Flexible resistance memory devices based on Cu/ZnO:Mg/ITO structure

Resistance memory devices based on a Cu/Mg‐doped ZnO/indium‐tin‐oxide structure on a PET (polyethylene terephthalate) flexible substrate were fabricated. The devices showed stable bipolar resistance switching property and good flexibility. The high to low resistance ratio was larger than 30 times, the endurance was more than 10² cycles, and the resistance retention was longer than 10⁴ s. The resistance values of both high and low resistance states were not significantly changed by bending in a radius (≥20 mm) for more than 10³ times. This resistance switching phenomenon of our devices can be explained by creation/rupture of metal conductive channels induced by electrochemical migration of Cu ions. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Fully room‐temperature‐fabricated TiN/TaOx/Pt nonvolatile memory devices

The reliable resistive switching properties of TiN/TaO_x/Pt structures fabricated with a fully room‐temperature process are demonstrated in this letter. The devices exhibited a low operation voltage of 0.6 V as well as good endurance up to 10⁵ cycles. No data loss was reported upon continuous readout for more than 10⁴ s at 125 °C. Multilevel storage is feasible due to the dependence of the low resistance state (LRS) on the initial “SET” (switch from high to low RS) compliance current. The values of LRS showed no dependence on the size of the device, which correlated with the localized conductive filament mechanism. This nonvolatile multilevel memory effect and the fully room‐temperature fabrication process make the TiN/TaO_x/Pt memory devices promising for future nonvolatile memory application. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The application of conventional photolithography to microscale organic resistive memory devices

We demonstrate the application of conventional photolithography to fabricate organic memory devices in an array structure with a cell area of 4 × 4 μm² without damaging the underlying organic memory layer. Applying photolithography to organic electronic devices is not trivial because the solvents used during lithography may dissolve and damage the previously coated organic layers. The application of photolithography to our organic devices was possible because of the introduction of polymethyl methacrylate (PMMA)/polyvinyl alcohol (PVA) onto the memory active layer, where PMMA functions as a buffer layer to prevent dissolution of the PVA layer during developing process, and PVA acts as a striped layer during metal lift-off process. Embedded Al bottom electrodes were particularly constructed to minimize the switching failure. The completed organic memory devices exhibited typical unipolar switching behavior and excellent memory performance in terms of their statistical memory parameters (ON and OFF currents and threshold voltages), ON/OFF ratio (>10²), endurance (>230 cycles), and retention (>10⁴ s). This convenient photolithography patterning technique is applicable for the further scaling of many types of organic devices.     

The effects of substrate temperature on ZnO-based resistive random access memory devices

Ag/ZnO/Zn/Pt structure resistive switching devices are prepared by radio frequency magnetron sputtering.The ZnO thin films are grown at room temperature and 400 C substrate temperature,respectively.By comparing the data,we find that the latter device displayed better stability in the repetitive switching cycle test,and the resistance ratio between a high resistance state and a low resistance state is correspondingly increased.After 104-s storage time measurement,this device exhibits a good retention property.Moreover,the operation voltages are very low:-0.3 V/-0.7 V(OFF state) and 0.3 V(ON state).A high-voltage forming process in the initial state is not required,and a multistep reset process is demonstrated.     

阻变存储器复合材料界面及电极性质研究

采用基于密度泛函理论的第一性原理对比研究了Cu（111）/HfO₂（001）,Cu（111）/HfO₂（010）,Cu（111）/HfO₂（100）三种复合材料界面模型的失配率、界面束缚能、电荷密度、电子局域函数以及差分电荷密度. 计算结果表明：Cu（111）/HfO₂（010）失配率最小,界面束缚能最大,界面体系相对最稳定;对比电荷密度及电子局域函数图显示,只有HfO₂（010）方向形成的复合材料体系出现了垂直Cu电极方向完整连通的电子通道,表明电子在此方向上具有局域性、连通性,与阻变存储器（RRAM）器件导通方向一致;差分电荷密度图显示,Cu（111）/HfO₂（010）复合材料体系界面处存在电荷密度分布重叠的现象,界面处有电子的相互转移、成键的存在;进一步计算了Cu（111）/HfO₂（010）体系距离界面不同位置的间隙Cu原子形成能,表明越靠近界面Cu原子越容易进入HfO₂ 体内,在外加电压下易发生电化学反应,从而导致Cu导电细丝的形成与断裂. 研究结果可为RRAM存储器的制备及性能的提高提供理论指导和设计工具. 关键词： 阻变存储器 复合材料 界面 电子通道     

