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The ferroelectric field-effect transistor (FeFET) is a promising memory technology due to its high switching speed, low power consumption, and high capacity. Since the recent discovery of ferroelectricity in Si-doped HfO 2 thin films, HfO 2 -based materials have received considerable interest for the development of FeFET, particularly considering their excellent complementary metal-oxide-semiconductor (CMOS) compatibility, relatively low permittivity, and high coercive field. However, the multilevel capability is limited by the device size, and multidomain switching tends to vanish when the channel length of the HfO 2 -based FeFET approaches 30 nm. Here, multiple nonvolatile memory states are realized by tuning the electric field gradient across the Hf 0.5 Zr 0.5 O 2 (HZO) ferroelectric thin film along the channel direction of FeFET. The multi-step domain switching can be readily and directionally controlled in the HZO-FeFETs, with a very low variation. Moreover, multiple nonvolatile memory states or multi-step domain switching can be effectively controlled in the FeFETs with a channel length less than 20 nm. This study suggests the possibility to implement multilevel memory operations and mimic biological synapse functions in highly scaled HfO 2 -based FeFETs.

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The multilevel memory performances of ferroelectric field effect transistor (FeFET) with Hf 0.5 Zr 0.5 O 2 (HZO) ferroelectric thin film are investigated. First, similar retention characteristics are observed for intermediate and saturated polarization states of HZO ferroelectric thin film, which enables memories for multi-bit data storage. And then, 2-bit/cell operation of HZO-based FeFET is demonstrated utilizing two NAND architecture compatible write schemes of varying program pulse amplitude and width. Low cycle-to-cycle variability, long retention to extrapolation of 10 years at 85 • C, and endurance of 500 cycles are achieved for the both schemes. Moreover, the mechanism for multilevel memory operations of the FeFET is illustrated based on the polarization switching dynamics of HZO ferroelectric thin film. INDEX TERMS FeFET memory, Hf 0.5 Zr 0.5 O 2 , 2-bit/cell, write schemes, retention, endurance. SHUAIZHI ZHENG received the B.S. degree in chemistry and the M.S. degree in inorganic chemistry from Beijing Normal University, China, in 2005 and 2008, respectively, and the academic degree of Doctor of Natural Sciences from the

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