Characteristic Switching Time Research Articles

Ferroelectric Al0.85Sc0.15N and Hf0.5Zr0.5O2 Domain Switching Dynamics.

The capability to reliably program partial polarization states with nanosecond programming speed and femtojoule energies per bit in ferroelectrics makes them an ideal candidate to realize multibit memory elements for high-density crossbar arrays, which could enable neural network models with a large number of parameters at the edge. However, a thorough understanding of the domain switching dynamics involved in the polarization reversal is required to achieve full control of the multibit capability. Transient current integration measurements are adopted to investigate the domain dynamics in aluminum scandium nitride (Al0.85Sc0.15N) and hafnium zirconium oxide (Hf0.5Zr0.5O2). The switching dynamics are correlated to the crystal structure of the films. The contributions of domain nucleation and domain wall motion are decoupled by analyzing the rate of change of the time-dependent normalized switched polarization. Thermally activated creep domain wall motion characterizes the Al0.85Sc0.15N switching dynamics. The statistics of independently nucleating domains and the domain wall creep motion in Hf0.5Zr0.5O2 are associated with the spatially inhomogeneous distribution of local switching field due to polymorphism, absence of preferential crystallite orientation, as well as defects and charges that can be located at the grain boundaries. The c-axis texture, single-phase nature, and strong likelihood of less fabrication process-induced defects contribute to the homogeneity of the local switching field in Al0.85Sc0.15N. Nonetheless, defects generated and redistributed upon bipolar electric field switching cycling result in Al0.85Sc0.15N domain wall pinning. The wake-up effect in Hf0.5Zr0.5O2 is explained thorough the continuous addition of switchable regions associated with two independent distributions of characteristic switching times.

Unraveling the Roles of Switching and Relaxation Times in Volatile Electrochemical Memristors to Mimic Neuromorphic Dynamical Features

AbstractComputing through ensembles of interacting dynamical elements is the next frontier of the diverse field of neuromorphic computing. Spiking neural networks are one of the possible examples. Computation through dynamics and through time requires the development of novel technologies for devices with rich dynamics. Among the various candidates, the most promising ones are volatile electrochemical memristive systems that switch from high to low resistance state by voltage application and self‐recover the high resistance state after a tunable relaxation time. Such devices can perform a wide variety of computational primitives. However, a clear comprehensive picture of their possible dynamics and their physical interpretation is still missing. In the present manuscript, prototypical electrochemical silver/silicon oxide/platinum (Ag/SiOx/Pt) memristive devices are characterized to identify dynamical aspects, like integrative effects and stochastic switching. Integrative effects are evidenced both in high and low resistance states, associated to wake‐up phase and cumulative switching. All the dynamical aspects are related to characteristic switching times and relaxation times, and with reference to the electrochemical and physical processes involved in device operation. The various dynamical aspects are linked to short‐term memory effects and basic temporal processing functions based on paired‐pulse effects that are relevant for neuromorphic applications.

Non-Mechanical Multiplexed Beam-Steering Elements Based on Double-Sided Liquid Crystal Metasurfaces

We propose, optimize, fabricate and test beam-steering elements based on double-sided liquid-crystal (LC) metasurfaces allowing for diffractive and spectral multiplexing, and thus covering three different beam deflection directions each. While straightforward parallel design requires one diffractive beam-steering element per a direction determined by Bragg’s law, double-sided LC-metasurfaces are remarkably flexible and allow optimization for three operation modes at different applied voltages: zero-voltage mode with dominant +1 order red light and +2 order blue light diffraction; intermediate mode at 1.4–1.6 V voltage with dominant +1 order blue light diffraction; and transparent mode at 5 V voltage. We comprehensively study three such elements with 6, 8 and 10 micrometer periods and verify their capability of deflecting blue and red light beams with 40–70% efficiencies in nine target directions with 10 ms characteristic switching times.

