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Epitaxial ferroelectric HfO2films are the most suitable to investigate intrinsic properties of the material and for prototyping emerging devices. Ferroelectric Hf0.5Zr0.5O2(111) films were epitaxially stabilized on La2/3Sr1/3MnO3(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 those expected for domain matching epitaxy, indicating that this unconventional mechanism could be the prevailing factor in the stabilization of ferroelectric Hf0.5Zr0.5O2with (111) orientation in the epitaxial Hf0.5Zr0.5O2(111)/La2/3Sr1/3MnO3(001) heterostructure.

Ferroelectric doped hafnium oxide constitutes, at present, an intensively investigated candidate material to develop outperforming non-volatile memory devices. We report reading and writing of the ferroelectric polarization with light in Hf0.5Zr0.5O2/Nb:SrTiO3structures, where light is absorbed at the interface between the two materials, thereby enabling both processes. Reading of ferroelectric polarization is accomplished through the induced short-circuit photocurrents, which is a pathway towards voltage-less-non-destructive reading. Optical writing allows remote and contact-less switching of ferroelectric polarization, without the need for external voltages. The presence or absence of a Pt capping layer is crucial for the aforementioned read/write operations. If top Pt is present, photocarriers flow, resulting in short-circuit photocurrent, whose magnitude is modulated by the induced depolarization field. Instead, if Pt is removed, photocarriers are accumulated at the top surface, eventually producing an optically induced switching of polarization, as revealed by piezoelectric force microscopy observations. These results are highly appealing for the design of novel energy-efficient ferroelectric memory devices actuated with light.

Conventional strain engineering of epitaxial ferroelectric oxide thin films is based on the selection of substrates with a suitable lattice parameter. Here, we show that the variation of oxygen pressure during pulsed laser deposition is a flexible strain engineering method for epitaxial ferroelectric BaTiO3films either on perovskite substrates or on Si(001) wafers. This unconventional growth strategy permits continuous tuning of strain up to high levels (ε > 0.8%) in films greater than one hundred nanometers thick, as well as selecting the polar axis orientation to be either parallel or perpendicular to the substrate surface plane.