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超晶格按能带结构分为三种:(1)以GaAs/AlGaAs为代表的I类超晶格,GaAs的禁带完全落入AlGaAs的内部,电子和空穴都被限制在材料GaAs中;(2)以InAs/GaSb为代表的T2SLs,InAs的禁带和GaSb的禁带错开,电子被限制在InAs中,而空穴被限制在GaSb中;(3)以HgTe/CdTe为代表的III类超晶格,其能带结构与I类超晶格类似,但其中一种组成材料为半金属,半金属的厚度对超晶格的能带结构起到了决定性的作用。
锑化物T2SLs,通常由窄带系的6.1Å族材料如InAs、GaSb、AlSb、InSb、GaAs和AlAs组成,通过改变周期厚度及材料组分,使得超晶格子带形成的禁带宽度小于组成的材料,吸收波长范围可覆盖短波到甚长波,展现出优异的能带可调节性,在红外探测器及激光器领域有着广泛的应用。目前,InAs/GaSb和InAs/InAsSb T2SLs被广泛认为最具潜力的两种T2SLs材料体系[29]。
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自20世纪70年代开始, InAs/GaSb II 类超晶格被广泛应用于红外探测器研究。近年来,InAs/GaSb II 类超晶格在很多性能参数上,比如外量子效率、响应率等已经可以与MCT相媲美,但仍然受限于少数载流子寿命,主要受限于材料体系内的缺陷态[30]。Ga被认为导致InAs/GaSb II 类超晶格的缺陷态主要来源之一。研究表明,InAs的少子寿命(~325 ns)要比GaSb (~100 ns)的寿命长[31],并且InAs0.80Sb0.20 的少子寿命(~250 ns)与InAs相当[32]。因此,GaSb或者含Ga的界面材料被认为是引起缺陷态的主要来源之一,其较高的缺陷态密度导致了较大的非辐射复合的暗电流,包括Shockley–Read–Hall (SRH) 非辐射复合,缺陷辅助隧穿 (Trap-assisted tunneling,TAT)电流等,严重制约了InAs/GaSb T2SLs的器件性能。相比于InAs/GaSb,InAs/InAsSb T2SLs具有以下优点:
(1)更长的少子寿命:InAs/InAsSb T2SLs不含Ga原子,杜绝了由于Ga产生的缺陷态,同时异质结界面更为简单,可实现更长的载流子寿命;研究表明,InAs/InAsSb T2SLs比InAs/GaSb T2SLs的少子寿命高了~1-2个数量级,在77 K达到~10 μs,可以和MCT材料相比拟[33]。
(2)更简化的异质外延过程:InAs/InAsSb异质结具有两种相同的元素(In和As),只有Sb是变量元素,具有更加简单的异质结界面结构。InAs/InAsSb T2SLs的能带结构依赖于层的厚度及As/Sb元素之比。由于MBE外延生长过程中,In和As可以一致保持开的状态,只需要控制Sb源挡板阀的开关,就可以完成整个超晶格的生长过程,有望在大规模生产中更好地控制界面及更高的良率[34-35]。
(3)更大的缺陷态容忍度: InAs/InAsSb T2SLs(导带差∆Ec ~142 meV,价带差∆Ev ~226 meV)比InAs/GaSb (导带差∆Ec ~930 meV, 价带差∆Ev ~510 meV)具有较小的导带差和价带差[36],两种超晶格的能带结构如图1所示。InAs/InAsSb T2SLs的导带较低,缺陷态能级位于导带边之上,具有较大缺陷容忍度,使得相同缺陷态密度下少子寿命更长[37]。
图 1 (a) InAs/GaSb能带结构; (b) InAs/InAsSb能带结构
Figure 1. (a) Band structure of InAs/GaSb; (b) Band structure of InAs/InAsSb
但InAs/InAsSb T2SLs也有诸多的局限性。图1中,两种超晶格都采用InAs电子量子阱,但它们在空穴量子阱方面有所不同:GaSb的价带边明显高于InAsSb的价带边,这将超晶格子价带边(HH1)拉得更高,从而更容易实现更小的超晶格带隙。也就是InAs/InAsSb需要较大的周期厚度达到与InAs/GaSb相同的吸收截止波长,减少了电子-空穴波函数的交叠,从而导致较弱的振子强度和较小的吸收系数[38]。同时,对于生长在GaSb衬底上的InAs/InAsSb T2SLs,InAs层处于轻微拉伸应变,而InAsSb层处于相对较高的压缩应变。通常需要相对较厚的InAs层来对InAs/InAsSb T2SLs中的InAsSb层进行应变平衡补偿。较厚的InAs层导致InAsSb价带量子阱的更大的分离,从而使得生长方向空穴有效质量增大[39-40],限制了n型吸收层的少子空穴的扩散长度,进而影响器件的量子效率[41],尤其是在长波红外波段会导致量子效率(QE)较低[42]。
为了提高InAs/InAsSb超晶格的吸收系数,研究人员发现,提高Sb组分,可有效减小周期厚度,增加波函数交叠。但由于严重的As-Sb置换,Sb组分的增加,需要足够高的Sb通量(束流),导致表面残留大量Sb原子,在InAs/InAsSb SLs界面处易诱导Sb偏析问题[43-44]。Sb偏析的产生,使超晶格出现缓变界面,不利于载流子的限制,又对应变的调控带来不确定因素,极大地提高了器件能带设计的难度[45]。
