Competing Built-In Electric Fields in Au/MoS₂/WSe₂ Dual Junction Photodetectors for Broadband VIS-IR Detection

Abstract

Van der Waals (vdW) heterostructures are attractive for optoelectronic devices due to their lattice-mismatch tolerance and tunable band structures. Here, we report a gate-tunable Au/MoS₂/WSe₂ dual junction photodetector featuring competing asymmetric built-in electric fields. Spatially resolved photocurrent measurements reveal that selective utilization of these built-in electric fields decouples the transport dynamics of dark and photogenerated carriers. Such decoupling allows for independent modulation of the dark current and photocurrent, enabling the concurrent realization of the ultralow dark current and high photocurrent. Moreover, gate-voltage modulation enhances the photoresponse by ~245%, yielding a detectivity of 1.98 × 10¹² Jones over the 532–940 nm range. Imaging and optical communication further verify the device’s practical potential. These results provide a viable route toward high-sensitivity and electrically reconfigurable broadband photodetectors.

1. Introduction

Visible (VIS) and infrared (IR) sensing provide complementary spectral information for target recognition in complex environments [1,2,3,4]. Two-dimensional (2D) materials are promising candidates for high-performance VIS-IR photodetectors, owing to their atomic thickness, high carrier mobility, and strong light–matter interactions. Among these materials, semiconducting transition-metal dichalcogenides (TMDs), such as MoS₂ and WSe₂, have been extensively explored for photodetection [5,6,7,8]. MoS₂ typically exhibits n-type transport behavior with a sizable bandgap (~1.8 eV in monolayer form) and relatively high carrier mobility (~200 cm² V⁻¹ s⁻¹), enabling efficient photoresponse in the visible spectral range [9,10]. For instance, Singh reported a monolayer MoS₂ MSM photodetector with a peak responsivity of 13.15 mA/W at 480 nm [11], while Zhou demonstrated a double-terminal MoS₂ photodetector exhibiting a fast response time of 224 μs and a maximum responsivity of 280 mA/W [12]. WSe₂, in contrast, often exhibits p-type transport behavior and possesses a bandgap of ~1.6 eV in its monolayer form, together with strong optical absorption and pronounced excitonic effects, making it attractive for high-sensitivity photodetection [13]. Li et al. reported a self-driven WSe₂ photodetector with bottom Schottky contacts, achieving a responsivity of 360 mA/W and a detectivity of 1.07 × 10¹² Jones [14], whereas Yang et al. demonstrated a WSe₂ homojunction photodetector with a responsivity of 0.35 A/W and a detectivity of 2.37 × 10¹¹ Jone [15].

Van der Waals (vdW) heterojunctions constructed from different 2D materials provide an effective strategy for enhancing photodetection performance by facilitating efficient carrier separation and tunable band alignment. Sun et al. reported a MoS₂/WSe₂ heterotransistor exhibiting a photoresponsivity of 17.8 A/W and a response speed of <80 ms under 660 nm illumination, and the device also showed a gate-dependent near-infrared photoresponse at 850 nm [16]. Shin et al. and Xiao et al. subsequently reported WSe₂/MoS₂ heterojunction photodetectors with broadband response and markedly improved sensitivity, confirming the considerable potential of MoS₂/WSe₂-based vdW heterostructures for high-performance photodetection [17,18]. However, in most reported heterojunction photodetectors, different interfaces are usually treated as a whole, and the roles of the metal/semiconductor contact and semiconductor/semiconductor heterojunction are rarely distinguished. As a result, the transport of dark carriers and photogenerated carriers remains strongly coupled, which limits the independent regulation of the dark current, photocurrent, and overall device performance. Therefore, developing more versatile photodetectors requires the precise regulation of each junction and effective manipulation of the carrier transport.

