和A6隧穿结区的电场分布;(b)注入电流为35 mA时,器件A3,A4,A5和A6有源区内空穴分布;(c)器件A3,A4,A5和A6的光输出功率和注入电流之间的关系
Fig. 3. Simulation results. (a) Electric field profiles in tunneling regions for devices A4, A5, and A6 at injection current of 35 mA; (b) hole concentration profiles in active region for devices A3, A4, A5, and A6 at current injection of 35 mA; (c) light output power for devices A3, A4, A5, and A6 versus injection current
值得注意的是,
图 4. 注入电流为35 mA时,器件A3,A4,A5和A6最后一个量子阱中的横向空穴分布
Fig. 4. Lateral hole concentration profiles in last quantum well for devices A3, A4, A5, and A6 at injection current of 35 mA
图 5. 器件A3,A4,A5和A6的电流和电压特性曲线
Fig. 5. Current versus applied voltage for devices A3, A4, A5, and A6
3.2 电场存储器
影响空穴注入效率的另一个重要的因素就是p区的空穴浓度及其结构组成。
图 6. p-AlxGa1-xN层界面耗尽区能带图和电场示意图。(a)能带图;(b)电场示意图。图片引自文献[ 58],已获得Optical Society of America的版权许可
Fig. 6. Energy band diagram and electric field profile in interface depletion region of p-AlxGa1-xN layer. (a) Energy band diagram; (b) schematic of electric field profile. Reproduced from Ref. [58] with permission of Optical Society of America
为此,研究人员设计了5组具有不同AlN组分的p-EBL和p-Al
表 1. 具有不同AlN组分的p-EBL和p-AlxGa1-xN层的器件结构。表格引自文献[ 58],已获得Optical Society of America的版权许可
Table 1. Devices with different AlN compositions for p-EBL and the p-AlxGa1-xN layers. Reproduced from Ref. [58] with permission of Optical Society of America
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图 7. 计算结果。(a) 5组DUV LED器件结构的空穴供给层中对应的电场分布;(b)注入电流为200 A·cm-2时,5组DUV LED器件理论计算得到的光功率。图片7(a)引自文献[ 58],已获得Optical Society of America的版权许可。图片7(b)引自文献[ 4],具体数值总结于文献[ 58]
Fig. 7. Simulation results. (a) Electric field profiles in hole supplier layer for five groups of DUV LED devices; (b) calculated power densities for five groups of DUV LED devices at injection current of 200 A·cm-2. (a) is reproduced from Ref. [58] with permission of Optical Society of America. (b) is reproduced from Ref. [4], and the value is summarized according to the report in Ref. [58]
图 8. I-V特性对比图。(a) Original device和Reference device;(b) Reference Device和器件B1;(c) Reference Device,器件B2和B3。图片引自文献[ 58],已获得Optical Society of America的版权许可
Fig. 8. I-V characteristic comparison. (a) Original device and Reference device; (b) Reference device and device B1; (c) Reference device, device B2 and device B3. Reproduced from Ref. [58] with permission of Optical Society of America
3.3 p-AlxGa1-xN/AlyGa1-yN/AlxGa1-xN(x>y)EBL
