Ultrasensitive Phototransistor Based on Laser-Induced P-Type Doped WSe₂/MoS₂ Van der Waals Heterojunction

Abstract

Out-of-plane p-n heterojunctions based on two-dimensional layered materials (2DLMs) with unusual physical characteristics are attracting extensive research attention for their application as photodetectors. However, the present fabrication method based on 2DLMs produces out-of-plane p-n homojunction devices with low photoresponsivity and detectivity. This work reports an ultrasensitive phototransistor based on a laser-induced p-doped WSe₂/MoS₂ van der Waals heterojunction. The laser treatment is used for p-doping WSe₂ nanoflakes using high work function WO_x. Then, an n-type MoS₂ nanoflake is transferred onto the resulting p-doped WSe₂ nanoflake. The built-in electric field of p-doped WSe₂/MoS₂ is stronger than that of pristine WSe₂/MoS₂. The p-n junction between p-doped WSe₂ and MoS₂ can separate more photogenerated electron–hole pairs and inject more electrons into MoS₂ under laser illumination than pristine WSe₂/MoS₂. Thus, a high photoresponsivity (R) of ~1.28 × 10⁵ A·W⁻¹ and high specific detectivity (D*) of ~7.17 × 10¹³ Jones are achieved under the illumination of a 633 nm laser, which is approximately two orders higher than the best phototransistor based on a WSe₂/MoS₂ heterojunction. Our work provides an effective and simple method to enhance photoresponsivity and detectivity in two-dimensional (2D) heterojunction phototransistors, indicating the potential applications in fabricating high-performance photodetectors based on 2DLMs.

1. Introduction

Photodetectors are critical elements of emerging technologies, such as autonomous vehicles, environmental monitoring systems, security systems, mobile phone cameras and telecommunications systems [1,2]. The most common types of photodetectors are photodiodes and phototransistors. A photodiode works by developing a p-n junction or Schottky junction, which separates the photoinduced electron–hole pairs, causing the generated electrons and holes to move in opposite directions, culminating in a photocurrent or photovoltage. In a phototransistor, electron–hole pairs are primarily separated by external bias and further regulated by a gate, resulting in a photocurrent. Photodetectors based on two-dimensional (2D) material heterostructures have remarkable responsivity, which has been confirmed through theoretical simulation models [3,4]. Most importantly, out-of-plane 2D van der Waals heterostructure phototransistors can combine the advantages of photodiodes and phototransistors. The operation schematics of a photodiode and phototransistor are shown in Figure S1a,b in the Supporting Information, respectively. In the heterostructure phototransistor configuration, the photodiode (p–n junction) part separates electron–hole pairs under light illumination, in which electrons are injected to MoS₂, resulting in maximizing its electrical response to light in terms of photogain, as shown in Figure S1c. The phototransistor also minimizes the noise in its electrical output by operating the device in the depletion regime using appropriate gate voltage modulation.

Recently, phototransistors based on WSe₂ and MoS₂ have shown promising avenues for optoelectronic devices, with promising carrier transport properties, high on/off ratio, high light absorption coefficient, scalable thickness down to the monolayer and, most significantly, easy to introduce doping [2,5,6,7,8,9,10,11]. However, these devices have shown limited photoresponsivity and detectivity due to the weak built-in electric field formed between WSe₂ and MoS₂ [6,9,10]. Therefore, further investigations regarding improvements in the strength of a built-in field for boosting the performance of the phototransistor based on WSe₂/MoS₂ are still needed.

Among 2D transition metal dichalcogenides (TMDs), p-doping techniques have been widely reported as an effective tool to manipulate the properties of heterostructure phototransistors, including chemical doping with XeF₂ vapors, PPh₃/AuCl₃, low-energy phosphorus implantation, Te and H₂ gas, electrostatic split-gate configuration, ozone treatment and UV/ozone treatments [11,12,13,14,15,16,17]. Additionally, n-doping techniques have also been reported, including electron beam lithography, self-assembled monolayer, triphenylphosphine, lithium fluoride, nitrogen, zinc oxide and alkali metal compound-promoted chemical vapor deposition (CVD) [18,19,20,21,22,23,24,25]. However, none of these methods can simultaneously achieve controllable, scalable and stable performance over an extended period without introducing chemical impurities. This severely limits the application of optoelectronic devices based on TMDs. Nevertheless, laser treatment has been used for defining p-n heterojunctions for fabricating high-performance photodetectors. It is a simple, efficient and selective p-doping approach for 2D TMDs [5,26,27].

