Unipolar resistive switching behavior of BiFeO3 thin films prepared by chemical solution deposition

Bismuth ferrite (BiFeO₃, BFO) thin films were spin-coated on Pt/Ti/SiO₂/Si substrates by a chemical solution deposition method. The ferroelectric BFO films annealed at 500 °C and 550 °C were found to possess unipolar resistive switching behaviors. The resistance ratio of the high resistance state (HRS) to the low resistance state (LRS) of the unipolar resistance switching is about 10³ for the ferroelectric BFO films. The conduction mechanisms are concluded to be space charge-limited conduction for the initial state and Ohmic conduction for the LRS. As for the HRS, the Poole-Frenkel emission fits well in the whole voltage region. Traps composed of oxygen vacancies are considered to play a key role in forming conducting paths. The relaxation time of electronic carriers is much shorter than that of ionic oxygen vacancies; therefore, the resistance switching is considered more probably due to carrier injection and emission through the Poole-Frenkel model after forming.     

Complementary Resistive Switching Using Metal–Ferroelectric–Metal Tunnel Junctions

Complementary resistive switching (CRS) devices are receiving attention because they can potentially solve the current‐sneak and current‐leakage problems of memory arrays based on resistive switching (RS) elements. It is shown here that a simple anti‐serial connection of two ferroelectric tunnel junctions, based on BaTiO₃, with symmetric top metallic electrodes and a common, floating bottom nanometric film electrode, constitute a CRS memory element. It allows nonvolatile storage of binary states (“1” = “HRS+LRS” and “0” = “LRS+HRS”), where HRS (LRS) indicate the high (low) resistance state of each ferroelectric tunnel junction. Remarkably, these states have an identical and large resistance in the remanent state, characteristic of CRS. Here, protocols for writing information are reported and it is shown that non‐destructive or destructive reading schemes can be chosen by selecting the appropriate reading voltage amplitude. Moreover, this dual‐tunnel device has a significantly lower power consumption than a single ferroelectric tunnel junction to perform writing/reading functions, as is experimentally demonstrated. These findings illustrate that the recent impressive development of ferroelectric tunnel junctions can be further exploited to contribute to solving critical bottlenecks in data storage and logic functions implemented using RS elements.     

Investigation of PbZr0.4Ti0.6O3 capacitors with room temperature as-grown LaNiO3 electrodes

LaNiO₃ (LNO) film grown at room temperature (RT) by RF magnetron sputtering is used as the electrode for integrating LaNiO₃/PbZr_0.4Ti_0.6O₃/LaNiO₃ (LNO/PZT/LNO) capacitor on SrTiO₃ (STO) substrate. For comparison, LNO film grown at 250 °C is also used as the electrode of PZT capacitor. Reflection high energy electron diffraction (RHEED) technique is used to characterize the LNO film, it is found that LNO film prepared at 250 °C is epitaxial although no diffraction pattern is found for RT as-grown LNO. Ferroelectric properties of PZT films strongly depend on the LNO bottom electrodes. The remanent polarization (P _r) and coercive voltage (V _c), measured at 5 V, for the capacitors with LNO bottom electrodes prepared at RT and 250 °C, are 20 and 37 μC/cm², 1.67 and 1.95 V, respectively. No obvious degradation of polarization for PZT capacitors with RT as-grown LNO electrodes can be found. Room temperature as-grown LNO as both bottom and top electrodes to fabricate ferroelectric capacitors can save 2/3 thermal budgets, which may pay a way to decrease the potential challenges of devices resulting from the oxidation, interdiffusion or reactions during integrating ferroelectric capacitors with Si technologies.     

Forming-Free Unipolar Resistive Switching in BiFe0.95Co0.05O3 Films

We report the forming-free unipolar resistive switching effects in polycrystalline BiFe_0.95Co_0.05O₃ films which were spin-coated on ITO/glass substrates by a chemical solution deposition method. The resistive ratio of the high resistive state (HRS) to the low resistive state (LRS) is more than 2 orders of magnitude. The conduction of the HRS is dominated by the space-charge-limited conduction mechanism, while Ohmic behavior dominates the LRS, which suggests a filamentary conduction mechanism. The oxygen vacancies are considered to play an important role in forming the conducting filaments.     

