Characteristics and mechanism of nano‐polycrystalline La2O3 thin‐film resistance switching memory

We investigated the characteristics and the mechanism of Pt/La₂O₃/Pt resistance switching memory with a set of measurements. The La₂O₃ films were determined as nano‐poly‐crystalline (diameter of the nanocrystals 5–10 nm) by XRD and HRTEM analysis. The Pt/La₂O₃/Pt device exhibited excellent resistive switching properties, including low switching voltage (<2 V), large low/high resistance ratio (>10⁸), and good cycling endurance property. The conduction mecha‐ nisms of the Pt/La₂O₃/Pt device were revealed with current–voltage characteristics, which are different in the low and high resistance states. Furthermore, XPS analysis and temperature‐dependent resistance measurement in the low resistance state showed that the conducting filaments in the Pt/La₂O₃/Pt device are mainly affected by oxygen defects rather than metallic La. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Coexistence of resistive switching and magnetism modulation in sol-gel derived nanocrystalline spinel Co3O4 thin films

We report the coexistence of resistive switching and magnetism modulation in the Pt/Co₃O₄/Pt devices, where the effects of thermal annealing and film thickness on the resistive and magnetization switching were investigated. The sol-gel derived nanocrystalline Co₃O₄ thin films obtained crack-free surface and crystallized cubic spinel structure. The 110 nm Co₃O₄ film based device annealed at 600 °C exhibited optimum resistive switching parameters. From I–V curves fitting and temperature dependent resistance, the conduction mechanism in the high-voltage region of high resistance state was dominated by Schottky emission. Magnetization-magnetic field loops demonstrated the ferromagnetic behaviors of the Co₃O₄ thin films. Multilevel saturation magnetization of the Co₃O₄ thin films can be easily realized by tuning the resistance states. Physical resistive switching mechanism can be attributed to the rejuvenation and annihilation of conductive filament consisting of oxygen vacancies. Results suggest that Pt/Co₃O₄/Pt device shows promising applications in the multifunctional electromagnetic integrated devices.     

Effects of the compliance current on the resistive switching behavior of TiO2 thin films

Nanocrystalline TiO₂ thin films were fabricated on Pt(111)/Ti/SiO₂/Si substrates by the thermal oxidation of evaporated Ti films. Effects of the compliance current on the resistive switching behavior of the Pt/TiO₂/Pt sandwich structures were studied in detail. The reset current increased when the compliance current increased from 10 mA to 20 mA. When the compliance current exceeded 20 mA, the switching behavior disappeared, which could be attributed to the change of the conducting behavior in the low-resistance state. A resistance change ratio of as high as 10² was obtained between the high-resistance state and the low-resistance state. The study of the effect of compliance current contributes to obtaining stable and reliable resistive switching behavior for nonvolatile memory applications.     

Nonvolatile bipolar resistance switching effects in multiferroic BiFeO3 thin films on LaNiO3-electrodized Si substrates

We report on reversible bipolar resistance switching effects in multiferroic BiFeO₃ thin films without electroforming. The BiFeO₃ thin films with (110) preferential orientation were prepared on LaNiO₃-electrodized Si substrates with a Pt/BiFeO₃/LaNiO₃ device configuration. The resistance ratio of high resistance state (HRS) to low resistance state (LRS) of the devices was as high as three orders of magnitude. The dominant conduction mechanisms of LRS and HRS were dominated by ohmic behavior and trap-controlled space charge limited current, respectively. The resistance switching mechanism of the devices was discussed using a modified Schottky-like barrier model taking into account the movement of oxygen vacancies.     

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.     

Bipolar resistive switching effect and mechanism of solution-processed orthorhombic Bi2SiO5 thin films

Orthorhombic Bi₂SiO₅ thin films with dense surface were synthesized by using a chemical solution deposition method. The crystallized films were first utilized to implement resistive memory cells with Pt/Bi₂SiO₅/Pt sandwich architecture. It exhibited outstanding switching parameters including concentrated distributions of low and high resistance states, uniform switching voltages, cycling endurance, and long retention. Furthermore, the model of formation and rupture of conductive filaments consisted of oxygen vacancies was used to well explain resistive switching behavior. The results revealed that the solution-processed Bi₂SiO₅ thin film devices have great potential for forefront application in nonvolatile memory.     

