Analysis and modeling of resistive switching mechanism oriented to fault tolerance of resistive memory based on memristor

With the progress of the semiconductor industry, resistive memories, especially the memristor, have drawn increasing attention. The resistive memory based on memrsitor has not been commercialized mainly because of data error. Currently, there are more studies focused on fault tolerance of resistive memory. This paper studies the resistive switching mechanism which may have time-varying characteristics. Resistive switching mechanism is analyzed and its respective circuit model is established based on the memristor Spice model.     

Highly improved resistive switching performances of the self-doped Pt/HfO2:Cu/Cu devices by atomic layer deposition

Metal-oxide electrochemical metallization (ECM) memory is a promising candidate for the next generation nonvolatile memory. But this memory suffers from large dispersion of resistive switching parameters due to the intrinsic randomness of the conductive filament. In this work, we have proposed a self-doping approach to improve the resistive switching characteristics. The fabricated Pt/HfO₂:Cu/Cu device shows outstanding nonvolatile memory properties, including high uniformity, good endurance, long retention and fast switching speed. The results demonstrate that the self-doping approach is an effective method to improve the metal-oxide ECM memory performances and the self-doped Pt/HfO₂:Cu/Cu device has high potentiality for the nonvolatile memory applications in the future.     

Flexible and transparent resistive switching devices using Au nanoparticles decorated reduced graphene oxide in polyvinyl alcohol matrix

We have investigated the resistive switching mechanism in solution processed Au-reduced graphene oxide-polyvinyl alcohol (PVA) nanocomposites on flexible substrates. Monodispersed gold nanoparticles (Au NPs) attached to reduced graphene oxide (RGO) in aqueous PVA solution have been synthesized using a novel one pot technique. The fabricated hybrid device showed high On/Off switching ratio more than 10³ with low operating voltages. The performance of hybrid device can be effectively enhanced over control RGO device. The switching mechanism occurs from the electrochemical reduction/oxidation process of partially reduced graphene oxide. The proposed devices reveal superior asymmetric bipolar resistive switching characteristics attractive for solution processable flexible and transparent non-volatile memory applications.     

Analysis of resistive switching behaviors of vanadium oxide thin film

We demonstrated the polarization of resistive switching for Cu/VO_x/Cu memory cell. Switching behaviors of Cu/VO_x/Cu cell were tested by semiconductor device analyzer (Agilent B1500A), and the relative micro-analysis of I-V characteristics of VO_x/Cu was characterized by conductive atomic force microscope (CAFM). The I-V test results indicated that both forming and the reversible resistive switching between low resistance state (LRS) and high resistance state (HRS) can be observed under either positive or negative sweep. The CAFM images for LRS and HRS directly exhibited evidences of the formation and rupture of filaments based on positive or negative voltage. Cu/VO_x/Cu sandwiched structure exhibits a reversible resistive switching behavior and shows potential applications in the next generation nonvolatile memory field.     

Investigation of resistive switching behaviours in WO3-based RRAM devices

In this paper, a WO₃-based resistive random access memory device composed of a thin film of WO₃ sandwiched between a copper top and a platinum bottom electrodes is fabricated by electron beam evaporation at room temperature. The reproducible resistive switching, low power consumption, multilevel storage possibility, and good data retention characteristics demonstrate that the Cu/WO₃/Pt memory device is very promising for future nonvolatile memory applications. The formation and rupture of localised conductive filaments is suggested to be responsible for the observed resistive switching behaviours.     

Mechanism analysis of switching direction transformation in an Er2O3 based RRAM device

The resistive random access memory (RRAM) based on resistive switching effect has considered to be the most advanced next generation memory, in which the switching direction determines the order of reading-writing. In this work, the rare-earth metal Er₂O₃ was used as functional layer, and Ag and indium-tin-oxide (ITO) are selected as top and bottom electrode to fabricate resistive switching device. Further, it is observed that the switching direction and memory window of resistive switching device can be regulated by exchanging top and bottom electrode. Moreover, the complementary switching memory behavior in Ag/Er₂O₃/ITO/Er₂O₃/Ag structure was also observed. Through mechanism analysis, it is expected that the barrier changes and metal-ions oxidation-reduction should be responsible for the conversion of switching direction and regulation of memory window. This work opens up a way to the development of next generation new concept memory.     

