Evaluation of piezoelectric Ta2O5 thin films deposited on sapphire substrates

X-axis-oriented tantalum pentoxide (Ta
₂
O
₅
) piezoelectric thin films were deposited on sapphire (Al
₂
O
₃
) substrates using an RF magnetron sputtering system. The crystallinity and Rayleigh-type surface acoustic wave (R-SAW) propagation properties of the thin films were evaluated. For a Ta
₂
O
₅
thin film deposited on the R-Al
₂
O
₃
substrate plane, a similar preferential (200) orientation to that for a Ta
₂
O
₅
thin film deposited on a SiO
₂
glass substrate was obtained. For the first mode of the R-SAW on the R-plane Al
₂
O
₃
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-Al
₂
O
₃
substrate plane, the possibility of the epitaxial growth of a hexagonal Ta
₂
O
₅
thin film with (203) orientation was shown. Furthermore, for the R-Al
₂
O
₃
substrate plane, the possibility of the homoepitaxial growth of an orthorhombic Ta
₂
O
₅
thin film with (201) orientation by using an oxide Ta thin film (TaO
_x
) 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 Ta
₂
O
₅
has no piezoelectricity and the Ta
₂
O
₅
thin film deposited on the TaO
_x
buffer layer had polycrystalline structure.