Two-dimensional transition-metal dichalcogenides (TMDs) and semi-metallic graphene are considered attractive candidates for applications in optoelectronics due to their intriguing optical and electrical properties [1–6]. In nonlinear optics, graphene has a wide range of applications such as graphene fiber lasers [7–9] and refractive index sensors [10]. MoS2, which is a type of TMD, attracts much attention due to its direct bandgap and high optical absorption coefficients [11,12], which compensate for the lack of bandgap and low optical absorption coefficients of graphene. Therefore, several research studies have reported various photoelectric applications based on a graphene-MoS2 heterostructure, including memory devices, phototransistors, and flexible photodetectors [13–16]. Recently, research has focused on vertical van der Waals heterostructures (vdWHs), which allow highly disparate materials to be integrated without the constraints of crystal lattice matching [17]. The advantages of TMD vdWHs are that various nanoscale materials can be freely combined, including two-dimensional materials, perovskite, and metal, to achieve functions that were not previously possible [17–23]. MoS2/WS2 vertical heterojunction photodetector arrays can be used for a self-driven photodetector without applying a source drain bias [24]. Black phosphorus (BP)/InSe photoconductive detectors have excellent responsivity in a wide response range from 405 to 1550 nm [25]. Hybrid perovskite has been demonstrated to form p-n junction diodes [26]. Additionally, the interaction between 2D materials and perovskite can achieve a photoelectric detection capability not previously seen with a responsivity of 3×109 A/W and an external quantum efficiency (EQE) of ∼1.2×1010% [27].

Various characteristics of metal-semiconductor contact have already been widely studied [28–30], and most studies are focused on improving ohmic contact between TMDs and metals. However, there are not many studies on metal/graphene/TMD vdWH photoelectric devices. Therefore, it may be valuable to investigate the electro-optical properties of vertical Au/graphene/TMDs, in which the Au is regarded as a functional material rather than simply electrodes. In addition, most TMD vdWH devices are produced using mechanical exfoliation and artificial alignment methods [31–33], which has disadvantages including rough stacking configuration controllability, messy interfaces, and low yield. The region of the device cannot be determined in advance, which makes devices fabricated by mechanical exfoliation difficult to produce and integrate in large quantities with high quality.

Due to this production method difficulty for TMD devices, and since the integration of vertically stacked TMDs and metal can produce novel effects, in this study we fabricate a vertical Au/graphene/MoS2 vdWHs bipolar phototransistor using Au as both the functional material and the electrodes. The Au/graphene/MoS2 vertically stacked devices exhibit an obvious photocurrent enhancement. Based on our experimental results, a photocurrent is generated in the reverse-biased graphene/MoS2 junction, and the forward-biased Au/graphene junction contributes to the enhancement of photocurrent. The maximum optical responsivity is 16,458 A/W with fast generation speed of the photocurrent (1.48×10−4 A/s) under 405 nm laser irradiation. The interfacial transport of the carriers and the optical response mechanisms are also studied.

Figure 1(a) shows an optical microscope image of the Au/graphene/MoS2 bipolar phototransistor based on a vertical vdWH architecture. The vertical Au/graphene/MoS2 vdWHs were produced with monolayer graphene transferred to the surface of an Au electrode and bilayer MoS2 then transferred on top of the graphene, with the Au and MoS2 thinly insulated by the monolayer graphene. The Raman spectra of the graphene/MoS2 heterojunction was characterized by laser excitation with a wavelength of 532 nm. The typical Raman spectrum of the device fabrication is shown in Fig. 1(b).

The employed technique allows reconfigurability to create the phototransistor, and the fabrication process of the phototransistor is schematically illustrated in Fig. 2. First, 100 nm/10 nm thick Au/Ti, used as the emitter electrode, was deposited on a clean P++-doped Si wafer coated with 300 nm SiO2 and followed by a photolithographic lift-off process. Then, chemical vapor deposition (CVD) monolayer graphene was transferred to the surface of the emitter electrode followed by a photolithographic process, which acts as the base region but without an extraction electrode. Next, the CVD bilayer MoS2 was transferred to the surface of the graphical graphene for the collector, which was also followed by a photolithographic process. Finally, 100 nm/10 nm thick Au/Ti electrode was deposited onto the MoS2 as a contact electrode. In summary, the Au is in contact with the graphene, the graphene is in contact with the MoS2, and the MoS2 is in contact with the Ti. By this “transfer-lithography” method, the positions of the electrodes and the stacked 2D material area are pre-fixed by the designer. Therefore, the devices can be mass-produced.

