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Complex electrical structures composed of nanomaterials are widely studied in the research of nanoelectronics. Characterizing the current distribution is important to understand the current conducting mechanism and optimize the device's design. In this work, we employed the nitrogen vacancy centers in diamond as quantum sensors to directly and noninvasively monitor currents in nanowire networks. The sub-micrometer magnetic field imaging was achieved by injecting microwave current into networks and detecting the magnetic resonate spins' population, revealing the internal current paths involved in electrical conduction during electrical annealing. The establishment, breakdown, and reform of current paths were imaged in detail, which are difficult to realize through conventional methods. The mechanism of resistance change and relocating of current pathways was subsequently analyzed. This work demonstrates that a diamond-based quantum microscope is a useful tool to unveil the nanoscale conducting properties of complex conductive networks and guide the design for potential applications. [ABSTRACT FROM AUTHOR]

Resistive random-access memory (RRAM) devices have been widely studied for neuromorphic, in-memory computing. One of the most studied RRAM structures consists of a titanium capping layer and a HfOx adaptive oxide. Although these devices show promise in improving neuromorphic circuits, high variability, non-linearity, and asymmetric resistance changes limit their usefulness. Many studies have improved linearity by changing materials in or around the device, the circuitry, or the analog bias conditions. However, the impact of prior biasing conditions on the observed analog resistance change is not well understood. Experimental results in this study demonstrate that prior higher reset voltages used after forming cause a greater resistance change during subsequent identical analog pulsing. A multiphysics finite element model suggests that this greater analog resistance change is due to a higher concentration of oxygen ions stored in the titanium capping layer with increasing magnitude of the reset voltage. This work suggests that local ion concentration variations in the titanium capping layer of just tens of atoms cause significant resistance variation during analog operation. [ABSTRACT FROM AUTHOR]

Flexible strain sensors that are currently available are mainly used in human motion recognition and medical health detection applications, and there is still an urgent need for sensors to realize real-time monitoring of the 3D deformation of industrial and agricultural products. In this work, a flexible strain sensor with a hat-shaped structure was fabricated using a molding technique to perform in situ measurement of 3D deformation. An algorithm for resistance change detection and linear calibration equations were proposed to enable analysis of the deformation data and calculation of local shape changes. The sensor was applied to monitor the growth deformation of a kumquat fruit, and the results were highly consistent with the algorithm. The proposed technique has great potential for application to 3D deformation detection of flexible objects. [ABSTRACT FROM AUTHOR]

Neuromorphic capabilities of a self-aligned complementary metal-oxide-semiconductor compatible W/WOx/HfO2/Ru cell in a 3D vertical memristive structure were investigated. We show that the device exhibits nonfilamentary forming-free multilevel resistive switching with gradual resistance change. In addition, the poor retention of a low resistance state allows integration of these structures in architectures that require short-term memory characteristics such as reservoir computing systems. The ability of the device to rely on the temporal sequence of the stream was tested with the digit recognition task. Since a WOx layer was obtained by thermal oxidization and HfO2 and Ru layers were grown by atomic layer deposition methods, the device is suitable for high-density systems with high connectivity within a neural network. [ABSTRACT FROM AUTHOR]

We demonstrate here an In2S3 based vacuum pressure sensor that can operate over a vacuum range, 103–10−3 mbar at room temperature. It is shown that vacuum pressure has a significant electrical impact on the sensitivity and the sensor response of the device. The sensor response of the device in terms of the rate of resistance change is 183 with a quick rise/fall time of 3.3/1.7 s. Even after being exposed to ambient conditions for 8 weeks, the device displays a consistent and periodic sensor response for 100 consecutive vacuum on/off cycles, demonstrating its durability. X-ray photoelectron spectroscopy was used to identify the surface adsorbed/chemisorbed groups, which are responsible for vacuum sensing properties. These surface effects were further confirmed by exposing the device to different environments such as humidity, high temperature, and high purity oxygen. Though In2S3 is known for its properties as a buffer layer in solar cells, this study reveals another potential application of In2S3 thin films as a vacuum pressure sensor. [ABSTRACT FROM AUTHOR]

One of the most important challenges in antiferromagnetic spintronics is the readout of the Néel vector state. High current densities up to 108 Acm−2 used in the electrical switching experiments cause notorious difficulty in distinguishing between magnetic and thermal origins of the electrical signals. To overcome this problem, we present a temperature dependence study of the transverse resistance changes in the switching experiment with CoO | Pt devices. We demonstrate the possibility to extract a pattern of spin Hall magnetoresistance for current pulses with a density of 5 × 10 7 A cm − 2 that is present only below the Néel temperature and does not follow a trend expected for thermal effects. This is the compelling evidence for the magnetic origin of the signal, which is observed using purely electrical techniques. We confirm these findings by complementary experiments in an external magnetic field. Such an approach can allow determining the optimal conditions for switching antiferromagnets and be very valuable when no imaging techniques can be applied to verify the origin of the electrical signal. [ABSTRACT FROM AUTHOR]

