Two-dimensional (2D) semiconductor materials recently have attracted considerable attention as future semiconductor devices due to their unique and outstanding physical properties [1–4]. Each layer interacts with weak van der Waals force and thus can be easily peeled off with mechanical exfoliation using Scotch tape [5]. Due to the thinness of 2D semiconductors, they exhibit interesting physical properties such as a layer dependent band gap, valley related transport induced by broken inversion symmetry, and giant magnetoresistance. 2D semiconductor materials have high carrier mobility due to their single crystalline structure and have no dangling bonds on the surface, and consequently are advantageous for high integration and hold promise as high-performance electronic devices [6–14]. In addition, owing to excellent interaction with light and the ease of forming heterojunctions, they have significant potential for use as next-generation optoelectronic devices. Representative 2D semiconductor materials include graphene, transition metal chalcogenides (TMDCs), black phosphorus, and Mxene [5,7,15,16]. 2D TMDCs with energy bandgaps in nearinfrared (NIR) regions are attracting attention as candidate materials for NIR photodetection [9,12,17–19].

Here, we fabricated a MoTe₂/SnSe₂ heterostructure photodiode for NIR photodetection. MoTe₂ has a smaller band gap than other TMDCs such as MoS2 and WSe2. The band gap of monolayer MoTe₂ is ~1.1 eV, which is similar to that of silicon. Few layer MoTe₂ meanwhile has a band gap of 0.95 eV, enabling NIR photodetection. SnSe₂ has excellent electrical conductivity and strongly n-doped characteristics. To characterize MoTe₂ and SnSe₂, we fabricated a MoTe₂ field effect transistor (FET) and a SnSe₂ FET. The MoTe₂ FET shows typical p-type characteristics in the transfer curve with a high on/off ratio and mobility of ~43 cm²/Vs. The SnSe₂ FET shows strong n-type characteristics in the transfer curve with a low on/off ratio and mobility of ~3.77 cm²/Vs. The fabricated MoTe₂/SnSe₂ heterojunction diode exhibits a high rectification ratio of 104, an ideality factor of 1.5, and a clear photoresponse under light emitting diode (LED) illumination from 528 to 1,300 nm. The short-circuit current (I_SC) is up to ~400 pA and the open circuit voltage (V_OC) is ~0.1 V. This photoresponse is due to the band alignment of MoTe₂ and SnSe₂. From temperature dependent I−V characteristics, the potential barrier height was extracted as 0.54 eV and this value appears to be reasonable, as demonstrated by the band alignment.

The 2D MoTe₂/SnSe₂ photodiode was fabricated on a glass substrate (Eagle XG). First, 2D MoTe₂ nanoflakes were mechanically exfoliated using a polydimethylsiloxane (PDMS) stamp and then transferred onto a glass substrate using a direct imprint system. Subsequently, SnSe₂ nanoflakes were exfoliated in the same manner and then transferred to partially overlap the MoTe₂ flakes. Patterned Ti (25 nm)/Au (25 nm) electrodes were deposited on SnSe₂ nanoflakes by direct current magnetron sputtering and conventional photolithography lift-off processes. A Pt (50 nm) electrode was also deposited on MoTe₂ in the same manner as applied for the Ti electrode. Finally, the device was annealed at 200 °C for 5 min under ambient conditions.

Figures 1(a) and 1(b) show an optical microscope (OM) image and a three-dimentional (3D) schematic of the p-MoTe₂/n-SnSe₂ heterojunction photodiode. MoTe₂ and SnSe₂ formed a vertical heterojunction via PDMS dry transfer onto the glass substrate. Ti/Au was used for the SnSe₂ contact electrode. Pt/MoTe₂ shows good ohmic contact characteristics. The junction area is estimated to be 30 μm². Figure 2(a) shows a 3D schematic of the n-SnSe₂ FET. The SnSe₂ FET with a Ti/Au electrode shows highly doped n-type characteristics, as presented in Fig. 2(b). The on/off ratio is only 1.5 under ± 50 V of gate voltage [inset Fig. 2(b)]. The electron mobility is calculated as ~3.77 cm²/Vs with the following Eq. (1).

Figure 1. OM image and (b) 3D schematic of MoTe₂/SnSe₂ heterojunction.

