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In this paper we present a study of the switching kinetics of SrTiO3 based resistive switching memory devices. A pulse scheme is used to cycle the cells between the high resistive state (HRS) and the low resistive state (LRS) thereby monitoring the transient currents for a precise analysis of the SET and RESET transitions. By variation of the width and amplitude of the applied pulses the switching kinetics are studied between 10-8 and 104 s. Taking the pre-switching currents into account, a power dependency of the SET is found that emphasizes the importance of local Joule heating for the nonlinearity of the switching kinetics.
Resistively switching devices have attracted great attention for potential use in future nonvolatile information storage. Among various oxide materials that show resistive switching (RS) behavior, SrTiO3 (STO) is regarded as a model material to study the effect of valence changes accompanying RS in the oxide . In this class of materials, the RS effect is attributed to rely on the migration of oxygen vacancies and an associated valence change in the cation sublattice. To achieve a switchable state, an initial electroforming step is typically required, which is believed to create conductive regions in the insulating material . Under high electrical stress, an oxygen-deficient region, often referred to as the virtual cathode (VC), is formed . The RS occurs across a very short distance between the VC and the anode, allowing for very short switching times. As the electroforming step greatly impacts the device performance and switching variability, its understanding is essential for device optimization. Electroforming is affected by multiple parameters, e.g. voltage, current, temperature, dopant and defect concentrations, ambient gas atmosphere and time. Distinguishing the influence of the particular parameters is a desirable aim and challenging task. Electrocoloration of Fe-doped STO single crystals has proven a valuable means to visualize valence changes of the Fe ions and is thus suitable to study the formation of the VC. Therefore, we performed electrocoloration experiments and used high resolution transmission light optical microscopy to make the redoxprocesses during electroforming visible. The influence of process driving parameters on the evolution of the VC region is studied. The evolution of the VC is interpreted by drift-diffusion simulation of the time evolution of the oxygen vacancy distribution.
A range of material systems exist in which nanoscale ionic transport and redox reactions provide the essential mechanisms for memristive switching. One class relies on mobile cations, which are easily created by electrochemical oxidation of the corresponding electrode metal, transported in the insulating layer, and reduced at the inert counterelectrode. These devices are termed electrochemical metallization (ECM) memories, also called conductive bridge random access memories. The memristive characteristics of the ECM cells provide opportunities for circuit design and computational concepts that go beyond those in traditional complementary metal oxide semiconductor (CMOS) technology. Passive memory arrays open up paths toward ultradense and 3D stackable memory and logic gate arrays. Furthermore, the multivalued conductance characteristics allow for potential exploitation of the cells as synapses in neuromorphic circuits in future energy efficient high-performance computer architectures. Despite exciting results obtained in recent years, many challenges have to be met before these physical effects can be turned into competitive industrial technology. Here, we briefly review the basic working principle, the different possible and potential material combinations, and the fundamental electrochemical processes in ECM cells and their implications for device operations. The prospects of ECM-based resistive random access memory as an emerging memory technology are also reviewed in terms of switching speed and scalability.
Resistively switching TiO2 thin films show a multitude of resistance states, which are achieved during the programming and erasing of a memory cell. These resistance states depend on the applied voltage and the allowed current. Additionally, the operation time has a relevant influence on the adjusted resistance. This parameterization points out a potential application in future multi-level cell memory systems, but also determines the persistence of the non-volatile nature and provides an additional insight into the physics of the resistance switching. Our devices consist of metal-insulator-metal stacks made of Pt/TiO2/Ti/Pt, which are built up in crosspoint junctions. The maximum programming current and the maximum erase voltage amplitude were used to tune in the low resistance and high resistance state, respectively, in combination with the operation time. The corresponding dependencies were determined by quasi-static voltage sweeps, pulse bursts and single pulses of up to 4 V and down to 10 ns.
Silicon dioxide based Electrochemical Metallization (ECM) cells were intensively studied as a promising candidate for CMOS compatible non-volatile memory devices. The resistance of ECM cells can be switched between a high resistive (OFF) state and a low resistive (ON) state by applying a sufficient voltage or current pulse. This resistance transition is attributed to the formation and rupture of a few nanometers in diameter metallic filament. However, the metal ion transport which is believed to be responsible for the filamentary switching mechanism is not understood in detail. In case of SiO2 we suppose protons or humidity may enhance the metal ion transport.
