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Epitaxial Bi4Ti3012 (BTO) films with Lao0.5Sr0.5Co03 (LSCO) or Pt bottom electrodes were grown on MgO(OOl) substrates by pulsed laser deposition. Surprisingly, a symmetric Pt/BTO/Pt capacitor showed a highly asymmetric polarization switching and an asymmetric Pt/BTO/LSCO capacitor revealed a nearly symmetric polarization switching. To understand these intriguing phenomena, Auger electron spectroscopy and x-ray photoemission spectroscopy depth-profiles were used. The evidences for interdiffusions at the bottom BTO/Pt interface were found. To get further understanding on the interfacial states, a capacitance-voltage (C-V) measurement was performed on the Pt/BTO/Pt capacitor. By fitting the C-V data with a back-to-back Schottky diode model, built-in voltages at the top and the bottom interfaces were determined to be 1.1 V and 3.2 V, respectively. From the obtained built-in voltages, an asymmetric band diagram for the Pt/BTO/Pt structure was suggested. Therefore, the imprint failure can be explained by existence of asymmetric interracial states.

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We report a carrier type dependence on the spatial asymmetry of unipolar resistive switching for various metal oxides, including NiOx, CoOx, TiO2−x, YSZ, and SnO2−x. n-type oxides show a unipolar resistive switching at the anode side whereas p-type oxides switch at the cathode side. During the forming process, the electrical conduction path of p-type oxides extends from the anode to cathode while that of n-type oxides forms from the cathode to anode. The carrier type of switching oxide layer critically determines the spatial inhomogeneity of unipolar resistive switching during the forming process possibly triggered via the oxygen ion drift.

We investigated the asymmetric current-voltage (I-V) characteristics and accompanying unipolar resistive switching of pure ZnO and Mn(1%)-doped ZnO (Mn:ZnO) films sandwiched between Pt electrodes. After electroforming, a high resistance state of the Mn:ZnO capacitor revealed switchable diode characteristics whose forward direction was determined by the polarity of the electroforming voltage. Linear fitting of the I-V curves highlighted that the rectifying behavior was influenced by a Schottky barrier at the Pt/Mn:ZnO interface. Our results suggest that formation of conducting filaments from the cathode during the electroforming process resulted in a collapse of the Schottky barrier (near the cathode), and rectifying behaviors dominated by a remnant Schottky barrier near the anode.

Heteroepitaxial Bi3.25La0.75Ti3O12 (BLT) films were grown by pulsed-laser deposition on n-GaN/Al2O3(0001) substrates. X-ray θ–2θ, rocking curve measurements and -scans revealed the degree of crystallinity and epitaxiality. The full width at half maximum for (028) BLT deposited on GaN was 0.93° due to multi-orientational states. Surface chemical bonding states of as-deposited and etched films were recorded by X-ray photoelectron spectroscopy. The remanent polarization and coercive field of the Au/BLT/n–GaN/Al2O3(0001) structure were 22 µC/cm2 and 150 kV/cm, respectively.

The transport properties of as-deposited and plasma-treated SrRuO3 thin films grown on SrTiO3(001) substrates were investigated. The SrRuO3 thin films were treated with two different kinds of plasma gases: O2 and H2. The difference in the transport properties of the two plasma-treated SrRuO3 films was explained by the different oxygen vacancy concentration, based on different values for the out-of-plane lattice constant. This oxygen vacancy was significantly reduced by recovery annealing under high oxygen pressure.

In this paper, vertically trapped charge location is investigated to understand the carrier-transport dynamics in chromium-doped strontium titanate (Cr-SrTiO3 (STO))-based charge trapping memory devices using a transient analysis method. The vertical location of trapped charges is found to move from the Cr-SrTiO3/Si3N4 interface to the bulk region of Si3N4 with an increasing of the electric field, and, particularly, available trap sites are limited at the Cr-SrTiO3/Si3N4 interface by hole injection from the Si substrate into the Si3N4 layer at a high electric field (EOX > 7 MV/cm). In addition, some of these charges passing across the SiO2 (OX) layer generate many Si-SiO2 interface traps (Dit: 1.58 × 1012 cm−2 eV−1) that may degrade the device. However, the trapping efficiency can be improved by using sufficiently thick ( > 10 nm) bottom layers and by preventing direct hole tunneling and thereby, reducing the interface trap density.

