Nonvolatile Applications Research Articles

Thermally tuned on-chip optical memory

Phase-change materials, known as Chalcogenide alloys, are a promising alternative to traditional random-access memory. They possess characteristics that are particularly beneficial for non-volatile storage applications. The features of the Phase change material Ge-Sb-Te alloy (GST) used for substrate-integrated optical memory include scaling, quick switching times, minimal switching energy, and exceptional thermal stability. The material has two tuneable states, amorphous and crystalline, with the amorphous layer for loading data optically. In contrast, the crystalline state holds the data longer without significant loss. The study designed a classic thermally tuned optical memory on a silicon substrate. It demonstrated a dependency of lattice structure on external voltage and revealed a large storage capacity for information in the form of an optical signal. The heat transport simulation utilized the Heat Transport (HEAT) solver of the Finite Element Eigenmode (FEEM) solver. At the same time, the optical response analysis involved the Finite Difference Time Domain (FDTD) solver of Lumerical. The proposed structure exhibits a memory-switching phenomenon when a temperature shift of about 60 °C from room temperature is induced by a change in the external voltage of 147 mV. These findings have substantial implications for non-volatile storage memory development, providing a potential solution for high-capacity, low-energy data storage.

Advances on MXene-Based Memristors for Neuromorphic Computing: A Review on Synthesis, Mechanisms, and Future Directions.

Neuromorphic computing seeks to replicate the capabilities of parallel processing, progressive learning, and inference while retaining low power consumption by drawing inspiration from the human brain. By further overcoming the constraints imposed by the traditional von Neumann architecture, this innovative approach has the potential to revolutionize modern computing systems. Memristors have emerged as a solution to implement neuromorphic computing in hardware, with research based on developing functional materials for resistive switching performance enhancement. Recently, two-dimensional MXenes, a family of transition metal carbides, nitrides, and carbonitrides, have begun to be integrated into these devices to achieve synaptic emulation. MXene-based memristors have already demonstrated diverse neuromorphic characteristics while enhancing the stability and reducing power consumption. The possibility of changing the physicochemical properties through modifications of the surface terminations, bandgap, interlayer spacing, and oxidation for each existing MXene makes them very promising. Here, recent advancements in MXene synthesis, device fabrication, and characterization of MXene-based neuromorphic artificial synapses are discussed. Then, we focus on understanding the resistive switching mechanisms and how they connect with theoretical and experimental data, along with the innovations made during the fabrication process. Additionally, we provide an in-depth review of the neuromorphic performance, making a connection with the resistive switching mechanism, along with a compendium of each relevant performance factor for nonvolatile and volatile applications. Finally, we state the remaining challenges in MXene-based devices for artificial synapses and the next steps that could be taken for future development.

Zeolitic-imidazolate framework (ZIF-67) based bipolar memristor for nonvolatile ReRAM application

In the realm of next-generation nonvolatile memories, there is a strong demand for resistive random access memory (ReRAM) devices that are affordable, stable, and simple to produce. Metal–organic frameworks (MOFs) have garnered extensive research attention across diverse fields due to their remarkable chemical adaptability, stability, and exceptional structural variability. In this work, a ReRAM device has been fabricated that incorporates a solution-processed thin film of zeolitic imidazolate framework (ZIF-67), a material of MOFs class, as the resistive switching (RS) layer. This Al/ZIF-67/ITO structured device possesses a consistent RS behavior with a high on/off resistance ratio of ∼104. The retention of low resistance state and high resistance state have been shown up to 104 s. Furthermore, the device also shows a consistent switching for 500 cycles of pulse switching signals of +6 V/−1.8 V (set/reset). Based on the impedance spectroscopy analysis, a filamentary switching mechanism has been established for the RS behavior of the device. The robust and enduring performance, coupled with the substantial on/off resistance ratio and high retention of the states, makes this device a promising candidate for nonvolatile ReRAM applications.

