D-Material Detector Performance – the Present Status

Development of pristine 2D-material high-sensitivity photodetectors is determined by two major challenges: the low optical absorption in thin active regions (~ 100-200 nm) and the short photocarrier lifetime. As with graphene-based detectors, the photodetectors are limited by tradeoffs between high responsivity, ultrafast response time, and broadband operation. Photodetectors based on 2D materials display a large variation in their current responsivity and response time [2,43], covering about nine orders of magnitudes, as is shown in Fig. 7.14. The current studies are mostly limited to exfoliated materials from bulk-layered crystals, with very limited yields, reproducibility, and scalability. In consequence, the divergence in the responsivity values is large. Moreover, the photopaging effect has a strong influence on the relationship between current responsivity and response time, which is shown in Fig. 7.14(b).

To enhance the infrared absorption, multiple layers, instead of a single layer, are selected. High photogain is often realized by using the 2D material as the fast transfer channel for charge carriers. However, as mentioned in Chapter 6, their general drawback is a very slow response time attributed to traps and enhanced capacitance. The response time at room temperature is typically longer than ~0.1 ms, which indicates a considerably

FIGURE 7.14 Room-temperature current responsivity against response time for 2D-material photodetectors: (a) in comparison with commercial silicon, InGa As, and HgCdTe photodiodes (many data are taken from Ref. [43]), and (b) for part low-dimensional photodetectors (after Ref. [44]); the dashed line represents a typical magnitude order of gain-bandwidth product for traditional high-performance thin-film photodetectors, and the white symbol indicates that this is a photogating-enhanced photodetector.

longer response time in comparison with commercial silicon, InGaAs, or HgCdTe photodiodes; for HOT photodiodes, the response time is typically between 1 and 10 ns. The upper-left blank panel of Fig. 7.14(a) indicates the lack of photodetectors with both ultrahigh responsivity and ultrafast response speed. This observation is also supported by Fig. 7.14(b). It is also well known that the high gain induced by a long carrier lifetime can also be caused by both a large generation-recombination noise and a slow speed, which are incompatible with improving the device detectivity. In detector focal plane arrays, the suitable response time depends on the pixel number, signal readout mode, driving integrated circuit design rules, and other factors.

Figure 7.15 summarizes the photoresponsivity and response time of different 2D-material photodetectors. It is shown that black phosphorus (bP) falls into a region between graphene and TMDs. However, degradation in air and other environments is an unresolved issue that may limit future applications of bP [45]. bP degrades rapidly under ambient conditions, affecting its structure and properties. In particular, the role of different ambient species has remained controversial. Considerably better stability is observed for noble transition metal dichalcogenides, with record room-temperature detectivity measured for PdSe₂/MoS₂ heterostructures of about 10¹⁰ Jones, in the long-wavelength infrared range [13].

FIGURE 7.15 Summary of the responsivity and response time of photodetectors based on different groups of 2D materials: graphene, black phosphorus (bP), and transition metal dichalcogenides (TMDs).

Another study on the 2D material gives the experimentally measured relationship between detectivity and response time, as shown in Fig. 7.16, for photodetectors operated mainly in the visible and SWIR spectral regions [46]. The large variation in responsivity and response time (Fig. 7.14) results in a change in the D'/product of about ten orders of magnitude, between 10⁷ and 10¹⁷ Jones Hz.

Although photodetection platforms based on 2D materials have demonstrated a variety of potential applications, outstanding challenges remain to be addressed in order to exploit the distinct advantages of these new materials. The prospects for commercialization will depend not only on the detector performance, but also on the distinct advantages in the ability to fabricate large-scale high-quality 2D materials at a low cost. The final goal is to establish large-scale integration of 2D crystals with existing photonic and electronic platforms, such as CMOS technologies.

Most of the high-performance 2D-material photodetectors, consisting of thin layers or heterostructures, are achieved by mechanical exfoliation. There is still a significant need for improvement of 2D-material technology in order to obtain well-developed devices of industrial applicability. One of the critically important issues is the stability and quality of 2D

FIGURE 7.16 The state-of-the-art graphene heterojunction devices (after Ref. [46]).

materials. Although many efforts have been devoted to synthesizing largesized single-crystal 2D materials, it is still hard to obtain single-crystals larger than millimeter size, and the grain size and sample size of TMDs are even smaller. Using the CVD-grown technique, relatively large size samples can be obtained, but they have defects and contaminations, which can cause sample degradation over time, resulting in devices which are not stable.

Generally, photodetectors made from TMD-layered semiconducting material operate at the visible and near-infrared regions, and generally their high sensitivity does not coincide with a fast response time. In comparison with a response time measured in the nanosecond range for HgCdTe photodiodes and type-II superlattice IB QCIPs operating at room temperature, the response time of TMD is typically in the millisecond range. This indicates that 2D-infrared photodetectors operating at room temperature are not ready for real-life applications. The performances of black phosphorus-arsenic alloy detectors are comparable to commercial HgCdTe photodiodes. However, the instability of the surface, due to chemical degradation under ambient conditions, remains a major impediment to its prospective applications. More promising are stable noble TMD photodetectors like PdSe₂/MoS₂ heterojunctions, with record detectivity in the LWIR range at room temperature. However, their practical application lies in perfect material synthesis and processing. To achieve their potential, 2D-material photodetectors still have a long way to go.