H. Funakubo

Epitaxial BiFeO3 thin films fabricated by chemical solution deposition

Epitaxial BiFeO3 (BFO) thin films were fabricated on (001)-, (110)-, and (111)-oriented single-crystal SrRuO3(SRO)∕SrTiO3(STO) structures by chemical solution deposition. X-ray diffraction indicates the formation of an epitaxial single-phase perovskite structure and pole figure measurement confirms the cube-on-cube epitaxial relationship of BFO‖SRO‖STO. Chemical-solution-deposited BFO films have a rhombohedral structure with lattice parameter of 0.395nm, which is the same structure as that of a bulk single crystal. The remanent polarization of approximately 50μC∕cm2 was observed in BFO (001) thin films at 80K.

Stabilizing the ferroelectric phase in doped hafnium oxide

The ferroelectric properties and crystal structure of doped HfO2 thin films were investigated for different thicknesses, electrode materials, and annealing conditions. Metal-ferroelectric-metal capacitors containing Gd:HfO2 showed no reduction of the polarization within the studied thickness range, in contrast to hafnia films with other dopants. A qualitative model describing the influence of basic process parameters on the crystal structure of HfO2 was proposed. The influence of different structural parameters on the field cycling behavior was examined. This revealed the wake-up effect in doped HfO2 to be dominated by interface induced effects, rather than a field induced phase transition. TaN electrodes were shown to considerably enhance the stabilization of the ferroelectric phase in HfO2 compared to TiN electrodes, yielding a Pr of up to 35 μC/cm2. This effect was attributed to the interface oxidation of the electrodes during annealing, resulting in a different density of oxygen vacancies in the Gd:Hf...

(111)-textured Mn-substituted BiFeO3 thin films on SrRuO3∕Pt∕Ti∕SiO2∕Si structures

(111)-textured 5%-Mn-substituted BiFeO3 (BFMO) thin films were fabricated on (111)-oriented SrRuO3(SRO)∕Pt∕Ti∕SiO2∕Si structures by chemical solution deposition. X-ray diffraction analyses (θ-2θ and pole figures) confirmed that SRO and BFMO films were highly (111) textured. High-resolution x-ray diffraction reciprocal space mapping showed that the crystal structure of the BFMO films was a rhombohedrally distorted perovskite structure. Transmission electron microscopy cross section images revealed that BFMO films were composed of columnar grains epitaxially aligned on the SRO grains. The remanent polarization of approximately 70μC∕cm2 and coercive field of approximately 300kV∕cm were observed at 900kV∕cm applied electric field in the (111)-textured BFMO thin films.

Real-space mapping of dynamic phenomena during hysteresis loop measurements: Dynamic switching spectroscopy piezoresponse force microscopy

Dynamic switching spectroscopy piezoresponse force microscopy is developed to separate thermodynamic and kinetic effects in local bias-induced phase transitions. The approaches for visualization and analysis of five-dimensional data are discussed. The spatial and voltage variability of relaxation behavior of the a-c domain lead zirconate-titanate surface suggest the interpretation in terms of surface charge dynamics. This approach is applicable to local studies of dynamic behavior in any system with reversible bias-induced phase transitions ranging from ferroelectrics and multiferroics to ionic systems such as batteries, fuel cells, and electroresistive materials.

Ultrafast switching of ferroelastic nanodomains in bilayered ferroelectric thin films

The dynamic switching of ferroelastic nanodomains in ferroelectric PbZr0.3Ti0.7O3/PbZr0.7Ti0.3O3 bilayers was investigated. Synchrotron microdiffraction using a high-speed pulse generator reveals that electric field pulses as short as 200 ns can switch the ferroelastic domain. Multiples of random distribution analysis of the field-induced changes in diffraction peak intensities finds a dynamic strain change from 0.2 to 1% with increasing the pulse width. Raman spectroscopy shows considerable decreases in A1(1TO) soft mode intensity after applications of short pulses, confirming the ferroelastic switching. The results demonstrate that ferroelastic domains can indeed move at time scales of the order of hundreds of nanoseconds.