Solution-processed high-k thin films as a resistive switching for ReRAM applications

Resistive switching characteristics of solution-processed high-k thin films (HfO_x and TaO_x) were investigated for ReRAM applications. The thickness of solution-processed high-k thin films can be easily controlled by simple spin coating. We optimized the critical thickness of solution-processed HfO_x and TaO_x thin films, for reliable ReRAM operations. A similar bipolar resistive switching behavior was observed from both solution-processed and sputter-processed HfO_x films. Furthermore, it was found that the solution-processed HfO_x and TaO_x films have a uniform resistive switching characteristic. The dominant conduction of these solution-processed films is described by Ohmic conduction in the low-resistance state. On the other hand, Ohmic conduction at low voltage and Poole–Frenkel emission at high voltage dominate in the high-resistance state. It was verified that the solution-processed HfO_x and TaO_x films have superior endurance and retention characteristics. Therefore, ReRAM devices based on solution-processed high-k materials are expected to be a promising candidate, for usage of resistive memory in glass substrate or flexible substrate based electronic devices.     

Random and localized resistive switching observation in Pt/NiO/Pt

Resistive memory switching devices based on transition metal oxides are now emerging as a candidate for nonvolatile memories. To visualize nano‐sized (10 nm to 30 nm in diameter) conducting filamentary paths in the surface of NiO thin films during repetitive switching, current sensing–atomic force microscopy and ultra‐thin (<5 nm) Pt films as top electrodes were used. Some areas (or spots), which were assumed to be the beginning of the conducting filaments, appeared (formation) and disappeared (rupture) in a localized and random fashion during the switching and are thought to contribute to resistive memory switching. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Resistive switching behaviour of a tantalum oxide nanolayer fabricated by plasma oxidation

We investigate the resistive switching behaviour of a tantalum oxide nanolayer‐based nonvolatile memory with Pt/TaO_5–x/TaN structure, which was prepared at room temperature through a processing compatible with CMOS technology. The tantalum oxide nanolayer with thickness of about 5 nm was fabricated by plasma oxidation of TaN films. The switching mechanism can be explained by the modulation of the local oxygen‐deficient conduction channel resulting from oxygen ions drift. This Letter represents a cost‐efficient method for developing nanoscale restive switching nonvolatile memories. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Atomic crystals resistive switching memory

Facing the growing data storage and computing demands, a high accessing speed memory with low power and non volatile character is urgently needed. Resistive access random memory with 4F~2 cell size, switching in sub-nanosecond cycling endurances of over 10~(12) cycles, and information retention exceeding 10 years, is considered as promising nex generation non-volatile memory. However, the energy per bit is still too high to compete against static random acces memory and dynamic random access memory. The sneak leakage path and metal film sheet resistance issues hinder th further scaling down. The variation of resistance between different devices and even various cycles in the same device hold resistive access random memory back from commercialization. The emerging of atomic crystals, possessing fin interface without dangling bonds in low dimension, can provide atomic level solutions for the obsessional issues. Moreove the unique properties of atomic crystals also enable new type resistive switching memories, which provide a brand-new direction for the resistive access random memory.     

Synthesis of hexagonal boron nitride thin film on Pt substrates for resistive switching memory applications

Hexagonal boron nitride (hBN), due to its high reliability as a two-dimensional (2D) dielectric material, has attracted much attention for its potential applications in nanoelectronic devices. Here, the use of radio frequency (RF) magnetron sputtering-grown hBN films to construct hBN-based resistive switching (RS) memory device is reported, and the RS mechanism is deduced. The hBN-based RS memory shows low operating voltage, reproducible write cycles, and long retention time. First-principles simulations further confirm the resistive switching. This work provides an important case to facilitate the future applications of 2D materials in the RS memory.     

