Photogating Effect of Atomically Thin Graphene/MoS2/MoTe2 van der Waals Heterostructures

The development of short-wave infrared photodetectors based on various two-dimensional (2D) materials has recently attracted attention because of the ability of these devices to operate at room temperature. Although van der Waals heterostructures of 2D materials with type-II band alignment have significant potential for use in short-wave infrared photodetectors, there is a need to develop photodetectors with high photoresponsivity. In this study, we investigated the photogating of graphene using a monolayer-MoS2/monolayer-MoTe2 van der Waals heterostructure. By stacking MoS2/MoTe2 on graphene, we fabricated a broadband photodetector that exhibited a high photoresponsivity (>100 mA/W) and a low dark current (60 nA) over a wide wavelength range (488–1550 nm).

In this study, we investigated the modulation of the charge-carrier density of graphene in a vertical graphene/MoS 2 /MoTe 2 vdW heterostructure. We fabricated a broadband graphene/MoS 2 photodetector with a MoS 2 /MoTe 2 vdW heterostructure as the gate stack. We found that the graphene was significantly doped by the photoexcited charge generated in the MoS 2 /MoTe 2 heterostructure and that type−II band alignment between MoS 2 and MoTe 2 resulted in a photoresponsivity greater than 100 mA/W with a dark current of 60 nA over a wide wavelength range. Thus, we were able to realize a SWIR graphene photodetector with high photoresponsivity. Figure 1a shows a schematic of the process for fabricating the photodetector. After monolayer graphene was exfoliated and placed on a wafer substrate, monolayer MoS 2 was transferred onto the graphene layer using the PMMA transfer method, such that it partially covered the graphene. The overlapping graphene/MoS 2 region formed the Schottky junction of the device. Next, monolayer MoTe 2 was transferred onto the graphene/MoS 2 junction. To passivate the Schottky junction region, we covered it with a thick hBN layer. The vdW heterostructures were then annealed in a vacuum. Finally, we deposited a source electrode on the graphene layer and a drain electrode on the MoS 2 layer using e−beam lithography. The source and drain electrodes were neither connected to the MoTe 2 layer nor to the hBN layer. Therefore, the device was a graphene/MoS 2 Schottky diode in which the MoTe 2 /hBN layer was stacked on the Schottky junction. Figure 1b,c show atomic force microscopy (AFM) images of the MoS 2 and MoTe 2 monolayers on the Si/SiO 2 wafer used to fabricate the photodetector. The height of the MoS 2 layer was 0.63 nm and that of the MoTe 2 layer was 0.66 nm; this confirmed that the MoS 2 and MoTe 2 structures were monolayers. To further assess the thicknesses of the graphene, MoS 2 , and MoTe 2 layers, we determined their Raman spectra before they were transferred (Figure 1d, 532 nm). The ratio of the intensity of the 2D peak (Pos(2D) = 2682 cm −1 ) of graphene to that of its G peak (Pos(G) = 1592 cm −1 ) was slightly larger than 1. The height of the graphene layer was smaller than 1 nm, which indicated that the graphene layer was a monolayer (see Figure S1) [29]. In addition, the D peak of graphene conventionally observed at approximately 1350 cm −1 , whose intensity is proportional to the defect density of graphene, was not present [30]. The two peaks of MoS 2 seen at 387.4 and 405.7 cm −1 were the E 1 2g and A 1g peaks, respectively. The distance between these two peaks is indicative of the number of MoS 2 layers, and it increases as the number of MoS 2 layers increases [31]. In this study, this distance was~18 cm −1 , which confirmed that the MoS 2 layer was also a monolayer ( Figure 1e). Figure 1f shows the Raman spectrum of the MoTe 2 layer. Only one distinctive peak was present at approximately 240 cm −1 . This was the E 1 2g peak of the MoTe 2 . The absence of a peak at approximately 280 cm −1 (B 2g ) indicated that this layer was also a monolayer [32].

