Osamu Nakagawara

Enhancement of remanent polarization in epitaxial BaTiO3/SrTiO3 superlattices with “asymmetric” structure

Enhancement of remanent polarization has been demonstrated in epitaxial “asymmetric” BaTiO3/SrTiO3 strained superlattices, in which the thickness ratio of BaTiO3 to SrTiO3 layers is changed at molecular layer order accuracy. The superlattices have been prepared on Nb-doped SrTiO3 (100) single-crystal substrates by a pulsed-laser deposition technique. The superlattice with a stacking periodicity of 15 unit cells BaTiO3/3 unit cells SrTiO3 shows the largest remanent polarization 2Pr of 46 μC/cm2, which is about three times that of the BaTiO3 single-phase film formed under the same condition. The increase in the remanent polarization is attributed both to the BaTiO3-rich structure and to the increase in lattice parameter c due to the mismatch of in-plane lattice parameters between BaTiO3 and SrTiO3.

Electrical properties of (Zr,Sn)TiO4 dielectric thin film prepared by pulsed laser deposition

We have been successful in obtaining temperature‐stable crystallized thin film of (Zr,Sn)TiO4. Preferential (111)‐oriented (Zr,Sn)TiO4 thin film was prepared by pulsed laser deposition. Effects of crystallization were elucidated based on a comparison of electric properties of crystallized and amorphous (Zr,Sn)TiO4 film. For crystallized film, the temperature coefficient of capacitance (TCC) was 20 ppm/°C at 3 MHz and the dielectric constant er=38 in the microwave range of 1–10 GHz. These values are superior to those for amorphous film (TCC=220 ppm/°C, er=27). The crystallization of this material was found quite effective for improving dielectrical properties. Atomic force microscope images showed the surface morphologies of crystallized and amorphous film of (Zr,Sn)TiO4 to differ.

Effects of buffer layers in epitaxial growth of SrTiO3 thin film on Si(100)

SrTiO3 thin film has been formed on Si(100) substrates with various single buffer layers such as SrO, CeO2, CaF2, CoSi2 and a multibuffer layer, YSZ/Y2O3/YBa2Cu3O7 by ArF excimer laser ablation. The relation of lattice orientation of buffer layers with SrTiO3 layer has been elucidated. The orientation of SrTiO3 film is influenced not only by lattice matching but by crystal structure and chemical bonding of the buffer layers. As well, the multibuffer layer more effectively forms preferential c‐axis oriented SrTiO3 film on Si(100), while CoSi2 buffer is more effective for improving the dielectric constant of SrTiO3 than other buffer layers.

Moisture-resistant ZnO transparent conductive films with Ga heavy doping

Moisture-resistant ZnO transparent conductive films were formed with Ga heavy doping by off-axis-type rf magnetron sputtering. The resistivity of 12.4wt% Ga-doped ZnO is 1.3×10−3Ωcm and changes by less than 3% over a 2000h reliability test at a temperature of 85°C and a humidity of 85%. The crystal structural analysis of the heavily Ga-doped ZnO films indicates that the c axis grows along various directions, which is quite different from the conventional c-axis oriented growth. The effect of heavy doping is discussed based on the crystal structural transformation and carrier compensation by excess Ga segregated in the film.

Epitaxial growth and dielectric properties of (111) oriented BaTiO3/SrTiO3 superlattices by pulsed-laser deposition

Epitaxial BaTiO3(111)/SrTiO3(111) multilayered thin films have been investigated with various periods of stacking layer between 0.45/0.45 and 10/10 nm on Nb-doped SrTiO3(111) substrates by a pulsed-laser deposition technique. Upon decreasing the period of each layer, the spacing of (111) plane (d111) of the multilayered film increases, and the relative dielectric constant goes up to 594 which is twice as large as that of (111) oriented (Ba0.5, Sr0.5)TiO3 solid-solution film. The expansion of d111, which might be attributed to an in-plane pressure effect due to the large lattice strain in the heteroepitaxial interface, contributes to the enlargement of relative dielectric constant. Remanent polarization observed in polarization versus applied voltage hysteresis loop is no more than 2.7 μC/cm2 with 2.0/2.0 nm period of layer.

EPITAXIAL GROWTH OF (SR, BA)NB2O6 THIN FILMS BY PULSED LASER DEPOSITION

Thin films of (Sr, Ba)Nb2O6 (SBN) were prepared on SrTiO3 (100) substrates by a pulsed laser deposition and their crystallographic properties were investigated. Epitaxially grown films with the c-axis perpendicular to the substrate were obtained at substrate temperatures higher than 650°C in an oxygen atmosphere (containing 8% O3) of 3 mTorr. An X-ray phi scan analysis indicates that the films have antiphase domains in the c-plane and the relationship is SBN //SrTiO3 . The lattice mismatch between the film and the substrate is less than 1%, which contributes to the desirable crystalline quality of SBN films. A smooth surface of roughness Rms=0.8 nm was obtained by controlling the morphology of the initial growth layer in a two-step deposition.

