Ultralow Subthreshold Swing of a MOSFET Caused by Ferroelectric Polarization Reversal of Hf0.5Zr0.5O2 Thin Films

The emergence of complementary metal-oxide semiconductor (CMOS)-compatible HfO2-based ferroelectric materials provides a promising way to achieve ferroelectric field-effect transistors (FeFETs) with a steep subthreshold swing (SS) reduced to below the Boltzmann thermodynamics limit (∼60 mV/dec at room temperature), which has important implications for lowering power consumption. In this work, a metal-oxide-semiconductor field-effect transistor (MOSFET) is connected with Hf0.5Zr0.5O2 (HZO)-based ferroelectric capacitors with different capacitances. By adjusting the capacitance of ferroelectric capacitors, an ultralow SS of ∼0.34 mV/dec in HfO2-based FeFETs can be achieved. More interestingly, by designing the sweeping voltage sequences, the SS can be adjusted to be 0 mV/dec with the drain current ranging over six orders of magnitude, and the threshold voltage for turning on the MOSFET can be further reduced. The manipulated SS could be attributed to the evolution of ferroelectric switching. Our work contributes to understanding the origin of ultralow SS in ferroelectric MOSFETs and the realization of low-power devices.

Single-Gate In-Transistor Readout of Current Superposition and Collapse Utilizing Quantum Tunneling and Ferroelectric Switching

In nanostructure assemblies, the superposition of current paths forms microscopic electric circuits, and different circuit networks produce varying results, particularly when utilized as transistor channels for computing applications. However, the intricate nature of assembly networks and the winding paths of commensurate currents hinder standard circuit modeling. Inspired by the quantum collapse of superposition states for information decoding in quantum circuits, the implementation of analogous current path collapse to facilitate the detection of microscopic circuits by modifying their network topology is explored. Here, the superposition and collapse of current paths in gate-all-around polysilicon nanosheet arrays are demonstrated to enrich the computational resources within transistors by engineering the channel length and quantity. Switching the ferroelectric polarization of Hf0.5 Zr0.5 O2 gate dielectric, which drives these transistors out-of-equilibrium, decodes the output polymorphism through circuit topological modifications. Furthermore, a protocol for the single-electron readout of ferroelectric polarization is presented with tailoring the channel coherence. The introduction of lateral path superposition results into intriguing metal-to-insulator transitions due to transient behavior of ferroelectric switching. This ability to adjust the current networks within transistors and their interaction with ferroelectric polarization in polycrystalline nanostructures lays the groundwork for generating diverse current characteristics as potential physical databases for optimization-based computing.

Effects of Charge Trapping on Memory Characteristics for HfO2-Based Ferroelectric Field Effect Transistors

A full understanding of the impact of charge trapping on the memory window (MW) of HfO₂-based ferroelectric field effect transistors (FeFETs) will permit the design of program and erase protocols, which will guide the application of these devices and maximize their useful life. The effects of charge trapping have been studied by changing the parameters of the applied program and erase pulses in a test sequence. With increasing the pulse amplitude and pulse width, the MW increases first and then decreases, a result attributed to the competition between charge trapping (CT) and ferroelectric switching (FS). This interaction between CT and FS is analyzed in detail using a single-pulse technique. In addition, the experimental data show that the conductance modulation characteristics are affected by the CT in the analog synaptic behavior of the FeFET. Finally, a theoretical investigation is performed in Sentaurus TCAD, providing a plausible explanation of the CT effect on the memory characteristics of the FeFET. This work is helpful to the study of the endurance fatigue process caused by the CT effect and to optimizing the analog synaptic behavior of the FeFET.

Why it is Unfortunate that Linear Machine Learning "Works" so well in Electromechanical Switching of Ferroelectric Thin Films

