Photodetectors play an important role in many fields such as remote sensing，reconnaissance，thermal imaging，and medical imaging. Narrow-spectrum photodetectors are unable to meet the increasingly complex needs of photodetection. The two-dimensional（2D）materials have received a lot of attention because of their unique structural，electrical and optical properties since the successful exfoliation of graphene in 2004^［1-4］. Up to now，most of the reported photodetectors based on 2D materials work in a narrow spectral band^［5］，and there are relatively few reports on broadband photodetectors，which affect the development of 2D material photodetectors. In recent years，InSe has been widely reported for its wide adjustable band gap ranging from 1.25 eV for bulk materials to 2.2 eV for monolayer materials^［6］. InSe-based photodetectors are very suitable for the detection in the spectral range of 400-1 000 nm^［7，8］，and InSe has high carrier mobility and small effective mass of electrons^［9］，all of which indicate that InSe is a promising candidate material for broadband response. However，the reported broadband InSe-based photodetectors have shown relatively low responsivity. For example，the WS₂/InSe heterojunction photodetector has a responsivity of 61 mA/W under 520 nm illumination^［10］. The SnSe/InSe heterojunction photodetector has a responsivity of 350 mA/W under 808 nm illumination^［11］. SnS₂ is an environment-friendly material with high carrier mobility，high switching ratio，and strong optical absorption，which makes it very suitable for photoelectric devices^{［12，13］}. However，the drawbacks of narrow spectrum response range of SnS₂ and the easy recombination of photogenerated carriers^［14］ have hindered its development. By combining the advantages of these two materials，it’s promising to construct a broadband.

In this work，a SnS₂/InSe vertical structure photodetector was prepared，in which InSe was used as the photo absorption layer and SnS₂ was used as the transmission layer. By building a Van der Waals（vdW）heterojunction to form an effective type-II energy band alignment structure，the electron-hole pairs can be separated effectively to extend the carrier lifetime and improve the responsivity. The device had excellent responsivity in the 365-965 nm range，whose responsivity reached 813 A/W under 365 nm illumination with gate voltage modulation. Under the same gate voltage，a maximum responsivity of 371 A/W under 965 nm illumination，which was much higher than other reported broadband photodetectors^［15］，was obtained. And the device had a specific detectivity of 3.17×10¹² Jones，a high external quantum efficiency of 1.3×10⁵% and a response time of 27 ms. These results demonstrate the successful preparation of a broadband SnS₂/InSe heterostructure photodetector with high performance.

The SnS₂/InSe heterojunction device was fabricated on SiO₂/ Si substrates using a dry transfer technique. Firstly，few-layer flakes of SnS₂ and InSe were mechanically exfoliated from commercial bulk crystals，and the exfoliated SnS₂ flakes were transferred onto a highly p-doped Si substrate with 300 nm SiO₂，subsequently. The same approach was then adopted to transfer the exfoliated InSe onto SnS₂ with the assistance of an optical microscope（OM，BX51，OLMPUS）. Finally，electrode patterns were prepared by electron-beam lithography system（EBL，Raith eLINE Plus）and then Ti/Au（10 nm/50 nm）metal stacks were deposited by electron beam evaporation（Ulvac Ei-5z）to form source and drain electrodes. Then Raman spectrometer（LABRAM HR，Japan Horriba-JY）and atomic force microscopy（AFM，Dimension ICON，American Bruker）were used to measure characteristic peaks and heights of materials. The atomic structure features，the composition and element distribution of the heterojunction were analyzed by high-resolution transmission electron microscopy（HRTEM，Talos）and energy dispersive X-ray spectroscopy（EDS），respectively.

