A plasma-based ferroelectric undoped HfOx capacitor is fabricated using plasma-enhanced atomic layer deposition (PE-ALD), and the multilevel characteristics of synaptic behavior are investigated. A gradual ferroelectric phase transition in the material is confirmed by grazing incidence X-ray diffraction (GI-XRD) for different O2 plasma periods. The metal-ferroelectric-metal (MFM) capacitor presents excellent remnant polarization (Pr) up to 13 μC/cm2 and a switching endurance of more than 108 cycles. Superior small potentiation/depression nonlinearity (αp/αd = -0.08/-1.66 for voltage modulation stimulation) for synaptic training and almost symmetry training curve (|αp - αd|) are obtained. High stability under multilevel operation with five consecutive cycles of alternating potentiation and depression is exhibited without significant polarization variation for each training step (pulse).
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