Satoru Utsugi

Experimental evidence for orientation property of Pb(Zr0.35Ti0.65)O3 by manipulating polar axis angle using CaF2 substrate

Perfectly oriented (001), (101), and (111) Pb(Zr0.35Ti0.65)O3 (PZT) films were grown on identical (111)CaF2 substrates by metal-organic chemical vapor deposition. These films exclude domains parallel to the surface; therefore, all domains are actively switchable under the electric field between top and bottom electrodes. Saturation polarization values, Psat(001), Psat(101), and Psat(111), for these PZT films were 75, 50, and 43 μC/cm2, respectively. This orientation dependency was in a good agreement with the theoretical relationship for a tetragonal PZT single crystal, where Psat(001)=Psat(101)/2=Psat(111)/3.

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.

Spontaneous polarization estimation from the soft mode in strain-free epitaxial polar axis-oriented Pb(Zr,Ti)O3 thick films with tetragonal symmetry

The ferroelectric property, crystal structure, and A1(1TO) soft mode of almost strain-free epitaxial polar axis-oriented tetragonal lead zirconate titanate thick films grown on CaF2 substrates were investigated by changing the Zr/(Zr+Ti) ratio and the temperature. The square of spontaneous polarization (Ps), Ps2, monotonically increased as tetragonal distortion, (c/a)−1, increased where a and c are the lattice parameters of the films and as the square of frequency in A1(1TO), ω2[A1(1TO)], increased. Additionally, the relationships of Ps2, (c/a−1), and ω2[A1(1TO)] were experimentally obtained, which confirmed the Ps value obtained from (c/a−1) and ω[A1(1TO)] without direct polarization measurements.

Complex domain structure in relaxed PbTiO3 thick films grown on(100)cSrRuO3//(100)SrTiO3 substrates

Domain structures of epitaxial PbTiO3 films grown on (100)cSrRuO3//(100)SrTiO3 substrates by metalorganic chemical vapor deposition were investigated by x-ray diffraction (XRD) and piezoresponse force microscopy (PFM) techniques. It was found that with increasing film thickness, the domain structure changed from simple (001) polarization orientation to a complicated mixture of (001) and (100) orientations. PFM mappings showed that in the thicker films (∼1100 nm), the zigzag (001)/(100) domain boundaries made an angle of approximately 87° instead of 90° typically observed in (001)/(100) domain patterns in thinner (<300 nm) films. Full-relaxed tilting angle θ1 + θ2 + θ3 = 3.4° obtained from cross-sectional profile analysis of topological step-terrace structure was in good agreement with 3.4° and 3.6° angle values obtained from XRD measurements and theoretical prediction, respectively.

Piezoelectric Properties of {100}-Oriented Epitaxial BiCoO3–BiFeO3 Films Measured Using Synchrotron X-ray Diffraction

Electric polarization and the strain of {100}-oriented epitaxial BiFeO3 and BiFeO3–BiCoO3 films grown on (100)cSrRuO3 ∥ (100)SrTiO3 substrates were simultaneously measured using synchrotron X-ray diffraction. The apparent piezoelectric coefficient d33,obs values of BiFeO3 and BiFeO3–BiCoO3 films were 28 and 63 pm/V, while the observed electrostrictive coefficient Q11,obs values of the films calculated from electric polarization and strain were 1.4 ×10-2 and 3.6 ×10-2 m4/C2, respectively. These results implied that the films become soft by the addition of BiCoO3 into BiFeO3.

Crystal orientation dependency of ferroelectric property in rhombohedral Pb(Zr,Ti)O3 films

Ferroelectric properties were investigated for (100), (110), and (111)-single-oriented rhombohedral Pb(Zr0.65Ti0.35)O3 films. Saturation polarization, Psat, was changed by the simple tilting angle of the polar axis from the film surface normal. On the other hand, (100)- and -oriented films prepared on (100) Si substrates showed similar Psat values due to the coexistence of orientation. The coercive field, Ec, of rhombohedral Pb(Zr0.65Ti0.35)O3 films was lower than that of tetragonal Pb(Zr0.4Ti0.6)O3 films. (100)-oriented rhombohedral films with SrRuO3 electrodes did not show noticeable degradation in polarization up to 1010 switching cycles. These results show that (100)-oriented rhombohedral Pb(ZrxTi1−x)O3 film is available to realize the low voltage operated ferroelectric random access memory instead of the present tetragonal Pb(ZrxTi1−x)O3 films.