Reset Current Research Articles

Localization of Joule Heating in Phase-Change Memory With Incorporated Nanostructures and Nanolayer for Reducing Reset Current

In this paper, we intensively investigated the proposed nanocontact phase-change memory (nano-C PCM) with incorporated nanostructures and high-resistivity nanolayer (nano-L) for reducing reset current. The high resistivity was able to be tuned by doping N into the conventional Ge2Sb2Te5 phase-change material. The analysis based on finite-element method exhibited that the current density in the nano-C PCM could be locally enhanced to about two times that of both conventional and nano-L PCM devices. This resulted in the localization of Joule heating in the nano-C PCM, making it have the highest temperature at the same programming current among the three types of PCM devices. Reset current of nano-C PCM could be greatly reduced to 8.2% of that of the conventional one, owing to its high-efficiency heating for amorphization.

Self-Structured Conductive Filament Nanoheater for Chalcogenide Phase Transition.

Ge2Sb2Te5-based phase-change memories (PCMs), which undergo fast and reversible switching between amorphous and crystalline structural transformation, are being utilized for nonvolatile data storage. However, a critical obstacle is the high programming current of the PCM cell, resulting from the limited pattern size of the optical lithography-based heater. Here, we suggest a facile and scalable strategy of utilizing self-structured conductive filament (CF) nanoheaters for Joule heating of chalcogenide materials. This CF nanoheater can replace the lithographical-patterned conventional resistor-type heater. The sub-10 nm contact area between the CF and the phase-change material achieves significant reduction of the reset current. In particular, the PCM cell with a single Ni filament nanoheater can be operated at an ultralow writing current of 20 μA. Finally, phase-transition behaviors through filament-type nanoheaters were directly observed by using transmission electron microscopy.

The Role of Oxygen Vacancies on Switching Characteristics of TiO(x) Resistive Memories.

Using oxygen vacancy rich (VO-rich) TiO(x) dielectric with high work function Ni electrode, large resistance window of > 10x and narrow current distribution were realized in the Ni/VO-rich TiO(x)/TaN resistive random access memory (RRAM) device. It can be ascribed to the formation and rupture of conducting filaments by the percolation of VOs and Ti interstitials. Moreover, the effects of annealing treatment and top electrode on resistive switching properties were investigated. The device with VO-deficient TiO(x) after annealing reduces the defects and exhibits small window and low switching currents. The device with low work function Ti top electrode provides low barrier to increase reset currents and the randomly distributed filamentary paths forms near the Ti causes wide current distribution.

The role of contact resistance in GeTe and Ge2Sb2Te5 nanowire phase change memory reset switching current

Nanowire (NW) structures offer a model system for investigating material and scaling properties of phase change random access memory (PCRAM) at the nanometer scale. Here, we investigate the relationship between nanowire device contact resistance and reset current (Ireset) for varying diameters of NWs. Because the reset switching current directly affects possible device density of PCRAM NWs, it is considered one of the most important parameters for PCRAM. We found that the reset switching current, Ireset, was inversely proportional to the contact resistance of PCRAM NW devices decreasing as NW diameter was reduced from 250 nm to 20 nm. Our observations suggest that the reduction of power consumption of PCRAM in the sub-lithographic regime can be achieved by lowering the contact resistance.

Oxygen-doped Sb2Te3 for high-performance phase-change memory

In this study, we intensively investigated oxygen doped Sb2Te3 (ST–O) for high-performance phase-change memory (PCM) based on X-ray diffraction analysis and electrical measurements. The crystal became more ordered with increasing annealing temperature and the lattice deformed with the interstitial sites occupied by doped O atoms. It also exhibited that mean crystal size decreased from 8nm to 3nm and thus crystal growth was significantly suppressed by fine oxides due to O-doping. The resistivity of crystalline O–ST could be around 3–4 orders of magnitude higher than that of crystalline ST, which enables great reduction in reset current. It was clear that the high resistivity was able to be tuned simply by doping O into the conventional ST phase-change material for improving the performance of PCM.

