Two-dimensional （2D） materials，such as graphene，BP，and transition metal dichalcogenides （TMDCs），the research of 2D materials has become a top priority in the field of optoelectronics due to their excellent optical and electrical properties^［1］. TMDCs materials have high mobility and high On/Off ratios because they can achieve large modulation through the gate field effect^［2］. These characteristics enable them to be widely used in photodetectors. However，most photodetectors based on TMDCs，such as those based on MoTe₂ and WSe₂，can only operate in the visible region due to their relatively large bandgap^［3-5］. Therefore，it is significant to explore and manufacture photodetectors with a broader spectrum，higher sensitivity and high responsivity using two-dimensional materials.

2D van der Waals （vdW） heterojunctions offer an ideal platform to overcome the limitations of single materials and enhance device performance. In vdW heterojunctions，the individual components are stacked using weak vdW forces between the layers^［6-7］，effectively avoiding the limitations of lattice matching and other factors in traditional heterojunctions. In recent years，the bandgap of MoTe₂ materials has been found to range from 0.83 eV for bulk materials to 1.1 eV for monolayers^［8-9］，which is narrower than that of other commonly used TMDCs materials such as MoS₂ and ReS₂^［10］. Broad-spectrum photodetectors （600-1550 nm） based on MoTe₂ materials have been successfully demonstrated. L. Yin^［11］ reported a multilayer MoTe₂ device with high responsivity to visible light at large back-gate voltages. However，the device's detectivity may be severely degraded due to its high dark current. Previous studies have utilized various materials，such as MoS₂^［12］，graphene^［13］，and Ge^［14］，to form heterojunction photodetectors with MoTe₂. The high carrier mobility of MoTe₂ enables the photodetectors to have faster response times^［15］. In addition，indium selenide （InSe） has recently gained attention in optoelectronics and nanoelectronics due to its high electron mobility and broadband optical absorption. The InSe-based photodetectors exhibit outstanding performance in broadband photodetection （400-1000 nm） and fast response times，as low as 87 µs^［16-18］. This combination of InSe and MoTe₂ advantages enhances the overall performance of the photodetector. Currently，there is limited research on the photodetection capability of InSe/MoTe₂ heterojunctions. Sun el.^［19］ proposed an InSe/MoTe₂ heterojunction photodetector for photodetection under two types of laser irradiation，405 nm and 635 nm. The photodetector achieved high detectivity，but the maximum responsivity was limited to 15.4 mA/W.

In this paper，a photodetector based on InSe/MoTe₂ vertical heterojunction is fabricated，in which MoTe₂ serves as the transmission layer and InSe serves as the grating layer to regulate the channel current. The photodetector exhibits excellent photodetection performance due to its vertical structure and high-quality interface. The detectivity of the photodetector shows an ultrahigh value of over 1.05×10¹³ Jones，surpassing that of other reported photodetectors based on 2D materials^{［12-14，20-25］}. Ultraviolet （100-400 nm） photodetectors have attracted extensive attention in many fields，such as space exploration，biological analysis，environmental sensors，communication，and imaging^［26］. In addition，the photodetectors reported in this paper have photoresponses ranging from ultraviolet （365 nm） to near-infrared （965 nm）. The photodetector exhibits an ultra-high external quantum efficiency （EQE） of 1.03 × 10⁵ %，resulting in extremely high photoelectric conversion. By modulating the gate voltage，the responsivity can reach 300.57 A/W. The heterojunction photodetector also exhibits outstanding detection performances with a fast response.

The vertically structured InSe/MoTe₂ heterojunction was fabricated using a deterministic dry transferred technique. First，300 nm silicon oxide insulation layer was deposited on a silicon wafer by plasma-enhanced chemical vapor deposition. The BN thin layer was removed by mechanical stripping and placed on the SiO₂/Si substrate to provide a clean and flat interface. Then MoTe₂ nanoflakes were mechanically exfoliated from the bulk crystals to the polydimethylsiloxane （PDMS） films and transferred onto the SiO₂/Si substrate. Next，several layers of mechanical peeling InSe flakes were artificially stacked on the MoTe₂ flakes under the optical microscope （OM，BX51，OLMPUS） assisted by an aligned transfer system. The two-dimensional materials mentioned in the text are commercially available bulk crystals. Finally，multiple electrode patterns were defined by standard electron beam lithography （EBL，Raith eLine Plus），then Ti/Au （10 nm/60 nm） metal stacks were deposited by electron beam evaporation （Ulvac Ei-5z） to form source and drain electrodes. The thickness of the photodetector was determined by AFM （Dimension ICON，American Bruker）. Raman spectra were carried out using a Raman spectrometer system （Raman，LABRAM HR，Japan Horriba-JY） with a 532 nm laser source. The atomic structure features of the heterojunction were examined using HRTEM （Talos）. The composition and element distribution of the heterojunction were analyzed via EDS mapping on the HRTEM. Before the HRTEM test，the photodetector’s surface was coated with a conductive layer of elemental Cr to facilitate the positioning of the cut sample and the deposition of the protective layer under the focused ion beam （FIB） microscope. The electrical transport properties of the photodetector were carried out by Keithley 2612B and 2400 at room temperature.