Fatigue and retention properties of Bi3.25La0.75Ti3O12 films using LaNiO3 bottom electrodes

Fatigue-free Bi_3.25La_0.75Ti₃O₁₂ (BLT) thin films were grown on LaNiO₃ bottom electrodes grown in a microwave furnace at 700 °C for 10 min from the polymeric precursor method. It was found that LaNiO₃ (LNO) bottom electrode with pseudocubic structure strongly promote the formation of (00l) texture of BLT films. The remanent polarization (P_r) and the drive voltage (V_c) were 11 μC/cm² and 1.3 V respectively, and are better than the values found in the literature. The polarization of the Au/BLT/LNO/SiO₂/Si (100) capacitors with a thickness of 280 nm exhibited no degradation after 1 × 10¹⁰ switching cycles at an applied voltage of 5 V with a frequency of 1 MHz. After several tests the capacitors retain 77% of its polarization upon a retention time of 10⁴ s.     

Low power and stable resistive switching in graphene oxide-based RRAM embedded with ZnO nanoparticles for nonvolatile memory applications

The present study reports the role of zinc oxide nanoparticles (ZnO NPs) embedded in graphene oxide (GO)-based RRAM for non-volatile memory applications. GO thin film embedded with different concentrations of ZnO NPs was deposited on bottom electrode, i.e., indium tin oxide (ITO) coated glass. Thermal evaporation technique was used for the fabrication of top electrodes for electrical measurements. Structural and morphological studies of synthesized GO and ZnO NPs were done by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). Switching characteristics of the RRAM devices were investigated using electrical measurements. It has been observed that the optimized concentration of ZnO NPs (20%) shows stable switching behavior with low SET (??0.61 V) and RESET (+?0.65 V) voltages as compared to pure GO devices. The switching of the fabricated memory devices from high resistance state (HRS) to low resistance state (LRS) has been found due to conductive filament formed between top and bottom electrodes. This conductive filament has been confirmed by the change in resistance as a function of temperature. The Al/GO-ZnO(20%)/ITO devices show stable endurance behavior for >?50 cycles and retention behavior for >?4?×?10³ s. In HRS, the dominated conduction mechanism was found to be space-charge limited conduction (SCLC), whereas in LRS, the Ohmic conduction mechanism was observed. The incorporation of ZnO NPs increased the number of oxygen vacancies in switching layer which eventually enhanced the formation of conductive filament. This phenomenon has been confirmed using XPS characterization of the switching layer. These optimized concentrations of ZnO embedded in GO switching layers provide a way for future low power non-volatile memory devices.

     

Mechanisms of current conduction in Pt/BaTiO3/Pt resistive switching cell

The 80-nm-thickness BaTiO₃ (BT) thin film was prepared on the Pt/Ti/SiO₂/Si substrate by the RF magnetron sputtering technique. The Pt/BT/Pt/Ti/SiO₂/Si structure was investigated using X-ray diffraction and scanning electron microscopy. The current-voltage characteristic measurements were performed. The bipolar resistive switching behavior was found in the Pt/BT/Pt cell. The current-voltage curves were well fitted in different voltage regions at the high resistance state (HRS) and the low resistance state (LRS), respectively. The conduction mechanisms are concluded to be Ohmic conduction and Schottky emission at the LRS, while space-charge-limited conduction and Poole-Frenkel emission at the HRS. The electroforming and switching processes were explained in terms of the valence change mechanism, in which oxygen vacancies play a key role in forming conducting paths.     

