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Wu Jingyuan, Liu Zhaoguo, Zhang Tong. Research progress of infrared single-photon detection with high gain (Invited)[J]. Infrared and Laser Engineering, 2021, 50(1): 20211016. doi: 10.3788/IRLA20211016
Citation: Wu Jingyuan, Liu Zhaoguo, Zhang Tong. Research progress of infrared single-photon detection with high gain (Invited)[J]. Infrared and Laser Engineering, 2021, 50(1): 20211016. doi: 10.3788/IRLA20211016

Research progress of infrared single-photon detection with high gain (Invited)

doi: 10.3788/IRLA20211016
  • Received Date: 2020-11-03
  • Rev Recd Date: 2020-12-14
  • Available Online: 2021-01-22
  • Publish Date: 2021-01-22
  • Ultra-sensitive single-photon detection is a key technology for the development of optical quantum information and quantum manipulation. It is of important scientific significance and application value to realize high-efficiency, high-sensitivity, low-power and low-cost single-photon photodetectors. There is still a large gap between visible single-photon detector based on silicon and infrared ones in terms of the cost and performance. Exploring the technology of infrared single-photon detection with novel materials and mechanism has become the urgent needs in the field of photodetection. In recent years, low-dimensional materials have offered a new possibility for realizing high-gain, room-temperature and broad-band photodetectors due to their unique physical and chemical properties. The research on the low-dimensional materials based photodetectors with good performance has also become a hot topic in the field of infrared photodetection. In this review, the basic principles of traditional avalanche infrared photodetectors were introduced firstly. On this basis, the latest development of avalanche devices based on novel low-dimensional materials was summarized. Then the new gain amplification mechanism of the photodetector based on photogating effect was discussed and the structure as well as the performance of the devices were reviewed. Finally, the future developing directions and challenges of the infrared single-photon detection technology were prospected.
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Research progress of infrared single-photon detection with high gain (Invited)

doi: 10.3788/IRLA20211016
  • 1. Department of Optoelectronic Science and Engineering, College of Science, Donghua University, Shanghai 201620, China
  • 2. Joint International Research Laboratory of Information Display and Visualization, School of Electronic Science and Engineering, Southeast University, Nanjing 210096, China

Abstract: Ultra-sensitive single-photon detection is a key technology for the development of optical quantum information and quantum manipulation. It is of important scientific significance and application value to realize high-efficiency, high-sensitivity, low-power and low-cost single-photon photodetectors. There is still a large gap between visible single-photon detector based on silicon and infrared ones in terms of the cost and performance. Exploring the technology of infrared single-photon detection with novel materials and mechanism has become the urgent needs in the field of photodetection. In recent years, low-dimensional materials have offered a new possibility for realizing high-gain, room-temperature and broad-band photodetectors due to their unique physical and chemical properties. The research on the low-dimensional materials based photodetectors with good performance has also become a hot topic in the field of infrared photodetection. In this review, the basic principles of traditional avalanche infrared photodetectors were introduced firstly. On this basis, the latest development of avalanche devices based on novel low-dimensional materials was summarized. Then the new gain amplification mechanism of the photodetector based on photogating effect was discussed and the structure as well as the performance of the devices were reviewed. Finally, the future developing directions and challenges of the infrared single-photon detection technology were prospected.

    • 单光子探测是一种光子尺度下的极限灵敏光电探测技术,在量子通信、激光雷达、精密测量以及超灵敏探测等前沿学科领域发挥着重要作用[1-4],尤其随着量子信息科学领域的迅猛发展,极大地推动了高性能单光子探测技术研究,部分可见光硅基单光子探测器已经实用化并得到广泛应用。然而,与可见光波段的探测器相比,红外单光子探测器由于探测材料的红外光吸收限制、制备工艺复杂以及热噪声影响,在探测效率、灵敏度等性能参数上仍有很大的提升空间。拓展单光子探测器的响应波长范围具有显著的研究价值[5-6],例如,低损耗光纤通讯窗口通常位于红外波段,且近红外单光子探测器在量子密钥分发领域具有广阔的应用前景;此外,在自由空间的激光测距等领域,通过拓展响应波长可以显著提高人眼安全阈值,更适合于主动探测成像领域等。因此,红外超灵敏探测也是目前光电检测技术领域的研究热点。

