Polarization Switching Characteristics Research Articles

Microstructure, dielectric storage, ferroelectric polarization, and non-ferroelectric resistive switching characteristics of sol–gel-derived BZT ferroelectric films with tailored Zr content

Microstructure, dielectric storage, ferroelectric polarization, and non-ferroelectric resistive switching characteristics of sol–gel-derived BZT ferroelectric films with tailored Zr content

Defects and oxygen impurities in ferroelectric wurtzite Al1−xScxN alloys

III-nitrides and related alloys are widely used for optoelectronics and as acoustic resonators. Ferroelectric wurtzite nitrides are of particular interest because of their potential for direct integration with Si and wide bandgap semiconductors and unique polarization switching characteristics; such interest has taken off since the first report of ferroelectric Al1−xScxN alloys. However, the coercive fields needed to switch polarization are on the order of MV/cm, which are 1–2 orders of magnitude larger than oxide perovskite ferroelectrics. Atomic-scale point defects are known to impact the dielectric properties, including breakdown fields and leakage currents, as well as ferroelectric switching. However, very little is known about the native defects and impurities in Al1−xScxN and their effect on the dielectric and ferroelectric properties. In this study, we use first-principles calculations to determine the formation energetics of native defects and unintentional oxygen incorporation and their effects on the polarization switching barriers in Al1−xScxN alloys. We find that nitrogen vacancies are the dominant native defects, and unintentional oxygen incorporation on the nitrogen site is present in high concentrations. They introduce multiple mid-gap states that can lead to premature dielectric breakdown and increased temperature-activated leakage currents in ferroelectrics. We also find that nitrogen vacancy and substitutional oxygen reduce the switching barrier in Al1−xScxN at low Sc compositions. The effect is minimal or even negative (increases barrier) at higher Sc compositions. Unintentional defects are generally considered to adversely affect ferroelectric properties, but our findings reveal that controlled introduction of point defects by tuning synthesis conditions can instead benefit polarization switching in ferroelectric Al1−xScxN at certain compositions.

Orientation-dependent tunneling electroresistance in Pt/Pb(Zr,Ti)O3/Nb:SrTiO3 ferroelectric tunnel junctions

Orientation anisotropy is a well-known essential character for polarization characteristics of ferroelectric materials, which has been widely investigated in conventional ferroelectric random access memories. In this work, we study the effects of orientation on the tunneling electroresistance (TER) of ferroelectric tunnel junctions (FTJs). Rhombohedral Pb(Zr[Formula: see text],Ti[Formula: see text])O3 (PZT) that has the polar axis along the [Formula: see text] orientation is adopted as potential barriers and two kinds of FTJs that are composed of (001)- and (111)-oriented PZT barriers and Nb:SrTiO3 (Nb:STO) electrodes, respectively, are fabricated. The (111)-oriented Pt/PZT/Nb:STO FTJ exhibits a giant ON/OFF ratio of [Formula: see text], about 30 times that of the (001)-oriented device, due to the lowered PZT barrier in the ON state and the widen Schottky barrier in the OFF state based on current and capacitance analyses. In addition, compared to the (001)-oriented device, the (111)-oriented FTJ shows a sharper and faster switching between the ON and OFF states according to the nucleation-limited-switching dynamics model, giving rise to good linearity in memristive behaviors for synaptic plasticity and reliable retention and endurance properties for the resistance switching. The improved TER properties are ascribed to larger effective polarizations and 180∘ switching in the (111)-oriented PZT barrier. These results facilitate the design and fabrication of high-performance FTJ devices with the optimization of crystallographic orientation and polarization switching characteristics.

Enhanced polarization switching characteristics of HfO2 ultrathin films via acceptor-donor co-doping

In the realm of ferroelectric memories, HfO2-based ferroelectrics stand out because of their exceptional CMOS compatibility and scalability. Nevertheless, their switchable polarization and switching speed are not on par with those of perovskite ferroelectrics. It is widely acknowledged that defects play a crucial role in stabilizing the metastable polar phase of HfO2. Simultaneously, defects also pin the domain walls and impede the switching process, ultimately rendering the sluggish switching of HfO2. Herein, we present an effective strategy involving acceptor-donor co-doping to effectively tackle this dilemma. Remarkably enhanced ferroelectricity and the fastest switching process ever reported among HfO2 polar devices are observed in La3+-Ta5+ co-doped HfO2 ultrathin films. Moreover, robust macro-electrical characteristics of co-doped films persist even at a thickness as low as 3 nm, expanding potential applications of HfO2 in ultrathin devices. Our systematic investigations further demonstrate that synergistic effects of uniform microstructure and smaller switching barrier introduced by co-doping ensure the enhanced ferroelectricity and shortened switching time. The co-doping strategy offers an effective avenue to control the defect state and improve the ferroelectric properties of HfO2 films.

