arXiv subjects

Alexei Gruverman

Publications and source records attributed to Alexei Gruverman.

18 recordsLinked to original sources

arXiv2026

Temperature-doping phase diagram and endurance in Ce-doped HfO2

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% <= x <= 20%) and temperature. A temperature-doping phase diagram is established for 10 nm films, showing a systematic evolution from the ferroelectric orthorhombic phase to tetragonal and cubic phases with increasing Ce content. The orthorhombic-tetragonal transition temperature decreases from ~800{\deg}C at x = 5% to $~300{\deg}C$ at x = 15%, indicating strong stabilization of higher-symmetry phases with doping. Consistently, the remanent polarization decreases from ~15 to $3.8 {\mu}C/cm2$ as x increases from 5% to 20%. In contrast, the endurance improves significantly, with higher Ce concentrations exhibiting markedly enhanced cycling stability up to 108 cycles. The opposing trends of polarization and endurance are correlated with reduced orthorhombic distortion, suggesting that fatigue mitigation in Ce-doped HfO2 is linked to structural evolution. These results provide a framework for optimizing composition and reliability in ultrathin ferroelectric HfO2 devices.

cond-mat.mtrl-sci↗

arXiv2026

Atom Probe Tomography as an Emerging Tool for Understanding Defect-driven Mechanisms in HfO$_{2}$-based Ferroelectrics

HfO$_{2}$-based ferroelectrics are essential for the next generation of CMOS-compatible memory and logic devices, yet their performance is governed by a complex interplay between oxygen vacancies, dopants, and structural defects that remains an active area of investigation. These defects shape the function-critical dynamic phenomena, such as polar phase stabilization, wake-up, fatigue, and imprint. In this Perspective, we review the limitations of established high-resolution structural characterization techniques and propose atom probe tomography (APT) as a powerful tool for the three-dimensional (3D), atomic-scale mapping of all constituent species in hafnia-based ferroelectric systems. By resolving individual dopants, vacancy clustering, and interfacial segregation, APT can facilitate a quantitative understanding of defect-property relations in hafnia-based ferroelectrics. We discuss current experimental challenges for APT application to ferroelectric oxides, demonstrate a proof-of-concept of atomic-scale reconstruction in a hafnia-based device stack, and highlight the potential of APT to guide the development of ferroelectric structures with enhanced reliability and performance.

cond-mat.mtrl-sci↗

arXiv2025

Improper Ferroelectricity at the Monolayer Limit

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.

cond-mat.mtrl-sci↗

arXiv2024

Antiferroelectric Hafnia Down to the 2D Limit

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.

cond-mat.mtrl-sci↗

arXiv2024

Imaging Local Effects of Voltage and Boron Doping on Spin Reversal in Antiferromagnetic Magnetoelectric Cr2O3 Thin Films and Devices

Chromia (Cr2O3) is a magnetoelectric oxide which permits voltage-control of the antiferromagnetic (AFM) order, but it suffers technological constraints due to its low Neel Temperature (TN ~307 K) and the need of a symmetry breaking applied magnetic field to achieve reversal of the Neel vector. Recently, boron (B) doping of Cr2O3 films led to an increase TN > 400 K and allowed the realization of voltage magnetic-field free controlled N\'eel vector rotation. Here, we directly image the impact of B doping on the formation of AFM domains in Cr2O3 thin films and elucidate the mechanism of voltage-controlled manipulation of the spin structure using nitrogen vacancy (NV) scanning probe magnetometry. We find a stark reduction and thickness dependence of domain size in B-doped Cr2O3 (B:Cr2O3) films, explained by the increased germ density, likely associated with the B doping. By reconstructing the surface magnetization from the NV stray-field maps, we find a qualitative distinction between the undoped and B-doped Cr2O3 films, manifested by the histogram distribution of the AFM ordering, i.e., 180 degree domains for pure films, and 90 degree domains for B:Cr2O3 films. Additionally, NV imaging of voltage-controlled B-doped Cr2O3 devices corroborate the 90 degeree rotation of the AFM domains observed in magnetotransport measurement.