Machine learning (ML) is relied on for materials spectroscopy. It is challenging to make ML models fail because statistical correlations can mimic the physics without causality. Here, using a benchmark band-excitation piezoresponse force microscopy polarization spectroscopy (BEPS) dataset the pitfalls of the so-called "better", "faster", and "less-biased" ML of electromechanical switching are demonstrated and overcome. Using a toy and real experimental dataset, it is demonstrated how linear nontemporal ML methods result in physically reasonable embedding (eigenvalues) while producing nonsensical eigenvectors and generated spectra, promoting misleading interpretations. A new method of unsupervised multimodal hyperspectral analysis of BEPS is demonstrated using long-short-term memory (LSTM) β-variational autoencoders (β-VAEs) . By including LSTM neurons, the ordinal nature of ferroelectric switching is considered. To improve the interpretability of the latent space, a variational Kullback-Leibler-divergency regularization is imposed . Finally, regularization scheduling of β as a disentanglement metric is leveraged to reduce user bias. Combining these experiment-inspired modifications enables the automated detection of ferroelectric switching mechanisms, including a complex two-step, three-state one. Ultimately, this work provides a robust ML method for the rapid discovery of electromechanical switching mechanisms in ferroelectrics and is applicable to other multimodal hyperspectral materials spectroscopies.

High Velocity, Low-Voltage Collective In-Plane Switching in (100) BaTiO3 Thin Films

Ferroelectrics are being increasingly called upon for electronic devices in extreme environments. Device performance and energy efficiency is highly correlated to clock frequency, operational voltage, and resistive loss. To increase performance it is common to engineer ferroelectric domain structure with highly-correlated electrical and elastic coupling that elicit fast and efficient collective switching. Designing domain structures with advantageous properties is difficult because the mechanisms involved in collective switching are poorly understood and difficult to investigate. Collective switching is a hierarchical process where the nano- and mesoscale responses control the macroscopic properties. Using chemical solution synthesis, epitaxially nearly-relaxed (100) BaTiO₃ films are synthesized. Thermal strain induces a strongly-correlated domain structure with alternating domains of polarization along the [010] and [001] in-plane axes and 90° domain walls along the [011] or [01 1 ¯ $\bar{1}$ ] directions. Simultaneous capacitance-voltage measurements and band-excitation piezoresponse force microscopy revealed strong collective switching behavior. Using a deep convolutional autoencoder, hierarchical switching is automatically tracked and the switching pathway is identified. The collective switching velocities are calculated to be ≈500 cm s^-1 at 5 V (7 kV cm^-1 ), orders-of-magnitude faster than expected. These combinations of properties are promising for high-speed tunable dielectrics and low-voltage ferroelectric memories and logic.

Atomic Visualization and Switching of Ferroelectric Order in β-In2 Se3 Films at the Single Layer Limit

2D ferroelectrics have received wide interest due to the remarkable quantum states of emerging physics at reduced dimensionality, associated with their exotic properties in high-performance and nonvolatile functional devices. Here, by combing molecular beam epitaxy synthesis and scanning tunneling microscopy characterization, two metastable phases of layered In₂ Se₃ films: β'- and β*-In₂ Se₃ are reported, which develop different types of in-plane spontaneous polarizations, thus resulting in different striped morphologies. The anti-ferroelectric order in β'-In₂ Se₃ and ferroelectric order of β*-In₂ Se₃ are identified, respectively, down to the 2D limit by comprehensive investigations of structural and spectroscopic signatures, including the lattice distortion, the spatial profile of images, the formation of domain structure, and the electronic band-bending by polarization charges at edges. The ferroelectric switching between those two phases are further controlled via applying an electric field generated from the scanning tunneling microscopy tip in a reversible manner. The intriguing tunability between the (anti-)ferroelectric orders in the 2D limit provides a promising platform for studying the interplay between electronic structure and ferroelectricity in van der Waals materials, and promotes potential development of miniaturized transistors and memory devices based on electric polarizations.

Ferroelectric field-effect transistors based on HfO2: a review

In this article, we review the recent progress of ferroelectric field-effect transistors (FeFETs) based on ferroelectric hafnium oxide (HfO₂), ten years after the first report on such a device. With a focus on the use of FeFET for nonvolatile memory application, we discuss its basic operation principles, switching mechanisms, device types, material properties and array structures. Key device performance metrics such as cycling endurance, retention, memory window, multi-level operation and scaling capability are analyzed. We also briefly survey recent developments in alternative applications for FeFETs including neuromorphic and in-memory computing as well as radiofrequency devices.

