Au/SrTiO3/Au界面电阻翻转效应的低频噪声分析

本文研究了Au/SrTiO₃/Au三明治结构中的双极电阻翻转效应, 观察到高、低阻态的电阻弛豫现象. 低频噪声测量表明高、低阻态的电阻涨落表现出1/f行为. 对比试验表明, 高阻态的低频噪声来源于反向偏置肖特基势垒和氧空位的迁移, 强度较大, 低阻态的噪声则源于类欧姆接触底电极区域的氧空位迁移导致的载流子涨落, 强度较低. 同时, 界面上氧空位浓度的弛豫导致了高、低阻态的弛豫过程. 关键词： 电阻翻转效应 低频噪声 氧空位     

Nanoscale resistive switching in SrTiO3 thin films

The local conductivity of SrTiO₃ thin films epitaxially grown on SrRuO₃‐buffered SrTiO₃ single crystals has been investigated in detail with an atomic force microscope equipped with a conducting tip (LC‐AFM). These experiments demonstrate that the conductivity of SrTiO₃ thin films originates from nanoscale well‐conducting filaments connecting the surface to the SrRuO₃ bottom electrode. The electrical conduction of the filaments is shown to be reversibly modulated over several orders of magnitude by application of an appropriate electrical field. We analyze the resistive switching by addressing individual filaments with the AFM tip as well as by scanning areas up to the µm scale. Temperature dependent measurements reveal that resistive switching on a macroscopic scale can be traced down to the insulator‐to‐metal transition of the independently switchable filaments. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Characteristics of the bipolar resistive switching behavior in memory device with Au/ZnO/ITO structure

In this work, reproducible and stable bipolar resistive switching behavior without the requirement of forming process is observed in the memory device with Au/ZnO/ITO structure. It shows a high R_on/R_off ratio, where R_on and R_off are the resistance at low resistance state (LRS) and high resistance state (HRS), respectively. The dominated transport mechanisms for LRS and HRS are related to space charge limited current and Ohmic behavior, respectively. This bipolar resistive behavior is attributed to the formation and rupture of conducting filaments which are constructed with oxygen vacancies. The Au/ZnO/ITO device discussed in this work shows huge potential applications in the next generation nonvolatile memory field.     

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.     

Any-polar resistive switching behavior in Ti-intercalated Pt/Ti/HfO2/Ti/Pt device

The special any-polar resistive switching mode includes the coexistence and stable conversion between the unipolar and the bipolar resistive switching mode under the same compliance current. In the present work, the any-polar resistive switching mode is demonstrated when thin Ti intercalations are introduced into both sides of Pt/HfO₂/Pt RRAM device. The role of the Ti intercalations contributes to the fulfillment of the any-polar resistive switching working mechanism, which lies in the filament constructed by the oxygen vacancies and the effective storage of the oxygen ion at both sides of the electrode interface.     

Electro- and opto-resistive switching behaviors of the Nb doped SrTiO3 films

Nb doped SrTiO₃ (Nb:STO) films were deposited on (100) SrTiO₃ substrates using a pulse laser deposition technique. The effects of deposition pressure on their structural, electrical and optical properties were investigated. Decrease in deposition pressure lead to decrease in grain size and average surface roughness. Various optical parameters such as refractive index, extinction coefficient, and band gap were calculated by applying the envelop or extrapolation methods using the transmittance data obtained from a UV/Vis spectrophotometer. A systematic decrease in resistivity and increment of negative charged carriers was observed with decreasing deposition pressure. Experimental results exhibited electro- and opto-resistive switching behaviors with a resistive transition from high resistance state to low resistance state on the application of current-pulse or UV light. Multi-level resistance states have also been demonstrated using a train of current pulses of different magnitudes or a simultaneous application of current and UV light. Such observed phenomenon makes Nb:STO a potential candidate to be used in future for the fabrication of multi-level memory devices and transparent thin films transistors.     