Coexistence of nonvolatile unipolar and volatile threshold resistive switching in the Pt/LaMnO3/Pt heterostructures

Coexistence of nonvolatile unipolar and volatile threshold resistive switching is observed in the Pt/LaMnO₃ (LMO)/Pt heterostructures. The nonvolatile unipolar memory is achieved by applying a negative bias, while the volatile threshold resistive switching is obtained under a positive bias. Additionally, the pristine low resistance state (LRS) could be switched to high resistance state (HRS) by the positive voltage sweeping, which is attributed to the conduction mechanism of Schottky emission. Subsequently, the insulator-to-metal transition in the LMO film due to formation of ferromagnetic metallic phase domain contributes to the volatile threshold resistive switching. However, the nonvolatile unipolar switching under the negative bias is ascribed to the formation/rupture of oxygen-vacancy conducting filaments. The simultaneously controllable transition between nonvolatile and volatile resistance switching by the polarity of the applied voltage exhibits great significance in the applications of in-memory computing technology.     

Resistive switching behaviors of Cu/TaOx/TiN device with combined oxygen vacancy/copper conductive filaments

Forming-free and self-compliant bipolar resistive switching is observed in Cu/TaO_x/TiN conductive bridge random access memory. Generally, Pt has been investigated as an inert electrode. However, Pt is not desirable material in current semiconductor industry for mass production. In this study, all electrodes are adapted to complementary metal-oxide-semiconductor compatible materials. The self-compliant resistive switching is achieved via usage of TiN bottom electrode. Also, dissolved Cu ions in TaO_x lead to forming-free resistive switching behavior. The resistive switching mechanism is formation and rupture of combined oxygen vacancy/metallic copper conductive filament. We propose that Cu/TaO_x/TiN is a promising candidate for a conductive bridge random access memory structure.     

基于Au/TiO_2/FTO结构忆阻器的开关特性与机理研究

采用简单的一步水热法在FTO导电玻璃上外延生长了锐钛矿TiO_2纳米线,制备了具有Au/TiO_2/FTO器件结构的锐钛矿TiO_2纳米线忆阻器,系统研究了器件的阻变开关特性和开关机理.结果表明,Au/TiO_2/FTO忆阻器具有非易失的双极性阻变开关特性.同时,在103s的时间内,器件在0.1 V的电阻开关比始终保持在20以上,表明器件具有良好的非易失性.此外,器件在低阻态时遵循欧姆导电特性,而在高阻态时则满足陷阱控制的空间电荷限制电流传导机制,同时提出了基于氧空位导电细丝形成与断开机制的阻变开关模型.研究结果表明Au/TiO_2/FTO忆阻器将是一种很有发展潜力的下一代非易失性存储器.     

Role of an oxide interface in a resistive switch

In the present era of data-driven architectures like 5G, Internet of things (IoT), Artificial Intelligence (AI), etc, the requirement of fast-switchable memory storage is more than ever. Oxide resistive switches are considered to be a primary choice in the non-volatile memory design. In this work, we have engineered the conventional metal-insulator-metal (MIM) structure of an oxide memristor (Ag/ZnO/ITO) by inducing an additional oxide layer La_0.7Sr_0.3MnO₃ (LSMO) at the interface between the active layer (ZnO) and Ag electrode. The presence of LSMO acts as a reservoir for the oxygen vacancies, easing the conducting filament formation process in ZnO, thereby enabling drastic improvement of the switching performance and offering reliable endurance over multiple switching cycles. First-principles-based calculations suggested the role of oxygen vacancies in controlling the electronic state of ZnO and formation of vacancies in the resistive switching process, which is in agreement with the experimental observation. The current results pave ways for improving the switching performance of resistive memory circuits through simple structural engineering incorporation, which lies at the heart of oxide electronics.     

