  The device concept of ferroelectric-based negative capacitance (NC)
transistors offers a promising route for achieving energy-efficient logic
applications that can outperform the conventional semiconductor technology,
while viable operation mechanisms remain a central topic of debate. In this
work, we report steep slope switching in MoS$_2$ transistors back-gated by
single-layer polycrystalline PbZr$_{0.35}$Ti$_{0.65}$O$_3$. The devices exhibit
current on/off ratios up to 8$\times$10$^6$ within an ultra-low gate voltage
window of $V_g$ = $\pm$0.5 V and subthreshold swing (SS) as low as 9.7 mV
decade$^{-1}$ at room temperature, transcending the 60 mV decade$^{-1}$
Boltzmann limit without involving additional dielectric layers. Theoretical
modeling reveals the dominant role of the metastable polar states within domain
walls in enabling the NC mode, which is corroborated by the relation between SS
and domain wall density. Our findings shed light on a hysteresis-free mechanism
for NC operation, providing a simple yet effective material strategy for
developing low-power 2D nanoelectronics.
