Promising Candidates For Non-volatile Memories Research Articles

Nonvolatile ferroelectric domain wall memory integrated on silicon

Ferroelectric domain wall memories have been proposed as a promising candidate for nonvolatile memories, given their intriguing advantages including low energy consumption and high-density integration. Perovskite oxides possess superior ferroelectric prosperities but perovskite-based domain wall memory integrated on silicon has rarely been reported due to the technical challenges in the sample preparation. Here, we demonstrate a domain wall memory prototype utilizing freestanding BaTiO3 membranes transferred onto silicon. While as-grown BaTiO3 films on (001) SrTiO3 substrate are purely c-axis polarized, we find they exhibit distinct in-plane multidomain structures after released from the substrate and integrated onto silicon due to the collective effects from depolarizing field and strain relaxation. Based on the strong in-plane ferroelectricity, conductive domain walls with reading currents up to nanoampere are observed and can be both created and erased artificially, highlighting the great potential of the integration of perovskite oxides with silicon for ferroelectric domain wall memories.

Bipolar resistive switching and non-volatile memory properties of MnO2-polyaniline (PANI) nanocomposite

Organic materials are becoming a promising candidate for non-volatile memory especially resistive switching memories. The present work reports bipolar resistive switching and memristive properties of polyaniline coated MnO2 nanocomposite. The hybrid nanocomposite was synthesized by oxidative polymerization of aniline in the presence of the MnO2 nanostructures. The XRD analysis confirms the α-MnO2 phase and the TEM micrograph reveals the formation of nanorods of the material. The results of Attenuated Total Reflection Infrared Spectroscopy (ATR-IR) and Thermogravimetric analysis (TGA) confirmed the presence of the polymer in the composite. The developed device shows bipolar resistive switching behavior with a memory window ~7 × 102, switches efficiently over 103 cycles, and can retain the data over 104 seconds. Space charge limited current conduction model along with the possible filamentary route of operation describes conduction and resistive switching mechanisms of the device.

Recent advances in metal nanoparticle‐based floating gate memory

AbstractNonvolatile memory is distinguished for the application in many electronic products due to its excellent charge storage ability. Nevertheless, as the device dimensions are scaled down, floating gate memory encounters various challenges: the increasing leakage current leading to a serious reliability issue and the decreasing of charge density. Hence, metal nanoparticle‐based floating gate memory has been proposed and become a promising candidate for nonvolatile memories due to its outstanding operation speed, excellent scalability, and favorable reliability. This review briefly introduces the classification of memory devices. The operation mechanisms, fabrication and characterization of metal nanoparticle‐based floating gate memory are discussed based on research activities reported in recent years.

ANALYSIS THE EFFECTS OF FERROELECTRIC ELECTRIC FIELD ON CURRENT-VOLTAGE CHARACTERISTICS OF THE FERROELECTRIC FIELD EFFECT TRANSISTOR USING SRBI2TA2O9 THIN FILM

This paper presents a new analytical expression for current-voltage characteristics of the Ferroelectric Field Effect Transistor (FeFET), a promising candidate for nonvolatile memories. For this research, a FeFET model using Pt/SrBi2Ta2O9/Insulators/Si thin film as an effect gate stack was proposed and assessed. Firstly, we have studied the effects of ferroelectric polarization on current-voltage characteristics of FeFET based on the analytical method of CMOS device, the polarization hysteresis properties versus electric field (P-E) of the ferroelectric material was explicitly analysed with two parameters of saturated and unsaturated polarization. Then, by mathematical analysis, the current-voltage values was calculated under different conditions such as using differences of substrate doping concentration, oxide thickness, ferroelectric thickness, working temperature, etc. Finally, the calculated results were simulated by the Matlab software that gave us an overview of the device properties.

