Hiroshi Nakaki

Ion Modification for Improvement of Insulating and Ferroelectric Properties of BiFeO3 Thin Films Fabricated by Chemical Solution Deposition

Ion modification techniques for improving the insulating and ferroelectric properties of BiFeO3 (BFO) thin films are reported. Rare-earth-substituted BFO films with chemical compositions of Bi1.00-xRExFe1.00O3 [RE = La and Nd] were fabricated on (111)Pt/TiO2/SiO2/(100)Si substrates using a chemical solution deposition technique. Well-saturated P-E curves were obtained for La3+- and Nd3+-substituted BFO films, while the curve of a nonsubstituted BFO film was distorted due to the leakage current. Remanent polarization (Pr) values measured at 10 K were respectively 44 and 51 µC/cm2, for La3+- and Nd3+-substituted BFO films, which are significantly superior to conventional Pb-free ferroelectrics.

Analysis for crystal structure of Bi(Fe,Sc)O3 thin films and their electrical properties

Thin films of Bi(Fe1−xScx)O3 (BFSO) system were fabricated on (111)Pt∕TiO2∕SiO2∕(100)Si substrates by chemical solution deposition to improve the electrical resistivity by substituting electrically unstable Fe3+ cations for stable Sc3+ cations. A single phase of perovskite was obtained in the range of x=0–0.30, in which selective replacement of Fe3+ and Sc3+ was confirmed in x=0–0.15 by using Raman measurement. The leakage current density of the BFSO films was reduced by increasing x. A well-saturated polarization–electric field hysteresis loop was obtained for BFSO films with x=0.15, showing remanent polarization of approximately 35μC∕cm2.

Strain-relaxed structure in (001)∕(100)-oriented epitaxial PbTiO3 films grown on (100) SrTiO3 substrates by metal organic chemical vapor deposition

The authors grew (001)- and (001)∕(100)-oriented epitaxial PbTiO3 films with various thicknesses on (100)SrTiO3 substrates. They used x-ray diffraction to measure the angles between surface normal [001] of (001)-oriented domains and [100] of (100)-oriented domains. The angles were found to be approximately 3.6° when the film thickness exceeded 1100nm. This value is consistent with the value obtained by a geometric calculation for strain-free PbTiO3. This result suggests that thick epitaxial PbTiO3 films grown on (100)SrTiO3 substrates have a fully strain-relaxed structure.

Thick Epitaxial Pb(Zr0.35,Ti0.65)O3 Films Grown on (100)CaF2 Substrates with Polar-Axis-Orientation

(100)/(001)-oriented tetragonal Pb(Zr,Ti)O3 (PZT) films, which were thicker than 2 µm, were epitaxially grown on SrRuO3-covered (100)Si, (100)KTaO3, (100)SrTiO3, and (100)CaF2 substrates by metal organic chemical vapor deposition. The volume fraction of the (001)-orientation almost linearly increased as the thermal strain increased during the cooling process from the deposition temperature to the Curie temperature. Consequently, perfectly (001)-oriented, i.e., polar-axis-oriented, thick films were obtained on CaF2 substrates with a remanent polarization of 71 µC/cm2. This approach to grow polar-axis-oriented PZT thick films enabled the intrinsic piezoelectricity of PZT itself to be clarified, and has potential in novel applications.

Crystal structure and electrical property comparisons of epitaxial Pb(Zr,Ti)O3 thick films grown on (100)CaF2 and (100)SrTiO3 substrates

Epitaxial Pb(Zr,Ti)O3 (PZT) thick films of 2.0–3.0 μm thickness were grown at 600 °C on (100)cSrRuO3∥(100)SrTiO3 and (100)cSrRuO3∥(100)LaNiO3∥(100)CaF2 substrates by pulsed-metal organic chemical vapor deposition. All films showed (100) and/or (001) orientations with tetragonal symmetry. It was found that there is an almost linear relationship between the estimated thermal strain from the deposition temperature to the Curie temperature and the volume fraction of (001) orientation in the mixture of (100) and (001) orientations for the films on both substrates. Consequently, the perfectly (001)-oriented, i.e., polar-axis-oriented, PZT thick films were obtained on CaF2 with the Zr/(Zr+Ti) ratio from 0.20 to 0.40. Moreover, the lattice parameter of a- and c-axes and their ratio (c/a) of those polar-axis-oriented films were almost the same as the reported data for the powder, suggesting that the large strain is not remaining in those films. The relative dielectric constant (er) of the polar-axis-oriented Pb(Zr...

