Evaluation of Piezoelectric Ta2O5 Thin Films Deposited on Sapphire Substrates

X-axis-oriented tantalum pentoxide (Ta2O5) piezoelectric thin films were deposited on sapphire (Al2O3) substrates using an RF magnetron sputtering system. The crystallinity and Rayleigh-type surface acoustic wave (R-SA W) propagation properties of the thin films were evaluated. For a Ta2O5 thin film deposited on the R-Al2O3 substrate plane, a similar preferential (200) orientation to that for a Ta2O5 thin film deposited on a SiO2 glass substrate was obtained. For the first mode of the R-SA W on the R-plane Al2O3 sample, a coupling factor of 1.78% and a phase velocity of 4,895 m/s were obtained for a normalized film thickness of 0.225. For the c-Al2O3 substrate plane, the possibility of the epitaxial growth of a hexagonal Ta2O5 thin film with (203) orientation was shown. Furthermore, for the R-Al2O3 substrate plane, the possibility of the homoepitaxial growth of an orthorhombic Ta(2)O5 thin film with (201) orientation by using an oxide Ta thin film (TaOx) as a buffer layer was also shown. However, no increase in coupling factor and no major improvement in propagation loss were observed upon these crystallizations because the hexagonal Ta2O5 has no piezoelectricity and the Ta2O5 thin film deposited on the TaOx buffer layer had poly crystalline structure.