      主流的单光子红外探测器主要分为三类:雪崩效应器件、频率上转换器件以及低温超导器件。其中雪崩效应光电二极管工作时通常需要施加几十伏到上千伏的驱动电压,利用巨大的载流子倍增增益实现室温单光子探测[7-8]。频率上转换器件则利用非线性光学晶体将红外光转换为可见光[9],进而通过可见光硅基单光子探测器件进行探测,但它普遍存在较大的非线性噪声等问题。超导单光子探测器主要包括超导纳米线单光子探测器[10-11]、超导相变边缘探测器[12]以及动态电感探测器[13]等,尽管超导类探测器件在探测效率、暗计数以及宽谱响应等性能方面超越了传统半导体单光子探测器,但由于它严苛的极低温工作条件(小于3 K),使得系统重量和体积剧增,不利于集成化,且成本昂贵,限制了超导单光子探测器的实际应用。总而言之,目前各类红外单光子探测技术仍面临着关键瓶颈问题需要突破,发展室温工作、高效率、低功耗和集成化的单光子红外探测器需求十分迫切。

      文中总结了基于高增益放大机制的红外单光子探测技术研究进展,在传统半导体雪崩类红外光电探测器工作机制和器件性能介绍的基础上,主要讨论了基于新兴的低维材料(如一维纳米线和二维材料)的高增益光电探测机制,总结了基于雪崩机制的低维材料光电探测器发展现状,并分析了这些低维材料器件相比于体材料的潜在优势,此外介绍了基于光诱导栅压效应的单一低维材料和异质结复合体系光电探测器的结构和性能,最后展望了基于低维材料的超灵敏红外探测技术的未来研究方向。

    • 雪崩光电二极管是目前最成熟的红外单光子探测器结构,它是基于内光电效应和内部电子碰撞电离增益机制实现超灵敏探测[5],如图1所示,雪崩光电二极管通常由p-n结构成,并工作在反向偏置电压条件下,当反向偏压大于器件的击穿电压时,耗尽层中的光生电子-空穴对被加速获得足够动能,通过与晶格碰撞电离实现电子空穴对的倍增,这种雪崩链式效应使得探测器内部电信号急速放大,此时倍增因子趋于无穷大,该过程也被称为盖革模式(Geiger mode),是实现单光子计数的必要条件。然而由于盖革模式下器件雪崩效应无法自行停止,因此需要通过设计外部淬灭电路来控制器件的雪崩过程。

      Figure 1.  Schematic diagram of the structure and working principle of avalanche photodiode[5]

    • 通常红外雪崩二极管单光子探测器根据半导体带隙要求采用Ge或InGaAs材料制备,Ge的极限响应波长为1.8 μm,但其光电二极管背景噪声非常大,与Ge相比,InGaAs作为直接带隙半导体,它的红外吸收系数更大,因此探测效率更高,且噪声更小,尤其适合工作在1 550 nm通信窗口[14]。最常见的红外雪崩二极管通常由InGaAs/InP异质结构成,其中InGaAs作为吸收层产生载流子,InP为倍增层实现载流子的碰撞电离,此外在吸收层和倍增层中会插入多级过渡层以实现电场的平滑过渡。对于单光子探测器来说,探测效率(Photon Detection Efficiency, PDE)和暗计数(Dark Count Rate, DCR)是关键性能指标,目前商用的InGaAs/InP红外雪崩光电探测器的PDE仅为20%,DCR约为100 kHz (在1 550 nm波长处),性能远低于红外超导探测器。近年来,研究人员围绕红外半导体雪崩探测器的结构设计、制备工艺以及测试电路方面开展了大量的研究工作以提升器件性能[3, 15-16]

      单光子探测器的PDE由器件的耦合效率、吸收效率以及雪崩碰撞产生概率等因素共同决定,最近,针对PDE改善问题,中国科学技术大学Fang等人对InGaAs/InP雪崩光电探测器进行了结构优化(见图2(a)[16],通过添加介质-金属反射层提升了器件对 1550 nm波长入射光子的吸收率,同时利用单片集成读出电路提取微弱的雪崩信号,并抑制寄生电容,最终器件室温下的PDE最高达到60.1%,DCR为340 kHz (见图2(b))。

      Figure 2.  (a) Schematic diagram of the InGaAs/InP avalanche photodetector; (b) Normalized DCR and PDE performance of the photodetector. The inset plots the effective gating width measurement with 50% PDE[16]

