Interface-Induced WSe2 In-plane Homojunction for High-Performance Photodetection.

2D transition metal dichalcogenides (TMDCs) have been extensively attractive for nano-electronics and nano-optoelectronics due to their unique properties. Especially, WSe2, having bipolar carrier transport ability and sizable bandgap, is a promising candidate for future photodetectors. Here, we report an in-plane WSe2 homojunction formed by the interface gate of the substrate. In this architecture, an insulated h-BN flake was used to make only part of WSe2 flake contact substrate directly. Finally, the structures of WSe2/substrate and WSe2/h-BN/substrate construct an in-plane homojunction. Interestingly, the device can operate in both photovoltaic and photoconductive modes at different biases. As a result, a responsivity of 1.07 A W−1 with a superior detectivity of over 1012 jones and a fast response time of 106 μs are obtained simultaneously. Compared with previously reported methods adopted by chemical doping or electrostatic gating with extra bias voltages, our design provides a more facile and efficient way for the development of high-performance WSe2-based photodetectors.


Introduction
In recent decade, 2D transition metal dichalcogenides (TMDCs) have drawn great attention owing to their particular properties. High in-plane mobility, tunable bandgap, mechanical flexibility, strong light-matter interaction, and easy processing make them very competitive for future nano-optoelectronics devices [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. Especially, tungsten diselenide (WSe 2 ), a bipolar semiconductor with facile carrier-type manipulation, allows remarkably potential applications in the junction-based photodetectors [21][22][23][24][25][26][27][28]. So far, the main strategies of constructing junction solely in WSe 2 include chemical doping and electrostatic gating. For example, recently, an intramolecular WSe 2 p-n junction was reported [26]. The n region and p region within WSe 2 were formed by polyethyleneimine chemical doping and back gate control, respectively. The p-n junction presented a responsivity of 80 mA W −1 and 200 μs response time. Sun et al. doped WSe 2 by using cetyltrimethyl ammonium bromide to form intramolecular p-n junction, in which the responsivity and the response time are 30 A W −1 and~7 ms, respectively [27]. Baugher et al. demonstrate a lateral WSe 2 p-n junction achieved by electrostatic gating through applying two gate biases with opposite polarity. The responsivity of 210 mA W −1 has been obtained [28]. However, due to the inevitable chemical impurities and the necessary multiple bias settings, these methods make the fabrication and application of junction-based devices complex and difficult. Assembling various 2D materials to build vertical van der Waals heterostructures like WSe 2 /MoS 2 junction [29] has become popular for the development of novel photodetectors. But, in this configuration, the process of carrier transport between different layered materials suffers from the interface defects, which restricts the device response speed. For the Schottky junction formed between metals and 2D materials, the Schottky barrier height is usually determined by Fermi-level pinning, which is uncontrollable and has a great impact on the responsivity of the devices. Additionally, the reported works cannot seem to possess both high responsivity and fast response speed.
Here, we demonstrate a facile and more efficient way to realize an in-plane WSe 2 homojunction. In the architecture, part of WSe 2 channel is on the Si/SiO 2 substrate and the other part is on the h-BN flake. This scheme is common in floating/semi-floating gate memories, in which the h-BN is adopted as gate dielectric layer [30,31]. The charges stored on one side of h-BN layer can regulate the conductivity of the material on the other side. In our work, however, the h-BN flake as a perfect isolator is used to eliminate the interface gating effect on the WSe 2 channel. The polarity of WSe 2 , which part is only on the Si/SiO 2 substrate, can be modulated by interface gate. As a result, the devices operate in photovoltaic (PV) mode well at zero bias. Meanwhile, it exhibits photoconductive (PC) characteristics at high bias. A responsivity of 1.07 A W −1 with a superior detectivity of over 10 12 jones and a fast response time of 106 μs are obtained simultaneously without the intricate device design and the risk of introducing additional chemical impurities. Figure 1a shows a schematic of the in-plane WSe 2 homojunction. It can be seen that part of WSe 2 flake is placed on h-BN flake (WSe 2 -h) and the other part contacts the Si/SiO 2 substrate directly (WSe 2 -S). The function of h-BN is to isolate the interface gate (IG) of the Si/SiO 2 substrate on the WSe 2 -h. So, the formation of homojunction between WSe 2 -h and WSe 2 -S mainly relies on the IG modulating the polarity of WSe 2 -S. The IG is produced by the trapped charges at the SiO 2 surface. This will be discussed below in detail. Figure 1b presents the optical picture of the device. Four electrodes (E1-E4, Ti/Au) were prepared by electron-beam lithography, metallization, and the lift-off process. The thickness of materials is characterized by atomic force microscope (AFM) (see Fig. 1c). The height of WSe 2 (h-BN) flake in direct contact with the Si/SiO 2 substrate (white dotted lines) was measured as 65 (23) nm (see Fig. 1d, e). It can be seen that there is a slope instead of sharp step in the height profile between the WSe 2 (h-BN) and the Si/SiO 2 substrate. This may be due to the residual photoresist at the edge of the material. Figure 1f shows the Raman spectra of WSe 2 and h-BN flakes. For the WSe 2 , the first order E 2g and A 1g Raman modes are clearly distinguished~250 cm −1 , suggesting that the WSe 2 has a multilayer morphology [32,33]. For the h-BN, the Raman peak of E 2g mode at~1370 cm −1 is observed. Due to the large bandgap of h-BN, the Raman signal is weak compared with that in WSe 2 [34].

