Memory Devices Research Articles

Room Temperature Magnetic Skyrmions in Gradient-Composition Engineered CoPt Single Layers.

Topologically protected magnetic skyrmions in magnetic materials are stabilized by an interfacial or bulk Dzyaloshinskii-Moriya interaction (DMI). Interfacial DMI decays with an increase of the magnetic layer thickness in just a few nanometers, and bulk DMI typically stabilizes magnetic skyrmions at low temperatures. Consequently, more flexibility in the manipulation of DMI is required for utilizing nanoscale skyrmions in energy-efficient memory and logic devices at room temperature (RT). Here, we demonstrate the observation of RT skyrmions stabilized by gradient DMI (g-DMI) in composition gradient-engineered CoPt single-layer films by employing the topological Hall effect, magnetic force microscopy, and nitrogen-vacancy scanning magnetometry. Skyrmions remain stable over a wide range of applied magnetic fields and are confirmed to be nearly Bloch-type from micromagnetic simulation and analytical magnetization reconstruction. Furthermore, we observe skyrmion pairs, which may be explained by skyrmion-antiskyrmion interactions. Our findings expand the family of magnetic materials hosting RT magnetic skyrmions by tuning g-DMI via gradient polarity and a choice of magnetic elements.

Role of sulphur in resistive switching behavior of natural rubber-based memory

The rising environmental awareness has spurred the extensive use of green materials in electronic applications, with bio-organic materials emerging as attractive alternatives to inorganic and organic materials due to their natural biocompatibility, biodegradability, and eco-friendliness. This study showcases the natural rubber based resistive switching memory devices and how varying sulphur concentrations (0 - 0.8 wt.%) in natural rubber thin films impact the resistive switching characteristics. The natural rubber was formulated and processed into a thin film deposited on an ITO substrate as the bottom electrode and with an Ag film as the top electrode. The addition of sulphur modifies the degree of crosslinking in the natural rubber thin film, from which the concentration of -C=C- group and density of defect site (S+) are affected, and hence the resistive switching behavior of the memory device. The devices exhibit bipolar resistance with symmetric switching characteristics which are attributed to the formation of conductive paths facilitate by electron transport along -C=C- and S+ defect sites between the two electrodes. Notably, a sample with 0.2 wt.% sulphur exhibits a high ON/OFF ratio (104), a large memory window (5.5 V), prolonged data retention (10 years), and reliable endurance (120 cycles). These findings highlight the potential of natural rubber as a promising material for eco-friendly resistive-switching random access memory applications.&#xD.

Epitaxial Thin Film Growth on Recycled SrTiO3 Substrates Toward Sustainable Processing of Complex Oxides.

Complex oxide thin films cover a range of physical properties and multifunctionalities that are critical for logic, memory, and optical devices. Typically, the high-quality epitaxial growth of these complex oxide thin films requires single crystalline oxide substrates such as SrTiO3 (STO), MgO, LaAlO3, a-Al2O3, and many others. Recent successes in transferring these complex oxides as free-standing films not only offer great opportunities in integrating complex oxides on other devices, but also present enormous opportunities in recycling the deposited substrates after transfer for cost-effective and sustainable processing of complex oxide thin films. In this work, the surface modification effects introduced on the recycled STO are investigated, and their impacts on the microstructure and properties of subsequently grown epitaxial oxide thin films are assessed and compared with those grown on the pristine substrates. Detailed analyses using high-resolution scanning transmission electron microscopy and geometric phase analysis demonstrate distinct strain states on the surfaces of the recycled STO versus the pristine substrates, suggesting a pre-strain state in the recycled STO substrates due to the previous deposition layer. These findings offer opportunities in growing highly mismatched oxide films on the recycled STO substrates with enhanced physical properties. Specifically, yttrium iron garnet (Y3Fe5O12) films grown on recycled STO present different ferromagnetic responses compared to that on the pristine substrates, underscoring the effects of surface modification. The study demonstrates the feasibility of reuse and redeposition using recycled substrates. Via careful handling and preparation, high-quality epitaxial thin films can be grown on recycled substrates with comparable or even better structural and physical properties toward sustainable process of complex oxide devices.

Improving the Nonvolatile Memory Characteristics of Sol-Gel-Processed Y2O3 RRAM Devices Using Mono-Ethanolamine Additives.