Figure 1（a）shows the schematic diagram of a SnS₂/InSe heterojunction photodetector. The mechanically exfoliated SnS₂ and InSe were sequentially covered on the SiO₂/Si substrate，and the Ti/Au electrodes were placed on the SnS₂. Figure 1（b）shows the Raman spectrum of the single SnS₂ and InSe as well as the region where the two were stacked to form a heterojunction. The single SnS₂^［16］（blue line）had a typical Raman feature main peak A_1g at 313.4 cm^-1，and the single InSe ^［6］（red line）had four Raman feature peaks at 116 cm^-1，178 cm^-1，200 cm^-1，and 227 cm^-1，corresponding in turn to A₁'，E "（TO），E"（LO），and A₁. All the above peaks were observed in the overlapping region of the SnS₂/InSe heterojunction（black line），indicating the formation of a Van der Waals heterojunction. The thicknesses of the SnS₂ and InSe were measured by AFM，as shown in Figure 1（c）. The thicknesses of SnS₂ and InSe were 12 nm and 10 nm，respectively，and the inset shows the surface topography of the heterojunction. Figure 1（d）shows the surface scanning electron microscope（SEM）image of the device，which had a regular shape and a contamination-free surface. The HRTEM characterized the interface of each layer of the device，as shown in Figure 1（e）. The interface of each layer of the device was clearly discernible and flat. Figure 1（f）shows the energy dispersive x-ray spectroscopy（EDS）of the device. The elements In，Se and S were uniformly distributed and no diffusion. Weak Sn elements signals was detected in the InSe layer，because Se and In are in adjacent positions in the periodic table，and the Sn-Lα peak overlaps with the In-Lβ peak，so the In-Lβ peak is sometimes mistaken for the Sn-Lα peak when detecting Sn element，so that it can be detected in the InSe layer. In fact，the Sn element was only detected in the bottom layer. All the above results indicate the successful fabrication of the high-quality SnS₂/InSe heterojunction.

Photoelectric characteristics of the SnS₂/InSe heterojunction photodetector was tested. Figure 2（a）shows Schematic diagram of the device measure setup. Bias voltages were applied to the electrodes connected to the SnS₂. Gate voltage（V_g）were applied through the highly doped silicon substrate. Figure 2（b）shows the output characteristic curves of the photodetector as the gate voltage varied from -60 V to 60 V under dark conditions. The inset was the output characteristic curves of SnS₂. Source-drain current（I_ds）increased with increasing gate voltage，indicating that the photodetector had effective gate voltage modulation. We thought that the nonlinear output curves of the SnS₂/InSe photodetector is mainly due to the additional barrier of heterojunction^［6］. Figure 2（c）shows the transfer characteristic curves of the photodetector. As the gate voltage changed from -80 V to 80 V，the device switched from the insulating state to the conducting state. Figure 2（d）shows a plot of the logarithmic curves of I_ds versus V_g when source-drain voltage（V_ds）was 5 V，which characterized the switching ratio of the photodetector，and the switching ratio could reach 10⁵，which indicated the device had good current regulation capability. Figure 2（e）shows the output characteristic curves at different incident optical power densities under 365 nm illumination when the gate voltage was 0 V. I_ds increased as the incident optical power density increased. Because with the increase of incident optical power density，more photogenerated carriers are generated in the channel，which lead to I_ds increase. To examine the gate voltage modulation capability of the device more intuitively，we tested the transfer characteristic curves at V_ds=5 V for different incident optical power densities. As shown in Figure 2（f），I_ds increased with increasing gate voltage，indicating that the gate voltage could effectively modulate the channel current，and a large gate voltage drive more photogenerated carriers through the heterojunction. In addition，I_ds increased with larger incident optical power density at the same gate voltage. Therefore，the large I_ds current was a result of the combined modulation of the gate voltage and the incident optical power density.

To characterize the detection performance of the SnS₂/InSe heterojunction photodetector under 365 nm illumination，the responsivity（R），specific detectivity（D*），external quantum efficiency（EQE），and noise equivalent power（NEP）were calculated according to the following equations：

where P_in，A，e， h，c，and λ are the incident optical power density，effective illuminated area，electron charge，Planck's constant，light speed，and incident light wavelength，respectively.

