Takafumi Kamo

RF Magnetron Sputtering Growth of Epitaxial SrRuO3 Films with High Conductivity

SrRuO3 films were grown on (001)SrTiO3 single crystal substrates by rf magnetron sputtering under various total pressures, and their crystal structure, room temperature resistivity, and temperature dependency of resistivity were investigated. High-resolution X-ray diffraction (XRD) analysis revealed that the unit cell volume of these films decreased with increasing total pressure from 1.3 to 27 Pa and was almost constant above 27 Pa corresponding to that of bulk SrRuO3. SrRuO3 films deposited under a total pressure of 27 Pa showed the lowest room temperature resistivity, i.e., 250 µΩcm, almost the same as the reported one of the SrRuO3 single crystal. This film also showed a positive temperature dependency of resistivity with a temperature dependency change at about 150 K, which was also in good agreement with the reported one of the SrRuO3 single crystal.

Strong growth orientation dependence of strain relaxation in epitaxial (Ba,Sr)TiO3 films and the resulting dielectric properties

The growth orientation dependence of strain relaxation and the dielectric properties were investigated for (001)- and (111)-epitaxial (Ba,Sr)TiO3 films. The films were deposited on SrRuO3/SrTiO3 and SrTiO3 substrates using rf magnetron sputtering. The residual strain was found to be remarkably different between the two orientations, although these lattice mismatches are identical; the strain relaxation of the (001)-epitaxial films is significantly slower than that of the (111)-epitaxial films and is promoted only when the growth rate is very low (≤5 nm/h). The observed orientation dependence is discussed with the surface energy for both growth orientations, which influences the growth mode of the films. Due to the large contrast of the strain in the (001)- and (111)-epitaxial films, the paraelectric to ferroelectric phase transition temperature of the (001)-epitaxial films is much higher than that of unstrained bulks, while the (111)-epitaxial films show a phase transition temperature corresponding to tha...

Effect of bottom electrode on dielectric property of sputtered-(Ba,Sr)TiO3 films

(Ba0.5Sr0.5)TiO3 (BST) films were deposited on (111)Pt/TiO2/SiO2/Al2O3 substrates by rf sputtering. By inserting a thin layer of SrRuO3 in between BST film and (111)Pt electrode, the BST films grew fully (111)-oriented without any other orientations. In addition, it enables us to reduce the growth temperature of BST films while keeping the dielectric constant and tunability as high as those of BST films directly deposited on Pt at higher temperatures. The dielectric loss of the films on SrRuO3-top substrates was comparable to that on Pt-top substrates for the same level of dielectric constant. The results suggest that the SrRuO3 thin layer on (111)Pt electrode is an effective approach to growing highly crystalline BST films with (111) orientation at lower deposition temperatures.

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.

Direct observation of intrinsic piezoelectricity of Pb(Zr,Ti)O3 by time-resolved x-ray diffraction measurement using single-crystalline films

Lead zirconate titanate, Pb(Zr,Ti)O3 or PZT, is one of the most widely investigated ferroelectric and piezoelectric materials due to its superior properties. However, the intrinsic properties of PZT have not been directly measured due to the lack of fabrication of single crystals even though a basic understanding of intrinsic properties has been of interest developing lead-free piezoelectric materials. We demonstrated the direct observation of the intrinsic piezoelectric property by means of the detection of electric-field induced crystal lattice distortion of thick Pb(Zr0.35Ti0.65)O3 single-crystalline films with single polar-axis orientation and negligible residual strain using the time-resolved X-ray diffraction (XRD) together with the polarization response. Consequently, the effective converse piezoelectric response was experimentally revealed; hence, the electrostrictive coefficient, which is the conversion coefficient between the electrical and mechanical response, was determined. The obtained effec...

