The critical impact of epitaxial stress on the stabilization of the ferroelectric orthorhombic phase of hafnia is proved. Epitaxial bilayers of Hf0.5Zr0.5O2 (HZO) andLa0.67Sr0.33MnO3 (LSMO) electrodes were grown on a set of single crystalline oxide (001)oriented (cubic or pseudocubic setting) substrates with lattice parameter in the 3.71 -4.21 Å range. The lattice strain of the LSMO electrode, determined by the lattice mismatch with the substrate, is critical in the stabilization of the orthorhombic phase of HZO. On LSMO electrodes tensile strained most of the HZO film is orthorhombic, whereas the monoclinic phase is favored when LSMO is relaxed or compressively strained. Therefore, the HZO films on TbScO3 and GdScO3 substrates present substantially enhanced ferroelectric polarization in comparison to films on other substrates, including the commonly used SrTiO3. The capability of having epitaxial doped HfO2 films with controlled phase and polarization is of major interest for a better understanding of the ferroelectric properties and paves the way for fabrication of ferroelectric devices based on nanometric HfO2 films.

a) Corresponding authors: ignasifinamartinez@gmail.com, fsanchez@icmab.es SrTiO3 templates have been used to integrate epitaxial bilayers of ferroelectric Hf0.5Zr0.5O2 and La2/3Sr1/3MnO3 bottom electrode on Si(001). The Hf0.5Zr0.5O2 films show enhanced properties in comparison to equivalent films on SrTiO3(001) single crystalline substrates. The films, thinner than 10 nm, have very high remnant polarization of 34 µC/cm 2 . Hf0.5Zr0.5O2 capacitors at operating voltage of 4 V present long retention time well beyond 10 years and high endurance against fatigue up to 10 9 cycles. The robust ferroelectric properties displayed by the epitaxial Hf0.5Zr0.5O2 films on Si(001) using SrTiO3 templates paves the way for the monolithic integration on silicon of emerging memory devices based on epitaxial HfO2.

Epitaxial ferroelectric HfO 2 films are the most suitable to investigate intrinsic properties of the material and for prototyping emerging devices. Ferroelectric Hf 0.5 Zr 0.5 O 2 (111) films have been epitaxially stabilized on La 2/3 Sr 1/3 MnO 3 (001) electrodes. This epitaxy, considering the symmetry dissimilarity and the huge lattice mismatch, is not compatible with conventional mechanisms of epitaxy. To gain insight into the epitaxy mechanism, scanning transmission electron microscopy characterization of the interface was performed, revealing arrays of dislocations with short periodicities. These observed periodicities agree with the expected for domain matching epitaxy, indicating that this unconventional mechanism could be the prevailing factor in the stabilization of ferroelectric Hf 0.5 Zr 0.5 O 2 with (111) orientation in the epitaxial Hf 0.5 Zr 0.5 O 2 (111)/La 2/3 Sr 1/3 MnO 3 (001) heterostructure.

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.