表1总结了InAs/GaSb与InAs/InAsSb超晶格各自的优缺点。总体来说,InAs/InAsSb超晶格材料在均匀性,稳定性和少子寿命上表现突出,高工作温度下暗电流小,且生长简单,是制造低成本、小尺寸、低重量和低功耗(C-SWaP)红外焦平面阵列(IRFPA)的优势材料。
表 1 InAs/GaSb与InAs/InAsSb超晶格优缺点对比
Table 1. Comparison of advantages and disadvantages of InAs/GaSb and InAs/InAsSb superlattices
InAs/InAsSb T2SLs InAs/GaSb T2SLs Longer minority carrier lifetime Higher absorption coefficient Advantages Simpler epitaxy process Greater offset of conduction band and valence band Better defect tolerance Larger cut-off wavelength range Lower absorption coefficient Short minority carrier lifetime Disadvantages Lower vertical hole mobility More complicated epitaxy process Shorter carrier diffusion length Intrinsic defects of Ga atom Sb segregation -
InAs/InAsSb T2SLs的发展路线[46]如图2所示,20世纪90年代Biefeld等首次生长了InAs/InAsSb超晶格[47],Zhang等报道了InAs/InAsSb T2SLs的光泵浦的中红外激光器,并提出InAs/InAsSb可应用于长波红外探测器[48]。自此之后,有大约10年的时间,InAs/InAsSb超晶格一直处于沉寂期,鲜有研究人员对其进行红外探测器的研究。直到2009年,加拿大西蒙菲莎大学的Lackner等人为了实现低噪声探测,再次研究了InAs/InAsSb超晶格,开展了在GaSb衬底外延生长晶格匹配的InAs和InAsSb的超晶格结构的研究[49]。2011年,Steenbergen等通过时间分辨光致发光(TRPL)研究发现InAs/InAsSb超晶格具有比InAs/GaSb超晶格更长的少子寿命[50]。自此以后,InAs/InAsSb超晶格得到了广泛关注。2015年,美国桑迪亚国家实验室与亚利桑那州立大学、伊利诺伊大学厄巴纳-香槟分校合作表征了InAs/InAsSb T2SLs的少数载流子输运,测量了少子扩散长度及生长方向上的空穴迁移率[51]。同年,Prins等人通过实验表明,InAs/InAsSb T2SLs的缺陷态能级位于导带边之上,证明这类超晶格具有更好的缺陷态容忍能力,在低暗电流红外探测器上具有良好的应用前景[37]。
基于InAs/InAsSb T2SLs优异的材料性能,2012年,Kim等在GaSb衬底上外延生长InAs/InAsSb T2SLs,制备了nBn型长波红外探测器[8]。之后InAs/InAsSb超晶格被应用于中波[52-55]、长波[8, 56-59]、甚长波[41]及双色探测器[60]。另外,由于InAs/InAsSb T2SLs较大的缺陷态容忍度等优点,GaAs[59]、Si[52, 54]、Ge-Si[61-62]等衬底外延生长的InAs/InAsSb T2SLs也被先后报道,有望应用于大尺寸、低成本的红外焦平面。
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目前,InAs/InAsSb T2SLs在中波红外探测方面有着十分优秀的表现,高工作温度下的量子效率与MCT探测器相当。在长波方面,与更为成熟的InAs/GaSb T2SLs相比,InAs/InAsSb T2SLs具有更易于生长,更长的少数载流子寿命等优点,但InAs/InAsSb T2SLs需要更长的超晶格周期来实现相同的截止波长[79]。同时,在长波波段,InAs/InAsSb T2SLs在生长方向上空穴迁移率低于InAs/GaSb T2SLs,进一步降低了少子扩散长度。而InAs/InAsSb T2SLs的电子迁移率明显优于空穴迁移率[80],因此,为了提高量子效率,长波需要采用p型InAs/InAsSb T2SLs作为吸收层,相关性能指标仍处于研发初期。总体来说,目前InAs/InAsSb T2SLs在高温工作中波红外探测器上优势明显,为进一步提升势垒型InAs/InAsSb T2SLs红外探测器性能,研究人员对其暗电流和探测率等关键性能参数进行了详细研究。
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暗电流是指在没有外加光辐射,探测器在外加偏压时产生的电流。暗电流一般由载流子的扩散或者器件表面和内部的缺陷以及杂质引起。对于T2SLs红外探测器其暗电流包含四种机制:扩散电流、产生-复合电流、隧穿电流、表面泄漏电流。