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.

Temperature and polarity dependence of the switching behavior of Ni/HfO2-based RRAM devices

In this work, the impact of temperature and switching polarity on the stability and variability of Ni/HfO2-based RRAM structures has been investigated for temperatures ranging from −40°C to 175°C. Special attention is given to the analysis of the variability and thermal behavior of the reset currents and voltages. The experimental results show that the temperature plays a key role in the reset operation, owing to the self-accelerated dissolution process of the conductive filament. Furthermore, for both polarities, a larger instability of the on-state current at high temperatures is evidenced.

Flexible one diode-one phase change memory array enabled by block copolymer self-assembly.

Flexible memory is the fundamental component for data processing, storage, and radio frequency communication in flexible electronic systems. Among several emerging memory technologies, phase-change random-access memory (PRAM) is one of the strongest candidate for next-generation nonvolatile memories due to its remarkable merits of large cycling endurance, high speed, and excellent scalability. Although there are a few approaches for flexible phase-change memory (PCM), high reset current is the biggest obstacle for the practical operation of flexible PCM devices. In this paper, we report a flexible PCM realized by incorporating nanoinsulators derived from a Si-containing block copolymer (BCP) to significantly lower the operating current of the flexible memory formed on plastic substrate. The reduction of thermal stress by BCP nanostructures enables the reliable operation of flexible PCM devices integrated with ultrathin flexible diodes during more than 100 switching cycles and 1000 bending cycles.

Effects of the fluctuation in a singly-connected conducting filament structure on the distribution of the reset parameters in unipolar resistance switching

The reset current (Ireset), voltage (Vreset), and resistance of the low resistance state, as functions of the compliance current (CC), were investigated in a Pt/NiO/Pt structure that showed unipolar resistance switching. Interestingly, the Ireset and the Vreset measured at low CCs were found to be widely distributed. In order to explain the behavior of the reset parameters for the singly-connected conducting filament (CF) structure, a simple model of CFs was employed whose width variation follows the Gaussian distribution. The wide distribution of the reset parameters can be attributed to the fluctuation in the number and/or the width of the CFs.

Hierarchically Self-Assembled Block Copolymer Blends for Templating Hollow Phase-Change Nanostructures with an Extremely Low Switching Current

Phase change memory (PCM) is one of the most promising candidates for next-generation nonvolatile memory devices because of its high speed, excellent reliability, and outstanding scalability. However, the high switching current of PCM devices has been a critical hurdle to realize low-power operation. Although one solution is to reduce the switching volume of the memory, the resolution limit of photolithography hinders further miniaturization of device dimensions. In this study, we employed unconventional self-assembly geometries obtained from blends of block copolymers (BCPs) to form ring-shaped hollow PCM nanostructures with an ultrasmall contact area between a phase-change material (Ge2Sb2Te5) and a heater (TiN) electrode. The high-density (approximately 0.1 terabits per square inch) PCM nanoring arrays showed extremely small switching current of 2–3 μA. Furthermore, the relatively small reset current of the ring-shaped PCM compared to the pillar-shaped devices is attributed to smaller switching volume, which is well supported by electro-thermal simulation results. This approach may also be extended to other nonvolatile memory device applications such as resistive switching memory and magnetic storage devices, where the control of nanoscale geometry can significantly affect device performances.