High power durable SAW filter with epitaxial aluminium electrodes on 38.5/spl deg/ rotated Y-X LiTaO/sub 3/ by two-step process sequence in titanium intermediate layer

High power durable electrodes with epitaxial aluminium (Al) films have been successfully grown on 38.5/spl deg/ rotated Y-X LiTaO/sub 3/ substrates. We reported epitaxial Al films on 64 rotated Y-X LiTaO/sub 3/ with titanium (Ti) intermediate layer in our previous papers. Despite a quite similar crystal structure, it has been difficult to obtain epitaxial Al on LiTaO/sub 3/ due to the different cut angle suitable for SAW devices. We found that a two-step sequence in the deposition temperature of Ti intermediate layer could make it possible for Al/Ti structure to grow epitaxially on 38.5/spl deg/ rotated Y-X LiTaO/sub 3/. It is most important for epitaxial growth to fabricate a Ti initial region, constructing the interface against the substrate, at high deposition temperature. Clear symmetrical spots were observed in x-ray pole figure from films prepared by a two-step process sequence, which suggests a twin structure of the Al crystal film. Power durability was evaluated from acceleration test as for ladder-type SAW filter with center frequency of 1.9GHz. Devices with epitaxial Al electrodes have had 3.3 /spl times/ 10/sup 9/ hours of estimated lifetime at input power of 0.8W and in ambient temperature of 50, which is drastically longer than those with polycrystalline electrode. Epitaxial electrodes with extremely less grain boundary can improve power durability because self-diffusion of Al atoms occurs mainly in the grain boundary of the film.

Influence of H2O partial pressure in the sputtering chamber on the crystallinity and relative dielectric constant of SrTiO3 thin film prepared at low substrate temperature

Abstract It has been investigated here how the crystallinity and relative dielectric constant of low-temperature sputtered SrTiO 3 film is influenced by H 2 O partial pressure the in deposition chamber. With increasing H 2 O partial pressure, the relative dielectric constant of SrTiO 3 film prepared at a substrate temperature of 280°C tends to degrade, and the amorphous layer in the interface between bottom electrode and dielectric film increases in thickness observed in TEM images. H 2 O partial pressure and relative dielectric constant can be estimated from in situ spectrum analysis of hydrogen in plasma emission. The monitoring and control of H 2 O partial pressure are necessary for the SrTiO 3 deposition process to achieve stable dielectric properties.

Crystallinity and Surface Morphology of Epitaxial (Sr,Ba)Nb 2 O 6 Thin Film Prepared by Pulsed Laser Deposition with Two-Step Growth Sequence

We have prepared epitaxially grown SBN thin film with c-axis orientation by an ArF pulsed laser deposition on SrTiO 3 (100). Pole figures show that the SBN film has a twin structure aligned at ±18.4: with the a-axis of SrTiO 3 . The epitaxial relationship of SBN //SrTiO 3 is determined. The lattice mismatch between SBN and SrTiO 3 is approximately 0.9%, which contributes to the desirable crystallinity of the SBN thin film. Furthermore, we have tried to form the SBN film with a two-step growth sequence in order to improve surface morphology. The amorphous initial growth region of 5 monolayers (2 nm thickness) is prepared with no substrate heating followed by post-annealing treatment at 720°C and additional growth on the initially crystallized layer. RHEED patterns of the SBN film with the two-step growth sequence have remained streaky throughout the film formation, compared with spotty patterns observed from films prepared by a conventional sequence. Atomic force microscope (AFM) images show that both the initial stage and final stage have extremely flat surfaces of rms≦ lnm which is a remarkably improved figure compared with the roughness rms ≧3nm for the film deposited at 720°C from the initial stage. These results suggest that the two-step growth sequence makes it possible to improve surface morphology to a nanometer level.

Preparation and electrical properties of (Zr,Sn)TiO{sub 4} dielectric thin films by laser ablation

(Zr,Sn)TiO{sub 4} is considered as a promising dielectric material for microwave devices owing to the temperature stability of capacitance and excellent microwave properties. Preferential (111)-oriented (Zr,Sn)TiO{sub 4} thin film was obtained by an ArF laser ablation. Properties of the crystallized film were as follows; the temperature coefficient of capacitance TCC was 17.6 ppm/C at 3 MHz and the dielectric constant {var_epsilon}{sub r}, 38 in the microwave range of 1GHZ--10GHz. It has turned out that the crystallization of this material is quite effective for improving dielectrical properties. Surface morphologies were observed by atomic force microscope (AFM). Grains grew on the crystallized film at 1 {micro}m x 1 {micro}m size.