Daichi Ichinose

Orientation control of epitaxial tetragonal Pb(ZrxTi1−x)O3 thin films grown on (100)KTaO3 substrates by tuning the Zr/(Zr + Ti) ratio

Pulsed metal organic chemical vapor deposition is used to epitaxially grow 30-nm thick tetragonal Pb(ZrxTi1−x)O3 thin films on (100) KTaO3 single crystal substrates. The in-plane misfit strain, which is controlled by the Zr/(Zr+Ti) ratio of Pb(ZrxTi1−x)O3 thin films, tunes the film orientation. When Zr/(Zr+Ti) = 0, which leads to the tensile strain from the substrate, a perfectly (100)-oriented film is formed, whereas when Zr/(Zr+Ti) = 0.45, which leads to the compressive strain from the substrate, a completely (001)-oriented film is realized. The volume fraction of the (001)-oriented domain almost linearly increases as the Zr/(Zr+Ti) ratio increases for films with (100) and (001) mixed orientations; this change in the volume fraction can be explained by considering the average lattice parameter matching between the Pb(ZrxTi1−x)O3 thin film and the KTaO3 substrate. These results demonstrate a method to systematically control the film orientation using the Zr/(Zr+Ti) ratio to tune the in-plane strain of Pb...

Fabrication of (110)-one-axis-oriented perovskite-type oxide thin films and their application to buffer layer

BaCe0.9Y0.1O3−δ (BCYO) and SrZr0.8Y0.2O3−δ (SZYO) thin films of perovskite-type oxides were deposited on (111)Pt/TiO x /SiO2/(100)Si substrates. X-ray diffraction patterns showed that the (110)-oriented BCYO and SZYO thin films were grown on (111)Pt/Si substrates directly without using any buffer layers. Thin films of SrRuO3 (SRO), a conductive perovskite-type oxide, were also deposited on those films and highly (110)-oriented SRO thin films were obtained. We believe that this (110)-oriented SRO works as a buffer layer to deposit (110)-oriented perovskite-type ferroelectric oxide thin films as well as a bottom electrode and can modify the ferroelectric properties of the oxide thin films by controlling their crystallographic orientations.

Polar-axis-oriented crystal growth of tetragonal PZT films on stainless steel substrate using pseudo-perovskite nanosheet buffer layer

Lead zirconate titanate (PZT) film with polar axis orientation was grown on a SUS 316L stainless steel substrate with the help of a Ca2Nb3O10 nanosheet (ns-CN) layer that had a pseudo-perovskite-type crystal structure. The ns-CN buffer layer was supported on a platinized SUS 316L (Pt/SUS) substrate, followed by chemical solution deposition (CSD) of the PZT films with tetragonal symmetry (Zr/Ti =40/60). The PZT films consisting of c-domain, with [001]-axis orientation of the perovskite unit cell, were deposited on the ns-CN/Pt/SUS substrate owing to (i) epitaxial lattice matching between the unit cell of PZT and substrate surface and (ii) in-plane thermal stress applied to the PZT film during cooling-down step of CSD procedure. The c-domain-oriented PZT film on ns-CN/Pt/SUS substrate exhibited enhanced remanent polarization of approximately 52 μC/cm2 and lowered dielectric permittivity of approximately 230, which are superior to those of conventional PZT films with random crystal orientation and comparable to those of epitaxial PZT films grown on (100)SrRuO3//(100)SrTiO3 substrates.

Effect of in-plane tensile strain in (100)/(001)-oriented epitaxial PbTiO3 films on their phase transition temperature and tetragonal distortion

(100)/(001)-oriented epitaxial lead titanate (PbTiO3) films with various thicknesses were grown on (100) KTaO3 substrates by pulsed metal–organic chemical vapor deposition. The change of crystal structure with film thickness and deposition temperature was investigated. The paraelectric phase of 50 and 1000 nm-thick films had a tensile strain of 0.5% and almost 0% at 700 °C, respectively. The phase change temperature from the paraelectric phase to the ferroelectric phase, the Curie temperature (Tc), increased with the in-plane strain of the paraelectric phase; that is, Tc increased with decreasing film thickness. In contrast, room-temperature tetragonal distortion decreased as the film became thinner. This study reveals the effect of in-plane tensile strain in (100)/(001)-oriented epitaxial PbTiO3 films with higher Tc and smaller tetragonal distortion at room temperature.

Fabrication of highly (110)-oriented BaCeO3-based proton-conductive oxide thin films by RF magnetron sputtering method

A proton-conductive BaCe0.9Y0.1O3−δ (BCYO) thin-film, one of the candidates for the electrolyte of intermediate-temperature solid oxide fuel cells (IT-SOFCs), was deposited on (111)Pt/TiO2/SiO2/(100)Si substrate by the RF magnetron sputtering method. The sputtering conditions, i.e., deposition temperature, pressure, and sputtering gases, were examined to improve the crystallinity of the films. The BCYO films deposited at more than 500 °C were well crystallized and showed only 110 diffraction. The addition of O2 into the sputtering gas causes adverse results of lower crystallinity and lower deposition rates. A wide-range XRD reciprocal space mapping also revealed that the (110)-oriented BCYO films can be obtained on (111)Pt/TiO2/SiO2/(100)Si substrates without the need to use any buffer layers. We consider that this BCYO film possibly shows high proton conductivity and may be a suitable material for the SOFC electrolyte because of its high crystallinity.

