# Trends in Performance Limits of the HOT Infrared Photodetectors
## 5 2D Material Infrared Photodetectors  
Graphene and other two-dimensional (2D) materials, due to uncommon electronic and optical properties, are considered to be promising candidates for IR photodetectors [52]. The further development of graphene-based photodetectors is a consequence of the high dark current the gapless material significantly limits the sensitivity. The discovery of new 2D materials with direct energy gaps in a wide spectral range (from the visible to the IR) has set a new direction for detector's design and fabrication. Although the technology readiness, the 2D materials are still at low level of development, the detectors' manufacturability and reproducibility have been challenging (this topic is widely studied in research laboratories around the globe). [PERSON] et al. [53] distinguished the different types of 2D materials and refined them into different families (see Figure 13) covering a broad range of electrical and optical properties:  
* transition metal dichalcogenides (TMDs),
* black phosphorus (bP), metal halides (e.g., PbI\({}_{2}\), MgBr\({}_{2}\)), metal oxides (such as MnO\({}_{2}\) and MnO\({}_{3}\)), double hydroxides, III-Vs (such as InSe and GaS), V-VIs (such as Bi\({}_{2}\)Te\({}_{3}\) and Sb\({}_{2}\)Se\({}_{3}\)), and
* atomically thin hexagonal boron nitride (h-BN, similar to hexagonal sheets of graphene),
* halide perovskites. 2D materials have their roots in layered van der Waals (vdW) solids. Atomic layers are built by in-plane atoms connected by ionic or tight covalent bonds along 2D directions. Each layer is bonded with another by a weak vdW interactions along out-of-plane direction. Such design causes that many of 2D materials could be mechanically exfoliated from bulk single crystals. What more, due to a weak bond between layers, a mixing of different 2D materials together is also possible with the flexibility of the heterostructures. Energy band profiles of the layered materials differ from their bulk counterparts. Since the material gets thinner from the bulk to the monolayer, e.g., for TMDs the band structure transits from smaller indirect transition to a larger direct due to quantum confinement effects, thus, the bandgap (operating wavelength) can be adjusted by the layers number. Moreover, large strains occurring in these materials strongly affect their optical  
Figure 13: Energy bandgap of the selected layered semiconductors versus wavelength. The energy bandgap exhibits the dependence on the layers number, strain level and chemical doping. FIR—Far infrared; LWIR—Long wavelength infrared; MWIR—Mid wavelength infrared; SWIR—Short wavelength infrared; NIR—Near infrared; UV—Ultraviolet. and electronic properties. A high absorption coefficient of TMDs (typically 10\({}^{4}\)-10\({}^{6}\) cm\({}^{-1}\)) results from dipole transitions between localized d-states and excitonic coupling of such transitions. Thanks to that, >95% of the sunlight is absorbed in sub-micrometer thickness TMD films, while carrier mobility is low (typically less than 250 cm\({}^{2}\)/Vs). Despite the fact that the mobility can be improved by increasing the number of TMDs layers, this disadvantage is difficult to circumvent. The carrier density depends on the doping levels and the number of recombination centers, and typically reaches 10\({}^{12}\) cm\({}^{-2}\)[54]. In comparison to graphene, TMDs, like molybdenum disulfide (MoS2), tungsten disulfide (WS2) and molybdenum diselenide (MoSe2) is characterized by higher absorption in the visible and NIR ranges. As presented in Figure 13, the 2D bandgap profile is so different what allows to cover a very broad range from the UV to IR. For current status of technology, only graphene-based, black phosphorus-arsenic (bPAs), noble metal dichalcogenides, and bismuthene (like Bi2 Te3 and Bi2 Se3) are treated as a main player in IR and also THz regions. Since 2D TMDs are limited to UV-NIR, bP can be adjusted to below 0.3 eV by As doping.