The metastable orthorhombic phase of hafnia is generally obtained in polycrystalline films, whereas in epitaxial films its formation has been much less investigated. We have grown Hf0.5Zr0.5O2 films by pulsed laser deposition and the growth window (temperature and oxygen pressure during deposition, and film thickness) for epitaxial stabilization of the ferroelectric phase is mapped. The remnant ferroelectric polarization, up to 24 C/cm 2 , depends on the amount of orthorhombic phase and interplanar spacing and increases with temperature and pressure for a fixed film thickness. The leakage current decreases with an increase in thickness or temperature, or when decreasing oxygen pressure. The coercive electric field (EC) depends on thickness (t) according the EC -t -2/3 scaling, which is observed by the first time in ferroelectric hafnia, and the scaling extends to thickness down to around 5 nm. The proven ability to tailor functional properties of high quality epitaxial ferroelectric Hf0.5Zr0.5O2 films paves the way toward understanding their ferroelectric properties and prototyping devices.

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.

Ferroelectric orthorhombic Hf0.5Zr0.5O2 (HZO) thin films have been stabilized epitaxially on La2/3Sr1/3MnO3/SrTiO3(001) by pulsed laser deposition. The epitaxial orthorhombic films, (111)-oriented and with very flat surface, show robust ferroelectric properties at room temperature. They present a remnant polarization around 20 μC/cm 2 without need of a wake-up process, a large coercive electric field of around 3 MV/cm, an extremely long retention extending well beyond 10 years, and an endurance up to about 10 8 cycles. Such outstanding properties in the nascent research on epitaxial HfO2based ferroelectric films, can pave the way to a better understanding of the effects of orientation, interfaces, strain and defects on ferroelectricity in HfO2.

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.

Ferroelectric HfO 2 is a promising material for new memory devices, but significant improvement of its important properties is necessary for practical application. However, previous literature shows that a dilemma exists between polarization, endurance and retention. Since all these properties should be simultaneously high, overcoming this issue is of the highest relevance. Here, we demonstrate that high crystalline quality sub-5 nm Hf 0.5 Zr 0.5 O 2 capacitors, integrated epitaxially with Si(001), present combined high polarization (2P r of 27 µC cm −2 in the pristine state), endurance (2P r > 6 µC cm −2 after 10 11 cycles) and retention (2P r > 12 µC cm −2 extrapolated at 10 years) using the same poling conditions (2.5 V). This achievement is demonstrated in films thinner than 5 nm, thus opening bright possibilities in ferroelectric tunnel junctions and other devices.This journal is