扩散电流与少数载流子密切相关。扩散电流Idiff随少子寿命τ延长而减少(Idiff ~ τ−1)。InAs/InAsSb T2SLs的少子寿命比InAs/GaSb T2SLs的延长了数十倍,理论上扩散电流将比InAs/GaSb T2SLs小数十倍,但实验上并没有得到证明。
器件的内部会存在许多不同的缺陷和杂质。在器件工作时,这些缺陷和杂质会捕获电子和空穴进行复合,产生G-R复合暗电流。G-R复合暗电流主要发生在耗尽层,也是pn型T2SLs红外探测器的主要组成部分。采用势垒型结构InAs/InAsSb T2SLs探测器可有效抑制G-R复合电流,降低器件G-R复合暗电流。
隧穿电流包括带间隧穿电流和陷阱辅助隧穿电流。隧穿电流与有效质量、掺杂浓度及外加电场密切相关。当器件的反向工作电压较大时,隧穿电流较大。势垒型结构InAs/InAsSb T2SLs探测器中降低器件工作电压是降低隧穿电流的有效途径。
表面漏电流是由于在蚀刻过程中器件侧壁形成的界面态产生的暗电流,势垒型结构能够有效地抑制表面泄漏暗电流[81]。
近年来,中波InAs/InAsSb T2SLs红外探测器的暗电流变化趋势如图9所示,在150~160 K的条件下,器件的暗电流已经接近Rule 07。这些具有如此低暗电流密度的势垒型 InAs/InAsSb T2SLs将在高温工作的中波红外探测器方面极具竞争潜力。
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光电探测器的噪声等效功率(NEP),是探测器的本征属性,和器件面积、测试环境有关。为了屏蔽这两者的影响,1953年Jones[89]定义了归一化探测率D*:
$$ {{{D}}^*} = D \times {({{{A}}_{{e}}} \cdot \Delta {{f}})^{1/2}}[{\text{cm}} \cdot {\text{H}}{{\text{z}}^{1/2}}/{\text{W}}] $$ (1) 式中:D为探测率即噪声等效功率的倒数;
${{A}}_{e}$ 为器件面积;$\mathrm{\Delta }{f}$ 为检测带宽。温度较高时,归一化探测性能受热噪声限制,此时归一化探测率D*为:$$ {D^*} = \frac{{q\lambda \eta }}{{2hc}}{\left( { \frac{{{R_0}A}}{{kT}} } \right)^{1/2}} $$ (2) 式中:q为电子电量;
$ \lambda $ 为波长;$ \eta $ 为量子效率;h为普朗克常数;c为光速;$ {R}_{0}A $ 为零偏动态电阻与器件面积的乘积;k为玻尔兹曼常数;T为温度。近年来,报道的中波InAs/InAsSb T2SLs探测率如图10所示。总体上,中波InAs/InAsSb T2SLs探测率在1011~1012 cm·Hz1/2/W,相关的提升机制有待进一步探索。
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近年来,势垒型InAs/InAsSb T2SLs的关键性能提升趋势明显。为方便比较,表2[68, 72-73, 75-76, 82-88, 90-93]汇总了近年来势垒型InAs/InAsSb T2SLs红外探测器的关键性能。2016~2018年,JPL在中波InAs/InAsSb T2SLs关键性能上进行了大量研究,通过蚀刻工艺降低表面态密度和暗电流,提高QE,设计了许多性能优异的器件。2021~2022年,该团队在中波的基础上,研究了长波、甚长波InAs/InAsSb T2SLs,基于pn-CBIRD势垒结构设计长波长InAs/InAsSb T2SLs,发现其暗电流特性受限于表面漏电流,需要在较低偏置条件下工作。
表 2 势垒型InAs/InAsSb T2SLs红外探测器关键性能对比表
Table 2. Key performance comparison chart of barrier-type InAs/InAsSb T2SLs infrared detectors
Time Structure Wavelength/
μmDetection rate/
cm· Hz1/2·W−1QE Working
temperature/KDark current/
A·cm−2Research
institutionsReference 2018 nBn 5.4 2.53×1011 49.1% 150 ~3×10−5 JPL [68] 2018 nBn 5.37 4.6×1011 52% 150 4.5×10−5 JPL [82] 2019 12.5 62 2.6×10−5 JPL [91] 2019 nBn 4.6 1.4×1011 150 1.6×10−4 CQD [83] 2019 nBn 5.5 56% 160 3.4×10−4 Air Force Research Lab [90] 2019 nBn 4.8 50% 150 5×10−6 JPL [88] 2020 pBn 4.4 7.1×1011 39% 150 1.16×10−5 CQD [84] 2020 nBn 5 1.82×1011 37.5% 150 1.55×10−4 Kunming Institute of Physics [85] 2020 pBn 4.8 4.43×1011 57.6% 5.39×10−5 Kunming Institute of Physics [72] 