All-dielectric terahertz metamaterial with polarization switching characteristic

All silicon dielectric metamaterial (SDM) composed of two outer symmetric semi-circular rings and two inner symmetric split-ring resonators (SRRs) is presented. The electromagnetic responses of SDM device with different structure heights in transverse electrical (TE) and transverse magnetic (TM) modes are studied by numerical simulations and experiments. By increasing structure height of SDM device, the resonances of the SDM devices exhibit red-shift and the resonant intensities become stronger. The shifting ranges of resonances are 0.36 THz in TE mode and 0.27 THz in TM mode. Furthermore, the SDM device with structure height of 100 μm demonstrates great resonant intensity, and two extra resonances are generated at 1.60 THz and 2.26 THz in TM mode. By further investigating the polarization characteristics of SDM device with structure height of 100 μm, there are two linear trends for the polarization switching function at 1.60 THz and 2.10 THz. The corresponding correction coefficients are 0.99608 and 0.99726 and the modulation depths are 74 % and 74.5 % for the resonance at 1.60 THz and 2.10 THz, respectively. This SDM design paves the way to the strategy of the development of frequency filtering, polarization switching, and resonance modulation characteristics in the THz-wave applications.

Time-fractional Landau–Khalatnikov model applied to numerical simulation of polarization switching in ferroelectrics

Ferroelectrics are complex structured media that exhibit fractal properties and memory effects in terms of a number of dynamic characteristics. The most relevant applications of ferroelectrics in science and technology are associated with the primary mechanisms of polarization switching and domain structure dynamics induced by external exposure. In this study, we propose a time-fractional modification of the Landau–Ginzburg–Devonshire–Khalatnikov model to describe the dynamics of ferroelectric polarization switching. To solve the time-fractional cubic-quintic partial differential equation numerically, a computational scheme is derived. The technique combines an iterative procedure and an implicit finite-difference scheme based on an approximation of the Caputo derivative. A series of computational experiments are presented to visualize polarization switching characteristics on the example of thin films of barium titanate. A variation of the order of fractional derivative as a numerical characteristic of the memory effect allows one to “adjust” suitable regimes of simulated dynamical system. The obtained findings indicate that the generalized time-fractional model can provide better reproduction of experimental data in comparison with the classical analogue.

Polarization switching characteristics in AFE/FE Double–Layer devices

Ferroelectric/anti-ferroelectric/Ferroelectric(FE/AFE/FE) stack-based multi-bit memory has better device-to-device variation control compared to single-layer FE devices. The interplay between FE and AFE layers hasn’t been studied which influences the switching current profile. We simulate the polarization switching characteristics in AFE/FE double layers and analyze their differences from stand-alone devices due to the interplay between the layers. The impacts of spontaneous polarization charges and film thicknesses on the current shift are evaluated, which provides design guidance for these multi-layer devices.

Ferroelectric switching and current characteristics dependence on ferroelectric polarization direction of microwave synthesized BiFeO3 nanocubes

Herein, we studied the ferroelectric switching and current characteristics of BiFeO3 (BFO) nanocubes dispersed on the surface of a Nb-doped SrTiO3 (Nb:STO) substrate based on the ferroelectric polarization orientation. The microwave synthesis method afforded BFO nanocubes with an average size of ∼50 nm, which were dispersed on the Nb:STO substrate surface and the substrate was subsequently subjected to heat treatment at 500 °C for 1 h. The piezoelectric d33 hysteresis loop, ferroelectric domain structure, and ferroelectric polarization switching characteristics of the 50-nm-sized BFO nanocubes were examined using piezoresponse force microscopy. Finally, atomic force microscopy confirmed the dependency of current characteristics on the ferroelectric polarization orientation of the BFO nanocubes, verifying the applicability of BFO nanocubes as storage media for ferroelectric polarization information.