Composition dependence of unipolar resistance switching in TaOx thin films

Amorphous TaO_x thin films were deposited at different temperatures, and the resistance switching properties of the Pt/TaO_x/Pt structure were investigated. X‐ray photoelectron spectroscopy showed that the amount of Ta₂O₅ in the film decreased and the content of Ta suboxides increased substantially when the growth temperature was increased. Unipolar resistance switching near the anode was stable only for TaO_x film grown at room temperature. The experimental results revealed the critical effect of the film composition on the resistance switching behavior of TaO_x films. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Resistive switching in Au/TiO2/Pt thin film structures on silicon

The conditions and mechanisms of preliminary treatment in strong electric fields (forming) and subsequent resistive switching in Au/TiO₂/Pt thin-film structures on silicon were investigated. The thin TiO₂ films in these structures were prepared by different methods, namely, vacuum evaporation of metallic titanium followed by thermal oxidation in air and radio-frequency cathode sputtering of titanium dioxide from a powder target. The current-voltage and voltage-capacitance characteristics of the structures, as well as the dependences of their conductivity on the time of exposure to a dc voltage of different polarities and on the temperature were measured. The data obtained permitted the conclusion that the physical mechanism underlying the forming process consists in a sharp increase in the density of surface states in TiO₂ films due to the electric breakdown of the Schottky barrier at the contact with the platinum electrode, whereas the resistive switching of the structures is governed by the variation in the population of surface states in the TiO₂ band gap and/or in the defect concentration in the barrier region of the structures acted upon by voltage pulses of different polarities.     

Effect of AlN layer on the resistive switching properties of TiO2 based ReRAM memory devices

The present study reports the resistive switching behaviour in Titanium Dioxide (TiO₂) material, with possible implementations in non volatile memory device. The Cu/TiO₂/Pt memory device exhibit uniform and stable bipolar resistive switching behaviour. The current-voltage (I-V) analysis shows two discrete resistance states, the High Resistance State (HRS) and the Low Resistance State (LRS). The effect of an additional AlN layer in the resistive memory cell is also investigated. The Cu/TiO₂/AlN/Pt device shows a multilevel (tri-state) resistive switching. Multilevel switching is facilitated by ionic and metallic filament formation, and the nature of the formed filaments is confirmed by performing a resistance vs. temperature measurement. The bilayer device shows improved reliability over the single layer device. The formation of high thermal conductive interfacial oxy-nitride (AlON) layer is the main reasons for the enhancement of resistive switching properties in Cu/TiO₂/AlN/Pt cell. The performance of device was measured in terms of endurance and retention, which exhibits good endurance over 10⁵ cycles and long retention time of 10⁵ s at 125 °C. The above result suggests the feasibility of Cu/TiO₂/AlN/Pt devices for multilevel non volatile ReRAM application.     

Effects of electrode polarity on filament ruptures during unipolar resistance switchings

It has been argued that in unipolar resistance switching of Pt/TiO₂/Pt capacitors, conducting filaments should propagate from the cathode interface, and the rupture and recovery of the filaments should occur near the anode interface [Kim et al., Appl. Phys. Lett. 91 (2007) 012907]. We investigated whether this “anode-interface localized filamentary mechanism” holds for Pt/NiO/Pt capacitors. We examined how the electrode polarity affects the rupturing of the conducting filaments in unipolar resistance switching by performing resistance switching measurements on 246 NiO capacitors. We found some dependencies on electrode polarity; however, they were not consistent with the anode-interface localized filamentary mechanism. We also found that Joule heating affects the rupturing process, suggesting that the weakest link of conducting filaments in NiO thin films plays an important role during the rupturing process.     