In this study, the laser treatment technique is used for the achievement of p-doping WSe₂, forming a strong built-in electric field between p-doped WSe₂ and untreated MoS₂ in the p-doped WSe₂/MoS₂ van der Waals heterojunction. The resulting p-doped WSe₂/MoS₂ heterojunction phototransistor has a high photoresponsivity of ~1.28 × 10⁵ A·W⁻¹ and specific detectivity of ~7.17 × 10¹³ Jones, which are significantly higher than those of the reported MoS₂ and WSe₂/MoS₂ field-effect transistors (FETs) [1,2,6,8,28]. Our work opens a new avenue for high-performance photodetectors based on two-dimensional heterojunction phototransistors.

2. Materials and Methods

2.1. Multilayer WSe₂/MoS₂ Preparation

The WSe₂ and MoS₂ nanoflakes were first exfoliated from their crystal (HQ graphene company) by using blue tapes. The multilayer WSe₂ was fabricated through repeated tape exfoliation and transferred to a p⁺⁺ Si substrate with a 90 nm SiO₂ capping layer. In contrast, the multilayer MoS₂ was directly prepared on a polydimethylsiloxane (PDMS) substrate, supported by a glass slide. Atomic force microscopy (AFM, Dimension 3100, Veeco, Plainview, NY, USA) and optical microscopy (Keyence VHX-600, Osaka, Japan) were used to measure and screen the shapes and thickness of WSe₂ (MoS₂), respectively. When conducting these experiments, the surface of WSe₂ and MoS₂ should be clean. The area of MoS₂ nanoflakes needs to be larger than WSe₂ to completely cover the underlying WSe₂.

2.2. Laser-Scanned Process

Multilayer WSe₂ was scanned via Raman spectrometer laser (Renishaw in via Raman microscope) to fulfill region-selectable p-type doping. To figure out the proper power and exposure time for p-doping, multilayer WSe₂ was scanned with 633 nm, 515 nm and 473 nm lasers. The laser power and exposure time were set in a series of gradients. The diameter (d) of the 633 nm laser beam was calculated as 0.61 µm by d = K·λ/NA, where K = 0.82 was used for a Gaussian beam, λ = 633 nm was the laser wavelength and NA = 0.85 was the numerical aperture of the objective; the diameters of 515 nm and 473 nm laser beam were 0.50 and 0.47 µm, respectively. The laser scanning step size was set as 0.3 µm. Then, the pristine WSe₂ and laser-scanned WSe₂ were measured via Raman spectrometer (Witec Alpha 300 Access, Ulm, Germany) with a 532 nm laser. Laser power was set to 0.5 mW to avoid doping effect on WSe₂. The integration time was 8 s, and the number of integrations was 3. The experiment details are shown in Supporting Information Figure S2 and Table S1. After the preliminary experiment with the laser-scanned process, another WSe₂ was scanned using 633 nm laser, with laser power of 4 mW, exposure time of 0.75 s and laser spot diameter of 0.61 µm.

2.3. Polydimethylsiloxane (PDMS)-Assisted Transfer

The PDMS supporting the specific MoS₂ was carefully cut into 1 mm × 1 mm pieces without tearing up the MoS₂ to avoid stress shredding MoS₂/MoS₂ heterojunction in the PDMS-assisted transfer process. After the laser-scanned process of WSe₂, the specific MoS₂ was accurately transferred onto the WSe₂ by using a dry transfer system (E1-T multifunctional high-precision 2D material transfer system from Metatest Corporation Company, Las Vegas, NV, USA). The fabricated WSe₂/MoS₂ heterojunction was soaked in acetone at 40 °C for ten mins to remove uncrosslinked oligomers of PDMS that remained on the surface of MoS₂, followed by isopropanol soaking and blow-dry process.

