Asymmetric Switching Behavior Research Articles

Investigation on asymmetric resistive switching (RS) characteristics in p-NiO/n-ZnO heterojunctions

In this research, we report asymmetric resistive switching (RS) characteristics using p-NiO and n-ZnO heterojunctions. A cost-effective spray pyrolysis technique is adopted for the deposition of n-ZnO and p-NiO layers. The structural analysis of ZnO and NiO layers confirms the hexagonal wurtzite and cubic structure, respectively. The morphological analysis of the p-NiO/n-ZnO heterojunction confirms the presence of a multilayered structure. The heterojunction was stacked between top silver (Ag) and bottom indium doped tin oxide (ITO) electrodes, and the RS characteristics were investigated. The RS device exhibits asymmetric current-voltage (I-V) characteristics with an ON/OFF ratio calculated as 2.7 and stable endurance characteristics of 50 cycles. It was analyzed that the formation and rupture of conductive filaments through intrinsic defects modulated by the p-NiO and n-ZnO layers contribute to the observed asymmetric resistive switching (RS) behavior in the fabricated p-NiO/n-ZnO heterojunction.

Insertion Trade-off Effects on the Spin-Transfer Torque Memory Explored by In Situ X-ray

Magnetoresistive random-access memory (MRAM) relies on magnetic tunnel junctions (MTJs) comprising heterostructures of CoFeB/MgO/CoFeB. Nonetheless, the dielectric breakdown of MgO limits the lifespan of MRAM devices. In the current study, we terminated MgO with a Mg surface facing the CoFeB free layer to reduce the mismatch in band alignment across the barrier. The Mg-modified interface was shown to enhance the breakdown voltage while reducing the switching current and asymmetric switching behavior, with a moderate sacrifice of perpendicular magnetic anisotropy, tunneling magnetoresistance, and resistance–area product. This performance trade-off was observed in all MTJs, regardless of cell size (180, 130, and 80 nm; each size has been tested with at least 20 cells). Visualization of the proposed junction via in situ X-ray absorption spectroscopy proved effective in elucidating the reconstruction of the interface within the context of energy barrier height, asymmetric switching behavior, and the voltage-driven movement of oxygen ions. A spin-dependent band diagram was constructed to correlate the tunneling/switching properties of the MTJ with such a trade-off scenario. The atomistic simulation of the magnetic properties revealed that Mg termination lowered the switching energy barrier with an effective domain wall motion within the CoFeB free layer.

Polarization behavior of` lead-free 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3 thin films with enhanced ferroelectric properties

(Bi0.5Na0.5)TiO3 based ferroelectric lead-free thin films have great potential for modern micro-devices. However, the multicomponent feature and volatile nature of Bi/Na makes the achievement of high quality films challenging. In this work, the morphotropic phase boundary composition, 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3 thin films were successfully prepared by CSD method. Dense films with low dielectric loss and low leakage current density were obtained. A well-defined polarization hysteresis loop with a high remnant polarization was observed in the thin films. Moreover, the polarization behavior of the film at original state, under electric field and upon heating was investigated by PFM. A self-polarization and asymmetric domain switching behavior were observed. High temperature induced depolarization and the self-polarization recovered upon cooling. The thin films with good quality show a promising potential for the application in electrical devices, and the in-depth investigation of the polarization behavior improves the understanding of ferroelectric and piezoelectric properties of thin films.

Secure STT-MRAM Bit-Cell Design Resilient to Differential Power Analysis Attacks

Spin-transfer torque magnetic random access memory (STT-MRAM) is a promising nonvolatile memory technology for various applications from low power to high-density memory. However, STT-MRAM is prone to power analysis attacks due to its asymmetric resistive states and switching behavior. This noninvasive class of attacks is a serious threat to system security. To reduce the correlation between the data and the power consumption of the memory, a countermeasure based on a resilient cell design with a symmetrical structure is proposed in this article. The standard cell and the proposed cell have been attacked and their resiliencies are compared. When attacked with a correlation power analysis (CPA), the proposed bit cell attacked with the hamming weight (HW) model is 100 times more resilient compared to the standard STT-MRAM cell. The proposed bit cell attacked with the hamming distance (HD) or the STT power models is more than 500 times more resilient compared to the standard STT-MRAM cell.

Nanoscale 3D Stackable Ag-Doped HfOx-Based Selector Devices Fabricated through Low-Temperature Hydrogen Annealing.