Experimental evidence of strain relaxed domain structure in (100)/(001)-oriented epitaxial lead titanate thick films grown by metal organic chemical vapor deposition

Epitaxial (100)/(001)-oriented PbTiO3 films with thickness of 2.8 μm were grown on Nb-doped (100) SrTiO3 substrates by pulsed metal organic chemical vapor deposition. Complex domain structures consisting of c-domains (c1) and three types of a-domains (a1, a2, and a3) were observed by piezoresponse force microscopy in conjunction with high-resolution x-ray diffraction analysis. The obtained results suggest that (a2/a3) and (a1/c1) domain configurations represent mostly strain-relaxed structures. This conclusion is in good agreement with theoretical predictions made for thick films [Phys. State., Solidi 37, 329 (1976)].

Domain structure of (100)/(001)-oriented epitaxial PbTiO3 thick films with various volume fraction of (001) orientation grown by metal organic chemical vapor deposition

(100)/(001)-oriented epitaxial PbTiO3 films thicker than 1 μm were grown on various types of substrates by chemical vapor deposition. The domain structures of these films with different volume fractions of (001) were investigated. Domain structures, consisting of (100)/(001)-oriented domains, were observed regardless of the type of substrate. However, the tilting angles of the a- and c-domains from the surface normal linearly changed with the volume fraction of the (001) orientation. These results suggest that the volume fraction of the (001) orientation is crucial in identifying the domain structure of PbTiO3 thick films.

Formation of BiFeO3–BiScO3 Thin Films and Their Electrical Properties

(1-x)BiFeO3–xBiScO3 (x=0–0.5) thin films were prepared on (111)Pt/TiO2/SiO2/(100)Si substrates by chemical solution deposition, and their crystal structure, leakage current density, and ferroelectric properties were investigated. X-ray diffraction analysis revealed that the single-phase perovskite was obtained for x from 0 to 0.4. The lattice parameter of the a-axis increased monotonically with increasing x, whereas that of the c-axis decreased. The surface of the films became smooth for the increase in x of up to 0.15, and the leakage current density at room temperature decreased to 5.1×10-8 A/cm2 at 100 kV/cm. Well-saturated polarization and electric field hysteresis loops were obtained for x=0.15 and 0.3, and the remanent polarization (Pr) and coercive field (Ec) of the films for x=0.15 and 0.3 were 66 and 70 µC/cm2 and 525 and 545 kV/cm, respectively.

In situ Raman spectroscopy for characterization of the domain contributions to electrical and piezoelectric responses in Pb(Zr,Ti)O3 films

We employed in situ Raman spectroscopy under electric field for (100)/(001)-oriented tetragonal Pb(Ti0.61Zr0.39)O3 films with (001)-volume fraction (VC) of 35%. The increase in VC was revealed above 200 kV/cm, which resulted in the larger remanent polarization. In addition, the application of high enough field also brings a feature, i.e., large reversible change in VC with/without electric field that can quantitatively explain the enhanced piezoelectric response. These demonstrate the usefulness of in situ Raman observation to probe the domain contributions to the electrical and piezoelectric responses.

Crystal structure and microstructure of epitaxial Pb(Zr,Ti)O3 films consisting of mixed phases with tetragonal and rhombohedral symmetries grown on (100)cSrRuO3// (100)SrTiO3 substrate by metalorganic chemical vapor deposition

(100)-/(001)-oriented epitaxial Pb(Zr,Ti)O 3 (PZT) films with various Zr/(Zr + Ti) ratios and film thicknesses were grown on (100) c SrRuO 3 //(100)SrTiO 3 substrates at 600 °C by metalorganic chemical vapor deposition. Their crystal structure and microstructure consisting of particularly mixed phases with tetragonal and rhombohedral symmetries were investigated in detail. Films with the mixed phases with tetragonal and rhombohedral symmetries were ascertained at a Zr/(Zr + Ti) ratio of around 0.50 at a thickness of more than 200 nm from x-ray diffraction (XRD) analysis, while no mixed phase region was observed for 50 nm thick film. The relative volume fraction of the tetragonal c domain decreased with increasing film thickness for 2-μm-thick films with mixed phases, while the tetragonal a domain and rhombohedral domain increased. These mixed phases were ascertained to have originated from the same cubic PZT grown above Curie temperature from results obtained by temperature-dependent high-resolution XRD reciprocal space mapping. The transmission electron microscopy images clearly revealed that the microstructure of the film gradually changed along the film thickness, despite the fact that the film was very uniform. Tetragonal phases were dominant in the bottom region (near the interface with the substrate) of the film, while rhombohedral phase was dominant in the upper region (near the surface), which was also ascertained by cross-sectional Raman analysis. In addition, the tetragonal a / c domain was wider in the bottom region than that in the upper region of film with mixed phases. These data clearly demonstrate that both the crystal structure and microstructure changed along the film thickness.