Keisuke Saito

Structural and multiferroic properties of BiFeO3 thin films at room temperature

BiFeO3 thin films have been prepared on Pt∕TiO2∕SiO2∕Si substrates under various oxygen pressures of 0.15–0.005Torr at a temperature of 450°C by pulsed-laser deposition. The effects of deposition pressure on their crystal structure and multiferroic properties have been investigated at room temperature. X-ray diffraction analysis (θ-2θ scans and 2-dimensional scans) shows that the BiFeO3 thin films consist of perovskite single phase with tetragonal crystal structure and space group P4mm. The c-axis lattice constant decreases (4.062–4.006A) and c∕a ratio of the films decreases from 1.032 to 1.014 with a decrease in the oxygen pressure. The surface roughness and grain size of the films depend dramatically on oxygen pressures. The dielectric constant of the films decreases with decreasing oxygen pressure. The film deposited at 0.05Torr shows a stable current density and well-saturated hysteresis loop with twice the remanent polarization (2Pr) of 136μC∕cm2 and coercive field (2Ec) of 109kV∕cm. The BiFeO3 thin ...

Dependence of electrical properties of epitaxial Pb(Zr,Ti)O3 thick films on crystal orientation and Zr∕(Zr+Ti) ratio

Epitaxial Pb(Zr,Ti)O3(PZT) films, 1.5–2.0μm in thickness, with a Zr∕(Zr+Ti) ratio ranging from 0.20 to 0.75 were grown on (100)c-,(110)c-, and (111)c-oriented SrRuO3∕∕SrTiO3 substrates at 600u2009°C by metal-organic chemical vapor deposition (MOCVD). The effects the Zr∕(Zr+Ti) ratio had on the crystal structure, dielectric and ferroelectric properties, and piezoelectric response of these films with different crystal orientations were systematically investigated. We ascertained from x-ray-diffraction reciprocal-space-mapping analysis that (001)T-∕(100)T-∕(100)R-,(101)T-∕(110)T-∕(110)R-∕(101¯)R-, and (111)T-∕(111)R-∕(111¯)R-oriented films had epitaxially grown on the respective (100)c-,(110)c-, and (111)c-oriented SrRuO3∕∕SrTiO3 substrates. The constituent phase changed from a tetragonal single phase, a mixture phase of a tetragonal and rhombohedral, to a rhombohedral single phase with increasing Zr∕(Zr+Ti) ratio irrespective of the orientation of the substrates. However, the range of the Zr∕(Zr+Ti) ratio of th...

Buffer-enhanced room-temperature growth and characterization of epitaxial ZnO thin films

The room-temperature epitaxial growth of ZnO thin films on NiO buffered sapphire (0001) substrate was achieved by using the laser molecular-beam-epitaxy method. The obtained ZnO films had the ultrasmooth surface reflecting the nanostepped structure of the sapphire substrate. The crystal structure at the surface was investigated in situ by means of coaxial impact-collision ion scattering spectroscopy. It was proved that the buffer-enhanced epitaxial ZnO thin films grown at room temperature had +c polarity, while the polarity of high-temperature grown ZnO thin films on the sapphire was −c. Photoluminescence spectra at room temperature were measured for the epitaxial ZnO films, showing only the strong ultraviolet emission near 380nm.

Spontaneous polarization change with Zr∕(Zr+Ti) ratios in perfectly polar-axis-orientated epitaxial tetragonal Pb(Zr,Ti)O3 films

