Shear Bulk Wave Transducer Made of (11\bar20)-Plane Epitaxial ZnO Film on R-Sapphire

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Abstract
A (110)-plane epitaxial ZnO film is deposited on an R-plane ((012)-plane) sapphire substrate. In order to generate a shear bulk wave using the ZnO film, metal electrodes are required at both planes of the ZnO film; namely, the top plane on the top surface of the ZnO film and the bottom plane at the boundary between the ZnO film and the R-sapphire substrate. A c-plane ((0001)-plane) polycrystal ZnO film is conventionally deposited on the metal film on the R-sapphire substrate. Thus, the (110)-plane epitaxial ZnO film for the shear bulk transducer has not been realized until now. The authors realized a (110)-plane epitaxial ZnO film with low electrical resistivity (ρ of the order of 10-4 Ωcm) on an R-sapphire substrate by doping with an impurity such as Al, Ga, or V as a substitute for the metal electrode, and a (110)-plane piezoelectric epitaxial ZnO film with high electrical resistivity (ρ>10+10 Ωcm) on this epitaxial ZnO film with low electrical resistivity for fabricating a shear wave transducer. An Al electrode was deposited on the piezoelectric ZnO film. By using the Al electrode as the top-plane electrode and a (110)-plane epitaxial ZnO film with low electrical resistivity as the bottom-plane electrode substituting for the metal film, strong shear bulk waves have been generated.

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