Reduction of RESET current in phase change memory devices by carbon doping in GeSbTe films

Phase Change Memory (PCM) has been proposed for use as a substitute for flash memory to satisfy the huge demands for high performance and reliability that promise to come in the next generation. In spite of its high scalability, reliability, and simple structure, high writing current, e.g., RESET current, has been a significant obstacle to achieving a high density in storage applications and the low power consumption required for use in mobile applications. We report herein on an attempt to determine the level of carbon incorporated into a GeSbTe (GST) film that is needed to reduce the RESET current of PCM devices. The crystal structure of the film was transformed into an amorphous phase by carbon doping, the stability of which was enhanced with increasing carbon content. This was verified by the small grain size and large band gap that are typically associated with carbon. The increased level of C-Ge covalent bonding is responsible for these enhancements. Thus, the resistance of the carbon doped Ge2Sb2Te5 film was higher than that for an undoped GST film by a factor of 2 orders of magnitude after producing a stable face-centered cubic phase by annealing. As a consequence, the PCM devices showed a significant reduction in RESET current as low as 23% when the carbon content was increased to 11.8 at. %. This can be attributed to the elevated SET resistance, which is proportional to the dynamic resistance of the PCM device, caused by the high resistance due to a carbon doped GST film.

Improved resistive switching phenomena and mechanism using Cu-Al alloy in a new Cu:AlOx/TaOx/TiN structure

Improved resistive switching phenomena such as device-to-device uniformity, lower formation voltage (2.8V) and RESET current, >500 program/erase cycles, longer read endurance of >106 cycles with a program/erase pulse width of 1μs, and data retention of >225h under a low current compliance of 300μA have been discussed by using Cu-Al alloy in Cu:AlOx/TaOx/TiN conductive bridging resistive random access memory (CBRAM) device for the first time. The switching mechanism is based on a thinner with dense Cu filament formation/dissolution through the defects in the Cu:AlOx/TaOx/TiN structure owing to enhance memory characteristics. These characteristics have been confirmed by measuring randomly picked 100 devices having via-hole size of 0.4×0.4μm2. The Cu-Al alloy becomes Cu:AlOx buffer layer and Ta2O5 becomes TaOx switching layer owing to Gibbs free energy dependency. All layers and elements are observed by high-resolution transmission electron microscope (HRTEM) image and energy dispersive X-ray spectroscopy (EDX). By developing a numerical equation in between RESET current and formation voltage, it is found that a higher rate of Cu migration is observed owing to both the defective switching layer and larger size, which results a lower formation voltage and RESET current of the Cu:AlOx/TaOx/TiN structure, as compared to Cu/Ta2O5/TiN under external positive bias on the Cu electrode. This simple Cu:AlOx/TaOx/TiN CBRAM device is useful for future nanoscale non-volatile memory application.

Asymmetric structure with high electric–thermal conversion efficiency for nanoscale phase change memory based on three‐dimensional simulation

How to decrease the reset current is a key point for improving the performance of phase change memory (PCM). Therefore, an asymmetric structure of PCM is proposed to achieve this aim. Using ANSYS, three-dimensional finite element models were established to simulate the efficiency of electric–thermal conversion in PCM cells with an asymmetric structure and a conventional symmetric structure. Simulation results show that the peak temperature in the asymmetric cell with a feature size (FS) of 16 nm is higher than that in the conventional symmetric cell with the same FS by 37.2% when the same current pulse is applied. In an asymmetric cell, the current path and heat dissipating conditions are quite different from those in a symmetric cell, which leads to the remarkable enhancement in electric–thermal conversion efficiency. Simulation results also suggest a proper offset value in asymmetric structures, with which the efficiency of electric–thermal conversion can be further improved. If this new structure is used in fabrication, no additional cost will be incurred because the only difference between symmetric and asymmetric structures is the position of the top opening of the electrodes, indicating that the new structure is highly compatible with modern fabrication processes of PCM.

Tuning the switching behavior of binary oxide-based resistive memory devices by inserting an ultra-thin chemically active metal nanolayer: a case study on the Ta2O5-Ta system.

