The fundamental acoustic properties of amorphous Ta 2O 5 and Nb 2O 5 thin films prepared by RF sputtering were evaluated using a Rayleigh-type surface acoustic wave (SAW) on 128° YX-LiNbO 3 and a shear-horizontal-type SAW on 36° YX-LiTaO 3. The elastic constants c 11 and c 44 of the thin films were determined from the measured phase velocities of two SAW modes with mutually perpendicular particle motion. Although the elastic constants of the Ta 2O 5 thin film were found to be approximately 14-17% larger than those of the Nb 2O 5 thin film, it was clarified that the Ta 2O 5 thin film has a stronger SAW trapping effect than the Nb 2O 5 thin film because the density of the former was measured to be 1.5 times higher than that of the latter. On the other hand, the Nb 2O 5 thin film has a smaller propagation loss for a Rayleigh-type SAW, a lower temperature coefficient of delay, and a higher refractive index than the Ta 2O 5 thin film.
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