In this study, Y2O3-based resistive random-access memory (RRAM) devices with a mono-ethanolamine (MEA) stabilizer fabricated using the sol-gel process on indium tin oxide/glass substrates were investigated. The effects of MEA content on the structural, optical, chemical, and electrical characteristics were determined. As the MEA content increased, film thickness and crystallite size decreased. In particular, the increase in MEA content slightly decreased the oxygen vacancy concentration. The decreased film thickness decreased the physical distance for conductive filament formation, generating a strong electric field. However, owing to the lowest oxygen vacancy concentration, a large electrical field is required. To ensure data reliability, the endurance cycles across several devices were measured and presented statistically. Additionally, endurance performance improved with the increase in MEA content. Reduced oxygen vacancy concentration can successfully suppress the excess formation of the Ag conductive filament. This simplifies the transition from the high- to the low-resistance state and vice versa, thereby improving the endurance cycles of the RRAM devices.

Control of ferroelectricity in Ta-doped HfO2 and its non-zero-crossing current–voltage hysteresis behavior

Hafnium oxide (HfO2)-based ferroelectrics are being explored as potential candidates for ferroelectric memory devices due to their highly compatibility with complementary metal-oxide-semiconductor (CMOS) technology. Enhancing the remanent polarization and investigating the underlying mechanism are crucial tasks. In the present study, tantalum (Ta) was introduced as a dopant to induce ferroelectric properties in HfO2, a large portion of orthorhombic phase was recognized in the as-grown Ta:HfO2 without further thermal treatment. The remanent polarization of Ta:HfO2 thin films can be optimized by adjusting the oxygen flow rates during the sputtering process. The influencing factors for enhanced ferroelectric performance include the control of Ta concentration, its valence state, and the presence of singly ionized oxygen vacancies, which are influenced by oxygen addition. Furthermore, the resistive switching behavior showing non-zero crossing current–voltage (I–V) hysteresis is associated with ferroelectricity and the presence of oxygen vacancies. A model has been proposed to explain the ferroelectric resistive switching with non-zero crossing I–V characteristics by considering the role of oxygen vacancies and polarization effects. This model suggests that the oxygen vacancies at the surface layer, along with ferroelectric polarization, play a crucial role in electron transport.

Metastable body-centered cubic CoMnFe alloy films with perpendicular magnetic anisotropy for spintronics memory

ABSTRACT A body-centered cubic (bcc) FeCo(B) is a current standard magnetic material for perpendicular magnetic tunnel junctions (p-MTJs) showing both large tunnel magnetoresistance (TMR) and high interfacial perpendicular magnetic anisotropy (PMA) when MgO is utilized as a barrier material of p-MTJs. Since the p-MTJ is a key device of current spintronics memory, i.e. spin-transfer-torque magnetoresistive random access memory (STT-MRAM), it attracts attention for further advance to explore new magnetic materials showing both large PMA as well as TMR. However, there have been no such materials other than FeCo(B)/MgO. Here, we report, for the first time, PMA in metastable bcc Co based alloy, i.e. bcc CoMnFe thin films which are known to exhibit large TMR effect when used for electrodes of MTJs with the MgO barrier. The largest intrinsic PMA’s were about 0.6 and 0.8 MJ/m3 in a few nm thick CoMnFe alloy film and multilayer film, respectively. Our ab-initio calculation suggested that PMA originates from tetragonal strain and the value exceeds 1 MJ/m3 with optimizing strain and alloys composition. The simulation of the thermal stability factor indicates that the magnetic properties obtained satisfy the requirement of the data retention performance of X-1X nm STT-MRAM. The large PMA and high TMR effect in bcc CoMnFe/MgO, which were rarely observed in materials other than FeCo(B)/MgO, indicate that bcc CoMnFe/MgO is one of the potential candidates of the materials for X - 1X nm STT-MRAM.