Top electrode effects on resistive switching behavior in CuO thin films

Top electrode (TE) material on the resistive switching behavior of (TE)/CuO/SnO₂:F/Si substrate has been studied. We investigated the switching properties of CuO films deposited by sol-gel process. Two kinds of top electrode (TE) material on the resistive switching behaviors have been studied. The nonpolar and bipolar resistive switching phenomenon was observed in CuO thin films with different top electrodes. The filamentary mechanism was used to explain the two kinds of resistive switching behaviors. For the Pt/CuO/ATO device, it showed the nonpolar resistive switching where conducting path is formed and disappear due to the oxygen vacancy. For the Cu/CuO/ATO device, the resistance reduction is due to the existing Cu to form conduction Cu-rich pathways. An opposite bias takes the existing Cu back to the Cu electrode to its high-resistance state. CuO thin films are also observed by XRD, AFM and XPS.     

Fatigue behavior of the electric pulse induced reversible resistance change effect in AgLa0.7Ca0.3MnO3Pt sandwich

La_0.7Ca_0.3MnO₃ (LCMO) films were prepared by a chemical solution deposition method on a Pt/Si substrate. Reversible resistance switching by electric pulses is observed in Ag/LCMO/Pt sandwich structures. This R switching behavior shows fatigue with time and applied pulse number. The voltage threshold, the stability and the durability of the reversible R switching are improved by annealing the LCMO films under oxygen atmosphere. PACS 73.40.-c; 75.70.Cn; 81.15.-z     

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.     

Top electrode-dependent resistance switching behaviors of lanthanum-doped ZnO film memory devices

Lanthanum-doped ZnO (Zn_0.99La_0.01O) polycrystalline thin films were deposited on Pt/Ti/SiO₂/Si substrates by a chemical solution deposition method. Metal/La-doped ZnO/Pt sandwich structures were constructed by depositing different top electrodes (Ag and Pt). Unipolar switching and bipolar switching characteristics were investigated in Pt/La-doped ZnO/Pt and Ag/La-doped ZnO/Pt structures, respectively. Compared with the undoped devices (Pt/ZnO/Pt and Ag/ZnO/Pt), the La-doped devices exhibits superior resistive switching performances, such as narrow distribution of the resistive switching properties (R _ON, R _OFF, V _Set, and V _Reset), higher R _OFF/R _ON ratio and sharp switching transition.     

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.     

Unipolar resistive switching properties of amorphous Pr0.7Ca0.3MnO3 films grown on a Pt/Ti/SiO2/Si substrate

Amorphous Pr_0.7Ca_0.3MnO₃ (APCMO) films were grown on a Pt/Ti/SiO₂/Si (Pt–Si) substrate at temperatures below 500 °C and the Pt/APCMO/Pt–Si device showed unipolar resistive switching behavior. Conduction behavior of the low resistance state (LRS) of the Pt/APCMO/Pt–Si device followed Ohm's law, and the resistance in LRS was independent of the size of the device, indicating that the conduction behavior in LRS can be explained by the presence of the conductive filaments. On the other hand, the resistance in the high resistance state (HRS) decreased with increasing the device size, and the conduction mechanism in the HRS was explained by Schottky emission.     

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.     

SiO_x(x=1.3)薄膜的优化阻变特性与退火温度的关系探究

采用电子束蒸发技术在Si衬底上制备了亚氧化硅SiOx(x=1.3)薄膜,研究了不同温度热退火处理的SiOx薄膜作为阻变层的ITO/SiOx/Si/Al结构的阻变特性.研究发现,在电极尺寸相同的条件下,随着退火温度的增加,该结构的高低阻态比显著提高,最高可达109.X射线光电子能谱和电子顺磁共振能谱的分析表明,不同退火温度下形成的不同价态的硅悬挂键是低阻态下细丝通道的主要来源.椭偏仪的测试结果表明,经过热退火处理的SiOx薄膜折射率的增大是导致高阻态下器件电阻增大的原因.     

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.     

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.