One of the key challenges of phase change memory (PCM) is its high power consumption during the reset operation, when the phase change material (typically Ge2Sb2Te5, i.e., GST) heats up to ∼900 K or more in order to melt. Here, we study the temperature-dependent behavior of PCM devices by probing the reset power at ambient temperatures from 80 to 400 K. We find that different device structures exhibit contrasting temperature-dependent behavior. The reset power in our confined-type PCM is nearly unchanged with ambient temperature, corresponding to a temperature-dependent thermal resistance, whereas results for mushroom-type PCM from the literature show a linear relation between power and temperature, suggesting a more constant thermal resistance. This discrepancy is ascribed to different temperature distributions and thermal properties of the dominant components of the PCM cell thermal resistance, as shown by electro-thermal modeling. In the confined cell, the thermal boundary resistance of the GST and the thermal conductivity of the bottom electrode dominate the thermal resistance, while for the mushroom cell, the GST thermal conductivity plays a greater role. These findings can help to design more power- and energy-efficient PCM devices by better focusing thermal management efforts on the key components of the device. [ABSTRACT FROM AUTHOR]

We show that a CoFeB/MgO-based magnetic tunnel junction (MTJ) formed directly on a flexible substrate has considerable potential as a high-sensitivity strain gauge. A gauge factor of ∼1000, which represents the highest sensitivity reported, thus far, for a film-type strain gauge and is ∼500 times larger than that of the most prevalent metal-foil strain gauge, is realized in the flexible MTJ with a pseudo-spin valve structure under the condition of external magnetic-field assistance. Additionally, using a flexible MTJ with strain-insensitive exchange-biased pinned and strain-sensitive free layers, we demonstrate that a large resistance change due to the strain application can be achieved under magnetic field-free conditions. According to simulations based on the coherent magnetization rotation model, conditions are suggested for improving the gauge factor, as well as for using the flexible MTJ as a strain gauge in practical applications. [ABSTRACT FROM AUTHOR]

Persistent photoconductivity (PPC) describes the light-induced persistent enhancement of conductivity after turning off the light, which is tunable by a gating voltage. Here, we report a bias-tunable PPC in van der Waals heterojunctions of black phosphorus/2D electron gas (BP/2DEG) on SrTiO3. With a light pulse applied, the resistance of a BP/2DEG heterojunction decreases dramatically over five orders of magnitude. Such a low resistive state is maintained over 5 days without any sign of recovery. The application of the negative bias voltage can tune the device into a high resistive state. By combining the light and bias voltages, the device enables it to change into any resistance value, indicating a potential application for multi-bit photoelectric memory. Further study demonstrates that the BP/2DEG heterojunction can achieve both optical writing/electric erasing and electric writing/optical erasing. Our work paves the way to control the PPC through the bias voltage. [ABSTRACT FROM AUTHOR]

Vanadium dioxide (VO2) thin films grown on hexagonal boron nitride (hBN) flakes show three orders of magnitude resistance change due to metal–insulator transition (MIT). The MIT property of VO2 thin films is strongly dependent on the metallic domain size, which should be identified to derive the resistance change owing to the single metallic domain. In this study, we investigated the relationship between the metallic domain size and the device-size-dependent MIT property of VO2 thin films grown on hBN. We observed by temperature-dependent Raman spectroscopy and optical microscopy the emergence of the metallic domains and determined the metallic domain size in VO2 thin films grown on hBN. The metallic domain size of the VO2 thin films grown on hBN was determined to be ∼500 nm on average in length and up to sub-micrometer scale. Electric transport measurements revealed that VO2/hBN microwires exhibit multi-level step-like resistivity changes that change by one to two orders when the length and width are ∼2 μm owing to the confined metallic domains in the micrometer scale. Our results open a way for VO2 devices, showing a steep and large resistance change even in the micrometer scale. [ABSTRACT FROM AUTHOR]