The authors report a silicon-based one-diode–type resistive-switching memory (RRAM) device with self-rectifying properties and high electrical properties. The RRAM cell consisted of Al/Cr–SrTiO3/Si and revealed intrinsic diode properties, so that unwanted sneaky currents could be removed from an RRAM crossbar array without extra switching devices. The insulator–metal transition property of the proposed device was explained using the space-charge–limited conduction mechanism. The memory device showed good characteristics including high ON/OFF ratio (~106), low reset current (~10-11 A), high speed at low voltage (200 ns, 2 V), and reasonable endurance (>104 cycles) and retention characteristics (>104 s).

We have developed a motion system with tripod piezo tube legs, which is called the beetle-type or Besocke-type system, in order to control the position of a sub-micropipette. The stick-slip lateral motion of the beetle-type robot achieved a minimum step size of 600±200 nm by applying a sawtooth pulse at a voltage of 30 V and a pulse width of 10 ms. The sliding motion for the insertion and extraction of the sub-micropipette was controlled by a piezoactuator, and inverse sawtooth pulses were applied to the actuator to have more precise step motion than the specifications of the actuator, and a minimum step size of 480±80 nm at a pulse width of 0.17 ms was achieved. Nonlinear responses of the step size with sawtooth pulse widths were observed in both lateral motion and pipette insertion/extraction motion.

We study properties of the line at 3.33 eV observed in photoluminescence (PL) emission spectra of various ZnO films prepared using pulsed laser deposition method. The influence of deposition parameters, such as oxygen pressure, laser fluence, post-annealing, and electric field exposure on intensity of this luminescence band has been investigated. The recombination characteristics are probed by temperature and excitation dependent PL spectroscopy. The obtained experimental data suggest that the 3.33 eV luminescence line in ZnO depends strongly on surface band bending and originates from recombination of bound excitons (BEs) complex located near the surface and grain boundaries. The anomalously small thermal activation energy of BE in comparison with the localization energy is explained by decreasing of the interface barrier. Possible nature of defects that bind free excitons and cause the 3.33 eV emission line in ZnO is proposed.

Ultrashort-period PbZrO3/PbTiO3 (PTO/PZO) ferroelectric/antiferroelectric superlattices are proposed for nano-domain engineering. They behaved like single ferroelectric metamaterials with monodomain excluding a 90° domain boundary, irrespective of their total thicknesses. Further, nanosized 180° domains, as small as ∼12 nm, were achieved by applying a short pulse voltage. Such domain structures of superlattices result from the lowering of the symmetry of supercell by specific atomic stacking in superlattices, contrary to the domain structures of ferroelectric thin films developed by the strain-induced ferroelectric instability. We suggest that ferroelectric superlattices with an ultrashort-period have a clear advantage for nano-domain engineering with greatly enhanced long term stability of the domains.

The local concentration of potassium ion in a single HeLa cell was observed with an ion-selective nano-pipette probe. Ion selectivity was achieved by using a polyvinyl chloride film with selected ionophores placed within the nano-pipette. Both alternating and constant bias voltages were applied to the counter electrode for the observation of local ion concentrations with a response time of less than 0.1 s. These measurements were enabled by a low-current detection system prepared specifically for this study. The difference in local potassium concentrations between inside a living HeLa cell and the surrounding solution was approximately 100 mM, while no difference in potassium ion concentration was observed between the interior of dead cells and the surrounding solution.

