基于二维材料限域或界面效应的非制冷红外光探测研究

光电探测器是一种将电磁波信号转化为电信号的仪器设备，根据其工作波段可以将其分为紫外光探测器、可见光探测器和红外光探测器三大类。目前，对光电探测器研究和产业化热点主要集中在红外光探测器上。传统的红外光探测器主要包括碲镉汞、量子阱以及超晶格等探测器。然而，这些传统红外光探测器具有成本高、应用性能较差等缺点，不能满足新一代小型化、宽谱、高灵敏度、透明和可印刷柔性光探测器的需求。

随着石墨烯的发现，越来越多的二维（2D）材料被大家所发现，其优异的光电性能以及独特的原子结构使得2D材料不仅在光电探测领域和半导体行业拥有巨大的应用前景，而且在新型的微电子器件中也展现出了巨大的应用潜能。提升2D材料光电探测器的光响应度和比探测率、拓宽其探测光谱范围，是将其应用于实际生产或生活中遇到的严峻挑战。为了探明2D材料光电探测器的响应机制从而提升其综合性能，本文在深入研究2D材料光学和电学性能基础上，设计并构筑了新型2D材料光电探测器：h-BN/MoTe₂/石墨烯/SnS₂/h-BN范德华（vdW）异质结宽谱探测器、h-BN保护的MoS₂/WSe₂ II型异质结光电探测器和FePSe₃辐射热探测器，并对它们的响应机理进行了系统研究。这些光电探测器不仅满足目前对光电探测器高灵敏度、高速、宽波段的要求，其工作机理的系统研究还可以为下一代的光通信技术提供新的设计思路。

首先，通过化学气相输运法合成了MoTe₂和SnS₂块体单晶。其次，利用机械剥离、干法转移和标准的电子束曝光工艺制备了h-BN/MoTe₂/石墨烯/SnS₂/h-BN p-g-n光电探测器。进一步，系统研究了h-BN/MoTe₂/石墨烯/SnS₂/h-BN p-g-n异质结的基本物性和光电响应性能。在该异质结光电探测器中包含几个独特的设计：1）利用p型MoTe₂与n型SnS₂界面处的内建电场有效分离光生载流子；2）MoTe₂和SnS₂间的石墨烯不仅拓宽了探测波谱范围而且还可以减少电荷陷阱并改善界面质量，增强光吸收以及范德华p-g-n结中的电荷传输；3）采用h-BN夹层结构降低器件的噪声，提升探测器性能。最后，研究了石墨烯中间层的厚度对探测器性能的影响，发现包含5-7层石墨烯中间层的vdW p-g-n器件的整体光响应性能最优。值得注意的是，在紫外到近红外超宽光谱范围内，该异质结光电探测器的响应度可以超过2.6×10³ AW⁻¹且其比探测率D*亦可达到10¹³ Jones，这比目前已报道基于MoTe₂的光电探测器高至少两个数量级。在短波红外光（SWIR）激发下，该探测器的D*有所降低，但也可以达到1.06×10¹¹ Jones，该结果可以与商用非制冷SWIR Ge-on-Si光电探测器相媲美。该工作可以为宽波段、超高灵敏度的新型光电探测器的设计提供一种可行的策略。

通过化学气相输运法合成了MoS₂和WSe₂块体单晶，并利用机械剥离、干法转移和标准的电子束曝光工艺分别制备MoS₂、WSe₂光电探测器以及h-BN保护的MoS₂/WSe₂ II型异质结光电探测器。进一步，研究了这三种探测器的光电性能。对比发现，MoS₂/WSe₂ II型异质结结构中的层间激子跃迁成功地将探测器的响应波谱范围拓宽至超越单一材料带隙的近红外波段。值得注意的是，在1064 nm近红外光（NIR）激发下，该探测器的响应度可以达到4.8×10³ AW⁻¹，这比目前已知的2D/2D范德华p-n结NIR光电探测器的响应度都高。该工作不仅获得了一种可以超越单一材料带隙限制的高响应度光电探测器，还为新一代、宽波段光电探测器提供了一种可行的设计思路。

