Hafnia (HfO2) is a promising material for emerging chip applications due to its high-k dielectric behaviour, suitability for negative capacitance heterostructures, scalable ferroelectricity, and silicon compatibility. The lattice dynamics along with phononic properties such as thermal conductivity, contraction, and heat capacity are under-explored, primarily due to the absence of high quality single crystals. Herein, we report the vibrational properties of a series of HfO2 crystals stabilized with yttrium (chemical formula HfO2:xY, where x = 20, 12, 11, 8, and 0%) and compare our findings with a symmetry analysis and lattice dynamics calculations. We untangle the effects of Y by testing our calculations against the measured Raman and infrared spectra of the cubic, antipolar orthorhombic, and monoclinic phases and then proceed to reveal the signature modes of polar orthorhombic hafnia. This work provides a spectroscopic fingerprint for several different phases of HfO2 and paves the way for an analysis of mode contributions to high-k dielectric and ferroelectric properties for chip technologies.
The question of whether one can systematically identify (previously unknown) ferroelectric phases of a given material is addressed, taking hafnia (HfO$_2$) as an example. Low free energy phases at various pressures and temperatures are identified using a first-principles based structure search algorithm. Ferroelectric phases are then recognized by exploiting group theoretical principles for the symmetry-allowed displacive transitions between non-polar and polar phases. Two orthorhombic polar phases occurring in space groups $Pca2_1$ and $Pmn2_1$ are singled out as the most viable ferroelectric…
Ferroelectric hafnia is being explored for next generation electronics due to its robust ferroelectricity in nanoscale samples and its compatibility with silicon. However, its ferroelectricity is not understood. Other ferroelectrics usually lose their ferroelectricity for nanoscopic samples and thin films, and the hafnia ground state is non-polar baddeleyite. Here we study hafnia with density functional theory (DFT) under epitaxial strain, and find that strain not only stabilizes the ferroelectric phases, but also leads to unstable modes and a downhill path in energy from the high temperature …
Here, we optimized the annealing temperature of HZO/TiN thin film heterostructure via multiscale analysis of remnant polarization, crystallographic phase, minimum ferroelectric domain size, and average grain size. We found that the remnant polarization was closely related to the relative amount of the orthorhombic phase whereas the minimum domain size was to the relative amount of the monoclinic phase. The minimum domain size was obtained at the annealing temperature of 500$^\cird$C while the optimum remnant polarization and capacitance at the annealing temperature of 600$^\circ$C. We conclude…
We performed density functional theory (DFT) calculations on epitaxially strained hafnia. We demonstrate the stabilization of the ferroelectric ($Pca2_{1}$) phase from the antiferroelectric ($Pbcn$) in bulk hafnia in the presence of electric field. We found that the polar ($Pca2_{1}$) phase can be efficiently stabilized with an adequate choice of film orientation. We show that for a (010)-oriented Pbcn, the ferroelectric $Pca2_{1}$ phase can be reached with a relatively small electric field ($\mid\varepsilon\mid\geq 150$ KV/m). We thus provide a simple explanation to the experimental observati…
Although technologically challenging, the integration of ferroelectric thin films with graphene spintronics potentially allows the realization of highly efficient, electrically tuneable, non-volatile memories. Here, the atomic layer deposition (ALD) of ferroelectric Hf$_{0.5}$Zr$_{0.5}$O$_2$ (HZO) directly on graphene (Gr)/Co/heavy metal (HM) epitaxial stacks is investigated via the implementation of several nucleation methods. With an in-situ method employing an Al$_2$O$_3$ layer, the HZO demonstrates a remanent polarization (2Pr) of 19.2 $μC/cm^2$. An ex-situ, naturally oxidized sputtered Ta…
Ferroelectric tunnel junctions (FTJ) based on hafnium zirconium oxide (Hf1-xZrxO2; HZO) are a promising candidate for future applications, such as low-power memories and neuromorphic computing. The tunneling electroresistance (TER) is tunable through the polarization state of the HZO film. To circumvent the challenge of fabricating thin ferroelectric HZO layers in the tunneling range of 1-3 nm range, ferroelectric/dielectric double layer sandwiched between two symmetric metal electrodes are used. Due to the decoupling of the ferroelectric polarization storage layer and a dielectric tunneling l…
Ferroelectricity in binary oxides including hafnia and zirconia have riveted the attention of the scientific community due to highly unconventional physical mechanisms and the potential for integration of these materials into semiconductor workflows. Over the last decade, it has been argued that behaviors such as wake-up phenomena and an extreme sensitivity to electrode and processing conditions suggests that ferroelectricity in these materials is strongly coupled with additional mechanisms, with possible candidates including the ionic subsystem or strain. Here we argue that the properties of …
Guidelines for the optimization of hafnia-based ferroelectrics through superlattice engineering
University of Groningen（格罗宁根大学）；Luxembourg Institute for Science and Technology (LIST)（卢森堡科学与技术研究所）；University of Twente（特温特大学）；University of Luxembourg（卢森堡大学）
