While the conductance of a first-order memristor is defined entirely by the external stimuli, in the second-order memristor it is governed by the both the external stimuli and its instant internal state. As a result, the dynamics of such devices allows to naturally emulate the temporal behavior of biological synapses, which encodes the spike timing information in synaptic weights. Here, we demonstrate a new type of second-order memristor functionality in the ferroelectric HfO 2 -based tunnel junction on silicon. The continuous change of conductance in the p + -Si/Hf 0.5 Zr 0.5 O 2 /TiN tunnel junction is achieved via the gradual switching of polarization in ferroelectric domains of polycrystalline Hf 0.5 Zr 0.5 O 2 layer, whereas the combined dynamics of the built-in electric field and charge trapping/detrapping at the defect states at the bottom Si interface defines the temporal behavior of the memristor device, similar to synapses in biological systems. The implemented ferroelectric second-order memristor exhibits various synaptic functionalities, such as paired-pulse potentiation/depression and spike-rate-dependent plasticity, and can serve as a building block for the development of neuromorphic computing architectures.

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Ferroelectric hafnium oxide films are attracting interest as a promising functional material for non-volatile ferroelectric memory due to a number of excellent advantages including perfect compatibility with Si technology, full scalability, low power consumption, high endurance, and a nanosecond switching speed. A high switching speed is inherent for all inorganic ferroelectrics, and it originates from the thermodynamics of polarization reversal. Another fundamental property of ferroelectric films is that the polarization switching speed depends on the electric field in the ferroelectric, as it is predicted by laws of polarization switching kinetics. Meanwhile, during the lifetime of a memory cell, the internal electric field changes due to the emergence of the built-in field associated with charge injection and charge accumulation in the nearby-electrode passive layer of the ferroelectric film. In this work, we demonstrate that the switching speed strongly depends on the entire prehistory of memory cells and may vary by several orders of magnitude. For this purpose, we study the switching kinetics in Hf0.5Zr0.5O2 thin films as a function of time, applied voltage, and temperature. Our experiment shows that the elevated operating temperature, as well as any time delays, and even write and read pulses themselves cause an apparent slowing down of the polarization switching in the ferroelectric film and a real decrease in the switching speed of ferroelectric memory. In particular, after long-term information storage, this effect can cause a readout failure in a memory chip designed for certain operating frequency. By means of theoretical simulations, we prove that the effect of degradation of the switching speed is caused by the charge injection induced by both the field of spontaneous polarization and the external electric field.

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