Tadasu Hosokura

Measurement of the dielectric constant of thin films by terahertz time-domain spectroscopic ellipsometry.

We present a reflection-type terahertz time-domain spectroscopic ellipsometry (THz-TDSE) technique for measuring the complex dielectric constants of thin-film materials without replacement of the sample. THz-TDSE provides complex dielectric constants from the ratio of the complex amplitude reflection coefficients between p- and s-polarized THz waves. The measured dielectric constants of doped GaAs thin films show good agreement with predictions of the Drude model, even though the film thickness is of the order of a tenth of the penetration depth of the THz waves. In addition, we demonstrate the measurements of soft-phonon dispersion in SrTiO(3) thin films deposited on a Pt layer. The obtained dielectric constants agree well with the predictions of a generalized harmonic oscillator model.

Analysis of Dielectric Response of TiO2 in Terahertz Frequency Region by General Harmonic Oscillator Model

The terahertz range dielectric response of polycrystalline TiO2 (rutile) ceramics and rutile single crystals were investigated by analyzing infrared-active phonons by both Fourier-transform far-infrared (FT-FIR) reflectivity measurement and terahertz time-domain spectroscopy (THz-TDS). The dielectric functions at room temperature measured by THz-TDS are well expressed using the general harmonic oscillator model. We found that the imaginary part of the dielectric permittivity of the TiO2 ceramic is threefold higher than that of the rutile single crystal at 300 K. The large dielectric loss is attributed to the additional damping of the A2u mode or/and an increase in the phonon damping. The low-temperature dielectric loss of the rutile single crystals is exceedingly low. However, TiO2 ceramics shows little temperature dependence. These results indicate that extrinsic scattering significantly affects the dielectric loss of the TiO2 ceramics in the THz region.

Fabrication and Electrical Characterization of Epitaxially Grown (Ba,Sr)TiO3 Thin Films Prepared by Sol-Gel Method

Epitaxially grown (Ba,Sr)TiO3 thin films were prepared on platinum-coated silicon substrate by sol-gel method using a (Ba,Sr)TiO3 sol derived from Ba(CH3COO)2, Sr(CH3COO)2 and Ti(O-i-C3H7)4. The morphology of the films was found to depend on the annealing condition. A columnar structure was obtained for (Ba,Sr)TiO3 thin film by annealing at 800 °C and a columnar grain was found to be single crystal by transmission electron microscope (TEM). The columnar grown film exhibits a preferred (111) orientation that follows the (111) orientation of Pt substrate. Measurement of the C-V in MFM was configured in order to demonstrate good dielectric properties. Obtained films showed high voltage tunability.

Dielectric properties of (Ba,Sr)TiO 3 thin films with varied microstructures prepared by the chemical solution deposition method for thin-film capacitors and ferroelectric varactors

We prepared epitaxially grown three-axis-oriented (Ba_0.7S_0.3)TiO₃ thin films on (100) platinum-coated (100) MgO single-crystal substrates by the chemical solution deposition method, using a solution derived from Ba(CH₃COO)₂, Sr(CH₃COO)₂, and Ti(O-i-C₃H₇)₄. Microstructures of fabricated thin films depend strongly on the fabrication process, especially on the annealing condition. A (Ba,Sr)TiO₃ thin film fabricated with an annealing temperature of 1073K was found to be a single crystal by transmission electron microscopy. The single-crystal (Ba,Sr)TiO₃ thin film exhibited a (100) three-axis-orientation, which followed the (100) orientation of the platinum electrode on the MgO single-crystal substrate. A (100) three-axis-oriented (Ba,Sr)TiO₃ thin film may be useful for preparing a thin-film capacitor.

Orientation-controlled BaTiO3 thin films fabricated by chemical solution deposition

We synthesized orientation-controlled (100), (110), and (111) BaTiO3 films by the chemical solution deposition method. The electric properties of the fabricated BaTiO3 thin films were evaluated, and dielectric constants of 2000 were found for the (110)-oriented and (111)-oriented BaTiO3 film, with temperature dependence stabilized from 20 °C to 150 °C.

Nanoscale characterization of ferroelectric materials by scanning probe microscope under ultrahigh vacuum

The downsizing trend in multilayer ceramic capacitors (MLCCs) will reach a limit as long as a conventional way of materials development continues. Creation of advanced materials and precise control of materials properties will be a key technology to break through the current situation. To address this issue, scanning probe microscopy under ultrahigh vacuum (UHV-SPM) is employed to clarify materials properties from nanoscale perspective. Kelvin probe force microscopy allows us to obtain surface potential mapping on degraded dielectrics with highly spatial resolution, which is promising to consider degradation mechanism of MLCCs. Piezoresponse force microscopy enables not only to evaluate size dependent properties of ferroelectric nanoislands but also to investigate temperature dependence of domain images in Sn doped strontium titanate ceramics. Characterization of ferroelectrics based on UHV-SPM will open a new way to design future MLCCs.