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†
To control the polarization switching characteristics of ferroelectric HfxZr1−xO2 (HZO) thin films, the effects of oxygen partial pressure (PO2) during the sputtering deposition of HZO and the area ratio (SI/SF) of metal–ferroelectric–metal–insulator–semiconductor (MFMIS) gate stacks were investigated for ferroelectric synapse transistors. An increase in PO2 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 SI/SF 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 PO2 and decreasing the SI/SF ratio by controlling the switching kinetics of the ferroelectric partial polarizations of the HZO gate insulators in the proposed synapse FeFETs.