The common nonpolar switching behavior of binary oxide-based resistive random access memory devices (RRAMs) has several drawbacks in future application, such as the requirements for a high forming voltage, a large reset current, and an additional access device to settle the sneak-path issue. Herein, we propose the tuning of the switching behavior of binary oxide-based RRAMs by inserting an ultra-thin chemically active metal nanolayer, and a case study on Ta2O5-Ta systems is provided. The devices are designed to be Pt/Ta2O5(5 - x/2)/Ta(x)/Ta2O5(5 - x/2)/Pt with x = 0, 2, or 4 nm. The reference devices without the Ta nanolayer exhibit an expected nonpolar switching behavior with a high forming voltage of ∼-4.5 V and a large reset current of >10 mA. In contrast, a self-compliance bipolar switching behavior with a low forming voltage of ∼-2 V and a small reset current of <1 mA is observed after inserting a 2 nm Ta nanolayer. When the Ta nanolayer is increased to 4 nm, a complementary resistive switching (CRS) behavior is found, which can effectively settle the sneak-path issue. The appearance of CRS behavior suggests that a thin Ta nanolayer of 4 nm is robust enough to act as an inner electrode. Besides, the behind switching mechanisms are thoroughly discussed with the help of a transmission electron microscope and temperature-dependent electrical measurements. All these results demonstrate the feasibility of tuning switching behavior of binary oxide-based RRAMs by inserting an ultra-thin chemically active metal nanolayer and might help to advance the commercialization of binary oxide-based RRAMs.

Growth-Dominant Superlattice-Like Medium and Its Application in Phase Change Memory

Lateral PCM is one structure with good thermal confinement and low RESET current. Ideal growth-dominant (GD) phase change material is needed for lateral Phase change memory (PCM). We propose the concept of GD superlattice-like (SLL) phase change structure. GeTe and Sb7Te3 were selected to form the GD SLL structure. Crystallization temperature and resistivity of GeTe and Sb7Te3 were thickness dependent. The crystallization temperature of GeTe/Sb7Te3 SLL phase change structure increase with increasing thickness ratio of GeTe/Sb7Te3. GeTe/Sb7Te3 SLL structure built with a thickness ratio of 1.6:1 (GeTe to Sb7Te3) exhibited the lowest melting temperature compared to other thickness ratio. For a typical lateral PCM employing such GeTe/Sb7Te3 SLL phase change structure, stable SET/RESET operations with a low RESET current of 1.5 mA, SET and RESET speed of 30 nanoseconds, a good endurance above 106 cycles and a high RESET/SET resistance ratio window of 20 were achieved, demonstrating strong potentials in next generation non-volatile memory application.

Statistical characteristics of reset switching in Cu/HfO2/Pt resistive switching memory.

A major challenge of resistive switching memory (resistive random access memory (RRAM)) for future application is how to reduce the fluctuation of the resistive switching parameters. In this letter, with a statistical methodology, we have systematically analyzed the reset statistics of the conductive bridge random access memory (CBRAM) with a Cu/HfO2/Pt structure which displays bipolar switching property. The experimental observations show that the distributions of the reset voltage (Vreset) and reset current (Ireset) are greatly influenced by the initial on-state resistance (Ron) which is closely related to the size of the conductive filament (CF) before the reset process. The reset voltage increases and the current decreases with the on-state resistance, respectively, according to the scatter plots of the experimental data. Using resistance screening method, the statistical data of the reset voltage and current are decomposed into several ranges and the distributions of them in each range are analyzed by the Weibull model. Both the Weibull slopes of the reset voltage and current are demonstrated to be independent of the on-state resistance which indicates that no CF dissolution occurs before the reset point. The scale factor of the reset voltage increases with on-state resistance while that of the reset current decreases with it. These behaviors are fully in consistency with the thermal dissolution model, which gives an insight on the physical mechanism of the reset switching. Our work has provided an inspiration on effectively reducing the variation of the switching parameters of RRAM devices.

3-D Resistance Model for Phase-Change Memory Cell

In this paper, a 3-D resistance model is proposed for phase-change (PC) memory cell based on cell geometry and RESET current. Explicit expression is developed for PC radius in terms of RESET current, cell geometry, and material property. Conformal mappings are used for SET and RESET resistance calculation in 2-D models, which solve the problem of current crowding in structures with complex boundary condition. In the 3-D model development, additional spreading resistance is considered, together with the bulk resistance stemmed from 2-D model to form the eventual complete expression. Models show good consistency with finite element simulation and experimental data.

