Because of their immense scalability and manufacturability potential, the HfO2-based ferroelectric films attract significant attention as strong candidates for application in ferroelectric memories and related electronic devices. Here, we report the ferroelectric behavior of ultrathin Hf0.5Zr0.5O2 films, with the thickness of just 2.5 nm, which makes them suitable for use in ferroelectric tunnel junctions, thereby further expanding the area of their practical application. Transmission electron microscopy and electron diffraction analysis of the films grown on highly doped Si substrates confirms formation of the fully crystalline non-centrosymmetric orthorhombic phase responsible for ferroelectricity in Hf0.5Zr0.5O2. Piezoresponse force microscopy and pulsed switching testing performed on the deposited top TiN electrodes provide further evidence of the ferroelectric behavior of the Hf0.5Zr0.5O2 films. The electronic band lineup at the top TiN/Hf0.5Zr0.5O2 interface and band bending at the adjacent n(+)-Si bottom layer attributed to the polarization charges in Hf0.5Zr0.5O2 have been determined using in situ X-ray photoelectron spectroscopy analysis. The obtained results represent a significant step toward the experimental implementation of Si-based ferroelectric tunnel junctions.

Visualization of domain structure evolution under an electrical bias has been carried out in ferroelectric La:HfO2 capacitors by a combination of Piezoresponse Force Microscopy (PFM) and pulse switching techniques to study the nanoscopic mechanism of polarization reversal and the wake-up process. It has been directly shown that the main mechanism behind the transformation of the polarization hysteretic behavior and an increase in the remanent polarization value upon the alternating current cycling is electrically induced domain de-pinning. PFM imaging and local spectroscopy revealed asymmetric switching in the La:HfO2 capacitors due to a significant imprint likely caused by the different boundary conditions at the top and bottom interfaces. Domain switching kinetics can be well-described by the nucleation limited switching model characterized by a broad distribution of the local switching times. It has been found that the domain velocity varies significantly throughout the switching process indicating strong interaction with structural defects.

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Because of their full compatibility with the modern Si-based technology, the HfO-based ferroelectric films have recently emerged as viable candidates for application in nonvolatile memory devices. However, despite significant efforts, the mechanism of the polarization switching in this material is still under debate. In this work, we elucidate the microscopic nature of the polarization switching process in functional HfZrO-based ferroelectric capacitors during its operation. In particular, the static domain structure and its switching dynamics following the application of the external electric field have been monitored with the advanced piezoresponse force microscopy (PFM) technique providing a nm resolution. Separate domains with strong built-in electric field have been found. Piezoresponse mapping of pristine HfZrO films revealed the mixture of polar phase grains and regions with low piezoresponse as well as the continuum of polarization orientations in the grains of polar orthorhombic phase. PFM data combined with the structural analysis of pristine versus trained film by plan-view transmission electron microscopy both speak in support of a monoclinic-to-orthorhombic phase transition in ferroelectric HfZrO layer during the wake-up process under an electrical stress.

Because of its compatibility with semiconductor-based technologies, hafnia (HfO2) 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, HfO2 has recently been predicted to display a negative longitudinal piezoelectric effect, which sets it apart from classic ferroelectrics (e.g., perovskite oxides like PbTiO3) 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 HfO2 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.

Ferroelectric (FE) HfO 2 -based thin films, which are considered as one of the most promising material systems for memory device applications, exhibit an adverse tendency for strong imprint. Manifestation of imprint is a shift of the polarization−voltage (P−V) loops along the voltage axis due to the development of an internal electric bias, which can lead to the failure of the writing and retention functions. Here, to gain insight into the mechanism of the imprint effect in La-doped HfO 2 (La:HfO 2 ) capacitors, we combine the pulse switching technique with high-resolution domain imaging by means of piezoresponse force microscopy. This approach allows us to establish a correlation between the macroscopic switching characteristics and domain time−voltage-dependent behavior. It has been shown that the La:HfO 2 capacitors exhibit a much more pronounced imprint compared to Pb(Zr,Ti)O 3 -based FE capacitors. Also, in addition to conventional imprint, which evolves with time in the poled capacitors, an easily changeable imprint, termed as "fluid imprint", with a strong dependence on the switching prehistory and measurement conditions, has been observed. Visualization of the domain structure reveals a specific signature of fluid imprintcontinuous switching of polarization in the same direction as the previously applied field that continues a long time after the field was turned off. This effect, termed as "inertial switching", is attributed to charge injection and subsequent trapping at defect sites at the film−electrode interface.