Performance Prospects of Deeply Scaled Spin-Transfer Torque Magnetic Random-Access Memory for In-Memory Computing

In recent years, Spin-Transfer-Torque Magnetic Random Access Memory (STT-MRAM) has been considered as one of the most promising non-volatile memory candidates for in-memory computing. However, system-level performance gains using STT-MRAM for in-memory computing at deeply scaled nodes have not been assessed with respect to more mature memory technologies. In this letter, we present perpendicular magnetic tunnel junction (pMTJ) STT-MRAM devices at 28nm and 7nm. We evaluate the system-level performance of convolutional neural network (CNN) inference with STT-MRAM arrays in comparison to Static Random Access Memory (SRAM). We benchmark STT-MRAM and SRAM in terms of area, leakage power, energy, and latency from 65nm to 7nm technology nodes. Our results show that STT-MRAM keeps providing $\sim 5\times $ smaller synaptic core area, $\sim 20\times $ less leakage power, and $\sim 7\times $ less energy than SRAM when both devices are scaled from 65nm to 7nm. With the emerging need for low power computation for a broad range of applications such as internet-of-things (IoT) and neural network (NN), STT-MRAM can offer energy-efficient and high-density in-memory computing.

Integration of external electric fields in molecular dynamics simulation models for resistive switching devices

Resistive switching devices emerged a huge amount of interest as promising candidates for non-volatile memories as well as artificial synapses due to their memristive behavior. The main physical and chemical phenomena which define their functionality are driven by externally applied voltages and the resulting electric fields. Although molecular dynamics simulations are widely used in order to describe the dynamics on the corresponding atomic length and time scales, there is a lack of models which allow for the actual driving force of the dynamics, i.e., externally applied electric fields. This is due to the restriction of currently applied models to solely conductive, non-reactive, or insulating materials, with thicknesses on the order of the potential cutoff radius, i.e., 10 Å. In this work, we propose a generic model, which can be applied in particular to describe the resistive switching phenomena of metal-insulator-metal systems. It has been shown that the calculated electric field and force distribution in case of the chosen example system Cu/a-SiO2/Cu are in agreement with the fundamental field theoretical expectations.

A Full-Sensing-Margin Dual-Reference Sensing Scheme for Deeply-Scaled STT-RAM

Spin transfer torque-random access memory (STT-RAM) has recently been regarded as one of the most promising non-volatile memory candidates for the next-generation computer architectures. However, the readability issue has become a new obstacle for STT-RAM in deeply-scaled technology nodes, owing to (a) the increasing process-voltage-temperature variations but reduced supply voltage, resulting in low sensing margin (SM); (b) reduced current margin between the read current and the critical write current of magnetic tunnel junction, leading to the high read disturbance (RD). Here, to deal with the readability issue of deeply-scaled STT-RAM, we propose a full-sensing-margin dual-reference sensing (FSM-DRS) scheme via exploiting analog signal processing within the sensing circuit design. The proposed FSM-DRS scheme improves SM significantly but with no increase of RD through: (a) including two reference cells which utilize the same structure of the data cells to provide two reference signals, thus reducing the reference mismatch or regularity problem; and (b) adding an analog signal pre-processing operation between the data and reference signals before decision, doubling SM without increasing RD. In comparison with the typical sensing schemes, our simulation results (under the 40 nm technology node) show that our FSM-DRS scheme has an ~70% enhancement in average SM as well as a $\sim 10\times $ decrease in bit error rate.

Electrode Dependence of Tunneling Electroresistance and Switching Stability in Organic Ferroelectric P(VDF‐TrFE)‐Based Tunnel Junctions

AbstractFerroelectric tunnel junctions (FTJs) are promising candidates for nonvolatile memories and memristor‐based computing circuits. Thus far, most research has focused on FTJs with a perovskite oxide ferroelectric tunnel barrier. As the need for high‐temperature epitaxial film growth challenges the technological application of such inorganic junctions, more easily processable organic ferroelectrics can serve as alternative if large tunneling electroresistance (TER) and good switching durability would persist. This study reports on the performance of FTJs with a spin‐coated ferroelectric P(VDF‐TrFE) copolymer tunnel barrier. The use of three different bottom electrodes, indium tin oxide (ITO), La0.67Sr0.33MnO3, (LSMO), and Nb‐doped SrTiO3 (STO) are compared and it is shown that the polarity and magnitude of the TER effect depend on their conductivity. The largest TER of up to 107% at room temperature is measured on FTJs with a semiconducting Nb‐doped STO electrode. This large switching effect is attributed to the formation of an extra barrier over the space charge region in the substrate. The organic FTJs exhibit good resistance retention and switching endurance up to 380 K, which is just below the ferroelectric Curie temperature of the P(VDF‐TrFE) barrier.

Overcoming the Fundamental Barrier Thickness Limits of Ferroelectric Tunnel Junctions through BaTiO3/SrTiO3 Composite Barriers.