Improved Bipolar Resistive Switching Memory Characteristics in Ge&lt;SUB&gt;0.5&lt;/SUB&gt;Se&lt;SUB&gt;0.5&lt;/SUB&gt; Solid Electrolyte by Using Dispersed Silver Nanocrystals on Bottom Electrode

Resistive switching random-access memory (ReRAM) devices based on chalcogenide solid electrolytes have recently become a promising candidate for future low-power nanoscale nonvolatile memory application. The resistive switching mechanism of ReRAM is based on the formation and rupture of conductive filament (CF) in the chalcogenide solid electrolyte layers. However, the random diffusion of metal ions makes it hard to control the CF formation, which is one of the major obstacles to improving device performance of ReRAM devices. We demonstrate the spin-coated metal nanocrystals (NCs) enhance the bipolar resistive switching (BRS) memory characteristics. Compared to the Ag/Ge0.5Se0.5/Pt structure, excellent resistive switching memory characteristics were obtained from the Ag/Ge0.5Se0.5/Ag NCs/Pt structure. Ag NCs improve the uniformity of resistance values and reduce the reset voltage and current. A stable DC endurance (> 100 cycles) and a high data retention (> 10(4) sec) were achieved by spin coating the Ag NCs on the Pt bottom electrode for ReRAMs.

Impact of device size and thickness of Al2O 3 film on the Cu pillar and resistive switching characteristics for 3D cross-point memory application.

Impact of the device size and thickness of Al2O3 film on the Cu pillars and resistive switching memory characteristics of the Al/Cu/Al2O3/TiN structures have been investigated for the first time. The memory device size and thickness of Al2O3 of 18 nm are observed by transmission electron microscope image. The 20-nm-thick Al2O3 films have been used for the Cu pillar formation (i.e., stronger Cu filaments) in the Al/Cu/Al2O3/TiN structures, which can be used for three-dimensional (3D) cross-point architecture as reported previously Nanoscale Res. Lett.9:366, 2014. Fifty randomly picked devices with sizes ranging from 8 × 8 to 0.4 × 0.4 μm2 have been measured. The 8-μm devices show 100% yield of Cu pillars, whereas only 74% successful is observed for the 0.4-μm devices, because smaller size devices have higher Joule heating effect and larger size devices show long read endurance of 105 cycles at a high read voltage of -1.5 V. On the other hand, the resistive switching memory characteristics of the 0.4-μm devices with a 2-nm-thick Al2O3 film show superior as compared to those of both the larger device sizes and thicker (10 nm) Al2O3 film, owing to higher Cu diffusion rate for the larger size and thicker Al2O3 film. In consequence, higher device-to-device uniformity of 88% and lower average RESET current of approximately 328 μA are observed for the 0.4-μm devices with a 2-nm-thick Al2O3 film. Data retention capability of our memory device of >48 h makes it a promising one for future nanoscale nonvolatile application. This conductive bridging resistive random access memory (CBRAM) device is forming free at a current compliance (CC) of 30 μA (even at a lowest CC of 0.1 μA) and operation voltage of ±3 V at a high resistance ratio of >104.

Understanding the early cycling evolution behaviors for phase change memory application

The RESET current of T-shaped phase change memory cells with 35 nm heating electrodes has been studied to understand the behavior of early cycling evolution. Results show that the RESET current has been significantly reduced after the early cycling evolution (1st RESET) operation. Compared the transmission electron microscope images, it is found that the hexagonal Ge2Sb2Te5 (GST) crystal grains are changed into the grains with face centered cubic structure after the early cycling evolution operation, which is taken as the major reason for the reduced RESET current, confirmed by a two-dimensional finite analysis and ab initio calculations.