Multilevel Switching Research Articles

Effect of AlN layer on the resistive switching properties of TiO2 based ReRAM memory devices

The present study reports the resistive switching behaviour in Titanium Dioxide (TiO2) material, with possible implementations in non volatile memory device. The Cu/TiO2/Pt memory device exhibit uniform and stable bipolar resistive switching behaviour. The current-voltage (I-V) analysis shows two discrete resistance states, the High Resistance State (HRS) and the Low Resistance State (LRS). The effect of an additional AlN layer in the resistive memory cell is also investigated. The Cu/TiO2/AlN/Pt device shows a multilevel (tri-state) resistive switching. Multilevel switching is facilitated by ionic and metallic filament formation, and the nature of the formed filaments is confirmed by performing a resistance vs. temperature measurement. The bilayer device shows improved reliability over the single layer device. The formation of high thermal conductive interfacial oxy-nitride (AlON) layer is the main reasons for the enhancement of resistive switching properties in Cu/TiO2/AlN/Pt cell. The performance of device was measured in terms of endurance and retention, which exhibits good endurance over 105 cycles and long retention time of 105 s at 125 °C. The above result suggests the feasibility of Cu/TiO2/AlN/Pt devices for multilevel non volatile ReRAM application.

Multi-level storage and ultra-high speed of superlattice-like Ge50Te50/Ge8Sb92 thin film for phase-change memory application

Superlattice-like Ge50Te50/Ge8Sb92 (SLL GT/GS) thin film was systematically investigated for multi-level storage and ultra-fast switching phase-change memory application. In situ resistance measurement indicates that SLL GT/GS thin film exhibits two distinct resistance steps with elevated temperature. The thermal stability of the amorphous state and intermediate state were evaluated with the Kissinger and Arrhenius plots. The phase-structure evolution revealed that the amorphous SLL GT/GS thin film crystallized into rhombohedral Sb phase first, then the rhombohedral GeTe phase. The microstructure, layered structure, and interface stability of SLL GT/GS thin film was confirmed by using transmission electron microscopy. The transition speed of crystallization and amorphization was measured by the picosecond laser pump-probe system. The volume variation during the crystallization was obtained from x-ray reflectivity. Phase-change memory (PCM) cells based on SLL GT/GS thin film were fabricated to verify the multi-level switching under an electrical pulse as short as 30 ns. These results illustrate that the SLL GT/GS thin film has great potentiality in high-density and high-speed PCM applications.

Dual functionality of threshold and multilevel resistive switching characteristics in nanoscale HfO2-based RRAM devices for artificial neuron and synapse elements

We demonstrate the dependency of dual functionality on the operating current with threshold and multilevel switching behaviors in HfO2-based resistive memory (RRAM) devices. These devices can be used to produce electronic neurons and synapses for neuromorphic computing applications. The control of the formation and rupture of a conductive filament (CF) driven by the movement of oxygen vacancies (V0) in a high-current (100μA) operated RRAM acting as synapse enables multilevel conductance states to be achieved. On the other hand, operation of the device in the low-current regime (≤10μA) leads to a transition from memory to threshold switching, which is activated only by applying voltage. This behavior is described by a weak CF composed of a few V0 created by using a Poole-Frenkel based analytical model. Thus, threshold switching in RRAM operated at a low current plays a role in generating output spikes as neurons when the accumulated inputs exceed the critical value.

(Invited) Volume Resistive Switching in Metallic Perovskite Oxides Driven By the Metal-Insulator Transition