Both bi-stable memory and mono-stable threshold switching are observed in amorphous NbOx films. In addition, the transition between memory and threshold switching can be induced by changing external electrical stress. Raman spectroscopy and transmission electron microscope data show that the NbOx film is self-assembled into a layered structure consisting of a top metal-rich region and a bottom oxygen-rich region. The volume ratio of the two regions depends on the film thickness. Our experimental results suggest that different characteristics of conducting filaments in the two regions result in thickness dependence of switching types and the transition between memory and threshold switching.

Selective ion currents in aqueous calcium chloride and sodium chloride solutions with concentrations of up to 1.0 M were observed with sub-micropipettes in which poly(vinyl chloride) (PVC) films containing ionophores selectively filtered cations. Calcium bis[4-(1,1,3,3-tetramethylbutyl)phenyl] phosphate (HDOPP-Ca) and bis[(12-crown-4)methyl]-2-dodecyl-2-methylmalonate [bis(12-crown-4)] were used as the ionophores to filter calcium and sodium ions, respectively. The selective ion current was observed using a low-current detection system developed from scanning tunneling microscopy. The approximate linear relationship between the ion concentration and ion current suggests that the sub-micropipette probe can be used to detect the intracellular local concentration of a specific ion up to 1.0 M.

By confining columnar grains of textured oxide film using anodized aluminum oxide template, we could obtain a grain-boundary-free (GB-free) cuprous oxide (Cu(2)O) nanowire arrays with a narrow diameter distribution and a high density under the same electrochemical deposition condition. A two-terminal device fabricated using an individual GB-free nanowire and Au/Cr electrodes exhibits bipolar resistive switching contrary to the unipolar one of a textured film, and Schottky-like conduction. On the other hand, a nanowire device with Pt electrodes reveals non-switching behavior and Ohmic conduction. Thus, we can propose that the bipolar switching of a nanowire device with Au/Cr electrodes may result from the modulation of Schottky barrier at the interface by migration of oxygen vacancies while the unipolar one of a textured film may be defined as the bulky filamentary switching along the GBs in the GB-embedded texture films.

A novel method of detecting sodium and potassium ions separately with a sub-micropipette probe of approximately 100 nm inner diameter has been demonstrated. A poly(vinyl chloride) film containing crown ether ligands in sub-micropipettes filtered the ions. Sodium ions were trapped with bis(12-crown-4), whereas potassium ions were trapped with bis(benzo-15-crown-5). Alternate and direct bias voltages were applied to the counter electrode in the sub-micropipette so that the local ion concentrations could be observed as current signals after conversion to milivolt output signals with our low-current detection system prepared for this study.

We observed cathode-interfaced electroreduction and a variation of oxygen content in an irreversible low resistance state of highly oriented cuprous oxide (Cu2O) films. These local microstructural and stoichiometric changes in hard breakdown of film allow speculation on the unipolar resistive switching mechanism: formation of metallic filaments originating from generation/migration of oxygen ions (O2−) from the cathode and release of oxygen gas through the anode. Based on the as-proposed switching model, endurance switching properties could be modulated from tens up to over 104 switching cycles by controlling ambient gas or the interface between Cu2O and anode.

Selective ionic currents in aqueous sodium and potassium chloride solutions with concentrations from 0.01 M to 1.0 M were measured using sub-micro pipette probes in which a poly(vinyl chloride) film containing crown ethers selectively filtered sodium or potassium ions. The selective ionic currents were monitored with a sub-picoampere current measurement system developed from the techniques of TΩ-gap impedance scanning tunneling microscopy. The ionic currents increased with the concentration of the corresponding solution, and thus these sub-micro pipette probes can be applied to detect local ionic concentration of a specific ion in living cells with ionic concentration higher than 0.1 M.

Optical and resistance switching properties in the Ag/ZnO/Pt structure were investigated. Two types of resistance switching (bipolar and monopolar) were observed. Photoluminescence measurements showed dissimilar emission spectra for the each type of switching that might be explained by different processes occurred during resistance switching. In case of the bipolar resistance switching, the change in the ultraviolet part of spectrum was observed and it was attributed to Ag+ ions migration. On the other hand, the monopolar switching demonstrated the changes in the visible luminescence emission that could be, in turn, interpreted by the generation/recovery of oxygen vacancies.

