Influence of oxygen treatment and temperature on electrical properties of the epitaxial Nb-doped SrTiO3 films on silicon

We have fabricated the epitaxial Nb-doped SrTiO3(NbSTO) thin films on Si substrates using a TiN film as the buffer layer.The oxygen-treatment and temperature dependence of electrical properties has been investigated.Oxygen treatment showed the surface change of NbSTO films has immense influence on the resistance switching.The resistance ratio of two resistance states decreased after oxygen treatment.With tested-temperature rising,the resistance and resistance ratio of two resistance states increased.The resistance switching of Pt/NbSTO junction as a function of oxygen-treatment and temperature can be explained by the charge trapping and detrapping process in the Pt/NbSTO interface,which will help understand the resistance switching mechanism of oxides.     

La施主掺杂SrTiO_3单晶的阻变性能研究

以La施主掺杂SrTiO_3(La STO)单晶为样品,制备了Pt/LaSTO/In结构存储器件.通过一系列电学测试,发现该器件具有稳定的多级阻变现象,最大开关比为10~4;高低阻电流-电压关系曲线的拟合分析表明,高阻时存在界面势垒,而低阻时满足电子隧穿模型特性.电子顺磁共振研究表明LaSTO单晶内存在带正电的空穴缺陷中心.综合分析证明器件的高低阻之间的转变由界面空位缺陷导致的电子俘获与去俘获引起.此外发现光照会对LaSTO单晶的阻值产生影响.该实验结果为LaSTO单晶在阻变存储器件中的应用提供了理论和技术指导.     

Low power consumption bipolar resistive switching characteristics of ZnO-based memory devices

Ag/ZnO/Pt structure resistive switching devices are fabricated by radio frequency (RF) magnetron sputtering at room temperature. The memory devices exhibit stable and reversible resistive switching behavior. The ratio of high resistance state to low resistance state can reach as high as 10 2 . The retention measurement indicates that the memory property of these devices can be maintained for a long time (over 10 4 s under 0.1-V durable stress). Moreover, the operation voltages are very low, -0.4 V (OFF state) and 0.8 V (ON state). A high-voltage forming process is not required in the initial state, and multi-step reset process is demonstrated. Resistive switching device with the Ag/ZnO/ITO structure is constructed for comparison with the Ag/ZnO/Pt device.     

Ti/HfO2/Pt阻变存储单元中的氧空位聚簇分布

为了研究阻变存储器导电细丝的形成位置和分布规律, 使用X射线光电子能谱研究了Ti/HfO₂/Pt阻变存储器件单元中Hf 4f的空间分布, 得到了阻变层的微结构信息. 通过I-V测试, 得到该器件单元具有典型的阻变特性; 通过针对Hf 4f的不同深度测试, 发现处于低阻态时, 随着深度的增加, Hf⁴⁺化学组分单调地减小; 而处于高阻态和未施加电压前, 该组分呈现波动分布; 通过Hf⁴⁺在高阻态和低阻态下组分含量以及电子能损失谱分析, 得到高阻态下Hf⁴⁺组分的平均含量要高于低阻态; 另外, 高阻态和低阻态下的O 1s谱随深度的演变也验证了Hf⁴⁺的变化规律. 根据实验结果, 提出了局域分布的氧空位聚簇可能是造成这一现象的原因. 空位簇间的链接和断裂决定了导电细丝的形成和消失. 由于导电细丝容易在氧空位缺陷聚簇的地方首先形成, 这一研究为导电细丝的发生位置提供了参考.     

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.     

Brain-like synaptic memristor based on lithium-doped silicate for neuromorphic computing

Artificial synapse is one of the potential electronics for constructing neural network hardware. In this work, Pt/LiSiO_x/TiN analog artificial synapse memristor is designed and investigated. With the increase of compliance current (C. C.) under 0.6 mA, 1 mA, and 3 mA, the current in the high resistance state (HRS) presents an increasing variation, which indicates lithium ions participates in the operation process for Pt/LiSiO_x/TiN memristor. Moreover, depending on the movement of lithium ions in the functional layer, the memristor illustrates excellent conduction modulation property, so the long-term potentiation (LTP) or depression (LTD) and paired-pulse facilitation (PPF) synaptic functions are successfully achieved. The neural network simulation for pattern recognition is proposed with the recognition accuracy of 91.4%. These findings suggest the potential application of the LiSiO_x memristor in the neuromorphic computing.     

