  Achieving robust and electrically controlled valley polarization in monolayer
transition metal dichalcogenides (ML-TMDs) is a frontier challenge for
realistic valleytronic applications. Theoretical investigations show that
integration of 2D materials with ferroelectrics is a promising strategy;
however, its experimental demonstration has remained elusive. Here, we
fabricate ferroelectric field-effect transistors using a ML-WSe2 channel and a
AlScN ferroelectric dielectric, and experimentally demonstrate efficient tuning
as well as non-volatile control of valley polarization. We measured a large
array of transistors and obtained a maximum valley polarization of ~27% at 80 K
with stable retention up to 5400 secs. The enhancement in the valley
polarization was ascribed to the efficient exciton-to-trion (X-T) conversion
and its coupling with an out-of-plane electric field, viz. the quantum-confined
Stark effect. This changes the valley depolarization pathway from strong
exchange interactions to slow spin-flip intervalley scattering. Our research
demonstrates a promising approach for achieving non-volatile control over
valley polarization and suggests new design principles for practical
valleytronic devices.