Enhanced performance of flexible BiFeO3 ferroelectric memory with Mica substrate via SrTiO3 buffer layer

BiFeO3 (BFO) application in flexible wearable devices is garnering interest because of its unique ferroelectric and magnetic properties. However, the integration of high-quality BFO films onto flexible substrates presents significant technical challenges. Here, we successfully fabricated high-quality BFO films on mica substrates by using pulsed laser deposition, and report the fatigue characteristics of BFO films on flexible substrates for the first time. The results demonstrated that, after 108 bipolar switching cycles, the polarization only degraded by 0.28%, indicating superior fatigue characteristics compared to previously reported BFO films. Additionally, the device ferroelectric properties remained largely unchanged, with a bending radius of 3.5 mm. The fabricated flexible Pt/BFO/La0.65Sr0.35MnO3(LSMO)/SrTiO3(STO)/mica non-volatile memory devices exhibited mechanical flexibility and fatigue resistance. These findings not only highlight the potential of flexible BFO films for wearable electronic devices and flexible memory devices, they also provide valuable insight for the future development of high-performance flexible ferroelectric materials.

Synthesis and Anisotropic Memristive Behavior of Borophene Nanosheets

AbstractNeuromorphic computing, marked by its parallel computational abilities and low power usage, has become pivotal in advancing artificial intelligence. However, the advancement of neuromorphic computing has faced significant obstacles due to the performance limitations of traditional memory devices struggling with high power consumption and limited reliability. Two‐dimensional (2D) materials have been extensively investigated as high‐performance memristive materials, but they are often restricted by fixed memristive properties, which complicate circuit design and limit flexibility. Here, we report that multilayer borophene nanosheets represent a breakthrough material, displaying anisotropic variable memristive properties. The nanosheets, comprising semiconductor α’‐4H‐borophene sheets and metal β12‐borophene sheets, have been synthesized on aluminum foil surface through chemical vapor deposition method. The multilayer borophene nanosheets exhibit volatile memory behavior in the vertical direction and non‐volatile memory behavior in the planar direction. This innovative class of 2D nanosheets not only overcomes the limitations of conventional memory devices but also expands the potential applications of borophene‐based memories in information storage and in‐memory computing.

Synthesis and Anisotropic Memristive Behavior of Borophene Nanosheets.

Neuromorphic computing, marked by its parallel computational abilities and low power usage, has become pivotal in advancing artificial intelligence. However, the advancement of neuromorphic computing has faced significant obstacles due to the performance limitations of traditional memory devices struggling with high power consumption and limited reliability. Two-dimensional (2D) materials have been extensively investigated as high-performance memristive materials, but they are often restricted by fixed memristive properties, which complicate circuit design and limit flexibility. Here, we report that multilayer borophene nanosheets represent a breakthrough material, displaying anisotropic variable memristive properties. The nanosheets, comprising semiconductor α'-4H-borophene sheets and metal β12-borophene sheets, have been synthesized on aluminum foil surface through chemical vapor deposition method. The multilayer borophene nanosheets exhibit volatile memory behavior in the vertical direction and non-volatile memory behavior in the planar direction. This innovative class of 2D nanosheets not only overcomes the limitations of conventional memory devices but also expands the potential applications of borophene-based memories in information storage and in-memory computing.

Decoding the magnetic bit positioning error in a ferrimagnetic racetrack.

Current-driven motion of magnetic domain walls is one of the key technologies for developing storage class memory devices. Extensive studies have revealed a variety of material systems that exhibit high-speed and/or lower power propagation of the domain walls driven by electric current. However, few studies have assessed the reliability of the operations of the memory technology. Here, we decode the errors associated with writing and shifting domain walls using nanosecond current pulses in a ~5-micrometer-wide wire composed of a Pt/GdFeCo bilayer. We find that writing a domain wall at the edge of the wire causes a bit positioning error of ~0.3 micrometers, whereas the shifting process induces an error of ~0.1 micrometers per a 2-nanosecond-long current pulse. The error correlation among successive shifting is negligible when the current drive is sufficiently large. These features allow reliable operation of highly packed domain walls in a ferrimagnetic racetrack.

The Fabrication of Polyimide-Based Tunable Charge Traps Ternary Memristors Doped with Ni-Co Coated Carbon Composite Nanofibers.