Electrochemical metallization-based threshold switching devices with active metal electrodes have been studied as a selector for high-density resistive random access memory (RRAM) technology in crossbar array architectures. However, these devices are not suitable for integration with three-dimensional (3D) crossbar RRAM arrays due to the difficulty in vertical stacking and/or scaling into the nanometer regime as well as the asymmetric threshold switching behavior and large variation in the operating voltage. Here, we demonstrate bidirectional symmetric threshold switching behaviors from a simple Pt/Ag-doped HfOx/Pt structure. While fabricating the Pt/Ag-doped HfOx/Pt film using a 250 nm hole structure, filaments composed of Ag nanoclusters were constructed through a low-temperature (∼200 °C) hydrogen annealing process where the shape of the film in a nanoscale via a hole structure was maintained for integration with 3D stackable crossbar RRAM arrays. Finite Ag filament paths in the HfOx layer led to uniform device-to-device performances. Moreover, we observed that the hydrogen annealing process reduced the delay time through the reduction of the oxygen vacancies in the HfOx layer. Consequently, the proposed Pt/Ag-doped HfOx/Pt-based nanoscale selector devices exhibited excellent performance of high selectivity (∼105), ultralow OFF current (∼10 pA), steep turn-on slope (∼2 mV/decade), and short delay time (3 μs).

Electrical and optical modulation on ferroelectric properties of P(VDF-TrFE) thin film capacitors

Modulating the ferroelectric properties of P(VDF-TrFE) polymers both electrically and optically could open up new opportunities for their applications in non-volatile memories and sensors. Here by using the Nb:SrTiO3 semiconductor as electrode compared with metal Au electrode, we report on the modulation of ferroelectric properties of P(VDF-TrFE) thin film capacitors both by electric field and UV light. A ferroelectric hysteresis loop shift together with the asymmetric switching behavior has been observed when using semiconducting electrode, which could be explained by the band alignment model based on interfacial charge screening. On the basis of band bending near the ferroelectric/semiconductor interface, we could further modulate the ferroelectric switching behaviors reversibly by UV light illumination. Our research provides a new route to engineer the ferroelectric properties of P(VDF-TrFE) polymer thin film capacitors, promising their applications in optoelectronic devices.

Oxygen-ion-migration-modulated bipolar resistive switching and complementary resistive switching in tungsten/indium tin oxide/gold memory device

In this paper, we report our investigation of room-temperature-fabricated tungsten/indium tin oxide/gold (W/ITO/Au) resistive random access memory (RRAM), which exhibits asymmetric bipolar resistive switching (BRS) behavior. The device displays good write/erase endurance and data retention properties. The device shows complementary resistive switching (CRS) characteristics after controlling the compliance current. A WOx layer electrically formed at the W/ITO in the forming process. Mobile oxygen ions within ITO migrate toward the electrode/ITO interface and produce a semiconductor-like layer that acts as a free-carrier barrier. The CRS characteristic here can be elucidated in light of the evolution of an asymmetric free-carrier blocking layer at the electrode/ITO interface.

Asymmetry in mechanical polarization switching

Recent demonstration of a mechanical 180° switching of ferroelectric polarization has enabled an alternative polarization control mechanism based on the flexoelectric coupling between polarization and strain gradient. Mechanical switching is a highly asymmetric phenomenon associated with the inhomogeneous strain induced by an atomic force microscope (AFM) tip pressed against the ferroelectric surface. Here, we demonstrate the asymmetric domain switching behavior in the vicinity of the 180° domain wall in PbTiO3 thin films with respect to the AFM tip scanning direction. The writing-direction-dependent asymmetric domain response has been modeled by molecular dynamics simulation showing asymmetry in domain wall displacement due to the difference in the volume of mechanically switched domains. The obtained results show that the mechanically induced switching dynamics is very different from the conventional 180° switching realized by an external electric field and has to be exploited differently. In particular, nanoscale domain engineering via the tip-induced flexoelectric effect requires careful consideration of asymmetric interaction between the existing domain structures and the strain gradient.

Size effect on polarization switching kinetics of P(VDF-TrFE) copolymer nanodots

We demonstrate polarization switching kinetics of poly(vinylidene fluoride-ran-trifluoroethylene) P(VDF-TrFE) nanodots depending on their sizes. Ferroelectric copolymer P(VDF-TrFE) nanodots, ranging from 20 to 250 nm in lateral size, were deposited on a Nb-doped SrTiO3 substrate by a position-controlled dip-pen nanolithography technique. Ferroelectric switching characteristics of P(VDF-TrFE) nanodots was investigated by piezoresponse force microscopy. Asymmetric switching behavior was observed during switching process, indicating that the threshold electric fields for polarization reversal were measured differently under the same conditions according to the switching polarity. Significantly, when the size of P(VDF-TrFE) nanodot approached around 250 nm, electric fields for polarization switching were compatible with those of the P(VDF-TrFE) thin film. We suggest that the electric fields required for polarization reversal of the P(VDF-TrFE) nanodots present asymmetric switching behaviors, and asymptotic behavior be observed as the nanodots increase in lateral size.

