Two dimensional (2D) van der Waals (vdWs) crystals, such as graphene^[1], black phosphorus (BP)^[2], silicene^[3], group-IV monochalcogenides^[4] and transition metal dichalcogenides (TMDCs)^[5], are attractive for use in various field, such as electronics and optoelectronics^[6–8], valleytronics^[9], flexible devices^[10], memory devices^[11], sensors^[12], due to their unique electronic, mechanical, thermal, optical properties. The vdWs crystals have atomically flat thickness and clean interfaces that are free of dangling bonds due to relatively weak van der Waals interactions among layers. This offers opportunities to stack different individual 2D materials into vdWs heterostructures without the limitations of lattice mismatch. The vdWs heterostructures have opened a new platform for fundamental research and device applications^[13]. Broadband photodetector have great potential for optoelectronic applications, such as optical communication, imaging, night vision, environmental monitoring, and remote sensing^[14–18]. Polarization properties is another degree of freedom for light that is largely uncorrelated with spectral and intensity images, and thus has the ability to carry more information^[16]. Therefore, the broadband polarization sensitive photodetector based on vdWs structure will promote the development of the next generation photodetectors.

Black phosphorus is a layered material, where each phosphorus atom is bonded covalently to three neighboring atoms forming a puckered orthorhombic lattice. The BP is a direct bandgap semiconductor. The band gap of BP is dependent on the number of layers, which increases from ~0.3 eV for bulk to ~2 eV for monolayer limit^[19]. The ultrahigh carrier mobility of ~1000 cm²V⁻¹s⁻¹ was obtained in BP field effect transistor (FET) for a thickness of ~10 nm^[2]. BP also has high in-plane anisotropy properties due to its different bond angles and lengths along the direction of armchair (AC) and zigzag (ZZ)^{[20, 21]}. The hole mobility of BP can theoretically reach 10 000 to 26 000 cm²V⁻¹s⁻¹ for the monolayer (ZZ direction)^[22]. Polarization sensitive broadband photodetectors based on BP FET have been demonstrated, which indicating BP is a promising candidate for broadband polarization sensitive photodetection^[23].

Rhenium disulphide (ReS₂) as a member of TMDCs also have been extensively investigated^{[24, 25]}. ReS₂ is a semiconductor with a single direct band gap of 1.55 eV that is nearly independent of the number of atomic layers^{[26, 27]}. It exhibits a strong in-plane anisotropy in nature due to its stable Peierls distorted octahedral (1T) phase where the buckled S layers and zigzag Re chains along one of the lattice vectors in the plane^{[26, 28]}. According to the reported ReS₂ photodetectors, it shows an n-type behavior with the electron mobility of ~40 cm²V^-1s^-1, and a photoresponsivity of 10³ A/W was achieved at low intensity of light, which implies ReS₂ has a potential for sensitive photodetection^{[29, 30]}.

Recently, the reported BP-on-WSe₂ heterostructure photodetector exhibits a broadband photoresponsivity in a spectral range of 400–1550 nm^[19]. A similar photodetectors based on BP/ReS₂ p–n diode were reported, which shows the photoresponse properties can be modified by adjusting the back gate voltage, however a high photoresponsivity achieved only in the ultraviolet (UV) band^[31]. Another photodetector based on BP/ReS₂ heterojunctions were stacked by controlling their crystal orientation, which exhibits a stronger polarization-dependent photocurrent characteristic in the visible region^[32]. In this work, we systematically studied the variation of photoresponsivity and the polarization sensitive photocurrent anisotropy ratio of the BP/ReS₂ heterojunction devices with the incident light power and wavelength. The devices realize broadband photoresponse from visible to near infrared (NIR) (400–1800 nm) with stable and repeatable photoswitch characteristics, and the photoresponsivities reaches 1.8 mA/W at 1550 nm. In addition, the polarization sensitive detection from the visible to NIR spectrum (532–1750 nm) was demonstrated, which shows a highly polarization sensitive photocurrent with an anisotropy ratio as high as 2.98 at 532 nm, 3.39 at 650 nm, 6.44 at 1064 nm, 5.5 at 1550 nm, and 4.3 at 1750 nm. Our study shows that the vdWs heterojunctions is an effective way to realize the broadband and polarized photodetection.