Superelastic oxide micropillars enabled by surface tension-modulated 90° domain switching with excellent fatigue resistance

Superelastic materials capable of recovering large nonlinear strains are ideal for a variety of applications in morphing structures, reconfigurable systems, and robots. However, making oxide materials superelastic has been a long-standing challenge due to their intrinsic brittleness. Here, we fabricate ferroelectric BaTiO₃ (BTO) micropillars that not only are superelastic but also possess excellent fatigue resistance, lasting over 1 million cycles without accumulating residual strains or noticeable variation in stress-strain curves. Phase field simulations reveal that the large recoverable strains of BTO micropillars arise from surface tension-modulated 90° domain switching and thus are size dependent, while the small energy barrier and ultralow energy dissipation are responsible for their unprecedented cyclic stability among superelastic materials. This work demonstrates a general strategy to realize superelastic and fatigue-resistant domain switching in ferroelectric oxides for many potential applications.

The time-resolved hard X-ray diffraction endstation KMC-3 XPP at BESSY II

The time-resolved hard X-ray diffraction endstation KMC-3 XPP for optical pump/X-ray probe experiments at the electron storage ring BESSY II is dedicated to investigating the structural response of thin film samples and heterostructures after their excitation with ultrashort laser pulses and/or electric field pulses. It enables experiments with access to symmetric and asymmetric Bragg reflections via a four-circle diffractometer and it is possible to keep the sample in high vacuum and vary the sample temperature between ∼15 K and 350 K. The femtosecond laser system permanently installed at the beamline allows for optical excitation of the sample at 1028 nm. A non-linear optical setup enables the sample excitation also at 514 nm and 343 nm. A time-resolution of 17 ps is achieved with the `low-α' operation mode of the storage ring and an electronic variation of the delay between optical pump and hard X-ray probe pulse conveniently accesses picosecond to microsecond timescales. Direct time-resolved detection of the diffracted hard X-ray synchrotron pulses use a gated area pixel detector or a fast point detector in single photon counting mode. The range of experiments that are reliably conducted at the endstation and that detect structural dynamics of samples excited by laser pulses or electric fields are presented.

Ferroelectric Switching in Trilayer Al2O3/HfZrOx/Al2O3 Structure

Since ferroelectricity has been observed in simple binary oxide material systems, it has attracted great interest in semiconductor research fields such as advanced logic transistors, non-volatile memories, and neuromorphic devices. The location in which the ferroelectric devices are implemented depends on the specific application, so the process constraints required for device fabrication may be different. In this study, we investigate the ferroelectric characteristics of Zr doped HfO₂ layers treated at high temperatures. A single HfZrO_x layer deposited by sputtering exhibits polarization switching after annealing at a temperature of 850 °C. However, the achieved ferroelectric properties are vulnerable to voltage stress and higher annealing temperature, resulting in switching instability. Therefore, we introduce an ultrathin 1-nm-thick Al₂O₃ layer at both interfaces of the HfZrO_x. The trilayer Al₂O₃/HfZrO_x/Al₂O₃ structure allows switching parameters such as remnant and saturation polarizations to be immune to sweeping voltage and pulse cycling. Our results reveal that the trilayer not only makes the ferroelectric phase involved in the switching free from pinning, but also preserves the phase even at high annealing temperature. Simultaneously, the ferroelectric switching can be improved by preventing leakage charge.

Back-End, CMOS-Compatible Ferroelectric Field-Effect Transistor for Synaptic Weights

Neuromorphic computing architectures enable the dense colocation of memory and processing elements within a single circuit. This colocation removes the communication bottleneck of transferring data between separate memory and computing units as in standard von Neuman architectures for data-critical applications including machine learning. The essential building blocks of neuromorphic systems are nonvolatile synaptic elements such as memristors. Key memristor properties include a suitable nonvolatile resistance range, continuous linear resistance modulation, and symmetric switching. In this work, we demonstrate voltage-controlled, symmetric and analog potentiation and depression of a ferroelectric Hf_0.57Zr_0.43O₂ (HZO) field-effect transistor (FeFET) with good linearity. Our FeFET operates with low writing energy (fJ) and fast programming time (40 ns). Retention measurements have been performed over 4 bit depth with low noise (1%) in the tungsten oxide (WO_x) readout channel. By adjusting the channel thickness from 15 to 8 nm, the on/off ratio of the FeFET can be engineered from 1 to 200% with an on-resistance ideally >100 kΩ, depending on the channel geometry. The device concept is using earth-abundant materials and is compatible with a back end of line (BEOL) integration into complementary metal-oxide-semiconductor (CMOS) processes. It has therefore a great potential for the fabrication of high-density, large-scale integrated arrays of artificial analog synapses.