2.4. Device Fabrication

Device fabrication consists of several steps: spin coating, photolithography, surface treatment, electron beam evaporation deposition, lift-off and annealing treatment. In the spin-coating process, the WSe₂/MoS₂ heterojunction was coated with ultraviolet photoresist (LOR, Microchem company, Austin, TX, USA) at 5000/min rotation for 1 min and baked at 180 °C for 10 min. Then, the WSe₂/MoS₂ heterojunction was coated with ultraviolet photoresist (RZJ-304-25, Ruihong Electronic Chemicals Co., Ltd., Milpitas, CA, USA) at 5000/min rotation for 1 min and baked at 115 °C for 1 min. In the photolithography process, source and drain electrodes are patterned via lithography (Microwriter ML3 laser direct-writing system) on MoS₂ at a laser dose of 160 mJ·cm⁻². Then, the WSe₂/MoS₂ heterojunction was soaked in developer solution for 30 s, followed by deionized water soaking and blow-dry process. In the surface treatment process, the WSe₂/MoS₂ heterojunction was treated in Ar-plasma with at 4 mTorr for 1 min by using a plasma cleaner (Harrick PDC-002, NY, USA) to remove the remained photoresist. In the electron beam evaporation deposition process, 5 nm Ni and 50 nm Au were deposited by the electron beam evaporator (GX1350 from Tianxingda Vacuum Coating Equipment Co., Ltd, Shenzhen City, China). In the lift-off process, the WSe₂/MoS₂ heterojunction was soaked in N-Methylpyrrolidone (NMP) at 40 °C for 30 min, followed by isopropanol soaking and blow-dry process. In the annealing treatment process, the WSe₂/MoS₂ heterojunction was annealed at 300 °C in an Ar atmosphere for 2 h by using a tube annealing furnace (OTF-1200X-S from HF-kejing Equipment Co., Ltd., Hefei City, China) to realize better contact. The device fabrication process is shown in Figure S3 in the Supporting Information.

2.5. Optical Characterization

The fabricated MoS₂/WSe₂ heterojunction on the SiO₂/Si substrate was characterized using Raman and photoluminescence (PL) spectroscopy (Witec Alpha 300 Access, Ulm, Germany) with a 532 nm laser. In the Raman measurements, the laser power was set to 0.2 mW to avoid doping effect on WSe₂, the integration time was 5 s and the number of integrations was 2. For the PL measurements, the laser power was set to 0.5 mW, the integration time was 8 s and the number of integrations was 3. Optical microscopy (Keyence VHX-600, Osaka, Japan) and atomic force microscopy (AFM, Dimension 3100, Veeco, Plainview, NY, USA) were used to identify the device’s surface topography and step height, respectively.

2.6. Electrical Characterization

All the electric measurements were conducted with a semiconductor parameter analyzer (Agilent, Santa Clara, CA, USA, B1500A). Scanning photocurrent microscopy (SPCM) analysis system combines the electrical characterization system (Agilent B1500A) and the Raman system (Renishaw in Via, Gloucestershire, UK) using a motorized XYZ stage. The semiconductor diode laser (633 nm, Renishaw in Via, Gloucestershire, UK) was focused onto the device’s channel region utilizing a long working distance objective lens with a 50× magnification (NA = 0.5). A mechanical chopper (BET-ES from Beijing Nbet Technology Co., Ltd., Beijing, China) and an oscilloscope (Agilent 86100A) were employed to analyze the time-dependent photoresponse. The hysteretic behavior of the device transfer curve in the air environment can be effectively removed by alternating current pulse scanning (Alternating Polarity, AP) by changing the polarity of the gate voltage, which was applied to transistors of various channel materials [29,30]. Short gate voltage (V_G) pulses were applied to the device to obtain the transfer characteristics with and without illumination.