Disentangling the ferroelectric phases of epitaxial hafnia
Zernike Institute for Advanced Materials, University of Groningen（格罗宁根大学泽尔尼克高级材料研究所）；The European Synchrotron Radiation Facility (ESRF)（欧洲同步辐射装置）；CogniGron centre, University of Groningen（格罗宁根大学认知科学中心）
Films of all-important compound hafnia (HfO2) can be prepared in an orthorhombic ferroelectric (FE) state that is ideal for applications, e.g. in memories or negative-capacitance field-effect transistors. The origin of this FE state remains a mystery, though, as none of the proposed mechanisms for its stabilization -- from surface and size effects to formation kinetics -- is fully convincing. Interestingly, it is known that doping HfO2 with various cations favors the occurrence of the FE polymorph; however, existing first-principles works suggest that doping by itself is not sufficient to stab…
The discovery of ferroelectric properties of binary oxides revitalized the interest in ferroelectrics and bridged the scaling gap between the state-of-the-art semiconductor technology and ferroelectric memories. However, before hitting the markets, the origin of ferroelectricity and in-depth studies of device characteristics are needed. Establishing a correlation between the performance of the device and underlying physical mechanisms is the first step toward understanding the device and engineering guidelines for a novel, superior device. Therefore, in this paper a holistic modeling approache…
Twist-induced Out-of-plane Ferroelectricity in Bilayer Hafnia
Institute of Materials Science and Engineering, Washington University in St. Louis（华盛顿大学圣路易斯分校材料科学与工程研究所）；Department of Mechanical Engineering and Material Science, Washington University in St. Louis（华盛顿大学圣路易斯分校机械工程和材料科学系）；Department of Physics and Astronomy, University of Nebraska, Lincoln（内布拉斯加大学林肯分校物理与天文学系）；Department of Physics, Washington University in St. Louis（华盛顿大学圣路易斯分校物理系）
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 fir…
Engineering Grain Boundary Commensurability for Ferroelectric Stabilization in Hafnia-Based Films
Rice University（莱斯大学）；Rice Advanced Materials Institute, Rice University（莱斯大学先进材料研究所）；University of Pennsylvania（宾夕法尼亚大学）；Texas A&M University（德克萨斯农工大学）；University of California, Berkeley（加州大学伯克利分校）；Advanced Photon Source, Argonne National Laboratory（阿贡国家实验室先进光子源）；Smalley-Curl Institute, Rice University（莱斯大学斯马利-柯尔研究所）
Atom Probe Tomography as an Emerging Tool for Understanding Defect-driven Mechanisms in HfO$_{2}$-based Ferroelectrics
Norwegian University of Science and Technology (NTNU)（挪威科技大学）；Helmholtz-Zentrum Berlin für Materialien und Energie（柏林亥姆霍兹材料能源中心）；Freie Universität Berlin（柏林自由大学）；University of Nebraska, Lincoln（内布拉斯加大学林肯分校）；University of Duisburg-Essen（杜伊斯堡-埃森大学）；Research Center Future Energy Materials and Systems（未来能源材料与系统研究中心）
Device-area selection of memristive transport regimes in epitaxial $Hf_{0.5}Zr_{0.5}O_{2}$-based ferroelectric devices
INN-CONICET-CNEA（阿根廷国家原子能委员会-国家科学研究委员会研究所）；Universidad Argentina de la Empresa (UADE)（阿根廷企业大学）；Instituto de Nanociencia y Materiales de Aragón (INMA-CSIC)（阿拉贡纳米科学与材料研究所）；Universidad de Zaragoza（萨拉戈萨大学）；Catalan Institute of Nanoscience and Nanotechnology (ICN2)（加泰罗尼亚纳米科学与技术研究所）；Centre de Nanosciences et de Nanotechnologies, CNRS, Université Paris-Saclay（巴黎萨克雷大学-CNRS纳米科学中心）；University of Groningen（格罗宁根大学）
Magnetoelectric coupling in ferromagnet/multiferroic systems is often manifested in the exchange bias effect, which may have combined contributions from multiple sources, such as domain walls, chemical defects or strain. In this study we magnetically "fingerprint" the coupling behavior of CoFe grown on epitaxial BiFeO3 (BFO) thin films by magnetometry and first-order-reversal-curves (FORC). The contribution to exchange bias from 71°, 109° and charged ferroelectric domain walls (DWs) was elucidated by the FORC distribution. CoFe samples grown on BFO with 71° DWs only exhibit an enhancement of t…
HfO$_2$ is an important high-$κ$ dielectric and ferroelectric, exhibiting a complex potential energy landscape with several phases close in energy. It is, however, a strongly anharmonic solid, and thus describing its temperature-dependent behavior is methodologically challenging. We propose an approach based on self-consistent, effective harmonic potentials and higher-order corrections to study the potential energy surface of anharmonic materials. The introduction of a reweighting procedure enables the usage of unregularized regression methods and efficiently harnesses the information containe…
Wake-up effect is still an obstacle in the commercialization of hafnia-based ferroelectric thin films. In this work, we investigate the effect of defects, controlled by ozone dosage, on the field cycling behavior of the atomic layer deposited Hf0.5Zr0.5O2 (HZO) films. A nearly wake-up free device was achieved after reduction of carbon contamination and oxygen defects by increasing the ozone dosage. The sample which was grown at 30 sec ozone pulse duration shows about 98% of the woken-up Pr at the pristine state while those grown below 5 sec ozone pulse time show a pinched hysteresis loop, unde…
Stress-Induced Ferroelectricity in Hafnium Oxide Core-Shell Nanoparticles
Institute of Physics of the National Academy of Sciences of Ukraine（乌克兰国家科学院物理研究所）；Frantsevich Institute for Problems in Materials Science of the National Academy of Sciences of Ukraine（乌克兰国家科学院弗拉特谢维奇材料科学问题研究所）；Department of Materials Science and Engineering, University of Tennessee, Knoxville（田纳西大学诺克斯维尔分校材料科学与工程系）；Zone 5 Technologies（第五区技术公司）
