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在过去的十几年里,新兴的二维层状材料促进了新型光电探测器的发展[1]。不同的二维材料通常具有不同带隙,覆盖了目前传统块状半导体材料所不能达到的几乎所有感兴趣的波长[1]。二维材料超薄的厚度使其静电调控的效果突出,局域栅压能够耗尽绝大多数本征载流子,抑制暗电流。另外,二维材料能够与绝大多数衬底以及其他二维材料进行集成和堆叠,而不用考虑传统材料晶格匹配的苛刻限制。再加上其制造工艺与目前的半导体技术兼容,二维材料在光电探测领域具有很大的应用前景。作为第一种被广泛研究的二维材料光电探测器[1],金属-二维材料-金属(Metal-2D Material-metal, M2M)光电探测器的结构类似于二维材料场效应晶体管。在零偏压操作下,自驱动光响应通常由金属-二维材料边界的局部光照产生,因为那里存在肖特基结[2–5]。其机制可能是光伏(PV)或光热电效应(PTE),这取决于入射波长以及二维材料的掺杂情况[2–5]。简单的体系结构允许这种类型的器件与其他系统兼容集成。因此,M2M光电探测器在实际应用中得到了广泛的研究。虽然这种器件的优点是显而易见的,但其自驱动模式存在两个瓶颈问题:1)在均匀的泛光照明下,没有净的自驱动光响应;2)由于结区的低光吸收,光响应有限。第一个问题是由于两个对称的电极-二维材料肖特基结区处的光电流大小相近,方向相反。许多研究致力于打破对称性,包括在沟道中进行不同的掺杂以形成结、使二维材料与异种金属接触、使二维材料与另一种材料形成异质结[6–10]。到目前为止,还没有可靠的二维材料掺杂方法,而且所有其他结构都需要复杂的制造工艺,从而增加二维材料损伤的风险。第二个问题来自于入射光的波长与二维材料原子厚度之间的巨大不匹配,这严重限制了光与物质相互作用的光学长度。纳米光子结构具有在亚波长尺度上产生强光场的能力,已被证明有望增强二维材料的吸收和光响应[11–16]。随着对器件工作原理的深入理解,笔者将根据光响应机理对器件结构上的光与物质相互作用进行更精细的控制,以获得更好的性能改进。对于M2M器件,需要一种能够增强光与物质在一个电极的相互作用而抑制光与物质在另一个电极的相互作用的纳米光子结构。此外,纳米光子结构应与器件很好地兼容,不应干扰其他功能,如栅控。近年来,人们尝试使用非对称集成的等离激元纳米结构同时解决这两个瓶颈问题。如表1所示,Echtermeyer T J等人通过在石墨烯上制备亚波长金属光栅,将石墨烯与等离激元纳米结构结合,在可见光波段获得了20倍的两端电极处光响应的差异[14];Shautsova V等人制备了等离激元纳米天线通过非对称分布,从而在石墨烯沟道上产生较大的电子温度梯度,极大地增强了PTE产生的光电流,在天线集成的电极附近的响应率得到了明显提高,比没有天线集成的电极附近的光响应提高了约5倍[16];Hou C等人通过光学纳米五聚体天线集成少层二硫化钼,实现了增强少层二硫化钼的近红外探测,通过研究光学纳米天线的位置分布,发现了光学天线集成的金属电极与无光学天线集成的金属电极处的光响应对比度为2.54倍[17]。结果表明,M2M器件在泛光照明下具有显著的自驱动光响应。与增强石墨烯光吸收的其他光子结构相比[11–15, 18–22],等离激元纳米谐振腔可以提供更有效的耦合、不灵敏的角度依赖性以及与M2M器件结构更好的兼容性[23–26]。近期,笔者课题组提出并实现了二维材料与等离激元纳米谐振腔的复合结构,并获得了两个电极处的光响应对比度超过100倍。其在泛光照射下的净响应率比金属光栅集成石墨烯的响应率高出一个数量级以上。后者是石墨烯吸收增强的常用结构。
表 1 非对称集成等离激元结构器件比较
Table 1. Comparison between asymmetrically integrated plasmonic structures
Progress on the study of two-dimensional material self-driven photoresponse enhancement by asymmetrically integrated plasmonic nanostructures (Invited)
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摘要: 金属-二维材料-金属是最常见的二维材料光探测器件的结构。由于结构简单、易于集成,该类器件受到最广泛的关注和研究。其自驱动光探测的模式具有很低的暗电流,有望成为高性能红外探测的新途径。然而金属-二维材料-金属的自驱动光探测存在两个瓶颈问题:(1)反对称的金属-二维材料结区引起的泛光照射下光响应的抵消;(2)二维材料有限光吸收导致的低响应率。文中介绍了利用等离激元纳米结构的非对称集成引入非对称的光耦合,从而打破泛光照射下二维材料与两端电极接触区域产生的光电流的对称性,实现净的自驱动光响应;同时利用等离激元纳米结构产生的局域强光场提高二维材料光吸收率和光响应率的一系列研究进展。在石墨烯等离激元纳米谐振腔复合结构中,实现两个电极附近的光响应对比度超过100倍,突破了对称光耦合导致的光响应抵消的难题。由于具有将入射光耦合成局域模式的优越能力,等离激元纳米谐振腔比亚波长金属光栅更有效地提高石墨烯响应率一个数量级以上。Abstract: Metal-2D material-metal photodetectors is the most common type of 2D material photodetectors. Due to the simple structure and the ease of integration with other systems, metal-2D material-metal photodetectors have received the widest range of attentions and research interest. The self-driven mode of this type of photodetectors has very low dark current, and then it is regarded as a promising new route for high performance infrared detection. However, there are two bottleneck problems for self-driven metal-2D material-metal photodetectors: (1) photoresponse cancellation caused by antisymmetric 2D material-contact junctions, (2) low responsivity due to limited light absorption of 2D materials. The recent progress on the study of metal-2D material-metal photodetectors with asymmetrically integrated plasmonic nanostructures was introduced, where asymmetrical light coupling was utilized to break the anti-symmetry between the photocurrents at the two contact-2D material junctions for self-driven net photoresponse, and the induced strong local field was utilized to enhance the absorptance and the responsivity of the 2D material. In the hybrid device of graphene and plasmonic nanocavities, the contrast between photoresponses at the two contacts is more than 100 times, which breaks through the problem of photoresponse cancellation caused by symmetric optical coupling. Due to the superior capability to couple the incident light into a localized mode, the plasmonic nanocavity can enhance the responsivity of graphene over one order of magnitude higher than a subwavelength metal grating.