Effects of drying temperature on preparation of pectin polysaccharide thin film for resistive switching memory

Commercially purchased pectin powder was formulated and processed into a thin film with different drying temperatures (60 °C–180 °C) and subsequently, sandwiched between Ag and ITO as top and bottom electrodes, respectively, for resistive switching memory study. Chemical functional groups, surface roughness, elemental chemical composition, and cross-sectional morphology of the dried pectin thin film were investigated via Fourier transform infrared spectroscopy, atomic force microscopy, and high-resolution transmission electron microscopy-energy dispersive X-ray spectroscopy. The higher the drying temperature, the rougher the surface with less residual moisture and more defect sites within the thin film was observed. Current–voltage measurements together with the chemical analysis results were used to establish the resistive switching mechanism. Resistive switching controlled by oxygen vacancy as conductive paths with ON/OFF ratio of 10⁴, memory window of 3.6 V, and retention time of 10 years with READ voltage as low as ? 0.01 V for pectin dried at 180 °C has been demonstrated.

     

Reproducible resistance switching for BaTiO3 thin films fabricated by RF-magnetron sputtering

BaTiO₃ (BTO) thin film was fabricated to investigate its non-volatile and reversible resistance switching phenomena by RF-sputtering method. The reversible resistance switching phenomenon was observed by DC voltage sweep and Pt/BTO/Pt metal-insulator-metal structure devices showed the bipolar resistance switching such as Pr_0.7Ca_0.3MnO₃ and Cr-doped SrTiO₃. The typical leakage current-voltage characteristic measurements were performed. High resistance state (HRS) and low resistance state (LRS) were maintained without power supply. The margin of the resistance between HRS and LRS is considerable during 120th cycles. The current emission mechanisms were suggested by double logarithm plot of leakage current vs. voltage. The comparison of the spreading current mapping images for two different resistance states showed that local conduction path was formed at LRS and was destroyed at HRS.     

室温下制备非晶ZnO薄膜及其电阻开关特性研究

室温下采用紫外固化的方法取代溶胶-凝胶方法中的高温退火制备了氧化锌薄膜, XRD分析结果表明薄膜为非晶的, XPS分析结果表明薄膜的主要成分是ZnO。在深紫外固化后的薄膜表面溅射Al作为顶电极获得Al/a-ZnO/FTO结构的器件,研究深紫外照射时间对器件电阻转变性能的影响, 进一步解释了深紫外固化的机制。研究表明: 经过充足时间(12 h)照射的器件表现出双极性电阻开关特性,阈值电压分布集中(-3.7 V<V_set<-2.9 V, 3.4 V< V_reset<4.3 V)且符合低电压工作的要求, 至少在4000 s内器件的高低阻态都没有发生明显的退化, 表现出了良好的存储器特性。Al/a-ZnO/FTO器件的这种电阻转变特性可以用空间电荷限制电流传导机制解释。     

Magnetism and electrode dependant resistive switching in Ca-doped ceramic bismuth ferrite

Here we report on the preparation and structural, magnetic and electrical characterization of BiFeO₃ and Bi_0.9Ca_0.1FeO₃ ceramic multiferroic samples. We suggest that Ca-doping creates oxygen vacancies and destabilizes the BiFeO₃ spiral magnetic structure. We also study resistive switching effects in Bi_0.9Ca_0.1FeO₃ with metallic electrodes, finding that the appearance of the effect is dependant on the fabrication procedure of the metallic electrode. On the basis of these observations, we critically revise some assumptions in currently available models of resistive switching of complex oxides.     

Influence of carrier injection on resistive switching of CaCu3Ti4O12 thin films with Ni electrode

The influence of electron injection on the electric-pulse-induced resistive switching of perovskite CaCu₃Ti₄O₁₂ (CCTO) films was studied by current-voltage (I-V) measurements. The electron injection was reduced by annealing the sample in an O₂ atmosphere. The switching from the high-resistance state HRS to the low-resistance state LRS by a filamentary mechanism was suppressed when the carrier injection occurs by Poole-Frenkel emission. The interfacial potential barrier plays a crucial role in determining the carrier injection.     