In this study, we report a gate-tunable Au/MoS₂/WSe₂ dual junction photodetector operating in the VIS–IR spectral range. Distinct from conventional single-junction devices, the present structure contains two junction regions with asymmetric built-in electric fields, enabling selective control of carrier transport at different interfaces. Owing to this competing built-in electric field configuration, the device enables the simultaneous suppression of the dark current and enhancement of the photocurrent under appropriate bias conditions. Furthermore, gate-voltage modulation provides an additional degree of freedom to engineer the interfacial band alignment and optimize the photoresponse. As a result, the device achieves a 2.45-fold enhancement in responsivity and a specific detectivity of 1.98 × 10¹² Jones at 532 nm. Proof-of-concept imaging and optical information encoding demonstrations highlight its potential for multiband optoelectronic applications.

2. Materials and Methods

2.1. Fabrication of the Au/MoS₂/WSe₂ Heterojunction

The fabrication procedure of the Au/MoS₂/WSe₂ dual junction photodetector is schematically illustrated in Figure 1. First, Cr/Au electrodes (10/60 nm) were pre-deposited on a SiO₂/p-type Si substrate to define the source and drain contacts. The WSe₂ and MoS₂ flakes were mechanically exfoliated from bulk crystals and subsequently assembled into the device via a dry transfer method. Specifically, a WSe₂ flake was first transferred onto the pre-patterned electrode region using a PDMS (Gel Pak, USA) stamp, forming the bottom semiconducting layer and establishing electrical contact with the metal electrode. Subsequently, a MoS₂ flake was aligned and stacked onto the WSe₂ flake using the same PDMS-assisted dry transfer process, forming a vertical MoS₂/WSe₂ Van der Waals heterojunction in the overlapping region, which, together with the metal/semiconductor contact, defines a dual junction device architecture. The heavily doped p-type Si substrate served as a global back-gate electrode, while the SiO₂ layer acted as the gate dielectric, enabling effective electrostatic tuning of the band alignment and carrier transport properties of the heterojunction photodetector.

2.2. Characterization and Measurements

The thicknesses of the exfoliated WSe₂ and MoS₂ flakes were measured by atomic force microscopy (AFM, Jupiter XR, Oxford Instruments, Abingdon, UK). The Raman spectra of WSe₂ and MoS₂ were characterized by Raman spectrometer with a 532 nm laser (WITec alpha300RA confocal microscopy Raman system, WITec GmbH, Ulm, Germany). The electrical and optoelectronic properties of the Au/MoS₂/WSe₂ heterojunction photodetector were measured using a source meter (Keithley 2602B, Keithley Instruments, Inc., Solon, OH, USA). The drain–source voltage (V_ds) and gate voltage (V_g) were applied to evaluate the transport characteristics of the device. For the photoresponse measurements, a 532 nm laser (CFS2-532-FC, Thorlabs, Inc., Newton, NJ, USA) was employed as the excitation source. The laser beam was focused to the micrometer scale through a confocal optical system to ensure localized illumination on the device. Photocurrent mapping was performed by scanning the focused laser spot across the device area while recording the corresponding photocurrent signal. Time-resolved photoresponse measurements were conducted by periodically modulating the laser illumination.

3. Results and Discussion

Figure 2a shows a 3D schematic of the device structure, with MoS₂ vertically stacked on top of WSe₂. The MoS₂ and WSe₂ layers are electrically connected to the source and drain electrodes, respectively. Figure 2b shows an optical image of the device, with the MoS₂, WSe₂, source, and drain regions outlined by dashed lines in different colors (scale bar: 20 μm). Raman spectroscopy was employed to characterize the heterostructure, as shown in Figure 2c. WSe₂ exhibits characteristic Raman peaks at approximately 246.7 cm⁻¹ and 254.9 cm⁻¹, corresponding to the in-plane $E_{2g}^1$ mode and the out-of-plane $A_{1g}$ mode, respectively [19]. Two additional weak peaks located at ~368.9 cm⁻¹ and ~390.3 cm⁻¹ are attributed to second-order phonon scattering processes in multilayer WSe₂ [20,21]. Similarly, MoS₂ exhibits two prominent Raman peaks at 383.6 cm⁻¹ and 407.1 cm⁻¹, corresponding to the in-plane $E_{2g}^1$ and out-of-plane $A_{1g}$ modes, respectively [22,23,24]. Steady-state photoluminescence (PL) spectra (Figure S1, Supporting Information) further reveal distinct excitonic emissions from MoS₂ and WSe₂, while the heterojunction region exhibits combined and modulated features, indicating interlayer coupling and charge transfer. The thicknesses of the MoS₂ and WSe₂ flakes were characterized by atomic force microscopy (AFM), yielding values of approximately 54 nm and 65 nm, respectively (Figure 2d and Figure S2 in the Supporting Information). Overall, a high-quality Au/MoS₂/WSe₂ dual junction was successfully fabricated.

