1.
Introduction

In addition to the intensity and wavelength, light polarization provides more information of the target objects, such as surface roughness, etc^{[1, 2]}. Therefore, polarization-resolved photodetectors have been a clearly defined need for a variety of significant applications, including object imaging, medical diagnosis, the military, and so on^[3]. However, there is still a challenge to advance the development of polarization detection due to the lack of high-performance polarization-sensitive materials. In recent years, two-dimensional (2D) has attracted tremendous attention in the field of electronics and optoelectronics due to their unique physical properties^{[4, 5]}. Black phosphorus (BP), a promising polarized 2D material with ultrahigh mobility and narrower direct bandgap, from 0.34 to 2.1 eV^[6], performs excellent polarization detection due to its intrinsic anisotropic lattice structure^[7]. The polarized-resolved photodetector based on BP has been demonstrated and the polarization extinction ratio of more than 100 under illuminated at 3 μm has been achieved^[8]. Despite the outstanding anisotropic optical properties, a BP-based photodetector still faces many challenges due to its rigorous growing conditions and poor stability in the atmospheric environment^{[9, 10]}.



Recently, both theoretical and experimental results have shown rhenium selenide (ReSe₂) has anisotropic optical properties and strong stability in the atmospheric environment^[11]. ReSe₂ photodetectors with high responsivity in the visible light wavelength at room temperature have also been reported^[12]. Additionally, the noble metal dichalcogenides (XY₂, X = Ni, Pt, Pd, and Y = S, Se) can achieve large-area growth by direct selenylation of noble metal atoms or metal original chemical vapor deposition (MOCVD) technology^{[13, 14]}. However, up to now, there are hardly reports about the high-performance ReSe₂ photodetectors with polarization-resolved photodetection. Benefiting from the weak van der Waals force in the interface, 2D materials can be easily exfoliated from their bulk materials and transferred to the substrate^[15]. Besides, different 2D materials can be vertically stacked to form artificially controllable heterostructures without considering lattice mismatch, due to the free of dangling bond on the material surface^{[16, 17]}. Different from the conventional heterostructure devices grown by molecular beam epitaxy, the layer-by-layer freely stacked structures have provided a platform to study new physical phenomena, such as the superconductivity in double-layer graphene^[18] and the ballistic avalanche effect in InSe/BP heterostructures^[19]. There have been many reports on 2D vdWHs that can be applied for numerous electronic and photoelectrical devices, including p–n junction diodes^[20], memory devices, and photodetectors^[21]. Compared with photoconductive devices, the vdWHs have many advantages, such as small dark current and fast response speed^{[22, 23]} due to the formation of the built-in electric field in 2D materials.



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 $ {A}_{
m{g}}^{1} $ mode^[24]. Likewise, the Raman peaks of pristine ReSe₂ are located at 127, 163, and 177 cm^–1, corresponding to $ {A}_{
m{g}}^{1},, E_{
m{g}}^{1},, E_{
m{g}}^{2} $ modes respectively^[25], which are very consistent with previous reports of ReSe₂. Notably, compared with the individual ReSe₂ and WSe₂ crystal, Raman peaks for the overlapped region of ReSe₂/WSe₂ have no shift, indicating the high quality of the ReSe₂/WSe₂ heterojunction after the dry transfer processes^[26]. The thickness of ReSe₂ and WSe₂ flakes are 12 and 25 nm, measured by atomic force microscope (AFM) measurement, as shown in Fig. 1(d).






onerror="this.onerror=null;this.src='http://www.jos.ac.cn/fileBDTXB/journal/article/jos/2021/3/PIC/20070019-1.jpg'"
class="figure_img" id="Figure1"/>



Download



Larger image


PowerPoint slide






Figure1.
(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 I_ds–V_ds curves under increasing gate voltages (V_g) 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 I_ds–V_ds 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:






onerror="this.onerror=null;this.src='http://www.jos.ac.cn/fileBDTXB/journal/article/jos/2021/3/PIC/20070019-2.jpg'"
class="figure_img" id="Figure2"/>