Analysis and modeling of resistive switching mechanisms oriented to resistive random-access memory

With the progress of the semiconductor industry,the resistive random-access memory(RAM) has drawn increasing attention.The discovery of the memristor has brought much attention to this study.Research has focused on the resistive switching characteristics of different materials and the analysis of resistive switching mechanisms.We discuss the resistive switching mechanisms of different materials in this paper and analyze the differences of those mechanisms from the view point of circuitry to establish their respective circuit models.Finally,simulations are presented.We give the prospect of using different materials in resistive RAM on account of their resistive switching mechanisms,which are applied to explain their resistive switchings.     

Interface‐engineered resistive memory using plasma‐modified electrode on polyimide substrate

In this study, we report a low power Ni/GeO_x /TiO_y /TaN resistive random access memory (RRAM) using plasma‐modified electrode. The low sub‐mA switching current, highly uniform switching cycles (only 4% variation for the set) and good high‐temperature current distribution at 125 °C are simultaneously achieved in this RRAM device. Such good performance can be ascribed to interface plasma treatment on TaN electrode where the resulting Ta–N ionic bond increases the oxidation resistance and reduces the oxygen vacancy concentration near TaN interface that is favorable to lower switching power and improve high‐temperature current distribution.

     

Non-volatile memory characteristics of polyimide layers embedded with ZnO nanowires

We fabricated 8 × 8 cross-bar array-type organic non-volatile memory devices of polyimide (PI) layers embedded with ZnO nanowires. The ZnO nanowires were synthesized by chemical vapor deposition and deposited into the PI layers by a solution coating process. The devices of PI layer without ZnO nanowires showed an insulating characteristic without exhibiting any memory behavior. The ZnO nanowires acted as carrier trapping sites in the insulating PI layers for our memory devices. The organic memory devices exhibited write-once-read-many-times-type non-volatile memory characteristics with an excellent ON/OFF switching ratio over 10⁶, good uniformity in cumulative probability, and stability without serious degradation over 10⁴ s.     

基于ITO/聚甲基丙烯酸甲酯/Al的有机阻变存储器SPICE仿真

探索了ITO/PMMA/Al器件的阻变机理及其SPICE电路仿真, 通过优化聚甲基丙烯酸甲酯(PMMA)层退火温度, 器件可实现连续擦-读-写-读操作. 基于不同退火温度PMMA薄膜的表面形貌研究, 构建了单层有机阻变器件的非线性电荷漂移模型, 以及描述该模型掺杂区界面移动的状态方程, 并通过反馈控制积分器建立了SPICE仿真电路. 最后, 代入器件实际测量参数, 得到与器件实际结果基本一致的电流-电压模拟曲线. 结果验证了单层有机器件的阻变机理, 说明该非线性电荷漂移模型的SPICE仿真在有机阻变器件仿真中同样适用. 关键词： 有机阻变存储器 非线性电荷漂移 SPICE仿真     

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.     

Investigation of light programmable non‐volatile memory in an organic phototransistor

Excellent non‐volatile memory characteristics have been demonstrated under the optoelectric conditions for organic phototransistors (OPTs). The high photosensitivity shown as reversible shifts in light‐induced V_TH exhibits a large memory window for programming caused by the excited immobile carriers (electron) trapped as a function of the electrical bias and the light intensity. The long life span of stored electrons also reveals promising behavior with respect to data retention as well as the electrical reliability to serve as a data storage medium with the non‐volatile memory characteristic in OPTs. The V_TH recovery accelerated by the reversible bias stress for the stored charges under irradiation shows that the erasing behavior is clearly brought by the discharge process of long‐lived electrons occupied in deep states. Plausible mechanisms in the energy band are discussed for the programming and erasing process, which provides a fundamental understanding of the intrinsic charge storage behavior in OPTs. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

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.     