Semiconducting Carbon Nanotubes with Light‐Driven Gating Behaviors in Phototransistor Memory Utilizing an N‐Type Conjugated Polymer Sorting

Semiconducting single‐walled carbon nanotubes (s‐SWNTs) have arisen a growing interest in field‐effect transistors (FETs) due to their advantages, such as lower fabrication temperature, flexibility, and solution processing applicability, compared to traditional silicon‐based FETs. In this study, diversifying the functionality of s‐SWNT‐based FETs is focused on, particularly emphasizing their use in nonvolatile photomemory applications. By selectively wrapping s‐SWNT with n‐type conjugated polymers (CPs), electron‐trapping and photoresponsive capabilities are endowed in the device. After optimizing the structure and aggregating behavior of n‐type CPs, a favorable supramolecular network comprising s‐SWNT and CPs is formed and applied in phototransistor memory. Accordingly, the device exhibits a high memory ratio and window of 105 and 75.7 V, representing its remarkable charge‐storage capabilities. In addition, the device demonstrates decent long‐term stability over 104 s and multilevel memory behavior driven by the varied gate bias or accumulated light‐gating periods. The proposed memory mechanism involves electrical writing and photoerasing processes with the existence of n‐type CPs on s‐SWNT, revealing the underlying principles of charge transfer between their heterojunction interfaces. Herein, this research contributes to developing advanced phototransistor memory, offering a promising avenue for future electronic applications.

An Electrically Resistive Switching Nonvolatile Memory System Based on PEDOT:PSS

The current density–voltage (J–V) curves of poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) are examined at room temperature, which reveal nonlinear behavior and hysteresis loop‐like patterns. Herein, the potential of the material for nonvolatile applications requiring resistive random access memory (ReRAM) is demonstrated. The resistive switching effect is explained using a Schottky barrier and the space‐charge‐limited conduction model. Additionally, dielectric and electrical characteristics are analyzed in the frequency range from 20 Hz to 10 MHz. The impedance, electric modulus, dielectric loss tangent, and AC conductivity are explored through On‐ and Off‐states of resistive switching. A modified Debye function and Jonscher's power law are used for data fitting. The resistive switching memory behavior of PEDOT:PSS are studied using J–V and chemical impedance spectroscopy (CIS) measurements. The Cu/PEDOT:PSS/Cu sandwich‐type devices show bipolar switching behavior, and an equivalent circuit model based on the impedance spectra is proposed to describe the CIS response in both states. The switching process is attributed to the electrical‐field‐induced effect of dipolar reorientation on the electrical conductivity of PEDOT:PSS at grain boundaries.

Multiscale simulations of growth-dominated Sb2Te phase-change material for non-volatile photonic applications

Chalcogenide phase-change materials (PCMs) are widely applied in electronic and photonic applications, such as non-volatile memory and neuro-inspired computing. Doped Sb2Te alloys are now gaining increasing attention for on-chip photonic applications, due to their growth-driven crystallization features. However, it remains unknown whether Sb2Te also forms a metastable crystalline phase upon nanoseconds crystallization in devices, similar to the case of nucleation-driven Ge-Sb-Te alloys. Here, we carry out ab initio simulations to understand the changes in optical properties of amorphous Sb2Te upon crystallization and post annealing. During the continuous transformation process, changes in the dielectric function are highly wavelength-dependent from the visible-light range towards the telecommunication band. Our finite-difference time-domain simulations based on the ab initio input reveal key differences in device output for color display and photonic memory applications upon tellurium ordering. Our work serves as an example of how multiscale simulations of materials can guide practical photonic phase-change applications.

Non-Volatile Logic Design Considerations for Energy Efficient Tolerant Variation

Systems design for the non-volatile application must work on less energy or power. The spin-transfer torque-magnetic tunnel junction (STT-MTJ) devices added to the flip-flops which are regarded as non-volatile storage devices. Those are addresses to save the energy of that system stated by the nonvolatile logic. The changes during the production of STT-MTJ and CMOS transistors decrease the yield, which leads to overdesign as well as more energy consumption. The total processes of driver circuitry design for the tradeoffs for backup and restore performance. A new method called the novel method is introduced for flawless energy drivers for given results. The design for the backup time determination and to reduce the energy wastage are mentioned. To get an efficient output of 98% this approach needs to dissipate 5 times more energy than initially required. This method can dissipate the energy up to 26%. It also contains the nonvolatile flip-flop (NVFF) which has energy consumption more when it is used in the functional blocks.

Retention Secured Nonlinear and Self-Rectifying Analog Charge Trap Memristor for Energy-Efficient Neuromorphic Hardware.