Conditional radiation tolerance of ferroelectric Hf0.5Zr0.5O2 thin film under 60Co gamma-ray irradiation

We report the effect of 60Co gamma-ray irradiation on the ferroelectric properties of metal–ferroelectric Hf0.5Zr0.5O2–metal thin film structures. The pristine Hf0.5Zr0.5O2 films showed strong radiation tolerance against gamma-rays with stable remanant polarization values. When Hf0.5Zr0.5O2 films were exposed to electric field cycling, or “wake-up” process, prior to irradiation, however, their ferroelectricity demonstrated a clear degradation of remanant polarization and coercive voltage shift of the hysteresis curves. The analysis of ferroelectric switching dynamics revealed faster polarization switching with broadening of Lorentzian distribution of characteristic switching time for higher radiation doses, which is contrary to the wake-up behavior. The relationship between the wake-up process and gamma-ray irradiation on the stability of ferroelectric Hf0.5Zr0.5O2 films was discussed in light of domain alignment and defect mechanisms, considering both the redistribution and trapping of defect charges.

Time-resolved piezoelectric response in relaxor ferroelectric (Pb0.88La0.12)(Zr0.52Ti0.48)O3 thin films

The domain switching dynamics in a relaxor ferroelectric lanthanum-modified lead zirconate titanate thin film with 12 mol. % La was investigated by time-resolved x-ray diffraction. While most frequently epitaxial thin films are investigated, the present work reports results on a polycrystalline thin film. Asymmetric butterfly loops of the strain as a function of the applied electric field evidenced a built-in electric field oriented toward the thin film–substrate interface. The piezoelectric coefficient d33 (in the film reference frame) diminishes with the increasing frequency of an applied AC electric field. From the strain transient during the application of positive-up negative-down voltage pulse sequences with frequencies of up to 100 kHz, characteristic times of the order of 100–200 ns were determined for these relaxor ferroelectric thin films. While switching times ranging from the picosecond to the millisecond range are reported in the literature for different materials, these characteristic switching times are comparable to epitaxial lead zirconate titanate thin films of various compositions despite the polycrystallinity of the present thin film. However, the evidenced built-in electric field significantly influences the switching behavior for different polarities.

Analysis of bistability at the coupling between waveguide and whispering gallery modes of a nonlinear hemicylinder

The optical bistability caused by the coupling between modes of the parallel-plate waveguide and a nonlinear hemicylindrical crystal is studied using theoretical and numerical analysis. In such a system a waveguide channel is parallelly coupled to whispering gallery modes (WGMs) of a hemicylindrical microresonator ensuring bistable behaviour at input intensities of the order of a few MW cm−2. The characteristic minimum switching time of the system (around 30 ps) can be controlled by varying the thickness of the metal layer which couples the waveguide and WGMs. This is conditioned by the change of the quality Q-factor, as well as the coupling coefficient of the resonator. The main advantages of the system are fabrication simplicity, small sizes of the order of 3 µm, integrability with other optical components and the possibility of adjusting the processes by making use of the electro-optical effect.

Local field inhomogeneity and ferroelectric switching dynamics in Hf1−xZrxO2 thin films

We report on the inhomogeneity of the internal bias according to the Zr alloy concentration confirmed by the ferroelectric switching dynamics of ${\mathrm{Hf}}_{1\ensuremath{-}x}{\mathrm{Zr}}_{x}{\mathrm{O}}_{2}$ thin films. The analytic model for the internal bias was considered in terms of the dipole-dipole interaction accompanied by Zr concentration variation. The ferroelectricity and switching dynamics of ${\mathrm{Hf}}_{1\ensuremath{-}x}{\mathrm{Zr}}_{x}{\mathrm{O}}_{2}$ thin films were investigated using conventional electrical measurements and local piezoresponse microscopy. Analysis of static and dynamic polarization reversal revealed a correlation between the activation field for polarization switching and Zr concentrations through the variation in local ferroelectric coercivity and the characteristic ferroelectric switching time. The spatially inhomogeneous local built-in electric field was attributed to the defect interaction in the ${\mathrm{Hf}}_{1\ensuremath{-}x}{\mathrm{Zr}}_{x}{\mathrm{O}}_{2}$ thin films.