cond-mat.mtrl-sci↗

arXiv2023

Dual mechanisms for transient capacitance anomaly in improper ferroelectrics

The recent discovery of transient negative capacitance has sparked an intense debate on the role of homogeneous and inhomogeneous mechanisms in polarizations switching. In this work, we report observation of transient negative capacitance in improper ferroelectric h-YbFeO3 films in a resistor-capacitor circuit, and a concaved shape of anomaly in the voltage wave form, in the early and late stage of the polarizations switching respectively. Using a phenomenological model, we show that the early-stage negative capacitance is likely due to the inhomogeneous switching involving nucleation and domain wall motion, while the anomaly at the late stage, which appears to be a reminiscent negative capacitance is the manifestation of the thermodynamically unstable part of the free-energy landscape in the homogeneous switching. The complex free-energy landscape in hexagonal ferrites may be the key to cause the abrupt change in polarization switching speed and the corresponding anomaly. These results reconcile the two seemingly conflicting mechanisms in the polarization switching and highlight their different roles at different stages. The unique energy-landscape in hexagonal ferrites that reveals the dual switching mechanism suggests the promising application potential in terms of negative capacitance.

cond-mat.mtrl-sci↗

arXiv2023

Observation of Antiferroelectric Domain Walls in a Uniaxial Hyperferroelectric

Ferroelectric domain walls are a rich source of emergent electronic properties and unusual polar order. Recent studies showed that the configuration of ferroelectric walls can go well beyond the conventional Ising-type structure. Néel-, Bloch-, and vortex-like polar patterns have been observed, displaying strong similarities with the spin textures at magnetic domain walls. Here, we report the discovery of antiferroelectric domain walls in the uniaxial ferroelectric Pb$_{5}$Ge$_{3}$O$_{11}$. We resolve highly mobile domain walls with an alternating displacement of Pb atoms, resulting in a cyclic 180$^{\circ}$ flip of dipole direction within the wall. Density functional theory calculations reveal that Pb$_{5}$Ge$_{3}$O$_{11}$ is hyperferroelectric, allowing the system to overcome the depolarization fields that usually suppress antiparallel ordering of dipoles along the longitudinal direction. Interestingly, the antiferroelectric walls observed under the electron beam are energetically more costly than basic head-to-head or tail-to-tail walls. The results suggest a new type of excited domain-wall state, expanding previous studies on ferroelectric domain walls into the realm of antiferroic phenomena.

cond-mat.mtrl-sci↗

arXiv2022

Ultra-High Carrier Mobilities in Ferroelectric Domain Wall Corbino Cones at Room Temperature

Recently, electrically conducting heterointerfaces between dissimilar band-insulators (such as lanthanum aluminate and strontium titanate) have attracted considerable research interest. Charge transport has been thoroughly explored and fundamental aspects of conduction firmly established. Perhaps surprisingly, similar insights into conceptually much simpler conducting homointerfaces, such as the domain walls that separate regions of different orientations of electrical polarisation within the same ferroelectric band-insulator, are not nearly so well-developed. Addressing this disparity, we herein report magnetoresistance in approximately conical 180o charged domain walls, which occur in partially switched ferroelectric thin film single crystal lithium niobate. This system is ideal for such measurements: firstly, the conductivity difference between domains and domain walls is extremely and unusually large (a factor of at least 1013) and hence currents driven through the thin film, between planar top and bottom electrodes, are overwhelmingly channelled along the walls; secondly, when electrical contact is made to the top and bottom of the domain walls and a magnetic field is applied along their cone axes (perpendicular to the thin film surface), then the test geometry mirrors that of a Corbino disc, which is a textbook arrangement for geometric magnetoresistance measurement. Our data imply carriers at the domain walls with extremely high room temperature Hall mobilities of up to ~ 3,700cm2V-1s-1. This is an unparalleled value for oxide interfaces (and for bulk oxides too) and is most comparable to mobilities in other systems typically seen at cryogenic, rather than at room, temperature.

cond-mat.mtrl-sci↗

arXiv2021

Intrinsic ferroelectricity in Y-doped HfO2 thin films

Ferroelectric HfO2-based materials hold great potential for widespread integration of ferroelectricity into modern electronics due to their robust ferroelectric properties at the nanoscale and compatibility with the existing Si technology. Earlier work indicated that the nanometer crystal grain size was crucial for stabilization of the ferroelectric phase of hafnia. This constraint caused high density of unavoidable structural defects of the HfO2-based ferroelectrics, obscuring the intrinsic ferroelectricity inherited from the crystal space group of bulk HfO2. Here, we demonstrate the intrinsic ferroelectricity in Y-doped HfO2 films of high crystallinity. Contrary to the common expectation, we show that in the 5% Y-doped HfO2 epitaxial thin films, high crystallinity enhances the spontaneous polarization up to a record-high 50 μC/cm2 value at room temperature. The high spontaneous polarization persists at reduced temperature, with polarization values consistent with our theoretical predictions, indicating the dominant contribution from the intrinsic ferroelectricity. The crystal structure of these films reveals the Pca21 orthorhombic phase with a small rhombohedral distortion, underlining the role of the anisotropic stress and strain. These results open a pathway to controlling the intrinsic ferroelectricity in the HfO2-based materials and optimizing their performance in applications.