We investigated the resistance switching characteristics of the Ag/ZnO/Pt/Si structure and their influence on the photoluminescence (PL) properties of the ZnO layer. Two resistance states were obtained by applying voltages of the same polarity. The resistance switching in the Ag/ZnO/Pt/Si device was ascribed to the formation/disruption of conducting filaments in the ZnO layer during switching processes. PL measurements of the ZnO film showed a significant increase in the visible emission from samples switched to the low resistance state (LRS). This PL signal was attributed to emission from defects generated in the ZnO layer during transition from the high resistance state (HRS) to the more conductive LRS.

Bipolar resistive switching behaviors have been observed in Pt/TiO2/SrRuO3 structures whose TiO2 has a highly oriented anatase phase. The resistive switching behaviors reveal a strong dependence on the duration time of the switching pulse, top electrode size, and amplitude of the switching voltage. We have also analyzed the conduction mechanisms of each resistance state in both polarities. All the resistive switching characteristics of our Pt/TiO2/SrRuO3 structures can be explained by memristor behavior based on locally induced ion migration.

We investigated the dynamics of reversible resistance switching of NiO nonvolatile memory devices by measuring the switching time as a function of applied voltage. A model of local switching induced by charge carrier fluence was proposed to explain the time-dependent switching behaviors. Charge carrier flow under Poole–Frenkel emission builds up local conductive paths inside NiO. The enhanced electric field at the ends of conductive clusters abruptly increases local Poole–Frenkel emission and accelerates the percolation propagation. The results of Monte Carlo calculation strongly supported the model of local switching induced by charge carrier fluence.

We have observed a switching transition between bistable memory switching and monostable threshold switching in Pt/NiO/Pt structure. Bistable memory switching could be changed to monostable threshold switching by applying a positive electrical pulse with height of 2 V and width between 10−2 and 10−4 s. The change is reversible by applying a negative electrical pulse with the same height and width. By considering polarity- and width-dependence of the switching transition and compositional difference on electrical properties in NiOx, we have proposed a model in which the migration of oxygen ions (O2−) is responsible for the switching transition in Pt/NiO/Pt structures.

We found different nonvolatile memory effects between ferroelectric and resistive switching in Pt/PbZr0.3Ti0.7O3(PZT)/La0.5Sr0.5CoO3 (LSCO) heterostructures, depending on thickness of epitaxial PZT films. As the film thickness decreased below 34 nm, leakage and/or tunneling currents increased and hindered ferroelectric switching of films; alternatively, bipolar resistive switching was observed. Analysis using fitting plot on resistive switching behaviors suggested that variable Schottky barrier at the interface between Pt electrode and the film may be responsible for the different nonvolatile memory switching.

We investigated a growth behavior of highly oriented and columnar grained cuprous oxide (Cu2O) films, which were obtained through a chelate-assisted electrochemical solution approach. It was demonstrated that the electrochemical growth of Cu2O films followed a layer-to-island growth mode with a critical thickness of ∼190 nm. The chelating agent induced the layer-growth of flat-surfaced films consisting of single-crystalline planar grains, and influenced the preferred orientation of films maintained within the island-growth mode. In particular, the single-crystalline columnar grains with stable interfaces and diameters of 100–200 nm provided highly localized areas of linear grain boundaries for filamentary resistive switching. We measured different conduction behaviors of flat-surfaced films showing nonswitching Ohmic conduction and unipolar memory switching in as-deposited and annealed films, respectively. These different conduction behaviors were found to originate from the microstructure changes generat...

We report resistive switching characteristics of single-crystalline Ni/NiO core/shell nanodisk arrays, in which the conducting filaments are highly localized on the surface of nanostructure. The local current distributions observed in such a single-grained nanodisk demonstrate that the contact area and the contact time between the conductive tip of conducting atomic force microscopy and the surface of nanodisk critically influence the voltage-stress-induced electroforming behaviors of nanofilaments in NiO switching nanoblocks. These contact parameters, such as the contact area and the contact time, are interpreted to the electrode size and the voltage-stress time for the formation of filaments in metal oxides.