通过化学气相输运法合成了三元的金属三卤磷系化合物FePSe₃块体单晶，并利用机械剥离、干法转移和标准的电子束曝光工艺制备了FePSe₃辐射热探测器。进一步，对不同厚度FePSe₃辐射热探测器的基础性能和光响应性能进行了深入研究。该辐射热探测器包含几个特点：1）室温下该材料的电阻温度系数（TCR）几乎不随厚度的变化而发生变化且可以达到−2.96% K⁻¹，这略高于商用辐射热探测器中VO_x的TCR值；2）FePSe₃材料中能量处于0.75-0.89 eV的⁵T_2g-⁵E_g跃迁成功地将材料的吸收波段拓宽至短波红外波段；3）FePSe₃材料的2D范德华堆垛形成了天然悬空结构减少了FePSe₃与外界环境的热交换，进一步提升了辐射热探测器的光响应性能。在紫外-可见光激发下FePSe₃辐射热探测器的响应度和D*分别位于10¹⁰-10¹¹ VW⁻¹和10¹¹-10¹² Jones范围内，且对FePSe₃材料厚度的依赖性较强。在1550 nm光激发下，FePSe₃辐射热探测器的响应度和D*分别为1×10⁸ VW⁻¹和1×10⁹ Jones，这比目前商用VO_x辐射热探测器高2-3个数量级。该工作不仅揭示了FePSe₃辐射热探测器的响应机理，还为新型的、高灵敏度的辐射热探测器提供了新方法。

通过上述研究，本文为具有宽谱、高灵敏度的2D材料光电探测器的制备提供了可行的设计思路。在深入研究其光响应机理的基础上，对其结构进行了优化，为下一代的光通信技术提供实验支撑和借鉴。

