Giant tunnelling electroresistance in atomic-scale ferroelectric tunnel junctions

Subangstrom ion beam engineering of buried ultrathin oxides for scalable quantum computing

Multilayer nanoscale systems incorporating ultrathin tunnel barriers, magnetic materials, amorphous oxides, and promising dielectrics are essential for next-generation logics, memory, quantum, and neuro-inspired computing. Still, an ultrathin film control at the atomic scale remains challenging. Here, we introduce a complementary metal-oxide semiconductor-compatible approach using focused ion beam irradiation for buried ultrathin films' engineering with subangstrom thickness control. Molecular dynamics simulations confirm the pivotal role of ion-induced crystal defects. Its performance is exemplified by Josephson junction resistance tuning in the range of 2 to 37% with a 0.86% standard deviation in completed chips. Furthermore, it enables ±17-megahertz frequency accuracy (±0.172 angstrom tunnel barrier thickness variation) in superconducting multiqubit processors, as well as qubit energy relaxation and echo coherence times exceeding 0.5 milliseconds.

Giant switchable ferroelectric photovoltage in double-perovskite epitaxial films through chemical negative strain

Double-perovskite films have been extensively studied in multifunctional fields due to their modifiability. Here, a controlled process strategy to induce chemical strain and anomalous Poisson deformation is proposed for perovskite-based films. The chemical negative strain in the local-ordering BiSmFe2O6 double-perovskite films can be regulated by oxygen engineering to cause the effectively tunable bandgap. We markedly increased the switchable open-circuit voltage to ~1.56 V from ~0.50 V for Pt/BiSmFe2O6/Nb-SrTiO3 devices, which is the highest in single-layer perovskite-based ferroelectric photovoltaic perpendicular devices under white light-emitting diode irradiation. The multifield composite action mechanism reveals the electrical cause of the large open-circuit voltage. The synergy of the optical fields and ferroelectric fields provides the possibility of multilevel storage. Structural characterizations indicate that the chemical strain offers a dual role of lattice distortion and vacancy migration. The strategy of controllable chemical strain facilitates further exploration of the application potential of ferroelectric materials for multifunctional electronic devices.

Flexible BaTiO3 Ferroelectric Nonvolatile Memory for Neuromorphic Computation

The use of BaTiO3 (BTO) ferroelectric thin films in flexible ferroelectric memory offers a promising pathway for next-generation nonvolatile memory applications, given BTO's excellent ferroelectric properties, stability, high dielectric constant, and strong fatigue resistance. However, the fabrication of BTO on flexible substrates presents a significant technical challenge. In this study, we achieved high-quality, single-crystalline (111)-oriented BTO films on mica substrates through the design of buffer layers. The BTO films exhibit strong polarization properties (remnant polarization, 2Pr ∼15.63 μC/cm2, and saturation polarization, 2Ps ∼36.61 μC/cm2), and the flexible BTO devices maintained exceptional stability under bending radii of 3.5 and 6 mm. After 107 bipolar switching cycles, polarization showed only minor changes, with a retention time exceeding 104 s. We further explored the application of flexible BTO ferroelectric memory in neuromorphic computing. The flexible BTO-based memory demonstrated adjustable synaptic behavior, effectively modulating EPSC (excitatory postsynaptic current) responses through pulse amplitude and width to simulate short-term memory. PPF (paired pulse facilitation) and LTP (long-term potentiation) behaviors verified its synaptic weight modulation capabilities, achieving 91.6% accuracy in neural network-based handwritten digit recognition after 103 training cycles. These findings underscore the potential of flexible BTO ferroelectric memory for memory devices and neuromorphic computing, offering promising applications for wearable AI systems.

Hybrid Ferroelectric Tunnel Junctions: State of the Art, Challenges, and Opportunities

Ferroelectric tunnel junctions (FTJs) harness the combination of ferroelectricity and quantum tunneling and thus herald opportunities in next-generation nonvolatile memory technologies. Recent advancements in the fabrication of ultrathin heterostructures have enabled the integration of ferroelectrics with various functional materials, forming hybrid tunneling-diode junctions. These junctions benefit from the modulation of the functional layer/ferroelectric interface through ferroelectric polarization, thus enabling further modalities and functional capabilities in addition to tunneling electroresistance. This Perspective aims to provide in-depth insight into the physical phenomena of several typical ferroelectric hybrid junctions, ranging from ferroelectric/dielectric, ferroelectric/multiferroic, and ferroelectric/superconducting to ferroelectric/2D materials, and finally their expansion into the realm of ferroelectric resonant tunneling diodes (FeRTDs). This latter aspect, i.e., resonant tunneling, offers an approach to exploiting tunneling behavior in ferroelectric heterostructures. We discuss examples that have successfully shown room-temperature ferroelectric control of parameters such as the resonant peak, tunnel current ratio at peak, and negative differential resistance. We conclude the Perspective by summarizing the challenges and highlighting the opportunities for the future development of hybrid FTJs, with a special emphasis on a possible type of FeRTD device. The prospects for enhanced performance and expanded functionality ignite tremendous excitement in hybrid FTJs and FeRTDs for future nanoelectronics.