In this work we report our studies on the proton incorporation in amorphous SiO2 thin films focused on the impact of hydrogen and humidity on the resistive switching effect. The switching behavior was analyzed by current-voltage measurements performed at different ambient conditions. The incorporation of hydrogen has been confirmed by Time-of-Flight Secondary-Ion-Mass-Spectroscopy (ToF-SIMS). The results led to an expansion of the defect model proposed in the literature.
In this study an electroforming free device structure based on 25nm thin TiO2 thin films is presented. The TiO2 films are deposited on CMOS compatible W plugs. The use of 5nm thick interlayers of Ti and W between the TiO2 and the Pt electrode turn out to be the key step to achieve the forming free performance. In these Pt/Ti/TiO2/W or Pt/W/TiO2/W samples the switching polarity can be repeatedly changed from “eightwise” to “counter-eightwise” in one device by a proper adjustment of the I-V measurement conditions. The most simple explanation for this observation is that the switching interface can be flipped back and forth from the bottom to the top electrode.
In this report, the fabrication and electrical characterization of fully vertically integrated complementary resistive switches (CRS), which consist of two anti-serially connected Cu-SiO2 memristive elements, is presented. The resulting CRS cells are initialized by a simple procedure and show high uniformity of resistance states afterwards. Furthermore, the CRS cells show high switching speeds below 50 ns, making them excellent building blocks for next generation non-volatile memory based on passive nanocrossbar arrays.
The storage principal of the Electrochemical Metallization Memory Cell is based on change of cell resistance induced by electro-chemical driven growth and rupture of a cupric or silver filament in an insulating matrix. This kind of switching was found in several materials as AgGeSe, CuGeS, silicon oxide or tungsten oxide .
During write operation copper or silver is oxidized at the corresponding electrode and copper or silver ions are driven out of the copper or silver anode into the insulating matrix due to the applied field, whereas the insulating matrix serves as solid electrolyte. The silver or copper ions migrate towards the cathode. At the cathode electrochemical reduction occurs, and deposition of metallic copper or silver takes place. Fast diffusion paths in the solid electrolyte matrix or preferred nucleation sites (seeds) at the boundary lead to filamentary growth. This growing cupric or silver dendrite finally reaches the anode and switches the device to a low resistance state.
Based on this switching mechanism a FEM simulation model was set up. To simplify the model space charges due to silver or copper migration are neglected. It is further assumed, that the conductivity in the solid electrolyte is only ionic. Hence, it is sufficient to solve the well-known Laplace equation to address the electric properties as well as ion migration. A “Level Set” method is used to track the boundary of the growing filament. The velocity of this boundary is proportional to the ionic current density calculated by Laplace equation. Based on this model simulations are applied to cell structures with multiple fast diffusion paths and seeds. Simulation results show that just one filament reaches the anode.
In a second step, Butler-Vollmer boundary conditions are introduced. This nonlinearity leads to an exponential dependence between switching time and switching voltage. As switching voltage increases, switching time decreases.
A simulation model capable of simulating ECM memory cells is presented. The model enables to simulate the behaviour of different cell geometries or different materials as solid electrolyte. Furthermore it gives deeper insight into the switching mechanism.
This work was supported by the European project EMMA “Emerging Materials for Mass storage Architectures” (FP6-033751).
This is a copy of the slides presented at the meeting but not formally written up for the volume.