3. Results and Discussion

3.1. Optical Characterization

A 3D (three-dimensional) schematic diagram, 2D schematic diagram of the operation principle and an optical image of the phototransistor based on laser-scanned WSe₂/MoS₂ van der Waals heterojunction are shown in Figure 1a–c, respectively. The MoS₂ and WSe₂ nanoflakes were exfoliated onto PDMS and SiO₂ substrates, respectively. The WSe₂ surrounded by a white dotted-line triangle in Figure 1c was fully scanned using a 633 nm wavelength laser. After the laser-scanned process, MoS₂ nanoflakes were transferred onto the surface of laser-scanned WSe₂. Source and drain electrodes were patterned via lithography on MoS₂ and then 5 nm Ni and 50 nm Au were deposited by the electron-beam evaporator, followed by the lift-off process. Figure 1d presents the height profiles measured using AFM (Supporting Information, Figure S4). The thicknesses of pristine MoS₂ and WSe₂ and the heterojunction were 5 nm. This shows a consistent thickness. It has been shown that the ambipolar transfer characteristic of the pristine WSe₂ field-effect transistor (FET) was converted into unipolar p-type behavior after laser-scanning treatment according to our former works [5,27]. The laser-oxidized product WO_x is responsible for the p-type doping of WSe₂ [5,27]. The laser-scanned method can also be used for p-doping MoTe₂ [26,31].

In order to establish the proper power and exposure time for p-doping, multilayer WSe₂ samples with laser treatment of different power and exposure time were measured (Supporting Information Figure S2 and Table S1). As shown in Figure S2c, the PL peak intensity of WSe₂ decreased with exposure time, suggesting effective oxidation originating from serious damage to the structure. As shown in Figure S2e, the PL peak intensity of WSe₂ increased with the 633 nm laser-scanned power, indicating a photo-induced enhancement in the electron concentration. After elaborative comparison, the 633 nm laser (laser power of 4 mW, exposure time of 0.75 s) was chosen for controllably p-doping the WSe₂.

Figure 2a shows the Raman spectra of multilayer WSe₂, MoS₂ and the overlapping region between laser-scanned WSe₂ and MoS₂. The corresponding fitting curves are shown in Supporting Information Figure S5; one strong peak (248 cm⁻¹) and four weak peaks (306, 357, 371 and 392 cm⁻¹) can be observed from WSe_2′s Raman spectra, and two strong peaks (381 and 406 cm⁻¹) can be observed from MoS_2′s Raman spectra. The Raman spectra of the overlapping area show three strong peaks at 248 cm⁻¹, 381 and 406 cm⁻¹, which means that the laser-scanned WSe₂/MoS₂ heterostructure is successfully conducted [6]. However, the Raman peak intensities of the MoS₂ are reduced for the laser-scanned WSe₂/MoS₂ heterojunction. These intensity reductions can be attributed to ultrafast charge transfers due to strong interfacial coupling in these structures between laser-scanned WSe₂ and MoS₂ [32,33]. Furthermore, we conducted the PL spectra of pristine WSe₂, MoS₂ and laser-scanned WSe₂/MoS₂ heterojunction, as shown in Figure 2b. Two prominent emission peaks of MoS₂ are observed at 1.82 and 1.95 eV, corresponding to emissions originating from the indirect transition and A exciton, respectively. PL of WSe₂ also exhibited an indirect transition and A exciton at 1.44 and 1.61 eV, respectively. In the laser-scanned WSe₂/MoS₂ heterojunction, the PL peak intensity of MoS₂ dropped, which indicates charge transfer between p-doped WSe₂ and MoS₂ [32]. In order to reveal the effect of doping, we fabricated a new WSe₂/MoS₂ heterojunction with undoped and doped regions. PL intensity of WSe₂ and MoS₂ both increase obviously in the doped region compared to the undoped WSe₂/MoS₂ heterojunction, which suggests this carrier density is enhanced through a controllable doping process (see Supporting Information Figure S6).

