Fabrication of KNbO_3 Thin Film and Application to SAW Device for Future Mobile Tohoku University Communication

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In this research, properties of KNbO_3 films, which was deposited on SrTiO_3 (STO) substrate using a 2 cathode sputtering method and a metal organic chemical vapor deposition (MOCVD) method, were investigated for surface acoustic wave (SAW) applications.1. We investigated the theoretical properties of SAW in KNbO_3 thin film on STO and Si substrates. As the results, we found that (001)<100>KNbO_3// (110)<1-10>STO was the best combination between the substrate and the orientation of the KNbO_3 thin film in the view of both SAW properties and a lattice matching. In this case, electromechanical coupling coefficient (k^2) is 10.4% for the film thickness of 1 wavelength.2. We investigated a best combination of the targets composition of the sputtering. Experimentally, we found that K_2CO_3 : Nb_2O_5=1 : 0 and K_2CO_3 : Nb_2O_5=3 : 1 was the best one, and we could successfully obtained (001) oriented KNbO_3 thin film on STO substrates.3. We observed the polarization distribution of the KNbO_3 thin films using scanning nonlinear dielectric microscope (SNDM), which was developed by one of the investigators of our research project. The results showed that the as-grown KNbO_3 film had an uniform polarization and its direction was along the surface normal if the film was fabricated by appropriate conditions. Also, as the result of the comparison between the polarization measurements and experimentally measured SAW properties, we could observe a large correlation between them. It means that we can estimate the SAW properties of thin films by SNDM without fabricate an actual SAW device