Preparing Pb(Zr,Ti)O3 films less than 100nm thick by low-temperature metalorganic chemical vapor deposition

Polycrystalline Pb(Zr,Ti)O3 (PZT) films 70–80nm thick on (111)Ir∕TiO2∕SiO2∕Si substrates were prepared at 415°C by metalorganic chemical vapor deposition (MOCVD). At 3V, the remanent polarization (Pr) of the as-deposited films was approximately 22μC∕cm2. Inserting PbTiO3 seeds between the PZT films and Ir bottom electrodes improved the crystallinity of the films markedly but improved their ferroelectric properties only slightly. Low-temperature postannealing, on the other hand, even at 400°C (i.e., below the deposition temperature), improved Pr values and hysteresis loop shapes without obviously improving the crystallinity of the films. The electrical properties were improved even more when the films were annealed at 500°C. These results suggest that the low-temperature processing and sub-100-nm film thickness needed for making three-dimensionally structured ferroelectric capacitors can be obtained by using low-temperature MOCVD to deposit PZT films, and then annealing those films at a temperature no grea...

Electrical properties and x-ray photoelectron spectroscopy studies of Bi(Zn0.5Ti0.5)O3 doped Pb(Zr0.4Ti0.6)O3 thin films

(1−x)Pb(Zr0.4,Ti0.6)O3–(x)Bi(Zn0.5,Ti0.5)O3 (PZT-BZT) (x=0, 0.03, 0.05, 0.08, and 0.1) films were deposited on Pt(111)/Ti/SiO2/Si(100) substrates by chemical solution deposition using spin-coating. All samples showed highly (111) oriented perovskite phase and no other phase was observed. The ferroelectric properties of PZT-BZT films were systematically investigated as a function of the content x of the BZT solution. It is found that BZT doping in PZT films could greatly enhance the remnant polarization (Pr), as well as improve the fatigue property. In a 3 wt % BZT-doped PZT film, the 2Pr and the coercive field (Ec) are 90 μC/cm2 and 95 kV/cm at 10 kHz, respectively, at an electric field of 500 kV/cm, and the leakage current density is less than 1×10−7 A/cm2. The impact of BZT doping on the structure of PZT has been investigated by x-ray photoelectron spectroscopy.

Influence of Epitaxial Growth Orientation on Residual Strain and Dielectric Properties of (Ba0.3Sr0.7)TiO3 Films Grown on In-Plane Compressive Substrates

Orientation dependence of the strain in epitaxial (Ba0.3Sr0.7)TiO3 films and the dielectric properties have been investigated using in-plane compressive substrates. (100)- and (111)-epitaxial films were grown on (100)- and (111)-cSrRuO3/SrTiO3 substrates by rf magnetron sputtering. The residual strain was found to be remarkably different in those films. The (100)-epitaxial films are fully constrained by the substrate, resulting in the large in-plane compressive strain. On the other hand, the (111)-epitaxial films are almost fully relaxed. As results, for the (100)-epitaxial films, the apparent Curie-Weiss temperature was much higher than that of unstrained bulks. This trend was also confirmed for the films on SrRuO3/(La0.3Sr0.7)(Al0.65Ta0.35)O3 substrates.

Comparison of BST film microwave tunable devices based on (100) and (111) MgO substrates

We have increased the figure-of-merit (FOM) of a (Ba,Sr)TiO3 (BST) film microwave tunable device by approximately three times for MgO(111) compared with a MgO(100) substrate at a frequency range of 20 GHz. Differences in permittivity and tunability in a BST film may be closely related to the difference in the film strain. The ratio of calculated permittivities of BST(100) and BST(111) films nearly corresponds to that of the FOM in the microwave range, which was rather unexpected because a higher permittivity leads to both larger tunability and dielectric loss in ferroelectrics. From a series of results, it is suggested that there are additional influences of orientation (other than the direct influence of strain itself) on the tunable properties in BST films especially in the high-frequency region.

Fabrication of tetragonal Pb(Zr,Ti)O 3 nanorods by focused ion beam and characterization of the domain structure

Recently, it has been revealed that properties of ferroelectric nanostructures depend on their size. The depolarization field has been considered as one of the major factors, which cause “size effect”. In this study, we investigated the influence of the depolarization field on the domain structure in tetragonal Pb(Zr,Ti)O 3 (PZT) nanorods. We fabricated PZT nanorods with 100 nm-4µm widths by focused ion beam, and evaluated their structures by synchrotron X-ray diffraction. There was clear difference in the domain fraction between film and nanorods.