p-EBL是造成DUV LED器件低空穴注入效率的另一个重要原因。由于p-AlGaN层的禁带宽度小于p-EBL,因此p-EBL的价带中存在一定的势垒高度,对空穴具有强烈的阻挡作用。而且,对于沿着[0 0 0 1]晶向的DUV LED器件而言,极化效应加剧了p-EBL对空穴的阻挡作用。研究人员发现采用超晶格EBL、组分渐变的EBL以及提高p-EBL中Mg杂质浓度均能够有效地减小EBL层中价带势垒高度,从而提高LED器件的空穴注入效率[59,72-75]。Zhang等[74]的最新研究成果表明,采用超晶格EBL的DUV LED器件的EQE随着大电流的注入几乎不存在性能衰减,这对于抑制LED器件的“droop effect”(效率衰减效应)具有重要的意义。除了超晶格EBL,Zhang等[59]还提出了p-Al
图 9. 示意图和计算结果。(a)传统的p-EBL和(b)具有p-AlxGa1-xN/AlyGa1-yN/AlxGa1-xN(x>y)EBL的DUV LED器件的能带示意图;(c) p-EBL和p-AlGaN层的空穴分布图。图片引自文献[ 59],已获得 American Chemical Society的版权许可
Fig. 9. Schematic and simulation results. Schematic of energy bands for (a) DUV LED with the conventional p-EBL and (b) DUV LED with the p-AlxGa1-xN/AlyGa1-yN/AlxGa1-xN (x>y) EBL; (c) hole concentration profiles in p-EBL and p-AlGaN layers. Reproduced from Ref. [59] with permission of American Chemical Society
图 10. 计算结果和实验结果。(a)器件C1和器件C2量子阱中空穴浓度分布图;(b)实验测得的器件C1和器件C2的光输出功率和EQE;(c)理论计算得到的器件C1和器件C2的光输出功率和EQE。图片引自文献[ 59],已获得American Chemical Society的版权许可
Fig. 10. Simulation and experimental results. (a) Hole concentration profiles in quantum wells for devices C1 and C2; (b) measured optical power and EQE for devices C1 and C2; (c) calculated optical power and EQE for devices C1 and C2. Reproduced from Ref. [59] with permission of American Chemical Society
3.4 p-EBL/p-AlGaN/p-GaN界面极化效应对空穴注入的影响
此外,极化效应对载流子的分布及输运也有着重要的影响。Tian等[79]发现[0 0 0 1]晶向DUV LED器件空穴浓度明显高于[0 0 0 -1]晶向的DUV LED器件,而且增加p-EBL/p-AlGaN/p-GaN界面处极化水平,可以提高DUV LED器件的空穴注入效率。
图 11. 注入电流为 35mA时计算结果。(a) DUV LED器件光输出功率与极化水平之间的关系;(b)器件D1,D2,D3,D4和D5的量子阱,及p-AlGaN层和p-GaN层中空穴分布图。图片引自文献[ 79],已获得Elsevier的版权许可
Fig. 11. Simulation results at injection current of 35 mA. (a) Light output power for DUV LEDs in terms of the polarization level; (b) hole concentration profiles in quantum wells, p-AlGaN layer and p-GaN layer for devices D1, D2, D3, D4, and D5. Reproduced from Ref. [79] with permission of Elsevier
研究结果表明,p-EBL/p-AlGaN/p-GaN界面处的极化水平是造成
图 12. 注入电流为35 mA时的计算结果。(a) DUV LED器件光输出功率和p-EBL/p-AlGaN/p-GaN界面处的极化水平之间的关系;(b)器件 D6,D7,D8和 D9量子阱和p-AlGaN/p-GaN层中空穴分布;(c)器件 D6,D7,D8和 D9的电场分布图。图片引自文献[ 79],已获得Elsevier的版权许可
Fig. 12. Simulation results at injection current of 35 mA. (a) Light output power for DUV LED versus polarization level at p-EBL/p-AlGaN/p-GaN interface; (b) hole concentration profiles in quantum well, p-AlGaN layer and p-GaN layer for devices D6, D7, D8, and D9; (c) electric field profiles for devices D6, D7, D8, and D9. Reproduced from Ref. [79] with permission of Elsevier
表 2. 注入电流为35 mA时,器件D6,D7,D8和D9的p-AlGaN/p-GaN层中的电场对空穴的做功。表格引自文献[ 79],已获得Elsevier的版权许可