Nanoscale study by piezoresponse force microscopy of relaxor 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 and 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 thin films grown on platinum and LaNiO3 electrodes

Relaxor 0.7Pb(Mg_1/3Nb_2/3)O₃-0.3PbTiO₃ (70/30 PMN-PT) and 0.9Pb(Mg_1/3Nb_2/3)O₃-0.1PbTiO₃ (90/10 PMN-PT) thin films have been grown by RF-sputtering on platinum (Pt) and lanthanum nickelate (LaNiO₃) bottom electrodes. For both electrodes, macroscopic measurements evidence lower coercive fields, remnant polarizations and piezoelectric coefficients d₃₃ for 90/10 PMN-PT films compared to 70/30 PMN-PT films. For both compositions, coercive fields and remnant polarizations are lower for films grown on LaNiO₃ compared to on Pt while piezoelectric coefficients d₃₃ are higher. For each electrode and composition, a similar behavior is revealed for electromechanical activity at the nanoscale when measuring local piezoelectric hysteresis loops; on the other hand, the voltages required for switching the domains are the highest for 90/10 PMN-PT films grown on LaNiO₃. The existence of large grain boundaries in the films grown on Pt and the presence of local random fields with polar nano-domains for the 90/10 composition could explain the differences measured in domains switching properties at the macroscale and nanoscale levels.     

退火温度对TiO2基电阻开关器件性能的影响

采用直流磁控溅射法在n+-Si上制备了TiO2薄膜,采用电子束蒸发镀膜仪在TiO2薄膜上沉积Au电极,获得了Au/TiO2/n+-Si结构的器件.研究了退火温度对薄膜结晶性能及器件电阻开关特性的影响.Au/TiO2/n+-Si结构的器件具有单极性电阻开关特性,置位(set)电压,复位(reset)电压、reset电流及功率的大小随退火温度的不同而不同,并基于灯丝理论对器件的电阻开关效应的工作机理进行了探讨.研究结果表明,500℃退火的器件具有良好的非易失性.器件高低阻态的阻值比大于103,其信息保持特性可达10年之久.在读写次数为100次时,器件仍具有电阻开关效应.     

Resistive hysteresis in BiFeO3 thin films

Capacitor-like Au/BiFeO₃/SrRuO₃ thin film with (1 1 1) orientation was grown on the SrTiO₃ (1 1 1) substrate by radio frequency magnetic sputtering. It shows a resistive switching behavior, where a stable hysteresis in current–voltage curve was well developed by applying an optimum voltage at room temperature, and it reached the saturation at a bias voltage of 8 V. The Child's law in V_max → 0 direction and the interface-limited Fowler–Nordheim tunneling in 0 → V_max direction, together with the polarization reversal in the BiFeO₃ barrier, are shown to involve in the observed resistive hysteresis.     

The Multiferroic Properties of Zn Doped BiFeO3 Thin Films

BiFe_1?x Zn _x O₃ thin films (x=0, 0.05, 0.1, 0.2, and 0.3) were prepared on LaNiO₃ buffered surface oxidized Si substrates using pulsed laser deposition. The effect of Zn dopants on the structural, electrical, magnetic properties, and exchange bias on NiFe layers for BiFeO₃ thin films has been systematically investigated. X ray diffraction and Raman spectra show that BiFe_1?x Zn _x O₃ thin films have polycrystalline rhombohedral structure with Zn concentration smaller than 5 %, and a large amount of the impurity phase of Bi₂Fe₄O₉ with further increasing Zn concentration above 10 %. With 5 % Zn doping in BiFeO₃, the film quality is improved with larger grain size, better ferroelectricity, and larger magnetization. The exchange bias of BiFe_1?x Zn _x O₃ on a 3.6 nm thick NiFe layer drastically decreases with increasing Zn doping concentration, and totally disappears with the Zn concentration larger than 10 %, indicating that the canted antiferromagnetic spin structure of BiFeO₃ was suppressed with increasing the Zn doping concentration.     

