The mechanism of the remnant polarization (P r) growth during the first stage of ferroelectric HfO2-based memory cell operation (the wake-up effect) is still unclear. In this work, we reveal the microscopic nature of the P r growth in functional ferroelectric capacitors based on a polycrystalline 10 nm thick (111) out-of-plane textured Hf0.5Zr0.5O2 film during electric cycling. We observe the cycle-by-cycle evolution of the domain structure with the piezoresponse force microscopy (PFM). During the early stage of the wake-up, three types of domains are found: (i) normal domains (polarization aligned along the applied electric field), (ii) nonswitchable domains with upward and downward polarization, and (iii) domains with anomalous polarization switching (polarization aligned against the applied electric field) that are commonly surrounded by nonswitchable domains. Initially, nonswitchable and “anomalous” domains are 200–300 nm in width, and they occupy ∼70% of the capacitor area. During electric field cycling, these domains reduce in area, which is accompanied by the P r growth. We attribute the domain pinning and the anomalous polarization reversal to the internal bias field of the oxygen vacancies. The local density of the oxygen vacancies decreases during electric cycling, thus producing the reduction of the internal bias field. The correlation of the PFM data with both the results of the structural analysis of fresh and cycled Hf0.5Zr0.5O2 film by transmission electron microscopy and the performance of the ferroelectric capacitor indicates that after the first cycle of the wake-up the P r growth is not associated with phase transformations, but only with the transformation of the domain structure. The obtained results elucidate the physical mechanism of the emergence of P r during the wake-up of the ferroelectric HfO2-based memory cell.

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.

No abstract

No abstract

New interest in the implementation of ferroelectric tunnel junctions has emerged following the discovery of ferroelectric properties in HfO 2 films, which are fully compatible with silicon microelectronics technology. The coercive electric field to switch polarization direction in ferroelectric HfO 2 is relatively high compared to classical perovskite materials, and thus it can cause the migration of non-ferroelectric charges in HfO 2 , namely charged oxygen vacancies. The charge redistribution would cause the change of the tunnel barrier shape and following change of the electroresistance effect. In the case of ambiguous ferroelectric properties of HfO 2 ultrathin films, this oxygen-driven resistive switching effect can mimic the tunnel electroresistance effect. Here, we demonstrate two separate resistive switching regimes, depending on the applied voltage, in the same memristor device employing a ferroelectric Hf 0.5 Zr 0.5 O 2 (4.5 nm) layer. The first regime originates from the polarization reversal, whereas the second one is attributed to the accumulation/depletion of the oxygen vacancies at the electrode interface. The modulation of the tunnel barrier causes the enhancement of R OFF /R ON ratio in ∼20 times compared to the tunnel electroresistance effect. The developed device was used to formulate the criteria for unambiguous discrimination between the ferroelectric-and non-ferroelectric resistive switching effects in HfO 2 -based ferroelectric tunnel junctions.