        Photodetector is a device that converts electromagnetic signals into electrical signals. According to the working-bands of photodetectors, it can be classified into three categories: ultraviolet detectors, visible light detectors and infrared detectors. At present, the research and industrialization hotspots of photodetectors are mainly focused on infrared detectors. The traditional infrared detectors mainly include HgCdTe, quantum wells, and superlattice photodetectors. However, these traditional infrared photodetectors have the disadvantages of high cost and poor performance, which cannot meet the requirements for the next generation photodetector with miniaturized, broad-spectrum, excellent sensitivity, transparent and printable flexible properties.
        With the discovery of graphene, more and more two-dimensional (2D) materials have been discovered. It’s excellent electronic and optical properties and unique atomic structure make 2D materials have tremendous potential for next-generation photodetector, semiconductor industry and microelectronic. These are the unavoidable challenges in applying 2D material photodetectors to practical production or life, such as simultaneously improving the photoresponsivity and specific detectivity, and broadening their detection spectral range. To explore the photoresponse mechanism and improve the comprehensive performance of 2D material photodetectors, the novel optical and electrical properties of 2D materials and their photodetectors were furtherly investigated in this dissertation, including: hBN/MoTe2/graphene/SnS2/h-BN heterojunction broad-spectrum detectors, h-BN protected MoS2/WSe2 type-II heterojunction photodetector and FePSe3 bolometer, have been designed and explored. In additon, the physical mechanism of the photodetor has been systematically studied. These photodetectors not only meet the current requirements for high sensitivity, fast response speed, wideband of photodetectors, but the systematic study of their working principle can also provide a viable approach for the next generation of ultrahigh-sensitivity and broadband optoelectronics.
        MoTe2 and SnS2 bulk single crystals were synthesized by chemical vapor transport method, and h-BN/p-MoTe2/graphene/n-SnS2/h-BN p-g-n photodeterctors were prepared by standard mechanical exfoliation, dry transfer, and electron beam lithography processes. The fundamental physical properties and photoresponse performance of h-BN/p-MoTe2/graphene/n-SnS2/h-BN p-g-n heterostructure were systematically investigated. There are several unique designs in the p-g-n junction: 1) there is an vertical built-in electrical field between p-MoTe2 and n-SnS2, which can reduce the charge trapping effect; 2) The zero-bandgap graphene interlayer is switched between the MoTe2 and SnS2 not only expending the detection spectrum range, but also enhancing light absorption and charge transport in van der Waals (vdW) p-g-n junctions by reducing charge traps and improving the interface quality; 3) The h-BN sandwich structure is used to reduce the response time, noise of the device and enhance the performance of the detector by shielding the detrimental impact of the charged impurities from the substrate surface or the environment contamination. Finally, the roles of the sandwiching graphene and the origin of the high performances were systematically studied, an optimized structure of the vdW h-BN/pMoTe2/graphene/n-SnS2/h-BN p-g-n junction containing 5-7-layer graphene interlayer have been achieved. The combined effect of these device designs is profound as illustrated in the extraordinary broadband responsivity exceeding 2.6×103 AW−1 and specific detectivity D* up to ~ 1013 Jones have been achieved in the ultra-wide spectrum from ultrvoilet to near infrared (NIR), which is at least two orders of magnitude higher than that of the MoTe2-based photodetectors reported so far. Under 1550 nm short wave infrared (SWIR) light illumination, the D* of the detector is decrease slightly, but it also can reach up to 1.06×1011 Jones, which is comparable to the commercial SWIR uncooled Ge-on-Si photodetector. This work can provide a feasible design idea for new type photodetectors with broadband and ultra-high sensitivity. 
        MoS2 and WSe2 bulk single crystals were grown by chemical vapor transport method. The h-BN protected WSe2, MoS2 and MoS2/WSe2 II type heterojunction photodetectors were prepared by mechanical exfoliation, dry transfer, and standard electron beam lithography processes, respectively. The photoresponse performance of three devices were measured separately. Notably, the interlayer exciton in the MoS2/WSe2 type II heterojunction structure successfully broadens spectral range to the near-infrared band exceeding the cutoff wavelengths of the individual MoS2 and WSe2. Comparing the photoresponse performance of the WSe2, MoS2 and MoS2/WSe2 II type heterojunction photodetector, the responsivity of h-BN-protected MoS2/WSe2 heterojunction photodetector can reach up to 105 AW−1 , which is also nearly one orders of magnitude larger than that of MoS2 photodetector and WSe2 photodetector under 532 nm light illumination. Under 1064 nm light illumination, the responsivity of the detector is up to 4.8×103 AW−1 , which represents the best performance so far achieved on the 2D/2D van der Waals p-n junction NIR photodetector. These results not only obtains a photodetector with high responsivity beyond the limit of intrinsic bandgap of single material, but also provides a scheme to design new type and high sensitivity photodetectors.
        FePSe3 single crystals were prepared by chemical vapor transport method, and FePSe3 bolometer with different thicknesses were prepared by standard mechanical exfoliation method, and electron-beam lithography process. The fundamental and photoresponse performance of FePSe3 bolometer with different thicknesses were investigated. The FePSe3 bolometer contains several features: 1) the temperature coefficient of resistance (TCR) of material is nearly independent of the sample thickness at room temperature and can reach up to −2.96% K−1 , which is slightly higher than the TCR value of the commercial VOx bolometer; 2) the 5T2g5Eg transition of Fe2+ in an octahedral ligand field, whose energy is 0.75-0.89 eV, broadens the absorption band of the material to the short-wave infrared band; 3) The 2D van der Waals stacking of FePSe3 material inherit a natural suspension structure that can reduce the heat exchange between FePSe3 and the external environment, which further enhances the photoresponse performance of FePSe3 bolometer. The responsivity and D* of the FePSe3 bolometer are 10 10-1011 VW−1 and 1011 -1012 Jones under the illumination wavelengths ranging from ultraviolet to visible spectra, respectively, and exhibit an inverted-bell shaped thickness dependence. The responsivity and D* of the FePSe3 bolometer exceeding ~ 1×108 VW−1 and ~ 1×109 Jones under 1550 nm light illumination, which is 2-3 orders of magnitude higher than that of current commercial VOx bolometer. This result not noly reveals the photoresponse mechanism of FePSe3 bolometer, but also paves the way toward new type of bolometers with high sensitivity.
        These works provide a viable route toward new type of 2D photodetectors with boradband and high sensitivity. Base on the researches on the photoresponse mechanisms, the structure of photodetectors were optimized, which underpins strategies and references for the next generation of optical communication technology

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