A memristive crossbar array (MCA) is an ideal platform for emerging memory and neuromorphic hardware due to its high bitwise density capability. A charge trap memristor (CTM) is an attractive candidate for the memristor cell of the MCA, because the embodied rectifying characteristic frees it from the sneak current issue. Although the potential of the CTM devices has been suggested, their practical viability needs to be further proved. Here, a Pt/Ta2 O5 /Nb2 O5- x /Al2 O3- y /Ti CTM stack exhibiting high retention and array-level uniformity is proposed, allowing a highly reliable selector-less MCA. It shows high self-rectifying and nonlinear current-voltage characteristics below 1µA of programming current with a continuous analog switching behavior. Also, its retention is longer than 105 s at 150°C, suggesting the device is highly stable for non-volatile analog applications. A plausible band diagram model is proposed based on the electronic spectroscopy results and conduction mechanism analysis. The self-rectifying and nonlinear characteristics allow reducing the on-chip training energy consumption by 71% for the MNIST dataset training task with an optimized programming scheme.

Spray deposited zinc tungstate thin film for non-volatile memory application

The Ag/ZWO/FTO resistive switching (RS) device was fabricated using the chemical spray pyrolysis method. The crystalline phase, morphology, and atomic composition were characterized effectively through different physicochemical techniques. The fabricated device exhibits bipolar resistive switching behavior with switching voltage below ± 2 V, an HRS/LRS ratio of >10, good endurance of 150 cycles, and retention over 104 s. The β and x63% factors of Weibull distribution suggest the good operational uniformity of the device. The results suggest that ZnWO4-based RRAM can be a potential candidate for non-volatile applications.

A novel tunable two dimensional ferroelectric material SbAs(CH2OCH3)2 with topological and valley polarized properties

In recent years, two-dimensional (2D) materials that have ferroelectric, valley polarization as well as topological order is a major focal topic of nanomaterials research. We studied the monolayer (ML) SbAs(CH2OCH3)2 based on first-principles calculations. The adsorption energy results and phono spectra results show that the structure are stable. The in-plane ferroelectricity can be adjusted by rotating functional groups, CH2OCH3-. Chosen a suitable substrate (for example, diagonal boron nitride), an in-plane electric field can be supplied to control the ferroelectric switch. The topological order can be adjusted by stress stretching. After 6% tensile stress, it represents the quantum spin Hall effect. Considering energy band of spin polarization, it shows that the spin-splitting Dirac states at the K and -K valleys have opposite spin moments. The discovery of coexisting ferroelectric, valley polarization and topology in functional 2D materials opens a way for non-volatile memory storage and spintronics applications.

Boolean Logic Function Realized by Phase-Change Blade Type Random Access Memory

Phase-change random access memory (PCRAM) has been considered as one of the most promising candidates for future nonvolatile storage applications. Recent advent of the blade-type PCRAM further shed a light on its commercial prospect due to its much smaller energy consumption than the conventional Lance-type PCRAM. However, the logic computing capability of the blade-type PCRAM remains vague up-to-date. To deeply study its function versatility, we developed a comprehensive three-dimensional model based on the classical nucleation-growth theory to mimic the crystallization and amorphization processes of the blade-type PCRAM device. According to this well-established model, the device resistance after phase transformation was calculated with respect to different crystallization and amorphization pulses. By pertaining high and low pulse magnitudes and device resistances to appropriate input and output logic levels, a variety of Boolean logic functions, including “ NAND,” NOR,” AND,” OR,” and “ NOT,” were successfully achieved on one single PCRAM cell. The feasibility of simultaneously realizing storage and logic computing functions on the designed PCRAM device was therefore demonstrated. Such intriguing merit, combined with its superb storage traits, makes the proposed blade-type PCRAM cell suitable for current in-memory computing and neuromorphic circuits as the key components of future digital computers operated on non-von Neumann mode.

Tailoring the Structural and Optical Properties of Germanium Telluride Phase-Change Materials by Indium Incorporation.

Chalcogenide phase-change materials (PCMs) based random access memory (PCRAM) enter the global memory market as storage-class memory (SCM), holding great promise for future neuro-inspired computing and non-volatile photonic applications. The thermal stability of the amorphous phase of PCMs is a demanding property requiring further improvement. In this work, we focus on indium, an alloying ingredient extensively exploited in PCMs. Starting from the prototype GeTe alloy, we incorporated indium to form three typical compositions along the InTe-GeTe tie line: InGe3Te4, InGeTe2 and In3GeTe4. The evolution of structural details, and the optical properties of the three In-Ge-Te alloys in amorphous and crystalline form, was thoroughly analyzed via ab initio calculations. This study proposes a chemical composition possessing both improved thermal stability and sizable optical contrast for PCM-based non-volatile photonic applications.

Quantum Spin-Valley Hall Kink States: From Concept to Materials Design.