Ferroelectric tunnel junctions (FTJs) have attracted increasing research interest as a promising candidate for nonvolatile memories. Recently, significant enhancements of tunneling electroresistance (TER) have been realized through modifications of electrode materials. However, direct control of the FTJ performance through modifying the tunneling barrier has not been adequately explored. Here, adding a new direction to FTJ research, we fabricated FTJs with BaTiO3 single barriers (SB-FTJs) and BaTiO3/SrTiO3 composite barriers (CB-FTJs) and reported a systematic study of FTJ performances by varying the barrier thicknesses and compositions. For the SB-FTJs, the TER is limited by pronounced leakage current for ultrathin barriers and extremely small tunneling current for thick barriers. For the CB-FTJs, the extra SrTiO3 barrier provides an additional degree of freedom to modulate the barrier potential and tunneling behavior. The resultant high tunability can be utilized to overcome the barrier thickness limits and enhance the overall CB-FTJ performances beyond those of SB-FTJ. Our results reveal a new paradigm to manipulate the FTJs through designing multilayer tunneling barriers with hybrid functionalities.

A 16 Kb Spin-Transfer Torque Random Access Memory With Self-Enable Switching and Precharge Sensing Schemes

Spin-transfer torque magnetic random access memory (STT-MRAM) is considered one of the most promising non-volatile memory candidates thanks to its excellent performance in terms of access speed, endurance, and compatibility to CMOS. However, high power supply voltage is required in the conventional STT-MRAM writing circuit, which results in high power consumption (e.g., ~10 pJ/bit). In addition, it suffers from stochastic switching behavior and process voltage temperature variations. These make power-efficient and reliable write/read circuits become critical challenges. In this paper, we present novel circuits and architectures to build a 16 kb STT-MRAM design with low power and high reliability. For example, the self-enable switching scheme reduces the power consumption effectively and the fore-placed sense amplifier improves the robustness to process variation. Using an accurate compact model of 65 nm STT-MRAM and a commercial CMOS design kit, mixed transient and statistical simulations have been performed to validate this design.

Emerging Nonvolatile Memories to Go Beyond Scaling Limits of Conventional CMOS Nanodevices

Continuous dimensional scaling of the CMOS technology, along with its cost reduction, has rendered Flash memory as one of the most promising nonvolatile memory candidates during the last decade. With the Flash memory technology inevitably approaching its fundamental limits, more advanced storage nanodevices, which can probably overcome the scaling limits of Flash memory, are being explored, bringing about a series of new paradigms such as FeRAM, MRAM, PCRAM, and ReRAM. These devices have indeed exhibited better scaling capability than Flash memory while also facing their respective physical drawbacks. The consequent tradeoffs therefore drive the information storage device technology towards further advancement; as a result, new types of nonvolatile memories, including carbon memory, Mott memory, macromolecular memory, and molecular memory have been proposed. In this paper, the nanomaterials used for these four emerging types of memories and the physical principles behind the writing and reading methods in each case are discussed, along with their respective merits and drawbacks when compared with conventional nonvolatile memories. The potential applications of each technology are also briefly assessed.

Gd-substituted bismuth titanate film capacitors having ferroelectric reliability and large non-volatile charges

Fatigue-free Gd-modified bismuth titanate (Bi 3.15Gd 0.85Ti 3O 12; BGdT) film capacitors having stable charge-retaining characteristics were grown on Pt/TiO 2/SiO 2/Si(1 0 0) substrates using the method of metal-organic sol decomposition. The BGdT film capacitor with a top Pt electrode showed significantly improved values of the remanent polarization (2 P r) and the non-volatile charge as compared to those of the Bi 4− x La x Ti 3O 12 (BLT; x=0.75) film capacitor, currently renowned as a promising candidate for non-volatile memories. The saturated 2 P r value of the BGdT capacitor was 75 μC/cm 2 while it remained essentially constant up to 4.5×10 10 read/write switching cycles at a frequency of 1 MHz. In addition to these, the capacitor demonstrated excellent charge-retention characteristics with its sensing margin of 52 μC/cm 2 and a strong resistance against the imprinting failure.