In recent years, huge efforts have been made in the research community to overcome the limitations in Flash and RAM memories, and consequently, new emerging technologies are competing in the race towards a new paradigm in information storage. In particular, nonvolatile memories based on the Resistive Switching (RS) effect, where two (or more) reversible resistance states can be induced upon application of an electric field, has emerged with excellent performance. This phenomenon has been observed in many oxide systems, in particular in perovskite oxides, which are materials showing outstanding properties giving rise to exotic physical phenomena due to the strong electronic correlation, such as metal-insulator transitions (MIT). This is the case of the metallic perovskite La1-xSrxMnO3 (LSMO), RENiO3 (RE=rare earth) and YBa2Cu3O7(YBCO) family compounds, which are able to display Volume RS effects induced by the MIT and therefore, small changes in carrier concentration can induce huge resistance changes. The mechanism underlying this phenomenon is still unclear although oxygen vacancies mobility plays an important role in the mechanism underneath this phenomenon. In this presentation we will discuss our studies on bipolar resistive switching of the mentioned perovskite oxides. Switching characteristics have been evaluated by C-SPM and I(V) curves with metal electrodes. Scanning tunneling microscopy and spectroscopy (STM/S), transport and resistive measurements were performed to gain insight into the local density of states of the material for different resistance states. Large resistance rations (102-104), over 100 cycles experiments, multilevel switching and switching dynamics have been evaluated. In addition, we have confirmed the strong influence of different atmospheres on the resistive switching properties of bare LSMO thin films. Remarkably, this influence disappears when the LSMO layer is capped with a CeO2thin layer, which acts as an oxygen reservoir, making this bilayer a proper material choice for encapsulation. A 3-terminal proof of principle device based on Ag/CeO2/LSMO/CeO2/Ag will be presented. We believe that these results contribute to a better understanding of the physical mechanism behind the robust RS effect of these metallic perovskite oxides films.

Optimized Design of GaN Switched-Capacitor-Converter-Based Envelope Tracker for Satellite Application

Envelope tracking and envelope elimination and restoration (EER) are two important techniques used to improve efficiency of power amplifiers. They employ a power supply called envelope tracker to provide a dynamic voltage that tracks the envelope of the transmitted signal. This paper presents an implementation of the envelope tracker based on a multilevel converter in series with a linear regulator for satellite application. In order to have highly efficient and light solution, a switched-capacitor-converter-based multilevel converter is proposed. For the multilevel converter, duty cycle, switching frequency, capacitors, and inductors are optimized based upon the precise power loss model. Two prototypes (GaN-based and Si-based) are fabricated, aiming for a 5-MHz radio frequency envelope signal tracking with a maximum output power of 40 W. The experimental results verify the effectiveness of the proposed configuration with 96.2% efficiency of the multilevel converter and 75% of the envelope tracker. Besides, comparison experiments are performed between the Si-based and GaN-based transistors, which show that the GaN transistors offer better efficiency and, therefore, are preferred for satellite application.

A computational study of hafnia-based ferroelectric memories: from ab initio via physical modeling to circuit models of ferroelectric device

The discovery of ferroelectric properties of binary oxides revitalized the interest in ferroelectrics and bridged the scaling gap between the state-of-the-art semiconductor technology and ferroelectric memories. However, before hitting the markets, the origin of ferroelectricity and in-depth studies of device characteristics are needed. Establishing a correlation between the performance of the device and underlying physical mechanisms is the first step toward understanding the device and engineering guidelines for a novel, superior device. Therefore, in this paper a holistic modeling approaches which lead to a better understanding of ferroelectric memories based on hafnium and zirconium oxide is addressed. Starting from describing the stabilization of the ferroelectric phase within the binary oxides via physical modeling the physical mechanisms of the ferroelectric devices are reviewed. Besides, limitations and modeling of the multilevel operation and switching kinetics of ultimately scaled devices as well as the necessity for Landau-Khalatnikov approach are discussed. Furthermore, a device-level model of ferroelectric memory devices that can be used to study the array implementation and their operational schemes are addressed. Finally, a circuit model of the ferroelectric memory device is presented and potential further applications of ferroelectric devices are outlined.

Multi-level switching in TiOxFy film with nanoparticles

A reliable bipolar resistive switching device was achieved with multi-level switching behavior in fluorine-doped titanium oxide (TiOxFy) film. Different resistance states can be precisely controlled by different pulse voltages, which reveals the device’s high potential in neuromorphic research. The characteristics of I–V curves in each resistance state were analyzed. Nanoparticles were observed in the TiOxFy film by HR-TEM. The underlying physical mechanisms during resistance switching are discussed and a model of a meshy conducting path is proposed.