We propose a general and tunable platform to realize high-density arrays of quantum spin-valley Hall kink (QSVHK) states with spin-valley-momentum locking based on a two-dimensional hexagonal topological insulator. Through the analysis of Berry curvature and topological charge, the QSVHK states are found to be topologically protected by the valley-inversion and time-reversal symmetries. Remarkably, the conductance of QSVHK states remains quantized against both nonmagnetic short- and long-range and magnetic long-range disorder, verified by the Green-function calculations. Based on first-principles results and our fabricated samples, we show that QSVHK states, protected with a gap up to 287meV, can be realized in bismuthene by alloy engineering, surface functionalization, or electric field, supporting nonvolatile applications of spin-valley filters, valves, and waveguides even at room temperature.

Ferroelectric properties and structural characterisation of lead titanate thin films prepared by polymeric precursor method

ABSTRACT Ferroelectric thin films of lead titanate (PbTiO3) have been prepared on SiO2/Si(100) and LaNiO3(110)/SiO2/Si(100) substrates by a sol-gel spin coating method, using lead acetate trihydrate, and titanium isopropoxide as precursors along with 2-methoxyethanol as a solvent and acetic acid as catalyst. X-Ray Diffraction patterns show the formation of the perovskites structure of PbTiO3 thin films deposited on SiO2/Si substrates at 650°C. A SEM micrograph of the film cross-section is used to measure the thickness, which gave a value of 0.65 μm. Analysis of its ferroelectric hysteresis measurements gave values of remanent polarisation of Pr = 18.37 μC/cm2 and a coercive field of Ec = 121.53 kV/cm which are essential for non-volatile memories application. The current-voltage characteristic was found to be space charge conduction in the high voltage region and its piezoelectric coefficient d33, as measured by a Laser Scanning Doppler vibrometry, showed small linear decreases from 75 pm/V at 45 kV/cm to 67 pm/V at the highest applied field (330 kV/cm).

Performances and Stability Analysis of a Novel 8T1R Non-Volatile SRAM (NVSRAM) versus Variability

Static Random-Access Memories (SRAMs) are an integral part of the chip industry, occupying a noticeable share of the memory market due to their high performance and compatibility with CMOS technology. Traditional SRAMs do not have the capacity to retain data after power-off, preventing their use in non-volatile applications. This paper presents a novel Non-Volatile SRAM (NVSRAM) device based on Resistive RAM (RRAM) technology. A comparison between SRAM and the proposed NVSRAM performances is proposed at both cell and memory array level. The comparison covers several metrics such as energy consumption, area and static noise margin (SNM). Moreover, this work proposes a deep analysis of the impact of RRAM variability as well as the CMOS subsystem variability on the NVSRAM performances. The proposed structure demonstrates robust NVSRAM performances in terms of stability and reliability despite RRAM variability.

First-principles study of structural and opto-electronic characteristics of ultra-thin amorphous carbon films

Most amorphous carbon (a-C) applications require films with ultra-thin thicknesses; however, the electronic structure and opto-electronic characteristics of such films remain unclear so far. To address this issue, we developed a theoretical model based on the density functional theory and molecular dynamic simulations, in order to calculate the electronic structure and opto-electronic characteristics of the ultra-thin a-C films at different densities and temperatures. Temperature was found to have a weak influence over the resulting electronic structure and opto-electronic characteristics, whereas density had a significant influence on these aspects. The volume fraction of sp3 bonding increased with density, whereas that of sp2 bonding initially increased, reached a peak value of 2.52 g/cm3, and then decreased rapidly. Moreover, the extinction coefficients of the ultra-thin a-C films were found to be density-sensitive in the long-wavelength regime. This implies that switching the volume ratio of sp2 to sp3 bonding can effectively alter the transmittances of ultra-thin a-C films, and this can serve as a novel approach toward photonic memory applications. Nevertheless, the electrical resistivity of the ultra-thin a-C films appeared independent of temperature. This implicitly indicates that the electrical switching behavior of a-C films previously utilized for non-volatile storage applications is likely due to an electrically induced effect and not a purely thermal consequence.

Unraveling the optical contrast in Sb2Te and AgInSbTe phase-change materials

Chalcogenide phase-change materials (PCMs) show a significant contrast in optical reflectivity and electrical resistivity upon crystallization from the amorphous phase and are leading candidates for non-volatile photonic and electronic applications. In addition to the flagship Ge2Sb2Te5 phase-change alloy, doped Sb2Te alloys, in particular AgInSbTe used in rewritable optical discs, have been widely investigated for decades, and nevertheless the theoretical insights on the optical properties of this important family of PCMs are scarce. Here, we carry out thorough ab initio simulations to gain an atomistic understanding of the optical properties of Sb2Te and AgInSbTe. We show that the large optical contrast between the amorphous and crystalline phase stems from the change in bond type in the parent compound Sb2Te. Ag and In impurities serve mostly the purpose of stabilization of the amorphous phase, and have marginal impact on the large variation in the dielectric function upon the phase transitions.