      尽管通过结构优化和电路设计等方法在传统半导体红外雪崩探测器的性能方面已取得一些进步,但由于后脉冲效应,即材料缺陷俘获载流子再次引发雪崩击穿效应的影响,使得器件的DCR难以进一步降低(通常在100 kHz量级)[17]。随着纳米技术的发展,研究人员开始尝试采用制备基于半导体低维纳米结构的雪崩光电探测器,有望在保持高增益的同时降低暗噪声,此外,纳米结构光电探测器具有更易于集成化和小体积的优点,满足红外光电探测器的发展趋势。下文针对一维纳米线和二维材料雪崩光电探测器的最新研究进展分别进行了介绍。

    • 一维纳米线由于较大的表面积-体积比以及亚波长直径尺寸展现出独特的物理性质,为高增益以及偏振敏感的光电探测带来了新的机遇,而且纳米线可以与入射光发生强耦合显著提升光吸收,并通过改变纳米线的尺寸形貌和组分对光吸收进行调控,非常有潜力实现室温红外探测[18-21]。早在2006年,哈佛大学C. M. lieber课题组首次提出了由Si-CdS纳米线构成的纳米级光电倍增二极管[18],器件探测光子数小于100,且具有亚波长空间分辨率,十余年来,研究人员通过探索基于不同半导体材料的纳米线以及优化纳米线阵列的形貌和结构,从而提升了单光子探测性能。

      2019年,美国加州大学Hyunseok Kim等人制备了工作在盖革模式下的垂直纳米线阵列雪崩探测器(见图3[21],纳米线中InGaAs层为吸收层,GaAs为倍增层,每个雪崩探测器包含4 400根纳米线,由于每根纳米线单独产生雪崩倍增过程,因此与体材料相比,该器件的后脉冲效应影响显著减小,最终优化的结果是器件的DCR低至10 Hz,时间抖动小于113 ps,这表明利用一维纳米线结构有望直接实现高性能InGaAs红外雪崩光电探测器,而不需要复杂的淬灭电路设计,但该器件仍需要低温制冷的工作条件。为了提高器件的工作温度,S. J. Gibson等人报道了采用锥形InP纳米线p-n结阵列,实现了室温下高效单光子探测[22],器件的增益高达105,时间抖动小于20 ps,并且通过对InP纳米线大小和形貌的设计优化,探测器在紫外-近红外宽光谱(450~900 nm)范围内具有几乎一致的探测效率,内量子效率超过70%。

      Figure 3.  (a) Measured DCR at different temperatures of InGaAs-GaAs avalanche photodetector. The inset shows the schematic drawing of device composed of InGaAs absorption layer, GaAs avalanche layer, and InGaP passivation shell; (b) SEM images of nanowire growth after each layer. Scale bar 500 nm[21]

      上述工作充分反映了低维半导体纳米线结构在红外单光子探测领域的应用潜力,与一维超导纳米线单光子探测器相比,基于雪崩增益机制的半导体纳米线探测器的工作温度显著提升,有望实现兼具室温工作、高效率、低噪声和快速响应等优异性能的单光子探测,通过选择适当纳米线材料以及设计阵列周期,能够进一步拓展探测器的响应波长范围和提升器件性能。

    • 二维材料由于其独特的层状结构和能带特征,近年来在光电探测领域得到了广泛关注,但目前将二维材料应用于雪崩光电器件的相关研究刚刚起步[23],二维材料雪崩光电探测器的优势在于由于其原子级厚度性质,所需要的碰撞电离击穿电压远小于三维半导体材料,此外,通过外场调控充分耗尽二维材料中的载流子,因此能够抑制器件的暗噪声。最早研制的二维材料雪崩光电探测器主要工作在可见光波段[24-25],采用的材料包括二维InSe以及MoS2等,不同于传统半导体构建的p-n结雪崩二极管,二维材料通常采用光电晶体管的器件形式,利用二维材料与金属电极之间的肖特基势垒产生电场形成载流子的雪崩效应。

      为了在红外波段响应,需要选取合适的窄带隙二维材料,黑磷(Black phosphorus, BP)由于高载流子迁移率以及窄带隙特性,非常有潜力应用于高增益红外探测领域。2019年,J. Jia等人报道了基于多层BP的超灵敏雪崩光电探测器(见图4(a)[26],由于多层BP的带隙约为0.33 eV,因此其光电倍增过程所需要的电场强度远小于其他二维材料。在外加电场较小时,电流随电场强度线性增加,当外加电场超过临界值以后,由于雪崩倍增机制光电流迅速增大到微安量级,如图4(b)和4(c)所示。对于二维材料探测器而言,它们的普遍性问题在于二维材料的原子级厚度限制了光吸收能力,表面等离激元结构由于局域场增强特性可以用来显著提升二维材料光电探测器件的光吸收[27-29]。J.Jia等人也采取这一策略,通过在BP雪崩探测器沟道上沉积金纳米颗粒,不仅提高了BP的光吸收率,同时还利用金颗粒的n型电子掺杂作用降低了器件的暗电流,将器件的信噪比提升了3倍。

