Sohei Okazaki

Characterization of LiNb1-xTaxO3 composition-spread thin film by the scanning microwave microscope

Dielectric property of a composition-spread LiNb 1-x Ta x O 3 thin film, fabricated by the combinatorial pulsed-laser deposition (PLD) method, was systematically characterized by the scanning microwave microscope (SμM). Measured frequency shift showed a broad maximum around x = 0.2-0.5, and gradually decreased with x, resulting in a lower dielectric constant in the LiTaO 3 side compared to the LiNbO 3 side. The trend of frequency shift has been revealed to possess a strong correlation with the sharpness of XRD peak, suggesting that lowering of dielectric constant is principally brought about by the degradation of crystallinity.

Wet Etching of Amorphous TiO2 Thin Films Using H3PO4–H2O2 Aqueous Solution

We report on the wet etching of amorphous undoped and Nb-doped TiO2 thin films using H3PO4–H2O2 etching solution. The etching rate (R) showed a maximum at a H3PO4 concentration of approximately 50 wt % at 80 °C, suggesting that H2PO4- and/or H3O+ is responsible for the etching reaction. The addition of H2O2 to H3PO4 solution significantly enhanced R, and an optimized solution exhibited an R of 13 nm/min at 80 °C, which is one order of magnitude higher than that using H2SO4. These results demonstrate that H3PO4–H2O2 aqueous solution is an effective etchant for TiO2-based amorphous thin films.

Fabrication of Nb-Doped TiO2 Transparent Conducting Films by Postdeposition Annealing under Nitrogen Atmosphere

Here, we report that highly conductive polycrystalline anatase Nb-doped TiO2 (TNO) thin films can be prepared via crystallization of amorphous precursors under N2 atmosphere. An optimized TNO film on a glass substrate exhibited a low resistivity of 8.4 ×10-4 Ω cm and an absorbance of 6% at a wavelength of 460 nm. These transport and optical properties were comparable to those of TNO films fabricated by vacuum annealing. This demonstrates the potential of TNO as an electrode for GaN-based light-emitting diodes.

Quantitative Conductivity Mapping of SrTiO3–LaAlO3–LaTiO3 Ternary Composition-Spread Thin Film by Scanning Microwave Microscope

We carried out quantitative mapping of conductivity σ for the SrTiO3–LaAlO3–LaTiO3 (STO–LAO–LTO) ternary composition-spread thin film by using the scanning microwave microscope (SµM). The σ was evaluated from the shifts in Q-value with reference to the standard Ti1-xNbxO2 composition-spread thin film. Results for the ternary system showed excellent agreement with the literature values of σ in the binary STO–LAO and STO–LTO systems, confirming the accuracy of the present analysis method. An electric phase diagram was proposed for the ternary system based on the quantitative film-conductivity data. Metallic conduction in the ternary system was observed in a wide area close to the STO–LTO side, which gave way to the hopping conduction if LAO component exceeded ca. 35%.

The Development of Scanning Microwave Microscope for High-Throughput Characterization of Dielectric and Conducting Materials at Low Temperatures

We developed a scanning microwave microscope (SμM) designed for characterizing local electric properties at low temperatures. A high-Q λ/4coaxial cavity was used as a sensor probe, which can detect the change of quality factor due to the tip-sample interaction with enough accuracy. From the measurements of combinatorial samples, it was demonstrated that this SμM system has enough performance for high-throughput characterization of sample conductance under variable temperature conditions.