The two-dimensional material is transferred using a wet method, and the steps are as follows. (a) The PMMA phenyl ether solution is spun onto the surface of the 2D material twice (first at 3000 r/min and baked at 90°C for 1 min and second at 1000 r/min and baked at 120°C for 20 min). The 2D material is on a Si/SiO2 substrate. (b) A 2 mol/L potassium hydroxide solution is used to etch the Si/SiO2 substrate until the PMMA-2D material film is completely separated from the Si/SiO2 substrate. (c) The surface of the 2D material is repeatedly cleaned with deionized water. (d) The target substrate is used to collect the PMMA-2D material film from the deionized water and the target substrate is dried. (e) The target substrate is baked at 150°C for 20 min. (f) The PMMA is removed by acetone.

We begin by discussing the bands of the Au/graphene and the graphene/MoS2 junctions. When the graphene comes into contact with the Au while forming the vdWHs, the electrons of graphene are injected into the Au inducing holes in the graphene. Dipoles are formed at the interface between the Au and the graphene. Meanwhile, the Fermi level of the graphene is strongly affected by the formation of interfacial dipoles, and therefore the graphene is p-type doped by the Au contact [34–37]. Dipoles are formed by the injected electrons in the Au and induced holes in the graphene, which can produce a barrier at the contact interface of Au and graphene. The barrier produced by the dipoles is different from a traditional p-n junction, which does not have a space charge region. A traditional p-n junction will turn “on” when there is an applied forward bias to weaken the built-in electric field. However, dipoles are different, since the metal itself contains many electrons, and the electrons injected by the graphene increase the number of metal surface carriers. Similarly, the concentration of holes on the surface of the graphene is also increased. When a forward bias is added to the graphene, due to the external electric field, electrons drift towards the positive pole and holes drift towards the negative pole. Electrons in the metal appear to move unimpededly into the low electron concentration graphene, and similarly for the holes. If the graphene is under a positive bias, the electrons drift against the field line of the built-in potential of the dipoles. In contrast, due to the accumulated induced charge at the interface, the electrons need to overcome the repulsive force of the induced charge. At this point, the interface displays characteristics similar to a p-n junction as shown in Fig. 3(a). In the graphene/MoS2 vdWHs, the MoS2 exhibits n-type semiconductor characteristics and the graphene exhibits p-type semiconductor characteristics. It is worth mentioning that contact with Au will affect the type of graphene due to the injection of the holes. When the p-type graphene connects with the n-type MoS2, the Fermi levels have to be consistent, and the energy bands will bend [38]. Therefore, the graphene/MoS2 vdWHs will demonstrate rectification characteristics as shown in Fig. 3(b).

To further interpret the photoelectric properties and photocurrent amplification mechanism of the bipolar phototransistor, a band diagram of the vertical Au/graphene/MoS2 vdWHs is used to understand its working principle induced by an incident laser. The band diagram of our device in its original state is shown in Fig. 3(c). For our device, the emitter contact ground and the collector are maintained under forward bias, e.g., the collector-emitter voltage (VCE)>0V. The Au/graphene junction is forward biased and the graphene/MoS2 junction is reversed biased. The electrons and holes are blocked by the barrier at the graphene/MoS2 junction. Therefore, the phototransistor has a small dark current. The resulting band diagram under irradiation and voltage bias is shown in Fig. 3(d). Photoexcitation induces the graphene/MoS2 junction, generating photogenerated electron-hole pairs and changing the chemical potential. Due to the existing external electric field, the photogenerated electron-hole pairs will separate and drift. The photogenerated electrons flow to the positive pole and the photogenerated holes flow to the graphene layer and will ultimately be compounded with the electrons in the emitter. However, the graphene base region is very thin, therefore the graphene only has a limited number of holes, and thus not all of the electrons will be compounded. It is well known that the concentration of electrons in metals is much greater than that in semiconductors under external electric fields. The electrons in the emitter will cross the base and enter the collector due to the drift. This mechanism is similar to bipolar transistors and thus will amplify the photocurrent of the device [5]. Additionally, although the work functions of MoS2 and Ti are similar, the metal-semiconductor contact produces a smaller Schottky barrier in the collector of the device because of the low doping concentration of MoS2. However, under normal working conditions (VCE>0V), the Schottky barrier is positively biased and can be considered equivalent to a contact resistance. Thus, our device has a large bias voltage, and therefore the effects of this barrier in positive bias are ignored.