Fabrication of the BP/ReS₂ heterojunction-based polarized photodetector devices. First, a BP (from HQ graphene) flake was exfoliated onto polydimethylsiloxane (PDMS) stamps by adhesive tape. The stamps are adhered to a glass slide. Under optical microscope, the BP flake with appropriate thickness and shape can be chosen. Afterwards, the target BP flake was transferred onto a 300 nm thick SiO₂/Si substrate by using a site-controllable dry transfer method^[33]. Then, using the same method, a ReS₂ (from HQ graphene) flake was transferred onto the BP flake to fabricate a 2D heterojunction. Notably, the whole transfer processes were performed in a glove box to ensure a clean interface. Finally, the source and drain electrode regions were patterned by standard electron beam lithography, and Ta/Au (5/45 nm) layers were deposited on an ultrahigh vacuum evaporative sputtering system, followed by a lift-off process. The lift-off process was also performed in a glove box to avoid BP degradation. The thickness of both BP and ReS₂ flakes were measured by atomic force microscopy (AFM) (Bruker Multimode 8). The device was stored in a small vacuum chamber with 2 × 10^–5 mbar during the whole electric and optoelectric measurement processes.

Electrical and optoelectrical properties were measured using a semiconductor characterization system (Agilent Technology B1500A) and a lock-in amplifier (SR830) with a light chopper. The visible to NIR light illumination was provided by a Fianium WhiteLase supercontinuum laser source. The output light was tuned into monochromatic light by an Omni-λ300 monochromator. For the spatially-dependent photocurrent measurement, a microscope objective lens (50×), a lock-in amplifier (SR830) with a light chopper and a micromechanical stage were combined. Linearly polarized light was obtained by making a laser beam pass through a Glan-Taylor prism (polarizer) and a half-wave plate (HWP). The polarization direction of the light was controlled by rotating the HWP while keeping the incident laser power a constant.

Calculation details of the FET device mobility: The field-effect mobility can be calculated using the equation μ = (L/W)(d/ε₀ε_rV_ds)(∆I_ds/∆V_bg), where L and W are the length and width of the channel respectively, d is the oxide thickness, ε₀ and ε_r are the permittivity of free space and the relative permittivity of the SiO₂, and the last term is the slope of the transfer curve in the linear regime of the ON state.

Characteristic of the BP/ReS₂ heterojunction device. Fig. 1(a) presents a schematic of our heterojunction device. All of the devices were prepared by mechanical exfoliation and site-controllable dry transfer method. The detailed fabrication processes has been provided in the experimental section. An AFM image of the p-BP/n-ReS₂ heterojunction is shown in Fig. 1(b), with the height profile along the red solid line being shown in the inset, indicating the thicknesses of BP (~10 nm) and ReS₂ (~12 nm). We first performed the electrical characterizations of the ReS₂ FET and BP FET at room temperature. Fig. 1(c) shows the transfer curve of the ReS₂ FET with a linear plot of the source-drain current (I_ds) versus back gate voltage (V_bg) at a fixed source–drain voltage (V_ds = 0.5 V). The device shows an n-type FET behaviors with electron mobility of ~29 cm²V^–1s^–1 (see the experimental section for the calculation details), which is consistent with the reported value^[30]. Optical microscope image of the ReS₂ FET covered by ~10 nm thick BN is shown in the top inset of Fig. 1(c). The thickness of the ReS₂ is about 6 nm. The I_ds–V_ds curves under different V_bg for the ReS₂ FET have been presented in the bottom inset of Fig. 1(c). The linear I_ds–V_ds curves under low drain voltage indicates negligible Schottky barrier at the ReS₂ and Au interfaces. Fig. 1(d) shows I_ds as a function of V_bg for BP FET at room temperature and V_ds = 0.5 V. This device showed a p-type FET behavior with hole mobility ~ 166 cm²V^–1s^–1. The top inset of Fig. 1(d) shows the optical microscope image of the BP FET covered by ~15 nm thick BN. The thickness of the BP is about 10 nm. The I_ds–V_ds curve of the BP FET is shown in the bottom inset of Fig. 1(d). The near-linear I_ds–V_ds curve shows a near ohmic contact between BP and Au.