Results and Discussion
To explore the effect of substrate on WSe 2 , transfer characteristics of WSe 2 -S and WSe 2 -h were studied separately. As shown in Fig. 2a, both transfer curves exhibit bipolar behavior and an obvious hysteresis can be observed in the curve of WSe 2 -S (black) compared with that of WSe 2 -h (red). The current of WSe 2 -h is higher than that of WSe 2 -S. The steep slope in the curve of WSe 2 -h indicates a relatively large transconductance, which is proportional to carrier mobility. For WSe 2 -S, the hysteresis is attributed to the charge trapping at the SiO 2 surface [35][36][37][38]. When V g was swept from − 30 to 0 V, the negative V g makes the WSe 2 populated with holes and drives some holes into the SiO 2 (see Fig. 2b). The trapped holes in SiO 2 generate a positive local gate, i.e., IG, to modulate the WSe 2 conductance in return (weak depletion effect). Therefore, the charge neutrality point of V g appears around − 5 V. Similarly, when V g was swept from 30 to 0 V, the positive V g makes the WSe 2 populated with electrons and also drives some electrons into the SiO 2 (see Fig. 2c). The trapped electrons in SiO 2 generate a negative IG to modulate the WSe 2 conductance in return (the same weak depletion effect). So, the charge neutrality point of V g appears around 5 V. For WSe 2 -h, the h-BN flake inhibits the carrier transfer between WSe 2 and SiO 2 under V g modulation. This is the reason for the non-obvious hysteresis in the WSe 2 -h curve. Therefore, an in-plane homojunction can be formed simply by taking advantage of the IG. Figure 3a shows the I d -V d curves of the device under dark and light conditions at V g = 0 V. The source-drain voltage is applied on electrodes E2 and E3 (see the inset). It can be seen that the short-circuit currents (at V d = 0 V) increase with the incident power, indicating a PV effect. Interestingly, the curves also present PC characteristics at V d = ± 1 V. For the former, the photocurrents are attributed to the homojunction. As shown in Fig. 3b, although V d and V g were set at 0 V, a few already trapped holes in SiO 2 form a small positive IG to modulate the WSe 2 -S. So, the n --type WSe 2 -S and intrinsic WSe 2 -h (without the effect of IG due to the isolation by h-BN flake) constitute an in-plane homojunction. Under illumination, the photoexcited electron-hole pairs will be separated by built-in field of the homojunction. Although I d -V d curves present PV characteristic well at zero bias, the homojunction did not show a rectifying behavior maybe due to the relatively weak built-in field compared with the externally applied V d . For the latter, the whole WSe 2 flake as a photoconductor responds the light signal at high bias. The photoexcited carriers will be driven to the electrodes by V d . Therefore, the photoresponse in Fig. 3a is the result of synergistic effect of PV and PC modes. The responsivities as a function of the light power for different V d are summarized in Fig. 3c, given by R = I ph /PA, where I ph is the photocurrent, P is the power intensity, and A is the effective photosensitive area of the detector [39,40]. During the calculation, the effective photosensitive area, i.e., the WSe 2 part between E2 and E3, is 115.75 μm 2 . The responsivities of 1.07 A W −1 and 2.96 A W −1 are obtained for V d of 0 V Fig. 2 Transfer characteristics. a I d -V g curves of WSe 2 -S (black line) and WSe 2 -h (red line). The sweep direction of V g is indicated by the arrows. b, c Physical explanation for the hysteresis phenomenon. The arrows indicate the direction of electric field induced by V g . The red and blue spheres represent holes and electrons, respectively and 1 V, respectively. The specific detectivity (D * ) as an important parameter determines the capability of a photodetector to response a weak light signal. Assuming that the shot noise from the dark current is the major contribution, D * can be defined as D * = RA 1/2 /(2eI dark ) 1/2 , where R is the responsivity, A is the effective photosensitive area, e is the electron charge, and I dark is the dark current [41,42]. Benefitting from the extremely low I dark , D * of 3.3 × 10 12 jones (1 jones = 1 cm Hz 1/2 W −1 ) and 1.78 × 10 11 jones are achieved for V d of 0 V and 1 V, respectively. Moreover, response time as a key figure of merit has been studied. As shown in Fig. 3d, a high and a low current state acquired at V d = 0 V have been obtained with the light modulation. The transient photoresponse exhibits highly stable and reproducible characteristics. Figure 3e gives a single modulation cycle of temporal response. The rising time (t r ), defined as the time necessary for the current to increase from 10% I peak to 90% I peak , was found to be~106 μs, and the falling time (t f ), defined analogously, was found to be~91 μs. Figure S1 shows temporal response of the device acquired at V d = 1 V. t r and t f were found to be~105 μs and~101 μs, respectively. Table 1 summarizes the reported WSe 2 homojunction formed by different methods. Obviously, the device in our work has high D * , comparable R, and relatively fast response speed. Moreover, Figure S2 presents the photoresponse characteristics of the other three devices. Distinct PV and PC currents can be observed at zero and high bias, respectively. The detectivity of all the WSe 2 homojunctions is higher than 10 12 jones, and the response time is a little more than 100 μs, proving that our devices can repeat the high-performance photodetection very well. Figure 4a and b present the I d -V d characteristics of WSe 2 -h and WSe 2 -S separately. The curves of both   Fig. 4c. At zero bias, the direction of net photocurrents originated from the Schottky junctions is opposite to that in the homojunction, and the experiment result shown in Fig.  3a is consistent with the latter. Therefore, the homojunction formed between WSe 2 -h and WSe 2 -S is the reason for the short-circuit photocurrents.
To further demonstrate that the photoresponse at zero bias is attributed to the homojunction, the output properties were investigated through measuring the I d -V d curves of the device with the source-drain voltage applied on electrodes E1 and E4. As shown in Figure S3a, the curves, same as the situation in Fig. 3a, also exhibit the PV and PC characteristics. As discussed above, for the former, the photocurrents are attributed to the builtin field of in-plane homojunction formed between WSe 2 -S and WSe 2 -h. For the latter, the photocurrents are attributed to the collection of photoexcited carriers by the externally applied V d . The responsivities as a function of the light power for different V d are summarized in Figure S3b. The responsivities (detectivities) of 0.51 A W −1 (2.21 × 10 12 jones) and 3.55 A W −1 (5.54 × 10 12 jones) are obtained for V d of 0 V and 1 V, respectively. During the calculation, the effective photosensitive area, i.e., the WSe 2 part between E1 and E4, is 519.4 μm 2 . The response time measured at zero bias is shown in Figure S3c and 3d, in which the rising time is 289 μs and the falling time is 281 μs. For the V d of 1 V ( Figure S3e and 3f), the rising and falling time are 278 μs and 250 μs, respectively. The response speed is a little slower than that measured between electrodes E2 and E3, because the relatively long conductive channel increases the photocarrier transmission distance and the probability for the interaction between photocarriers and defects.

