Risako Ueno

Crystal structure and ferroelectric properties of rare-earth substituted BiFeO3 thin films

The influence of ion modification using rare-earth cations on crystal structures, along with the insulating and ferroelectric properties of BiFeO3 (BFO) thin films was investigated. Rare-earth-substituted BFO films with chemical compositions of (Bi1.00−xREx)Fe1.00O3 (x=0–0.15, RE=La and Nd) were fabricated on (111)Pt∕TiO2∕SiO2∕(100)Si substrates using a chemical solution deposition technique. A crystalline phase of rhombohedral BFO was obtained by heat treatment in a N2 atmosphere at 500°C for 5min. The crystal anisotropy and the Curie temperature of BFO were degraded continuously with increasing contents of La3+ or Nd3+ cations. Ion modification using La3+ and Nd3+ cations up to x=0.05 lowered the leakage current density of the BFO film at room temperature from approximately 10−3 down to 10−6A∕cm2. A polarization (P)-electrical field (E) hysteresis loop measured at 10K revealed that the intrinsic remanent polarization of La3+- and Nd3+-substituted BFO films with x=0.05 (44 and 51μC∕cm2, respectively) was...

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.

Dependence of Ferroelectric Properties on Thickness of BiFeO3 Thin Films Fabricated by Chemical Solution Deposition

Bismuth ferrite (BiFeO3) thin films were fabricated by depositing sol–gel solutions on Pt/Ti/SiO2/Si(100) structures. X-ray diffraction (XRD) showed that a polycrystalline phase and also a small fraction of a secondary phase, Bi2Fe4O9, were present in the film. The nonperovskite secondary phase decreased with increasing thickness, which showed the influence of volume effects on the film. Improved leakage current density and enhanced polarization in BiFeO3 films were observed. A 400-nm-thick film showed a leakage current on the order of 10-8 A/cm2 at room temperature. The remanent polarization was approximately 90 µC/cm2 at 80 K and the piezoelectric coefficient d33 was approximately 50 pm/V.

Crystal Structure and Electrical Properties of Epitaxial BiFeO3 Thin Films Grown by Metal Organic Chemical Vapor Deposition

BiFeO3 thin films with various thicknesses were grown on (100)cSrRuO3∥(100)SrTiO3 substrates at 620°C by metalorganic chemical vapor deposition (MOCVD) for the first time. X-ray diffraction analysis revealed cube-on-cube epitaxial growth of phase-pure BiFeO3 films with (001) orientation irrespective of the film thickness. Out-of-plane lattice parameter decreased with increasing the film thickness up to 100 nm, however this change with the film thickness became small above this thickness. The well saturated hysteresis loops with the remanent polarization and the coercive field values of 51 µC/cm2 and 166 kV/cm, respectively were obtained at 80 K up to 400 kV/cm for the 480-nm-thick film.

Domain distributions in tetragonal Pb(Zr,Ti)O3 thin films probed by polarized Raman spectroscopy

We have investigated polarized Raman spectra of (001)/(100)-oriented tetragonal epitaxial Pb(ZrxTi1−x)O3 (PZT) thin films (x=∼0.35) in which the volume fraction of the polar c domain is systematically varied from 4% to 96%. From polarization analyses using high epitaxial quality films, we have successfully isolated the A1 and B1 phonons from the E phonons, thus offering a distinctive evaluation of the c domains. As increasing c-domain volume, the A1(TO) modes linearly increase in their intensity. A remarkable correlation is found between the A1(1TO)-mode intensity and the c-domain volume for PZT films. We suggest that this correlation as well as the A1(1TO)-mode intensity provide a simple and useful probe for characterization of c-domain volume and ferroelectric properties in PZT-based devices.