cond-mat.mtrl-sci↗

arXiv2021

Piezoelectricity in hafnia

Because of its compatibility with semiconductor-based technologies, hafnia (HfO$_{2}$) is today's most promising ferroelectric material for applications in electronics. Yet, knowledge on the ferroic and electromechanical response properties of this all-important compound is still lacking. Interestingly, HfO$_2$ has recently been predicted to display a negative longitudinal piezoelectric effect, which sets it apart form classic ferroelectrics (e.g., perovskite oxides like PbTiO$_3$) and is reminiscent of the behavior of some organic compounds. The present work corroborates this behavior, by first-principles calculations and an experimental investigation of HfO$_2$ thin films using piezoresponse force microscopy. Further,the simulations show how the chemical coordination of the active oxygen atoms is responsible for the negative longitudinal piezoelectric effect. Building on these insights, it is predicted that, by controlling the environment of such active oxygens (e.g., by means of an epitaxial strain), it is possible to change the sign of the piezoelectric response of the material.

cond-mat.mtrl-sci↗

arXiv2021

Persistent Ionic Photo-responses and Frank-Condon Mechanism in Proton-transfer Ferroelectrics

Photoexcitation is well-known to trigger electronic metastable states and lead to phenomena like long-lived photoluminescence and photoconductivity. In contrast, persistent photo-response due to ionic metastable states is rare. In this work, we report persistent structural and ferroelectric photo-responses due to proton metastable states via a nuclear quantum mechanism in ferroelectric croconic acid, in which the proton-transfer origin of ferroelectricity is important for the ionic metastable states. We show that, after photoexcitation, the changes of structural and ferroelectric properties relax in about 1000 s, while the photoconductivity decays within 1 s, indicating the dominant ionic origin of the responses. The photogenerated internal bias field that survives polarization switching process suggests another proton transfer route and metastable state, in addition to the metastable states resulting from proton transfer along the hydrogen bonds proposed previously. Analysis based on Frank Condon principle reveals the quantum mechanical nature of the proton-transfer process both within the hydrogen bonds and out of the hydrogen bonds, where small mass of proton and significant change of potential landscape due to the excited electronic states are the key. The demonstration of persistent photo-responses due to the proton metastable states unveils a nuclear quantum mechanism for photo-tunability of materials, which is expected to impact many material properties sensitive to ionic positions.

cond-mat.mtrl-sci↗

arXiv2017

Transient Nature of Negative Capacitance in Ferroelectric Field-Effect Transistors

Negative capacitance (NC) in ferroelectrics, which stems from the imperfect screening of polarization, is considered a viable approach to lower voltage operation in the field-effect transistors (FETs) used in logic switches. In this paper, we discuss the implications of the transient nature of negative capacitance for its practical application. It is suggested that the NC effect needs to be characterized at the proper time scale to identify the type of circuits where functional NC-FETs can be used effectively.

cond-mat.mes-hall↗

arXiv2017

On the persistence of polar domains in ultrathin ferroelectric capacitors

The instability of ferroelectric ordering in ultra-thin films is one of the most important fundamental issues pertaining realization of a number of electronic devices with enhanced functionality, such as ferroelectric and multiferroic tunnel junctions or ferroelectric field effect transistors. In this paper, we investigate the polarization state of archetypal ultrathin (several nanometres) ferroelectric heterostructures: epitaxial single-crystalline BaTiO$_3$ films sandwiched between the most habitual perovskite electrodes, SrRuO$_3$, on top of the most used perovskite substrate, SrTiO$_3$. We use a combination of piezoresponse force microscopy, dielectric measurements and structural characterization to provide conclusive evidence for the ferroelectric nature of the relaxed polarization state in ultrathin BaTiO$_3$ capacitors. We show that even the high screening efficiency of SrRuO$_3$ electrodes is still insufficient to stabilize polarization in SrRuO$_3$/BaTiO$_3$/SrRuO$_3$ heterostructures at room temperature. We identify the key role of domain wall motion in determining the macroscopic electrical properties of ultrathin capacitors and discuss their dielectric response in the light of the recent interest in negative capacitance behaviour.