In the dynamic fields of information science and electronic technology, there is a notable trend towards leveraging carbon materials, favored for their ease of synthesis, biocompatibility, and abundance. This trend is particularly evident in the development of memristors, benefiting from the unique electronic properties of carbon to enhance device performance. This study utilizes sensitized chemical evaporation and spin-coating carbonization techniques to fabricate nickel-cobalt coated carbon composite nanofibers (SC-NCMNTs). Novel polyimide (PI) matrix composite memory devices were fabricated using in situ polymerization technology. Transmission electron microscopy (TEM) and micro-Raman spectroscopy analyses validated the presence of dual interface structures located between the Ni-Co-MWNTs, carbon composite nanofibers, and PI matrix, revealing a significant number of defects within the SC-NCMNTs/PI composite films. Consequently, this results in a tunable charge trap-based ternary resistive switching behavior of the composite memory devices, exhibiting a high ON/OFF current ratio of 104 and a retention time of 2500 s at an operating voltage of less than 3 V. The mechanism of resistive switching is thoroughly elucidated through a comprehensive charge transport model, incorporating molecular orbital energy levels. This study provides valuable insights for the rational design and fabrication of efficient memristors characterized by multilevel resistive switching states.

SuFEx‐based Transparent Polysulfates and their Applications in Tunable Resistant Electrical Memory Materials

AbstractVisibly transparent electron devices are current research highlights, which are found at the “neotype” stage of technical development for the usage in the next generation “see‐through” electronic devices. However, less attention is paid to transparent semiconductor memory devices, and hence, achieving such “see‐through” electronic devices are still partially limited by lacking the easily achievable and cost‐effective transparent memory materials. Herein, three visible light transparent polysulfate‐based memory devices are reported, e.g. ITO/P‐BPS/Al, ITO/P‐TPA/Al, and ITO/P‐TPABPS/Al, that displayed DRAM, WORM, and FLASH effects, respectively. The mechanisms of the observed memory behavior of each memory material are proposed on the basis of computational simulation. Remarkably, these polysulfates‐based memory materials are obtained by simply using different main‐chain repeating units, suggesting a wide application potential of polysulfate as functionalized materials.

Single-molecule Magnets (SMM) spin channels connecting FeMn antiferromagnet and NiFe ferromagnetic electrodes of a tunnel junction

The integration of single-molecule magnets (SMMs) into magnetic tunnel junctions (MTJs) offers significant potential for advancing molecular spintronics, particularly for next-generation memory devices, quantum computing, and energy storage technologies such as solar cells. In this study, we present the first demonstration of SMM-induced spin-dependent properties in an antiferromagnet-based MTJ molecular spintronic device (MTJMSD). We engineered cross-junction-shaped devices comprising FeMn/AlOx/NiFe MTJs. The AlOx barrier thickness where the exposed junction edges meet was comparable to the SMM length, facilitating the incorporation of SMM molecules as spin channels for spin-dependent transport. The SMM channels enabled long-range magnetic moment ordering around molecular junctions, which were precisely engineered via fabrication processes. The SMM, composed of a [Mn6(μ3-O)2(H2N-sao)6(6-atha)2(EtOH)6] (H2N-saoH = salicylamidoxime, 6-atha = 6-acetylthiohexanoate) complex, featured thioester groups at the ends that upon hydrolysis they form bonds with the magnetic electrodes. SMM-treated junctions demonstrated a significant current enhancement, reaching up to 7 μA at an input voltage of 60 mV. Furthermore, SMM-doped junctions exhibited current stabilization in the μA range at lower temperatures, whereas the bare electrodes showed current suppression to the picoampere range. Magnetization measurements conducted at 55 K and 300 K on pillar-shaped devices revealed a reduction in magnetic moment at low temperatures. Additionally, Kelvin probe atomic force microscopy (KPAFM) measurements confirmed that SMM integration transformed the electronic properties over long ranges.These findings are attributed to the spin channels formed between magnetic metal electrodes, which enhance spin polarization at each magnetic electrode. Our research highlights the potential of using antiferromagnetic materials, characterized by minimal stray fields and zero net magnetization, to transform MTJMSD devices.

Supercritical CO2-Guided Passivation Strategies for Oxygen Vacancy Modulation in LaMnO3.