Results and Discussion
Raman spectroscopy is a powerful tool, not only for the characterization of isolated 2D materials, but also for the vdW stacking of 2D materials, because the Raman active phonon modes of 2D materials are sensitive to changes in the degree of doping and strain of the materials, as well as their vdW interactions with other layers. To evaluate the quality of the vdW stacking at the Schottky junction, we performed a Raman spectroscopy analysis after fabricating the device, as is shown in Figure 1. First, we obtained the Raman intensity maps of the graphene G peak, MoS 2 A 1g peak, and MoTe 2 E 1 2g peak, which allowed for the delineation of the edges of the graphene, MoS 2 , and MoTe 2 layers with precision ( Figure 2a). Because monolayered MoS 2 and MoTe 2 can be degraded by exposure to air, we focused on the MoS 2 , graphene, graphene/MoS 2 , and graphene/MoS 2 /MoTe 2 regions that were passivated by hBN ( Figure 2b). The integration time for one spot in the Raman image was 130 ms, which was sufficiently long for the photoexcited charge carriers in one material to transfer to the other materials. Figure 2c,d show the positions of the MoS 2 E 1 2g (Pos(E 1 2g )) and A 1g (Pos(A 1g )) peaks, respectively. The Raman maps clearly show that the differences in Pos(E 1 2g ) and Pos(A 1g ) depended on the stacking. Pos(E 1 2g ) and Pos(A 1g ) were 387.2 and 405.5 cm −1 , respectively, in the case of the MoS 2 region; these values were similar to those for MoS 2 before the transfer process. On the other hand, in the region where MoS 2 was stacked on graphene, Pos(A 1g ) was blue−shifted by 2 cm −1 , while Pos(E 1 2g ) remained the same (Figure 2e). Thus, Pos(E 1 2g ) and Pos(A 1g ) could be changed by changing the degree of doping [33], strain [34], and vdW interactions with the neighboring materials [35]. However, a shift in the A 1g peak of the MoS 2 by 1 cm −1 would require the removal of electrons in a density of 1 × 10 13 . In addition, the biaxial strain in MoS 2 changes both Pos(E 1 2g ) and Pos(A 1g ), which was not the case in this study. Therefore, we attributed the blue−shift of the Pos(A 1g ) to the stiffening of the A 1g phonon by the graphene−MoS 2 vdW interaction [35]. This was indicative of interlayer coupling between graphene and MoS 2 . For graphene/MoS 2 /MoTe 2 , the E 1 2g and A 1g peaks were red-shifted by~3 and 2 cm −1 , respectively, from those of graphene/MoS 2 , which was consistent with previous reports [36]. We believe that the red-shifting of the peaks was attributable to the tensile strain or the relaxation of the compressive strain owing to the mismatch in the lattice constants of the MoS 2 (0.316 nm) and MoTe 2 (0.352 nm).
We also monitored Pos(G) (Figure 2f) and Pos(2D) (Figure 2g) for graphene. Because Pos(G) and Pos(2D) are highly sensitive to the strain and doping level of graphene, these factors can be estimated from the peak positions (Figure 2h) [37]. Pos(G) and Pos(2D) for the graphene−only region were 1585.7 and 2671.5 cm −1 , respectively; these values correspond to a tensile strain of~0.05% and hole doping level of~6 × 10 12 cm −2 . However, in the graphene/MoS 2 region, the G and 2D peaks were both blue-shifted compared with those in the graphene-only region, which indicated that interlayer coupling with MoS 2 induced a compressive strain of 0.1% and the electron doping of graphene at thẽ 3 × 10 12 cm −2 level. The compressive strain induced in graphene by MoS 2 also suggests that the graphene/MoS 2 heterostructure was well formed in the analyzed area [38]. The electron doping of graphene implies that electrons were transferred from MoS 2 to graphene. After MoTe 2 was stacked on graphene, the compressive strain in graphene was relaxed slightly, probably because of the tensile strain generated or the relaxation of the MoS 2 −induced compressive strain by MoTe 2 . In addition, the graphene became less hole-doped compared with the graphene/MoS 2 region, which meant that the stacking of MoTe 2 enhanced the transfer of electrons to graphene. The underlying mechanism of graphene doping under the different heterostructures is discussed in more detail later in this paper. To investigate the charge transfer at the graphene/MoS 2 and MoS 2 /MoTe 2 interfaces, we performed photoluminescence (PL) spectroscopy ( Figure 3). The PL spectrum of monolayered MoS 2 originates from the radiative recombination of three types of quasiparticles: A excitons (~1.85 eV), B excitons (~2.03 eV), and A− trions (~1.80 eV) [39]. Figure 3a shows the band diagrams of the three quasiparticles. The relative contributions of the A excitons and A− trions to the PL spectrum of MoS 2 depend on the Fermi level of MoS 2 [40]. When MoS 2 is hole-doped or less electron-doped, the PL peak from the A excitons is very strong compared with that of the A− trions. On the other hand, the A exciton peak decreases as MoS 2 becomes n-doped, because an excessive number of electrons in MoS 2 bind to the photoexcited electron-hole pairs to form trions. Figure 3b,c show the total PL intensity and position of the A peak (A exciton + A− trion, respectively). The efficient PL quenching of graphene differentiates the graphene/MoS 2 /MoTe 2 region from the MoS 2 /MoTe 2 region. The MoTe 2 PL peak at approximately 1 eV was not observed, thus confirming that efficient charge separation had occurred between MoS 2 and MoTe 2 (Figure 3d).
