Tunneling Electroresistance Effect with Diode Characteristic for Cross-Point Memory

Self-Rectifying Resistive Memory with a Ferroelectric and 2D Perovskite Lateral Heterostructure

Integration of resistive switching and rectification functions in a single memory device is promising for high writing/readout accuracy with a simplified device architecture, but the realization remains challenging, especially with a low voltage operation. Herein, we developed self-rectifying resistive memory with a single memristive layer that can be operated at ultralow voltages with an excellent rectification ratio. The memristive layer consisted of a phase-separated lateral heterostructure of a ferroelectric polymer, poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-TrFE)], and a 2D halide perovskite, butylammonium lead iodide (BA2PbI4), which could be readily fabricated by spin-casting. Systematic characterization revealed that a lateral ferroelectric polarization from self-poled P(VDF-TrFE) could rectify the current flow into the BA2PbI4 channel. The resistive memory consisting of Ag/P(VDF-TrFE):BA2PbI4/indium tin oxide exhibited a high resistance switching ratio of >106 programmable at ±0.4 V and an excellent rectification ratio of >106 at ±0.1 V, along with a long data retention and stable endurance cycles.

Oxygen reservoir effect of Tungsten trioxide electrode on endurance performance of ferroelectric capacitors for FeRAM applications

The effect of W and WO3 electrodes on the ferroelectric characteristics of HZO (Zr-doped HfO2)-based MFM (metal-ferroelectric-metal) capacitors was investigated. During the deposition of tungsten, the W electrode was formed using only Ar gas, while the WO3 electrode was formed using a mixture of Ar and O2 gases. The W-based MFM capacitors exhibited superior remnant polarization (2Pr of 107.9 µC/cm2 at 700 oC) compared to the WO3-based capacitors; however, their endurance performance was degraded. In contrast, the WO3-based capacitors showed endurance performance enhanced by three orders of magnitude due to the oxygen-rich reservoir effect. The oxygen introduced during the deposition of WO3 prevented the oxygen scavenging effect of tungsten. Consequently, excessive generation of oxygen vacancies in the HZO layer was suppressed, resulting in improved endurance performance. These results were quantitatively confirmed through TEM, XPS, and XRD analyses.

High-Reliability and Self-Rectifying Alkali Ion Memristor through Bottom Electrode Design and Dopant Incorporation

Ionic memristor devices are crucial for efficient artificial neural network computations in neuromorphic hardware. They excel in multi-bit implementation but face challenges like device reliability and sneak currents in crossbar array architecture (CAA). Interface-type ionic memristors offer low variation, self-rectification, and no forming process, making them suitable for CAA. However, they suffer from slow weight updates and poor retention and endurance. To address these issues, the study demonstrated an alkali ion self-rectifying memristor with an alkali metal reservoir formed by a bottom electrode design. By adopting Li metal as the adhesion layer of the bottom electrode, an alkali ion reservoir was formed at the bottom of the memristor layer by diffusion occurring during the atomic layer deposition process for the Na:TiO2 memristor layer. In addition, Al dopant was used to improve the retention characteristics by suppressing the diffusion of alkali cations. In the memristor device with optimized Al doping, retention characteristics of more than 20 h at 125 °C, endurance characteristics of more than 5.5 × 105, and high linearity/symmetry of weight update characteristics were achieved. In reliability tests on 100 randomly selected devices from a 32 × 32 CAA device, device-to-device and cycle-to-cycle variations showed low variation values within 81% and 8%, respectively.

Ferroelectric Tunnel Junctions: Modulations on the Potential Barrier

Recently, ferroelectric tunnel junctions (FTJs) have attracted considerable attention for potential applications in next-generation memories, owing to attractive advantages such as high-density of data storage, nondestructive readout, fast write/read access, and low energy consumption. Herein, recent progress regarding FTJ devices is reviewed with an emphasis on the modulation of the potential barrier. Electronic and ionic approaches that modulate the ferroelectric barriers themselves and/or induce extra barriers in electrodes or at ferroelectric/electrode interfaces are discussed with the enhancement of memory performance. Emerging physics, such as nanoscale ferroelectricity, resonant tunneling, and interfacial metallization, and the applications of FTJs in nonvolatile data storage, neuromorphic synapse emulation, and electromagnetic multistate memory are summarized. Finally, challenges and perspectives of FTJ devices are underlined.

Nondestructive Readout Complementary Resistive Switches Based on Ferroelectric Tunnel Junctions

Recently, complementary resistive switches (CRSs) have attracted considerable attention because of the effective suppression of the sneak leakage that is an inherent problem of crossbar memory arrays. In this work, we propose a new CRS device enabling nondestructive readout based on back-to-back in-series Pt/BaTiO₃/Nb:SrTiO₃ ferroelectric tunnel junctions (FTJs). The FTJ elements exhibit not only a nonvolatile resistance switching but also a typical diode-like transport in the high-resistance state (HRS) because of the ferroelectric enhancement on the Schottky barrier of the BaTiO₃/Nb:SrTiO₃ interface. With the rectifying characteristic, the complementary HRS + LRS (low-resistance state) and LRS + HRS states can be well-distinguished and nondestructively read out by a subthreshold voltage. In addition, the sneak current is significantly suppressed in the Pt/BaTiO₃/Nb:SrTiO₃ CRS crossbar array, and the maximum scaling size is increased by about 50 times, in comparison to the array constituted by only the single-FTJ devices. These results facilitate the design of high-performance resistive memories based on the crossbar architecture.

Ti-Doped GaOx Resistive Switching Memory with Self-Rectifying Behavior by Using NbOx/Pt Bilayers

Crossbar arrays (CBAs) with resistive random access memory (ReRAM) constitute an established architecture for high-density memory. However, sneak paths via unselected cells increase the total power consumption of these devices and limit the array size. To eliminate such sneak-path problems, we propose a Ti/GaO_x/NbO_x/Pt structure with a self-rectifying resistive-switching (RS) behavior. In this structure, to reduce the operating voltage, we used a Ti/GaO_x stack to increase the number of trap sites in the RS GaO_x layer through interfacial reactions between the Ti and GaO_x layers. This increase enables easier carrier transport with reduced electric fields. We then adopted a NbO_x/Pt stack to add rectifying behavior to the RS GaO_x layer. This behavior is a result of the large Schottky barrier height between the NbO_x and Pt layers. Finally, both the Ti/GaO_x and NbO_x/Pt stacks were combined to realize a self-rectifying ReRAM device, which exhibited excellent performance. Characteristics of the device include a low operating voltage range (-2.8 to 2.5 V), high on/off ratios (∼20), high selectivity (∼10⁴), high operating speeds (200-500 ns), a very low forming voltage (∼3 V), stable operation, and excellent uniformity for high-density CBA-based ReRAM applications.