The structural and ferroelectric properties of epitaxial Hf1-xCexO2 (CHO) thin films in the ultrathin regime are investigated as a function of Ce concentration (5

Probing novel properties, arising from twisted interfaces, has traditionally relied on the stacking of exfoliated two-dimensional materials and the spontaneous formation of van der Waals bonds. So far, investigations involving intimate covalent or ionic bonds have not been a focus. Yet, we show here that an established technique, involving thermocompressional wafer bonding, works well for creating twisted non-van der Waals interfaces. We have successfully bonded z-cut lithium niobate single crystals to create ferroelectric oxide interfaces with strong polar discontinuities and have mapped the associated emergent interfacial conductivity. In some instances, a dramatic change in microstructure occurs, involving local dipolar switching. A twist-induced collapse in the capability of the system to effec8tively screen interfacial bound charge is implied. Importantly, this only occurs around specific moiré twist angles with sparse coincident lattices and associated short-range aperiodicity. In quasicrystals, aperiodicity is known to induce pseudo-bandgaps and we suspect a similar phenomenon here. Wafer bonding has allowed the synthesis of twisted interfaces which support polar discontinuities in ferroelectric lithium niobate. Two-dimensional sheet conductivity arises but is suppressed when twist angles lead to interfacial lattice aperiodicity.

Improper ferroelectrics, in which electric polarization emerges as a secondary effect of primary order parameters such as nonpolar structural distortions, are theoretically predicted to exhibit no intrinsic thickness limit. Yet, this prediction is inconsistent with experiment where ferroelectricity is observed to disappear in improper ferroelectric films under six formula units thick. We transcend this limitation by moving beyond the conventional epitaxial design paradigm that focuses largely on in-plane lattice mismatch and chemical bonding. By considering structural compatibility in the out-of-plane direction, we realize undiminished improper ferroelectricity at the monolayer limit. Our findings confirm the theoretically predicted absence of a critical thickness for improper ferroelectrics and establish a broadly applicable strategy for designing additional ultrathin ferroelectric materials.

Recent advances in the exploration of two-dimensional (2D) van der Waals (vdW) ferroelectrics revealed not only a wealth of fundamentally exciting properties but also a strong potential for nanoelectronic applications facilitated by their semiconducting nature and tunable polarization-coupled physical properties. Here, using scanning probe microscopy techniques, we investigate the effects of mechanical stress and optical illumination on the transport behavior of one of the most actively studied 2D ferroelectrics, α-In2Se3. Local I–V measurements reveal a strongly asymmetric polarization-dependent conductivity of α-In2Se3, which can be continuously tuned by the tip-induced mechanical pressure. While the local conductivity increases up to two orders of magnitude for both polarization states, the upward polarization displays a much sharper change. Further enhancement of conductivity by an order of magnitude is observed under optical illumination, resulting from a cumulative modulation of the junction barrier via polarization, strain, and optical excitation. The obtained results make α-In2Se3 a promising material for application in electronic devices with optomechanical functionality.

Emerging wurzite-structured ferroelectrics can help satisfy the demand for high-performance ferroelectrics compatible with III-nitride and Si technology. One of their particularly appealing properties is related to the presence of conducting domain walls, which can be used as functional elements in the devices with electrically tunable resistance-memristors. Using a combination of piezoresponse force microscopy (PFM) and conductive atomic force microscopy (CAFM) techniques, the electrical conductivity of the head-to-head (H-H) domain walls in the Al0.85Sc0.15N thin films on the n-GaN substrate is directly demonstrated. Transmission electron microscopy (TEM) studies of the Al0.85Sc0.15N films reveal that the conducting nature of these domain walls is likely related to their inclination with respect to the polar axis, resulting in polarization discontinuity at the domain junctions. On the other hand, no increased conductivity has been detected for the tail-to-tail (T-T) domain walls reflecting a semiconducting nature of Al0.85Sc0.15N thin films. Modulation of the domain wall density by voltage pulses with varying amplitude or duration allows realization of multiple stable resistance states with the maximum ON/OFF ratio of over 1500. These findings pave the way for the next-generation of ferroelectric electronic devices compatible with III-nitride technology.

In this study, using a set of scanning probe microscopy techniques, we investigate the electronic properties of the domain walls in the layered ferroelectric semiconductor of the transition metal oxide dihalide family, NbOI2. Although the uniaxial ferroelectricity of NbOI2 allows only 180° domain walls, the pristine 2D flakes, where polarization is aligned in-plane, typically exhibit a variety of as-grown domain patterns outlined by the electrically neutral and charged domain walls. The electrically biased probing tip can modify the as-grown domain structures. Piezoresponse force microscopy (PFM) domain imaging along with electrical measurements by conductive atomic force microscopy (C-AFM) reveals a conductive nature of the charged domain walls. It is shown that while modification of the pristine domain structure by the biased probing tip is possible, it is hampered by the high conductivity of the charged domain walls generated during the switching process.