The Au/MoS₂/WSe₂ dual junction was systematically evaluated across the visible to near-infrared range. As shown in Figure 3a, zero-bias time-resolved photocurrent measurements under periodic 532, 638, and 940 nm illumination exhibit distinct and reproducible current switching, demonstrating effective self-powered photodetection. The extracted on/off ratios are 10,444, 10,822, and 5377 for 532, 638, and 940 nm, respectively. Figure 3b,c present the output characteristics of the device under 532 and 940 nm spatially selective illumination, respectively, at different light intensities. The photocurrent increases monotonically with incident power over the entire bias range, indicating a pronounced power-dependent photoresponse. Notably, the dark current is larger under positive bias than under negative bias, whereas the photocurrent exhibits the opposite behavior, showing higher values under negative bias. The opposite rectification characteristics of the photocurrent and dark current suggest distinct carrier transport mechanisms under illuminated and dark conditions. The time-resolved photocurrent under different light powers is shown in Figure S3 in the Supporting Information. Figure 3d shows the time-resolved photocurrent under 532 nm light illumination at different bias voltages. The on/off ratios at −5 V, 0 V, and 5 V are 838, 77,619, and 3.8, respectively. Under negative bias, the suppressed dark current, together with the enhanced photocurrent, leads to a higher on/off ratio. To gain deeper insight into the microscopic photoresponse mechanism, scanning photocurrent spectroscopy was performed (Figure 3e). A pronounced negative photocurrent (blue region) is observed at the MoS₂/WSe₂ interface, whereas the positive signal (red region) is localized at the Au/MoS₂ interface. The spatial polarity reversal originates from oppositely oriented built-in electric fields at these junctions, which drive photogenerated carriers in opposite directions. The scanning photocurrent maps under other bias conditions are shown in Figure S4 in the Supporting Information. Figure 3f schematically depicts the energy band alignment of the Au/MoS₂/WSe₂ dual junction. The Au/MoS₂ contact establishes a built-in electric field directed from MoS₂ toward Au, while the MoS₂/WSe₂ interface forms an oppositely oriented internal field pointing from MoS₂ to WSe₂ [25,26,27]. The transport of dark carriers is jointly governed by the two built-in electric fields, whereas the transport of photogenerated carriers is primarily dominated by the electric field in the illuminated region. Therefore, by employing spatially selective illumination, the dark current and photocurrent can be effectively decoupled. KPFM measurements across the Au/MoS₂, Au/WSe₂, and MoS₂/WSe₂ regions further reveal interface-dependent surface-potential variations (Figure S5, Supporting Information). The evolution of the energy-band structure under different drain biases is further illustrated in Figure S6 in the Supporting Information. The carrier transport under different drain biases is determined by the competition between the oppositely oriented built-in electric fields at the Au/MoS₂ and MoS₂/WSe₂ junctions. Positive drain bias mainly facilitates transport through the Au/MoS₂ Schottky junction and leads to a relatively larger dark current, whereas negative drain bias more effectively cooperates with the built-in field at the MoS₂/WSe₂ heterojunction, enhancing photocarrier separation while maintaining a low dark current.