3.2. Electrical Characterization

We further conducted electrical measurement of the device. The sublinear I_DS (drain current) −V_DS (drain voltage) characteristics of a p-doped WSe₂/MoS₂ heterojunction transistor in Figure 3a indicate the ohmic contacts at source and drain terminals. The I_DS−V_G characteristic of the p-doped WSe₂/MoS₂ heterojunction transistor in Figure 3b shows an on/off of ~10⁷, which is comparable to that of the best MoS₂ transistors [2,6,8,17,28,34]. The gate insulator capacitance, C_g, for the 90 nm SiO₂ and 5 nm WSe_2, is calculated to be 3.77 × 10⁻⁸ F·cm⁻², using the following formula:

$$\frac{1}{C_g}=\frac{1}{C_{Si{O}_2}}+\frac{1}{C_{WS{e}_2}}$$

(1)

where C_SiO2 = 3.84 × 10⁻⁸ F·cm⁻² is capacitance of 90 nm SiO₂, C_WSe2 = 2.07 × 10⁻⁶ F·cm⁻² is capacitance of 5 nm WSe₂ (C_OX = ɛ₀ɛ_OX/t_OX; ɛ_SiO2 = 3.9; ɛ_WSe2 = 11.7; t_SiO2 = 90 nm; t_WSe2 = 5 nm; where ɛ₀ is the vacuum permittivity, ɛ_OX is relative permittivity, t_OX is gate insulator thickness). The field-effect mobility (µ) of the phototransistor is 32.74 cm²/V·s according to the following formula:

$$\mu =\frac{L·g_{m-max}}{W·C_g·V_{DS}}$$

(2)

where L = 8.95 µm is channel length of the device, W = 24.07 µm is channel width of the device and g_m-max = 1.66 × 10⁻⁵ S is maximum transconductance according to the dark transfer curve, V_DS = 5 V. We further examined the transfer characteristics of a few-layer WSe₂ FET on the 90 nm SiO₂/Si substrate, and the I_DS−V_G transfer characteristics before and after laser-scanned process (using 633 nm laser, with laser power of 4 mW, exposure time of 0.75 s and laser spot diameter of 0.61 µm) are shown in Supporting Information Figure S7. The I_DS−V_G transfer characteristics are transferred from ambipolar to unipolar p-type apparently, which indicates that WSe₂ is effectively p-type doped through the laser-scanned process. This can help to promote a strong built-in electric field between the laser-scanned WSe₂ and MoS₂.

3.3. Photoresponse

After the electrical characterization of a p-doped WSe₂/MoS₂ heterojunction transistor, its photoresponse measurements were also conducted. The device was measured under spot illumination with a 633 nm laser at different power intensities to investigate its photoresponsivity. The scanning photocurrent microscopy (SPCM) method was used to obtain a spatial-resolved photoresponse of the device. SPCM is performed at V_DS = 0 V, V_G = 0 V and an incident power of 1.94 μW. As shown in Figure S8, the photocurrent of the p-doped WSe₂/MoS₂ heterojunction is small because the photogenerated carriers cannot be effectively collected using source and drain electrodes under no bias (V_DS = 0 V) conditions. To obtain more obvious photocurrent responsivity, SPCM was performed at V_DS = 5 V, V_G = −10 V and an incident power of 1.94 μW. As shown in Figure 4a, the I_DS at a p-doped WSe₂/MoS₂ heterojunction is much stronger than that at nearby areas, owing to the stronger capability of separating the photogenerated electron–hole pairs in the built-in electric field of the p-doped WSe₂/MoS₂ heterojunction. Figure 4b shows the transfer curves of a p-doped WSe₂/MoS₂ heterojunction transistor without and with laser-beam illumination. The I_DS of the p-doped WSe₂/MoS₂ heterojunction increases with the laser-beam illumination intensity. The increasing laser-beam illumination intensity can produce more photoexcited electron–hole pairs after separating through the built-in electric field of the p-doped WSe₂/MoS₂ heterojunction, more electrons can transfer to MoS₂ and then can be effectively collected with the drain electrode, eventually forming the increasing photocurrent. The photoexcited holes are transferred to p-doped WSe₂, modulating the conductance of the MoS₂ channel, resulting in the photogating effect, which induces the threshold voltage leftshift. In such a heterostructure phototransistor configuration, the photodiode separates electron–hole pairs under light illumination and then cumulated holes in p-doped WSe₂, modulating the current in the channel in the phototransistor. It maximizes the electrical response to light in terms of photogain.

