Coexistence of unipolar and bipolar modes in Ag/ZnO/Pt resistive switching memory with oxygen-vacancy and metal-Ag filaments

In this study, the unipolar resistive switching(URS) and bipolar resistive switching(BRS) are demonstrated to be coexistent in the Ag/Zn O/Pt memory device, and both modes are observed to strongly depend on the polarity of forming voltage. The mechanisms of the URS and BRS behaviors could be attributed to the electric-field-induced migration of oxygen vacancies(VO) and metal-Ag conducting filaments(CFs) respectively, which are confirmed by investigating the temperature dependences of low resistance states in both modes. Furthermore, we compare the resistive switching(RS)characteristics(e.g., forming and switching voltages, reset current and resistance states) between these two modes based on VO- and Ag-CFs. The BRS mode shows better switching uniformity and lower power than the URS mode. Both of these modes exhibit good RS performances, including good retention, reliable cycling and high-speed switching. The result indicates that the coexistence of URS and BRS behaviors in a single device has great potential applications in future nonvolatile multi-level memory.     

Rectifying resistance switching behaviors of SnO2 microsphere films modulated by top electrodes

Based on the bipolar resistive switching (RS) characteristics of SnO₂ films, we have fabricated a new prototypical device with sandwiched structure of Metal/SnO₂/fluorine-doped tin oxide (FTO). The SnO₂ microspheres film was grown on FTO glass by template-free hydrothermal synthesis, which was evaporated with various commonly used electrodes such as aluminium (Al), silver (Ag), and gold (Au), respectively. Typical self-rectifying resistance switching behaviors were observed for the RS devices with Al and Au electrodes. However, no obvious rectifying resistance switching behavior was observed for the RS device with Ag electrode. Above results were interpreted by considering the different interface barriers between SnO₂ and top metal electrodes. Our current studies pave the ways for modulating the self-rectifying resistance switching properties of resistive memory devices by choosing suitable metal electrodes.     

Synergistic effects of electrical and optical excitations on TiO_2 resistive device

The influences of electrical and optical excitations on the conductivity characteristic are investigated in bulk and edge devices of ITO/TiO_2/ITO structure. Driven by the electrical and optical stimuli independently, the conductivity relaxation behaviors of the pristine resistive state(PRS) are observed and ascribed to the electron trapping and the oxygen transport processes. For a resistive switching(RS) device, the conductance change under optical illumination is about two orders of magnitude smaller than the conductance change corresponding to the variation of background current due to the emergence of a great number of oxygen vacancies in the RS device. With the illumination being off, the conductance slowly decays,which suggests that the oxygen diffusion process dominates the conductance relaxation. The difference in conductance relaxation between the bulk and edge devices indicates that the oxygen exchange plays a critical role in the relaxation process of conductivity. The synergistic effects of both electrical and optical excitations on the RS devices could be used for novel applications in integrated optoelectronic memory devices.     

Flexible and degradable resistive switching memory fabricated with sodium alginate

Transient electronics has attracted interest as an emerging technology to solve electronic-waste problem,due to its physically vanishing ability in solution.Here in this work,we demonstrate a flexible and degradable transient resistive switching(RS) memory device with simple structure of Cu/sodium alginate(SA)/ITO.The device presents excellent RS characteristics as well as high flexibility,including low operating voltage(1.5 V) and multilevel RS behavior.No performance degradation occurs after bending the device 50 times.Moreover,our device can be absolutely dissolved in deionized water.The proposed SA-based transient memory device has great potential for the development of green and security memory devices.     

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.     

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.     

