Shutaro Asanuma

Electric-field-induced resistance switching universally observed in transition-metal-oxide thin films

We show that polarity-dependent, nonvolatile resistance switching by electric field occurs in the thin film of various transition-metal oxides in almost the same manner. This result indicates that, contrary to the general acceptance, perovskite manganite is by no means a special compound for this phenomenon. It is also suggested that the resistance switching is not dominated by a detailed electronic structure of each sample, but dominated by a more general origin, e.g., crystalline defect.

Origin of colossal dielectric response of CaCu3Ti4O12 studied by using CaTiO3∕CaCu3Ti4O12∕CaTiO3 multilayer thin films

To elucidate the origin of the colossal dielectric response (CDR) of CaCu3Ti4O12 (CCTO), multilayer thin films of CCTO interposed in insulating CaTiO3 (CTO) were synthesized using a pulsed laser deposition technique. The capacitance C of CTO/CCTO/CTO films with different layer thicknesses is measured. After removing the capacitance of CTO by extrapolating C to zero CTO thickness, the real part of dielectric constant of CCTO is estimated to be 329–435, which is much smaller than the reported value for CCTO thin films. This fact indicates that the CDR of CCTO is extrinsic and originates from an internal barrier layer capacitor.

Structural and dielectric characterizations of relaxor/ferroelectric superlattice films Pb(Sc1/2Nb1/2)O3/PbTiO3 fabricated on a single-lattice scale

Relaxor/ferroelectric superlattice (SL) films Pb(Sc1/2Nb1/2)O3(PSN)/PbTiO3(PT) with nine different PT concentrations x are fabricated by a pulsed laser deposition method, which keeps the total film thickness at 100 nm. The structure of the SL films is examined by out-of-plane and in-plane x-ray diffractions. All films are epitaxial and their SL structures are coherent over the whole sample. SL reflections up to the fourth order are observed. Lattice constants of the PT and PSN layers in the SL are determined, and the PT lattice constant exhibits a marked increase at xm=32%. At this PT concentration, this phenomenon is due to a polarization reorientation from the a-axis oriented to the c-axis oriented domain. The complex dielectric constants of all samples are measured in the frequency region from 1 to 106 Hz, and the intrinsic dielectric constant e of the SL is derived by an analysis using the equivalent electric circuit model. e takes the maximum at xm. The possible origin of this peculiar phenomenon is ...

Observation of Rotation of Polarization in Thin Films of Pb(Sc1/2Nb1/2)O3?PbTiO3 via a Monoclinic Phase

Using a simple methodology based on the classical Bragg–Brentano X-ray geometry we have evidenced at room temperature a pseudo-cubic phase and a monoclinic (Pm) phase in thin films of PbSc1/2Nb1/2O3 (PSN) and (PbSc1/2Nb1/2O3)0.57–(PbTiO3)0.43 (PSN–PT) respectively, the latter compound corresponding to a morphotropic phase boundary (MPB) composition. Depending on the substrate and the thickness, different domains configuration are obtained. Competition between epitaxial and thermal stress induces a complex structural evolution with temperature which is different from that of ceramic samples. Indeed PSN films transform from a pseudo-cubic (triclinic) phase towards a cubic phase whereas in PSN–PT, a succession of transformations from a pure Pm phase toward a tetragonal (possibly non polar) phase is observed, a cubic phase never being reached up to 950 K.

Temperature evolution of lattice strains in relaxor PbSc1∕2Nb1∕2O3 thin films

Temperature evolution of the lattice parameters of relaxor PbSc1∕2Nb1∕2O3 (PSN) thin films have been measured and compared to the evolution of bulk PSN. In PSN films, a room temperature tetragonal polar phase transforms at higher temperature into a nonpolar tetragonal phase. Critical and Burns temperatures, associated, respectively, to the onset of the ferroelectric phase and to the nucleation of polar nanoregions have been assigned from strong anomalies in the temperature evolution of the film. These complex behaviors are explained by a quantitative study of the strains lying in the film.