We present a high-resolution transmission electron microscopy study, on the unit-cell scale, of the degree of tetragonality and the displacements of cations away from the centrosymmetry positions in an ultra-thin epitaxial PbZr0.2Ti0.8O3 film on a SrRuO3 electrode layer deposited on a SrTiO3 substrate. TEM results show that the lattice is highly tetragonal at the centre of the film with a c/a ratio of about 1.08, while it shows a reduced degree of tetragonality in the regions close to the interfaces. Most strikingly, we find that the maximum off-centre displacements for the central area of the film do not scale with the tetragonality in comparison with the bulk materials. The calculated switched polarization from the measured cationic displacement is 80 ìC/cm2 , and thus only half of the nominal bulk value. It is in very good agreement with electrical measurements of the switched polarization obtained via the PUND method. Furthermore, a systematic reduction of the atomic displacements is measured at the interfaces. This suggests that interface-induced suppression of the ferroelectric polarization plays a critical role in the size effect of nanoscale ferroelectrics. These issues will be discussed further in this presentation. This work was partially supported by the National Science Foundation (NSF) under Grants DMR-0132918, NSF-MRSEC DMR-0080008, and an NSF US-Europe program DMR-0244288. V.N also acknowledges the support of the Alexander von Humboldt Foundation for his stay in Germany and the financial support of an Australian Research Council Discovery Grant 0666231.
High quality Pb(Zr,Ti)O3 [PZT] and (Pb1-xBax)(ZryTi1-y)O3 (x ≤ 0.15, 0.25 ≤ y ≤ 0.50) [PBZT] thin films were grown on Pt (111) and Ir (111) coated silicon substrates by means of a pulsed liquid injection metal organic chemical vapor deposition (MOCVD) technique. The precursor solutions of Pb(DPM)2, Ba(DPM)2, Zr(IBPM)4, and Ti(OiPr)2(DPM)2 dissolved in butylacetate were separately injected into an AIX-200 reactor using a TriJet™ vaporizer. Stoichiometric films (0.98 ≤ A/B ≤ 1.06) with thickness between 80 nm and 150 nm were deposited at a susceptor temperature of 615 °C to 660 °C. Pure PZT films grown on platinum coated substrates show a randomly oriented perovskite structure accompanied with formation of a PbPtx alloy at the PZT/Pt interface. On the Ir(111) coated substrates the pure PZT films also exhibit a random orientation possibly due to oxidation of the Ir surface layer during the deposition process. Ferroelectric properties of Pr = 35 µC/cm2 and Ec = 90 kV/cm were obtained for a PZT (30/70) film of 150 nm thickness grown on Ir/Si. In contrast, PBZT films with a Ba content of about 5 to 15% show lower tendency for formation of a PbPtx interfacial layer, and a preferred (111) texture was observed for PBZT films grown on the Ir (111) substrates under optimized process conditions. Tetragonal and rhombohedral PBZT films with 15% Ba and a Zr-content of about 0.35 and 0.50, respectively, show an orientation dependence of the ferroelectric properties in the way that Ec is highest for <111> textured films in comparison to Ec determined for <110> textured films. The remanent polarization of 85 nm thick tetragonal PBZT films changes from 17 µC/cm2 for <111> orientation to 13.5 µC/cm2 for <110> texture. The relative permittivity changes in the same way from 600 to 540, respectively. The rhombohedral films exhibit a nearly independent Pr value of about 11 µC/cm2 while the switching field changes from 75 kV/cm for an <111> textured film to 46 kV/cm for an (110) textured one. The relative permittivity values of both films are 890 and 715 for the (110) and the (111) textured films, respectively. The trends observed for the textured PBZT films grown on Si substrates reflect the behaviour reported for epitaxial films 
Piezoresponse force microscopy (PFM) is the method of choice to investigate piezoactivity on a nanometer scale. A careful distinction between intrinsic and extrinsic effects are mandatory, especially when measuring ferroelectric nanostructures. We focus on two omnipresent extrinsic contributions with a substantial impact: firstly adsorbates on the surface of perovsike materials and secondly the dependence of the lateral piezoresponse on the topography. A thorough understanding of these extrinsic contributions is essential in order to avoid ambiguities in the analysis of PFM measurements.