Nonvolatile Resistive Switching and Physical Mechanism in LaCrO_3 Thin Films

Polycrystalline LaCrO_3(LCO) thin films are deposited on Pt/Ti/SiO_2/Si substrates by pulsed laser deposition and used as the switching material to construct resistive random access memory devices. The unipolar resistive switching(RS) behavior in the Au/LCO/Pt devices exhibits a high resistance ratio of ~104 between the high resistance state(HRS) and low resistance state(LRS) and exhibits excellent endurance/retention characteristics.The conduction mechanism of the HRS in the high voltage range is dominated by the Schottky emission, while the Ohmic conduction dictates the LRS and the low voltage range of HRS. The RS behavior in the Au/LCO/Pt devices can be understood by the formation and rupture of conducting filaments consisting of oxygen vacancies,which is validated by the temperature dependence of resistance and x-ray photoelectron spectroscopy results.Further analysis shows that the reset current I_R and reset power P_R in the reset processes exhibit a scaling law with the resistance in LRS(R_0), which indicates that the Joule heating effect plays an essential role in the RS behavior of the Au/LCO/Pt devices.     

Recent development of studies on the mechanism of resistive memories in several metal oxides

Resistive switching random access memories(RRAM)have been considered to be promising for future information technology with applications for non-volatile memory,logic circuits and neuromorphic computing.Key performances of those resistive devices are approaching the realistic levels for production.In this paper,we review the progress of valence change type memories,including relevant work reported by our group.Both electrode engineering and in-situ transmission electron microscopy(TEM)high-resolution observation have been implemented to reveal the influence of migration of oxygen anions/vacancies on the resistive switching effect.The understanding of resistive memory mechanism is significantly important for device applications.     

Light-induced negative differential resistance effect in a resistive switching memory device

The negative differential resistance (NDR) effect was observed in a Pt/BiFeO₃/TiO₂/BiFeO₃/Pt memory cell by using light-illumination as extra stimulation. Further, the coexistence appearances and gradually becomes obvious when the device is exposed to light-illumination, which display an excellent stability and reversibility of the coexistence of NDR and resistive switching (RS) at room temperature. Through analysis of the physical conduction mechanism, it is expected that a large number of photo-generated charge carriers are induced under light-illumination on the surface and interface of the heterojunction is responsible for the appearance of this coexistence phenomenon. Importantly, the NDR effect is strengthened by the competition transfer of charge carrier in the polarized electric field under light-illumination. This work shows that the coexistence of light-modulated NDR and RS can deeply explore the potential applications of light-controlled multifunctional devices.     

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.     

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.     

Materials,technologies, and circuit concepts for nanocrossbar-based bipolar RRAM

The paper reports on the characterization of bipolar resistive switching materials and their integration into nanocrossbar structures, as well as on different memory operation schemes in terms of memory density and the challenging problem of sneak paths. TiO₂, WO₃, GeSe, SiO₂ and MSQ thin films were integrated into nanojunctions of 100×100 nm². The variation between inert Pt and Cu or Ag top electrodes leads to valence change (VCM) switching or electrochemical metallization (ECM) switching and has significant impact on the resistive properties. All materials showed promising characteristics with switching speeds down to 10 ns, multilevel switching, good endurance and retention. Nanoimprint lithography was found to be a suitable tool for processing crossbar arrays down to a feature size of 50 nm and 3D stacking was demonstrated. The inherent occurrence of current sneak paths in passive crossbar arrays can be circumvented by the implementation of complementary resistive switching (CRS) cells. The comparison with other operation schemes shows that the CRS concept dramatically increases the addressable memory size to about 10¹⁰ bit.     

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.     

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.     

Resistive switching of in situ and ex situ oxygen plasma treated ZnO thin film deposited by atomic layer deposition

In situ and ex situ oxygen plasma treatment (OPT) were applied to treat ZnO thin films deposited by thermal atomic layer deposition (T-ALD), and the resistive switching (RS) behaviors of the films were investigated. For the in situ OPT, the treatment was applied after each T-ALD cycle. For the ex situ OPT, the treatment was applied on the as-grown film. The T-ALD-grown ZnO films were defect-rich and conductive with no RS behavior. After the OPT process, the resistivity of the films increased drastically, which is believed to be mainly due to the removal of hydrogen impurities, and the films showed bipolar RS characteristics. The dominant conduction mechanisms are the trap-controlled space charge limited current and ohmic behavior at different field regions. The RS behavior was induced upon the formation/disruption of the conducting filaments. Owing to the homogeneous chemical composition and fewer defects, the resistance ratio of the in situ OPT ZnO film is higher than that of the ex situ OPT film, implying that the in situ OPT method is an efficient way to fabricate resistive random access memory devices using the ALD-grown ZnO films.     