Experimental discovery of structure–property relationships in ferroelectric materials via active learning

Emergent functionalities of structural and topological defects in ferroelectric materials underpin an extremely broad spectrum of applications ranging from domain wall electronics to high dielectric and electromechanical responses. Many of these functionalities have been discovered and quantified via local scanning probe microscopy methods. However, the search has until now been based on either trial and error, or using auxiliary information such as the topography or domain wall structure to identify potential objects of interest on the basis of the intuition of operator or pre-existing hypotheses, with subsequent manual exploration. Here we report the development and implementation of a machine learning framework that actively discovers relationships between local domain structure and polarization-switching characteristics in ferroelectric materials encoded in the hysteresis loop. The hysteresis loops and their scalar descriptors such as nucleation bias, coercive bias and the hysteresis loop area (or more complex functionals of hysteresis loop shape) and corresponding uncertainties are used to guide the discovery of these relationships via automated piezoresponse force microscopy and spectroscopy experiments. As such, this approach combines the power of machine learning methods to learn the correlative relationships between high-dimensional data, as well as human-based physics insights encoded into the acquisition function. For ferroelectric materials, this automated workflow demonstrates that the discovery path and sampling points of on- and off-field hysteresis loops are largely different, indicating that on- and off-field hysteresis loops are dominated by different mechanisms. The proposed approach is universal and can be applied to a broad range of modern imaging and spectroscopy methods ranging from other scanning probe microscopy modalities to electron microscopy and chemical imaging.

Characteristics of Polarization Switching and Bistability in Vertical-Cavity Surface-Emitting Laser Subject to Continuous Variable-Polarization Optical Injection

Characteristics of Polarization Switching and Bistability in Vertical-Cavity Surface-Emitting Laser Subject to Continuous Variable-Polarization Optical Injection

Designs of metareflectors based on nanodisk and annular hole arrays with polarization independence, switching, and broad bandwidth characteristics

We propose two tunable metareflectors (MRs) composed of a suspending nanodisk and an annular hole on silicon (Si) substrate with aluminum (Al) mirrors atop. They are denoted as MR-1 and MR-2 for the former and latter, respectively. The proposed MRs exhibit high-efficient cyan-magenta-yellow (CMY) color filtering, and ultrabroad tuning range characteristics. The electromagnetic energy of the resonant wavelength is confined within the suspending nanostructure and bottom Al mirror and then performed a perfect absorption. By changing the height between suspending nanostructure and the bottom Al mirror, MRs exhibit active tuning and single-/dual-resonance switching characteristics spanning the entire visible spectra range. Furthermore, the resonant wavelengths of MRs are sensitive to the surrounding ambient media, which are red-shifted and modulated from single- to dual-resonance by changing the environmental refraction index. The corresponding sensitivities are 500 nm/RIU and 360 nm/RIU for MR-1, 289 nm/RIU and 270 nm/RIU for MR-2, respectively. These results provide an effective strategy for use in high-resolution displays, high-sensitive sensors, optical switches, optical communications, and flexible virtual reality (VR)/augmented reality (AR) applications.

Imprint phenomenon of ferroelectric switching characteristics in BaTiO3/PbTiO3 multilayer thin films

Materialsin which ferroelectric polarization can be locally controlled have potential for high-density information storage applications. Herein, we investigated the ferroelectric polarization switching characteristics of epitaxial BaTiO3/PbTiO3 (BTO/PTO) multilayer thin films by using piezoresponse force microscopy. Pulsed laser deposition was used to form an epitaxial BTO layer on an epitaxial ferroelectric PTO single thin film. This epitaxial BTO/PTO multilayer thin film exhibited an imprint phenomenon, in which the internal electric field was asymmetrically induced and the ferroelectric hysteresis loop was shifted. Owing to this imprint phenomenon, a large difference occurred between the two coercive electric fields of the ferroelectric hysteresis loop. Because the coercive electric field was reduced in one direction, a relatively small switching voltage was required to form ferroelectric polarization nanobits in a local region and the size of the switched nanobits was minimized. This behavior can be utilized as the basis for developing high-density ferroelectric-based storage media.

Experimental Investigations on Polarization Switching and Bistability in a 1550 nm VCSEL Subject to Orthogonal Optical Injection With Time-Varying Injection Power

Polarization switching (PS) and polarization bistability (PB) characteristics of a 1550 nm vertical-cavity surface-emitting laser (VCSEL) subject to orthogonal optical injection (OOI) with time-varying injection power are experimentally investigated for the first time. To implement the optical injection, a Mach-Zehnder modulator (MZM) modulated by a triangular radio wave is adopted, and the influences of some typical parameters on the performances of PS and PB are analyzed. Under suitable operation conditions, PB in the VCSEL subject to OOI with time-varying injection power can be observed only for the input power P0 of MZM locating within a determined range, and the width of hysteresis loop gradually decreases with the increase of P0 from the minimum P0, min to the maximum P0, max. Under different frequency detuning Δ ν between the injection light and the X polarization mode of the VCSEL, the determined ranges of P0 required for PB are different. With the variation of Δ ν from a negative value to a positive value, both P0, min and P0, max present a trend of decrease first and then increase. Additionally, with the increase of the modulation frequency, the hysteresis loop width firstly increases rapidly and then gradually tends saturation.