Epitaxial Pb(Zr,Ti)O3 (PZT) films 50 and 250nm thick with the Zr∕(Zr+Ti) ratio from 0.13 to 0.65 were grown on (100)cSrRuO3∕∕(100)SrTiO3 substrates at 540°C by pulsed-metalorganic chemical vapor deposition. The crystal orientation, lattice parameter, axial angle, and relative volume fraction of constituent domains were determined by high-resolution x-ray diffraction reciprocal space mapping. We grew (100) and∕or (001)-oriented epitaxial PZT films for the whole Zr∕(Zr+Ti) ratio. A tetragonal single phase was obtained for the Zr∕(Zr+Ti) ratio ranging from 0.13 to 0.54 and 0.19 to 0.45 with the 50 and 250nm thick films, respectively, while mixed phases of a tetragonal and rhombohedral from 0.45 to 0.60 for the 250nm thick films was obtained. For the films consisting of a tetragonal single phase, lattice parameters of a- and c-axes and their ratio (c∕a) were almost the same as the reported data for the PZT powder, suggesting a small amount of residual strain in them. In addition, perfectly polar-axis (c-axis)...

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.

Crystal Structure and Electrical Properties of {100}-Oriented Epitaxial BiCoO3–BiFeO3 Films Grown by Metalorganic Chemical Vapor Deposition

xBiCoO3–(1 - x)BiFeO3 (x = 0–0.22) films of 400 nm thickness were grown on (100)c SrRuO3∥(100) SrTiO3 substrates by metalorganic chemical vapor deposition. The changes in the crystal structure and electrical properties of the films with x were investigated. The constituent phase changed from rhombohedral to a mixture of rhombohedral and tetragonal, and to tetragonal with increasing x, but the x for this transition is different from that of 200-nm-thick films grown on (100) SrTiO3 substrates. The x of the morphotropic phase boundary that consisted of a mixture of tetragonal and rhombohedral symmetries depended on the film thickness. The remanant polarization continuously decreased with increasing x, in good agreement with the results obtained with the {100}-oriented Pb(Zr,Ti)O3 epitaxial films owing to the decrease in the crystal anisotropy of the films.

Crystal structure, electrical properties, and mechanical response of (100)-/(001)-oriented epitaxial Pb(Mg1∕3Nb2∕3)O3–PbTiO3 films grown on (100)cSrRuO3‖(100)SrTiO3 substrates by metal-organic chemical vapor deposition

Relaxor-type ferroelectric (1−x)Pb(Mg1∕3Nb2∕3)O3–xPbTiO3 (PMN-PT) films, 2–3μm in thickness, with a PbTiO3 content (x) ranging from 0 to 1 were grown on (100)cSrRuO3‖(100)SrTiO3 substrates at 650°C by metal-organic chemical vapor deposition. The effects the x value had on the crystal structure, dielectric and ferroelectric properties, and mechanical response of these films were systematically investigated. Epitaxial growth having (100)∕(001) orientation irrespective of x and the constituent phase change with x were ascertained from both x-ray diffraction reciprocal space mapping analysis and Raman spectroscopy. The constituent phase changed from a rhombohedral (pseudocubic) single phase, a mixture phase of rhombohedral (pseudocubic) and tetragonal phases, and a tetragonal single phase, with increasing x. The mixed phase region was found to exist at x=0.40–0.55, which was different from that reported for single crystals (x=0.31–0.35). The dependencies of relative dielectric constant and remanent polarizati...

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.

Metalorganic chemical vapor deposition of epitaxial perovskite SrIrO3 films on (100)SrTiO3 substrates

(001)c-oriented 60–70-nm-thick SrIrO3 films with a high-pressure-stable perovskite phase were epitaxially grown on (100)SrTiO3 substrates by metalorganic chemical vapor deposition, and the crystal structure and the resistivity of the films were investigated. X-ray diffraction techniques including general θ–2θ scan, the rocking curve and high-resolution reciprocal space mapping, were used to determine crystal quality, lattice constant, and crystal structure of the SrIrO3 films. It was demonstrated that the film crystal structure was tetragonal distorted by strong mechanical constraints due to the in-plane matching with the lattice parameters of SrTiO3 substrates, and maintained almost the same unit cell volume as the reported one for the perovskite cubic SrIrO3 phase. The resistivity of the film at room temperature was 740 µΩ cm, and it decreased with decreasing temperature down to 100 K. Finally, an atomically flat surface was obtained on atomically stepped (100)SrTiO3 substrates.