2020 double barrier 4.5 6.9×1011 45% 150 1.21×10−5 CQD [86] 2020 nBn 3.35 9.12×1011 23.5% 77 1.23×10−6 CQD [92] 2021 XBn 5 50% 150 4.5×10−5 ANR [87] 2021 pn-CBIRD 10.3 1.3×1011 77 5.4×10−5 JPL [75] 2021 nBn 3 50% 80 2×10−9 Korea i3 system [93] 2022 pn-CBIRD 13.3 53% 60 6.6×10−5 JPL [76] 2022 pBn 5.0 1.2×1011 29% 150 1.2×10−4 Northwestern University of China [73] 美国西北大学量子器件中心CQD的Razeghi团队2017年报道了势垒型InAs/InAsSb T2SLs的在多色探测器上的应用。在基于M结构的双带/多带集成探测上,设计了随偏压改变而探测不同波长的光电探测器[94]。偏压可选双频带器件通常由两个T2SLs吸收层和插入两个吸收层之间的薄势垒层组成,双带结构可以通过切换所施加的偏置电压来交替地处理两个吸收层。2020年,CQD使用锌离子注入设计了 nBn型T2SLs中波红外光电探测器[92],其比探测率可达9.12×1011 cm·Hz1/2/W。
其他国外研究机构如韩国i3system,法国国家研究署(ANR)也对势垒型InAs/InAsSb T2SLs展开了研究,都取得了不错的器件性能。2021年,韩国i3system通过干法刻蚀和等离子体处理制备了InAs/InAsSb T2SLs nBn型中波红外探测器[93],暗电流达到2×10−9 A/cm2。
表3[78, 95-105]汇总了近年来势垒型InAs/GaSb T2SLs红外探测器的关键性能。2016年,美国CQD设计的nBn型InAs/GaSb T2SLs红外探测器的探测率最高达到1.12×1013 cm·Hz1/2/W且其暗电流可以降低到9.5×10−9 A/cm2。
表 3 势垒型InAs/GaSb T2SLs红外探测器关键性能对比表
Table 3. Key performance comparison chart of barrier-type InAs/GaSb T2SLs infrared detectors
Time Structure Wavelength/
μmDetection rate/
cm·Hz1/2·W-1QE Working
temperature/KDark current/
A·cm-2Research
institutionsReference 2008 nBn 4.8 2.8×1011 23% 250 3.1×10−6 University of New Mexico [95] 2012 p-CBIRD 11.5 1.1×1011 21% 80 JPL [96] 2012 pBiBn 4.2/8.7 8.9×1012
/7.9×101138%/23.5% 77 1.6×10−7/
1.42×10−5University of New Mexico [97] 2014 pMp 4.9 1.2×1012 67% 150 1.2×10−5 Northwestern University [100] 2015 nBn 2.7 2.5×1010 77 2.5×10−6 Kunming Institute of Physics [99] 2017 pBn 5 50% 80 2×10−5 University of New Mexico [102] 2017 pBp 2.3/
2.9/
4.41×1011/
6.3×1011/
2×101120%/
22%/
34%150 5.5×10−8/
1.8×10−6/
8.7×10−5CQD [101] 2018 pBn 4.5 50% 80 4.7×10−6 The Ohio State University [103] 2019 pBn 6.4 7.6×1011 77 2.9×10−5 Shanghai University of
Science and Technology[78] 2020 nBn 5.6 2.5×1011 77 1.44×10−5 Shanghai University of
Science and Technology[98] 2021 nBn 5.3 2.6×1011 77 3.5×10−3 Korean Academy of Sciences [104] 2022 nBn 10.4/
12.21.7×1010/
1.5×10109×10−4/
2×10−2University of Science and
Technology of China[105] 对比表2、表3中的探测率和暗电流,InAs/GaSb T2SLs在探测率上优于InAs/InAsSb T2SLs,但在暗电流方面,InAs/InAsSb T2SLs得益于没有Ga原子带来的缺陷,高温工作下暗电流低于InAs/GaSb T2SLs。
Research progress of barrier InAs/InAsSb type-II superlattice infrared detectors (invited)