Multi-Domain Negative Capacitance Effects in Metal-Ferroelectric-Insulator-Semiconductor/Metal Stacks: A Phase-field Simulation Based Study

We analyze the ferroelectric domain-wall induced negative capacitance (NC) effect in Metal-FE-Insulator-Metal (MFIM) and Metal-FE-Insulator-Semiconductor (MFIS) stacks through phase-field simulations by self-consistently solving time-dependent Ginzburg Landau equation, Poisson’s equation and semiconductor charge equations. Considering Hf0.5Zr0.5O2 as the ferroelectric material, we study 180° ferroelectric domain formation in MFIM and MFIS stacks and their polarization switching characteristics. Our analysis signifies that the applied voltage-induced polarization switching via soft domain-wall displacement exhibits non-hysteretic characteristics. In addition, the change in domain-wall energy, due to domain-wall displacement, exhibits a long-range interaction and thus, leads to a non-homogeneous effective local negative permittivity in the ferroelectric. Such effects yield an average negative effective permittivity that further provides an enhanced charge response in the MFIM stack, compared to Metal-Insulator-Metal. Furthermore, we show that the domain-wall induced negative effective permittivity is not an intrinsic property of the ferroelectric material and therefore, is dependent on its thickness, the gradient energy coefficient and the in-plane permittivity of the underlying insulator. Similar to the MFIM stack, MFIS stack also exhibits an enhanced charge/capacitance response compared to Metal-Oxide-Semiconductor (MOS) capacitor. Simultaneously, the multi-domain state of the ferroelectric induces non-homogeneous potential in the underlying insulator and semiconductor layer. At low applied voltages, such non-homogeneity leads to the co-existence of electrons and holes in an undoped semiconductor. In addition, we show that with the ferroelectric layer being in the 180° multi-domain state, the minimum potential at the ferroelectric-dielectric interface and hence, the minimum surface potential in the semiconductor, does not exceed the applied voltage (in-spite of the local differential amplification and charge enhancement).

Polar Switching of Dipolar Molecules Confined in Submicron- and Micron-sized Pores in Polymer Films

p-Nitroanisole (pNA) confined in submicron- and micron-sized pores in polyimide films shows enhanced dipolar polarization and polar switching characteristics in the liquid state, as revealed by hys...

Switching Dynamics Analysis by Various Models of Hf0.5Zr0.5O2 Ferroelectric Thin Films

Recent discoveries of ferroelectric properties in ultrathin doped hafnium oxide (HfO2) have led to the expectation that HfO2 could overcome the shortcomings of perovskite materials and be applied to electron devices such as Fe-Random access memory (RAM), ferroelectric tunnel junction (FTJ) and negative capacitance field effect transistor (NC-FET) device. As research on hafnium oxide ferroelectrics accelerates, several models to analyze the polarization switching characteristics of hafnium oxide ferroelectrics have been proposed from the domain or energy point of view. However, there is still a lack of in-depth consideration of models that can fully express the polarization switching properties of ferroelectrics. In this paper, a Zr-doped HfO2 thin film based metal-ferroelectric-metal (MFM) capacitor was implemented and the polarization switching dynamics, along with the ferroelectric characteristics, of the device were analyzed. In addition, a study was conducted to propose an applicable model of HfO2-based MFM capacitors by applying various ferroelectric switching characteristics models.