In addition to the conductive type of the materials, the energy band structure of the materials is also very important for construction of a p–n junction. The energy band diagrams of the heterojunction in the isolated and different bias states are illustrated in Figs. 2(a)–2(d). Based on previous reports, the electron affinity and bandgap (E_g) of multilayer BP are around 4.2 and 0.4 eV, respectively^{[20, 22, 34]}. For multilayer ReS₂, the corresponding values are about 4.4 and 1.55 eV^{[26, 32]}. Therefore, the BP/ReS₂ heterojunction has a type-II band alignment because the conduction band minimum (E_C) of BP lies above that of ReS₂ by ∆E_C = 0.2 eV, whereas the valence band maximum (E_V) of ReS₂ lies below (∆E_V = 1.35 eV) that of BP. In addition, the valence band edge of BP lies below that of the conduction band edge of ReS₂ by only 0.2 eV, which means the BP/ReS₂ heterojunction can be easily electrostatic tuned to a broken-gap heterojunction with type-III band alignment, which can be used to explore the TFETs^{[35, 36]}. Fig. 2(a) presents the band diagram for isolated few layers p-BP and n-ReS₂. The built-in potential is formed when the two-layered materials are stacked together, which is indicated by blue arrow in Fig. 2(b). At V_ds = 0 V, the minority carrier (electron) in BP crosses the small barrier and forms a small reverse current. As shown in Fig. 2(c), under positive bias, the electrons (majority carriers in ReS₂) will transport to BP by overcoming the barrier height ∆E₁. Meanwhile, the holes (majority carriers in BP) will travel to ReS₂ by overcoming the barrier height ∆E₂. Due to the larger ∆E₂ compare to ∆E₁, the current is dominated by the over-barrier free-electron transport (blue arrow). Fig. 2(d) shows the electrical field in the heterojunction is strongly enhanced under reverse bias. The minority carrier (electrons in BP and holes in ReS₂) contributes to the conduction process.

To investigate the responsivity of the device dependence on the illumination intensities, different illumination intensities P ranging from 0 to 500 W/cm² are applied to the heterojunction device. Fig. 3(a) shows the |I_ds|–V_ds curves of the BP/ReS₂ heterojunction device at room temperature in the dark and under illumination with a 1550 nm laser at various excitation intensities (P = 10, 25, 50, 100, 250, 500 W/cm²). The |I_ds|–V_ds curve of the device in the dark exhibits a clear rectifying behavior. The rectification ratio, defined as the ratio of the forward/reverse current, is about 50 at V_ds = +1/–1 V. This behavior can be attributed to the formation of p–n diode within the atomically thin BP/ReS₂ heterojunction (more details see band alignment of the BP/ReS₂ heterojunction). The device exhibits a low dark current (~1.0 nA) at V_ds = –1 V. Under a light illumination, electron-hole pairs are generated and separated by the electric field in the heterojunction, which generate the photocurrents. Photocurrent (I_ph) is defined as the difference between drain currents with and without light-illumination (I_ph = |I_light – I_dark|). Under reverse bias (V_ds < 0 V), I_ph increases with increasing the light intensity P. I_ph can also be enhanced by increasing the reverse bias (shown in Fig. 3(b)). It can be explained by the increased electrical field in the heterojunction under reverse bias (Fig. 2(d)). The increased electrical field decreases the carrier transit time, thus reduces the recombination probability of the photo-generated electrons and holes. In addition, I_ph exhibits a sublinear dependence on the incident light intensity P and follows a power law of I_ph ∝ P^α, where α = 0.61, 0.68, and 0.73 at V_ds = 0, –0.4 and –1 V, respectively. Larger value α can be obtained by applied a larger reverse bias, which also benefits from the shorter carrier transit time. The similar sublinear behavior has also been reported for other similar structures, such as ReS₂ FETs^[29], WSe₂/graphene^[37], MoS₂/WS₂ heterostrctures^[38], and several of our previous works (WS₂/GaSe, InSe/GaSe, WSe₂/GaSe, MoTe₂ photodetectors)^[39–42]. To quantify the photoresponse performance, the photoresponsivity (R = I_ph/PS) of the heterojunction as a function of the illumination intensity at different applied voltages were extracted and shown in Fig. 3(c). The corresponding external quantum efficiency (EQE = hcR/eλ) of the device at different V_ds (V_ds = 0, –0.4, –1 V) were presented in Fig. 3(d). Here S is the in-plane area (~20 µm²) of the device, e is the electron charge, c is the speed of the light and h is Planck constant. All of two important factors increase with decreasing light intensity, consistent with the sublinear behavior of the photocurrent. Our device exhibits a photoresponsivity of R = 1.8 mA/W with corresponding EQE = 0.14% at V_ds = –1 V under the wavelength 1550 nm. The photoresponsivity is comparable to most of similar vdWs heterojunction photodetectors reported recently, such as 4.36 mA/W in MoS₂/WS₂^[43].

