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.     

Bipolar resistive switching in BiFe_(0.95)Zn_(0.05)O_3 films

Bipolar resistive switching is studied in BiFe0.95Zn0.05O3 films prepared by pulsed laser deposition on （001） SrTiO3 substrate, with LaNiO3 as the bottom electrode, and Pt as the top electrode. Multiple steps of resistance change are ob- served in the resistive switching process with a slow voltage sweep, indicating the formation/rupture of multiple conductive filaments. A resistive ratio of the high resistance state （HRS） to the low resistance state （LRS） of over three orders of mag- nitude is observed. Furthermore, the conduction mechanism is confirmed to be space-charge-limited conduction with the Schottky emission at the interface with the top Pt electrodes in the HRS, and Ohmic in the LRS. Impedance spectroscopy demonstrates a conductive ferroelectric/interfacial dielectric 2-layer structure, and the formation/rupture of the conductive filaments mainly occurs at the interfacial dielectric layer close to the top Pt electrodes.     

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.     

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.     

Analysis of resistive switching behaviors of vanadium oxide thin film

We demonstrated the polarization of resistive switching for Cu/VO_x/Cu memory cell. Switching behaviors of Cu/VO_x/Cu cell were tested by semiconductor device analyzer (Agilent B1500A), and the relative micro-analysis of I-V characteristics of VO_x/Cu was characterized by conductive atomic force microscope (CAFM). The I-V test results indicated that both 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 exhibited evidences of the formation and rupture of filaments based on positive or negative voltage. Cu/VO_x/Cu sandwiched structure exhibits a reversible resistive switching behavior and shows potential applications in the next generation nonvolatile memory field.     

Temperature-dependent resistive switching behavior in the structure of Au/Nb:SrTiO3/Ti

Au/Nb:SrTiO₃/Ti structures were fabricated by depositing Au and Ti electrodes on a single crystal 0.5 wt% Nb:SrTiO₃ (NSTO) using rf-magnetron sputtering technique. Resistive switching properties at different temperature were investigated. The Ti/NSTO interface was ohmic contact, which indicated that the resistive switching behavior was attributed to Au/NSTO interface. The resistive switching behavior happened only at the temperature above 180 K, which was possibly caused by the increase of Schottky barrier height with the increase of temperature. The structure showed a semiconductor behavior at high-resistance state (HRS) and a metallic behavior at low-resistance state (LRS). The switching conduction mechanism of Au/NSTO/Ti device is primarily described as space-charge-limited conduction (SCLC) according to the electrical transport properties measurement.     

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.     

电泳沉积法制备的Zn1-xCxO薄膜的结构及阻变性能

采用电泳沉积法在FTO导电玻璃基片上制备Zn_1-xCu_xO薄膜,并对其微观结构、光致发光谱、伏安特性、保持特性和转换电压分布进行探讨。PL谱表明,Cu掺杂在禁带中引入深受主能级,降低氧空位浓度,导致ZnO薄膜的紫外发光、蓝光发光和绿光发光峰强度降低。所得薄膜的晶粒细小、致密、均匀,具有稳定的双极性阻变特性,开关比R_off/R_on最高达到10⁵,其低阻态（LRS）和高阻态（HRS）的阻变机理分别符合欧姆定律和空间电荷限制传导理论。器件经100次循环测试后开关比无明显变化,呈现出较为良好的抗疲劳特性。Cu掺杂对LRS影响不大,但显著改善了HRS的分散性以及转换电压V_SET的分散性。当Cu掺杂量x=0.04时,器件表现出良好的综合性能：R_off≈10⁶ Ω,R_off/R_on≈10⁴,V_SET介于0.4~3.03 V之间。     

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.     