A Stretchable Terahertz Parabolic‐Shaped Metamaterial

AbstractA stretchable parabolic‐shaped metamaterial (PSM) coated onto polydimethylsiloxane (PDMS) is proposed and demonstrated for operation in the terahertz (THz) frequency range. By stretching the PDMS‐based PSM device along different directions, ultranarrowband, polarization dependent, switchable optical characteristics are obtained. By stretching the PSM width and length in the transverse electric (TE) and transverse magnetic (TM) modes, resonant tuning ranges of 0.55 and 0.32 THz, respectively, are demonstrated for the PSM device. In these deformation ranges, the Q‐factors of the PSM device for different widths and lengths are quite stable and maintained in the range of 9 to 14 for the TE mode, with a high Q‐factor of 50 obtained at resonance for the TM mode. The integration of the PSM on a mechanically deformable PDMS substrate provides potential for use in flexible electronics applications. Furthermore, the PSM device exhibits single‐/dual‐band switching and polarization switching characteristics. These multifunctional states for the PSM device can be determined by mechanical inputs to represent binary digits that can then be used to perform logic operations. Such a stretchable PSM device offers an effective approach for the realization of programmable metamaterials with enhanced optical‐mechanical performance due to its high flexibility, applicability, and cost‐effectiveness.

Phase field modeling of domain dynamics and polarization accumulation in ferroelectric HZO

In this work, we investigate the accumulative polarization (P) switching characteristics in ferroelectric (FE) thin films under the influence of sequential sub-coercive electric-field pulses. Performing the dynamic phase-field simulation (based on time-dependent Landau-Ginzburg model) and experimental measurement on Hf0.4Zr0.6O2 (HZO), we analyze the electric field induced domain-wall (DW) motion and the resultant P accumulation process in FE. According to our analysis, even in the absence of an applied electric field, the DW can potentially undergo spontaneous motion. Such a DW instability leads to spontaneous P-excitation and relaxation processes, which play a pivotal role in accumulative P-switching in an FE grain. We show that the extent of such P accumulation increases with the increase in the applied electric field, increase in excitation time and decrease in relaxation time. Finally, by considering an ensemble of grains with local and global coercive field distributions, we model the P-accumulation process in a large area HZO sample. In such a multi-grain scenario, the dependency of P accumulation on the applied electric field pulse attributes follows similar features as that of a single-grain, although the spontaneous processes (excitation/relaxation) are less prominent in large area sample.

Frequency-induced polarization switching and bistability in a 1550 nm VCSEL subject to parallel optical injection

We experimentally investigate frequency-induced polarization switching (PS) and bistability (PB) characteristics in a 1550 nm vertical-cavity surface-emitting laser (1550 nm VCSEL) subject to parallel optical injection, for which the polarization direction of the injection light is along with the direction of dominant polarization mode in the solitary 1550 nm VCSEL. The results show that, for a 1550 nm VCSEL under parallel optical injection with frequency detuning ∆ν (= νinj − ν0, where νinj is the frequency of injection light and ν0 is the frequency of dominant polarization mode of the solitary 1550 nm VCSEL), type I PS (the dominant polarization mode switches from Y-polarization mode with a shorter wavelength to X-polarization mode with a longer wavelength) and type II PS (the dominant polarization mode switches from X-polarization mode to Y-polarization mode) can be observed through continuously scanning the frequency of injection light. Moreover, the values of ∆ν required for generating PSs are dependent on the variation route, namely frequency-induced PB phenomenon occurs. In the region with negative ∆ν, there exist two hysteresis loops, whose widths are seriously influenced by the injection power and the biased current.

Dielectric properties and resistive switching characteristics of lead zirconate titanate/hafnia heterostructures

We report the dielectric response and resistive switching properties of bilayers of PbZr0.40Ti0.60O3 [PZT (40:60)] and HfO2 of varying thickness on platinized Si substrates. PZT (40:60) and HfO2 films were grown using chemical solution deposition and atomic layer deposition, respectively. We show here that the addition of an interposed linear dielectric layer with a high permittivity between the ferroelectric film and the top electrode modifies the polarization and resistive switching characteristics of the multilayer stack. We observe an increase in the coercive field by 45% for PZT films of 250 nm thickness with a 20 nm HfO2 layer compared to 250 nm thick PZT films grown under identical conditions. Simultaneously, the dielectric constant decreases by 43% from 409 to 175 for a 250 nm PZT film with 20 nm HfO2, accompanied by a significant improvement in the leakage current density from 5.6 × 10−4 A/cm2 to 8.7 × 10−8 A/cm2. Our resistance measurements show that there are two separate resistance states that are accessible with at least an order of magnitude in resistance difference from 5 × 108 to 5 × 109 Ω. We show that the dielectric response and the coercivity of the bilayer system can be explained by a capacitors-in-series model. This indicates that the PZT and the HfO2 layer could effectively be considered decoupled, presumably due to bound/trapped charges at the interlayer interface. This charged ferroelectric/dielectric interface could be the reason for the intermediate resistance states which could be used as multistate resistive memories in neuromorphic computing applications.