The development of 2D magnetic materials and the modulation of intrinsic magnetism are essential for the exploration of new materials in the field of information storage. Despite its strong ferromagnetic properties, LaMnO3 is hindered by a high number of oxygen defects, which result in a relatively short lifetime when employed in electronic memory devices. Here the successful transformation of bulk LaMnO3 into a 2D structure using supercritical carbon dioxide.is reported. This technique enables the successful modulation of the magnetic properties of the material. Interestingly, it is found that the oxygen defect is repaired, which is in sharp contrast to conventional perovskites. These promising results demonstrate the potential of using the magnetic properties of LaMnO3, which is of great importance in the context of expanding its application in electronic devices.

Crystal Chemistry and Design Principles of Altermagnets.

Altermagnetism was very recently identified as a new type of magnetic phase beyond the conventional dichotomy of ferromagnetism (FM) and antiferromagnetism (AFM). Its globally compensated magnetization and directional spin polarization promise new properties such as spin-polarized conductivity, spin-transfer torque, anomalous Hall effect, tunneling, and giant magnetoresistance that are highly useful for the next-generation memory devices, magnetic detectors, and energy conversion. Though this area has been historically led by the thin-film community, the identification of altermagnetism ultimately relies on precise magnetic structure determination, which can be most efficiently done in bulk materials. Our review, written from a materials chemistry perspective, intends to encourage materials and solid-state chemists to make contributions to this emerging topic through new materials discovery by leveraging neutron diffraction to determine the magnetic structures as well as bulk crystal growth for exploring exotic properties. We first review the symmetric classification for the identification of altermagnets with a summary of chemical principles and design rules, followed by a discussion of the unique physical properties in relation to crystal and magnetic structural symmetry. Several major families of compounds in which altermagnets have been identified are then reviewed. We conclude by giving an outlook for future directions.

Epitaxial Orientation-Controlled High Crystallinity and Ferroelectric Properties in Hf0.5Zr0.5O2 Films.

Hafnium-based binary oxides are essential for fabricating nanoscale high-density ferroelectric memory devices. However, effective strategies to control and improve their thin-film single crystallinity and metastable ferroelectricity remain elusive, hindering potential applications. Here, using NdGaO3 (NGO) substrates with four crystalline orientations, we report a systematic study of the structural characterizations and ferroelectric properties of epitaxial Hf0.5Zr0.5O2 (HZO) films, demonstrating orientation-controlled high crystallinity and enhanced ferroelectric properties. HZO films grown on NGO(001) and NGO(110) substrates exhibit relatively low crystallinity and a significant presence of the monoclinic phase. In contrast, HZO films grown on NGO(100) and NGO(010) possess high single crystallinity and a dominant ferroelectric phase. These differences are attributed to the surface symmetry of the NGO substrate, which favors the formation of 4- or 2-fold domain configurations. Moreover, the optimized HZO films exhibit a large polarization (2Pr) of ∼50 μC/cm2, enhanced fatigue behavior up to 1011 cycles, improved retention of 2Pr ∼ 40 μC/cm2 after 10 years, and characteristic polarization switching speeds in the submicrosecond range. Our results highlight the importance of modulating the single crystallinity and ferroelectric phase fraction of HfO2-based films to enhance ferroelectric properties, further revealing the potential of epitaxial symmetry engineering.

Enhancing Charge Trapping Performance of Hafnia Thin Films Using Sequential Plasma Atomic Layer Deposition.

We aimed to fabricate reliable memory devices using HfO2, which is gaining attention as a charge-trapping layer material for next-generation NAND flash memory. To this end, a new atomic layer deposition process using sequential remote plasma (RP) and direct plasma (DP) was designed to create charge-trapping memory devices. Subsequently, the operational characteristics of the devices were analyzed based on the thickness ratio of thin films deposited using the sequential RP and DP processes. As the thickness of the initially RP-deposited thin film increased, the memory window and retention also increased, while the interface defect density and leakage current decreased. When the thickness of the RP-deposited thin film was 7 nm, a maximum memory window of 10.1 V was achieved at an operating voltage of ±10 V, and the interface trap density (Dit) reached a minimum value of 1.0 × 1012 eV-1cm-2. Once the RP-deposited thin film reaches a certain thickness, the ion bombardment effect from DP on the substrate is expected to decrease, improving the Si/SiO2/HfO2 interface and thereby enhancing device endurance and reliability. This study confirmed that the proposed sequential RP and DP deposition processes could resolve issues related to unstable interface layers, improve device performance, and enhance process throughput.