Table 2. Work done to holes by electric field within p-AlGaN/p-GaN structure for devices D6, D7, D8, and D9 at injection current of 35 mA. Reproduced from Ref. [79] with permission of Elsevier
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3.5 不同AlN组分的量子垒
由于空穴具有低迁移率的特点,因此空穴在量子阱中的分布与量子垒价带中的势垒高度有着密切的关系。为削弱量子垒对空穴的阻碍作用,研究人员建议沿着[0 0 0 -1]方向逐渐增加量子垒的厚度,从而使空穴在量子阱中均匀分布[81-82]。最新研究结果表明,均匀的空穴分布对DUV LED器件性能的影响不再像蓝光LED中那么明显[4]。Zhang等[83]以AlGaN基UVA LED作为研究基点,当增加AlGaN量子垒中的AlN组分,尽管空穴分布均匀性变差,但量子阱中的空穴浓度显著提高,UVA LED器件性能明显改善[83]。器件E1,E2和E3对应的量子垒材料分别为Al0.02Ga0.98N,Al0.08Ga0.92N和Al0.10Ga0.90N。
图 13. 注入电流密度为100 A/cm2时,器件E1,E2和E3量子阱中的载流子分布。(a)电子分布;(b)空穴分布。图片引自文献[ 83],已获得Optical Society of America的版权许可
Fig. 13. Carrier concentration profiles for devices E1, E2, and E3 at injection current of 100 A/cm2. (a) Electron profiles; (b) hole profiles. Reproduced from Ref. [83] with permission of Optical Society of America
图 14. 能带示意图与光输出功率。(a) UVA LED器件能带示意图;(b)器件E1,E2和E3实验(曲线)和理论(散点)计算得到的光输出功率。图片引自文献[ 84],已获得Optical Society of America的版权许可
Fig. 14. Energy band diagram and optical output power. (a) Schematic of energy bands for UVA LED; (b) measured (curve) and calculated (scatterplot) optical powers for devices E1,E2, and E3 in terms of current. Reproduced from Ref. [84] with permission of Optical Society of America
4 结论
空穴注入效率对基于AlGaN材料的DUV LED器件的IQE有着重要的影响。一方面,AlGaN材料很难实现高效的Mg杂质掺杂;另一方面,空穴具有低迁移率的特点,而且在输运的过程中需要克服价带中的势垒高度。因此,本工作比较系统地总结分析了提高DUV LED空穴注入效率的诸多措施,揭示了一些重要的物理机理,对DUV LED器件的研究具有一定参考性。首先,研究人员提出了p+-GaN/AlGaN/n+-GaN介电调控隧穿结,其中n+-GaN层作金属接触层,可以实现提高空穴注入效率的同时降低DUV LED器件的工作电压。这主要是因为AlGaN插入层具有低介电常数的优点,增强了隧穿结区的电场强度。其次,关于DUV LED器件空穴供给层的结构组成(p-AlGaN/GaN),研究人员创新性地介绍了电场存储器的概念,即p-AlGaN层和p-GaN层的界面处存在的势垒高度使得p-AlGaN层中产生一个方向沿着[0 0 0 -1]晶向的耗尽电场。该耗尽电场增加了空穴的动能和势能,提高了空穴注入至有源区的概率。而针对p-EBL价带中的势垒高度,研究人员提出p-Al
随着外延生长技术不断革新与发展,上述几种器件结构设计能够最大程度地提高空穴注入效率,改善DUV LED的器件性能。但是,相较于蓝光LED中成熟的物理理论体系,目前关于DUV LED的物理机理的研究还比较少,尚处在摸索阶段。载流子在AlGaN/AlGaN MQWs中的输运机制的研究还比较匮乏,基于高Al组分AlGaN/AlGaN MQWs中的QCSE更加严重,高Al组分的p-AlGaN层电导性较差,极易产生电流拥挤效应以及附加的热效应。基于高Al组分的p-AlGaN和n-AlGaN层较难实现优良的欧姆接触。由于电子的迁移率较高,电子的注入效率需要谨慎对待,深紫外发光二极管独特的光极化特性使光提取效率较低,这些影响深紫外发光二极管光效的因素都需要投入研究精力,因此关于DUV LED器件的物理研究任重道远,需要科研人员进行大量的理论分析和实践检验,直至达成普遍的认知。
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Article Outline
田康凯, 楚春双, 毕文刚, 张勇辉, 张紫辉. AlGaN基深紫外发光二极管空穴注入效率的提高途径[J]. 激光与光电子学进展, 2019, 56(6): 060001. Kangkai Tian, Chunshuang Chu, Wengang Bi, Yonghui Zhang, Zihui Zhang. Hole Injection Efficiency Improvement for AlGaN-Based Deep Ultraviolet Light-Emitting Diodes[J]. Laser & Optoelectronics Progress, 2019, 56(6): 060001.