Influence of Epitaxial Growth Orientation on Residual Strain and Dielectric Properties of (Ba0.3Sr0.7)TiO3 Films Grown on In-Plane Compressive Substrates

Orientation dependence of the strain in epitaxial (Ba0.3Sr0.7)TiO3 films and the dielectric properties have been investigated using in-plane compressive substrates. (100)- and (111)-epitaxial films were grown on (100)- and (111)-cSrRuO3/SrTiO3 substrates by rf magnetron sputtering. The residual strain was found to be remarkably different in those films. The (100)-epitaxial films are fully constrained by the substrate, resulting in the large in-plane compressive strain. On the other hand, the (111)-epitaxial films are almost fully relaxed. As results, for the (100)-epitaxial films, the apparent Curie-Weiss temperature was much higher than that of unstrained bulks. This trend was also confirmed for the films on SrRuO3/(La0.3Sr0.7)(Al0.65Ta0.35)O3 substrates.

Comparison of BST film microwave tunable devices based on (100) and (111) MgO substrates

We have increased the figure-of-merit (FOM) of a (Ba,Sr)TiO3 (BST) film microwave tunable device by approximately three times for MgO(111) compared with a MgO(100) substrate at a frequency range of 20 GHz. Differences in permittivity and tunability in a BST film may be closely related to the difference in the film strain. The ratio of calculated permittivities of BST(100) and BST(111) films nearly corresponds to that of the FOM in the microwave range, which was rather unexpected because a higher permittivity leads to both larger tunability and dielectric loss in ferroelectrics. From a series of results, it is suggested that there are additional influences of orientation (other than the direct influence of strain itself) on the tunable properties in BST films especially in the high-frequency region.

Experimental study of effect of strain on electrocaloric effect in (001)-epitaxial (Ba,Sr)TiO3 thin films

The effect of strain on the electrocaloric effect in (001)-epitaxial (Ba,Sr)TiO3 (BST) thin films was experimentally investigated via indirect estimation by measuring the electric field dependence of polarization at various temperatures. Compressively strained (Ba0.3Sr0.7)TiO3 films were deposited on (001)cSrRuO3/(001)SrTiO3 and (001)cSrRuO3/(001)(La0.3Sr0.7)(Al0.65Ta0.35)O3 (LSAT) by RF magnetron sputtering. The BST film on (001)cSrRuO3/(001)SrTiO3 showed a higher ferroelectric-to-paraelectric phase transition temperature than that on (001)cSrRuO3/(001)LSAT owing to the larger compressive strain. It was revealed that the electrocaloric effect is maximized at around the ferroelectric-to-paraelectric phase transition temperature modified by the compressive strain in the films.

Nonlinear electric field dependence of electrocaloric effect in (001)-epitaxial (Ba, Sr) TiO 3 thin films

Recently, electrocaloric (EC) effect in ferroelectric thin films has been intensively studied for emerging cooling devices. The pyroelectric coefficient, which mainly determines the magnitude of the EC effect, depends not only on the temperature but also on the applied electric field. Therefore, we investigated the influence of the electric field on the EC effect in a Ba<inf>0.5</inf>Sr<inf>0.5</inf>TiO<inf>3</inf> thin film fabricated on SrRuO<inf>3</inf>/SrTiO<inf>3</inf> (001) substrate. It was found that the temperature showing the largest EC effect, T<inf>max</inf>, increased with the electric field change ΔE when ΔE was below 115 kV/cm. However, it decreased with further ΔE. The observed tendency was different from the theoretical prediction.

Interfacial dislocations in (111) oriented (Ba0.7Sr0.3)TiO3 films on SrTiO3 single crystal

We have investigated the interfacial structure of epitaxial (Ba,Sr)TiO3 films grown on (111)-oriented SrTiO3 single-crystal substrates using transmission electron microscopy (TEM) techniques. Compared with the (100) epitaxial perovskite films, we observe dominant dislocation half-loop with Burgers vectors of a⟨110⟩ comprised of a misfit dislocation along ⟨112⟩, and threading dislocations along ⟨110⟩ or ⟨100⟩. The misfit dislocation with Burgers vector of a⟨110⟩ can dissociate into two ½a⟨110⟩ partial dislocations and one stacking fault. We found the dislocation reactions occur not only between misfit dislocations, but also between threading dislocations. Via three-dimensional electron tomography, we retrieved the configurations of the threading dislocation reactions. The reactions between threading dislocations lead to a more efficient strain relaxation than do the misfit dislocations alone in the near-interface region of the (111)-oriented (Ba0.7Sr0.3)TiO3 films.