Subthreshold Current Suppression in ReS2 Nanosheet-Based Field-Effect Transistors at High Temperatures

Two-dimensional rhenium disulfide (ReS2), a member of the transition-metal dichalcogenide family, has received significant attention due to its potential applications in field-effect transistors (FETs), photodetectors, and memories. In this work, we investigate the suppression of the subthreshold current during the forward voltage gate sweep, leading to an inversion of the hysteresis in the transfer characteristics of ReS2 nanosheet-based FETs from clockwise to anticlockwise. We explore the impact of temperature, sweeping gate voltage, and pressure on this behavior. Notably, the suppression in current within the subthreshold region coincides with a peak in gate current, which increases beyond a specific temperature but remains unaffected by pressure. We attribute both the suppression in drain current and the presence of peak in gate current to the charge/discharge process of gate oxide traps by thermal-assisted tunnelling. The suppression of the subthreshold current at high temperatures not only reduces power consumption but also extends the operational temperature range of ReS2 nanosheet-based FETs.


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Experimental Section/Methods S2 The metal leads are fabricated by using standard photolithography and lift-off processes of evaporated Cr/Au (5 nm/110 nm) as shown in Figure 1a.The AFM image of ReS2 flakes is acquired by a Nanosurf A.G AFM (see Figure 1c).The Raman spectra are performed with a Raman Spectrometer (RENISHAW InVia).All measurements are performed with an excitation wavelength of 532 nm, with an output power of 10% of incident laser, and grid sizes of 1800 gmm -1 (see Figure 1d in the main text).
The electrical measurements on ReS2 nanosheet-based FETs are carried out in two-probe configuration, connected to a Keithley 4200 semiconductor characterization system (Tektronix Inc.) in a Janis ST-500 Probe Station (Lake Shore Cryotronics) provided with nanoprobes connected to the source/drain leads.The vacuum at 2.3 mbar is achieved through a rotary pump and controlled by a pressure gauge.

Conductivity at 2.3 mbar and different temperatures
To investigate the effects of pressure on suppression of the subthreshold current in ReS2 nanosheet-based FETs, we performed electrical measurements at a pressure of 2.3 mbar.
Figure S1a shows the output characteristics at 2.3 mbar and room temperature, for gate voltages ranging from -60 to 60 V, with a step of 10 V. Also in this case, the Id vs Vds curves are linear, showing the formation of good ohmic contacts 1 .The slight increase of drain currents in vacuum in the output curves, see panel a, is caused by desorption of adsorbates such as O2 and H2O which act as p-dopants 2 .
Furthermore, as already demonstrated for other 2D materials 3 , the desorption of adsorbates at low pressure causes a left shift in the transfer curve, which is shown in panel b), and a S3 reduction in the threshold voltage (Vth ~ -53 V), related to the increase of n-type doping of the materials.A slight increase in the mobility, compared to the ambient pressure, is also observed.In fact, we calculated the mobility at ambient pressure, finding the value of  = 8  2  −1  −1 that is consistent with the fact that a lower pressure reduces adsorbates on the surface of the ReS2 channel that act as scattering centres 4 .
We investigated the gate current behavior during the acquisition of the drain current.The gate current at 2.3 mbar, as we would expect, is also temperature dependent.We observed a similar peak as at atmospheric pressure, that increases when the temperature rises.In fact, the peak goes from ~10 -11 A at 290 K to ~10 -9 A at 402 K, just like at ambient pressure (Figures S1c-d).The gate current is not influenced by the pressure because it is related to the charge/discharge of the gate oxide (see model in the main text).
Finally, also at 2.3 mbar, we are interested in investigating the temperature above which the subthreshold current suppression and the transition CW -ACW occurs.Figure S1e shows the transfer curves at Vds = 0.1 V by increasing the temperature from 290 to 390 K. Like at atmospheric pressure, the shape and slope of the transfer curves do not change up to 360 K, where a suppression and a CW -ACW transition are observed (see red arrows).This is also confirmed by the peak in the transconductance vs Vgs shown in the inset of Figure S1f.
ReS2 flakes were exfoliated from a bulk ReS2 single crystal and transferred over a highly doped n-type Si substrate (resistivity 0.001-0.005Ω cm) covered by 290 nm thermal SiO2.