It is quite interesting to develop a multifunctional device using a single material with a simple structure. One of the possible candidates could be multiferroics, which are both ferroelectric and magnetic. By taking advantage of the strong spin-charge coupling, the electric field can control the magnetic polarization and the magnetic field can control the electric polarization. However, these multiferroics are not yet attractive for practical applications because none of the existing materials combine large and robust electric and magnetic polarizations at room temperature. Here, we report an unusual functional material showing a magnetic phase strongly coupled to an electric memory state at room temperature. An oxygen-vacant ZrO2 thin film generates bistable resistive switching between high-resistance (HR) and low-resistance (LR) states by applying external voltage, and it is ferromagnetic at the HR state but nonmagnetic at the LR state. This unique feature is applicable to unusual functional devices, s...

Time-dependent electroforming phenomena in NiO capacitors are investigated. Different current-voltage characteristics between dc sweep mode and pulse mode indicate that electroforming is time-dependent process. Statistical time-dependent dielectric breakdown (TDDB) measurements show the exponential dependence of electroforming time on applied voltage, confirming that forming process is not a spontaneous process at some critical voltage, but an upsurge process resulting from stress-induced defects. The statistical TDDB analysis explains not only the nature of electroforming process but also the anomalous forming and large variations in forming parameters.

Flat-surfaced and fully (111)-oriented Cu2O films were grown through a chelate-assisted electrochemical approach. Based on key roles of chelating agent, the flat surface of films controlled over the columnar-grained growth was obtainable with a root-mean-square roughness value below 3 nm. Cu2O films treated by a rapid-thermal-annealing process at 200 °C exhibited unipolar switching I-V characteristics, presenting the bistable resistance states with a high resistance ratio (Roff/Ron) over 3 orders of magnitude and considerably stable switching properties within 100 switching cycles.

At an applied voltage, anodic oxide alumina (AOA) films exhibit reproducible resistance switching between two states: a high-resistance state with insulating properties and a metallic low-resistance state. This resistance switching does not depend on the polarity of the applied voltage. A reduction in photoluminescence signal intensity of points switched to the low-resistance state was observed. The filamentary electronic model was employed to explain the resistance switching in the AOA films, which assumed that rupture and recovery of filaments are caused by the migration of electrons from and to filaments, respectively.

Ferroelectric Na0.52K0.48NbO3 (NKN) thin film on Pt/Ti/SiO2/Si substrate was prepared using the radio frequency magnetron sputtering method. The single phase and grain morphologies of NKN were confirmed by x-ray diffraction and atomic force microscopy analysis, respectively. The remnant polarization Pr and coercive electric field Ec of NKN film were 22.5 μC/cm2 and 90 kV/cm, respectively. The NKN film displayed low frequency dielectric dispersion in the temperature range 25–500 °C. The leakage current density of the film was 3.0×10−7 A/cm2 at 100 kV/cm. The piezoelectric constant d33 was estimated to be 45 pm/V using the piezoelectric force microscopy.

Resistance switching memory devices constructed on flexible plastic substrates via the spin-coating of titanium oxide solution were characterized in this study. The resistance switching memory device exhibited a ratio of the high resistance to low resistance states of more than 102, and this large resistance ratio was maintained even after 104 s. These memory characteristics are comparable to those of resistance switching memory devices based on titanium oxide films deposited on Si substrates. Moreover, the endurance of the flexible memory device investigated by means of a continuous substrate bending test revealed that the ratio of the high resistance to low resistance states was negligibly changed up to two hundred cycles. Its resistance switching characteristics were not degraded by the bending of the substrate, due to its short length channel and the high ductility of the electrode.

Reproducible resistance switching behaviors induced by the external voltage were investigated in anodic oxide alumina (AOA) thin films. Electric-transport measurements showed that a low resistance state in the Ag/AOA/A1 structures might be caused by the formation of the conductive filaments. Switching to a high resistance state occurred due to the rupture of these filaments. Change in the photoluminescence properties of the AOA films depending on the resistance states was observed.