Comparison of approaches for parameter estimation on stochastic models: Generic least squares versus specialized approaches

Parameter estimation for models with intrinsic stochasticity poses specific challenges that do not exist for deterministic models. Therefore, specialized numerical methods for parameter estimation in stochastic models have been developed. Here, we study whether dedicated algorithms for stochastic models are indeed superior to the naive approach of applying the readily available least squares algorithm designed for deterministic models.We compare the performance of the recently developed multiple shooting for stochastic systems (MSS) method designed for parameter estimation in stochastic models, a stochastic differential equations based Bayesian approach and a chemical master equation based techniques with the least squares approach for parameter estimation in models of ordinary differential equations (ODE). As test data, 1000 realizations of the stochastic models are simulated. For each realization an estimation is performed with each method, resulting in 1000 estimates for each approach. These are compared with respect to their deviation to the true parameter and, for the genetic toggle switch, also their ability to reproduce the symmetry of the switching behavior. Results are shown for different set of parameter values of a genetic toggle switch leading to symmetric and asymmetric switching behavior as well as an immigration-death and a susceptible-infected-recovered model. This comparison shows that it is important to choose a parameter estimation technique that can treat intrinsic stochasticity and that the specific choice of this algorithm shows only minor performance differences.

Direct observation of asymmetric domain wall motion in a ferroelectric capacitor

We report in situ transmission electron microscopy observations of the 180° polarization switching process of a PbZr0.2Ti0.8O3 (PZT) capacitor. The preferential, but asymmetric, nucleation and forward growth of switched c-domains were observed at the PZT/electrode interfaces, arising due to the built-in electric field induced at each interface. The subsequent sideways growth of the switched domains was inhibited by the depolarization field due to the imperfect charge compensation at the counter-electrode and also at the boundaries with preexisting a-domains, which contributed further to the asymmetric switching behavior. It was found that the preexisting a-domains split into fine a- and c-domains constituting a 90° stripe domain pattern during the 180° polarization switching process, revealing that these domains also actively participated in the out-of-plane polarization switching. The real-time observations uncovered the origin of the switching asymmetry and further clarified the importance of charged domain walls and the interfaces with electrodes in the ferroelectric switching processes.

Domain wall nucleation in epitaxial exchange-biased Fe/IrMn bilayers with highly misaligned anisotropies

The angular dependence of the magnetization reversal in epitaxial Fe/IrMn bilayers with collinear and non-collinear cubic and unidirectional anisotropies is investigated. Multistep loops with different magnetization reversal processes are observed for either positive or negative angles with respect to the Fe easy axis. The angular dependence of the switching fields displays the broken symmetry of the induced non-collinearity. The experimental results are reproduced with a generalized domain wall nucleation model that includes the induced anisotropy configuration and the peculiar asymmetric magnetic switching behavior. These results highlight the importance of the relative angle between anisotropies in epitaxial exchange bias systems with incoherent rotation reversal mechanism, opening a new pathway for tailoring the magnetic properties of such systems.

Asymmetric switching behavior in perpendicularly magnetized spin-valve nanopillars due to the polarizer dipole field

We report the free layer switching field distributions of spin-valve nanopillars with perpendicular magnetization. While the distributions are consistent with a thermal activation model, they show a strong asymmetry between the parallel to antiparallel and the reverse transition, with energy barriers more than 50% higher for the parallel to antiparallel transitions. The inhomogeneous dipolar field from the polarizer is demonstrated to be at the origin of this symmetry breaking. Interestingly, the symmetry is restored for devices with a lithographically defined notch pair removed from the midpoint of the pillar cross-section along the ellipse long axis. These results have important implications for the thermal stability of perpendicular magnetized magnetic random access memory bit cells.

Structural and resistance switching properties of ZnO/SrTiO3/GaAs heterostructure grown by laser molecular beam epitaxy

ZnO thin films were epitaxially grown on (001) GaAs substrate by laser molecular beam epitaxy with SrTiO3 (STO) as a buffer layer. The interface properties of ZnO/GaAs heterostructure are greatly improved by inserting STO buffer layer. The interfacial effects on the transport and dielectric characteristics of the heterostructure have been investigated. The current-voltage characteristic of the heterostructure reveals an asymmetric and resistance switching behavior, exhibiting a temperature-dependent resistance hysteresis in the temperature range of 50–300 K. These measured properties could be attributed to the charge effect at the interface of the heterostructure.