Biological matter is attracting increasing attention because it shows innovative features that have found several applications in technology, from highly sensitive sensors for medical treatments to devices for energy harvesting. Furthermore, most of its phenomenology remains unclear, thus giving hints for speculative investigations. In this Letter, we explore the possibility to use a well-known photosensitive protein, the Reaction Center of Rhodobacter sphaeroides, to build up an electrical pH sensor, i.e., a device able to change its resistance depending on the pH of the solution in which it crystalizes. By using a microscopic model previously tested on analog proteins, we investigate the electrical response of the reaction center single protein under different conditions of applied bias, showing the feasibility of the bio-rheostat hypothesis. As a matter of fact, the calculated resistance of this protein grows about 100% when going from a pH = 10 to a pH = 6.5. Moreover, calculations of the current voltage characteristics well agree with available experiments performed with current atomic force microscopy under neutral conditions. All findings are in qualitative agreement with the known role of pH in biochemical activities of reaction center and similar proteins, therefore supporting a proof-of-concept for the development of innovative electron devices based on biomaterials. [ABSTRACT FROM AUTHOR]

We obtain a large in-plane magnetic uniaxial anisotropy in the soft magnetic Fe60Co20B20 (FeCoB) thin films prepared by rotational sputtering. The anisotropy field (Ha) of 75 to 175 Oe was found in the films with wide ranged thickness from 2.5 to 100 nm, which was attributed to the magnetoelastic energy in association with anisotropic tensile stress. This stress-induced anisotropy has outstanding thermal stability that survives up to 350 °C in the annealing process. The similar large uniaxial anisotropy can be realized in other soft magnetic thin films, such as Fe, Co, Ni, FeCo, and NiFe, with the same synthesis technique. The anisotropic FeCoB film was used as a free layer in a synthetic spin valve. A linear resistance change against external field with a range wider than ±100 Oe together with a significantly reduced coercivity of ∼1.1 Oe (∼8.5 Oe in the case with isotropic free layer) was observed in the transfer curve. The results of this work not only confirm the feasibility of films with large stress-induced magnetic anisotropy as a functional layer in spin-valve devices but also demonstrate a simple synthesis route to induce the magnetic anisotropy, which provides an additional control parameter for the spintronic device design. [ABSTRACT FROM AUTHOR]

Spin torque oscillators (STOs) that generate a persistent magnetization oscillation by spin-transfer torque are promising spintronic devices for various applications. In this study, we propose and experimentally demonstrate a method to characterize the magnetization dynamics of STOs using injection locking. By placing the STO near an antenna that generates a microwave magnetic field of a variable frequency ( f M W ), injection locking of the STO to the microwave field occurs when f M W is in the locking range around the intrinsic oscillation frequency. During injection locking, the dc resistance of the STO exhibits a peak-and-valley dependence on f M W , which originates from the modification of the magnetization trajectory induced by injection locking. Based on this principle, the oscillation frequency can be estimated by measuring the dc resistance change. Because this method does not require measuring the high-frequency magnetoresistance signal from the STO, which is used in the conventional method, it is advantageous in cases where the high-frequency magnetoresistance signal is undetectable or exhibits additional frequency components different from the oscillation frequency. [ABSTRACT FROM AUTHOR]

Relaxation in resistance switching (RS) has severely hindered the stability of memory devices based on oxide-thin-film/ferroelectric heterostructures. In this work, Zn doped SnO2 (ZTO) thin films were deposited on (111)-0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-0.3PT) substrates to investigate the effect of oxygen vacancies (OVs) on resistance relaxation. The polarization-dependent resistance relaxation is observed in ZTO/PMN-0.3PT heterostructures. The largest relative change in resistance up to 53.6% within 20 min is ascribed to OV diffusion. Local aggregation of positively charged OVs with the application of poling electric field would enhance the Coulomb repulsion and strain in distorted lattice, which could accelerate the OV back-diffusion. Partial positively charged OVs are neutralized in the positive polarization state with the application of +10 kV/cm poling voltage, which weakens the relaxation compared to the negative polarization state. Our work helps to understand the interrelation between OVs and polarization-dependent relaxation of RS properties. [ABSTRACT FROM AUTHOR]

Elastic, deformable electrodes have attracted substantial interest owing to their applicability in unconventional stretchable devices. In this study, we investigate the effect of tensile strain on the conductive properties of gold nanowire (AuNW) network electrodes with macroscale mesh structures (mesh-shaped AuNW network electrodes) and the mechanism by which the mesh structure affects its conductivity, with the aim of maintaining a stable conductivity as the electrodes are stretched. At 150% elongation, the proposed AuNW macroscale mesh exhibited a relative resistance change (Δ R / R 0 ) of 11, as opposed to 30 for AuNW network electrodes without a mesh structure. Microstructural analysis revealed that resistance changes in the mesh-shaped AuNW network electrodes depend on the electrode stretching direction and the current direction within the network. Our experimental and analytical results provide a basis for optimizing the mesh design for metal nanowire networks used in stretchable electrodes. [ABSTRACT FROM AUTHOR]