Enhanced switching stability in Ta2O5 resistive RAM by fluorine doping

The effect of fluorine doping on the switching stability of Ta2O5 resistive random access memory devices is investigated. It shows that the dopant serves to increase the memory window and improve the stability of the resistive states due to the neutralization of oxygen vacancies. The ability to alter the current in the low resistance state with set current compliance coupled with large memory window makes multilevel cell switching more favorable. The devices have set and reset voltages of <1 V with improved stability due to the fluorine doping. Density functional modeling shows that the incorporation of fluorine dopant atoms at the two-fold O vacancy site in the oxide network removes the defect state in the mid bandgap, lowering the overall density of defects capable of forming conductive filaments. This reduces the probability of forming alternative conducting paths and hence improves the current stability in the low resistance states. The doped devices exhibit more stable resistive states in both dc and pulsed set and reset cycles. The retention failure time is estimated to be a minimum of 2 years for F-doped devices measured by temperature accelerated and stress voltage accelerated retention failure methods.

Multilevel Nonsuperconducting Fault Current Limiter: Analysis and Practical Feasibility

A new topology for nonsuperconducting fault current limiter (NSFCL) based on multilevel switching strategy is proposed in this paper. The proposed multilevel NSFCL circuit aims to control the high-level fault current by inserting different resistances in the circuit. Due to interrupting the fault current based on multilevel switching strategy which results in the decrement of switching numbers, power loss will be reduced and lifetime of the utility devices will be increased. The proposed NSFCL structure has the capability of limiting the fault current based on obtained quantities from current sensors. The proposed NSFCL topology which has no effect on voltage and load currents of the utility, ensures a high level system protection. The analysis of normal and fault conditions of the proposed multilevel NSFCL have been provided and experimental results are demonstrated to prove feasibility of the proposed structure.

Effect of top electrode material on resistive switching properties of WN based thin films for non volatile memory application

The influence of top electrode (TE) material on resistive switching properties of DC magnetron sputtered tungsten nitride (WN) thin film with TE (Ti, Al and Cu)/WN/Pt stack configuration has been investigated. Bipolar resistive switching behavior has been clearly found for all the three top electrodes. The memory cells with Ti and Al top electrodes exhibit two resistance states i.e. low resistance state (LRS) and high resistance state (HRS) switching which is caused by formation and rupture of ionic filaments. The formation of additional Cu filaments in WN layer are responsible for three resistance states (or say multilevel) switching for Cu top electrode. Temperature vs resistance measurement confirms the formation of ionic and metallic filaments. The WN thin film based ReRAMs show an excellent endurance over 105 cycles and non volatile long retention of 105 s along with ∼102 resistance ratio between HRS/LRS. This study suggests that the electrode engineering of WN based ReRAM devices have potential in nonvolatile and multilevel resistive switching memory.

Forecasting in Industrial Process Control: A Hidden Markov Model Approach

The forecasting of information in industrial process control can assist in predicting the occurrence of future events. This is useful when such events are undesired and may lead to costly shutdowns of an industrial process. Thus, the forecasting of information can assist in determining preparatory measures to be taken to mitigate or prevent the occurrence of undesired events. This paper addresses the forecasting of information in industrial process control using a hidden Markov model approach. It defines three types of information signals from data of industrial process control (namely, low-dynamic, fast-dynamic, and multi-level switching). The paper discusses the complete methodology for forecasting of the signals; namely, pre-processing of the data, detection of the explanatory variables, determining the order of the forecasting model, implementation of the forecasting model, and generation and validation of the forecasted information. In addition, the paper implements the proposed methodology for two different practical systems with adjusting the parameters of the prediction model to demonstrate its applicability.