To quantitatively assess the device’s photoresponse performance, two crucial parameters, photoresponsivity (R) and detectivity (D*), can be calculated according to the following formulas:

$$R=\frac{I_{light}-I_{dark}}{P}$$

(3)

where I_light is drain current, I_dark is dark current and P is incident laser power.

$$D^{*}=\frac{\sqrt{A}·R}{\sqrt{2q{I}_{dark}}}$$

(4)

where q is the element charge and A is the effective illuminated area. Figure 5a,b present photoresponsivity (R) and detectivity (D*) under the illumination of a 633 nm laser with different power intensities from 1 nW to 3.28 μW, respectively. As shown in Figure 5a, the photoresponsivity increases as gate voltage. The increasing gate voltage induces more electrons accumulated in the MoS₂ channel. The photoresponse saturation emerges at V_G of 20 V. A high responsivity of 1.28 × 10⁵ A·W⁻¹ is shown at an illumination power of 1 nW, V_G of 20 V and V_DS of 5 V. As shown in Figure 5b, a high specific detectivity of ~7.17 × 10¹³ Jones is observed at an illumination power of 1 nW, V_G of −5.5 V and V_DS of 5 V, which are higher than the values reported in previous studies [1,2,5,6,8,28]. As shown in Figure 5c,d, the photocurrent (I_PH = I_light − I_dark) and the logarithm of photoresponsivity of the device have a sublinear dependence on the logarithm of incident laser power from 1 nW to 3.28 μW, for V_G of 20 V, V_DS of 5 V. The photocurrent increases with laser power intensity, while photoresponsivity decreases with laser power intensity, which is similar to other 2D-based photodetectors [1,6,28,35].

In our work, the responsivity and specific detectivity of the device are much higher than those of conventional out-of-plane WSe₂/MoS₂ phototransistors [6], out-of-plane MoS₂ p-n homojunctions [2], monolayer MoS₂ phototransistors [28], multilayer MoS₂ phototransistors [8] and laser-induced p-type doped WSe₂ phototransistors [5] (

Table 1. Performance comparison between phototransistors based on 2D-material semiconductors.

Device Structure	R (A·W−1)	D* (Jones)	Rise/Decay Time	Spot Size (Diameter)	Reference
Laser-induced p-type doped WSe2/MoS2 (5 nm)	1.28 × 105	7.17 × 1013	-	1038 nm	This work
Out-of-plane WSe2/MoS2 (9 nm)	2700	5 × 1011	~10 ms	872 nm	[6]
Out-of-plane MoS2 p-n homojunction (7 nm)	7 × 104	3.5 × 1014	~10 ms	2 mm	[2]
Monolayer MoS2 (0.7 nm)	0.0075	-	50/50 ms	10 µm	[28]
Multilayer MoS2 (10 nm)	0.18	3 × 1010	-	-	[8]
Laser-induced p-type doped WSe2	0.8	-	139/39 ms	1038 nm	[5]

). The responsivity and specific detectivity are approximately two orders higher than the out-of-plane WSe₂/MoS₂ phototransistor [6], demonstrating laser treatment attributes to enhancing photoproperties. The out-of-plane MoS₂ p-n homojunction [2] has shown high photo responsivity and specific detectivity because they utilize a global illumination laser with a spot size of 2 mm, while we use a Raman laser spot (size of 1.04 µm) of Gaussian beam (NA = 0.5). Though they have approximately ten orders lower incident power intensity of 73 pW·cm⁻² compared to ours of 1.13 × 10¹¹ pW·cm⁻², we have higher responsivity, so we have better performance.