Modeling of Negative Capacitance in Ferroelectric Thin Films

The negative capacitance (NC) effect in ferroelectric thin films has attracted a great deal of attention from the material and semiconductor device communities because it could be a possible solution to the impending problems related to field-effect transistor power consumption and dynamic random-access memory charge loss. A short discussion on the fundamental premise of the NC effect is presented. A phase-field model based on the time-dependent Ginzburg-Landau (TDGL) formalism in conjunction with the Chensky-Tarasenko (C-T) formalism for multidomain configuration is then developed to reveal the subtle correlation between the domain wall motion and NC effect for different thicknesses of ferroelectric and dielectric films. When a ferroelectric film becomes thin enough, a stripe domain structure can be achieved through competition between the electrostatic energy and domain wall energy. This stripe domain structure is quite resilient to transition to a homogeneous polarization state, making it very useful for (quasi-)static NC operation. Finally, the physical implications of the numerical results are explored with analytical modeling. It is identified that the domain wall motion in the stripe domain structure remains dominated by the external field, even when the entire film is in the (quasi-)static NC state.

Elasticity Modulation Due to Polarization Reversal and Ionic Motion in the Ferroelectric Superionic Conductor KTiOPO4

The coupling between ionic degrees of freedom and ferroelectricity has received renewed attention in recent years, given that surface electrochemical processes have been shown to be intrinsically linked to ferroelectric phase stability in ultrathin ferroelectric films. However, the coupling between bulk ionic transport and local polarization switching has received less attention, as typically the bulk ionic mobilities are low for common ferroelectrics at room temperature. Here, we use the coupled band-excitation method in conjunction with site-correlated time-of-flight secondary ion mass spectrometry, to determine the coupling between ferroelectric switching and ionic motion in single crystal KTiOPO₄. The local scanning probe measurements indicate a substantial softening, as determined by resonant frequency changes, during reversal of polarization along one direction. These changes are correlated with the mass spectrometry measurements, showing a polarization-dependent accumulation of K ions at the polar surfaces, thus corroborating their role in the screening process. These studies shed light on the interplay between ionic dynamics and bulk ferroelectric switching and have implications for studies on domain wall conductivity, chemical switching, and bulk and surface-screening phenomena.

Accumulative Polarization Reversal in Nanoscale Ferroelectric Transistors

The electric-field-driven and reversible polarization switching in ferroelectric materials provides a promising approach for nonvolatile information storage. With the advent of ferroelectricity in hafnium oxide, it has become possible to fabricate ultrathin ferroelectric films suitable for nanoscale electronic devices. Among them, ferroelectric field-effect transistors (FeFETs) emerge as attractive memory elements. While the binary switching between the two logic states, accomplished through a single voltage pulse, is mainly being investigated in FeFETs, additional and unusual switching mechanisms remain largely unexplored. In this work, we report the natural property of ferroelectric hafnium oxide, embedded within a nanoscale FeFET, to accumulate electrical excitation, followed by a sudden and complete switching. The accumulation is attributed to the progressive polarization reversal through localized ferroelectric nucleation. The electrical experiments reveal a strong field and time dependence of the phenomenon. These results not only offer novel insights that could prove critical for memory applications but also might inspire to exploit FeFETs for unconventional computing.

Ferroelectricity in Hf0.5Zr0.5O2 Thin Films: A Microscopic Study of the Polarization Switching Phenomenon and Field-Induced Phase Transformations

Because of their full compatibility with the modern Si-based technology, the HfO₂-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 Hf_0.5Zr_0.5O₂-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 Hf_0.5Zr_0.5O₂ 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 Hf_0.5Zr_0.5O₂ layer during the wake-up process under an electrical stress.

Effect of Polarization Reversal in Ferroelectric TiN/Hf0.5Zr0.5O2/TiN Devices on Electronic Conditions at Interfaces Studied in Operando by Hard X-ray Photoemission Spectroscopy