The coupling of electronic degrees of freedom in materials to create "hybridized functionalities" is a holy grail of modern condensed matter physics that may produce versatile mechanisms of control. Correlated electron systems often exhibit coupled degrees of freedom with a high degree of tunability which sometimes lead to hybridized functionalities based on external stimuli. However, the mechanisms of tunability and the sensitivity to external stimuli are determined by intrinsic material properties which are not always controllable. A Mott metal-insulator transition (MIT) is technologically attractive due to the large changes in resistance, tunable by doping, strain, electric fields, and orbital occupancy but not, in and of itself, controllable with light. Here, an alternate approach is presented to produce optical functionalities using a properly engineered photoconductor/strongly correlated hybrid heterostructure. This approach combines a photoconductor, which does not exhibit an MIT, with a strongly correlated oxide, which is not photoconducting. Due to the intimate proximity between the two materials, the heterostructure exhibits giant volatile and nonvolatile, photoinduced resistivity changes with substantial shifts in the MIT transition temperatures. This approach can be extended to other judicious combinations of strongly correlated materials. [ABSTRACT FROM AUTHOR]

The fast development of high-accuracy neuromorphic computing requires stable analog memristors. While filamentary memory switching is very common in binary oxides, their resistive switching usually involves abrupt changes due to the rupture or reformation of metallic filaments. In this work, we designed a memristor consisting of dual-layer HfOy/HfOx, with different concentrations of oxygen vacancies (y > x). During the electroforming process, both the migration of existing oxygen vacancies in HfOx and the generation of new oxygen vacancies in HfOy occur simultaneously, leaving a semiconducting part close to the HfOy/HfOx interface. The resulting filament is not metallic as a whole, as revealed by first principles calculations. Such a device demonstrates excellent switching uniformity as well as highly gradual resistance change, ideal for neuromorphic computing. Through fine tuning of the filament structure, the device achieves low variation, high speed, gradual SET and RESET processes, and hundreds of stable multi-level state behaviors. The long-term synaptic plasticity was further achieved, showing good linearity and large analog switching window (ΔG as high as 487.5 μS). This works affords a route toward a gradual resistance change in oxide-based memristors. [ABSTRACT FROM AUTHOR]

NiO–Ni composite films were heteroepitaxially grown on (111)-oriented ferroelectric 0.31Pb(In1/2Nb1/2)O3-0.35Pb(Mg1/3Nb2/3)O3-0.34PbTiO3 (PIN-PMN-PT) single-crystal substrates by pulsed laser deposition. The NiO films prepared in high vacuum are n-type conducting and possess room-temperature ferromagnetism, which originates from oxygen vacancies and the presence of the second Ni phase, respectively. Taking advantage of the electric-field-induced ferroelectric polarization charges, we realized in situ reversible and nonvolatile modulation of both the electrical resistance and magnetism of the film. A relative resistance change of ∼470% is obtained at room temperature, while an appreciable magnetization change of ∼15% was achieved at 50 K by switching the polarization states of PIN-PMN-PT. The coexistence of charge-density-tunable electronic and magnetic properties of NiO–Ni/PIN-PMN-PT heterostructures may provide a strategy to design charge-mediated multiferroic devices for nonvolatile memory and spintronic applications. [ABSTRACT FROM AUTHOR]

Manipulating the photoresponse of two-dimensional electron gas at the interface of complex oxides is attracting tremendous interest because of its potential applications in photoelectric devices. In this study, transport behaviors of LaAlO3/SrTiO3 heterointerfaces tuned by a nickelate buffer layer under light irradiation have been investigated. With the increasing thickness of the buffer layer, the LaAlO3/SrTiO3 interfaces exhibit the metal-to-insulator transition. More importantly, we also observe a substantial enhancement in the photoresponse under a 360 nm light irradiation and the relative change in the resistance of heterointerfaces is enhanced from 6.8% (without a buffer layer) to 50 139% (with a 1.6 u.c. buffer layer) at 300 K. Moreover, the heterointerfaces exhibit a state of change from persistent to transient photoconductivity. These emerging results are analyzed in view of the relevant mechanisms of band bending and lattice effects. This work provides experimental support for the application of perovskite oxides in interface optoelectronics. [ABSTRACT FROM AUTHOR]

