Noble Transition Metal Dichalcogenide Photodetectors
Narrow-bandgap 2D noble metal dichalcogenides could be novel platforms for room-temperature LWIR photodetectors. Theoretical simulations predict that group X transition-metal dichalcogenides (TMDs) (Ni, Pd, Pt) are promising narrow-bandgap semiconductors, with ~0-0.25 eV [22-25]. The room-temperature mobility of bulk materials is greater than
1000 cm2/Vs. For 2D materials, carrier mobilities greater than 200 cm2/Vs and air-stable properties have been demonstrated in recent years [22,26].
The first few-layered PtS2 phototransistor on a hBN substrate, operating within the visible range, was demonstrated in 2017 [27]. A high pho- toresponsivity, up to 1.56 X 103 A/W, and an ultrahigh photogain, 2x 106, was achieved, benefiting from the short-channel and hole-trap states. Yu et al. reported an Ar-plasmon-treated bilayer PtSe2, with a responsivity and response time of 4.5 A/W and 1 ms, respectively, at a wavelength of 10 pm [12]. The detectivity approaches 7 X 10s cmHz1/2/W, which is higher than that of commercial bolometers operating in this wavelength range.
Black phosphorus photodetectors are challenging because of their instability in air. Investigations into air-stable group X TMD materials for LWIR applications have just started. A new step in their development has been provided by Long et al. in a recently published paper [13]. This paper is devoted to two types of PdSe2 photodetectors, based on phototransistors and their heterostructure.
The PdSe2 FETs were fabricated by a conventional electron-beam lithography process, using flakes with a thickness of 5-20 nm to ensure high carrier mobility and a relatively high absorption. Figure 7.6 shows the long-wavelength infrared photoresponse characteristics of a typical PdSe2 phototransistor at room temperature. The responsivity decreases from 42.1 to 13.8 A/W at Vds= 1 V under laser illumination of 10.6 pm, when the light power is increased from 1.42 to 5.67 nW. The photogating effect plays a crucial role in high photoconductive gain due to the long lifetime of combination induced by the trap states and the short carrier transit time. As shown in Fig. 7.6, the photogating effect decreases from 49 to 16 as the illumination power increases. This effect is particularly apparent when the responsivities of HgCdTe photodiodes, where g= 1, are compared with these of PdSe2 phototransistors [Fig. 7.6(b)]. In the case of PdSe2 phototransitors, the responsivity is about one order of magnitude higher than for FIgCdTe photodiodes in the wide infrared spectral range. The photoresponse speed, one of the most important figures of merit, is rather slow. The rise/delay time, defined as the time required to transition from 10/90% of the stable photocurrent during the illumination on/off cycle, is 74.5 ms/93.1 ms under 10.6-pm laser illumination. The response time under shorter-wavelength light is much faster, by about two orders of magnitude.
To decrease the dark current density, a heterojunction device is usually employed. The p-n heterojunction could be fabricated by stacking p-type
FIGURE 7.6 Long-wavelength infrared photoresponse of a typical PdSe2 phototransistor at room temperature: (a) power dependence responsivity and gain at Vds= 1 V, (b) responsivity and photocurrent gain as a function of wavelength at 1 V bias voltage (after Ref. [13]).
FIGURE 7.7 PdSe2/MoS2 heterostructure device: (a) schematic structure of the heterojunction, (b) Tfs(Vrfs) dark and illumination characteristics at room temperature (after Ref. [13]).
PdSe2 with other n-type 2D materials. Long et al. [13] selected n-type MoS2, deposited on p-type PdSe2, to form vdW heterojunction. A schematic structure of this device is shown in Fig. 7.7(a). Standard electron-beam evaporation was used for fabrication of the metal electrodes (5-nm Ti and 50 nm-Au).
Figure 7.7(b) shows a satisfactory device rectification effect of the output curve under dark conditions. The rectification ratio is typically about 100 at a 2 V bias, which indicates that a built-in electrical field exists at the interface. This is characterized by a broadband photoresponse from 450 nm to 10.6 pm, which is shown in Fig. 7.6(b). In the spectral range between 1 and 10.6 pm, the responsivity becomes stable at ~4 A/W.
The current noise of the PdSe2/MoS2 heterostructure is significantly suppressed, relative to that of the PdSe2 field-effect transistor (FET). At the low-frequency range, 1//noise dominates the noise current contribution. It is well known that the low-frequency flicker noise originates from the fluctuation of carriers being trapped and detrapped by defects and disorder and exists widely in 2D materials [28]. If the frequency increases beyond 1 kPIz, the noise current of the PdSe2/MoS2 heterostructure decreases to the Johnson noise level, whereas, for the PdSe2 FET devices, the current noise is three orders of magnitude higher than the Johnson noise level. This result indicates that the built-in electric field at the heterojunction effectively depresses the noise level, an effect which is highly desirable. In consequence, in spite of the higher responsivity of PdSe2 FET, in comparison with the heterostructure [Fig. 7.6(b)], the detectivity of the PdSe2/MoS2 heterojunction is higher, at 8 X 109 cmHz1/2/W at X = 10.6 m and T= 300 К (Figs. 3.15 and 7.5). This is a record achievement, because this value of detectivity is more than one order of magnitude larger than that of commercial HgCdTe photodiodes.
