Multilevel Operations In Hf_0.5Zr_0.5O₂-based FeFET Based On Gradient Electric Field Control

Ferroelectric field effect transistor(FeFET)is an emerging non-volatile memory which uses the spontaneous polarization properties of ferroelectric materials to store information,possessing the merits of low power consumption,non-destructive readout,and fast write/read speed.The FeFETs integrated with novel Hf O₂-based ferroelectric thin films(i.e.,Hf O₂-based FeFETs)have demonstrated the great potential in the field of non-volatile memories,due to its simple structure,superior silicon process compatibility,and high-density integration.However,the interface between Hf O₂-based ferroelectric films and silicon substrate still need to be improved,and the multilevel capability of Hf O₂-based FeFET is limited by the device size.Thus,the HZO-based FeFETs with a metal/ferroelectric/insulator/semiconductor(MFIS)gate stack of W/TiN/Hf_0.5Zr_0.5O₂(HZO)/ZrO₂/Al₂O₃/p-Si are fabricated in this paper,and performance improvements are realized.In addition,for the multilevel memory operations of highly scaled Hf O₂-based FeFETs,a new method of gradient electric field is proposed,and the performances of multilevel memory operations of HZO-based FeFET are investigated.The main research contents and results are listed as follow:(1)TiN/HZO/TiN ferroelectric capacitors are fabricated by atomic layer deposition(ALD)method,and the effects of ALD cycle numbers and annealing process on the ferroelectric properties of HZO thin films are systetmatically investiaged.It is found that the HZO thin film deposited with 70 cycles and annealed at 500^oC for 30s shows good ferroelectric properties,with the positive remnant polarization(P_r⁺)and negative remnant polarization(P_r^-)of 17.6?C/cm² and-17.3?C/cm² at 3 V,respectively.(2)The HZO-based FeFETs with the crystalline ZrO₂ high-k layer are fabricated,and their electrical characteristics are investigated.The FeFET with the crystalline ZrO₂high-k layer shows good switching characteristics and a larger memory window,such as a memory window of 1.56 V after applying a program pulse of 5 V/100 ns and an erase pulse of-5 V/100 ns,a large 10-year extrapolated memory window of 1.26 V,and a memory window of 0.63 V after 150 program/erase cycles.Moreover,the device packaging has little influence on the electrical characteristics of the HZO-based FeFETs,demonstrating the potential for practical application.(3)An electric field gradient scheme is proposed to realize multilevel memory operations in HZO-based FeFETs,and the multilevel memory performances of the HZO-based FeFETs are investigated.2-bit/cell operation of the HZO-based FeFETs shows low cycle-to-cycle variability,and has long retention to extrapolation of 10 years.Than,the mechanism of modulating multiple nonvolatile memory states in the HZO-based FeFETs by gradient electric field is discussed.It is found that the multiple memory states is related to the multi-step polarization switching of the HZO thin film along the channel of the FeFETs.Moreover,the gradient electric field scheme shows the great feasibility for the multilevel memory operations of Hf O₂-based FeFETs with a gate length of 20 nm or less.