One of the general features of ferroelectric systems is a complex nature of polarization reversal, which involves domain nucleation and motion of domain walls. Here, time‐resolved nanoscale domain imaging is applied in conjunction with the integral switching current measurements to investigate the mechanism of polarization reversal in yttrium‐doped HfO2 (Y:HfO2)—currently one of the most actively studied ferroelectric systems. More specifically, the effect of film microstructure on the nucleation process is investigated by performing a comparative study of the polarization switching behavior in the epitaxial and polycrystalline Y:HfO2 thin film capacitors. It is found that although the epitaxial Y:HfO2 capacitors tend to switch slower than their polycrystalline counterparts, they exhibit a significantly higher nucleation density and rate, suggesting that this is a rate‐limiting mechanism. In addition, it is observed that under the external fields approaching the activation field value, the switching kinetics can be described equally well by the nucleation limited switching and the Kolmogorov‐Avrami‐Ishibashi models for both types of capacitors. This signifies convergence of two different mechanisms implying that the polarization reversal proceeds via a homogeneous nucleation process unaffected by the film microstructure, which can be considered as approaching the intrinsic switching limit.

hysteresis observed in macroscopic polarization-voltage (P-V) measurements in HfO 2 and ZrO 2 thin films is a signature of AFE device functionality and originates from a complex interplay of favorable crystallization conditions. [3,5] Underlying electric field-induced FE structural phase transitions are hypothesized to be responsible for generating the distinctive features of antiferroelectricity in the fluorite material system, yet little is known about how to best exploit such phase transitions in fluorites. [4,6] Alternative theories of AFE-like behavior, such as oppositely imprinted FE domains and paraelectric-to-paraelectric phase transitions, need to be evaluated since the approach to achieve superior AFE device performance will depend on the underlying cause of macroscopic antiferroelectricity. [7][8][9][10] Due to the simplicity of controlling the film thickness, the size effect may be easily leveraged to influence and gain a better understanding of antiferroelectricity in ZrO 2 , but previous film thickness studies on ZrO 2 and Hf 1−x Zr x O 2 have not quantified the size-effect influence on supercapacitor performance. [11][12][13][14] A theory of AFE crystals was first formulated by Kittel in which two sub-lattices of opposing polarization are described by a Landau-Devonshire free energy model. [15] There is also increasing acceptance for a more general definition of antiferroelectricity to include reversible electric field induced first-order phase transitions between a paraelectric (nonpolar) and a FE (polar) phase that do not necessarily involve an antipolar crystal structure. [3,5,16,17] In this work, AFE is used in the expanded sense of the term to include field-induced phase transitions that may not specifically involve an antipolar crystal phase.While the diverse polymorphism of HfO 2 and ZrO 2 ceramics has been known for decades, [18] the emergence of a FE phase in thin films of these materials over the past decade has revealed fresh insight on new structural phases and interrelationships. [19][20][21] With the identification of the Pca2 1 polar orthorhombic (o) phase in HfO 2 thin films, [22] the possibility of electric-field induced phase transitions in fluorites surfaced with functional electronic AFE properties. [3,6,20] First-order electric field driven phase transitions between the nonpolar tetragonal and polar orthorhombic phases in HfO 2 and ZrO 2 are postulated to be the origin of antiferroelectricity inThe unique nonlinear dielectric properties of antiferroelectric (AFE) oxides are promising for advancements in solid state supercapacitor, actuator, and memory technologies. AFE behavior in high-k ZrO 2 is of particular technological interest, but the origin of antiferroelectricity in ZrO 2 remains questionable. The theory of reversible electric field-induced phase transitions between the nonpolar P4 2 /nmc tetragonal phase and the polar Pca2 1 orthorhombic phase is experimentally tested with local structural and electromechanical characterization of AFE ZrO 2 thin films. Piezoresponse f...