Resistive switching functional quantum-dot light-emitting diodes

This study demonstrates quantum-dot light-emitting diodes (QD-LEDs) with a function of resistive switching memory, capable of on/off operation at the same driving current depending on reset/set state. The QD-LEDs were fabricated by spin-coating process and experienced two different annealing conditions, which yielded defective or less-defective V₂O_5–x layer. One of the annealing conditions produced QD-LEDs with the unusual electrical behaviors of negative differential resistance (NDR), capacitance oscillation, and voltage–current hysteresis curves, signifying so-called resistive switching characteristics. X-ray and ultraviolet photoelectron spectroscopies were used to examine the chemical state of the differently annealed V₂O_5–x layers. The less stoichiometric V₂O_5–x layer was found to be responsible for the resistive switching behaviors of the NDR and the low and high resistance states (LRS and HRS, respectively). We discuss the LRS/HRS of V₂O_5–x for resistive switching in terms of a conductive filament effect, induced by microstructural changes caused by oxygen drift and vacancy annihilation processes in the high defect density V₂O_5–x layer.     

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.     

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.     

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.     

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.     

Resistive switching behavior and mechanism of room-temperature-fabricated flexible Al/TiS2-PVP/ITO/PET memory devices

The mixture of two-dimensional (2D) TiS₂ nanoflakes and polyvinylpyrrolidone (PVP) exhibits a nonvolatile, bipolar resistive switching behavior with a low resistance state (LRS)/high resistance state (HRS) current ratio of ～10² in the devices with a flexible Al/TiS₂-PVP/indium tin oxide (ITO)/polyethylene terephthalate (PET) structure. The polymer-assistant liquid-phase exfoliation of 2D nanoflakes from TiS₂ bulk material is processed in low-boiling solvent. And the fabrication process of these devices is performed entirely at room temperature. Such an energy-saving and scalable production process indicates a huge potential of large-scale industrial application. The AFM and TEM characterizations showed that the exfoliated 2D TiS₂ are flakes at micrometer scale with a layer-number of mostly 7 or 8. Both the HRS and the LRS can be kept for more than 10⁴ s. The endurance of devices was obtained over 100 direct current (DC) sweeping cycles with remarkable separations between different resistive states. The distributions of writing (set) and erasing (reset) voltages show that set and reset voltages are small (<2 V). Also, the resistive switching characteristics of the devices are stable during 1000 bending cycles. The switching behavior is explained by the thinning and recovery of Schottky barriers within devices.     

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.     

Bipolar switching properties of amorphous TiO2 thin film grown on TiN/Si substrate

Pt/TiO₂/TiN device with the amorphous TiO₂ film grown at room temperature under an oxygen partial pressure of 1.0 mTorr showed reliable bipolar switching behavior. During the electroforming process, a large number of oxygen vacancies formed in the TiO₂ film and accumulated at the Pt/TiO₂ interface. The barrier height of the Schottky contact of the Pt/TiO₂ interface was reduced owing to the presence of these oxygen vacancies, resulting in the low-resistance state (LRS). Moreover, oxygen ions diffused into the TiN electrode during the electroforming and set processes. On the other hand, the oxygen ions in the TiN electrode diffused out and reacted with oxygen vacancies in the TiO₂ film during the reset process, and the device changed from the LRS to the high-resistance state (HRS). Conduction in the LRS and HRS can be attributed to Ohmic conduction and the trap controlled space charged limited mechanism, respectively.     

Role of sweep direction and substrate work function on resistive switching of titanium-di-oxide [TiO2] nanoparticles

The resistive switching mechanism in titanium-dioxide nanoparticles (TiO₂ NP) is studied using the current-voltage (I–V) measurements. The TiO₂ NP are spin-coated on different substrates like FTO, ITO, Gold, and p-Silicon. The I–V measurements are carried out by changing the initial potential of the substrates to either 0 V (sweep1) or −1 V (sweep2). Resistive switching (RS) was observed only for FTO/TiO₂ NP and ITO/TiO₂NP devices in sweep1 direction. Whereas, in sweep2 direction, no such RS was observed in any of the devices. The detailed I–V analysis infers the Ohmic conduction followed by space charge limited conduction (SCLC) during the RS forming process for FTO/TiO₂ NP and ITO/TiO₂NP devices. The Au and p-Si substrates act as blocking contact for TiO₂ and exhibit Schottky/thermionic emission at lower voltages and SCLC at higher voltages. The TiO₂ NP coated on p-Si substrate exhibits rectifying behaviour with a current ratio of 3 orders of magnitude.     

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.     

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.