  Ferroelectric field-effect-transistor (FEFET) has emerged as a scalable
solution for 3D NAND and embedded flash (eFlash), with recent progress in
achieving large memory window (MW) using
metal-insulator-ferroelectric-insulator-semiconductor (MIFIS) gate stacks.
Although the physical origin of the large MW in the MIFIS stack has already
been discussed, its retention characteristics have not been explored yet. Here,
we demonstrate MIFIS FEFET with a maximum MW of 9.7 V, and show that MIFIS
FEFET has unstable retention characteristics, especially after erase. We
discover the origin of the unstable retention characteristics and prove our
hypothesis with absolute polarization measurement and different operation
modes, showing that the unstable retention characteristics is a fundamental
issue. Based on the understanding, we discuss a novel charge compensation model
and promising engineering methodologies to achieve stable retention in MIFIS
FEFET.