To further investigate charge transfer at the interfaces, we analyzed the PL spectra of the MoS2-only (Figure 3e (Figure 3e). Specifically, the A− trion peak was stronger than the A exciton peak, which was in keeping with the fact that MoS 2 is an n-type semiconductor. The PL spectrum of MoS 2 changed when it was placed over graphene. In this case, the exciton PL peak was stronger than the A− trion peak. This means that the photoexcited electrons in MoS 2 were transferred to graphene, while the photoexcited holes were accumulated in MoS 2 . This charge transfer can inhibit the radiative recombination of A− trions in MoS 2 by spatially separating the photogenerated electrons and holes. In addition, it simultaneously induced the electron doping of graphene during the optical measurements. This was consistent with the Raman spectroscopy analysis, which showed that the formation of the graphene/MoS 2 structure resulted in the n-type doping of graphene (Figure 2h). The work function of the MoS 2 was larger than that of graphene [41]. Thus, the electrons in the graphene were transferred to the MoS 2 layer after contact, resulting in an electric field whose direction was toward MoS 2 at the graphene/MoS 2 interface, as is shown in Figure 3h. Therefore, the photogenerated electrons in MoS 2 could be easily transferred to graphene. In the region where MoTe 2 was stacked on the graphene/MoS 2 structure, resulting in graphene/MoS 2 /MoTe 2 , the A exciton peak was not observed. This implies that holes did not accumulate in the MoS 2 layer in the resulting structure. This can be explained by the fact that MoS 2 and MoTe 2 exhibited type-II band alignment (Figure 3i) and that the photogenerated holes (electrons) in MoS 2 (MoTe 2 ) were transferred to MoTe 2 (MoS 2 ). The transfer of holes from MoS 2 to MoTe 2 aided the separation of the photoexcited electrons and holes in graphene, resulting in efficient PL quenching (Figure 3b). The type−II band alignment between MoS 2 and MoTe 2 also explains the increased electron doping of graphene in graphene/MoS 2 /MoTe 2 after the stacking of MoTe 2 on graphene/MoS 2 , as per the Raman spectroscopy analysis (Figure 2h). The photoexcited holes trapped in MoTe 2 could enter graphene through MoS 2 , resulting in the additional electron doping of graphene. This is because the MoS 2 layer was too thin to screen for holes in MoTe 2 . Figure 4a shows the current-voltage (I-V) characteristics of the photodetector under continuous illumination with a 50 nW light over the graphene/MoS 2 /MoTe 2 region. The laser spot was placed away from the metal electrodes to exclude the photocurrent from the graphene/metal and MoS 2 /metal junctions. In the dark, the I-V characteristics were similar to those of a typical Schottky diode with series resistances ( Figure S2). The series resistances can be attributed to the resistances of the graphene and MoS 2 layers and the contact resistances between graphene, MoS 2 layers, and the metal electrodes. The current increased under illumination, whose wavelength range was 488-1550 nm. A finite photocurrent was observed at zero voltage, showing the photovoltaic effect of the device [42]. However, the photocurrent was almost absent at zero voltage and increased as V was increased. It implies that the photocurrent mainly originated from the photogating effect rather than the photovoltaic effect. For photodetectors based on 2D materials, the photoresponsivity, R, and specific detectivity, D*, are generally used as the figures of merit [42]. R is the photocurrent per incident unit optical power, and D* is a measure of the smallest detectable signal from the photodetector and is given by D * = RA 1/2 /I n 2 1/2 , where A is the illumination area (1200 µm 2 ) and I n 2 1/2 is the root-mean-square noise current. The area of the whole