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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。
Progress on the study of two-dimensional material self-driven photoresponse enhancement by asymmetrically integrated plasmonic nanostructures (Invited)
doi: 10.3788/IRLA20211011
- Received Date: 2020-11-10
- Rev Recd Date: 2020-12-15
- Available Online: 2021-01-22
- Publish Date: 2021-01-22
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Key words:
- plasmonic nanostructures /
- asymmetric photoelectric coupling /
- self-driven photoresponse /
- flood illumination /
- infrared detection
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.