      Figure 4.  (a) Schematic of BP avalanche photodetector; (b) Photocurrent vs wavelength for different electric fields; (c) Operation principles of BP device at different electric fields. When E>Ecrit, carrier multiplication occurs due to the avalanche effect[26]

      除了传统的雪崩效应以外,二维材料独特的电子输运特性为探索全新的雪崩机制原理性器件提供了可能。2019年,南京大学A. Gao等人首次在二维材料原子级厚度异质结中发现了一种新型p-n结弹道雪崩击穿机制,他们利用弹道输运过程中电荷几乎无散射以及保持相位相干的量子特性,有望解决传统雪崩器件需要强电场激发的问题,并突破传统器件的理论噪声极限[30]。器件的具体结构由二维InSe/BP垂直异质结构成,响应波长可达到4 μm中红外波段,由于弹道雪崩机制,器件的亚阈值摆幅仅为0.25 mV/dev,展现了低功耗的优异性能,同时器件的雪崩阈值电压小于1 V,显著降低了器件的噪声。该工作为实现高性能红外雪崩光电探测器提供了新的研究思路。

    • 利用photogating (光诱导栅压调控)效应也是实现高增益灵敏探测的手段之一,不同于前面介绍的雪崩增益依赖于强电场,photogating效应通过光照引起的电势调控晶体管沟道的电导率,也可以理解为在场效应晶体管中产生了额外的栅压导致的光电响应[31-33],晶体管中俘获的载流子对沟道层产生强局域作用,进而形成极大光增益(见图5)。量子点场效应晶体管单光子探测器即利用了这一机制,该类器件最早是由剑桥大学A.J.Shields等人提出的[34],器件由GaAs二维电子气场效应晶体管和InAs量子点浮栅结构组成,通过InAs量子点高效俘获光生载流子,进而改变沟道二维电子气的输运特性,该技术可以实现一至三个光子的分辨。

      Figure 5.  Schematic of photogating effect of the phototransistor with high gain

      随着近年来低维材料研究热潮的兴起,基于photogating效应的光电探测器再次引起了人们的广泛关注。一方面,由于低维材料比表面积大,材料的表面态或缺陷态都可以作为载流子的俘获中心,从而调控沟道的电导率;另一方面,通过构建低维材料异质结复合体系,利用其中一种材料作为高效的载流子俘获中心,在界面载流子积累形成的内建电场作用下,另一种材料中的电导能够被有效调控,从而产生高增益。下文针对上述两种情形分别进行了描述。

    • 2014年,中国科学院上海技术物理研究所Weida Hu研究团队设计和制备了核壳结构的InAs纳米线,利用壳层对光生载流子的俘获调制纳米线沟道的电导,从而实现了可见-近红外波段的室温高增益光电探测器,器件的基本结构和工作原理示意图如图6(a)~6(b)所示[35]。他们进一步制备了基于单根CdS纳米线的场效应晶体管,并实现了457 nm波长下的室温可分辨的单光子探测[36],该工作不仅展示了基于photogating机制的核壳结构纳米线在室温单光子探测领域的应用潜力,未来还有望通过改变半导体材料将响应波长拓展到红外波段。

      Figure 6.  (a) Schematic diagram and (b) photogating mechanism of InAs nanowire photodetector with high gain at room temperature[35]; (c) AFM image of mid-infrared tellurene high-gain photodetector and the photocurrent at different incident wavelengths[37]

      二维材料探测器中同样存在photogating效应,在可见光波段已有关于MoS2、In2Se2等二维材料基于photogating效应的高增益探测器报道,这类器件的特点在于由于极高的响应度,器件能够对微弱光信号(有效光功率为皮瓦量级)灵敏探测[32-33, 38-39]。在红外波段,Q. Guo等人报道了基于BP的高增益室温探测器[40],器件响应波长达到3.39 μm,他们分析了由于浅杂质能级缺陷对光生载流子的俘获机制导致器件具有近105的高增益。然而,由于存在BP在空气中不稳定等问题,研究人员也在探寻其它窄带隙的二维材料,最近C. Shen等人研制了碲烯中红外探测器件(见图6(c)),在3.39 μm红外光入射条件下,基于photogating机制的器件增益为3.15×104,且碲烯的稳定性远高于BP,被认为在中红外高增益探测领域具有重要的应用前景[38]