Solution processed bilayer junction of silk fibroin and semiconductor quantum dots as multilevel memristor devices

Silkworm protein is integrated with semiconductor quantum dots (CdSe) to form bi-layer electrical memory devices. These films are individually spin coated on ITO surface to form ITO-silk fibroin:CdSe-metal junctions. These materials are characterized by DSC, TGA, FTIR, UV–visible absorbance, X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) and TEM to understand their physical properties. The charge transport is carried out on the above devices by cyclic voltage scan in the sequence zero-positive-zero-negative-zero to study their electrical memory switching behavior. Devices exhibit multilevel electrical switching both in the forward and in the reverse regions that are quite symmetric, the ON/OFF ratios are well resolved. These junctions posses a high repeatability with a good endurance and hence can be important interfaces for electrical information storage applications. The observed results of the above memristors are explained in terms of their energy band diagram, it is observed that in these interfaces monopolar charge transport of holes is the dominant mechanism that is further supported by multiple hole trapping centers both in CdSe and silk fibroin energy gaps.

Antiferromagnetic CuMnAs multi-level memory cell with microelectronic compatibility

Antiferromagnets offer a unique combination of properties including the radiation and magnetic field hardness, the absence of stray magnetic fields, and the spin-dynamics frequency scale in terahertz. Recent experiments have demonstrated that relativistic spin-orbit torques can provide the means for an efficient electric control of antiferromagnetic moments. Here we show that elementary-shape memory cells fabricated from a single-layer antiferromagnet CuMnAs deposited on a III–V or Si substrate have deterministic multi-level switching characteristics. They allow for counting and recording thousands of input pulses and responding to pulses of lengths downscaled to hundreds of picoseconds. To demonstrate the compatibility with common microelectronic circuitry, we implemented the antiferromagnetic bit cell in a standard printed circuit board managed and powered at ambient conditions by a computer via a USB interface. Our results open a path towards specialized embedded memory-logic applications and ultra-fast components based on antiferromagnets.

Realization of Multilevel States in Phase‐Change Thin Films by Fast Laser Pulse Irradiation

Multilevel storage techniques are promising for increasing storage density and for reducing energy consumption in the application of phase‐change materials based memory devices. However, accurately controlling the phase transitions as well as understanding the underlying switching mechanisms are still under investigation. In this study, nonvolatile optical multilevel switching in single‐layered GeTe phase‐change films prepared by laser ablation is demonstrated to be feasible and accurately controllable at a time scale of nanoseconds. For this purpose, an ns UV laser pump‐probe setup is adapted for the investigations. It is found that each laser pulse excitation (duration: 20 ns, wavelength: 248 nm) with a fluence of 26 mJ cm−2 induced a partial crystallization whereas complete crystallization is achieved by superposition of single pulses. In the reverse process, a single pulse excitation at a fluence of 112 mJ cm−2 leads to reamorphization of GeTe thin films. Regarding the switching times, complete crystallization and reamorphization are determined to be accomplished after ≈300 ns and ≈3 ns, respectively. Furthermore, it is shown that the local microstructure phase transformation processes are correlated to the observed reversible switching of the optical reflectivity, which are important for understanding the underlying mechanisms of the phase‐change‐materials‐based multilevel phase‐change memory.

Characteristics and transport mechanisms of triple switching regimes of TaOx memristor

While tantalum oxide (TaOx) memristors have shown superior multilevel switching performances and emerged as one of the leading candidates for analog memory and neuromorphic applications, its microscopic switching mechanisms at different resistance regimes remain obscure. In this work, electron transport mechanisms of TaOx memristors have been revealed by analyzing transport characteristics in three different resistance switching regimes. A quantum point contact model coupled with a first principles calculation has been validated to account for different conduction behaviors, which further sheds light on the evolution of the conducting filaments during switching processes. The high endurance observed in the low resistance switching regime is rationalized based on the observation of an enhanced instability and noise in the presence of a single conduction channel, i.e., quantum conductance G0=2e2/h.