The vertically stacked heterostructure based on InSe/MoTe₂ is shown in Fig. 1（a）. The Ti/Au electrodes were placed on the MoTe₂. The thicknesses of the MoTe₂ and InSe flakes are 8 nm and 15 nm，respectively，as shown in Fig. 1（b） and （c）. The morphological characteristics of the InSe/MoTe₂ heterojunction are shown in the inset of Fig. 1（b），which displays a flat surface that did not sustain any damage during material peeling and transfer. Fig. 1（d） shows the results of high-resolution transmission electron microscopy （HRTEM）. The interfaces of all layers are clear，flat，and uncontaminated，indicating good interface quality. The thickness of each layer has been verified，and is consistent with the AFM test results. Fig. 1（e） shows the detailed energy-dispersive X-ray spectroscopy （EDS） elemental mapping，which demonstrates uniform distribution of all elements in the MoTe₂ and InSe layers without diffusion. Raman spectra of individual materials and the overlapped region are displayed in Fig. 1（f）. Specifically，for pristine InSe （red line），four prominent peaks are centered at 116 cm^-1，178 cm^-1，200 cm^-1 and 227 cm^-1，corresponding to A₁′，E′′（TO），E′′（LO），and A₁ modes^［27］. The Raman signatures of MoTe₂ （green line） are typically observed at 232 cm^-1 （E¹_2g），171 cm^-1 （A_1g），and 288 cm^-1 （B¹_2g）^［28］. These peaks were also observed in the spectra of the overlapped region，indicating good quality of thin flakes in the junction region after layer exfoliation and device fabrication.

The electrical properties of the InSe/MoTe₂ heterojunction photodetector were tested under dark conditions. Fig.2 shows the electrical characteristic curve of the InSe/MoTe₂ heterojunction photodetector with an increase in the back gate voltage from -60 to 60 V. The output characteristic curve （I_ds-V_ds） of the InSe/MoTe₂ heterojunction photodetector is shown in Fig.2（a）. As the current increases and the temperature rises，the resistance of the device also increases^{［27，29］}，resulting in nonlinearity in the output characteristics. Fig.2（b） shows the transfer characteristic curve （I_ds-V_g） of the InSe/MoTe₂ heterojunction device in the dark state. The current of the device initially decreases and then increases with the gate voltage changing，indicating that the device exhibits bipolar behavior.

To investigate the optoelectronic performance of InSe/MoTe₂ heterojunction photodetectors under the illumination，we measured the I_ds-V_ds curves （Fig. 3（b）） and the I_ds-V_g curves （Fig. 3（c）） of InSe/MoTe₂ heterojunction devices at different incident optical powers density under 365nm light source （V_g=0 V）. Fig. 3（a） shows a schematic diagram of the device under laser irradiation. The output I-V curve in Fig. 3（b） demonstrates that the current in the channel increases as the incident optical power density increases. This indicates that more photogenerated carriers are produced in the channel with an increase in incident optical power density. Additionally，the photocurrent I_ph （I_ph=I_illumination-I_dark） is positively correlated with V_ds （where I_illumination and I_dark are I_ds with and without illumination）. Fig. 3（c） shows the I_ds-V_g curves. The current in the channel is positively correlated with the incident optical power density. The I_ds of the device increases significantly under laser irradiation，indicating that the photocurrent always dominates throughout the operating range of the device. In the conducting state （when V_g>V_th），the built-in field of the heterojunction increases as the Fermi level of MoTe₂ shifts in the conduction band due to the accumulation of electrons，leading to more efficient electron-hole pairs separation and an increase in the optical response^［30］. The energy band diagram of the electrical transport mechanism of the device is shown in Figure S1 （Supporting Information）.

The non-linear relationship between the photocurrent and optical power density can be well fitted with the power law equation^［31］：

where a is a constant for a certain wavelength，P is the incident power density and the exponent θ_{determines the photoelectric conversion efficiency. Fig. 3（d） shows the relationship between photocurrent and different incident power density at Vds=6V. As the incident power density increases，the photocurrent in the channel also increases. The photocurrent data can be well fitted with incident power density，and the obtained θ value of 0.93 （<1） verifies the presence of the photogating effect^［32］. Demonstrating the presence of carrier trapping in the top InSe layer to photocurrent as a grating layer，inducing more electron production in the channel to further modulate the channel conductance.}

To evaluation the performance of the heterojunction photodetector，the responsivity （R），detectivity （D^*），external quantum efficiency （EQE），and response time （τ） as evaluation metrics，which are be defined by the following equations：

where Pin is the incident optical power density，A is the effective illumination area，c is the speed of light，λ is the incident light wavelength and h is Planck's constant.