The photoelectric characteristics of the device can be illustrated with a more simplified schematic, which is shown at the top of Fig. 4(a). A device consisting of one back-to-back barrier and a laser irradiating the device from above is shown at the bottom of Fig. 4(a). The forward bias (VDD>0) sets the graphene/MoS2 barrier in the reverse direction, and the Au/graphene barrier in the forward direction. The current–voltage (I–V) characteristics of the Au/graphene/MoS2 bipolar phototransistor under 405 nm laser irradiation with different incident laser power densities are shown in Fig. 4(b). In darkness, due to the existence of the back-to-back barrier, the dark current is small regardless of whether the device is in reversed or forward bias. Under irradiation, there is a rapid increase of the photocurrent at low bias. This can be observed clearly at positive bias but is inconspicuous at negative voltage bias since the photocurrent cannot be amplified when the MoS2/Ti barrier is negatively biased, and the carrier concentration of MoS2 is less than that of Au. The collector-emitter photocurrent (ICE) increases as VCE increases until it reaches saturation, since the graphene/MoS2 barrier gets wider as VCE increases. As the graphene/MoS2 barrier width increases, the electrons in the emitter will drift more readily to the collector. Thus, ICE also increases as VCE increases until the voltage increases to a certain value, i.e., VCE=34V in Fig. 4(b), and ICE will not increase any further, as due to existing parasitic resistance in the device, the voltage difference of the graphene/MoS2 barrier decreases as ICE increases. As VCE increases, the voltage difference of the graphene/MoS2 barrier decreases and a dynamic balance of the voltage difference is maintained, which means that ICE does not further increase as VCE increases. Therefore, the device has different photocurrent gains at different voltages. For example, as VCE increases until it exceeds 46 V in Fig. 4(b), ICE decreases. The graphene/MoS2 barrier is broken, which means the device becomes inoperable.

The photocurrent and laser power density curves of the Au/graphene/MoS2 device at different values of VCE are shown in Fig. 4(c). It can be seen that the photocurrent has a linear relationship with the laser power density. The fastest increases in the photocurrent occur when VCE is between 10 V and 20 V, which corresponds to Fig. 4(b). The amplitude and polarity of the photocurrent in the gated vertical graphene/MoS2/graphene heterostructures can be readily modulated by the electric field of an external gate [39]. Figure 4(d) shows ICE versus the back-gate voltage (VG) under different VCE with 1.05mW/cm2 irradiance intensity. VG can affect the Fermi level of MoS2 significantly and the barrier height of graphene/MoS2 junction, thus changing ICE. In our device, the saturation region appears at a higher voltage bias due to the higher contact resistance of the MoS2/Ti contact.

The responsivities of our Au/graphene/MoS2 vdWH bipolar phototransistor are plotted in Figs. 5(a) and 5(b) and were directly extracted from the I–V curves and the photoinduced transfer curves [Figs. 4(b)–4(d)], using R=Iph/PS, where Iph is the photocurrent, P is the laser power density, and S is the effective laser absorption area. From Fig. 5(a), it can be seen that the responsivity rises linearly as VCE increases in region I, when the device is operating in the saturation region. In region II, the device is operating in the amplifier region, and the responsivity no longer changes with increasing VCE. This property may be used in an optical computing device [40]. As shown in Fig. 5(b), the laser power density has little effect on responsivity once the laser power density is large enough [>0.4mW/cm2 in Fig. 5(b)]. This indicates that the photocurrent of the device is linear as the laser power density increases. The maximum responsivity can be as high as 16,458 A/W in our device when VCE=36V, VG=0V and the laser power density is 0.45mW/cm2. Due to environmental factors and the accuracy of the measurement equipment, the measured value will deviate slightly, and further study of these specific factors and functions is required.

Figure 5(c) directly presents the relationship between the photocurrent and the dark current of the bipolar phototransistor at different values of VCE according to the ratio of Ilaser/Idark (irradiated under 405 nm 0.25mW/cm2 laser power density and VG=0V). The maximum laser/off ratio is above 1.8×103, indicating that the photocurrent is more than 3 orders of magnitude higher than the dark current. The different Ilaser/Ioff ratios are due to the different photocurrent gains and dark current values at different VCE. The highest detectivity value was 1.75×1014 Jones at VCE=17V as shown in Fig. 5(c). The detectivity is defined by Detectivity=R[S/(2qIdark)]0.5, where q is the unit electron charge. The high values are due to the high responsivities and the low dark current [41]. We measured and calculated the responsivity curves of Au/graphene/MoS2 vdWHs under laser radiation at three different wavelengths, 635, 532, and 405 nm, with the same laser power. The results are shown in Fig. 5(d). The main laser-absorbing layer of the device is MoS2, and the photoexcitation ability of the MoS2 decreases obviously as the wavelength increases. The optical excitation range of MoS2 is the main factor that affects the absorption wavelength of the proposed device.