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摘要: 红外探测技术在卫星侦察、军事制导、天文观测、医疗检测、现代通信等重要领域发挥着关键作用。II类超晶格(T2SLs)红外探测器作为继碲镉汞探测器之后的新一代红外探测材料,在稳定性、可制造性和成本等方面具有独特优势。势垒型InAs/InAsSb T2SLs红外探测器是最具潜力的T2SLs红外探测器之一,近年来其关键性能得到了稳步提高,但仍受吸收系数低、异质外延生长困难和暗电流大等因素的制约。文中综述了III-V族T2SLs的发展历程,分析了势垒型InAs/InAsSb T2SLs红外探测器的不同势垒结构、关键性能和发展趋势,指出了势垒型InAs/InAsSb T2SLs红外探测器需要解决的关键问题和未来发展方向。
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关键词:
- 红外探测器 /
- T2SLs /
- InAs/InAsSb /
- 势垒结构
Abstract: Infrared detection technology plays a key role in many important fields, such as satellite reconnaissance, military guidance, astronomical observation, medical detection, and modern communications. Type-II superlattices, as a new generation of infrared detection materials after HgCdTe detectors, have unique advantages in terms of stability, manufacturability, and cost. The barrier-type InAs/InAsSb type-II superlattice infrared detectors are one of the most promising type-II superlattice infrared detectors. Their key performance has been steadily improved in recent years but is still constrained by factors such as low absorption coefficient, difficult heteroepitaxial growth, and large dark current. Herein, this article reviews the development history of III-V type-II superlattices, analyzes the different barrier structures, key properties and development trends of barrier-type InAs/InAsSb type-II superlattice infrared detectors, and points out the potential key problems and future development directions of barrier type InAs/InAsSb type-II superlattice infrared detectors.-
Key words:
- infrared detector /
- Type-II superlattice /
- InAs/InAsSb /
- barrier structure
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表 1 InAs/GaSb与InAs/InAsSb超晶格优缺点对比
Table 1. Comparison of advantages and disadvantages of InAs/GaSb and InAs/InAsSb superlattices
InAs/InAsSb T2SLs InAs/GaSb T2SLs Longer minority carrier lifetime Higher absorption coefficient Advantages Simpler epitaxy process Greater offset of conduction band and valence band Better defect tolerance Larger cut-off wavelength range Lower absorption coefficient Short minority carrier lifetime Disadvantages Lower vertical hole mobility More complicated epitaxy process Shorter carrier diffusion length Intrinsic defects of Ga atom Sb segregation 表 2 势垒型InAs/InAsSb T2SLs红外探测器关键性能对比表