Nucleation, dielectric, and ferro electric studies of potassium succinate succinic acid (KSSA) crystals

The semi-organic material has strong optical non-linearity as well as thermal stability, both of organic and inorganic semiconductors. CNT and DFT are used to evaluate the nucleation kinetics of KSSA. With respect to varying temperatures, the kinetic parameters enthalpy, Gibbs free energy, real heat power, and entropy of activation are measured. the optimized geometric configuration of the compound is computed from DFT-B3LYP gradient calculations using the 6-31G (d,p) basis set, and its vibrational frequencies are noted. Vibrational parameters are assessed, and correlative equations pertaining to the variations in these thermodynamic properties are computed. The width of KSSA's Meta stable zone as a function of supersaturation is also calculated. Horowitz–Metzger, Coats–Redfern, and Piloyan–Novikova use thermo gravimetric methods to evaluate kinetic parameters. P-E hysteresis loop analysis identifies the crystal's Ferro electric origin. The loss tangent and dielectric constant range is registered between 35 K and 155 K, making it suitable for optoelectronic and non-volatile application.

Multi‐Level Electro‐Thermal Switching of Optical Phase‐Change Materials Using Graphene

Reconfigurable photonic systems featuring minimal power consumption are crucial for integrated optical devices in real‐world technology. Current active devices available in foundries, however, use volatile methods to modulate light, requiring a constant supply of power and significant form factors. Essential aspects to overcome these issues are the development of nonvolatile optical reconfiguration techniques which are compatible with on‐chip integration with different photonic platforms and do not disrupt their optical performances. Herein, a solution is demonstrated using an optoelectronic framework for nonvolatile tunable photonics that uses undoped‐graphene microheaters to thermally and reversibly switch the optical phase‐change material Ge2Sb2Se4Te1 (GSST). An in situ Raman spectroscopy method is utilized to demonstrate, in real‐time, reversible switching between four different levels of crystallinity. Moreover, a 3D computational model is developed to precisely interpret the switching characteristics, and to quantify the impact of current saturation on power dissipation, thermal diffusion, and switching speed. This model is used to inform the design of nonvolatile active photonic devices; namely, broadband Si3N4 integrated photonic circuits with small form‐factor modulators and reconfigurable metasurfaces displaying 2π phase coverage through neural‐network‐designed GSST meta‐atoms. This framework will enable scalable, low‐loss nonvolatile applications across a diverse range of photonics platforms.

Three distinct optical-switching states in phase-change materials containing impurities: From physical origin to material design

Ge2Sb2Te5 is the most widely utilized chalcogenide phase-change material for non-volatile photonic applications, which undergoes amorphous-cubic and cubic-hexagonal phase transition under external excitations. However, the cubic-hexagonal optical contrast is negligible, only the amorphous-cubic phase transition of Ge2Sb2Te5 is available. This limits the optical switching states of traditional active displays and absorbers to two. We find that increasing structural disorder difference of cubic-hexagonal can increase optical contrast close to the level of amorphous-cubic. Therefore, an amorphous-cubic-hexagonal phase transition with high optical contrast is realized. Using this phase transition, we have developed display and absorber with three distinct switching states, improving the switching performance by 50 %. Through the combination of first-principle calculations and experiments, we reveal that the key to increasing structural disorder difference of amorphous, cubic and hexagonal phases is to introduce small interstitial impurities (like N) in Ge2Sb2Te5, rather than large substitutional impurities (like Ag) previously thought. This is explained by the formation energy and lattice distortion. Based on the impurity atomic radius, interstitial site radius and formation energy, C and B are also potential suitable impurities. In addition, introducing interstitial impurities into phase-change materials with van der Waals gaps in stable phase such as GeSb4Te7, GeSb2Te4, Ge3Sb2Te6, Sb2Te3 will produce high optical contrast amorphous-metastable-stable phase transition. This research not only reveals the important role of interstitial impurities in increasing the optical contrast between metastable-stable phases, but also proposes varieties of candidate matrices and impurities. This provides new phase-change materials and design methods for non-volatile optical devices with multi-switching states.