We investigated the resistive switching characteristics of two types of Pt∕NiO∕Pt structures with epitaxial and polycrystalline NiO layers. Both of these Pt∕NiO∕Pt structures exhibited unipolar resistive switching. Pt/epitaxial-NiO∕Pt showed unstable switching or no resistance state change after several repeated runs. Pt/polycrystalline-NiO∕Pt showed very reproducible switching. The experimental data indicated that microstructural defects (e.g., grain boundaries) played crucial roles in the reliability of the unipolar resistive switching behavior. This was further supported by first-principles calculations.

We perform first-principles calculations on metal-HfO2 interfaces in the presence of oxygen vacancies. Pt, Al, Ti, and Ag are considered as electrodes. It is found that oxygen vacancies are strongly attracted to the interface with binding energies of up to several eVs. In addition, the vacancy affinity of interfaces is proportional to the work function of metals, which is understood by the transition level of the vacancy and metal-Hf bonding. Interfacial segregation of vacancies significantly affects effective work functions of p metals. Our results are consistent with flatband shifts in p-type field effect transistors employing high-k dielectrics and metal gates.

Resistance change random access memory devices using NiOx films with resistance switching properties have immense potential for high-density nonvolatile memory exceeding currently used flash memory. The only critical failure of a NiOx film is to write wrong information due to large fluctuations of switching voltages during successive resistance switching operations. The authors show that failure-free NiOx film can be grown directly on Pt electrode just by process control. Intensive analyses show that the superior resistance switching behaviors of their simple Pt∕NiOx∕Pt structure may result from a very thin Ni–Pt layer self-formed at the bottom interface during deposition of NiOx.

We report a first-principles study on SrRuO3∕SrTiO3 interface in the presence of the oxygen vacancy. While the oxygen vacancy on the side of SrTiO3 significantly lowers the Schottky barrier height, the oxygen vacancy close to the interface or inside the metallic electrode results in a Schottky barrier comparable to that of the clean interface. Based on these results, we propose a model for resistance-switching phenomena in perovskite oxide∕metal interfaces where electromigration of the oxygen vacancy plays a key role. Our model provides a consistent explanation of a recent experiment on resistance switching in SrRuO3∕Nb:SrTiO3 interface.

We have measured the DC resistance R(T) and AC dielectric constant ε(ω) for the bistable high-R and low-R states of NiO thin film. The high-R state shows thermally activated resistance and Debye relaxation of ε(ω). In the low-R state, R(T) exhibits a metallic temperature dependence of R(300 K)/R(5 K)=1.6. The value of ε(ω) is drastically different from that of the high-R state, and we interpret it in terms of the free-carrier Drude dielectric response. The plasma frequency ω p 2 in the metallic low-R state is estimated to be 1.2×109/cm3.

Conductivity switching phenomena controlled by external voltages have been investigated for various NiO films deposited by dc reactive sputtering methods. Pt∕NiO∕Pt capacitor structures with top electrodes of different diameters have showed increasing off-state current with the diameter of a top electrode and nearly the same on-state current independent of the diameter. Local conductivity switching behaviors have been observed in a series structure consisting of two Pt∕NiO∕Pt capacitors with different resistance values. By reasoning out conductivity switching mechanisms from the switching characteristics and introducing multilayers consisting of NiO layers with different resistance values, we have reduced the reset current by two orders of magnitude.

ZrO2 films have been deposited on Pt/Ti/SiO2/Si substrates by pulsed laser deposition methods. Giant and stable conductivity switching behaviors with maximum on/off ratio of 106 and switching endurance of 105 times are observed in a typical Pt/ZrO2/Pt structure. The Pt/ZrO2/Pt structure exhibits two ohmic behaviors in the low-voltage region (V < 1.4 V) depending on the value of previously applied voltage and Schottky-type conduction in the high-voltage region (1.4 V< V