Simulation of a Central Pattern Generator Using Memristive Devices

The central pattern generator (CPG) is one of the key elements of the nervous system of Vertebrates, determining the motor, breathing, swallowing, etc. cyclic motor patterns of a living organism, which require constant maintenance and prompt (or immediate) recovery in the case of injury. In this context, the construction of neuroprosthetic systems requires the development of electronic analogs (the corresponding CPGs). It is proposed to use memristive devices as adaptive elements with continuous tunable characteristics in these systems, because they exhibit a property, similar to bio-synaptic plasticity. The CPG model is developed, which yields an output activity, similar to that of biological objects. The requirements for the temporal characteristics of tunable elements for their successful use as artificial synapses are also specified. Memristive devices of a new type are fabricated according to the Langmuir–Schaefer technique using benzothieno[3,2-b] [1]-benzothiophene (BTBT) dimer as an active layer. BTBT-based devices make it possible to switch the conductivity at low voltages and currents, which is important in the development of energy-efficient neuroprosthetic systems, and the characteristic switching times are about several hundred milliseconds, which indicates that they can be used as analogs of synapses upon instrumental realization of the CPG.

Tuning Negative Capacitance in PbZr0.2Ti0.8O3/SrTiO3 Heterostructures via Layer Thickness Ratio

In this work, we exploit the ferroelectric-dielectric layer thickness ratio r as an effective tuning parameter to control the ferroelectric polarization and transient negative capacitance (NC) state in epitaxial ${\mathrm{Pb}\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_{3}/{\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_{3}$ bilayer heterostructures. The remnant polarization decreases monotonically with decreasing r, with the system exhibiting an abrupt transition from ferroelectric to dielectric dominated behavior at a critical ratio ${r}_{c}$ of 8 to 7, which is consistent with the evolution of the free-energy profile modeled via Landau theory. For samples with large r, the polarization switching dynamics during the transient NC regime can be well described by the nucleation and growth model, with the narrow distribution of the characteristic switching time (${t}_{0}$) pointing to a domain-wall-motion-limited behavior. Right below ${r}_{c}$, we observe a significantly broadened distribution of ${t}_{0}$, which can be attributed to the emergence of a multidomain state. The transient NC mode is quenched in samples with r 6, confirming that the ferroelectric order is suppressed. Our study provides critical information for optimizing the materials design for complex oxide-based NC transistors, paving the path for their application in low-power nanoelectronics.

Nonlinear domain wall velocity in ferroelectric Si-doped HfO2 thin film capacitors

We investigate the nonlinear response of the domain wall velocity (v) to an external electric field (Eext) in ferroelectric Si-doped HfO2 thin film capacitors using piezoresponse force microscopy (PFM) and switching current measurements. We verified the reliability of the PFM images of ferroelectric domain switching by comparing the switched volume fraction in the PFM images with the time-dependent normalized switched polarization from the switching current data. Using consecutive time-dependent PFM images, we measured the velocity of the pure lateral domain wall motion at various Eext. The Eext-dependent v values closely follow the nonlinear dynamic response of elastic objects in a disordered medium. The thermally activated creep and flow regimes were observed based on the magnitude of Eext. With a dynamic exponent of μ = 1, our thin film was found to have random-field defects, which is consistent with the Lorentzian distribution of characteristic switching time that was indicated in the switching current data.

Electric field cycling-mediated variations in defect distributions associated with wake-up and split-up behaviors of a ferroelectric Si-doped HfO2 thin film

We report variations in the characteristic ferroelectric switching time and interface free carrier concentration associated with wake-up and split-up behaviors of a 4.2 mol. % Si-doped HfO2 thin film. Prior to the development of the split-up behavior, the thin films exhibited the wake-up behavior; the remanent polarization increased on repeated electric field cycling. After the wake-up behavior, Si-doped HfO2 films exhibited degradation of the remanent polarization values and splitting of the ferroelectric switching current peaks when a lower electric field was swept; this is referred to as the split-up behavior. An investigation of the ferroelectric switching dynamics revealed retardation, followed by recovery, of the characteristic switching time for ferroelectric nucleation, coincident with the wake-up and split-up behaviors, respectively. We analyzed the interface free carrier concentration from capacitance–voltage characteristics and revealed that the wake-up and split-up behaviors were associated with changes of numbers of defects as electric field cycling continued.

