Ultra-thin Hf_1-xZr_xO₂ films have attracted tremendous interest since they show ferroelectric behavior at the nanoscale, where other ferroelectrics fail to stabilize the polar state. Their promise to revolutionize the electronics landscape comes from the well-known Si-compatibility of HfO₂ and ZrO₂, which (in amorphous form) are already used as gate oxides in MOSFETs. However, the recently discovered crystalline ferroelectric phases of hafnia-based films have been grown on Si only in polycrystalline form. Better ferroelectric properties and improved quality of the interfaces have been achieved in epitaxially-grown films, but these are only obtained on non-Si substrates. Here we report direct epitaxy of polar Hf_1-xZr_xO₂ phases on Si, enabled via in situ scavenging of the native a-SiO_x layer, using pulsed laser deposition under ballistic deposition conditions. We investigate the effect of substrate orientation and films composition to provide fundamental insights into the conditions that lead to the preferential stabilization of polar phases, namely the rhombohedral (r-) and the orthorhombic (o-) phases, against the non-polar monoclinic (m-), on Si.

Nanoscaled direct and converse piezoelectric responses from sub-100 nm thick epitaxial BiFeO₃ thin films grown by a water-based chemical method.

Hafnia-based thin films exhibit unconventional ferroelectricity. These materials also show rich polymorphism, and thus temperature and field-driven phase transitions, as well as oxygen migration. In a bigger context of exploring the synergy between ferroelectricity and diffusion-based structural phenomena, here we study temperature-dependent phase transitions in epitaxial Hf_0.5Zr_0.5O₂(HZO)/La_0.67Sr_0.33MnO₃ (LSMO, bottom electrode) heterostructures. We report topotactic phase transitions and their clear pathways in both LSMO and HZO layers upon heating under vacuum, using in situ scanning transmission electron microscopy (STEM). Specifically, we directly image oxygen and cationic columns using integrated differential phase contrast STEM and follow their evolution with temperature. We also perform in situ high temperature x-ray diffraction in air and show that the LSMO layer undergoes reversible thermal expansion and contraction when heated up to 850 °C, whereas HZO undergoes strain relaxation beyond 750 °C without any reversible phase transition. Our results provide a comprehensive and direct understanding of temperature-dependent structure, defect, and property correlations in these systems.

Despite its potential for CMOS applications, atomic layer deposition (ALD) of GeO₂ thin films, by itself or in combination with SiO₂, has not been widely investigated yet. Here, we report the ALD growth of SiO₂/GeO₂ multilayers on si1icon substrates using a so far unexplored Ge precursor. The characterization of multilayers with various periodicities reveals layer-by-layer growth with electron density contrast and the absence of chemical intermixing, down to a periodicity of two atomic layers.

The growth of $\alpha$-quartz-based piezoelectric thin films opens the door to higher-frequency electromechanical devices than those available through top-down approaches. We report on the growth of SiO${}_2$/GeO${}_2$ thin films by pulsed laser deposition and their subsequent crystallization. By introducing a devitrifying agent uniformly within the film, we are able to obtain the $\alpha$-quartz phase in the form of platelets with lateral sizes above 100 μm at accessible temperatures. Films containing different amounts of devitrifying agent are investigated, and their crystallinity is ascertained with X-ray diffraction and electron back-scatter diffraction. Our work highlights the difficulty in crystallization when competing phases arise that have markedly different crystalline orientation.

Memristive devices made of silicon compatible simple oxides are of great interest for storage and logic devices in future adaptable electronics and non-digital computing applications. A series of highly desirable properties observed in an atomic-layer-deposited hafnia-based stack, triggered our interest to investigate their suitability for technological implementations. In this paper, we report our attempts to reproduce the observed behaviour within the framework of a proposed underlying mechanism. The inability of achieving the electrical response of the original batch indicates that a key aspect in those devices has remained undetected. By comparing newly made devices with the original ones, we gather some clues on the plausible alternative mechanisms that could give rise to comparable electrical behaviours.