Because of their compatibility with modern Si-based technology, HfO₂-based ferroelectric films have recently attracted attention as strong candidates for applications in memory devices, in particular, ferroelectric field-effect transistors or ferroelectric tunnel junctions. A key property defining the functionality of these devices is the polarization dependent change of the electronic band alignment at the metal/ferroelectric interface. Here, we report on the effect of polarization reversal in functional ferroelectric TiN/Hf_0.5Zr_0.5O₂/TiN capacitors on the potential distribution across the stack and the electronic band line-up at the interfaces studied in operando by hard X-ray photoemission spectroscopy. By tracking changes in the position of Hf_0.5Zr_0.5O₂ core-level lines with respect to those of the TiN electrode in both short- and open-circuit configurations following in situ polarization reversal, we derive the conduction band offset to be 0.7 (1.0) eV at the top and 1.7 (1.0) eV at the bottom interfaces for polarization, pointing up (down), respectively. Energy dispersive X-ray spectroscopy profiling of the sample cross-section in combination with the laboratory X-ray photoelectron spectroscopy reveal the presence of a TiO_x/TiON layer at  both interfaces. The observed asymmetry in the band line-up changes in the TiN/Hf_0.5Zr_0.5O₂/TiN memory stack is explained by different origin of these oxidized layers and effective pinning of polarization at the top interface. The described methodology and first experimental results are useful for the optimization of HfO₂-based ferroelectric memory devices under development.

Switching Kinetics in Nanoscale Hafnium Oxide Based Ferroelectric Field-Effect Transistors

The recent discovery of ferroelectricity in thin hafnium oxide films has led to a resurgence of interest in ferroelectric memory devices. Although both experimental and theoretical studies on this new ferroelectric system have been undertaken, much remains to be unveiled regarding its domain landscape and switching kinetics. Here we demonstrate that the switching of single domains can be directly observed in ultrascaled ferroelectric field effect transistors. Using models of ferroelectric domain nucleation we explain the time, field and temperature dependence of polarization reversal. A simple stochastic model is proposed as well, relating nucleation processes to the observed statistical switching behavior. Our results suggest novel opportunities for hafnium oxide based ferroelectrics in nonvolatile memory devices.

Ultrathin Hf0.5Zr0.5O2 Ferroelectric Films on Si

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.

Novel biphenyl-substituted 1,2,4-oxadiazole ferroelectric liquid crystals: synthesis and characterization

Two novel series of unsymmetrically substituted 1,2,4-oxadiazole viz., R.Ox.C^*C_n compounds are synthesized and characterized. An optically active, (S)-(+)-methyl 3-hydroxy-2-methylpropionate is used to introduce a chiral center in the molecule. A biphenyl moiety prepared by Suzuki coupling reaction is directly attached to the oxadiazole core at C-5 position. Investigations for the phase behavior revealed that the series with a benzyl group on one end of the oxadiazole core exhibits an 1D orthogonal smectic-A phase while the second series with dodecyl flexible end chain shows orthogonal smectic-A and tilted chiral smectic-C (SmC*) phases over a wide range of temperatures. The smectic-C phase exhibits ferroelectric (FE) polarization switching. The mesomorphic thermal stabilities of these compounds are discussed in the domain of the symmetry and the flexibility of the alkyloxy end chain length attached to the chiral center.

Space-charge-mediated anomalous ferroelectric switching in P(VDF-TrEE) polymer films

We report on the switching dynamics of P(VDF-TrEE) copolymer devices and the realization of additional substable ferroelectric states via modulation of the coupling between polarizations and space charges. The space-charge-limited current is revealed to be the dominant leakage mechanism in such organic ferroelectric devices, and electrostatic interactions due to space charges lead to the emergence of anomalous ferroelectric loops. The reliable control of ferroelectric switching in P(VDF-TrEE) copolymers opens doors toward engineering advanced organic memories with tailored switching characteristics.

Ferroelectric Switching of Vinylidene and Trifluoroethylene Copolymer Thin Films on Au Electrodes Modified with Self-Assembled Monolayers

The ferroelectric switching characteristics of a vinylidene fluoride and trifluoroethylene copolymer were significantly changed via the chemical modification of a gold electrode with an alkanethiol self-assembled monolayer (SAM). The alkanethiol SAM-Au electrode successfully suppressed the leakage current (dark current) from the electrode to the bulk ferroelectric. Smaller leakage currents led to the formation of an effective electric field in the bulk ferroelectric. At switching cycles ranging from 10 to 100 kHz, significant changes in the ferroelectric properties were observed. At 100 kHz, a remanent polarization (Pr) of 68 mC·m⁻² was measured, whereas a very small Pr value of 2.4 mC·m⁻² was measured for the sample without a SAM. The switching speed of the SAM-Au electrode is as twice as fast as that of the unmodified electrode. A large potential barrier was formed via the insertion of a SAM between the Au electrode and the ferroelectric, effectively changing the ferroelectric switching characteristics.