Tip-force-induced ultrafast polarization switching in ferroelectric thin film: A dynamical phase field simulation

Dynamical phase field simulation is performed to reveal the dynamic characteristics of the tip-force-induced polarization switching in ferroelectric thin films. We demonstrate nontrivial influences of kinetic coefficient μ related to the second-order time derivative term in the dynamic equation of polarization on the mechanical switching behavior. It is found that such a term causes an oscillation feature of the switching process. Two characteristic switching times, i.e., the time when the inversed polarization begins to appear (denoted as τS1) and the time when the fraction of switched (c−) domain is largest during the loading process (denoted as τS2), can be defined to describe the tip-force-induced switching behavior. Both τS1 and τS2 are found to be affected by factors like misfit strain, temperature, and film thickness. Remarkably, the mechanical switching of polarization can be rather fast, with the switching time comparable to that of electrical switching. Due to the nontrivial dynamical effects, other important phenomena are observed: (a) the size and the pattern of switched domain (i.e., cylinder vs ring) in a single-point switching event strongly depend on the loading time, (b) the critical force of mechanical switching may be largely decreased by choosing a proper loading time, and (c) a large and stable domain pattern can still be written by a sweeping tip despite that the switched domain is not stable in the single-point switching event. Our study should provide new insights into the ultrafast phenomena in ferroelectric polarization switching under mechanical stimuli.

Effect of wake-up on the polarization switching dynamics of Si doped HfO2 thin films with imprint

We investigated the effect of wake-up on the polarization switching dynamics of Si doped HfO2 thin films with an imprint. From the polarization-voltage hysteresis loops, wake-up and imprint effects were observed. We measured the polarization switched by the voltage pulse at various durations and voltages. The switching behaviors could be explained by the nucleation-limited-switching model with a Lorentzian distribution of the characteristic switching time. Due to the wake-up phenomena, the center of the distribution increased, and the width became narrower, which could be related to improvement of the uniformity of the film. The polarization switching data from the positive and negative bias pulses was analyzed by using the same empirical equation with the effective electric-field corrected for the imprint effect. The increase of the characteristic time due to wake-up was carried out by an increase in the field-independent coefficient, τ1, without a change in the activation field, α.

Memristive State Equation for Bipolar Resistive Switching Devices Based on a Dynamic Balance Model and Its Equivalent Circuit Representation

A memory state equation consistent with a number of experimental observations is presented and discussed within the framework of Chua's memristive systems theory. The proposed equation describes the evolution of the memory state corresponding to a bipolar resistive switching device subject to a variety of electrical stimuli. It is shown that the memory equation agrees with: i ) the characteristic switching time associated with the ion/vacancy hopping mechanism within the dielectric film, ii ) the SET/RESET voltage logarithmic dependence on the voltage sweep ramp rate, iii ) the hysteretic behavior of the remnant conductance for cycled input signals, iv ) the generation of self-similar conductance loops for arbitrary initial conditions, and v ) the collapse of the resistive window with the increment of the input signal frequency. It is also shown that the proposed equation admits a circuital representation suitable for circuit simulations.

Sub-nanosecond spin-torque switching of perpendicular magnetic tunnel junction nanopillars at cryogenic temperatures

Spin-transfer magnetic random access memory devices are of significant interest for cryogenic computing systems where a persistent, fast, low-energy consuming, and nanometer scale device operating at low temperature is needed. Here, we report the low-temperature nanosecond duration spin-transfer switching characteristics of perpendicular magnetic tunnel junction (pMTJ) nanopillar devices (40–60 nm in diameter) and contrast them to their room temperature properties. Interestingly, the characteristic switching time decreases with temperature, with the largest reduction occurring between room temperature and 150 K. The switching energy increases with decreasing temperature, but still compares very favorably with other types of spin-transfer devices at 4 K, with <300 fJ required per switch. Write error rate (WER) measurements show highly reliable switching with WER ≤ 5 × 10–5 with 4 ns pulses at 4 K. Our results demonstrate the promise of pMTJ devices for cryogenic applications and show routes to further device optimization.

Ultrafast measurements of polarization switching dynamics on ferroelectric and anti-ferroelectric hafnium zirconium oxide.

