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Single-crystal films of ZnO have been epitaxially grown on the (0001) and (011̄2) planes of sapphire by rf sputtering. Crystalline structures and electrical properties of the films were investigated. Surface acoustic wave (SAW) properties, including a phase velocity, a coupling coefficient, a propagation loss, and a temperature coefficient of delay, were measured for SAW propagating along the c-axis of the ZnO films, on the (011̄2) planes of sapphire. Availability of this structure for high-frequency SAW devices has been demonstrated by a filter with a 1050-MHz center frequency.

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Crystallographic structure of as-grown epitaxial Pb͑Zr,Ti͒O 3 ͑PZT͒ films was investigated with regard to the Zr/Ti ratio and crystalline orientation. PZT films with ͑001͒ and ͑111͒ orientation were epitaxially grown on ͑100͒ and ͑111͒SrTiO 3 substrates respectively using radio-frequency ͑rf͒ sputtering. Four circle x-ray diffraction measurements revealed that the crystallographic dependence on Zr/Ti composition in PZT films was much different from bulk PZT. In particular, ͑001͒-oriented PZT films showed tetragonal structure even in the Zr/Ti composition of 70/30 where the bulk PZT ceramics are rhombohedral phase. In addition, although ͑001͒-oriented PZT films with Zr/Ti ratio of 53/47 and 70/30 showed tetragonal structure, ͑111͒-oriented PZT films with the same Zr/Ti ratio were identified as the rhombohedral structure. The cell volume of the PZT films with both orientations increased, suggesting the excess Pb atoms in the films due to the impinging energetic sputtered particles induces the anomalous crystalline structure of the PZT films. Dielectric properties of the PZT films exhibited stable value independent of Zr/Ti ratio and characteristic increase of dielectric constant near Zr/Tiϭ53/47 could not be observed. These results suggest that the internal stress due to the sputter deposition plays an important roll in the unique characteristics of crystallographic and electrical properties of the epitaxial PZT films.

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Piezoelectric thin films are of increasing interest in low-voltage micro electromechanical systems for sensing, actuation, and energy harvesting. They also serve as model systems to study fundamental behavior in piezoelectrics. Next-generation technologies such as ultrasound pill cameras, flexible ultrasound arrays, and energy harvesting systems for unattended wireless sensors will all benefit from improvements in the piezoelectric properties of the films. This paper describes tailoring the composition, microstructure, orientation of thin films, and substrate choice to optimize the response. It is shown that increases in the grain size of lead-based perovskite films from 75 to 300 nm results in 40 and 20% increases in the permittivity and piezoelectric coefficients, respectively. This is accompanied by an increase in the nonlinearity in the response. Band excitation piezoresponse force microscopy was used to interrogate the nonlinearity locally. It was found that chemical solution-derived PbZr(0.52)Ti(0.48)O(3) thin films show clusters of larger nonlinear response embedded in a more weakly nonlinear matrix. The scale of the clusters significantly exceeds that of the grain size, suggesting that collective motion of many domain walls contributes to the observed Rayleigh behavior in these films. Finally, it is shown that it is possible to increase the energy-harvesting figure of merit through appropriate materials choice, strong imprint, and composite connectivity patterns.

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