Electrical properties and carrier transport mechanisms of nonvolatile memory devices based on randomly oriented ZnO nanowire networks

We report reproducible bipolar resistive memory devices based on Au/ZnO nanowire networks (ZnO NWNs)/ITO, which show a high R_on/R_off ratio (～10⁴) and narrow dispersion of set and reset threshold voltages. The dominant conduction mechanisms of low resistance state and high resistance state were verified by Ohmic behavior and space charge limited current, respectively. The switching mechanism is confirmed in terms of the formation and rupture of conductive filaments, with oxygen vacancies localized on the ZnO NWNs surface involved in. The Au/ZnO NWNs/ITO devices presented in our work show potential applications in the next generation nonvolatile memory field. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Hafnia-based resistive switching devices for non-volatile memory applications and effects of gamma irradiation on device performance

Non-volatile memory (NVM) devices were fabricated as a Metal– Insulator–Metal (MIM) structures by sandwiching Hafnium dioxide (HfO₂) thin film in between two metal electrodes. The top and bottom metal electrodes were deposited by using the thermal evaporation, and the oxide layer was deposited by using the RF magnetron sputtering technique. The Resistive Random Access Memory (RRAM) device structures such as Ag/HfO₂/Au/Si were fabricated and I-V characteristics for the pristine and gamma-irradiated devices with a dose 24?kGy were measured. Further we have studied the thermal annealing effects, in the range of 100°–400°C in a tubular furnace for the HfO₂/Au/Si samples. The X-ray diffraction (XRD), Rutherford Backscattering Spectrometry (RBS), field emission-scanning electron microscopy (FESEM) analysis measurements were performed to determine the thickness, crystallinity and stoichiometry of these films. The electrical characteristics such as resistive switching, endurance, retention time and switching speed were measured by a semiconductor device analyser. The effects of gamma irradiation on the switching properties of these RRAM devices have been studied.     

Ni/HfO_2/Pt阻变单元特性与机理的研究

研究了Ni/HfO2(10 nm)/Pt存储单元的阻变特性和机理.该器件具有forming-free的性质,还表现出与以往HfO2(3 nm)基器件不同的复杂的非极性阻变特性,并且具有较大的存储窗口值(105).存储单元的低阻态阻值不随单元面积改变,符合导电细丝阻变机理的特征.采用X射线光电子能谱仪分析器件处于低阻态时的阻变层HfO2薄膜的化学组分以及元素的化学态,结果表明,Ni/HfO2/Pt阻变存储器件处于低阻态时的导电细丝是由金属Ni导电细丝和氧空位导电细丝共同形成的.     

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.     

基于HfO2的阻变存储器中Ag导电细丝方向和浓度的第一性原理研究

采用基于密度泛函理论的第一性原理方法, 研究了基于HfO₂的阻变存储器中Ag 导电细丝浓度以及方向性. 通过计算Ag杂质5种方向模型的分波电荷态密度等势面图、形成能、 迁移势垒和分波电荷态密度最高等势面值, 发现[-111]方向最有利于Ag导电细丝的形成, 这对器件的开启电压、形成电压和开关比有很大影响. 本文基于最佳的[-111]导电细丝方向, 设计了4 种Ag 浓度结构. 计算4种Ag浓度结构的分波电荷态密度等势面图, 得出Ag浓度低于4.00 at.% 时晶胞结构中无导电细丝形成且无阻变现象. 当Ag浓度从4.00 at.%增加到4.95 at.% 时, 晶胞结构中发现有导电细丝形成, 表明Ag浓度高于4.00 at.%时, 晶胞中可以发生阻变现象. 然而, 通过进一步对比计算这两种晶胞结构中Ag的形成能、分波电荷态密度最高等势面值、总态密度与Ag的投影态密度发现, Ag浓度越大, 导电细丝却不稳定, 并且不利于提高阻变存储器的开关比. 本文的研究结果可为改善基于HfO₂的阻变存储器的性能提供一定理论指导.     

Scaling dependence of memory windows and different carrier charging behaviors in Si nanocrystal nonvolatile memory devices

Based on the charge storage mode,it is important to investigate the scaling dependence of memory performance in silicon nanocrystal(Si-NC) nonvolatile memory(NVM) devices for its scaling down limit.In this work,we made eight kinds of test key cells with different gate widths and lengths by 0.13-μm node complementary metal oxide semiconductor(CMOS) technology.It is found that the memory windows of eight kinds of test key cells are almost the same of about1.64 V @ ±7 V/1 ms,which are independent of the gate area,but mainly determined by the average size(12 nm) and areal density(1.8×10~(11)/cm~2) of Si-NCs.The program/erase(P/E) speed characteristics are almost independent of gate widths and lengths.However,the erase speed is faster than the program speed of test key cells,which is due to the different charging behaviors between electrons and holes during the operation processes.Furthermore,the data retention characteristic is also independent of the gate area.Our findings are useful for further scaling down of Si-NC NVM devices to improve the performance and on-chip integration.