Interface-engineered resistive switching in Ag/SrTiO3/Nd0.7Ca0.3MnO3/YBa2Cu3O7 heterostructures

For Ag/Nd_0.7Ca_0.3MnO₃/YBa₂Cu₃O₇ (Ag/NCMO/YBCO) heterostructures, we investigate effects of an SrTiO₃ (STO) buffer layer inserted into the Ag/NCMO interface upon the room-temperature resistive switching. In comparison with the non-buffered (Ag/NCMO/YBCO) structure, the insertion of the STO buffer layer can greatly enhance the electric-field-induced-resistance (EPIR) effect. The STO-buffered (Ag/STO/NCMO/ YBCO) device can be switched on-and-off between the higher to lower resistance states at an EPIR ratio of 253% with pulsed voltage ±1.5 V and 405% with pulsed voltage ±3.0 V. The enhanced EPIR ratio is attributed to a combined effect of the migration of oxygen vacancies near the interface and ferroelectric polarization of the STO buffer.     

Metal and annealing atmospheres dependence of resistive switching in metal/Nb0.7wt%-SrTiO3 interfaces

We investigated the resistance switching (RS) effect of metal/Nb-doped SrTiO₃ interfaces under different treating conditions. Two types of I–V characteristics appeared due to the modification of Schottky-like barrier and the formation of insulating layer. According to X-ray photoelectron spectroscopy analysis, the change in interface potential barrier was contributed to the migration of oxygen vacancies and electrons trapping/detrapping of carriers in the vicinity of interface. Nonlinear fitting is applied to the curves to study the conduction mechanism of metal/NSTO. For “barrier height” style, Schottky emission and Poole–Frenkel (P–F) emission are dominating; for “insulating layer” style, space-charge-limited current, controls the conduction.     

Bipolar tri-state resistive switching characteristics in Ti/CeO_x/Pt memory device

Highly repeatable multilevel bipolar resistive switching in Ti/Ce Ox/Pt nonvolatile memory device has been demonstrated. X-ray diffraction studies of Ce O2 films reveal the formation of weak polycrystalline structure. The observed good memory performance, including stable cycling endurance and long data retention times（〉10^4s） with an acceptable resistance ratio（～10^2）, enables the device for its applications in future non-volatile resistive random access memories（RRAMs）. Based on the unique distribution characteristics of oxygen vacancies in Ce Ox films, the possible mechanism of multilevel resistive switching in Ce Ox RRAM devices has been discussed. The conduction mechanism in low resistance state is found to be Ohmic due to conductive filamentary paths, while that in the high resistance state was identified as Ohmic for low applied voltages and a space-charge-limited conduction dominated by Schottky emission at high applied voltages.     

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.     

Resistance switching effect in LaAlO3/Nb-doped SrTiO3 heterostructure

The authors report on the fabrication and electronic transport property of LaAlO₃/Nb-doped SrTiO₃ heterostructure. The current–voltage curves of this heterostructure show hysteresis and a remarkable resistance switching behavior, which increase dramatically with decreasing temperature. Multiresistance states were realized by voltage pulses with different amplitudes and polarities and the ratio of the electrical pulse induced resistance change is larger than 10⁴. More interestingly, the relaxation of junction current after switching follows the Curie–von Schweidler law J∝t ⁻ⁿ with an exponential increase of n with temperature. The results were discussed in terms of the trap-controlled space charge limited conduction process via defects near the interface of the heterostructure.     

Conversion of two types of bipolar switching induced by the electroforming polarity in Au/NiO/SrTiO3/Pt memory cells

The resistive switching characteristics of Au/p-NiO/n-SrTiO₃(STO)/Pt memory cells are investigated. Two types of bipolar switching with opposite polarity coexist in the cell and can be repeatedly adjusted by the electroforming polarity. The conduction mechanisms of low resistance and high resistance states are dominated by electron tunneling and interface barrier effect, respectively. The impact of electroforming polarity on the switching mechanism and the distribution of defects are discussed. The results indicate that these two types of switching originate from a variation of interface barrier respectively at the NiO/STO p–n junction and STO/Pt Schottky contact.     