Table 2. Key performance comparison chart of barrier-type InAs/InAsSb T2SLs infrared detectors
Time Structure Wavelength/
μmDetection rate/
cm· Hz1/2·W−1QE Working
temperature/KDark current/
A·cm−2Research
institutionsReference 2018 nBn 5.4 2.53×1011 49.1% 150 ~3×10−5 JPL [68] 2018 nBn 5.37 4.6×1011 52% 150 4.5×10−5 JPL [82] 2019 12.5 62 2.6×10−5 JPL [91] 2019 nBn 4.6 1.4×1011 150 1.6×10−4 CQD [83] 2019 nBn 5.5 56% 160 3.4×10−4 Air Force Research Lab [90] 2019 nBn 4.8 50% 150 5×10−6 JPL [88] 2020 pBn 4.4 7.1×1011 39% 150 1.16×10−5 CQD [84] 2020 nBn 5 1.82×1011 37.5% 150 1.55×10−4 Kunming Institute of Physics [85] 2020 pBn 4.8 4.43×1011 57.6% 5.39×10−5 Kunming Institute of Physics [72] 2020 double barrier 4.5 6.9×1011 45% 150 1.21×10−5 CQD [86] 2020 nBn 3.35 9.12×1011 23.5% 77 1.23×10−6 CQD [92] 2021 XBn 5 50% 150 4.5×10−5 ANR [87] 2021 pn-CBIRD 10.3 1.3×1011 77 5.4×10−5 JPL [75] 2021 nBn 3 50% 80 2×10−9 Korea i3 system [93] 2022 pn-CBIRD 13.3 53% 60 6.6×10−5 JPL [76] 2022 pBn 5.0 1.2×1011 29% 150 1.2×10−4 Northwestern University of China [73] 表 3 势垒型InAs/GaSb T2SLs红外探测器关键性能对比表
Table 3. Key performance comparison chart of barrier-type InAs/GaSb T2SLs infrared detectors
Time Structure Wavelength/
μmDetection rate/
cm·Hz1/2·W-1QE Working
temperature/KDark current/
A·cm-2Research
institutionsReference 2008 nBn 4.8 2.8×1011 23% 250 3.1×10−6 University of New Mexico [95] 2012 p-CBIRD 11.5 1.1×1011 21% 80 JPL [96] 2012 pBiBn 4.2/8.7 8.9×1012
/7.9×101138%/23.5% 77 1.6×10−7/
1.42×10−5University of New Mexico [97] 2014 pMp 4.9 1.2×1012 67% 150 1.2×10−5 Northwestern University [100] 2015 nBn 2.7 2.5×1010 77 2.5×10−6 Kunming Institute of Physics [99] 2017 pBn 5 50% 80 2×10−5 University of New Mexico [102] 2017 pBp 2.3/
2.9/
4.41×1011/
6.3×1011/
2×101120%/
22%/
34%150 5.5×10−8/
1.8×10−6/
8.7×10−5CQD [101] 2018 pBn 4.5 50% 80 4.7×10−6 The Ohio State University [103] 2019 pBn 6.4 7.6×1011 77 2.9×10−5 Shanghai University of
Science and Technology[78] 2020 nBn 5.6 2.5×1011 77 1.44×10−5 Shanghai University of
Science and Technology[98] 2021 nBn 5.3 2.6×1011 77 3.5×10−3 Korean Academy of Sciences [104] 2022 nBn 10.4/
12.21.7×1010/
1.5×10109×10−4/
2×10−2University of Science and
Technology of China[105] -
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