3.4. Time Dependence of Photoresponse

The time-dependent photoresponse was also measured to investigate the dynamic photoresponse of the device. A mechanical chopper and an oscilloscope (Agilent 86100A) were used to analyze the time-dependent photoresponse. Figure 6a,b present the time-dependent photoresponse of the device, which was measured using an optical chopper with a frequency of 1/30 Hz, incident power of 164 nW, at V_G = −10 to 10 V with a step of 5 V and V_DS = 5 V, respectively. The photocurrent of our device decreases as the time increases under positive gate voltage and increases as the time increases under negative gate voltage, which means our device can work in both bright and dark backgrounds. Figure 6c presents response time as a function of V_G. Rise time decreases as V_G increases. The occupied trap states in WSe₂ increase as V_G, leading to more photoelectrons participating in channel conductance.

3.5. Floating Gate and Photogating of Photoresponse

We further conducted I_DS − V_G transfer characteristics of forward sweep and backward sweep at 1 V/s of the device to investigate floating gate and photogating of the photoresponse. Figure 7a shows the I_DS − V_G transfer characteristics in dark conditions. Figure 7b shows the I_DS − V_G transfer characteristics under illumination intensity from 0.025 to 4 nW (power density of less than 1 nW was calculated from the transmittance of the filter). Figure 7c shows the I_DS − V_G transfer characteristics under illumination intensity from 2.14 to 4.48 µW. A clockwise hysteresis loop behavior was observed in both dark and illuminated conditions, indicating the charge trapping and de-trapping process at the interface between p-type doped WSe₂ and MoS₂. The trap charge density (N_t = 3.79 × 10¹² cm⁻²) can be calculated using following formula:

$$N_t=\frac{\Delta V·C_g}{q}$$

(5)

where q is the element charge, ΔV = 16.1 V in Figure 7a is the maximum voltage shift between the transfer curves in the forward and backward sweeps and C_g = 3.77 × 10⁻⁸ F·cm⁻² is the total capacitance of the 90 nm thick SiO₂ layer and 5 nm WSe₂ layer. The N_t is comparable with carrier density in MoS₂ [36,37,38,39,40,41], which indicates that the trap states can substantially enhance the current under different illumination conditions [38,41]. Threshold voltage left-shift is observed in I_DS − V_DS transfer characteristics of forward sweep and backward sweep under increasing optical powers, which indicates that the trapping at the interface of p-doped WSe₂ increases as illumination power, resulting in a photogating effect. Therefore, our device has high photoresponsivity and detectivity. In addition, our device can be used as optoelectronic memory for recording the perceived light information, benefiting from the large memory window.

4. Conclusions

In this study, we developed an ultrasensitive phototransistor based on a laser-induced p-doped WSe₂/MoS₂ van der Waals heterojunction. A simple, efficient and selective p-doping approach was used for WSe₂. The interface between p-doped WSe₂ and MoS₂ can separate the photogenerated electron–hole pairs and inject electrons into the MoS₂ channel layer under laser illumination due to the strong built-in electric field. The p-doped WSe₂/MoS₂ heterojunction phototransistor shows a high photoresponsivity of ~1.28 × 10⁵ A·W⁻¹ and a high specific detectivity of ~7.17 × 10¹³ Jones. Moreover, our phototransistor can operate in two modes. In the first mode, the photocurrent of our phototransistor decreases as the time increases under positive gate voltage, which means that our device can work in bright backgrounds. In the second mode, the photocurrent of our phototransistor increases as the time increases under negative gate voltage, indicating that our device can work in dark backgrounds. The capability of two work modes in our device indicates its potential application in bioinspired in-sensor visual adaptations [42]. By introducing trap states into the interface between p-doped WSe₂ and MoS₂, our device can be used for optoelectronic memory, benefiting from the large memory window. Together, these open a new avenue for future photoelectronic devices based on emerging 2D van der Waals heterojunctions with optimum device engineering.