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图 1 (a)等离激元纳米结构集成石墨烯器件的SEM图像(蓝色为石墨烯;紫色为SiO2(300 nm);黄色为Ti/Au电极。比例尺为20 μm);(b)金属条带阵列的SEM图像(L和TR 为入射光的偏振方向。比例尺为1 μm);(c)纳米结构处的光电压分布图(入射波长514 nm,偏振为TR方向,线宽110 nm,周期300 nm);(d)归一化光电流和最大增强系数[14]
Figure 1. (a) Scanning electron microscopy micrographs of the graphene devices with plasmonic nanostructures (Blue, graphene; purple, SiO2 (300 nm); yellow, Ti/Au electrodes. Scale bar, 20 μm); (b) SEM image of the metal strip array (L and TR incident light polarizations are indicated. Scale bar, 1 μm); (c) Photovoltage maps of one of the nanostructured contacts (The incident wavelength is 514 nm, TR polarization, the line width is 110 nm and the period is 300 nm); (d) Normalized photocurrent and maximum enhancement coefficient[14]
图 2 (a)具有等离激元纳米结构非对称分布的石墨烯器件原理图;(b)器件以及石墨烯/金纳米结构的SEM图像(上标尺代表2 μm,下标尺代表300 nm);(c)计算了入射波长740 nm时纵向(L)极化和横向(TR)极化的电磁场分布;(d)具有等离激元纳米结构(左)和无等离激元纳米结构(右)的石墨烯器件的暗场显微图像;(e)在不同的激发波长下,光电压以(d)箭头所示的方向扫描整个器件;(f)计算积分电磁场的线扫描[16]
Figure 2. (a) Schematic of the graphene device with plasmonic and nonplasmonic contacts; (b) Scanning electron microscopic image of the device (upper panel, scale bar is 2 μm) and graphene/Au nanostructures (lower panel, scale bar is 300 nm); (c) Calculated electromagnetic field distributions for longitudinal (L) and transverse (TR) polarization at 740 nm; (d) Dark field microscopic image of the graphene device with plasmonic (left) and nonplasmonic contacts (right); (e) Photovoltage line scans across the device in the direction indicated by the arrow in (d) taken at different excitation wavelengths; (f) Calculated line scan of the integrated electromagnetic field[16]
图 3 (a) MoS2光电探测器的SEM图像(刻度条代表5.0 μm);(b)放大的单个光学纳米天线的SEM俯视图(标尺代表100 nm);(c) 830 nm波长激发下单元Au纳米天线阵列处的电场分布俯视图(xy平面);(d)比较入射激光束在不同位置激发的光电流[26]
Figure 3. (a) SEM image of the MoS2 photodetector (The scale bar represents 5.0 μm); (b) Enlarged SEM top view of a single optical nano-antenna (The scale bar represents 100 nm); (c) Top view (xy plane) of the electric field distribution of an Au nano-antenna array under light excitation at 830 nm; (d) Comparison of the photocurrents measured with the incident laser beam at different positions[26]
图 4 (a)石墨烯与等离激元纳米谐振腔复合器件示意图;(b)单个等离激元纳米谐振腔集成石墨烯的示意图(器件的沟道长度为20 μm);(c)光响应率作为垂直和平行于x轴偏振的激光光斑照明位置的函数(激光波长为1.55 μm,光斑尺寸约为2.25 μm,功率为2.18 mW);(d)测量了两种器件的自驱动光响应光谱(两种器件的沟道长度均为10 μm);(e)光电压作为栅控电压的函数;(f)四种器件的光响应光谱(这四种复合结构器件都有不同的金属条带宽度:215、237、256、283 nm。周期保持不变:~590 nm)[29]
Figure 4. (a) Sketch of the graphene and plasmonic nanocavity hybrid structure; (b) Sketch of a single plasmonic nanocavity with graphene (The channel length of the device is 20 μm); (c) Photoresponsivity as a function of the laser spot illuminating position for polarizations perpendicular and parallel to the x-axis (The laser wavelength is 1.55 μm, the spot size is about 2.25 μm, and the power is 2.18 mW); (d) Measured self-driven photoresponse spectra of the two devices (The channel lengths of both devices are 10 μm); (e) Photovoltage as a function of the gating voltage; (f) Photoresponse spectra of the four devices (Each of the four hybrid devices has a different metal patch width: 215, 237, 256 and 283 nm. The period is kept the same: ~590 nm)[29]
表 1 非对称集成等离激元结构器件比较
Table 1. Comparison between asymmetrically integrated plasmonic structures
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