Considering the gate-tunable properties of MoS₂ and WSe₂, we further employed gate-voltage modulation to optimize the device performance. Figure 4a shows the transfer characteristics of the device measured under dark conditions at V_ds = ±5 V. Under positive drain bias, the dark current initially increases with the gate voltage, reaching a local maximum at approximately V_g = −7.5 V, followed by a sharp decrease near V_g = 0 V, and it subsequently rises rapidly under positive gate bias. In contrast, under negative drain bias, the dark current remains negligible over the entire gate voltage range, exhibiting very weak gate modulation. Figure 4b presents the transfer characteristics of the device under 532 nm illumination. The photocurrent exhibits gate-dependent behavior that is markedly different from that observed under dark conditions, owing to the distinct transport mechanisms of dark carriers and photogenerated carriers. Under positive drain bias, the local peak at V_g = −7.5 V becomes less pronounced, whereas the current enhancement at positive gate voltages is significantly strengthened. In contrast, under negative drain bias, the photocurrent increases as the gate voltage decreases, reaches a maximum at V_g = −7.5 V, and then gradually declines with further gate sweeping. The gate-voltage-dependent output characteristics under 532 nm illumination are presented in Figure 4c. Evidently, positive gate bias predominantly enhances the photocurrent under positive drain bias, whereas negative gate bias mainly strengthens the photocurrent under negative drain bias.

Time-resolved photoresponse measurements further elucidate the gate-tunable transport behavior of the device. Figure 4d compares the time-resolved photoresponse of the device at V_ds = 5 V with and without gate-voltage modulation. At V_g = 0 V, the dark current remains around 0.05 μA, and the photocurrent reaches only about 0.11 μA. Upon applying positive gate bias (V_g = 20 V), the dark current increases slightly to approximately 0.22 μA, whereas the photocurrent rises sharply to approximately 2.1 μA. Figure 4e compares the time-resolved photoresponse of the device at V_ds = −5 V with and without gate-voltage modulation. Under negative drain bias, the dark current remains nearly unchanged (from 1 to 2 nA) after gate modulation, whereas the photocurrent increases by approximately 238% (from 0.63 μA to 1.50 μA). The gate voltage modulates the interfacial band alignment and built-in electric field at the MoS₂/WSe₂ heterointerface, thereby tuning the carrier separation efficiency. As a result, the photoresponse under different drain-bias conditions can be effectively tuned through the electrostatic control of the local heterojunction band structure. The evolution of the energy-band structure under different gate voltages is further illustrated in Figure S7 in the Supporting Information.

The performance of the Au/MoS₂/WSe₂ dual junction device for VIS–IR photodetection was further evaluated in terms of responsivity (R) and detectivity (D*). Responsivity represents the ratio of the photocurrent to the incident optical power and evaluates the photocurrent conversion capability of the device. It is defined as $R={\displaystyle \frac{I_{\mathrm{p}\mathrm{h}}}{P_{\mathrm{i}\mathrm{n}}\times S}}$, where $I_{\mathrm{p}\mathrm{h}}$ is the photocurrent, $P_{\mathrm{i}\mathrm{n}}$ is the incident power density, and $S$ is the effective illuminated area. Detectivity is another important parameter that reflects the capability of a photodetector to detect weak optical signals. It can be expressed as $D^{\mathrm{\ast }}={\displaystyle \frac{R\times S^{1/2}}{i_{\mathrm{n}}}}$, where R is the responsivity, S is the effective photosensitive area of the device and i_n is the current noise spectral density, extracted from the dark-noise spectrum measured under the corresponding bias conditions (Figure S8, Supporting Information) [28,29]. The calculated responsivity and detectivity at V_ds = −5 V as a function of incident optical power, with and without gate-voltage modulation, are shown in Figure 4f. For all incident optical power densities, gate-voltage modulation leads to a multiple-fold enhancement in the photoresponse. A negative gate bias (V_g = −7.5 V) significantly enhances the responsivity, reaching a maximum value of approximately ~1.48 A/W. In contrast, at V_g = 0 V, the responsivity remains below 0.6 A/W. Under an illumination intensity of 3.6 mW cm⁻², the device achieves a maximum detectivity of 1.98 × 10¹² Jones. The calculated results at V_ds = 5 V exhibit a similar gate-voltage enhancement effect (Figure S9, Supporting Information). A comparison of the performance of our Au/MoS₂/WSe₂ photodetector with several state-of-the-art heterojunction photodetectors reported in the recent literature is summarized in

Table 1. Comparison of the performance of the Au/MoS₂/WSe₂ photodetector with state-of-the-art heterojunction photodetectors.