Flux-closure chirality control and domain wall trapping in asymmetric magnetic ring

A technique for flux-closure chirality control and domain wall trapping at the narrowest position in asymmetric magnetic ring is proposed. Micromagnetic simulation work was performed on permalloy asymmetric magnetic rings to observe its magnetic switching behavior. By controlling the lateral geometric features, the ring asymmetry, and the thickness of the film, a local vortex-free reversal process and well-controlled chirality of flux closure can be achieved. Furthermore, a domain wall trapping feature is also observed at the narrow arm of the asymmetric ring, which corresponds to the phenomenon that the magnetic domain wall does not annihilate until the magnetization in the wide arm reversed in a relatively large magnetic field. A phase diagram of the asymmetric ring switching behavior shows that the switching regimes (e.g., the domain wall propagation or nucleation annihilation) of the asymmetric rings and the domain wall pinning depend mainly on the film thickness.

Structure Effects on Resistive Switching of $ \hbox{Al/TiO}_{x}/\hbox{Al}$ Devices for RRAM Applications

Resistive switching characteristics are investigated for Al/TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> /Al devices, particularly for the structural effects in crossbar and via-hole-type devices. The via-hole structure shows more reliable switching characteristics than the crossbar structure, owing to the elimination of possible edge effects. The asymmetric switching behavior is analyzed with top Al/TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> , and bottom Al/TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> interfaces. A trap-controlled space-charge- limited-current model is proposed as a possible switching mechanism, and it is verified that switching mainly occurs on the top electrode/TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> , interface side.

Local switching behavior and electrical polarization of ferroelectric thin films under nanoindentation

The local polarization state and switching behavior of ferroelectric thin films under external mechanical loading were investigated. A nanoindentation technique has been devised to impose local mechanical stress. The results are presented for a 700 nm thick, Pb(Zr 0.30Ti 0.70)O 3 (PZT), film prepared by a sol–gel technique on a platinized Si substrate. A hysteric behavior was found in the local direct piezoelectric response of the ferroelectric thin films within the subcoercive stress range, and an enhanced piezoelectric activity is attributed to the effect of stress-induced domain-wall movement. Upon nanoindentation, voltage shift was observed in the Q–V hysteresis loops along the voltage axis, indicating an asymmetric switching behavior of the local polarization in the loaded films. The parameter of horizontal loop asymmetry increased and the Q–V loop shifted gradually to the positive voltage side as the indentation force increased. The changes in local switching behavior are suggested to result from a variation in residual stress state, asymmetric distribution of charged defects and asymmetric lattice distortion produced by the inhomogeneous indentation stress field.

Spin-dependent transport in multiwalled carbon nanotubes

We report on the observation of spin-polarized tunneling magnetoresistance (TMR) in Co-contacted multiwalled carbon nanotubes (MWCNT). At 4.2 K, we find an increase of the TMR with decreasing junction bias. When the junction bias current is 0.5 nA, the highest value of −36% for the TMR ratio is found. The enhanced TMR ratio is explained by the onset of co-tunneling effects. We also find an asymmetric switching behavior which is tentatively attributed to an additional exchange biasing in the electrode material. For different samples, the TMR can assume both negative and positive values, indicating that the sign of the spin polarization of the tunneling electrons can be reversed. Apparently, the spin polarization sign depends sensitively on the detailed electronic configuration of the contact between Co and MWCNT.

Asymmetric switching behavior of Ni/Pb 1.1(Zr 0.3Ti 0.7)O 3/Pt thin films

An asymmetric behavior of P–E response was observed in Ni/Pb 1.1(Zr 0.3Ti 0.7)O 3/Pt multilayer thin films where Ni was used as top electrode. This interesting phenomenon was investigated by comparative analysis of the hysteresis loops and dynamic pyroelectric responses. The PZT thin films were prepared under the same conditions but different Ni and Pt top electrodes were used in the hysteresis loop measurement, and the Pt was used as the common bottom electrode. It is believed that this asymmetric behavior in hysteresis loop is originated from the strong domain pinning near the top Ni electrode. Dynamic pyroelectric response to 633 nm radiation of the Ni/Pb 1.1(Zr 0.3Ti 0.7)O 3/Pt thin films with polarization up and down was carried out. Higher dynamic pyroelectric response was observed in the positively poled film, in which the direction of the polarization was from the top to bottom electrode. It provides further evidence that the domain pinning near the top electrode dominates the asymmetric switching behavior.

Current-driven reversal in annular vertical giant magnetoresistive devices

Experiments are reported that demonstrate the asymmetric switching behavior reported earlier in vertical giant magnetoresistance devices arises from the solid disk geometry of the device that produces a magnetic singularity at the disk center. Annular devices having a 0.1 μm center hole and 0.5 μm outer diameter are shown to switch symmetrically with an Amperian field.