Temperature Dependent Magnetic, Dielectric Studies of Sm-Substituted Bulk BiFeO3

Bismuth ferrite- (BiFeO₃) ceramic is the most studied and attractive multiferroic material with low magnetization, moderate leakage current, and low polarization. Samarium substituted bulk BiFeO₃ prepared at low synthesis temperature ??600?°C by the sol?Cgel process. Room temperature X-ray diffraction (XRD) patterns confirmed the formation of perovskite structure Bi_0.9Sm_0.10FeO₃ (BSFO) phases. Present compositions possess high dielectric constant (????199) and low dielectric loss (tan????0.009) at room temperature for 100?Hz frequency. Room temperature dielectric permittivity and dielectric loss decreased with increasing frequency from 100?Hz to 10 MHz. As the temperature increased, an enormous increase in both dielectric permittivity and dielectric loss is observed at all frequency regions. Temperature dependent M?CH hysteresis loops were saturated. Spin glass-like ferromagnetic behavior is retained in M?CH hysteresis loops measured from the low temperature region and normal ferromagnetic behavior is observed in the high temperature region, both at room temperature and above ??350?K, 400?K, respectively. The origin of the ferromagnetic property in BSFO may be due to the presence of rare earth metal ions at the lattice sites of?BFO.     

Fabrication and polarization-modulated resistive switching behavior of predominantly (110)-oriented BiFeO3 thin films on indium tin oxide/glass substrates

Predominantly (110)-oriented BiFeO₃ (BFO) thin films were deposited on indium tin oxide/glass substrates using a metal organic decomposition process by controlling the preheating temperature. The structure evolution with film thickness at different preheating temperatures was investigated to clarify the growth mode of (110)-predominant BFO film. The formation of the (110)-oriented BFO film is due to the low-temperature nucleation of (110)-oriented grains preheated at 425 °C. In the Au/BFO(110)/ITO heterostructure, a polarization-modulated bistable resistive switching behavior with high ratio of resistance and large diode current characteristics was observed, which makes the heterostructure attractive for application in resistive ferroelectric memory.     

Dielectric and impedance spectroscopy of Ni doped BiFeO<Subscript>3</Subscript>-BaTiO<Subscript>3</Subscript> electronic system

A nickel modified BiFeO₃–BaTiO₃ electronic system has been fabricated by using a high-temperature solid-state reaction process. Preliminary X-ray structural analysis has confirmed the formation of a single-phase material in the orthorhombic crystal system. The dielectric and impedance characteristics of the prepared material have been studied in a wide range of frequency (1 kHz-1 MHz) at different temperatures (25–500 °C) for the better understanding of the frequency-temperature dependence of its capacitive and resistive behavior respectively. A significant effect of grains and grain boundaries of the resistive characteristics of the material is observed at high temperatures. The electrical conductivity of the material increases with increase in frequency in the low-temperature region. Preliminary study of a small amount of Ni doping in the above binary system (i.e., BiFeO₃–BaTiO₃) has provided many interesting results which may be useful for the fabrication of an electronic device.     

The epitaxial growth of germanium and silicon on an Ag(111) film on a mica substrate

With the aim of finding a method of obtaining self-supporting single-crystal films of silicon for solar cells we studied the epitaxial growth of silicon and germanium prepared by evaporation in ultrahigh vacuum onto an Ag(111) film evaporated in situ onto a mica substrate cleaved in air. The films were examined mainly by reflection high energy electron diffraction. Silicon and germanium films 50–200 Å thick were composed of crystallites with two main orientations relative to the substrate and unoriented crystallites in varying proportions depending on the substrate temperature T_s and the previous heat treatment temperature T_H of the mica. Nearly single-crystal films of silicon could be obtained for T_s = 350 °C and T_H = 250 °C. The sticking coefficient for silicon on silver was found to decrease almost to zero for T_s = 420 °C with T_H = 250 °C. No single-crystal films of germanium were obtained.