Ultrathin ferroelectric films with out-of-plane polarization and high Curie temperatures are key to miniaturizing electronic devices. Most ferroelectrics employed in devices are proper ferroelectrics, where spontaneous polarization is the primary order parameter. Unfortunately, the Curie temperature of proper ferroelectrics is drastically reduced as the ferroelectric becomes thin; nearly all proper ferroelectrics need to be thicker than several unit cells. The absence of an ultrathin limit has been predicted, but not verified for improper ferroelectrics. These are ferroelectrics where the polarization emerges secondary to the primary order parameter, such as a structural distortion. Here we report improper ferroelectricity with an undiminished Curie temperature in a 0.75-unit-cell-thick hexagonal LuFeO3 (h-LuFeO3) film grown on a SrCo2Ru4O11 bottom electrode with an atomically engineered monolayer bridging layer. Our results demonstrate the absence of a critical thickness for improper ferroelectricity and provide a methodology for creating ultrathin improper ferroelectrics by stabilizing their primary order parameters.

Impacts of post-heat treatment in a H2-contained (97% Ar and 3% H2) atmosphere on the switching kinetics of (001)-oriented AlScN capacitors are investigated. Fully saturated polarization can be obtained for all capacitors at frequencies of 50 Hz–50 kHz. In contrast to other ferroelectric materials, the domain wall movement behavior does not change after the heat treatment with frequency. The switching behavior follows the Kolmogorov–Avrami–Ishibashi (KAI) model, and the dimension of domain growth, n, values are ∼1–2 for all films before and after the H2-contained heat treatment. The domain nucleation and lateral domain growth in both the electric field- and time-dependent domain dynamics visualized by piezoresponse force microscopy are consistent with the KAI model. The activation energy increases slightly for the film after the heat treatment. This highly resilient switching behavior brings the AlScN films one step closer to realizing the next generation of nonvolatile memory technologies.

The continuous dimensional scaling of semiconductor and logic photoelectric device requires ferroelectrics to possess robust photoelectric activity and switchable polarization at the nanoscale. However, traditional ferroelectrics such as oxide perovskites generally suffer from relatively large bandgap and deteriorated ferroelectricity in ultrathin forms, while the polarization in many transition metal dichalcogenides is related to inter-layer effects, leading to ferroelectricity that only exists in flakes with a certain layer number and particular stacking forms. The associated challenging fabrication and high-cost synthesis of inorganic ferroelectrics currently render mass industrial production of ultrathin ferroelectric semiconductors impossible. Here with (isopentylammonium)2(ethylammonium)2Pb3I10, we report an organic-inorganic hybrid perovskite nanoflake with cheap solution synthesis, switchable polarization, a narrow bandgap (1.86 eV to 2.21 eV form bulk to monolayer), and robust photoelectric properties down to the monolayer. The present work reveals the great potential of 2D hybrid perovskite ferroelectrics as low-cost ferroelectric semiconductors at the nanoscale.

The miniaturization of ferroelectrics with lateral size reduction is crucial for technological advancement but requires an understanding of the fundamental behavior of ferroelectrics at the nanoscale. While much attention has been focused on vertical scaling of perovskite ferroelectrics with thickness reduction, lateral scaling remains less explored. In this study, ferroelectricity is investigated in 20 nm thick single-crystalline BaTiO3 nanodisks with a diameter ranging from approximate to 400 down to 100 nm. They are fabricated by Ne ion milling of a 20 nm BaTiO3 film epitaxially grown on SrTiO3-buffered silicon. The nanodisks are ferroelectric with a Curie temperature in the range 230-270 degrees C as determined by temperature-dependent piezoresponse force microscopy. In 100 nm-diameter nanodisks, the vertical polarization component adopts three distinct patterns in the pristine state, aligning with theoretical predictions. The most prevalent pattern features a uniformly up-oriented vertical component. The rotational invariance of these domain patterns in the plane suggests a combination of center-type and flux-closure domains. Additionally, the up polarization can be switched progressively to down polarization upon application of a pulsed bias of increasing time width. The control of the polarization in nanostructures and of their progressive switching is of particular interest for memory applications.