Figure 3（a）shows the photocurrent I_ph-V_g curves of the device. I_ph increased first and then decreased with increasing gate voltage. Figure 3（b）shows the responsivity dependence of the gate voltage under various incident power densities at V_ds = 5 V. The responsivity decreased with the increase of the incident optical power density. The highest responsivity of 813 A/W was obtained for the photodetector at P_in=1.269 mW/cm² and V_g=12.5 V. The high responsivity of the device is due to the large number of photogenerated carriers generated in InSe under illumination，which are attracted to the SnS₂ layer by the gate voltage，thereby increasing the current in SnS₂ and improving the responsivity of the photodetector. Figure 3（c）shows the specific detectivity and noise equivalent power of the photodetector at V_ds=5 V and V_g=0 V. The specific detectivity reached a maximum value of 6.74×10¹² Jones at P_in=1.269 mW/cm² and the noise equivalent power reached a maximum value of 9.1×10^-16 W/Hz^1/2 at P_in=16.75 mW/cm². Figure 3（d）shows the external quantum efficiency of the detector at V_g=12.5 V and V_ds=5 V，reaching a maximum of 2.8×10⁵% at P_in=1.269 mW/cm².

The response time is an important parameter for evaluating the performance of the photodetector. Figure 4（a）showed the optical switching characteristic curve of the SnS₂/InSe heterojunction photodetector under 365 nm illumination. I_ds did not decay significantly after several times of optical switching，which indicated that the device had good stability. Figure 4（b）showed the response time of the detector，where the rise time was about 27 ms and the fall time was about 54 ms.

In addition，the device had a high optical responsivity and sensitivity from UV to NIR. Figure 5（a）showed the optical switching characteristic curves of the device under the incident wavelength of 365-965 nm. I_ds of the device could change stably after several times of optical switching under different wavelength irradiation，which proved that the device had good detection for broadband，and the response time was also stable at about 27 ms. What’s more I_ds was negatively correlated with wavelength，which is due to the fact that shorter wavelength light had higher energy. To verify the reliability of the experiment，we plotted the 2D image of the variation of responsivity with gate voltage at the same light power，as shown in Figure 5（b）. The responsivity could also reach 371 A/W at V_g=12.5 V under 965 nm illumination，which was much higher than other 2D broadband photodetectors^［17-19］. The specific detectivity and noise equivalent power versus different incident light wavelengths were shown in Figure 5（c）. The detectivity of the device were of the order of 10¹² Jones in the spectral range of 365-965 nm，and also 2-3 orders of magnitude higher than other 2D broadband photodetectors^{［10，19，20］}. And the noise equivalent power were as low as 10^-16 W/Hz^1/2. Figure 5（d）showed the external quantum efficiency versus the incident light wavelength，and a photovoltaic conversion capacity of 1.3×10⁵% was also obtained under 965 nm illumination. Therefore，our device had a good optical response performance in the 365-965 nm broadband spectral range.

To compare with other broadband heterojunction photodetectors，table 1 lists the results of other research groups^{［11，21-28］}. According to the comparison and analysis in the table. SnS₂/InSe has excellent photoelectric performance，and it provides a direction for improving the comprehensive performance of the broadband photodetector.

In summary，we have successfully prepared a SnS₂/InSe photodetector. Using the wide band gap of InSe，the photodetector could detect the spectral range from UV to NIR. The device achieved a high responsivity of 813 A/W and 371 A/W under 365 nm and 965 nm illumination，respectively，which was higher than some other reported 2D broadband photodetectors. And the detectivity were the order of 10¹² Jones in the spectral range of 365-965 nm. The photodetector also had an external quantum efficiency of 1.3×10⁵% and a response time of 27 ms under 965 nm illumination. The SnS₂/InSe heterojunction photodetector provides a new way for developing broadband and high responsivity photodetectors.