Materials	Spectral Range	Responsivity (A/W)	Detectivity (Jones)	Response Time	Reference
PdSe2/NbSe2	405–980 nm	0.027	9.8 × 107	1.6/1.9 μs	[30]
MoTe2/MoSe2	210–1690 nm	0.729	3.69 × 108	6.13/8.56 ms	[31]
Sb2O5/Ga2O3	303 nm	1.05 × 10−3	-	190/190 ms	[32]
WSe2/MoS2/WSe2 double vdWHs	532 nm	0.715	1.59 × 1013	45 μs	[33]
GeSe/MoS2	380–1064 nm	0.105	1.46 × 1010	-	[34]
WSe2/Bi2Te3	375–1550 nm	20.5	-	210 μs	[35]
Te/Wse2	405–1550 nm	0.196	7.5 × 1011	18 μs	[36]
Au/MoS2/WSe2	532–940 nm	1.48	1.98 × 1012	639/371 μs	This Work

. The response time, external quantum efficiency (EQE) and linear dynamic range (LDR) of the Au/MoS₂/WSe₂ photodetector under 532 nm illumination are presented in Figure S10 in the Supporting Information.

The Au/MoS₂/WSe₂ dual junction exhibits outstanding photoresponse performance, highlighting its strong potential for imaging and encoded optical communication applications. To experimentally validate this capability, a sophisticated single-pixel optoelectronic signal conversion system was constructed, as illustrated in Figure 5a. A laser beam passes through a patterned mask and irradiates the Au/MoS₂/WSe₂ dual junction photodetector. The mask, mounted on a computer-controlled motorized translation stage, scans along the X and Y directions. Meanwhile, the computer records the photocurrent together with the corresponding spatial position. Figure 5b presents the reconstructed image of the letters “LHX” under 532 nm illumination, showing clearly distinguishable contours with a spatial resolution of approximately 0.5 mm. The preserved structural features demonstrate effective pixel-by-pixel signal acquisition and reconstruction. The optical image captured by a commercial camera is provided in Figure S11 (Supporting Information). Image reconstruction under 638 nm illumination was also demonstrated (Figure S12, Supporting Information), further supporting the feasibility of multiband image sensing. Figure 5c presents the optical encoding communication results of the letters “LHX.” The letters L, H, and X are represented by their corresponding 8-bit ASCII codes: 01001100, 01001000, and 01011000, respectively. Information is encoded through the illumination state, where “0” and “1” represent the light-off and light-on states, respectively. Benefiting from the fast and stable photoresponse of the device, the received signals accurately reflect the illumination states. Based on the empirical relationship between the response time ($t_r$) and the 3 dB bandwidth ($f_{3\mathrm{d}\mathrm{B}}$), $f_{3\mathrm{d}\mathrm{B}}\approx 0.35/t_r$, the bandwidth is estimated to be ~548 Hz for a rise time of 639 μs. In addition, proof-of-concept optical encoding communication under 638 nm illumination was also demonstrated (Figure S13, Supporting Information), further supporting the multiband communication capability of the device. These results mainly serve as a proof-of-concept demonstration of the optical encoding capability of the device. The successful demonstrations of imaging and optical communication confirm the applicability of the Au/MoS₂/WSe₂ dual junction across multiple spectral bands.

4. Conclusions

In summary, we have demonstrated a gate-enhanced Au/MoS₂/WSe₂ dual junction broadband photodetector. The spatially resolved photocurrent measurements reveal that the selective utilization of asymmetric built-in electric fields decouples the transport of dark and photogenerated carriers, simultaneously enabling a low dark current and high photocurrent. Furthermore, gate-voltage modulation of different electric-field regions enables distinct modulation of the dark current and photocurrent. Within the 532–940 nm spectral range, the device achieves a high detectivity of 1.98 × 10¹² Jones. High-resolution imaging and optical communication demonstrations further highlight the potential of Au/MoS₂/WSe₂ dual junction photodetectors for broadband and high-speed optoelectronic applications. These results provide a viable strategy for realizing high-performance, customizable Van der Waals heterostructure photodetectors via multijunction band engineering.