Improvements in the synaptic operations of ferroelectric field-effect transistors using Hf0.5Zr0.5O2 thin films controlled by oxygen partial pressures during the sputtering deposition process

Abstract

To control the polarization switching characteristics of ferroelectric Hf_xZr_1−xO₂ (HZO) thin films, the effects of oxygen partial pressure (P_O2) during the sputtering deposition of HZO and the area ratio (S_I/S_F) of metal–ferroelectric–metal–insulator–semiconductor (MFMIS) gate stacks were investigated for ferroelectric synapse transistors. An increase in P_O2 resulted in a relative decrease in the ferroelectric orthorhombic phase owing to the compensation of oxygen vacancies into the HZO films. The introduction of an MFMIS gate stack with a smaller S_I/S_F ratio effectively reduced the electric field applied across the HZO gate insulator. The polarization switching times for the HZO thin films could be modulated in a wide range by means of these two strategies, which were clearly examined to facilitate the synaptic operations of ferroelectric field-effect transistors (FeFETs) using HZO gate insulators. Typical synaptic operations, including paired-pulse facilitation and spike timing-dependent plasticity, were clearly demonstrated to exhibit gradual modulations of the channel conductance of the FeFETs with the evolution of spike signals, and these behaviors were enhanced upon increasing P_O2 and decreasing the S_I/S_F ratio by controlling the switching kinetics of the ferroelectric partial polarizations of the HZO gate insulators in the proposed synapse FeFETs.