Strong coupling between polarization (P) and strain (ɛ) in ferroelectric complex oxides offers unique opportunities to dramatically tune their properties. Here colossal strain tuning of ferroelectricity in epitaxial KNbO3 thin films grown by sub-oxide molecular beam epitaxy is demonstrated. While bulk KNbO3 exhibits three ferroelectric transitions and a Curie temperature (Tc) of ≈676 K, phase-field modeling predicts that a biaxial strain of as little as -0.6% pushes its Tc > 975 K, its decomposition temperature in air, and for -1.4% strain, to Tc > 1325 K, its melting point. Furthermore, a strain of -1.5% can stabilize a single phase throughout the entire temperature range of its stability. A combination of temperature-dependent second harmonic generation measurements, synchrotron-based X-ray reciprocal space mapping, ferroelectric measurements, and transmission electron microscopy reveal a single tetragonal phase from 10 K to 975 K, an enhancement of ≈46% in the tetragonal phase remanent polarization (Pr), and a ≈200% enhancement in its optical second harmonic generation coefficients over bulk values. These properties in a lead-free system, but with properties comparable or superior to lead-based systems, make it an attractive candidate for applications ranging from high-temperature ferroelectric memory to cryogenic temperature quantum computing.

Cr2TiC2Tx is an ordered double-transition-metal MXene with peculiar magnetic properties. Previous studies produced sub-1-mu m sheets of Cr2TiC2Tx, which prevented complete characterization of its intrinsic properties at a single-flake level. We report the synthesis of high-quality Cr2TiC2Tx monolayers with lateral sizes exceeding 15 mu m for single-flake measurements. These measurements establish Cr2TiC2Tx as a unique material among the MXenes experimentally tested so far. Field-effect electrical measurements on Cr2TiC2Tx monolayers revealed an average conductivity of 180 S cm(-1) and p-type transport, while established MXenes, such as Ti3C2Tx and Nb4C3Tx, demonstrated n-type behavior. In contrast to negative photoresponse reported for Ti3C2Tx flakes, Cr2TiC2Tx devices show positive photoresponse to visible and infrared light. Nanoindentation measurements of monolayer Cr2TiC2Tx membranes yielded an effective Young's modulus of 220 +/- 22 GPa. Density functional theory calculations provide insights into the p-type character of Cr2TiC2Tx and predict its potentially tunable p-/n-type behavior depending on the concentrations of Cr vacancies, oxygens substituting carbon atoms, and surface terminations.

In this paper, using a combination of pulse testing measurements and piezoresponse force microscopy (PFM), an investigation of the polarization reversal behavior and the accompanying resistive switching in the Al0.72Sc0.28N thin film capacitors is reported. The obtained results reveal a transition from the nucleation-limited switching (NLS) in the low field range toward the more uniform switching described by the Kolmogorov-Avrami-Ishibashi (KAI) model in the high field range. It is found that the Al0.72Sc0.28N capacitors exhibit an unusually steep change in the switching time- it decreases by five orders of magnitude with a moderate increase of the applied field. This feature is caused by a significantly higher activation field value (approximate to 126 MV cm(-1)) in comparison with the conventional perovskite ferroelectrics. PFM visualization of the field-induced domain dynamics has allowed the evaluation of the nucleation rate and domain wall velocity. Furthermore, capacitors in the polydomain state generated by partial switching of polarization exhibit a significant (up to two orders of magnitude) increase in the steady-state conductance. This effect is likely caused by the injection of strongly inclined conducting 180 degrees domain walls. Resistance tunability offers additional functionalities to the Al1-xScxN devices where conductive domain walls are used as active elements.

HfO2-based thin films hold huge promise for integrated devices as they show full compatibility with semiconductor technologies and robust ferroelectric properties at nanometer scale. While their polarization switching behavior has been widely investigated, their electromechanical response received much less attention so far. Here, we demonstrate that piezoelectricity in Hf0.5Zr0.5O2 ferroelectric capacitors is not an invariable property but, in fact, can be intrinsically changed by electrical field cycling. Hf0.5Zr0.5O2 capacitors subjected to ac cycling undergo a continuous transition from a positive effective piezoelectric coefficient d(33) in the pristine state to a fully inverted negative d(33) state, while, in parallel, the polarization monotonically increases. Not only can the sign of d(33) be uniformly inverted in the whole capacitor volume, but also, with proper ac training, the net effective piezoresponse can be nullified while the polarization is kept fully switchable. Moreover, the local piezoresponse force microscopy signal also gradually goes through the zero value upon ac cycling. Density functional theory calculations suggest that the observed behavior is a result of a structural transformation from a weakly-developed polar orthorhombic phase towards a well-developed polar orthorhombic phase. The calculations also suggest the possible occurrence of a non-piezoelectric ferroelectric Hf0.5Zr0.5O2. Our experimental findings create an unprecedented potential for tuning the electromechanical functionality of ferroelectric HfO2-based devices.

