Control of contact properties in ferroelectric heterojunction T-NbTe<sub>2</sub>/Ga<sub>2</sub>S<sub>3</sub>

A monolayer ferroelectric semiconductor, Ga<sub>2</sub>S<sub>3</sub>, has received extensive attention because of its outstanding ductility, extremely high carrier mobility and unique out-of-plane asymmetric polarization characteristics. In this work, T-NbTe<sub>2</sub>/Ga<sub>2</sub>S<sub>3</sub> ferroelectric heterojunctions are constructed using out-of-plane asymmetric polarization characteristics of Ga<sub>2</sub>S<sub>3</sub>. The structural stability, preparation possibility and electrical contact properties for various ferroelectric heterojunction T-NbTe<sub>2</sub>/Ga<sub>2</sub>S<sub>3</sub> ferroelectric heterojunctions with the different polarization directions of Ga<sub>2</sub>S<sub>3</sub> are systematically studied by the first-principles calculations. It is found that heterojunctions T-NbTe<sub>2</sub>/Ga<sub>2</sub>S<sub>3</sub> exhibit sensitive responses to out-of-plane asymmetric polarization characteristics of Ga<sub>2</sub>S<sub>3</sub>. The two heterojunctions with the most stable energy, PD1 (<inline-formula><tex-math id="M3">\begin{document}$ \boldsymbol P_{\downarrow}$\end{document}</tex-math></inline-formula>) and PU2 (<inline-formula><tex-math id="M4">\begin{document}$ \boldsymbol P_{\uparrow} $\end{document}</tex-math></inline-formula>), in the intrinsic state form N-type and P-type Schottky contact, respectively. The polarization characteristics of the ferroelectric semiconductor Ga<sub>2</sub>S<sub>3</sub> are dependent on the contact type of the Schottky barrier in the ferroelectric heterojunction T-NbTe<sub>2</sub>/Ga<sub>2</sub>S<sub>3</sub>, which provides a practical approach for designing multifunctional Schottky devices. Specifically, the electrical contact depends on the external electric field. For the heterojunction, PD1 (and PU2), the contact can transition from Schottky contact to Ohmic contact at an electric field strength of +0.5 V/Å (+0.6 V/Å). Besides electric field, the contact properties of both heterojunctions PD1 and PU2 may also be tuned by an external biaxial strain. For the heterojunction, PD1, the contact can transition from Schottky contact to Ohmic contact at a biaxial strain tensile of 8%. And for the heterojunction, PU2, the contact can transition from P-type Schottky contact to N-type Schottky contact at a biaxial strain tensile of 2%, then from N-type Schottky contact to Ohmic contact at a strain tensile of 10%. These results provide a theoretical reference for designing two-dimensional ferroelectric nanodevices with high-performance electrical contact interfaces.