Abstract
1. Introduction
In addition to the intensity and wavelength, light polarization provides more information of the target objects, such as surface roughness, etc[
Recently, both theoretical and experimental results have shown rhenium selenide (ReSe2) has anisotropic optical properties and strong stability in the atmospheric environment[
Here, we report a ReSe2/WSe2 vdWH photodiode for polarization-resolved photodetection. Our devices show an obvious polarization-sensitivity and the detectivity exceeding 1.1 × 1012 Jones at 520 nm. Moreover, these devices perform a high reverse rectification ratio (up to 103). Our research opens opportunities for the exploration of polarization-sensitive detection in novel atomically thin electronic and optoelectronic applications.
2. Methods
2.1. ReSe2/WSe2 fabrication
The few-layer ReSe2 and WSe2 are exfoliated from flake-like single crystals by adhesive tapes. First, WSe2 is transferred onto Si/SiO2 (300 nm) substrate. Then, ReSe2 is vertically stacked on WSe2 via a micro-alignment transfer platform. We note that PDMS acts as a carrier during this process. Subsequently, a PMMA layer is coated on ReSe2/WSe2 heterojunction and the source/drain electrode patterns were defined by EBL (JEOL 6510 with a nano pattern generation system (NPGS) system), thermal evaporation and lift-off processes.
2.2. Electrical and photoelectric characterization
Optical microscopy images of devices are obtained by an optical microscope (BX51, Olympus). Raman spectra were measured by a confocal Raman/photoluminescence (PL) system (LabRAM HR800) equipped with a 532 nm laser. The thickness of the few-layer ReSe2 and WSe2 was measured by the atomic force microscope (AFM, Bruker Multimode 8). Electrical and photoelectrical properties of the fabricated devices were measured in a probe station (Lake Shore CPX-VF) combined with a semiconductor device parameter analyzer (Agilent, B1500). The polarization-resolved photoresponse measurements were performed by a home-made optical system where an oscilloscope (DPO 5204 Tektronix) was used to monitor the time dependence of the current, half-wave plats and polarizers are used to adjust the polarized states of the 520 nm laser. All the devices were measured at room temperature and in ambient conditions.
3. Results and discussion
3.1. Vertical stacked heterostructure ReSe2/WSe2 photodiodes design
Fig. 1(a) shows an optical microscopy image of the ReSe2/WSe2 vdWH photodetector (the schematic diagram is displayed in Fig. 1(b)), with ~ 25 nm WSe2 in thickness, ~ 12 nm ReSe2, and ~ 150 μm2 in the overlapped area of WSe2 and ReSe2. To fabricate such devices, we firstly mechanically exfoliated the WSe2 and ReSe2 flakes and physically transferred them onto a Si/SiO2 (300 nm) substrate via adhesive tapes, respectively. Subsequently, the ReSe2 flake is transferred onto the surface of WSe2, employing the polydimethylsiloxane (PDMS) in a micro-alignment transfer platform. Finally, a series of standard fabrication processes, including electron beam lithography (EBL), thermal evaporation, and lift-off, are applied to defined source/drain electrodes (More details can be found in the Methods section). Note that all electrodes are individually deposited on ReSe2 or WSe2 regions, rather than the overlapped regions. Raman spectra show that the main peak of pristine WSe2 is located at 247 cm–1, corresponding to
Figure 1.(Color online) Schematic diagram of ReSe2/WSe2 photodetector. (a) Optical microscopy image of a ReSe2/WSe2 vdWH device. (b) Schematic diagram of the ReSe2/WSe2 vdWH device. (c) Raman spectra of the WSe2, ReSe2, and ReSe2/WSe2 vdWH. (d) Thickness of individual ReSe2 and WSe2 layers.
3.2. Electrical and photoelectrical characterizations of ReSe2/WSe2 devices
The electrical properties of individual ReSe2 and WSe2 field-effect-transistors (FET) are characterized, as shown in Fig. 2(a). ReSe2 device exhibits a typical n-type conductive behavior, while the WSe2 device exhibits an obvious ambipolar conductive behavior (the inset in Fig. 2(a)), which is very consistent with previous reports. In Fig. 2(b), semi-logarithm Ids–Vds curves under increasing gate voltages (Vg) indicated the good rectification characteristic of ReSe2/WSe2 vdWH photodiodes. It is worth noting that the rectification ratio reaches 1 × 103 when a gate voltage of 30 V is applied. To further investigate the photoelectric properties of devices, we measured the Ids–Vds curves of ReSe2/WSe2 FETs in both dark and illuminated conditions, as shown in Fig. 2(c) (Devices were working at zero gate voltage unless additional specified). The schematics of the working mechanism are demonstrated in Fig. 2(d). The bandgap of ReSe2 and WSe2 are 1.5 and 1.2 eV, forming a type-II band alignment. Due to the disparity of work functions between ReSe2 and WSe2, a big built-in electric field are generated in the heterojunction interference. Under the illumination of the 520 nm laser, photogenerated electron-hole pairs can be quickly separated by the built-in field and collected by an external electric circuit. Therefore, the photocurrent mainly generated from the ReSe2/WSe2 overlapped area (heterojunction region). To verify the origin of the photoresponse, we further carried out the scanning photocurrent mapping (SPCM) measurements, as indicated in Fig. 3(c). One can find that the photocurrent is mainly contributed by the heterojunction region of ReSe2 and WSe2, which indicates that the origin of photoresponse is the photovoltaic effect of the heterojunction. To quantitatively evaluate the performance of ReSe2/WSe2 vdWH photodetector, we have calculated the responsivity (R) and detectivity (D*) by the following equations:
Figure 2.(Color online) Electrical and photoelectric properties of ReSe2/WSe2 vdWH FET. (a) Transfer characteristic curves of the individual ReSe2 FET (the inset is the WSe2 FET). (b)
Figure 3.(Color online) Performence characterization of ReSe2/WSe2 vdWH photodetector. (a) Responsivity and detectivity as a function of incident power. (b) Output electrical power
where
3.3. Polarization-resolve properties of ReSe2/WSe2 photodiodes
Next, we evaluate the anisotropic optical properties of the ReSe2/WSe2 vdWH device to examine its potential applications in polarization-resolved photodetectors. The performance of this device can be quantified by an extinction ratio
Figure 4.(Color online) Polarized properties of ReSe2/WSe2 device. (a) SPCM images of device under 520 nm light with different polarization directions (marked by orange arrows). (b) Corresponding photocurrent as a function of polarization angle.
4. Conclusion
In summary, we have successfully designed and fabricated ReSe2/WSe2 FET that can function as a polarized photodetector at room temperature. By designing the artificially vertical stacked ReSe2/WSe2 heterojunction, our device exhibits a high responsivity of 0.28 A/W and a high detectivity of 1.1 × 1012 Jones under the illumination of 520 nm laser at room temperature. More importantly, the SPCM results of the ReSe2/WSe2 photodiodes under different polarized angle illumination are investigated for the first time, showing the photoresponse has a closely dependent on the polarized state of light. We note that polarized photoresponse of all devices is measured at zero bias voltage and zero gate voltage, the low-power dissipation technology is also essential for future electronics. This work paves the way to develop polarization-resolve photodetectors based on van der Walls vertical stacked 2D materials.
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