      然而根据大多数基于单一低维材料的photogating光电探测器件实验报道结果,由于载流子被俘获会延长过剩少子的寿命,因此高增益是以延迟了器件的响应时间为代价获得的,只适用于追求高响应度而不要求快速响应的应用场合。

    • 通过构建不同低维材料的异质结复合体系,有望在探测器中充分发挥photogating机制的高增益特性的同时获得快速响应,量子点由于出色的光吸收能力通常作为光敏层,二维材料则作为沟道层,此时器件的探测波长由量子点的吸收波段决定,因此利用红外吸收量子点可以拓展可见光二维材料探测器的响应波段,即响应波长不再受到二维材料带隙的限制。近年来,不断有零维-二维材料复合结构光电探测器被提出[41-43],包括零维PbS-二维石墨烯、零维HgTe-二维MoS2等,不仅探测波长可以拓展到中红外波段,而且器件具有极高的增益和外量子效率。为了进一步提升复合体系界面的电场和载流子收集效率,I. Nikiskiy等人提出了在石墨烯光电晶体管垂直方向集成PbS量子点光电二极管[44],器件的结构如图7(a)~7(b)所示,此时二极管特性决定了器件的响应速度,其3 dB带宽达到1.5 kHz,同时利用石墨烯优异的载流子输运特性,器件的增益仍高达105,外量子效率超过70% (见图7(c))。除了常见的胶体量子点以外,研究人员也在不断尝试其他新型零维纳米结构,2017年,浙江大学Z. Ni等人利用重掺杂Si量子点与石墨烯场效应晶体管复合(见图7(d)~7(f),在紫外-近红外波段基于photogating效应器件的光增益高达1012,同时在中红外波段利用重掺杂Si量子点的表面等离激元共振增强机制提升了器件的响应度[45]。上述研究工作为研制兼具高增益和快速响应性能的红外探测器件提供了新的途径,未来有望实现室温工作的高效红外单光子探测。

      Figure 7.  (a) Optical microscope image and (b) schematic of the phototransistor consisted of colloidal PbS QDs and graphene, in which ITO-PbS-graphene form photodiode in the vertical direction; (c) responsivity and EQE performance of the device[44]; (d) Schematic of photoresponse mechanism, (e) responsivity and (f) gain of the hybrid photoransistor based on Si QDs and graphene[45]

    • 红外单光子探测器对于量子信息技术领域的发展至关重要,半导体雪崩效应光电探测器是目前发展最成熟的单光子探测器,基于雪崩倍增机制可以工作在室温条件,并具有高带宽和高增益的优点。然而,基于半导体的红外雪崩效应探测器由于后脉冲和噪声等问题,其探测率、暗噪声等性能难以突破是其面临的关键瓶颈问题。与此同时,人们也在探索基于新型材料的高增益、超灵敏红外探测技术,低维材料,包括一维纳米线和二维材料,由于它们巨大的比表面积以及独特的能带结构等新颖的物理特性,被认为最有潜力实现新一代高性能红外光电探测器。

      近年来,人们围绕着低维材料的制备、器件设计以及新型增益放大机制等方面开展了一系列研究工作,为研制高增益、低成本、低功耗和集成化的室温工作红外单光子探测器提供了新的可能。但不可否认的是,基于低维材料的单光子探测技术发展仍不成熟,大部分研究工作仅展示了器件的高增益特性及其单光子探测理论可行性,离红外单光子探测实用化仍有很远的距离,未来可从以下几方面加以关注:第一,基于单一低维材料以及复合体系的高增益机理仍需要进一步探索,例如通过结合微观多物理场表征手段,从而阐明载流子的动力学过程,为构建低维材料高增益红外探测器件结构提供理论依据。第二,针对器件的红外单光子探测性能提升问题,需要在材料制备、电学接触特性以及有源层与红外光相互作用的增强等方面进行优化设计。第三,探测器与量子点等新型单光子源的单片集成技术也是未来实现量子信息系统芯片化和柔性化的重要研究方向。通过理论研究的深入以及器件加工测试水平的提升,低维材料高增益探测技术对于推动高性能红外单光子探测领域的发展将发挥关键作用。

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