Pb(DPM)2 and Pb(TMOD)2 dissolved in ethylcyclohexane were evaluated as precursors for future atomic layer deposition (ALD) of Pb(Zr,Ti)O3 films. PbO films were deposited by a liquid injection atomic layer deposition on Pt-covered Si substrates at different deposition temperature and precursor volume per cycle. Pb(DPM)2 and Pb(TMOD)2 started thermal decomposition at deposition temperature of around 270°C and 320°C, respectively. Against increasing Pb(DPM)2 injection at 240°C, the deposition rate of PbO films saturated at around 1 Å/cycle, but kept increasing at 300°C, which is above the thermal decomposition temperature. The deposition rate of PbO films at 240°C dropped to a constant value with enough purge time after precursor injection and reactant supply. A saturated deposition rate of PbO films was also observed for Pb(TMOD)2 below the thermal decomposition temperature. However, the saturation behavior observed for Pb(TMOD)2 was slower and the saturated growth rate was higher comparing to Pb(DPM)2. In addition, the film thickness of the PbO films had an apparent gradient over the substrates. These results indicate that Pb(DPM)2 shows more reactive and stable chemisorption comparing to Pb(TMOD)2 for the self-limiting growth rate.
Processing of BST thin films is becoming more and more important for microwave electronics and for its probable incorporation in future high density DRAM's. A co-relation between processing and final device characteristics is of utmost importance. Differences in microstructure and electrical properties were observed when chemical solution deposited thin films were annealed using a conventional diffusion furnace, rapid thermal annealing furnace with different heating ramps, and a hot plate for pyrolysis prior crystallization. The solution was made with the propionate route and then deposited on Pt coated silicon wafers. Cross-sectional SEM's were performed on the different films. It was found that the microstructure depended on the annealing method of the film. The electrical properties of the films were also found to vary considerably. Frequency dependence of the dielectric constant was studied. The leakage study on different films was performed at different temperatures.
SrTa2O6 thin films with thickness between 6 and 150nm were deposited in a multi-wafer planetary MOCVD reactor combined with a TRIJET® liquid delivery system using a single source precursor, strontium-tantalum-(methoxyethoxy)-ethoxide dissolved in toluene. A rather narrow process window for the deposition of stoichiometric SrTa2O6 was found for this precursor at low pressures and a susceptor temperature around 500°C. Films were grown on Pt/TiO2/SiO2/Si, TiNx/Si, and SiO2/Si substrates. The as-deposited films were X-ray amorphous and could be crystallized by post-annealing at a temperature ≥700°C. The SrTa2O6 phase was dominating within a broad range of compositions (Sr/Ta: 0.4–0.7) and a perovskite type phase was observed for Sr/Ta > 0.7. The electrical properties have been investigated with MIM and MIS capacitors after sputter deposition of Pt top electrodes. The amorphous films had a relative permittivity, ε, in the range of 25–45, and low leakage currents. Crystallized films were investigated with Pt MIM capacitors. For stoichiometric SrTa2O6 the dielectric permittivity reached values of ε = 100–110, but the leakage currents were increased. Remarkably, the permittivity is not very sensitive to deviations from the exact stoichiometry of the SrTa2O6 phase (Sr/Ta: 0.40.7), but a decrease to values of ε = 30–40 is observed along with the phase transition at high Sr contents.
A new type of alkoxide precursor was developed to avoid the formation of the oxo-carbonate phase as an intermediate phase during crystallization. The precursors were synthesized from Ba metal, Sr metal and aminoethanol. The new precursors were found to be quite stable and soluble in a number of organic solvents. Films were prepared by depositing the precursor solution on Pt coated Silicon wafers and crystallized between 550 and 700°C. The films were found to crystallize only above 600°C. After crystallization Pt top electrodes were deposited by sputtering and lift off processing. FT-IR studies were performed on the films to check for any oxo-carbonate phase. Microstructural studies involving XRD, SEM and AFM were performed on the films. The films were found to have a dielectric constant of around 400 with a tunability of around 37%. The frequency and temperature dependence of the dielectric constant were also studied. In addition leakage studies were performed on the films at various temperatures.
In this contribution, the influence of different substrates and textures on the reversible and irreversible polarization in Pb(Zr,Ti)O3 (PZT) thin films will be presented. One possible scenario to explain the origin of the ferroelectric hysteresis is the notion that the domain walls move through a potential generated by their interaction with randomly distributed defects of the matrix. This potential then gives rise to reversible and irreversible changes in the ferroelectric polarization. The exact features of the interaction potential also depend on the stress state of the material which can be influenced by a suitable choice of the substrate.