Unipolar resistive switching in Au/Cr/Mg0.84Zn0.16O2?δ /p+-Si

Unipolar resistive switching memory cells were fabricated using a Mg_0.84Zn_0.16O_2?δ thin film, sandwiched between p⁺-Si (100) substrate and Cr/Au top electrodes. Electrical measurements showed a large memory window and memory window margin of 10⁷ and 10⁴, respectively. Furthermore, a wide switching voltage distribution gap of 3.6?V between the switching-ON and -OFF processes was obtained for different sweeping cycles. Gas bubbles at four different stages were observed on the top electrodes after electrical stimulus, indicating that conducting filaments consisting of oxygen vacancies are responsible for the resistive switching characteristics. Conductive atomic force microscopy results show that the highly conductive areas are along the edge of the gas bubble or the edge of the device. This phenomenon suggests the potential of scaling down the device area to lower than 32?nm.     

Resistive switching effect in metal–oxide–metal structures with ZnO:Li oxide layer

The resistive switching effect in metal–oxide–metal (MOM) structures has been investigated, where the 10% Li-doped ZnO layer was used as an oxide layer, as well as Pt and 20% fluorine doped SnO₂ (SnO₂:F) were used as a bottom electrodes. The current–voltage (I–V) and switching (I–t) characteristics of Ag/ZnO:Li/Pt and Ag/ZnO:Li/SnO₂:F structures were investigated. The unipolar resistive switching is detected in the structures with the Pt, while the use of transparent conductive SnO₂:F electrode instead of Pt, results to the bipolar memory effect.     

Irradiation of glass with infrared femtosecond laser pulses

We report a novel resistive random access memory using tri-layer dielectrics of GeO _x /nano-crystal TiO₂/TaON and low cost top Ni and bottom TaN electrodes. Excellent device performance of ultra-low 720 fJ switching energy, tight distributions of set/reset currents, and exceptionally long endurance of 5×10⁹ cycles were achieved simultaneously. Such excellent endurance may create new applications such as those used for Data Centers that are ascribed to the higher-κ nano-crystal TiO₂, hopping pass via grain boundaries, and fast switching speed of 100 ns to improve the dielectric fatigue during endurance stress.     

Analysis of the resistive switching behaviors of vanadium oxide thin film

We demonstrate the polarization of resistive switching for a Cu/VOx/Cu memory cell.The switching behaviors of Cu/VOx/Cu cell are tested by using a semiconductor device analyzer(Agilent B1500A),and the relative micro-analysis of I-V characteristics of VOx/Cu is characterized by using a conductive atomic force microscope(CAFM).The I-V test results indicate that both the forming and the reversible resistive switching between low resistance state(LRS) and high resistance state(HRS) can be observed under either positive or negative sweep.The CAFM images for LRS and HRS directly exhibit evidence for the formation and rupture of filaments based on positive or negative voltage.The Cu/VOx/Cu sandwiched structure exhibits reversible resistive switching behavior and shows potential applications in the next generation of nonvolatile memory.     

Effect of defect content on the unipolar resistive switching characteristics of ZnO thin film memory devices

In this study, unipolar resistive switching (URS) characteristics in ZnO thin film memory devices were systematically investigated with variable defect content. ZnO films displayed typically URS behavior while oxygen-deficient ZnO_1?x films did not show resistive switching effects. The devices with two intentional Ohmic interfaces still show URS. These results show that appearance of URS behavior can be dominated by initial oxygen vacancy content in ZnO thin films. Modest increase in oxygen vacancy content in ZnO films will lead to forming-free and narrower distributions of switching parameters (set and reset voltage, high and low resistance states). It indicates that controlling the initial oxygen vacancy content was an effective method to enhance the URS performance.