photodetector was larger than the laser spot size. Figure 4b shows the estimated R value as a function of V for various wavelengths. The device exhibited the maximum R value at V = 3 V; however, R increased when we applied a higher V. For instance, R exceeded 10 4 mA/W at 488 nm and 10 2 mA/W at all the other wavelengths. Notably, R was 3 × 10 3 and 1.8 × 10 2 mA/W at 980 and 1550 nm, respectively; these are the wavelengths at which MoS 2 is optically inactive. When the noise current is dominated by shot noise, D* can be estimated using the following equation [43]: D* was estimated to be 3 × 10 11 , 9 × 10 10 , and 5 × 10 9 Jones at 488, 980, and 1550 nm, respectively. Figure 4c shows the time-resolved photocurrent of the device under illumination with a 980 nm light at V = 3 V. The rise and decay times were 0.37 and 1.32 s, respectively (see also Table S1 [15,22,24,[44][45][46][47][48][49]).
In the present study, the photogating effect was a photoinduced change in the Fermi level of the material in question, namely, the graphene under MoS 2 . Under the photogating effect, the value of ϕ b at the graphene/MoS 2 junction was modulated by light, which, in turn, changed the device current. The photogating of the graphene-neighboring MoS 2 layer was well known (Figure 4d To confirm the role of the MoTe 2 layer, we fabricated a graphene/MoS 2 Schottky diode without a MoTe 2 layer on the Schottky junction and measured its photoresponse. As shown in Figure S3a, the current of the device barely changed under infrared light (980 and 1550 nm). Time-resolved current measurements were performed under infrared light illumination ( Figure S3b). However, no change in the current was observed during the measurements. The absence of a photoresponse excludes the possibility that the detrapping of charge carriers near the graphene/MoS 2 interface was responsible for the photoresponse under infrared light. Thus, these results clearly indicate that the absorption of infrared light by graphene did not contribute to the photocurrent of the device and that monolayered MoS 2 alone did not induce the photogating of graphene under infrared light. It is well known that photogenerated charge carriers recombine within a few picoseconds because of plasmon emission and carrier-phonon scattering [51]. Consequently, the photogenerated charge carriers in graphene were annihilated before the charges at the graphene/MoS 2 interface were separated. In addition, the bandgap of MoS 2 inhibits the absorption of infrared light, making the graphene/MoS 2 junction inactive under infrared light illumination. In summary, the observed photoresponse of the photodetector shown in Figure 4 was attributable to the MoTe 2 layer deposited on the MoS 2 /graphene Schottky junction.
In conclusion, we realized a graphene/MoS 2 barristor-based photodetector that exploited the photogating of graphene based on the type−II band alignment in the monolayered MoS 2 /monolayered MoTe 2 structure. The device showed a photoresponsivity as high as 10 4 mA/W and a detectivity of 3 × 10 11 Jones under visible light. More importantly, we were able to simultaneously achieve a photoresponsivity of more than 10 2 mA/W and detectivity of more than 5 × 10 9 Jones in the 980-1550 nm range.
Supplementary Materials: The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/mi14010140/s1, Figure S1: AFM image (left) and corresponding cross-sectional height profile (right) of graphene used in the photodetector; Figure S2: Dark current-voltage characteristics of the photodetector; Figure S3: a, Current-voltage curves of graphene/MoS2 photodetector without MoTe2 layer under illumination with 1-µW laser and b, timeresolved photocurrent of device under illumination with 1-µW laser at 980 nm; Table S1: Comparison of the performance parameters for photodetectors.