Lead-Free Layered Perovskite Film Capacitor for Ferroelectric Random Access Memory

Lead-free Bi 3.15 Sm 0.85 Ti 3 O 12 (BSmT) thin films were grown on Pt/TiO 2 /SiO 2 /Si(100) substrates using the method of metal-organic sol decomposition. The BSmT film capacitor with a top Pt electrode showed significantly improved values of the remanent polarization (2P r ) and the nonvolatile charge as compared to those of the Bi 4-x La x Ti 3 O 12 (BLT; x=0.75) film capacitor, recently known as the most promising candidate for nonvolatile memories. 2P r value of the BSmT capacitor was 52 μC/cm 2 at an applied voltage of 12 V while the net nonvolatile switching charge was as high as 20 μC/cm 2 and remained essentially constant up to 4.5×10 10 read/write switching cycles at a frequency of 1 MHz. In addition to these, the capacitor demonstrated excellent charge-retention characteristics with its sensing margin of 17 μC/cm 2 and a strong resistance against the imprinting failure.

Ferroelectric properties and crystal structure of praseodymium-modified bismuth titanate

Fatigue-free and highly c-axis oriented Bi4−xPrxTi3O12 (BPrT, x=0.85) thin films were grown on Pt/TiO2/SiO2/Si (100) substrates using the method of metalorganic sol decomposition. The BPrT film capacitor with a top Pt electrode showed significantly improved values of the remanent polarization (Pr) and the nonvolatile charge as compared to those of the Bi4−xLaxTi3O12 (x=0.75) film capacitor, recently known as a promising candidate for nonvolatile memories. The 2Pr value of the BPrT capacitor was 40 μC/cm2 at an applied voltage of 10 V while the net nonvolatile charge was as high as 20 μC/cm2 and remained essentially constant up to 4.5×1010 read/write switching cycles at a frequency of 1 MHz. In addition to these, the capacitor showed excellent charge-retention characteristics with its sensing margin of 16 μC/cm2 and a strong resistance against the imprinting failure.

Fatigue-free samarium-modified bismuth titanate (Bi4−xSmxTi3O12) film capacitors having large spontaneous polarizations

Fatigue-free and highly c-axis oriented Bi3.15Sm0.85Ti3O12 (BSmT) thin films were grown on Pt/TiO2/SiO2/Si(100) substrates using the method of metalorganic sol decomposition. The BSmT film capacitor with a top Pt electrode showed significantly improved values of the remanent polarization (2Pr) and the nonvolatile charge as compared to those of the Bi4−xLaxTi3O12 (x=0.75) film capacitor, recently known as the most promising candidate for nonvolatile memories. The 2Pr value of the BSmT capacitor was 49 μC/cm2 at an applied voltage of 10 V while the net nonvolatile switching charge was as high as 20 μC/cm2 and remained essentially constant up to 4.5×1010 read/write switching cycles at a frequency of 1 MHz. In addition to these, the capacitor demonstrated excellent charge–retention characteristics with its sensing margin of 17 μC/cm2 and a strong resistance against the imprinting failure.

MOCVD Process of Ferroelectric Lead Germanate Thin Films and Bottom Electrode Effects

AbstractPb5Ge3O11 is a new promising candidate for nonvolatile memory such as one-transistor memory applications because of its moderate polarization and relative low dielectric constant. But this material is a low symmetry ferroelectric material and it is said the spontaneous polarization exists only along the c-axis. Furthermore, in the PbO- GeO 2 system, the stability range for the Pb5Ge3O11 phase is very limited and that even a relatively small deviation in composition or in growth temperature could lead to the formation of other lead germanate compounds. Many methods have been used to grow Pb5Ge3O11 film. This is the first time the Pb5Ge3O1 thin film has been successfully grown by advanced MOCVD technique with a liquid delivery system. This paper will discuss the MOCVD process for growing Pb5Ge3O11 thin films on different electrodes (Pt, Ir and Pt/Ir). The properties of these electrodes, the microstructures of the Pb5Ge3O11 thin films on these electrodes and their ferroelectric properties will be discussed. The advanced two-step deposition technique has been used to improve the uniformity of the Pb5Ge3O11 film. Pure c-axis oriented Pb5Ge3O11 thin film and polycrystalline Pb5Ge3O11 thin film with preferred c-axis orientation and some Pb3GeO5 secondary phase have been successfully grown on these electrodes with good ferroelectric properties.