The ultrafast measurements of polarization switching dynamics on ferroelectric (FE) and antiferroelectric (AFE) hafnium zirconium oxide (HZO) are studied. The transient current during the polarization switching process is probed directly on the nanosecond scale. The switching time is determined to be as fast as 10 ns to reach fully switched polarization with characteristic switching times of 5.4 ns for FE HZO and 4.5 ns for AFE HZO by the nucleation limited switching model. The limitation by the parasitic effect on capacitor charging is found to be critical in the correct and accurate measurements of intrinsic polarization switching speed of HZO.

Stochastic model of dispersive multi-step polarization switching in ferroelectrics due to spatial electric field distribution

A stochastic model for polarization switching in tetragonal ferroelectric ceramics is introduced, which includes sequential 90°- and parallel 180°-switching processes and accounts for the dispersion of characteristic switching times due to a nonuniform spatial distribution of the applied field. It presents merging of the recent multistep stochastic mechanism with the earlier nucleation limited switching and inhomogeneous field mechanism models. The model provides a much better description of simultaneous polarization and strain responses over a wide time window and a deeper insight into the microscopic switching mechanisms, as is exemplarily shown by comparison with measurements on lead zirconate titanate.

Sub-nanosecond switching in a cryogenic spin-torque spin-valve memory element with a dilute permalloy free layer

We present a study of pulsed current switching characteristics of spin-valve nanopillars with in-plane magnetized dilute permalloy and undiluted permalloy free layers in the ballistic regime at low temperatures. The dilute permalloy free layer device switches much faster: the characteristic switching time for a permalloy (Ni0.83Fe0.17) free layer device is 1.18 ns, while that for a dilute permalloy ([Ni0.83Fe0.17]0.6Cu0.4) free layer device is 0.475 ns. A ballistic macrospin model can capture the data trends with a reduced spin-torque asymmetry parameter, reduced spin polarization, and increased Gilbert damping for the dilute permalloy free layer relative to the permalloy devices. Our study demonstrates that reducing the magnetization of the free layer increases the switching speed while greatly reducing the switching energy and shows a promising route toward even lower power magnetic memory devices compatible with superconducting electronics.

Ultrafast All-Optical Light Control with Tamm Plasmons in Photonic Nanostructures

Ultrafast all-optical modulators are crucial parts of prospective photonic devices. A number of plasmonic and dielectric nanostructures were nominated as candidates for integrated all-optical circuits. The key principle in the design of such devices is to engineer artificial optical resonances to increase the magnitude of modulation or to change the characteristic switching time. The major drawback is that the manufacturing becomes rather sophisticated. Here, we propose a method to tailor the ultrafast response of photonic crystal–metal nanostructures by employing a spectral shift of the Tamm-plasmon resonance. We show that for the absorbed pump fluence of 6 pJ reflectance of the sample at the near-infrared probe wavelength in the vicinity of the Tamm-plasmon resonance changes 25× stronger as compared with a bare metal film. Additionally, we show that by choosing a proper wavelength around the resonance a background-free reflectance modulation can be achieved. The characteristic pulse-limited switching time, in this case, is 150 fs.

Ferroelectric switching dynamics in 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 thin films

In this work, the ferroelectric characteristics of 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 (BCZT) thin films grown on 0.7 wt. % Nb-doped (001)-SrTiO3 (Nb:STO) single-crystal have been investigated. High-resolution transmission electron microscopy and electron energy loss spectroscopy revealed a very sharp Nb:STO/BCZT interface, while selected area electron diffraction revealed the epitaxial growth of the BCZT layer on the Nb:STO substrate. The ferroelectric nature of the BCZT films have been investigated by piezoresponse force microscopy and hysteresis loops. The effect of electric field on polarization switching kinetics has been investigated and has been analyzed by the nucleation limited switching model with a Lorentzian distribution function. The local field variation was found to decrease with the increase in the electric field, and thus, the switching process becomes faster. The peak value of the polarization current and the logarithmic characteristic switching time exhibited an exponential dependence on the inverse of electric field. This model gave an excellent agreement with the experimental polarization reversal transients throughout the whole time range.