Fig. 4（a） shows the relationship between responsivity and gate voltage under different optical power density when V_ds=6 V. The responsivity decreases with increasing incident light power，which is consistent with previous studies^［32］. It’s mainly due to the enhancement of carrier scattering and recombination rates under higher incident light power densities. The InSe/MoTe₂ heterojunction photodetectors exhibit a responsivity of 300.57 A/W when V_g=-80 V and P_in=1.269 mW/cm². Fig. 4（b） shows the relationship between detectivity and incident light power density. Detectivity decreases as incident light power density increases. The highest detectivity of 1.05 × 10¹³ Jones was obtained at V_ds = 6 V，V_g = -15 V，and P_in = 1.269 mW/cm². The high detectivity can be mainly attributed to the arrangement of the heterojunction energy bands，which allows the electrons in MoTe₂ to naturally flow into InSe，reducing the I_dark. Fig. 4（c） shows the variation of EQE with respect to the gate voltage under different power irradiation. The EQE of the photodetector reaches a maximum of 1.03×10⁵% at the lowest incident power density of 1.269 mW/cm². The results demonstrate a strong photoelectric conversion capability，as evidenced by the high EQE. Fig. 4（d） shows the dependence of NEP on the incident power density at V_ds=6 V and V_g = -15 V. The achieved NEP value is 4.75 × 10^-17 WHz^-1/2. The photoswitching characteristics at different incident powers are shown in Fig. 4（e）. Under 365 nm light，the photocurrent rapidly increases and stabilises at a high value. When the light source is switched off，the photocurrent rapidly disappears. The device remains stable and variable，even after several tests，demonstrating the excellent stability and reliability of this heterojunction photodetector. This paper presents the switch characteristic curve with an optical power of 16.75 mW/cm² （Fig. 4（f））. As shown in Fig.4（f），the response time （rise and fall） of the InSe/MoTe₂ heterojunction photodetector is 21.6 ms. This response speed is much improved compared to other reported heterojunction photodetectors based on grating effect^{［13，23，34-36］}.

The band gaps of MoTe₂ and InSe materials can be adjusted depending on their thickness，which extends the light detection range of the InSe/MoTe₂ heterojunction photodetector to the near-infrared. Fig. 5 shows the output characteristic curves （V_g=0 V），and the transfer characteristic curves （V_ds=6 V） of the InSe/MoTe₂ heterojunction photodetector under different incident light wavelengths. The output I-V curves indicate that the photocurrent of the heterojunction photodetector increases as the drain voltage increase，The maximum photocurrent is achieved under 365 nm light irradiation.

The optical switching characteristics of InSe/MoTe₂ heterojunction photodetectors under different laser wavelengths are shown in Fig. 6（a）. Under 365-965 nm light irradiation，the current sharply increases and remains at a high value. When the light source is switched off，the current rapidly decreases and the test results remain stable during repeated operation （see Figure S2 in the Supporting Information）. It demonstrates that the heterojunction photodetector maintains excellent stability and reliability in the 365-965 nm wavelength range. The dependence of responsivity on gate voltage at different wavelengths is shown in Fig.6（b）. The device achieves its highest responsivity value at 365 nm wavelength laser within the range of -80 V to 80 V with gate voltage modulation. And the maximum value of 300.57 A/W obtained at V_g=-80 V，indicating that the InSe/MoTe₂ heterojunction produces the most photogenerated electron-hole pairs under 365 nm wavelength light. Fig. 6（c） shows the dependence of D^* and EQE on different incident different light wavelengths at V_ds=6 V and V_g = 0 V. The detectivity at 365 nm and 965 nm are 1.25 × 10¹³ Jones and 1.12×10¹¹ Jones，respectively. The photodetector based on 2D materials has a higher result than most reported broad-spectrum photodetectors^{［13，35-36］}. The EQE curve has a similar dependency to the detectivity curve，with a maximum EQE value of 1.03 × 10⁵% at 365 nm incident wavelength，which exceeds most of the reported photodetectors^［38-39］，demonstrating excellent photo conversion capability. Fig. 6（d） shows the NEP as a function of wavelength at V_ds = 6 V，demonstrating an ultralow noise equivalent power.

In summary，the vertically stacked InSe/MoTe₂ heterojunction photodetector has been fabricated and systematically investigated，where InSe serves as the grating layer to regulate the channel photocurrent through localized or released holes. In terms of photoelectric performance，the InSe/MoTe₂ heterojunction photodetector has a fast response time of 21.6 ms at 365 nm，and by modulating the gate voltage and incident optical power，it can achieve a response rate of 300.57 A/W，a maximum detectivity of 1.05 × 10¹³ Jones，and an external quantum efficiency value of 1.03 × 10⁵%. The photodetector has excellent performance with broadband photodetection from 365 to 965 nm. Under the irradiation of 965 nm laser，the detectivity can reach of 8.99×10⁹ Jones. Our results open a way to improve photoresponsivity and reduce response time in high performance 2D optoelectronic devices.