Domain structure control of (001)∕(100)-oriented epitaxial Pb(Zr,Ti)O3 films grown on (100)cSrRuO3∕(100)SrTiO3 substrates

The domain structure and electrical properties were systematically investigated in (001)∕(100)-oriented tetragonal epitaxial Pb(Zr0.35,Ti0.65)O3 thin films grown on (100)cSrRuO3∕(100)SrTiO3 substrates by metalorganic chemical vapor deposition. The wide range of domain structures with various volume fractions of c-domains from 0.05 to 1.0 was systematically controlled by changing the growth temperature from 380to540°C and the film thickness from 50to380nm, while keeping the other processing conditions the same. Reducing the deposition temperature and∕or increasing the film thickness reduced the population of c-domains. The relative dielectric constant (≈500) of the a-domain dominant films (volume fraction of c-axis-oriented domains, VC≈0.05) was larger than that (≈300) of the c-domain dominant ones (VC≈1.0). In the c-domain dominant region, the saturation and remanent polarization changed linearly with the volume fraction of c-domains. In the a-domain dominant region, they showed extraordinary large values...

Domain structures and piezoelectric properties in epitaxial Pb(Zr0.35,Ti0.65)O3 thin films

The ferroelastic twin-domain structures and piezoelectric properties were investigated in epitaxial Pb(Zr0.35,Ti0.65)O3 thin films with various c-domain volume fractions by metal organic chemical vapor deposition on (100)cSrRuO3∕(100)SrTiO3 substrates. The domain structures could be characterized into three regions: a-domain dominant, c-domain dominant, and their mixed domain regions. The mixed domain structure is quite unusual but formed a bilayer with two regions of different a∕c domain proportions: the bottom layer is the a1-a2 domain structure and top layer is the a-c domain structure. The films of the mixed region have attained largest piezoelectric coefficients, d33, presumably due to enhanced a-to-c domain switching.

Raman Spectroscopic Characterization of Tetragonal PbZrxTi1-xO3 Thin Films: A Rapid Evaluation Method for c-Domain Volume

We present the use of Raman spectroscopy as a rapid and convenient evaluation tool for domain distribution of tetragonal PbZrxTi1-xO3 (PZT) thin films. From polarized Raman analyses of epitaxial PZT thin films with various c-domain volumes, we found that the intensity of the A1(TO) modes linearly scales with the c-domain volume. These observations, as well as the quick and nondestructive characteristics of this technique, clearly imply that Raman spectroscopy has enormous potential to quantify the fraction of c-domain volumes in a wide range of PZT devices.

A new method to characterize a relative volume to the c-domain in PZT films based on Raman spectra

ABSTRACT We have investigated polarized Raman spectra of epitaxial Pb(ZrxTi1-x)O3 (PZT) thin films (x = ∼0.35) in which the volume ratio of the polar c-domains is systematically varied from 4 to 96% relative to the non-polar a-domains, and a new method using polarized Raman spectroscopy to characterize a relative volume of c-domain in a PZT film is proposed. This method depends on discovery that an intensity of A 1 (1TO) is proportional to the relative volume of c-domain. Polarized Raman spectroscopy is necessary to separate the peaks from A 1(1TO) and B 1 modes to any other modes.

Observation of Domain Switching in Fatigued Epitaxial Pb(Zr,Ti)O3 Thin Films

We performed x-ray diffraction measurements by using highly brilliant synchrotron radiation on epitaxial Pb(Zr 0.35 Ti 0.65 )O 3 film capacitor structures. Small regions of 300-nm-thick epitaxial Pb(Zr,Ti)O 3 thin films with Pt and SrRuO 3 top electrodes were measured after applying various numbers of switching cycles of the electric field. Epitaxial Pb(Zr,Ti)O 3 thin films were prepared on epitaxial (100) c SrRuO 3 /(100)SrTiO 3 substrates by pulsed-metalorganic chemical vapor deposition. The volume faction of c -domain and remanent polarization was plotted against the number of switching cycles. In the both capacitors, the V c increased as the switching cycle increased independent of fatigue behavior.