Photodetectors based on two-dimensional semiconductors: Progress, opportunity and challenge

Abstract

Two-dimensional (2D) materials have attracted increasingly great interest in recent years due to their unique structures and novel physical and chemical properties. 2D semiconductors, the brightest member of the family of 2D materials, allow for realizing versatile electronic and optoelectronic devices. Their layered structure and atomic thickness render them have promising applications in monolithic integration, flexible devices and wearables electronics. Moreover, their bandgaps are usually related to the thickness and external strain, which mean the electronic optoelectronic properties can be tuned by changing the number of layers or strain engineering. Interestingly, some 2D semiconductors such as black phosphorus, GeS, and ReS2 have low symmetry crystal structures, their electronic and optical properties are highly anisotropic, such characteristics give us another degree of freedom to photodetections using polarized light. This feature article reviews the recent research activities that focus on applications of 2D semiconductors as photodetectors. It begins with survey of photocurrent generation mechanisms, which include photoconductive effect, photogating effect, photovoltaic effect, photo-thermoelectric effect and bolometric effect. These mechanisms are systematically introduced and discussed. Then, the general meaning of figure of merit that evaluates the performance of photodetectors is introduced, including responsivity, external quantum efficiency, time response, signal to noise ratio, noise equivalent power, detectivity. Furthermore, the recent photodetectors based on 2D semiconductors including transition metal dichalcogenides (TMDs), black phosphorus, ternary chalcogenides and their hybrid structures such as 2D-0D, 2D-1D, 2D-2D and 2D-3D structures are presented. MoS2 is the most studied TMD semiconductor, photodetectors based on monolayer and multilayers MoS2 are widely studied, some strategies including doping, encapsulation, and device design for improving photoelectronic performance are presented. Another interesting TMD is ReS2, due to its direct bandgap nature regardless of thickness and the low symmetry structure, it is supposed to have promising optoelectronic properties including in plane anisotropy. Black phosphorus, a p-type direct bandgap semiconductor with ultrahigh room temperature mobility beyond 1000 cm2/V s, have also received much attentions in recent years. Unlike most 2D semiconductor have bandgap range from 1–2 eV, black phosphorus have a much wide tunable bandgap from 0.3–1.5 eV, depending on the thickness, which means it can be used for infrared detection. Besides, like ReS2, black phosphorus also have in plane anisotropy in optic and electronic properties, proving us another additional opportunity by using polarization techniques to control the photoelectronic properties. The no dangling bonds nature at surface of 2D materials make the mixing of 2D materials with each other and other materials possible without the constraints of crystal lattice matching possible. These 2D based hybrid structures are also used in photodetectors to broaden the response range, increase response speed, and even investigate new physics. The most challenges of 2D semiconducting photodetectors are the low absorptions, slow response and narrow detect range. We also summarize some strategies to improve the above mentioned problems. Plasmon antenna, optical waveguides and optical microcavities can help to improve light absorption of 2D semiconductors. p-n Diode devices not only have fast response, but also wide detect range, other strategies such as encapsulation, surface modification, electrode design and so on might also help. Finally, the article ends with a summary and outlook on the future developments in this growing field.