Syntheses of relaxor/ferroelectric superlattice thin films Pb(Sc1∕2Nb1∕2)O3∕PbTiO3 and their dielectric properties

Superlattice (SL) thin films with different periods and composition ratios of Pb(Sc1∕2Nb1∕2)O3 and PbTiO3 (PT), inside and outside the morphotropic phase boundary, are synthesized by a pulsed laser deposition method. High order SL reflections are observed by x-ray diffraction, from which the SL period and correlation length are determined. The coherency of the SL structure is found to extend over the whole film in the thinner films. A maximum dielectric constant of 1800 with a dielectric loss of 0.05 is obtained in the frequency range of 1–104Hz for a SL with average composition of 25% of PT.

Phase Transitions in Ferroelectric Thin Films and Superlattices : Thermophysical Approach

It is shown that the information about thermophysical parameters of the nanostructured ferroelectrics is a useful supplement to the traditional x-ray and dielectric data. The results of the study of the temperature dependence of heat capacity of BaTiO 3 /MgO epitaxial thin films by dynamical 3ω method for the thickness range 50–450 nm are presented. The blurred specific heat anomalies corresponding to the phase transitions were observed, and the temperature of the transitions increases with decreasing of film thickness compared with T c in a bulk sample. The causes of the severe smearing of specific heat anomalies as well as pronounced nonlinearity of the dependence of the transition temperature on the film thickness are discussed. Specific heat data for ferroelectric superlattices (BaTiO 3 ) n /(Sr TiO 3 ) m /Sr TiO 3 were obtained for the first time for the films with the average thicknesses near 100 nm.

Comparative Study of Phase Transitions in Polycrystalline and Epitaxial BaTiO3 Thin Films by Specific Heat Measurements

The experimental data on the thermal properties of the nanostructured perovskite ferroelectrics are presented and analysed. The ability of the modified 3ω method for specific heat measurements is discussed and illustrated by the thermal properties of the thin polycrystalline (BaTiO 3 on fused SiO 2 , 50–1100 nm) and epitaxial (BaTiO 3 on MgO, 50–500 nm) films. The sharp decrease of the transition temperature, surplus entropy and spontaneous polarization were obtained for the polycrystalline films. The critical thickness and crystallite size are estimated for this case from the variation of both crystallite sizes at constant thickness and film thickness at constant crystallite size. The similarity of specific heat, phase transition excess entropy evolution in polycrystalline films and nanograined ceramics is revealed. The data on the specific heat of epitaxial films show the extremely diffused anomaly near 400 K, the phase transition has the weak tendency of shifting to higher temperatures with the decreasing of the film thickness. No anomalies were detected for the thinnest (50 nm) film up to 480 K.

Impact of inserted Ta ultrathin layer and postdeposition annealing on the forming voltage of Ir/Ti/Ta/HfO2/TiN/Ti/SiO2/Si resistive switching devices

We fabricated resistive switching (RS) devices that possess the Ir/Ti/Ta/HfO2/TiN/Ti/SiO2/Si structure with different Ta and Ti layer thicknesses by a magnetron sputtering method. The lowering of forming voltage was observed when the combination of Ti and Ta layer thicknesses was Ti (45 nm)/Ta (5 nm) and the device was annealed after the deposition of the top electrode. In this sample, the migration of oxygen ions from the HfO2 layer to the Ti layer through the Ta layer upon thermal annealing was confirmed by secondary ion mass spectroscopy measurements. On the basis of the dependence of the forming voltage on the combination of the top electrode thicknesses and the presence or absence of postdeposition annealing, we discuss a method of reducing the oxide layer more effectively without changing the material combination of the metal electrode and the oxide layer.

Features of Phase Transitions in Nanostructured Ferroelectric Oxides Revealed from Calorimetric Data

A short review of the study of thermal properties of the nanostructured ferroelectrics is given. An application of the modern calorimetric technique to a comparative study of phase transitions in the ferroelectric nanostructured materials (polycrystalline and epitaxial thin films, superstructures, ferroelectric heterostructures, substrate-induced ferroelectricity) is considered.