The role of nitrogen doping in ALD Ta2O5 and its influence on multilevel cell switching in RRAM

The role of nitrogen doping on the stability and memory window of resistive state switching in N-doped Ta2O5 deposited by atomic layer deposition is elucidated. Nitrogen incorporation increases the stability of resistive memory states which is attributed to neutralization of electronic defect levels associated with oxygen vacancies. The density functional simulations with the screened exchange hybrid functional approximation show that the incorporation of nitrogen dopant atoms in the oxide network removes the O vacancy midgap defect states, thus nullifying excess defects and eliminating alternative conductive paths. By effectively reducing the density of vacancy-induced defect states through N doping, 3-bit multilevel cell switching is demonstrated, consisting of eight distinctive resistive memory states achieved by either controlling the set current compliance or the maximum voltage during reset. Nitrogen doping has a threefold effect: widening the switching memory window to accommodate the more intermediate states, improving the stability of states, and providing a gradual reset for multi-level cell switching during reset. The N-doped Ta2O5 devices have relatively small set and reset voltages (< 1 V) with reduced variability due to doping.

Manipulating resistive states in oxide based resistive memories through defective layers design

In this work, multilevel switching was achieved by a strategically designed alternative multi-layer structure with pure and Mn-doped SnO2.

Mathematical Derivation of Switching Angles of Multilevel Voltage Source Inverter based on Alternative Phase Opposition Disposition (APOD)

Modular structured multilevel inverter is very useful for electrical application especially in high voltage and high power applications. The main function of this multilevel inverter is to produce multilevel AC output voltage from several separate DC sources. This project is to derive a newmathematical formulation of multilevel voltage source inverter switching instants. The proposed method for this project is based on the sinusoidal natural sampling PWM (SPWM) by comparing several modified modulation signal with a triangular carrier signal. This resulting intersection points between this modulation and carrier signal become the switching instants of the PWM pulses. Derivation also based on Alternative Phase opposition disposition (APOD). A cascaded multilevel inverter is selected as a topology for this project due to major advantages compare with other topology. The derived formula is analyzed by using MATLAB simulation software. It is found that the results that use the derived formula are almost identical to simulation result.

Multilevel hysteresis loop engineered with ferroelectric nano-metamaterials

Polarization switching in ferroelectric and multiferroic materials provides a means of designing a broad range of multifunctional devices. To date, such applications have typically exploited 180° ferroelectric polarization switching, where the polarization vector can be alternated between parallel or anti-parallel to the external electric field, i.e., bi-states. However, the coupling between the direction of switching and the long-range order parameter puts a limit on the well-defined polarization state, hindering the use of polarization switching and the advancement of novel applications. Using the ferroelectric nano-metamaterial concept, we demonstrate here that successive polarization switching through multiple well-defined stable states with a long-range order parameter decoupled from the direction of switching can be achieved, and consequently, controllable and multilevel hysteresis loops are obtained under a unidirectional electric field. The equilibrium polarization configuration of stable states are tailored by designing a nanoporous ferroelectric structure based on the ferroelectric nano-metamaterial concept. The orientation of the homogeneous electric field is then controlled to achieve the desired polarization switching behavior and hence the hysteresis loop level, which can pass through all stable polarization states. In addition, a general rule for the nanostructure-state-level of switching is established, from which a high level of hysteresis loop can be designed and achieved. The present study provides a foundation for novel devices based on multilevel ferroelectric switching, such as multilevel data storage memory, multi-logic gates, and electromechanical devices with multilevel control.

Unipolar resistive switching characteristics of W/Si3N4/Si memory devices with doped silicon bottom electrodes

In this paper, we report the effects of two different types of dopants (n- and p-type semiconductors) on the unipolar resistive switching characteristics of a fully CMOS-compatible W/Si3N4/Si structure. The W/Si3N4/p+-Si device displayed a gradual reset transition, while the W/Si3N4/n+-Si device showed abrupt reset switching. The different reset switching behaviors of the devices can be explained by their different properties immediately before set switching, which were experimentally demonstrated by the statistical relationships between resistive switching parameters such as the set and reset voltages, the set and reset currents, and the resistances of the high resistance state (HRS) and low resistance state (LRS). The different reset switching properties of the devices were further evidenced by the dynamics of the conducting paths in the LRS immediately before reset switching. The W/Si3N4/p+-Si device showed superior resistive switching performance to the W/Si3N4/n+-Si one because of the gradual transition with low reset current. Finally, we demonstrated multi-level switching, which is attractive for application in high-density memory, by altering the reset voltage.