Conclusion
In summary, we have demonstrated an in-plane WSe 2 homojunction by electrically tuning partial WSe 2 flake through interface gate. Compared with existing approaches like chemical doping and electrostatic gating by taking advantage of two gate biases, this design gives a more facile rout to realize WSe 2 homojunction. With light illumination, the device produces distinct shortcircuit photocurrents with a detectivity of 3.3 × 10 12 jones. At high bias, the device presents photoconductive characteristic and generates photocurrents with a detectivity of 1.78 × 10 11 jones. A response time as fast as 106 μs is also obtained simultaneously. Our study provides an efficient and reliable way for the development of high-performance WSe 2 -based photodetectors.

Methods
Both WSe 2 and h-BN bulk materials were purchased from Shanghai Onway Technology Co., Ltd. First, the h-BN and WSe 2 flakes were mechanically exfoliated onto a p + -Si/SiO 2 (300 nm) substrate and a poly-dimethyl siloxane (PDMS) layer, respectively. Then, a micromanipulator was used to put the WSe 2 flake, which is adhered to PDMS, onto the target h-BN flake through the microscope to locate the position. Part of WSe 2 flake overlaps the h-BN flake. Finally, the WSe 2 flake was released from PDMS through heating the substrate. The electrodes (Ti/Au) were prepared by electron-beam lithography, metallization, and the lift-off process. Photoresponse measurements were conducted using Agilent B1500
Additional file 1: Figure S1. Temporal response of the device acquired at V d = 1 V for 637 nm illumination. Figure S2. Photoresponse of the other three devices under 637 nm illumination. Figure S3 Photoresponse performance of the homojunction acquired between E1 and E4.