The time versus photo-response is also characterized. The transient behavior of the current of the Au/graphene/MoS2 bipolar phototransistor in a dark–light–dark cycle, under a 405 nm 0.45mW/cm2 incident laser at VCE=17V and VG=0V, is shown in Figs. 6(a) and 6(b). The rise time trise is approximately 20 ms, and the fall time tfall is approximately 92 ms. trise is lower than tfall, due to a slower photocurrent dissipation process than the photocurrent multiplication process. Further verification is performed by comparing the optical responsivity measurements of the Au/graphene/MoS2 bipolar phototransistor and the graphene/MoS2 photodiode. Although the comparison experiment uses the same process, the structural parameters of the two devices are different. Figure 6(c) shows the photocurrent of the graphene/MoS2 photodiode, which does not include the dark current. The photocurrent density of the devices is measured to ensure the accuracy of comparative experiments, which is defined as βphoto=Jph1/Jph2, where Jph1 is the photocurrent density of the Au/graphene/MoS2 phototransistor and Jph2 is the photocurrent density of the graphene/MoS2 photodiode. The photocurrent density is obtained by dividing the photocurrent across the effective adsorption area, which can avoid any possible error caused by a difference in device scale [42]. The photocurrent densities of the Au/graphene/MoS2 phototransistor and the graphene/MoS2 photodiodes under the same laser power density are shown in Fig. 6(d). The Au/graphene/MoS2 phototransistor demonstrates a larger photocurrent density, verifying the superiority of its 18-fold higher responsivity than the graphene/MoS2 photodiode. The amplification factor β is calculated by dividing the two photocurrent densities. When VCE>44V, the photocurrent density of the graphene/MoS2 photodiode will decrease and may break down.

Based on the previously discussed optoelectronic properties of an Au/graphene/MoS2 van der Waals heterojunction, the photoelectric performance was evaluated by calculating the EQE using the equation EQE=(∫Iphdt/q)/(∫PdtS/hv)×100%, where h is Planck’s constant, v is the frequency of the laser, and t is the irradiation time. Since photon-generated carriers in MoS2 have a long life, the photon-generated carrier contains not only the present but also the previous moment. Therefore, the values of EQE will be different under different conditions.

The ultrahigh responsivity and EQE in our proposed device can be explained based on multiple aspects. First, the presence of barriers in the graphene/MoS2 interface reduces the exciton recombination rate and increases the carrier lifetime, which is very useful to enhance the photocurrent [27]. Second, because the device is fabricated by the van der Waals force contact, there will be many defective energy levels in the device. Photon harvesting using these defective energy levels enhances the lifetime of the photogenerated carriers, which reduces carrier recombination. Third, a vertically stacked Au/graphene/MoS2 device has a broad junction area for efficient photon absorption. In general, the responsivity and EQE are produced by the superposition of photoexcited carriers over a period of time.

A summary of the comparison of Au/graphene/MoS2 vdWHs with other 2D material heterostructures based on graphene or MoS2 is listed in Table 1. The comparison results demonstrate the proposed device’s high-performance optoelectronic characteristics and the potential of the device. A graph comparing the responsivity and generation speed of the photocurrent of the proposed device with other reported devices based on MoS2 is shown in Fig. 7 [20,22,31,43–46]. Due to the different static operating points and responsivity in different devices, the response rate of the rising and falling edges (10%–90%) of the photo-response pulse is extracted. The responsivity and the transient response rate of the photocurrent are compared. The high optoelectronic performance of our device is evident from this summary of results. Although the Au/graphene/MoS2 bipolar phototransistor does not have the highest responsivity, it offers the fastest response rate and a good overall performance. The value of Iph/max(trise) is defined as the generation speed of the photocurrent. The high optoelectronic performance is most likely due to the novel photoelectric effect mechanism of the Au/graphene/MoS2 van der Waals heterojunction.

In this paper, we demonstrated a vertical Au/graphene/MoS2 van der Waals heterojunction bipolar phototransistor. Due to the excellent optical properties of perovskite and other 2D materials, devices may be fabricated using these materials. For example, BP can replace MoS2 as a laser-absorbing layer to increase the bandwidth, and perovskite can be used to improve the responsivity. However, it needs to be tested experimentally to determine whether the base region of a vertical phototransistor can be used with graphene. Meanwhile, the stability of the new material and the work function of metal also need further study.

In summary, this paper presented a mass-produced Au/graphene/MoS2 vdWHs bipolar phototransistor with dipoles as an emitter junction. Under 405 nm laser irradiation, the phototransistor can reach a responsivity of 16,458 A/W, and the maximum Ilight/Idark ratio can be more than 1.8×103. Additionally, the maximum value of detectivity is 1.75×1014 Jones. Additionally, the proposed phototransistors have obtained a photocurrent gain enhancement of 18. Compared with recent reports, the phototransistor achieves the fastest generation speed of photocurrent when generating photocurrent under laser irradiation between 380 nm and 550 nm. Due to its characteristics of excellent performance, such as simple preparation and ability to be mass-produced, it can be highly integrated with silicon semiconductor processes. This study provides new insights in the design of photonic functional metal-graphene-semiconductor vdWHs and photoelectric devices.