The image of the BP/ReS₂ heterojunction device shown in Fig. 4(a), where the boundaries of each layer are indicated by dotted lines with different colors. The corresponding scale bar is 10 µm. Fig. 4(b) shows the normalized photocurrent as a function of the illumination wavelength at V_ds = –1 V and illumination intensity P = 100 W/cm², which demonstrates the BP/ReS₂ heterojunction device exhibits obvious photoresponse over a broad spectral range (400–1800 nm), from the visible to near infrared. The maximum normalized I_ph ~ 1 is obtained at wavelength of 550 nm, and the minimum normalized I_ph ~ 0.013 is obtained at wavelength of 1025 nm. Therefore, the device can be used for broadband photodetection. ReS₂ has the strongest absorption in the ultraviolet and visible band, but weaker absorption in the near infrared bands^[44]. Meanwhile, BP has a natural narrow band gap, which has a strong absorption in visible and near infrared bands. As a result, a strong absorption at 550 nm of the device is induced by the light absorption properties of ReS₂ and BP. Another strong absorption at 1710 nm of the device is dominated by the light absorption properties of BP. According to the reported results, the absorption peak position for a N-layer BP can be described using tight-binding model through the expression E_N,n = 1.8 – 1.46cos(nπ/(N + 1)), where the subband index n = 1…N^[45]. The absorption peaks of 550 nm (2.25 eV) and 1720 nm (0.72 eV) in our device can be explained by n = 13, 5 when L = 20. In order to explore the origin of the photoresponse, scanning photocurrent maps were measured at both zero and negative biases. The corresponding photocurrent maps at zero and negative biases with 1550 nm laser excitation (illumination intensity P = 100 W/cm²) are shown in Figs. 4(c) and 4(d), respectively. The photocurrent map shows that the photoresponse of the overlapping region is strongly enhanced compared with the non-overlapping regions. It also demonstrates the formation of a p–n junction across the area of the BP/ReS₂ interface where the photo-generated electrons and holes are separated and extracted more efficiently.

The excellent stability is an important condition for the application of photodetectors. For investigate the stability of the device, the device was illuminated by a laser with different wavelength (or under different V_ds) and light intensity P = 50 W/cm². Fig. 5(a) presents the time dependences of |I_ds| during incident light switched on/off at V_ds = –1 V with different wavelength (λ = 400, 532, 1064, 1320, 1550 nm), which shows the device can steady working over a broad spectral range, from visible to near infrared. In addition, the photoswitch ratio (I_light/I_dark) of the device is about 23 at 400 nm, 25 at 532 nm, 12 at 1064, 13 at 1320, and 7 at 1550 nm. Source–drain current |I_ds| as a function of time with photoswitching at different V_ds (V_ds = 0, –0.2, –0.4, –0.6, –0.8, –1 V) shown in the Fig. 5(b). A stabilized photoresponsivity ~ 0.4 mA/W was achieved at λ = 1550 nm under zero bias, which shows the device can work as a self-driven near infrared photodetectors. Moreover, after many cycles, the photocurrent still responds in a similar fashion to the light, which exhibits excellent operation reversibility and stability.

