Rikyu Ikariyama

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.

Raman Spectroscopy Study of Oxygen Vacancies in PbTiO3 Thin Films Generated Heat-Treated in Hydrogen Atmosphere

Raman spectroscopy is utilized to study the evaluation of vacancies in PbTiO3 thin films subjected to hydrogen atmosphere heat treatment. The B1 mode consisted of only oxygen ion vibration was shifted to lower frequency with increasing heat treatment temperature. It was considered that the oxygen ions were lacking from PbTiO3, and that oxygen vacancies were generated. It was found that the frequency of the B1 mode is proportional to oxygen ion content. Pb ions were also lacking in the PbTiO3 thin films with oxygen ions and the number of oxygen vacancies was larger than that of Pb ions. We suggest that the Raman spectroscopy is a suitable tool for the evaluation of oxygen vacancies in PbTiO3 thin films.

Low strain sensitivity of the dielectric property of pyrochlore Bi–Zn–Nb–O films

Thermal strain sensitivity of the dielectric property was investigated for (111)-textured polycrystalline Bi-Zn-Nb-O films. Cubic pyrochlore films with similar crystallinity and the degree of the orientation were obtained on various substrates, and their 111 lattice spacing monotonously increased with the increasing thermal expansion coefficient of the substrates. The dielectric constant at 0 kV/cm and its tunability were almost independent of the residual strain, unlike highly sensitive (Ba0.5Sr0.5)TiO3 films. This strain independent characteristic of the dielectric property of pyrochlore Bi-Zn-Nb-O film is a big advantage in actual applications. 0 2008 American Institute of Physics.

Suppressed polar distortion with enhanced Curie temperature in in-plane 90°-domain structure of a-axis oriented PbTiO3 Film

PbTiO3 thin films were epitaxially grown on (001) KTaO3 single crystal substrates by metalorganic chemical vapor deposition. The coherent epitaxial growth introduced a large in-plane tensile strain to the PbTiO3 film. This tensile strain increased TC and directed the polarization to one of the in-plane ⟨100⟩ axes below TC, resulting in the formation of perfect a1/a2/a1/a2 domain structure. We found that the polar distortion is appreciably suppressed in such a1/a2/a1/a2 domain structure while TC is enhanced due to the strain.

Impact of 90?-Domain Wall Motion in Pb(Zr0.43Ti0.57)O3 Film on the Ferroelectricity Induced by an Applied Electric Field

Using scanning X-ray microdiffraction with a 50 µm micro-beam and a newly developed two-dimensional detector, the crystal and domain structures were analyzed for a 2.4-µm-thick epitaxial Pb(Zr0.43Ti0.57)O3 film directly underneath a 200 µm Pt top-electrode before and after applying an electric field. An obvious extrinsic 90°-domain rotation was observed and the volume fraction of c-domain was increased from ~42 to ~71% by an applied electric field over the coercive field. This phenomenon induced a large ferroelectric remanent polarization (Pr~60 µC/cm2) which is an unforeseeable value from an as-grown state of Pb(Zr0.43Ti0.57)O3 film.

Epitaxially grown ferroelectric thin films for memory applications (ferroelectric random access memories)

Tetragonal Pb(Zr, Ti)O3 films were epitaxially grown on SrRuO3-coated SrTiO3 substrates by metal organic chemical vapor deposition. Perfect polar-axis-oriented 50 nm thick tetragonal films were successfully grown on (100)SrRuO3//(100)SrTiO3 substrates. The single phase of the tetragonal symmetry region was expanded to a Zr/(Zr + Ti) ratio of around 0.6 for these films but was reduced to around 0.4 when (100)/(001)-mixture oriented 250 nm thick films were obtained. The dependence of spontaneous polarization (Ps) on the lattice parameter of c-axis to a-axis (c/a ratio) was demonstrated using perfectly polar-axis-oriented films with various Zr/(Zr + Ti) ratios. The 250 nm thick (100)/(001), (110)/(101), and (111)-oriented tetragonal films were grown on (100), (110), and (111)-oriented substrates. The domain structure was analyzed in detail, and it was found that the observed saturation polarization (P sat) value can be explained by the Ps value and the 90° domain contribution. These data clearly demonstrate the importance of epitaxial film research for actual ferroelectric random access memories applications.