Figure 2. 3D schematic and (b) transfer curve of SnSe₂ FET. (c) 3D schematic and (d) transfer curve of MoTe₂ FET.

where I_D is the drain current, V_GS is the gate voltage, V_DS is the drain voltage, L is the length of the FET, W is the width of the FET, and COX is the oxide capacitance. We used the linear mobility equation due to the low drain voltage of 0.1 V. SiO₂ with 285 nm thickness was used for the gate oxide and COX is calculated as 1.211 × 10⁻⁸ F/cm. Figure 2(c) exhibits a 3D schematic of the p-MoTe₂ FET with a Pt electrode. Unlike the SnSe₂ FET, the MoTe₂ FET shows a high on/off ratio of ~10⁵. Figure 2(d) shows that the MoTe₂ FET has a high on-current of about 10⁻⁶ A and a low off-current of about 10⁻¹¹ A under ± 50 V of V_GS. The hole mobility is calculated as 43 cm²/Vs. Figure 3(a) shows the current–voltage (I−V) characteristics of the MoTe₂/SnSe₂ heterojunction diode under a dark state. This heterojunction diode showed typical PN junction diode behavior without any additional field effect, operating effectively at a low voltage of ±2 V. The ideal factor of the diode was estimated to be approximately η~1.5, extracted using the equation $I=I_0{e}^{\frac{qV}{\eta kT}}$, where q is the charge of electrons, k is Boltzmann constant, and T is temperature [18]. Under reverse bias, the diode demonstrated an extremely low leakage current (IOFF) on the order of 10⁻¹¹ A, while the ON current (I_ON) reached approximately 10⁻⁶ A, resulting in a high rectification (ON/OFF) ratio exceeding 10⁵. To further investigate the photodiode properties, photo-induced I−V characteristics were measured under illumination from LEDs with wavelengths of NIR (1,300, 1,200, 1,050, 940, and 870 nm), red (630 nm), and green (528 nm). Figures 3(b) and 3(c) display dark and photo-induced I−V characteristics of the MoTe₂/SnSe₂ heterojunction diode on a logarithmic scale and a linear scale under LED illumination, revealing a broadband spectral response extending from 528 to 1,300 nm. This wide spectral response of the photodiode makes it potentially valuable for optoelectronic applications that require broadband detection capabilities. It is worth noting that the SnSe₂/MoTe₂ heterojunction diode also showed photovoltaic (PV) behaviors, as presented in Fig. 3(c). V_OC of 0.1 V and I_SC of 50−400 pA were clearly observed, indicating that our device also has potential as an efficient component in low-power, high-sensitivity photodetectors or solar cells. The spectral photoresponsivity was evaluated from I_SC and the light power density as function of wavelength [Fig. 3(d)]. The photoresponsivity is calculated using the following equation: $R=\frac{I_{\text{ph}}-I_{\text{dark}}}{P_{\text{in}}}$. It was observed that the responsivity of the MoTe₂/SnSe₂ diode increased as the illumination wavelength decreased (higher photon energies). Notably, under red LED illumination at 630 nm, the device exhibited the highest responsivity value of 0.18.

Figure 3. I−V characteristics of MoTe₂/SnSe₂ heterojunction diode. (b) Dark and (c) photoinduced I−V characteristics of the MoTe₂/SnSe₂ heterojunction photodiode on logarithmic and linear scales, respectively, under NIR (1,300, 1,200, 1,050, 940, and 870 nm) and visible (630 and 528 nm) LED illumination. (d) Spectral responsivity as a function of the illumination wavelength. Responsivity values were extracted from photoinduced I−V curves at V = 0 V.

I−V curves were measured at various temperatures to determine the potential barrier of the MoTe₂/SnSe₂ heterojunction diode. Figure 4(a) shows the temperature dependent I−V curves from 180 to 270 K. As expected, the current decreases as the temperature decreases due to the reduced carrier concentration and decreased thermal energy to overcome the barrier. As seen in Fig. 4(b), the activation energy for thermionic emission was extracted from the slope of the reverse current versus a q/kBT plot based on the temperature-dependent I−V curves. The activation energy is approximately 0.55 eV, this value that is related to the band alignment of MoTe₂ and SnSe₂. Figure 4(c) illustrates the band alignment of MoTe₂ and SnSe₂. SnSe₂ has a large electron affinity of 5 eV and an ionization energy of 6.2 eV, whereas MoTe₂ has an electron affinity of 4.22 eV and an ionization energy of 5.1 eV. The theoretical potential barrier for electron transfer is approximately 0.78 eV, which aligns closely with the experimental value.

Figure 4. Temperature-dependent I−V characteristics of MoTe₂/SnSe₂ heterojunction diode. (b) Arrhenius plots of the reverse current. (c) Energy band diagram of SnSe₂/MoTe₂ heterostructure.

This work demonstrates a MoTe₂/SnSe₂ heterojunction photodiode for broadband photodetection. The device exhibits a high rectification ratio over 10⁵, with low leakage current and high on current. We determined a significant activation energy of approximately 0.55 eV, which closely aligns with the theoretical potential barrier of 0.78 eV based on band alignment, indicating a well-matched heterojunction interface. The photodiode shows a broad spectral response (528 to 1,300 nm) with peak responsivity at 630 nm and PV behavior was observed, indicating its suitability for low-power, high-sensitivity optoelectronics as well as its potential for next-generation photodetectors and solar cells.

This research was supported by Changwon National University in 2023−2024.

The authors declare no conflicts of interest.