To examine the polarization sensitivity of the device, the device was illuminated by linearly polarized light with different polarized angle. The photocurrent as a function of polarized angle under different illumination wavelength is shown in Fig. 6. The optical microscope image of the BP/ReS₂ heterojunction device is exhibited in Fig. 6(a). The angles of zero degree and 90 degree represents the rotation direction of the linearly polarized light. The anisotropic response in |I_ph| plotted in the polar coordination at visible light wavelength of 532 nm (Fig. 6(b)) and 650 nm (Fig. 6(c)). The visible light intensity P = 25 W/cm². Figs. 6(d)–6(f) presents the evolution of the |I_ph| plotted in the polar coordination at near infrared light wavelength of 1064 nm (Fig. 6(d)), 1550 nm (Fig. 6(e)), and 1750 nm (Fig. 6(f)). The near infrared light intensity P = 100 W/cm². The results show the device can act as a polarized photodetector over a broad spectral range (from 532 to 1750 nm). The double symmetry has been observed. The direction of the maximum photocurrent is perpendicular to the direction of the minimum photocurrent. The polarization sensitive absorption of the BP/ReS₂ heterojunction photodetector is induced by crystal structure anisotropy. Because of the intrinsic low carrier mobility and high electron–hole recombination rate in ReS₂^[29], the polarization sensitivity of ReS₂ is relatively low. Thus, the polarization sensitive absorption of the BP/ReS₂ heterojunction device is mainly induced by crystal structure anisotropy of BP. The absorption of armchair-polarized light is always stronger than zigzag-polarized light for BP with all thicknesses, thus the two directions of the maximum photocurrent difference correspond to the orientation of BP crystal^{[45, 46]}. It is worth noting the polarized photocurrent ratio I_ph.max/I_ph.min ≈ 2.98 at 532 nm, 3.39 at 650 nm, 6.44 at 1064 nm, 5.5 at 1550 nm, 4.3 at 1750 nm, which is larger than some other polarized sensitive anisotropy photodetectors, such as GeSe (1.09 at 532 nm)^[47] and BP vertical p-n junction (3.5 at 1200 nm)^[23]. The electrons transition process must meet the transition selection rule of momentum conservation, which can be expressed by the formulas: E_g = ћω and ћk_i ± ћq_phonon = ћk_f, where E_g is the bandgap of the semiconductor, k_i and k_f are the electron wave vectors at the valence band maximum (VBM) and conduction band minimum (CBM), respectively, ω is the angular frequency of the incident photon. In addition, the electric dipole transition probability P(ω) for photon absorption per unit time is introduced by Fermi Golden Rule: P(ω) = 2π/ћ|<φ_f|H_eR|φ_i>|²δ(E(k_f) – E(k_i) – ћω), where ћ is the reduced Plank constant, H_eR is the interaction of electromagnetic excitation. Here the transition occur from the initial states |φ_i> to the finial states |φ_f> with corresponding energy levelE(k_i) and E(k_f). Moreover, the distribution and the overlap of VBM and CBM wave function is different along zigzag and armchair direction in BP, which will result in the difference of the |<φ_f|H_eR|φ_i>|² value^[47] and wavelength dependent polarized photoresponse. In the near infrared region, BP has a large photoresponse to the polarized light along the armchair direction while the photoresponse of polarized light along the zigzag direction is small and changes little^[23]. Thus the large polarized photocurrent ratios are observed in the near infrared region and the maximum polarized photocurrent ratio is 6.44 at 1064 nm.

In conclusion, we have fabricated BP/ReS₂ vdWs heterojunction devices by dry-transfer of 2D flakes method. The devices realize broadband photoresponse from visible to near infrared (NIR) (400–1800 nm) with stable and repeatable photoswitch characteristics, and the photoresponsivities reached 1.8 mA/W at 1550 nm. In addition, the polarization sensitive detection form the visible to NIR spectrum (532–1750 nm) was demonstrated, which shows a highly polarization sensitive photocurrent with an anisotropy ratio as high as 2.98 at 532 nm, 3.39 at 650 nm, 6.44 at 1064 nm, 5.5 at 1550 nm, and 4.3 at 1750 nm. Because of the intrinsic low carrier mobility and high electron–hole recombination rate in ReS₂, the polarization sensitivity of ReS₂ is relatively low. Thus, the polarization sensitive absorption of the BP/ReS₂ heterojunction device is mainly induced by crystal structure anisotropy of BP. Our study shows that van der Waals heterojunction is an effective way to realize the broadband polarization sensitive photodetection, which is of great significance to the application of multi-functional devices based on 2D vdWs heterostructures.

This work was supported by the National Key R&D Program of China (Grant No. 2017YFA0303400 and No.2017YFB0405700). This work was also supported by the NSFC Grant Nos. 61774144 and 11474272. The project was sponsored by Chinese Academy of Sciences, grant No. QYZDY-SSW-JSC020, XDPB12, and XDB28000000.