Tunnel devices based on ferroelectric Hf 0.5 Zr 0.5 O 2 (HZO) barriers hold great promises for emerging data storage and computing technologies. The resistance state of the device can be changed by a suitable writing voltage. However, the microscopic mechanisms leading to the resistance change are an intricate interplay between ferroelectric polarization controlled barrier properties and defect-related transport mechanisms. Here is shown the fundamental role of the microstructure of HZO films setting the balance between those contributions. The oxide film presents coherent or incoherent grain boundaries, associated to the existence of monoclinic and orthorhombic phases in HZO films, which are dictated by the mismatch with the substrates for epitaxial growth. These grain boundaries are the toggle that allows to obtain either large (up to ≈ 450 %) and fully reversible genuine polarization controlled electroresistance when only the orthorhombic phase is present or an irreversible and extremely large (≈ 10 3 -10 5 %) electroresistance when both phases coexist.are the resistances after polarizing the junction with writing voltages V W + or V W and R min (V W +,-) is the minimum resistance among these states. Accordingly, binary high (OFF) and low resistance (ON) states can be written in a ferroelectric memory cell and read by probing its resistance. It has also been shown that by performing minor polarization loops, ferroelectric tunnel devices can store information in different resistive states, mimicking the functioning of a memristive element. [4,5] This approach has been successfully achieved by using ferroelectric perovskites such as BaTiO 3 , [6][7][8][9] Pb(Zr 0.2

In the quest for reliable and power-efficient memristive devices, ferroelectric tunnel junctions are being investigated as potential candidates. Complementary metal oxide semiconductor-compatible ferroelectric hafnium oxides are at the forefront. However, in epitaxial tunnel devices with thicknesses around ≈4−6 nm, the relatively high tunnel energy barrier produces a large resistance that challenges their implementation. Here, we show that ferroelectric and electroresistive switching can be observed in ultrathin 2 nm epitaxial Hf 0.5 Zr 0.5 O 2 (HZO) tunnel junctions in large area capacitors (≈300 μm 2 ). We observe that the resistance area product is reduced to about 160 and 65 Ω•cm 2 for OFF and ON resistance states, respectively. These values are 2 orders of magnitude smaller than those obtained in equivalent 5 nm HZO tunnel devices while preserving a similar OFF/ON resistance ratio (210%). The devices show memristive and spike-timing-dependent plasticity behavior and good retention. Electroresistance and ferroelectric loops closely coincide, signaling ferroelectric switching as a driving mechanism for resistance change.

Films of Hf 0.5 Z 0.5 O 2 (HZO) contain a network of grain boundaries. In (111) HZO epitaxial films on (001) SrTiO 3, for instance, twinned orthorhombic (o-HZO) ferroelectric crystallites coexist with grain boundaries between o-HZO and a residual paraelectric monoclinic (m-HZO) phase. These grain boundaries contribute to the resistive switching response in addition to the genuine ferroelectric polarization switching and have detrimental effects on device performance. Here, it is shown that, by using suitable nanometric capping layers deposited on HZO film, a radical improvement of the operation window of the tunnel device can be achieved. Crystalline SrTiO 3 and amorphous AlO x are explored as capping layers. It is observed that these layers conformally coat the HZO surface and allow to increase the yield and homogeneity of functioning ferroelectric junctions while strengthening endurance. Data show that the capping layers block ionic-like transport channels across grain boundaries. It is suggested that they act as oxygen suppliers to the oxygen-getters grain boundaries in HZO. In this scenario it could be envisaged that these and other oxides could also be explored and tested for fully compatible CMOS technologies.

Advanced use of ferroelectric capacitors in data storage and computing relies on the control of their electrical resistance (electroresistance, ER) by the change of the electrostatic potential profile across the capacitor occurring upon electric field–driven polarization switching. Here it is reported the observation that BaTiO3‐based capacitors, sandwiched between Pt and La2/3Sr1/3MnO3 electrodes, display a large ER, whose magnitude (near 104% at room temperature) and sign (ER > 0, ER < 0) are determined by the writing pulse duration and temperature. Temperature‐dependent measurements have been instrumental to obtain evidence of the presence of a thermally activated process coexisting with the electronic changes produced by ferroelectric polarization switching, both contributing to ER. Detailed analysis allows concluding that the thermally activated process can be attributed to field‐assisted ionic motion. It is argued that the relative balance between purely electronic and ionic diffusion processes modulate the height of the interfacial Schottky barriers and, consequently, are responsible for the observed variations of magnitude and sign of ER.

Oxygen self-diffusion was investigated in TiO 2 layers employed for resistiveswitching memories using resonant nuclear reaction profiling (NRP) and 18 O labeling. The layers were grown using physical vapor deposition technique (sputtering) and were polycrystalline. The diffusivity was measured over the temperature range 600-800 • C and the activation energy for oxygen selfdiffusion in sputter-deposited TiO 2 films determined to be 1.09 ± 0.16 eV, a value consistent with results obtained by previous studies [1].