Intrinsic anionic rearrangement by extrinsic control: transition of RS and CRS in thermally elevated TiN/HfO2/Pt RRAM

Attaining inhibition of sneak current and versatile logic operations in a singular halide perovskite memristive device by introducing appropriate interface barriers

Emerging resistive switching devices hold the potential to realize densely packed passive nanocrossbar arrays, suitable for deployment as random access memory devices (ReRAMs) in both embedded and high-capacity storage applications. In this study, we have engineered ReRAMs comprising ITO/(UVO-treated) amorphous ZnO (a-ZnO)/MAPbI3/Ag which effectively mitigate cross-talk currents without additional components. Significantly, we successfully executed a comprehensive set of 12 distinct 2-input sequential logic functions in a single halide perovskite ReRAM unit for the first time. Furthermore, these logic functions are devoid of any dependency on external light sources, entail merely 1 or 2 logic steps, and showcase symmetrical operability. A superior resistive switching behavior was achieved by harmonizing the charge transport within the bulk MAPbI3 and the tunneling barriers at the interfaces. The outcomes indicate progress in mitigating cross-talk and executing multiple logic functions within a single halide perovskite ReRAM unit, offering a new perspective for the advancement of halide perovskite ReRAMs.

Transformed Filaments by Oxygen Plasma Treatment and Improved Resistance State

The simple structure and operation method of resistive random-access memory (RRAM) has attracted attention as next-generation memory. However, as it is greatly influenced by the movement of oxygen atoms during switching, it is essential to minimize the damage and adjust the defects. Here, we fabricated an ITO/SnO_X/TaN device and investigated the performance improvement with the treatment of O₂ plasma. Firstly, the change in the forming curve was noticeable, and the defect adjustment was carried out effectively. By comparing the I-V curves, it was confirmed that the resistance increased and the current was successfully suppressed, making it suitable for use as a low-power consumption device. Retention of more than 10⁴ s at room temperature was measured, and an endurance of 200 cycles was performed. The filaments' configuration was revealed through the depth profile of X-ray photoelectron spectroscopy (XPS) and modeled to be visually observed. The work with plasma treatment provides a variety of applications to the neuromorphic system that require a low-current level.

Complementary Resistive Switching Behavior in Tetraindolyl Derivative-Based Memory Devices

Complementary resistive switching (CRS) devices are more advantageous compared to bipolar resistive switching (BRS) devices for memory applications as they can minimize the sneak path problem observed in the case of BRS having a crossbar array structure. Here, we report the CRS behavior of 1,4-bis(di(1H-indol-3-yl)methyl)benzene (Indole1) molecules. Our earlier study revealed that Au/Indole1/Indium tin oxide (ITO) devices showed BRS under ambient conditions. However, the present investigations revealed that when the device is exposed to 353 K or higher temperatures, dynamic evolution of the Au/Indole1/ITO device from BRS to CRS occurred with a very good memory window (∼10³), data retention (5.1 × 10³ s), stability (50 days), and device yield (∼ 60%). This work explores the application possibility of indole derivatives toward future ultradense resistive random access memory.

Hafnium Oxide (HfO2 ) - A Multifunctional Oxide: A Review on the Prospect and Challenges of Hafnium Oxide in Resistive Switching and Ferroelectric Memories

Hafnium oxide (HfO₂ ) is one of the mature high-k dielectrics that has been standing strong in the memory arena over the last two decades. Its dielectric properties have been researched rigorously for the development of flash memory devices. In this review, the application of HfO₂ in two main emerging nonvolatile memory technologies is surveyed, namely resistive random access memory and ferroelectric memory. How the properties of HfO₂ equip the former to achieve superlative performance with high-speed reliable switching, excellent endurance, and retention is discussed. The parameters to control HfO₂ domains are further discussed, which can unleash the ferroelectric properties in memory applications. Finally, the prospect of HfO₂ materials in emerging applications, such as high-density memory and neuromorphic devices are examined, and the various challenges of HfO₂ -based resistive random access memory and ferroelectric memory devices are addressed with a future outlook.

Bipolar Resistive Switching in Lanthanum Titanium Oxide and an Increased On/Off Ratio Using an Oxygen-Deficient ZnO Interlayer

The present study pioneered an oxygen migration-driven metal to insulator transition Mott memory, a new type of nonvolatile memory using lanthanum titanium oxide (LTO). We first show the reset first bipolar property without an initial electroforming process in LTO. We used oxygen-deficient ZnO as an interlayer between LTO and a W electrode to clarify whether oxygen migration activates LTO as the Mott transition. ZnO oxygen deficiency provides oxygen ion migration paths as well as a reservoir, facilitating oxygen migration from LTO to the W electrode. Thus, including the ZnO interlayer improved oxygen migration between LTO and the W electrode, achieving a 10-fold increased on/off current ratio. The current research contributes to a better understanding of valence change Mott memory by exploring the LTO resistive switching mechanism and ZnO interlayer influences on the oxygen migration process.