Fabrication of printed memory device having zinc-oxide active nano-layer and investigation of resistive switching

An all printed resistive memory device, a 9-bit memristor, has been presented in this study consisting of 3 × 3 memristor crossbars deposited via electrohydrodynamic inkjet printing process at room conditions. Transparent zinc oxide active nano-layers, directly deposited by electrospray process, are sandwiched between the crossbars to complete the metal–insulator metal structure consisting of copper–zinc oxide–silver, where Cu and Ag are used as bottom and top electrodes respectively. The 9-bit memristor device has been characterized using current–voltage measurements to investigate the resistive switching phenomenon thereby confirming the memristive pinched hysteresis behavior signifying the read–write and memory characteristics. The memristor device showed a current bistability due to the existence of metal–oxide layer which gives rise to oxygen vacancies upon receiving the positive voltage hence breaking down into doped and un-doped regions and a charge transfer takes place. The maximum ON/OFF ratio of the current bi-stability for the fabricated memristor was as large as 1 × 10³, and the endurance of ON/OFF switchings was verified for 500 read–write cycles. The metal–insulator–metal structure has been characterized using X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscope techniques.     

Reversible resistance switching properties in Ti-doped polycrystalline Ta2O5 thin films

Unipolar reversible resistance switching effects were found in 5 at% Ti-doped polycrystalline Ta₂O₅ films with the device structure of Pt/Ti–Ta₂O₅/Pt. Results suggest that the recovery/rupture of the conductive filaments which are involved in the participation of oxygen vacancies and electrons leads to the resistance switching process. Ti-doped Ta₂O₅ thin films possess higher resistance whether in low-resistance state or high-resistance state and higher resistance switching ratio than Ta₂O₅ thin films, where Ti addition plays an important role in the resistance switching process by suppressing the migration of oxygen vacancies via forming an electrically inactive Ti/O–vacancy complex. Excellent retention properties of the high and low resistances under constant stress of applied voltage were obtained.     

SPICE modeling of memristors with multilevel resistance states

With CMOS technologies approaching the scaling ceiling, novel memory technologies have thrived in recent years, among which the memristor is a rather promising candidate for future resistive memory (RRAM). Memristor’s potential to store multiple bits of information as different resistance levels allows its application in multilevel cell (MCL) technology, which can significantly increase the memory capacity. However, most existing memristor models are built for binary or continuous memristance switching. In this paper, we propose the simulation program with integrated circuits emphasis (SPICE) modeling of charge-controlled and flux-controlled memristors with multilevel resistance states based on the memristance versus state map. In our model, the memristance switches abruptly between neighboring resistance states. The proposed model allows users to easily set the number of the resistance levels as parameters, and provides the predictability of resistance switching time if the input current/voltage waveform is given. The functionality of our models has been validated in HSPICE. The models can be used in multilevel RRAM modeling as well as in artificial neural network simulations.     

Resistive switching characteristic and uniformity of low-power HfO_x-based resistive random access memory with the BN insertion layer

In this letter,the Ta/HfO_x/BN/TiN resistive switching devices are fabricated and they exhibit low power consumption and high uniformity each.The reset current is reduced for the HfO_x/BN bilayer device compared with that for the Ta/HfO_x/TiN structure.Furthermore,the reset current decreases with increasing BN thickness.The HfO_x layer is a dominating switching layer,while the low-permittivity and high-resistivity BN layer acts as a barrier of electrons injection into TiN electrode.The current conduction mechanism of low resistance state in the HfO_x/BN bilayer device is space-chargelimited current(SCLC),while it is Ohmic conduction in the HfO_x device.     

“Positive” and “negative” electric-pulse-induced reversible resistance switching effect in Pr<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> films

“Negative” electric-pulse-induced reversible resistance (EPIR) switching phenomenon was found in In/PCMO/Pt sandwich, in which the high resistance can be written with positive voltage pulses, and the low resistance can be reset using negative voltage pulses (the positive voltage direction is defined as going from the top electrode to the bottom electrode). This is just the opposite from the “positive” EPIR effect in Ag/PCMO/Pt sandwich, in which the high resistance can be written only with negative voltage pulses, and the low resistance can be reset using positive voltage pulses. The I–V hysteresis curves of In/PCMO/Pt and Ag/PCMO/Pt sandwiches also show opposite directions, i.e., counterclockwise and clockwise under a negative voltage region for indium and Ag electrode systems, respectively. C–V characteristics show that the barrier does not exist in Ag/PCMO/Pt sandwich, while In/PCMO/Pt sandwich exhibits an obvious Schottky-like barrier. We suggest that in the negative EPIR behavior in In/PCMO/Pt structure, the resistance states are mainly controlled changing the Schottky-like barrier at the interface with the weak effect of carrier trapping process, while the positive EPIR behavior in Ag/PCMO/Pt sandwich mainly depends on the carrier trapping process at the interface. PACS 72.80.-r; 73.40.-C; 75.70.-i     

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.     

