Low Reset Voltages Research Articles

Enhancing the Uniformity of a Memristor Using a Bilayer Dielectric Structure.

Resistive random access memory (RRAM) holds great promise for in-memory computing, which is considered the most promising strategy for solving the von Neumann bottleneck. However, there are still significant problems in its application due to the non-uniform performance of RRAM devices. In this work, a bilayer dielectric layer memristor was designed based on the difference in the Gibbs free energy of the oxide. We fabricated Au/Ta2O5/HfO2/Ta/Pt (S3) devices with excellent uniformity. Compared with Au/HfO2/Pt (S1) and Au/Ta2O5/Pt (S2) devices, the S3 device has a low reset voltage fluctuation of 2.44%, and the resistive coefficients of variation are 13.12% and 3.84% in HRS and LRS, respectively, over 200 cycles. Otherwise, the bilayer device has better linearity and more conductance states in multi-state regulation. At the same time, we analyze the physical mechanism of the bilayer device and provide a physical model of ion migration. This work provides a new idea for designing and fabricating resistive devices with stable performance.

Excellent performance Ruthenium doped Sb2Te3 alloy for phase change memory

The contradictory between thermal stability and operation speed has traditionally been the restriction to apply phase change memory (PCM) in the harsh condition. For providing a feasible solution, in this paper, Ru-modification is introduced to boost the data retention of Sb2Te3 without sacrificing the operation speed over the conventional Ge2Sb2Te5 compound. The both experimental and simulation results indicate that Ru locates at the grain boundary to inhibit the formation of large grains. Several performance of the device was enhanced, including: ultra-high 10-year data retention above 200 °C, lower Reset voltage about 1.9 V at 100 ns pulse width and a fast operation speed of 6 ns. Refined grains and smaller density change contributed to the reduced voltage and increased endurance. These findings demonstrated that PCM based Ru doped Sb2Te3 will be a potential application in high performance memories.

Switching-behavior improvement in HfO2/ZnO bilayer memory devices by tailoring of interfacial and microstructural characteristics

We investigated the effect of top contact interface and microstructural characteristics of the insulating layers on resistive switching behaviors by fabricating and characterizing the HfO2/ZnO bilayer heterostructures. Different thickness of ZnO underlying layer and different deposition temperatures of the upper HfO2 layer were designed to analyze the intrinsic contribution of the crystalline microstructure of the insulating bilayer. Pt and Ti top electrodes were used to demonstrate the extrinsic contribution of the interface configuration. It was observed that all devices show bipolar RS characteristics. Unlike the device composed of Pt/HfO2/ZnO/Pt that exhibit an abrupt switching, a gradually continuous switching in the reset process was identified in the device composed of Ti/HfO2/ZnO/Pt. Interfacial charge migration process/characteristic plays a key role in the RS process as well as its conduction mechanism. The RS performance of the former is significantly better than that of the latter, including much lower reset voltage, two orders of magnitude larger OFF/ON ratio and HRS resistance. In addition, as compared to the intrinsic contribution arising from the microstructure of the HfO2/ZnO bilayer to the RS performances and current transport mechanism, the extrinsic effect contributed from the electrode characteristics (and its interface) is dominant.

Reset-voltage-dependent precise tuning operation of TiOx/Al2O3 memristive crossbar array

In this Letter, we present reset-voltage-dependent precise tuning operation of TiOx/Al2O3-based memristive devices. For the high resistance state (HRS) with high reset voltage, abrupt set operations are observed with a large variation, while the HRS obtained by low reset voltage provides gradual and uniform switching behaviors. The improvement of gradual switching and the programming accuracy are analyzed regarding cycle-to-cycle as well as device-to-device variations. We believe that these results can be applied to operate TiOx/Al2O3-based memristors in areas requiring highly accurate tuning characteristics.

Effects of SiO2 interlayers on the phase change behavior in the multilayer Zn15Sb85/SiO2 materials

In this study, the multilayer Zn15Sb85/SiO2 materials were investigated for phase change memory application with the intention of improving thermal stability and switching speed. The Zn15Sb85/SiO2 materials exhibited a high crystallization temperature (Tc∼230 °C), good data retention (T10 ∼ 160 °C), small density change (∼3.3%), flat surface and rapid amorphization speed (∼1.34 ns). T-shaped phase change devices based on Zn15Sb85/SiO2 materials achieved low RESET voltage (VRESET∼1.3 V) in programming operation. The results showed that the multilayer Zn15Sb85/SiO2 material was a promising phase change material with high thermal ability, low power consumption and rapid switching speed.