Chemical and Resistive Switching Properties of Elaeodendron buchananii Extract-Carboxymethyl Cellulose Composite: A Potential Active Layer for Biodegradable Memory Devices.

Biodegradable electronic devices play a crucial role in addressing the escalating issue of electronic waste accumulation, which poses significant environmental threats. In this study, we explore the utilization of a methanol-based extract of the Elaeodendron buchananii plant blended with a carboxymethyl cellulose biopolymer to produce a biodegradable and environmentally friendly functional material for a resistive switching memory system using silver and tungsten electrodes. Our analyses revealed that these two materials chemically interact to generate a perfect composite with near semiconducting optical bandgap (4.01 eV). The resultant device exhibits O-type memory behavior, with a low ON/OFF ratio, strong endurance (≥103 write/erase cycles), and satisfactory (≥103) data retention. Furthermore, through a comprehensive transport mechanism analysis, we observed the formation of traps in the composite that significantly improved conduction in the device. In addition, we established that altering the voltage amplitude modifies the concentration of traps, leading to voltage amplitude-driven multiple resistance states. Overall, our findings underscore the potential of functionalizing polymers that can be functionalized by incorporating plant extracts, resulting in biodegradable and nonvolatile memory devices with promising performance metrics.

Plasma Etch of IGZO Thin Film and IGZO/SiO2 Interface Diffusion in Inductively Coupled CH4/Ar Plasmas

ABSTRACTIn this work, the etching characteristics of InGaZnO4 (IGZO) thin films were systemically investigated in inductively coupled CH4/Ar plasmas with various gas ratios, bias voltages, and surface temperatures. The ion flux in the CH4/Ar plasmas was analyzed using bias power and voltage, whereas the relative densities of CH and H radicals were quantified through optical actinometry. The change in CH3 radical density was also estimated based on plasma kinetics analysis. The correlations between the plasma properties and IGZO etch rate were investigated. In CH4/Ar plasmas with CH4 ratios below 80%, the IGZO etch rate increases with a higher CH4 proportion, aligning with the ascending trend of CH3 radical density, which suggests that CH3 radical density acts as a limiting factor (radical‐limited regime). However, the IGZO etch rate decreases when the CH4 ratio exceeds 80%, corresponding to the reduction in ion flux, which indicates that ion flux becomes a limiting factor in this ion‐limited regime. The IGZO etch rate shows a linear relationship with bias voltage and follows the Arrhenius equation with varying surface temperature in plasmas without bias voltage. A spontaneous etch model was proposed based on these relationships. In this model, the IGZO etch rate depends on CH3 radical density and IGZO surface reactivity, where ion bombardment contributes more significantly to surface reactivity than high surface temperature. IGZO thin film was deposited on SiO2 during the fabrication of a 3D stacked ferroelectric field‐effect transistor (FeFET) memory device. The interaction between IGZO and SiO2 during CH4/Ar plasma processing was investigated using time‐of‐flight secondary ion mass spectrometry. The efficacy of removing IGZO atoms that had diffused into the SiO2 network was enhanced when subjected to plasma with a bias voltage, compared to the process without a bias voltage. However, exposure to CH4/Ar plasma resulted in a deeper diffusion of IGZO atoms into the SiO2 network, primarily attributed to ion bombardment rather than elevated surface temperature. A significant improvement in surface smoothness was achieved after IGZO and SiO2 etch by introducing BCl3 into the SiO2 etching chemistry.

Nonvolatile logic gate and full adder based on tri-terminal oxide resistive switching devices

Today's on-chip computing power is constrained by the “memory wall” and “power wall” caused by the Von Neumann bottleneck. As a potential solution, this work has developed nonvolatile logic gates based on field-effect tri-terminal oxide resistive switching memory devices (3T-RRAM). A compact circuit model using a polynomial control source (PCS) is proposed to describe the behavior of the fabricated 3T-RRAM. The 3T-RRAM can be regarded as a nonvolatile transmission gate for constructing nonvolatile logic gates. Additionally, a full adder with input storage functionality has been designed using only eight 3T-RRAMs (four nonvolatile logic gates), and a binarized neural network (BNN) based on 3T-RRAM logic gate arrays has been proposed. This demonstrates the great potential of nonvolatile logic gates in computing-in-memory applications.