点接触金属/Pr$lt;sub$gt;0.7$lt;/sub$gt;Ca$lt;sub$gt;0.3$lt;/sub$gt;MnO$lt;sub$gt;3$lt;/sub$gt;/Pt结构稳定的低电流电阻开关特性

利用自主开发的导电原子力显微镜控制Pt,W探针构成点接触金属/Pr_0.7Ca_0.3MnO₃（PCMO）/Pt三明治结构,对其电流-电压（I-V）及脉冲诱导电阻开关（EPIR）特性进行了研究.研究发现,在10 nA限流下两种电极对应结构的I-V都表现出相当稳定的双极性电阻开关特性,以及大于100的电阻开关比.进一步测试发现,点接触W/PCMO/Pt器件具有在10 nA限流下稳定的EPIR特性以及100 pA限流下重复的双极性电阻开关特性.此电流比已报道的电流低3个数量级,表明此结构在低功耗存储器件方面的潜在应用.通过对比样品不同位置、不同限流、不同接触面积点接触Pt/PCMO/Pt的I-V回滞特性,把点接触器件在低电流下稳定、显著的电阻开关效应归结于小的器件面积导致强的局域电场加强了O离子迁移效应. 关键词： 脉冲诱导电阻开关 电场下氧离子迁移 电阻开关     

基于Cu/SiO_x/Al结构的阻变存储器多值特性及机理的研究

采用氧化硅材料构建了Cu/SiOx/Al的三明治结构阻变存储器件.用半导体参数分析仪对其阻变特性进行测量,结果表明其具有明显的阻变特性,并且通过调节限制电流,得到了四个稳定的阻态,各相邻阻态的电阻比大于10,并且具有良好的数据保持能力.在不同温度条件下对各个阻态进行电学测试及拟合,明确了不同阻态的电子传输机理不尽相同:阻态1和阻态2为欧姆传导机制,阻态3为P-F(Pool-Frenkel)发射机制,阻态4为肖特基发射机制.根据电子传输机制,建立了铜细丝导电模型并对Cu/SiOx/Al阻变存储器件各个阻态的电致阻变机制进行解释.     

氧分压对Ni/HfO_x/TiN阻变存储单元阻变特性的影响

采用射频磁控溅射的方法,基于不同氧分压制备的氧化铪构建了Ni/HfO_x/TiN结构阻变存储单元.研究发现,随着氧分压的增加,薄膜表面粗糙度略有降低;另一方面,阻变单元功耗降低,循环耐受性能可达10~3次,且转变电压分布的一致性得到改善.结合电流-电压曲线线性拟合结果及外加温度测试探究了器件的转变机理,得出在低阻态的传导机理为欧姆传导机理,在高阻态的传导机理为肖特基发射机理,并根据氧空位导电细丝理论,对高低阻态的阻变机理进行了详细的理论分析.     

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.     

Digital and analog memory devices based on 2D layered MPS_3 (M= Mn,Co, Ni) materials

We demonstrate digital and analog devices with an Ag/MPS_3/Au structure based on layered MPS_3(M=Mn,Co,Ni)2 D materials.All devices show the bipolar behavior of resistive switching.In addition,Ag/MnPS3/Au and Ag/NiPS_3/Au devices show synaptic characteristics of potentiation and depression.The digital and analog characteristics of resistance states enable Ag/MPS_3/Au devices to work as both binary memory and artificial synapse devices.The Ag/MPS_3/Au memory devices are promising for applications of flexible eye-like and brain-like systems on a chip when they are integrated with photodetectors and FETs composed of full MPS_3 materials.