SiC‐Doped Ge2Sb2Te5 Phase‐Change Material: A Candidate for High‐Density Embedded Memory Application

AbstractPhase‐change memory is one of the most attractive solutions for embedded applications, thanks to the low cost of integration with current manufacturing processes and the good scaling behavior. Despite intensive research efforts that are devoted to the modification of Ge2Sb2Te5 (GST) phase‐change material, the obtained improvements are still unsatisfactory for high‐performance applications. In this paper, SiC‐modification is first introduced to enhance the amorphous stability of GST without sacrificing its transition speed. Several enhancements can be demonstrated, including better thermal stability (10‐year data retention above 120 °C), lower RESET voltage (about 3.0 V at 20 ns pulse width), reduced volume change (smaller than 3.0%), and expected operation cycles (more than 1.0 × 105). According to the findings, the local bonding nature of Ge/Te atoms can be tuned by C/Si dopants through forming CGe and SiTe bonds. Apart from strengthening the atomic binding network for desirable data retention, the element interdiffusion is also effectively controlled and suppressed. Together with finer grains, smaller density change, and more uniform morphology, the modified structure transition finally contributes to the reduced switching voltage and long‐term endurance. Hence, it is envisaged that SiC‐doped GST with such advantages will give a competitive option for high‐density and high‐performance embedded memory.

Ultralow Power Consumption Flexible Biomemristors.

Low power consumption is the important requirement in memory devices for saving energy. In particular, improved energy efficiency is essential in implantable electronic devices for operation under a limited power supply. Here, we demonstrate the use of κ-carrageenan (κ-car) as the resistive switching layer to achieve memory that has low power consumption. A carboxymethyl (CM) group is introduced to the κ-car to increase its ionic conductivity. Ag was doped in CM:κ-car to improve the resistive switching properties of the devices. Memory devices based on Ag-doped CM:κ-car showed electroforming-free resistive switching. This device exhibited low reset voltage (∼0.05 V), fast switching speed (50 ns), and high on/off ratio (>103) under low compliance current (10-5 A). Its power consumption (∼0.35 μW) is much lower than those of the previously reported biomemristors. The resistive switching may be a result of an electrochemical redox process and Ag filament formation in the CM:κ-car under an electric field. This biopolymer memory can also be fabricated on flexible substrate. This study verifies the feasibility of using biopolymers for applications to future implantable and biocompatible nanoelectronics.

Resistive switching properties and physical mechanism of europium oxide thin films

A forming-free resistive switching effect was obtained in Pt/Eu2O3/Pt devices in which the Eu2O3 thin films were fabricated by a chemical solution deposition method. The devices show unipolar resistive switching with excellent switching parameters, such as high resistance ratio (107), stable resistance values (read at 0.2 V), low reset voltage, good endurance, and long retention time (up to 104 s). On the basis of the analysis of the current–voltage (I–V) curves and the resistance-temperature dependence, it can be concluded that the dominant conducting mechanisms were ohmic behavior and Schottky emission at low resistance state and high resistance state, respectively. The resistive switching behavior could be explained by the formation and rupture of conductive filament, which is related to the abundant oxygen vacancies generated in the deposition process. This work demonstrates the great potential opportunities of Eu2O3 thin film in resistive switching memory applications, which might possess distinguished properties.

Simultaneously High Thermal Stability and Low Power Based on Ti-Doped Ge2Sb2Te5 Thin Films

Compared with Ge2Sb2Te5, Ti-doped Ge2Sb2Te5 has better amorphous thermal stability (activation energy of 4.65 eV) and data retention (165°C for 10-year). The crystallization is restrained by the added Ti atoms with smaller grains. A faster SET/RESET switching speed (12 ns) and a lower RESET voltage (2.67 V) are obtained. The results indicate that Ti-doped Ge2Sb2Te5 material has the excellent potential for high-density application in phase change memory.

Effect of RESET Voltage on Distribution of SET Switching Time of Bipolar Resistive Switching in a Tantalum Oxide Thin Film

The distribution of SET switching time of bipolar switching tantalum oxide thin films is studied using pulse measurement techniques. SET switching times are measured by repeating SET and RESET operations in a single cell. It is found that the distribution measured with a high RESET voltage can be well described by a steep Weibull distribution with a shape parameter >1, but lowering the RESET voltage results in a broad distribution at high cumulative frequencies. Statistical analysis shows that this broadening of the distribution can be attributed to the variation of initial conditions for SET, whereas a steep Weibull distribution points to an aging process leading to SET under a voltage stress. It is also shown that although the power of the leakage current before SET determines the fastest limit of the SET switching speed, those SET in the broadened distributions are delayed due to gradual current increase prior to the SET. The waiting time for the start of the gradual current increase has a correlation with the power of the leakage current, showing that Joule heating effect is still a significant factor in the SET mechanism even if the SET time distribution is broadened due to the variation of initial states programmed by low RESET voltages.

