Epitaxial thin films of HfO₂ doped with La have been grown on SrTiO₃(001) and Si(001), and the impact of the La concentration on the stabilization of the ferroelectric phase has been determined. Films with 2–5 at. % La doping present the least amount of paraelectric monoclinic and cubic phases and exhibit the highest polarization, having a remanent polarization above 20 μC/cm². The dopant concentration results in an important effect on the coercive field, which is reduced with increasing La content. Combined high polarization, high retention, and high endurance of at least 10¹⁰ cycles is obtained in 5 at. % La-doped films.

Systematic studies on polycrystalline Hf_1–xZr_xO₂ films with varying Zr contents show that HfO₂ films are paraelectric (monoclinic). If the Zr content is increased, films become ferroelectric (orthorhombic) and then antiferroelectric (tetragonal). HfO₂ shows very good insulating properties and it is used in metal-oxide-semiconductor field-effect devices, while ZrO₂ shows good piezoelectric properties, but it is antiferroelectric. In between, Hf_0.5Zr_0.5O₂ shows good ferroelectric properties at the expense of poorer insulating and piezoelectric properties than HfO₂ and ZrO₂, respectively. Here, we explore the ferroelectric, insulating, and piezoelectric properties of a series of epitaxial films of Hf_1–xZr_xO₂ with different compositions. We show that epitaxial growth permits the stabilization of the ferroelectric phase in a whole range of Zr content (from x = 0 to x = 1). In epitaxial ZrO₂ films, ferroelectricity coexists with better piezoelectric and insulating properties than Hf_0.5Zr_0.5O₂, and in HfO₂ epitaxial films, ferroelectricity coexists with better insulating properties than Hf_0.5Zr_0.5O₂. For the case of ZrO₂ films, large electroresistance is also observed. In both cases, the ferroelectric endurance is poorer than that for Hf_0.5Zr_0.5O₂.

With SrTiO₃(110) substrates, the ferroelectric polarization of Hf_0.5Zr_0.5O₂ improved by 50%, with remanent polarization as high as 33 μC cm⁻² in a film of less than 7 nm thickness.

Endurance of ferroelectric HfO₂ needs to be enhanced for its use in commercial memories. This work investigates fatigue in epitaxial Hf_0.5Zr_0.5O₂ (HZO) instead of polycrystalline samples. Using different substrates, the relative amount of orthorhombic (ferroelectric) and monoclinic (paraelectric) phases is controlled. Epitaxial HZO films almost free of parasitic monoclinic phase suffer severe fatigue. In contrast, fatigue is mitigated in films with a greater amount of paraelectric phase. This suggests that fatigue can be intrinsically pronounced in ferroelectric HZO. It is argued that the enhancement of endurance in films showing coexisting phases results from the suppression of pinned domain propagation at ferroelectric–paraelectric grain boundaries, in contrast with a rapid increase of the size of the pinned domains in single ferroelectric regions.

Control of the final state of polarization switched by illumination is achieved in BaTiO₃ films by the manipulation of the internal electric fields by using suitable bottom electrodes.

The recent discovery of ferroelectricity in doped HfO₂ has opened perspectives on the development of memristors based on ferroelectric switching, including ferroelectric tunnel junctions. In these devices, conductive channels are formed in a similar manner to junctions based on nonferroelectric oxides. The formation of the conductive channels does not preclude the presence of ferroelectric switching, but little is known about the device ferroelectric properties after conduction path formation or their impact on the electric modulation of the resistance state. Here, we show that ferroelectricity and related sizable electroresistance are observed in pristine 4.6 nm epitaxial Hf_0.5Zr_0.5O₂ (HZO) tunnel junctions grown on Si. After a soft breakdown induced by the application of suitable voltage, the resistance decreases by about five orders of magnitude, but signatures of ferroelectricity and electroresistance are still observed. Impedance spectroscopy allows us to conclude that the effective ferroelectric device area after the breakdown is reduced, most likely by the formation of conducting paths at the edge.