cond-mat.mtrl-sci↗

arXiv2016

Theoretical Approach to Electroresistance in Ferroelectric Tunnel Junctions

In this paper, a theoretical approach, comprising the non-equilibrium Green's function method for electronic transport and Landau-Khalatnikov equation for electric polarization dynamics, is presented to describe polarization-dependent tunneling electroresistance (TER) in ferroelectric tunnel junctions. Using appropriate contact, interface, and ferroelectric parameters, measured current-voltage characteristic curves in both inorganic (Co/BaTiO$_{3}$/La$_{0.67}$Sr$_{0.33}$MnO$_{3}$) and organic (Au/PVDF/W) ferroelectric tunnel junctions can be well described by the proposed approach. Furthermore, under this theoretical framework, the controversy of opposite TER signs observed experimentally by different groups in Co/BaTiO$_{3}$/La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ systems is addressed by considering the interface termination effects using the effective contact ratio, defined through the effective screening length and dielectric response at the metal/ferroelectric interfaces. Finally, our approach is extended to investigate the role of a CoO$_{x}$ buffer layer at the Co/BaTiO$_{3}$ interface in a ferroelectric tunnel memristor. It is shown that, to have a significant memristor behavior, not only the interface oxygen vacancies but also the CoO$_{x}$ layer thickness may vary with the applied bias.

cond-mat.mes-hall↗

arXiv2016

Room Temperature Ferroelectricity in Continuous Croconic Acid Thin Films

Ferroelectricity at room temperature has been demonstrated in nanometer-thin quasi 2D croconic acid thin films, by the polarization hysteresis loop measurements in macroscopic capacitor geometry, along with observation and manipulation of the nanoscale domain structure by piezoresponse force microscopy. The fabrication of continuous thin films of the hydrogen-bonded croconic acid was achieved by the suppression of the thermal decomposition using low evaporation temperatures in high vacuum, combined with growth conditions far from thermal equilibrium. For nominal coverages >=20 nm, quasi 2D and polycrystalline films, with an average grain size of 50-100 nm and 3.5 nm roughness, can be obtained. Spontaneous ferroelectric domain structures of the thin films have been observed and appear to correlate with the grain patterns. The application of this solvent-free growth protocol may be a key to the development of flexible organic ferroelectric thin films for electronic applications.

cond-mat.mtrl-sci↗

arXiv2016

Hydrodynamics of Domain Walls in Multiferroics: Impact on Memory Devices

We show that switching in ferroelectric lead germanate and lead iron tantalate zirconate titanate (PZTFT) does not resemble the equilibrium domain structure evolution of the Landau-Lifshitz-Kittel model but is instead highly nonequilibrium and similar, respectively, to the Richtmyer-Meshkov instability in liquids and the Helfrich-Hursault sliding instability in liquid crystals. The resulting nano-domain structures in PZTFT are circular or parabolic and involving folding bifurcations. These may have an undesirable impact on ferroelectric thin-film memoriesthat are also ferroelastic.

cond-mat.mtrl-sci↗

arXiv2011

Ultrathin BaTiO3 templates for multiferroic nanostructures

Structural, electronic and dielectric properties of high-quality ultrathin BaTiO3 films are investigated. The films, which are grown by ozone-assisted molecular beam epitaxy on Nb-doped SrTiO3 (001) substrates and having thicknesses as thin 8 unit cells (3.2 nm), are unreconstructed and atomically smooth with large crystalline terraces. A strain-driven transition to 3D island formation is observed for films of of 13 unit cells thickness (5.2 nm). The high structural quality of the surfaces, together with the dielectric properties similar to bulk BaTiO3 and dominantly TiO2 surface termination, make these films suitable templates for the synthesis of high-quality metal-oxide multiferroic heterostructures for the fundamental study and exploitation of magneto-electric effects, such as a recently proposed interface effect in Fe/BaTiO3 heterostructures based on Fe-Ti interface bonds.

cond-mat.mtrl-sci↗

arXiv2008

Imaging Mechanism of Piezoresponse Force Microscopy in Capacitor Structures

The image formation mechanism in Piezoresponse Force Microscopy (PFM) of capacitor structures is analyzed. We demonstrate that the spatial resolution is a bilinear function of film and top electrode thicknesses, and derive the corresponding analytical expressions. For many perovskites, the opposite contributions of d31 and d33 components can result in anomalous domain wall profiles. This analysis establishes the applicability limits of PFM for polarization dynamics studies in capacitors, and applies to other structural probes, including focused X-ray studies of capacitor structures.

cond-mat.mtrl-sci↗