Improvement in R off/R on ratio and reset current via combining compliance current with multilayer structure in tantalum oxide-based RRAM

Improvements in the R off/R on ratio and reset current are required prior to the practical application of RRAM. To achieve this improvement, tantalum oxide-based RRAM devices with multilayer structure (bi-layer and tri-layer) were fabricated and various compliance currents were adopted. The reset current of 40 µA was observed; the R off/R on ratio increased to more than 20 in the tri-layer structure device. Resistance changes in two types of devices under voltage pulses with different pulse width were also conducted. The tri-layer device exhibited lower reset voltage and higher R off/R on ratio than the bi-layer device under voltage pulses. X-ray photoelectron spectroscopy demonstrated the formation of Ta2O5 via plasma oxidation, and there was an oxygen gradient in the multilayer devices. The results demonstrated that the tri-layer structure with oxygen gradient was an effective method for achieving better device performance. Additionally, it is implied that reasonable control of the proportion of TaO2 and Ta2O5 and compliance current can improve device performance.

Phase transition behaviors of Ga 30Sb 70/Sb 80Te 20 nanocomposite multilayer films for application in phase change random access memory

Phase transition behaviors of Ga 30Sb 70/Sb 80Te 20 nanocomposite multilayer films were investigated for potential use in phase change random access memory (PCRAM). The temperature dependence of sheet resistance measurements were employed to study the phase transition behaviors of the multilayer films. It was found that the crystallization temperature of the multilayer films increased monotonously with increasing the layer thickness of Ga 30Sb 70. The exponential increase of crystallization temperature can reach as high as 130 °C when the layer thickness of Sb 80Te 20 was reduced to 4 nm. Furthermore, by adjusting the layer thicknesses of Ga 30Sb 70 and Sb 80Te 20, the crystallization temperature of the multilayer films can be accurately controlled. The results of cross-sectional analysis using transmission electron microscopy (TEM) confirmed that Ga 30Sb 70/Sb 80Te 20 multilayer films had layered structures with clear interfaces. Phase transition from crystalline (SET) to amorphous (RESET) states was observed to occur at relatively lower RESET voltages, as compared with a device using Ge 2Sb 2Te 5 film.

Colossal resistance switching effect in Pt/spinel-MgZnO/Pt devices for nonvolatile memory applications

We reported the discovery of colossal resistance switching effect in polycrystalline spinel-like structure MgZnO thin films with high Mg contents sandwiched by Pt electrodes. The ultrahigh resistance ratio of high resistance state to low resistance state of about seven to nine orders of magnitude with a low reset voltage of less than 1 V was obtained in this thin film system. The resistance ratio shows an increase of several orders of magnitude compared with those of previously reported resistance switching material systems including metal oxides, semiconductors, and organic molecules. This colossal resistance switching effect will greatly improve the signal-to-noise ratio and simplify the process of reading memory state for nonvolatile memory applications. Our study also provides a material base for studying the origin of resistance switching phenomenon.

High‐speed driving waveform based on reset and address discharge analysis using a Vt closed curve in ACPDPs

Abstract— Driving waveforms having both high‐speed address and low background were developed using Vt closed curve analysis. To prevent a misfired discharge during a sustain period, the voltage difference at the end of the ramp setup between the X‐Y electrodes remains constant. Because background luminance becomes higher (although the address discharge time lag becomes shorter) as the voltage difference between the A‐Y electrodes becomes larger, a low reset voltage and an address bias voltage were adopted during the ramp‐up period.

Electrical Switching Characteristics of Nitrogen Doped Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> Based Phase Change Random Access Memory Cell

Ge2Sb2Te5 (GST) has been widely studied for PRAM as reversible phase change material. GST is expected to reduce RESET (crystalline → amorphous) operation power, which is one of important issues for PRAM technology. In order to investigate the effect of nitrogen doping on electrical switching characteristics, we fabricated two kinds of PRAM cells with nitrogen-doped (N-doped) and un-doped GST, which were different bottom electrode contact size (0.80~1.00 ). N-doped GST PRAM cells have higher dynamic resistance with small sized bottom electrode contact and lower RESET voltage (about 1.2 V, 50 ns) than un-doped GST PRAM cells (about 1.6 V, 50 ns). The resistance switching ratio (RRESET to RSET) was about 100. The results of this study indicate that nitrogen doping into GST film and smaller size of bottom electrode contact reduce RESET power for PRAM operation.