Hafnia-based ferroelectric (FE) thin films are promising candidates for semiconductor memories. However, a fundamental challenge that persists is the lack of understanding regarding dimensional scaling, including thickness scaling and area scaling, of the functional properties and their heterogeneity in these films. In this work, excellent ferroelectricity and switching endurance are demonstrated in 4 nm-thick Hf0.5Zr0.5O2 (HZO) capacitors with molybdenum electrodes in capacitors as small as 65 nm × 45 nm in size. The HZO layer in these capacitors can be crystallized into the ferroelectric orthorhombic phase at the low temperature of 400 °C, making them compatible for back-end-of-line (BEOL) FE memories. With the benefits of thickness scaling, low operation voltage (1.2 V) is achieved with high endurance (>1010 cycles); however, a significant fatigue regime is noted. We observed that the bottom electrode, rather than the top electrode, plays a dominant role in the thickness scaling of HZO ferroelectric behavior. Furthermore, ultrahigh switched polarization (remanent polarization 2Pr ∼ 108 μC cm-2) is observed in some nanoscale devices. This study advances the understanding of dimensional scaling effects in HZO capacitors for high-performance FE memories.

Ferroelectric wurtzite-type aluminum scandium nitride (Al1-xScxN) presents unique properties that can enhance the performance of non-volatile memory technologies. The realization of the full potential of Al1-xScxN requires a comprehensive understanding of the mechanism of polarization reversal and domain structure dynamics involved in the ferroelectric switching process. In this work, transient current integration measurements performed by a pulse switching method are combined with domain imaging by piezoresponse force microscopy (PFM) to investigate the kinetics of domain nucleation and wall motion during polarization reversal in Al0.85Sc0.15N capacitors. In the studied electric field range (from 4.4 to 5.6 MV cm(-1)), ferroelectric switching proceeds via domain nucleation and wall movement. The currently available phenomenological models are shown to not fully capture all the details of the complex dynamics of polarization reversal in Al0.85Sc0.15N. PFM reveals a non-linear increase of both domain nucleation rate and lateral wall velocity during the switching process, as well as the dependency of the domain pattern on the polarization reversal direction. A continuously faster N- to M-polar switching upon cycling is reported and ascribed to an increasing number of M-polar nucleation sites and density of domain walls.

Antiferroelectricity is a material property characterized by alternating electric dipoles spontaneously ordered in antiparallel directions. Antiferroelectrics are promising for energy storage, solid-state cooling, and memory technologies; however, these materials are scarce, and their scalability remains largely unexplored. In this work, we demonstrate that single-crystalline hafnia, a lead-free CMOS-compatible material, exhibits antiferroelectricity under compressive-strain conditions. We observe antiparallel sublattice polarization and stable double-hysteresis in single-crystalline (111)-oriented epitaxial La-doped hafnia films grown on yttrium-stabilized zirconia and show that the antipolar orthorhombic phase of hafnia adheres to the Kittel model of antiferroelectricity. Notably, compressive strain strengthens the antiferroelectric order in thinner La-doped hafnia films, achieving an unprecedented 850 C ordering temperature in the two-dimensional limit, highlighting hafnia's potential for advanced antiferroelectric devices.

One of the exceptional features of the van der Waals (vdW) ferroelectrics is the existence of stable polarization at a level of atomically thin monolayers. This ability to withstand a detrimental effect of the depolarization fields gives rise to complex domain configurations characterized, among others, by the presence of layered "antipolar" head-to-head (H-H) or tail-to-tail (T-T) dipole arrangements. In this study, tomographic piezoresponse force microscopy (TPFM) is employed to study the 3D polarization arrangement in vdW ferroelectric alpha-In2Se3. Sequential removal of thin layers from the polar surface using the PFM tip reveals a complex 3D profile of the domain walls in the alpha-In2Se3 crystals. Antiparallel domain layers stacked along the polar direction are also observed by PFM imaging of the non-polar surfaces showing that H-H and T-T domain boundaries are commonly present in alpha-In2Se3. Application of TPFM to the electrically written domains allows evaluation of their geometrical lateral-to-vertical size aspect ratio, which shows a strong prevalence for the sidewise expansion in comparison to the forward growth. Local I-V measurements reveal a strong polarization direction dependence of conductivity due to the modulation of the energy barrier height as corroborated by theoretical modeling.