Highly Sensitive, Ultrafast, and Broadband Photo‐Detecting Field‐Effect Transistor with Transition‐Metal Dichalcogenide van der Waals Heterostructures of MoTe2 and PdSe2

Abstract Recently, van der Waals heterostructures (vdWHs) based on transition‐metal dichalcogenides (TMDs) have attracted significant attention owing to their superior capabilities and multiple functionalities. Herein, a novel vdWH field‐effect transistor (FET) composed of molybdenum ditelluride (MoTe2) and palladium diselenide (PdSe2) is studied for highly sensitive photodetection performance in the broad visible and near‐infrared (VNIR) region. A high rectification ratio of 6.3 × 105 is obtained, stemming from the sharp interface and low Schottky barriers of the MoTe2/PdSe2 vdWHs. It is also successfully demonstrated that the vdWH FET exhibits highly sensitive photo‐detecting abilities, such as noticeably high photoresponsivity (1.24 × 105 A W−1), specific detectivity (2.42 × 1014 Jones), and good external quantum efficiency (3.5 × 106), not only due to the intra‐TMD band‐to‐band transition but also due to the inter‐TMD charge transfer (CT) transition. Further, rapid rise (16.1 µs) and decay (31.1 µs) times are obtained under incident light with a wavelength of 2000 nm due to the CT transition, representing an outcome one order of magnitude faster than values currently in the literature. Such TMD‐based vdWH FETs would improve the photo‐gating characteristics and provide a platform for the realization of a highly sensitive photodetector in the broad VNIR region.

. a) Atomic force microscopy (AFM) characterization of a MoTe 2 nanoflake with the corresponding height profile. b) A transfer characteristic curve of a p-MoTe 2 FET with Pd metal electrodes with the source-drain voltage V ds = 1 V. . The charge carrier mobility ( of the FET was estimated using the relationship [1] , (1) where , , and indicate the length, width of the channel, and gate capacitance of the FET, respectively. The estimated hole mobilities s of the p-MoTe 2 FETs with Pd, Ni, and

Schottky barrier heights of the p-MoTe 2 FET devices
The energy band diagrams of the p-MoTe 2 FET devices are shown in Figure S2a before and after contact. To investigate the Schottky barrier heights between the metal electrodes (Pd, Ni, and Cr) and the p-MoTe 2 TMD material (metal-TMD junction), electrical measurements of the current-voltage (I ds -V ds ) characteristics were taken at several different temperatures (Ts) in the dark, as shown in Figure S2b for the FET with Pd electrodes. The Schottky barrier heights ( s) between the metal electrodes (Pd, Ni, and Cr) and p-MoTe 2 were estimated using the standard thermionic emission model, expressed as [2] ( ) * ( ) +, where the symbol A represents the junction area, A* denotes the Richardson constant, q is the elementary charge, and k B is the Boltzmann's constant. Based on the above relationship, plots against were used to estimate the value at the metal-TMD junctions for the Pd, Ni, and Cr electrodes ( Figure S2c). The estimated values are 28, 45, and 90 meV for the Pd, Ni, and Cr electrodes, respectively, as summarized in Table 1.

Measurements of the electronic properties of vdWH FETs in the dark
The current-voltage (I ds -V ds ) characteristics of the MoTe 2 /PdSe 2 vdWH FETs at different back-gate voltages (V bg ) in the dark are shown in Figure S9a. The forward and reverse currents were measured at the forward and reverse regions, respectively. At V ds < 0 V, the barrier height was increased across the MoTe 2 /PdSe 2 junction and I r was decreased. At V ds > 0 V, I f increased due to the reduction of the barrier height across the MoTe 2 /PdSe 2 junction.
Here, the rectification ratios between the forward current (I f ) measured at V ds = +5.0 V and the reverse current (I r ) measured at V ds = -5.0 V (rectification ratio (RR) = I f /I r ) were determined, [3] as shown in Figure S9b. Comparisons of the rectification ratios with previously reported values are also shown in the figure. Moreover, the reverse current through the PdSe 2 is low as compared to that with MoTe 2 , which also causes an increase in the rectification ratio at a negative gate voltage.
The drain current (I ds ) of the MoTe 2 /PdSe 2 FET can be analyzed by the Shockley diode equation, as [4] * ( ) +, where is the reverse saturation current, q is the elementary charge, ( ) ( ) is the ideality factor, denotes the Boltzmann constant ( J/K), and T indicates the absolute temperature. The ideality factor is determined by finding the slope of the curve in the linear forward-bias region at a given gate voltage ( ) ( Figure S9c). Figure

Effect of the thickness of the TMD material on the device performance of MoTe 2 /PdSe 2 vdWH FETs
We studied the effect of the TMD material thickness on the performance of MoTe 2 /PdSe 2 vdWH FETs. In order to investigate the effect of the thickness, we fabricated and characterized MoTe 2 /PdSe 2 vdWH FETs with different thicknesses of each TMD material.
First, when assessing the MoTe 2 /PdSe 2 vdWHs, the thickness of MoTe 2 was varied from 0.7 nm to 10 nm while the thickness of PdSe 2 remained constant (13 nm For the single and bi-layer cases, the barrier width is very narrow and electrons can easily tunnel through the thin depletion region. Thus, ineffective rectification occurs. In contrast, in the multi-layer case, rectifying behavior with a large rectification ratio (> 10 5 ) was clearly observed. It was also reported that the highest mobility of PdSe 2 can be obtained at a thickness of 10-20 nm.

Stability of the MoTe 2 /PdSe 2 vdWH FET devices
The fabricated MoTe 2 /PdSe 2 vdWH FETs were stored in a vacuum desiccator and long-term stability measurements were taken in a vacuum to avoid electrical degradation of the devices from the ambient environment. The photocurrent (I ds -V ds ) characteristics of the stored MoTe 2 /PdSe 2 vdWH FETs were measured up to 15 days with an interval of five days under incident laser light (532 nm) at 20-100 nW ( Figure S14a). The observed rectification ratios, RRs, of the FETs were found to have decreased from RR = 6.5 × 10 5 to RR = 9.5 × 10 4 after 15 days ( Figure S14b). The observed I ph -power dependences of the stored FETs are also shown Figure S14c.  Figure S14d shows the responsivities, Rs, of the FETs, indicating the decrement of R from R = 1.2 × 10 5 A W -1 to R = 4.9 × 10 4 A W -1 after 15 days.