*3.3. Electric Properties of Pt/CuInP2S6/Au FTJ*

CIPS by itself is the only two-dimensional ferroelectric material with a ferroelectric transition temperature (*Tc*) just over room temperature. Based on the above studies, we have observed the paraelectric phase in CIPS flakes, but it is rarely reported whether the paraelectric phase affects the device's performance. In this work, we used conductive AFM to study the electric properties of a CIPS FTJ as shown in Figure 5. The electrical characterization of an Au/CIPS/Pt vdW FTJ is shown in Figure 5a for a device with a 2-nm-thick CIPS layer. Figure 5b,c shows the topography and current images of the CIPS flakes. The leakage current scanning was performed within a 3 × <sup>3</sup> <sup>μ</sup>m2 area at a read voltage of 10 mV. The observed local conductive path regions indicated good electrical conductivity in the CIPS flakes.

**Figure 5.** Electrical characterization of a Si/Pt/CIPS/Au diode with1.7 nm CIPS. (**a**) Schematic representation of the experimental setup for C-AFM measurements. (**b**) Topography image of the CIPS nano flake with a thickness of 2 nm on the Si/SiO2/Ti/Pt substrate. The inset shows the height map of the CIPS flake. (**c**) The corresponding current phase diagram of CIPS flakes. (**d**) *I*–*V* curves measured with increasing sweep voltages, where *VMax* is from 2 to 3.5 V. (**e**) *Ion* and *Ioff* are the on and off current of the FTJ with resistance switching behavior, which are also corresponding to the low and high resistance states, read from (**d**) under different scanning voltages, with the inset the calculated switching ratio based on (**d**).

Figure 5d presents the *I*–*V* curves of a CIPS FTJ, measured with varying sweep ranges (*V*max from 2 V to 3.5 V). We can observe resistive switching in both positive and negative voltage ranges, demonstrating that FTJ has superior continuous current modulation and self-rectification functions. The *I*–*V* curves show a nonsymmetrical contour, and worse symmetry appears with increased voltage. This corresponds to the Cu2+ migration process. Before the Cu2+ migration, a Schottky barrier must be overcome. Due to the difference in the work functions between Au and Pt, the current of the *I*–*V* curve under a positive voltage and a negative voltage is asymmetric. The current limiting behavior in the negative

range is similar to the rectifying effect of diodes. As the applied voltage increases, the *I*–*V* curves show more obvious hysteresis in the positive range, which is always regarded as a resistance switching behavior. As shown in Figure 5e, the Ion and Ioff correspond to the on-current and off-current when applying different voltages during the resistance switching. As shown in the inset of Figure 5e, the *Ion/Ioff* of the FTJ with an ultra-thin CIPS film is over 200, which is comparable with the previous results [10], indicating that CIPS has good development prospects in the research on and application of nonvolatile memory devices.