To study the substrate influence, PZT thin films have been deposited on commercial Si wafers, MgO and SrTiO3 single crystals. Electrical characterization methods (hysteresis and small signal capacitance measurements) have been used to extract information on reversible and irreversible polarization contributions.
Piezoelectric and electrostrictive materials are potential candidates for integrated micro systems. They can be used in cantilever laminated structures for different applications, e.g. active vibration control or agile transducers [1-3]. Due to the necessity of miniaturization of MEMS devices and the reduction of process costs and time, the use of chemical solution deposition (CSD) technique with microlithography and reactive ion etching (RIE) are essential.
Within this work we used these techniques in combination with silicon bulk micro machining technique to fabricate piezoelectric Pb(Zr,Ti)O3 (PZT) and electrostrictive Pb(Mg1/3,Nb2/3)O3-PbTiO3 (PMN-PT) coated micro cantilevers with different lengths which can be used in micro switch or micro mirror applications. In general PMN shows no elastic hysteresis and a better aging behavior than PZT ceramics. Since the electrostrictive effect is smaller than the piezoelectric effect the tip deflection of PMN-PT coated beams is much lower. Cantilevers with two ceramic thin film layers and an internal electrode (bimorph) were designed and compared to such with single ceramic thin film layers (monomorph). For fabrication control and electrical characterization SEM, polarization hysteresis-, and CV-measurements were performed. Laser interferometry measurements were used to characterize the electromechanic performance of the ceramic thin films and cantilevers.
Imprint describes an aging effect in ferroelectric thin films which manifests itself by a shift of the P-V hysteresis loop on the voltage axis. In this paper a mechanism is described which attributes imprint to the screening of a large electric field within a thin surface layer by electronic charges. The field at the surface arises due to the existence of a thin surface layer in which the spontaneous ferroelectric polarization is suppressed. In the course of aging this field is gradually screened by electronic charges which are generated by a Frenkel-Poole effect and then become trapped near the electrode-thin-film interface causing the shift of the hysteresis loop. A numerical simulation based on this model allows a quantitative description of the imprint effect as a function of various experimental parameters.
Micromachined silicon cantilever beams actuated by the converse piezoelectric effect are of great interest for actuator applications, e.g. micro relays or micro mirrors. For the miniaturization and cost saving aspects the combination of silicon bulk micromachining and chemical solution deposition (CSD) technique for the ceramic thin films is very promising.
This paper presents the results of such a fabrication process for a PbZr0.45Ti0.55O3 (PZT) thin film micro actuator for a switch application. The actuator was designed with lengths of 190-990 μm, widths of 60-120 μm, and a complete thickness of 1.5 μm. Wherein the piezoelectric PZT function layer has a thickness of 350 nm. For a distance of 10 μm between the switch contacts and an applied voltage of 10 V a finite element analysis simulation (FEA) was carried out to obtain the principal stress contribution, the optimum cantilever length, the sensitivity, the resonance frequency and the switch contact force. The bending beams were characterized by laser interferometry, resonance frequency, and force measurements. These characterization results are compared to the FEM analyses and to an analytical approach.
The understanding of the polarization switching process of ferroelectric capacitors is highly relevant for the development and optimization of FeRAM devices. We report on the characterization of Pb(Zr,Ti)O3 thin films which have been studied by means of dedicated rectangle pulse measurements. Decreasing the voltage level of the excitation pulses decelerates the polarization switching significantly to the range of milliseconds and reduces the switchable polarization. In this work the influence of niobium (Nb) doping on the switching properties of PZT thin films prepared by CSD are investigated to reach the aspired conditions of low voltage operation, read and write access pulses in the range of nanoseconds. For the implementation of the transient behavior of ferroelectric capacitors in circuit design and simulation tools it is necessary to develop a model which precisely describes the polarization hysteresis, the pulse switching behavior as well as the small signal capacitance. The fundamental considerations for this model are presented, based on an ideal ferroelectric capacitor, taking into account the Curie-von Schweidler behavior. The latter is observed in non-ferroelectric high-K materials as well as in ferroelectric thin films.