Domain structure transition from two to three dimensions in tensile strained (100)/(001)-oriented epitaxial tetragonal PZT film

Pb(Zr0.05Ti0.95)O3 thin films of 30 nm thickness were epitaxially grown at 600 °C on (001) KTaO3 single crystal substrates by metal organic chemical vapor deposition. Films consist of the mixture of the a/c-domain composed of the c-domain with out-of-plane polarization and the a-domain with in-plane polarization and the a1/a2-domain formed by the combination of a-domains whose in-plane polar axes are orthogonal to each other. Films above the Curie temperature (Tc) of 550 °C were found to be under the in-plane tensile strain and transformed to a1/a2-domains at Curie temperature when the temperature decreased. This a1/a2-domain further changed to the mixture of the a/c-domain and a1/a2-domain below 350 °C. This indicated that the two-dimensional domain structures of the a1/a2-domain changed to three dimensional ones below Tc. Although the a/c-domain was induced to relax the strain, the a1/a2-domain was still unrelaxed. This is different from the widely investigated in-plane compressive strained films that have a one to three dimensional domain structure.Pb(Zr0.05Ti0.95)O3 thin films of 30 nm thickness were epitaxially grown at 600 °C on (001) KTaO3 single crystal substrates by metal organic chemical vapor deposition. Films consist of the mixture of the a/c-domain composed of the c-domain with out-of-plane polarization and the a-domain with in-plane polarization and the a1/a2-domain formed by the combination of a-domains whose in-plane polar axes are orthogonal to each other. Films above the Curie temperature (Tc) of 550 °C were found to be under the in-plane tensile strain and transformed to a1/a2-domains at Curie temperature when the temperature decreased. This a1/a2-domain further changed to the mixture of the a/c-domain and a1/a2-domain below 350 °C. This indicated that the two-dimensional domain structures of the a1/a2-domain changed to three dimensional ones below Tc. Although the a/c-domain was induced to relax the strain, the a1/a2-domain was still unrelaxed. This is different from the widely investigated in-plane compressive strained films that h...

Preparation of preferentially (111)-oriented Mg2Si thin films on (001)Al2O3 and (100)CaF2 substrates and their thermoelectric properties

Mg2Si thin films were deposited at 320 °C on (001)Al2O3 and (100)CaF2 substrates by radio-frequency magnetron sputtering. Both films showed a preferential (111) out-of-plane orientation with an in-plane random orientation irrespective of post-heat treatment. Mg2Si films on (001)Al2O3 substrates were under in-plane tensile strain, while those on (100)CaF2 substrates were under in-plane compressive strain both before and after heat treatment. Heat-treated films showed p-type conduction up to 500 °C. Their electrical conductivity and Seebeck coefficient were almost independent of the kind of substrate within the limit of the present study, from 0.22% compressive strain to 0.34% tensile strain at room temperature.

Polarization switching behavior of one-axis-oriented lead zirconate titanate films fabricated on metal oxide nanosheet layer

For the application of electronic devices using ferroelectric/piezoelectric components, one-axis-oriented tetragonal Pb(Zr0.40Ti0.60)O3 (PZT) films with thicknesses of up to 1 µm were fabricated with the aid of a Ca2Nb3O10 nanosheet (ns-CN) template for preferential crystal growth for evaluating their polarization switching behavior. The ns-CN template was supported on ubiquitous silicon (Si) wafer by a simple dip coating technique, followed by the repetitive chemical solution deposition (CSD) of PZT films. The PZT films were grown successfully with preferential crystal orientation of PZT(100) up to the thickness of 1020 nm. The (100)-oriented PZT film with ~1 µm thickness exhibited unique polarization behavior of ferroelectric polarization, i.e., a marked increase in remanent polarization (P r) up to approximately 40 µC/cm2 induced by domain switching under high electric field, whereas the film with a lower thickness showed only a lower P r of approximately 11 µC/cm2 even under a high electric field. The ferroelectric property of the (100)-oriented PZT film after domain switching on ns-CN/Pt/Si can be comparable to those of (001)/(100)-oriented epitaxial PZT films.

Electric-field-induced lattice distortion in epitaxial BiFeO3 thin films as determined by in situ time-resolved x-ray diffraction

Time-resolved X-ray diffraction (XRD) with synchrotron radiation while applying continuous voltage pulses was employed to investigate the electric-field-induced lattice distortion of an epitaxial BiFeO3 (BFO) thin film in a Pt/BFO (1 μm)/SrRuO3 (50 nm)/vicinal SrTiO3 (001) structure. XRD-reciprocal space maps based on the BFO 003, 114, and 1 1¯4 diffraction spots with and without the application of +15 V (150 kV/cm) to the capacitor demonstrated simultaneous electric-field-induced lattice distortion and crystallographic rotation in the BFO thin film. In response to the application of +15 V, the BFO lattice elongated by 0.08% along the [001]BFO direction and compressed by 0.05% along the [110]BFO direction. In addition, the BFO crystals were rotated by 0.01° along the [ 1¯1¯0]STO direction as a result of electric-field-induced lattice distortion under epitaxial strain along the vertical direction at the step edges of the vicinal substrate.

Angular dependence of Raman spectrum for Pb(Zr,Ti)O3 epitaxial films

Herein, we investigate the angular dependence of the Raman spectra of (001)/(100)-oriented tetragonal and (111)/-oriented rhombohedral epitaxial lead zirconate titanate (PZT) films. The A 1 and E modes have the same angular dependence for the (111)/-oriented rhombohedral epitaxial PZT film, but not for the (001)/(100)-oriented tetragonal epitaxial PZT film. These results agree well with theoretical predictions. On the basis of the fact that the angular dependence of the phonon mode in PZT films is linked to the crystal orientation and structure, the angular dependence of the Raman modes can be used to investigate the orientation and structure of PZT films.