Facile Achievement of Complementary Resistive Switching in Block Copolymer Micelle-Based Resistive Memories

Interest in resistive random access memory (RRAM) has grown rapidly in recent years for realizing ultrahigh density data storage devices. However, sneak currents in these devices can result in misreading of the data, thus limiting the applicability of RRAM. Complementary resistive switching (CRS) memory consisting of two antiserial RRAMs can considerably reduce sneak currents; however, complicated device architectures and manufacturing processes still remain as challenges. Herein, an effective and simple approach for fabricating CRS memory devices using self-assembled block copolymer micelles is reported. Cu ions are selectively placed in the core of polystyrene-block-poly(2-vinylpyridine) spherical micelles, and a hexagonally packed micelle monolayer is prepared through spin-coating. The micelle monolayer can be a symmetrical resistive switching layer, because the micelles and Cu act as dielectric and active metals in memory devices, respectively. The locally enhanced electric field and Joule heating achieved by the structured Cu atoms inside the micelles promote metal ionization and ion migration in a controlled manner, thus allowing for position selectivity during resistive switching. The micelle-based memory device exhibits stable and reliable CRS behavior, with a nonoverlapping and narrow distribution of threshold voltages. Therefore, this approach is promising for fabricating CRS memory devices for high-performance and ultrahigh-density RRAM applications.

Stacked Two-Dimensional MXene Composites for an Energy-Efficient Memory and Digital Comparator

Two-dimensional MXene has enormous potential for application in industry and academia owing to its surface hydrophilicity and excellent electrochemical properties. However, the application of MXene in optoelectronic memory and logical computing is still facing challenges. In this study, an optoelectronic resistive random access memory (RRAM) based on silver nanoparticles (Ag NPs)@MXene-TiO₂ nanosheets (AMT) was prepared through a low-cost and facile hydrothermal oxidation process. The fabricated device exhibited a typical bipolar switching behavior and controllable SET voltage. Furthermore, we successfully demonstrated a 4-bit in-memory digital comparator with AMT RRAMs, which can replace five logic gates in a traditional approach. The AMT-based digital comparator may open the door for future integrated functions and applications in optoelectronic data storage and simplify the complex logic operations.

Transformation of threshold volatile switching to quantum point contact originated nonvolatile switching in graphene interface controlled memory devices

Resistive switching devices based on binary transition metal oxides have been widely investigated. However, these devices invariably manifest threshold switching characteristics when the active metal electrode is silver, the dielectric layer is hafnium oxide and platinum is used as the bottom electrode, and have a relatively low compliance current (<100 μA). Here we developed a way to transform an Ag-based hafnium oxide selector into quantum-contact originated memory with a low compliance current, in which a graphene interface barrier layer is inserted between the silver electrode and hafnium oxide layer. Devices with structure Ag/HfO x /Pt acts as a bipolar selector with a high selectivity of >108 and sub-threshold swing of ∼1 mV dec-1. After introducing a graphene interface barrier, high stress dependent (forming at +3 V) formation of localized conducting filaments embodies stable nonvolatile memory characteristics with low set/reset voltages (<±1.0 V), low reset power (6 μW) and multi-level potential. Grain boundaries of the graphene interface control the type of switching in the devices. A good barrier can switch the Ag-based volatile selector into Ag-based nonvolatile memory.

Tailored nanoplateau and nanochannel structures using solution-processed rutile TiO2 thin films for complementary and bipolar switching characteristics

We synthesized two different nanostructures of rutile TiO₂ (r-TiO₂) thin films on a fluorine-doped tin oxide (FTO) substrate at the lowest temperature reported until now and fabricated resistive random access memory (RRAM) devices with these r-TiO₂ thin films having the stacking sequence of Ag/r-TiO₂/FTO. Complementary resistive switching (CRS) and bipolar resistive switching (BRS) were observed in different thicknesses of r-TiO₂ based devices. Benefiting from the in situ growth of the solution processed thin films and modulating the reaction growth rates, we successfully attained two different morphologies of r-TiO₂ with a nanoplateau at a controlled deposition rate and pre-defined nanochannels at a higher deposition rate. The RRAM devices with nano-plateaus of r-TiO₂ showed excellent CRS as well as unprecedented simultaneous observations of BRS. These CRS and BRS characteristics were reversible and reproducible. On the other hand, the tailored pre-defined nanochannels in r-TiO₂ led to forming-free BRS with a pulse endurance higher than 10⁷ without any degradation in the high and low resistance states. We propose a plausible switching mechanism of these unprecedented events using various physical and electrical characterization studies of low-temperature processed r-TiO₂ RRAM devices. This work suggests the importance of solution-processed thin film engineering for RRAM switching with reliable and reproducible characteristics.

Full imitation of synaptic metaplasticity based on memristor devices

Neuromorphic engineering is a promising technology for developing new computing systems owing to the low-power operation and the massive parallelism similarity to the human brain. Optimal function of neuronal networks requires interplay between rapid forms of Hebbian plasticity and homeostatic mechanisms that adjust the threshold for plasticity, termed metaplasticity. Metaplasticity has important implications in synapses and is barely addressed in neuromorphic devices. An understanding of metaplasticity might yield new insights into how the modification of synapses is regulated and how information is stored by synapses in the brain. Here, we propose a method to imitate the metaplasticity inhibition of long-term potentiation (MILTP) for the first time based on memristors. In addition, the metaplasticity facilitation of long-term potentiation (MFLTP) and the metaplasticity facilitation of long-term depression (MFLTD) are also achieved. Moreover, the mechanisms of metaplasticity in memristors are discussed. Additionally, the proposed method to mimic the metaplasticity is verified by three different memristor devices including oxide-based resistive memory (OxRAM), interface switching random access memory, and conductive bridging random access memory (CBRAM). This is a further step toward developing fully bio-realistic artificial synapses using memristors. The findings in this study will deepen our understanding of metaplasticity, as well as provide new insight into bio-realistic neuromorphic engineering.