Cost-effective fabrication of memristive devices with ZnO thin film using printed electronics technologies

A prototype memristive device has been presented in this paper, for which the top and bottom electrodes have been fabricated through a simple and cost-effective technique, i.e. electrohydrodynamic printing. For deposition of the bottom electrode pattern, a silver ink containing 60 wt% silver by content was subjected to controlled flow through a metal capillary exposed to an electric field at the ambient temperature to generate an electrohydrodynamic jet, thereby depositing uniform patterns of silver on glass substrate at a constant substrate speed. The top electrode has been deposited in a similar fashion. In between the top and bottom electrodes, a uniform layer of ZnO is fabricated using spin-coating technique. The nanoscale ZnO memristor stack has a channel length of 370 μm and channel width of 82 μm. A memristor thus fabricated was characterized and its current voltage curves were analyzed. The device showed a typical nonvolatile resistive switching behavior present in memristor devices thus highlighting the EHD printed patterning as a reliable method for the fabrication of memory devices.     

忆阻逾渗导电模型中的初态影响

逾渗网格模型是当前忆阻器件机理分析研究领域的热点之一, 但现有模型缺乏对初态设定的讨论. 本文对逾渗网格模型进行了简化, 并基于此, 通过电压激励步进的方式, 研究了不同初态对单极性忆阻开关元件中逾渗导电通道形成的影响, 分析了形成通道的动态过程以及相应的物理意义, 验证了忆阻开关元件高低阻态的阻值实际表现为高斯分布而非理想双值稳态; 而不同初态条件下, 忆阻开关元件导电通道的形状存在着不同的"树形"结构, 进而影响着其阻值的分布. 研究成果有助于进一步揭示忆阻器尚未明确的导电机理, 为今后对具体不同类型的忆阻元件的初态分析提供指导性作用. 关键词： 忆阻器 开关元件 逾渗模型 初态分析     

Charge transport and bipolar switching mechanism in a Cu/HfO_2/Pt resistive switching cell

Bipolar resistance switching characteristics are investigated in Cu/sputtered-HfO_2/Pt structure in the application of resistive random access memory(RRAM).The conduction mechanism of the structure is characterized to be SCLC conduction.The dependence of resistances in both high resistance state(HRS) and low resistance state(LRS) on the temperature and device area are studied.Then,the composition and chemical bonding state of Cu and Hf at Cu/HfO_2 interface region are analyzed by x-ray photoelectron spectroscopy(XPS).Combining the electrical characteristics and the chemical structure at the interface,a model for the resistive switching effect in Cu/HfO_2/Pt stack is proposed.According to this model,the generation and recovery of oxygen vacancies in the HfO_2 film are responsible for the resistance change.     

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.     

Understanding the conduction and switching mechanism of Ti/AlOx/TaOx/Pt analog memristor

In this letter, the conduction and bipolar switching mechanism of the ultrathin AlO_x(3 nm)/TaO_x(5 nm) memristor are investigated through the electrical characterization and elemental analysis. The AlO_x/TaO_x memristor exhibits high uniformity and excellent analog property after initial reset process. The following experiments and analyses demonstrate that the switching behavior could take place over the whole area of TaO_x/Pt interface and is dominated by the tunneling barrier modulation induced by oxygen ions migration.     

Colossal resistance switching in Pt/BiFeO3/Nb:SrTiO3 memristor

We report reversible resistance switching behaviors in Pt/BiFeO₃/Nb:SrTiO₃ memristor. The resistance of the junctions can be tuned up to about five orders of magnitude by applying voltage pulses at room temperature, which exhibits excellent retention and anti-fatigue characteristics. The high performances are promising for employing ferroelectric junctions in nonvolatile memory and logic devices. The nonvolatile resistance switching behaviors could be attributed to the formation and annihilation of trap centers in the BFO films, resulting in Poole–Frenkel emission for low resistance state and the thermionic emission for high resistance state, respectively.     

Flexible NiO nanocrystal-based resistive memory device fabricated by low-temperature solution-process

In this study, a nickel oxide (NiO) nanocrystal (NC) based flexible resistive memory device is demonstrated at temperature as low as 180 °C by ligand exchange process. The fabricated device for flexible application with structure Ni/NiO/Ni on PI substrate exhibits excellent switching characteristics with low set/reset voltages and stable resistance values in both ON and OFF states for over 100 switching cycles of memory operation. Also, this flexible memory device shows stable resistive switching properties under compressive stress with bending radius to 10 mm and consecutive bending cycles. The ReRAM fabricated by a low-temperature solution-process shows potential for next generation flexible electronics.     

Enhanced resistance switching stability of transparent ITO/TiO2/ITO sandwiches

We report that fully transparent resistive random access memory(TRRAM) devices based on ITO/TiO2/ITO sandwich structure,which are prepared by the method of RF magnetron sputtering,exhibit excellent switching stability.In the visible region(400-800 nm in wavelength) the TRRAM device has a transmittance of more than 80%.The fabricated TRRAM device shows a bipolar resistance switching behaviour at low voltage,while the retention test and rewrite cycles of more than 300,000 indicate the enhancement of switching capability.The mechanism of resistance switching is further explained by the forming and rupture processes of the filament in the TiO 2 layer with the help of more oxygen vacancies which are provided by the transparent ITO electrodes.