Download



Larger image


PowerPoint slide






Figure2.
(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) I_ds–V_ds curves of ReSe₂/WSe₂ vdWH FET under the increasing gate voltage from –10 to 40 V. (c) I_ds–V_ds curves of ReSe₂/WSe₂ photodetector under illumination of 520 nm laser. (d) Energy band diagrams of devices in dark and under illumination.






onerror="this.onerror=null;this.src='http://www.jos.ac.cn/fileBDTXB/journal/article/jos/2021/3/PIC/20070019-3.jpg'"
class="figure_img" id="Figure3"/>



Download



Larger image


PowerPoint slide






Figure3.
(Color online) Performence characterization of ReSe₂/WSe₂ vdWH photodetector. (a) Responsivity and detectivity as a function of incident power. (b) Output electrical power P_el versus V_ds. (c) SPCM images of the device. (d) Time-resolved photoresponse of the device.

$$ R =frac{{I}_{ m{ph}}}{{P}_{ m{in}} A},$$

$$ {D}^{*}=frac{R {A}^{frac{1}{2}}}{{(2e {I}_{ m{dark}})}^{frac{1}{2}}}, $$

where $ {I}_{
m{ph}} $ is the net photocurrent, $ {P}_{
m{in}} $ is the incident light power intensity, A is the effective photo-sensitive area, e is the elementary charge, and I_dark is the dark current, respectively. The calculated responsivity and detectivity are 0.28 A/W and 1.1 × 10¹² Jones under the illumination of 520 nm laser. Fig. 3(b) has demonstrated the output electrical power P_el, which is defined as ${P}_{
m{el}}={I}_{
m{ds}} {V}_{
m{ds}}$. It is obvious that the device has a photovoltaic phenomenon under the illumination of 520 nm laser, and the P_el increase with increasing laser power density. There is no doubt that electron-hole pairs generate from both ReSe₂ and WSe₂ layers when the laser illuminates the device. These photogenerated electron-hole pairs would be separated by the built-in electric field on the overlap area and then collected by source and drain electrodes, respectively. The response speed is also a key parameter for photodetectors and defined as the time needed to reach the stable state (the photocurrent increase/decreases from 10% to 90% of the peak). Fig. 3(d) has demonstrated several cycles of photoresponse under periodically on–off 520 nm light. One can find the current periodically rises and decays rapidly. Moreover, we also have extracted the response speed via a signal oscilloscope. Response speed usually includes rise time and decay time. There are and just the reverse. The ReSe₂/WSe₂ FET performs a rise time of 4.7 ms and a decay time of 4.1 ms. The fast response speed can be attributed to the built-in electric field of the heterojunction, which can effectively separate the electron-hole pairs and improve the carriers transit time.

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 $ {r}_{
m{e}} $, which can be defined as $ {r}_{
m{e}}={{R}_{x}}/{{R}_{y}} $, where $ {R}_{x} $ and $ {R}_{y} $ are the measured photocurrent with x-polarized illumination and y-polarized illumination, respectively. Fig. 4(a) shows SPCM images of the device under the illumination at 520 nm laser with an increasing polarized angle (the illumination polarized angle is determined by half-wave plates and polarizers). One can find that the photoresponse closely depends on the polarized angle of the incident light. We further extracted the photocurrent from SPCM images and the relationship between the photocurrent and polarized angle is shown in Fig. 4(b). An extinction ratio of 6 is calculated for the ReSe₂/WSe₂ vdWH device, demonstrating a good polarized detection ability in visible light wavelength.






onerror="this.onerror=null;this.src='http://www.jos.ac.cn/fileBDTXB/journal/article/jos/2021/3/PIC/20070019-4.jpg'"
class="figure_img" id="Figure4"/>



Download



Larger image


PowerPoint slide






Figure4.
(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.