Out-of-plane polarization induces a picosecond photoresponse in rhombohedral stacked bilayer WSe2

Constructing van der Waals materials with spontaneous out-of-plane polarization through interlayer engineering expands the family of two-dimensional ferroelectrics and provides an excellent platform for enhancing the photoelectric conversion efficiency. Here, we reveal the effect of spontaneous polarization on ultrafast carrier dynamics in rhombohedral stacked bilayer WSe2. Using precise stacking techniques, a 3R WSe2-based vertical heterojunction was successfully constructed and confirmed by polarization-resolved second harmonic generation measurements. Through output characteristics and the scanning photocurrent map under zero bias, we reveal a non-zero short-circuit current in the graphene/3R WSe2/graphene heterojunction region, demonstrating the bulk photovoltaic effect. Furthermore, the out-of-plane polarization enables the 3R WSe2 heterojunction region to achieve an ultrafast intrinsic photoresponse time of approximately 3 ps. The ultrafast response time remains consistent across varying detection powers, demonstrating environmental stability and highlighting the potential in optoelectronic applications. Our study presents an effective strategy for enhancing the response time of photodetectors.

Assessment of functional performance in self-rectifying passive crossbar arrays utilizing sneak path current

Self-rectifying memristive devices have emerged as promising contenders for low-power in-memory computing, presenting numerous advantages. However, characterizing the functional behavior of passive crossbar arrays incorporating these devices remains challenging due to sophisticated parasitic currents stemming from rich memristive dynamic behavior. Conventional methods using read margin assessments to evaluate functional behavior in passive crossbars are hindered by the voltage divider effect from the pull-up resistor. In this study, we propose a novel performance metric, Δ SC, harnessing sneak path currents to assess functional behavior. Through the application of a pair of negative rectification factors,RF n, L andRF n, H , we comprehensively delineate dynamic rectification behavior in both positive and negative bias regimes, as well as in low-resistance state and high-resistance state, deviating from conventional metrics such as on/off ratios, nonlinearity, and rectifying factors. Notably, Δ SC provides a quantitative evaluation of the interaction between sneak path currents and read margin, demonstrating its efficacy and addressing a pivotal research gap in the field. For instance, employing self-rectifying BiFeO3 memristive cells featuringRF n, L = 1.22E3 andRF n, H = 9.27, we showcase the successful functional performance of a passive crossbar array, achieving Δ SC < 2.19E-2, while ensuring a read margin > 0.

Ultralow Energy Consumption and Fast Neuromorphic Computing Based on La0.1Bi0.9FeO3 Ferroelectric Tunnel Junctions

Low-power and fast artificial neural network devices represent the direction in developing analogue neural networks. Here, an ultralow power consumption (0.8 fJ) and rapid (100 ns) La0.1Bi0.9FeO3/La0.7Sr0.3MnO3 ferroelectric tunnel junction artificial synapse has been developed to emulate the biological neural networks. The visual memory and forgetting functionalities have been emulated based on long-term potentiation and depression with good linearity. Moreover, with a single device, logical operations of "AND" and "OR" are implemented, and an artificial neural network was constructed with a recognition accuracy of 96%. Especially for noisy data sets, the recognition speed is faster after preprocessing by the device in the present work. This sets the stage for highly reliable and repeatable unsupervised learning.

Growth of emergent simple pseudo-binary ferroelectrics and their potential in neuromorphic computing devices

Ferroelectric memory devices such as ferroelectric memristors, ferroelectric tunnel junctions, and field-effect transistors are considered among the most promising candidates for neuromorphic computing devices. The promise arises from their defect-independent switching mechanism, low energy consumption and high power efficiency, and important properties being aimed for are reliable switching at high speed, excellent endurance, retention, and compatibility with complementary metal-oxide-semiconductor (CMOS) technology. Binary or doped binary materials have emerged over conventional complex-composition ferroelectrics as an optimum solution, particularly in terms of CMOS compatibility. The current state-of-the-art route to achieving superlative ferroelectric performance of binary oxides is to induce ferroelectricity at the nanoscale, e.g., in ultra-thin films of doped HfO2, ZrO2, Zn1-xMgxO, Al-xScxN, and Bi1-xSmxO3. This short review article focuses on the materials science of emerging new ferroelectric materials, including their different properties such as remanent polarization, coercive field, endurance, etc. The potential of these materials is discussed for neuromorphic applications.