Formation of Schottky-type metal/SrTiO3 junctions and their resistive properties

Motivated by the successful use of strontium titanate with different doping metals for memory cells on the basis of resistive switching and the recent findings on the major importance of oxygen vacancy redistribution in this compound, the present work shows the possibility of a non-volatile resistance change memory based on vacancy-doped SrTiO₃. The formation of corresponding metal/SrTiO_3−δ junctions (δ>0) in an electric field will be discussed as well as the switching between ohmic and Schottky-type contact behavior. A notable hysteresis in the current–voltage characteristics is used to carry out Write, Read, and Erase operations exemplifying the memory cell properties of such junctions. But whereas the electric field-induced formation of Schottky-type junctions is explainable by oxygen vacancy redistribution, the resistive switching needs to be discussed in terms of vacancies serving as electron trap states at the metal/oxide interface.     

Conductance spectroscopy of resistive switching Pt/Nb:STO single crystal Schottky junctions in air and vacuum

We investigated the admittance spectra of resistive switching Pt/Nb-doped SrTiO₃ single-crystal junctions at different resistance states in air and vacuum. The analyses showed that the carrier lifetime at the traps was largely varied depending on the resistance state, indicating the surface potential modification. The ambient dependence suggested that the charges at the trap states were affected by the oxygen adsorption/desorption at the surface. The conductance spectroscopy method clearly revealed the importance of the interface trap states in the resistive switching behavior.     

Correlation between crystallinity and resistive switching behavior of sputtered WO3 thin films

The as-deposited WO₃ thin films were post-annealed at different temperatures (300 °C and 600 °C) in air to investigate a correlation between crystallinity and switching behavior of WO₃ thin films. Associating the results of XRD, FTIR, XPS and FESEM measurements, the annealing-caused crystallinity change contributes to the variation of the switching behaviors of the WO₃ thin films. The as-deposited WO₃ films with low crystalline structure are preferred for random Ag conducting path, resulting in large switching ratio but fluctuating I–V hysteresis, whereas the annealed WO₃ films with crystallized compact structure limits Ag conducting path, favoring the stable I–V hysteresis but small switching ratio. It is therefore concluded that electrochemical redox reaction-controlled resistance switching depends not only on electrode materials (inert and reactive electrodes) but also on crystallinity of host oxide.     

Transport properties of Pr0.7Ca0.3MnO3/Nb:SrTiO3 heterojunctions

Current–voltage (J–V) characteristics of epitaxial hetero-junctions composed of Pr_0.7Ca_0.3MnO₃ and Nb:SrTiO₃ were studied under forward and reversed bias conditions. Detailed analysis showed that the J–V characteristics of these heterojunctions can be well-fitted by the thermally-assisted tunnelling model. While the dielectric constant of Nb:SrTiO₃ extracted under the forward bias was about one order of magnitude smaller than that of bulk SrTiO₃, the value obtained under reverse bias was very close to that of the bulk SrTiO₃. The result can be explained by the existence of an interface layer on the Nb:SrTiO₃ substrate with a smaller effective dielectric constant. The current finding suggested that the properties of interface layer should be taken into account in order to accurately simulate the J–V characteristics of such heterojunctions.     

Ferroelastic transition and the relaxor state in SrTiO3-PbMg1/3Nb2/3O3

Dielectric and acoustic properties of the (1 ? x)SrTiO₃?xPbMg_1/3Nb_2/3O₃ solid solutions (0 ? x ? 1) have been studied at temperatures ranging from 4.2 to 350 K. It has been found that the improper ferroelastic transition exists up to a concentration x = 0.4 and that the phase transition temperature grows with x increasing from 0 to 0.4. As the concentration of the second component is further increased, the transition becomes suppressed by the relaxor phase forming at x _c ≈ 0.22. The results obtained are discussed in terms of the current concepts of relaxor ferroelectrics.