Interfacial charge and strain are two coupling effects in semiconductor/ferroelectric epitaxial heterostructures, which are pivotal for use in tailoring functionalities in devices. In this work, La0.04Ba0.96SnO3/0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 heterostructures with varying film thicknesses were prepared in order to understand both charge and strain's contributions to the electric-field induced resistance change. The relative resistance change to the lattice strain remains almost unchanged in those thicker films, while increases a little bit in those thinner films. This slight increase is related to the substrate constraint near the interface and follows Freund's strain relaxation model during the dynamic strain induced by the piezoelectric switch. A depletion layer model was also established to simulate the electroresistance variation from the interfacial charge effect. The depletion layer involves an equilibrium between capture and release of electrons by the acceptor-like defects near the interface region. The resistance change vs electric field evolves from a butterfly-like shape to a square-like when decreasing the film thickness, due to the joint effect of strain and interfacial polarization screening charge. This study provides an insight into understanding heteroepitaxial coupling and exploring their potential applications in oxide electronic devices. [ABSTRACT FROM AUTHOR]

Spin-based memories are attractive for their non-volatility and high durability but provide modest resistance changes, whereas semiconductor logic transistors are capable of providing large resistance changes, but lack memory function with high durability. The recent availability of multiferroic materials provides an opportunity to directly couple the change in spin states of a magnetic memory to a charge change in a semiconductor transistor. In this work, we propose and analyze the spin–orbit torque field-effect transistor, a device with the potential to significantly boost the energy efficiency of spin-based memories and to simultaneously offer a palette of functionalities. [ABSTRACT FROM AUTHOR]

A cross-point memory architecture will be adopted to make the most of the suitability of resistance change memories for high integration. To achieve this, the development of a selector device that is connected in series to the memory and helps to remove the sneak path current flowing through unselected and half-selected cells is an urgent requirement. In this Letter, we developed a selector based on a simple sandwiched structure of Pt/CoO/ITO and confirmed the nonpolar threshold resistance switching that does not require the first fire process. The selector showed an excellent performance of a very small dispersion of the threshold voltages and resistance in the OFF state with the relative standard deviations of less than 2% each, as well as the quick transition between the OFF and ON states within 50 ns. The scaling law was confirmed in the current, both in the ON and OFF states, I ON and I th . It allows us to analyze I ON and I th , based on which we can reduce the area of the device. The current conduction of the Pt/CoO/ITO selector is ruled by the trap-controlled space charge limited current (SCLC), and the threshold switching from ON to OFF states and vice versa is caused by the transition from the trap-unfilled to the trap-filled SCLC and vice versa, respectively. [ABSTRACT FROM AUTHOR]

Filamentary adaptive oxide devices based on HfOx are a promising technology for neuromorphic computing applications. The resistance of these devices depends on the concentration of oxygen vacancies in the filament region. A local temperature rise from joule heating plays a significant role in the movement of oxygen ions, making thermal management crucial to reliable performance. In this work, the role of the substrate thermal conductivity on the analog performance was investigated at biologically realistic pulse widths. Au/Ti/HfOx/Au adaptive oxide devices were fabricated on substrates with two orders of magnitude difference in thermal conductivity. A lower thermal conductivity substrate dissipates heat more slowly, resulting in a large initial change in resistance from a single operation pulse, which is detrimental to the desired analog behavior. The results were validated by a COMSOL Multiphysics® model that models the flow of heat in both samples. [ABSTRACT FROM AUTHOR]

Wearable electronics that can deform with human skin and monitor physical information are promising next-generation electronics. Here, we demonstrate a self-powered smart patch for temperature sensing by integrating a triboelectric nanogenerator, power management circuit and temperature sensor together. Benefitting from the functional nanomaterial and fabrication process of spray coating, the main part of the device shows extraordinary mechanical flexibility and visible transparency. The temperature sensor presents a high sensitivity of 0.54% per Kelvin scaled by relative resistance change. Owing to the high efficiency of the implemented power management circuit, a capacitor of 100 μF can be easily charged to 1 V within 105 s through the triboelectric nanogenerator contacting with cotton, which can drive the sensor continuously to work for more than 100 s. The large-scalable fabrication process and integration design give this smart patch potential applications in human-machine interfaces and soft electronic skins. [ABSTRACT FROM AUTHOR]

The reversible resistance switching (RS) effect of the Al/TiOx/TiO2/Al heterostructure is investigated in this study. This RS was attributed to space-charge-limited conduction (SCLC) as controlled by localized traps in the TiOx layer. The preexisting SCLC theory was extended to describe the abrupt resistance transition. An analytical model was developed with consideration of the ratio of free and trapped carrier density, which was extracted from the experimental data to show exponentially distributed traps in energy. The proposed model can be applicable to RS phenomena induced by interface-type traps in other material system. [ABSTRACT FROM AUTHOR]