Piezoelectricity in ZnO-based multilayer structures for sensor applications

Zinc oxide (ZnO) is a multifunctional material with promising applications. Through proper doping, ZnO can be made transparent and conductive, piezoelectric, or ferromagnetic. Piezoelectric ZnO which possess large electromechanical coupling coefficients has been deposited on high velocity and low loss substrates to develop surface acoustic wave (SAW) and bulk acoustic wave (BAW) devices. Such devices can be used in both communications and sensors fields. This dissertation focuses on the integration of piezoelectric ZnO with various important semiconductor materials, including GaN, SiC, and Si, for the acoustic wave based sensor applications. The piezoelectric properties in ZnO/AlxGa1-xN/c-Al2O3 structure are investigated. The layered structure provides the flexibility to tailor acoustic properties by varying the Al composition in AlxGa1-xN, and the thickness ratio of ZnO to AlxGa1-xN. It is found that a wide thickness-frequency product region where coupling is close to its maximum value can be obtained. This multilayer structure is promising to design the high velocity, low loss and wide bandwidth SAW devices, which can be used in both the communications and sensors fields. A new hybrid deposition technology is developed by using metalorganic chemical vapor deposition (MOCVD) of a thin ZnO buffer layer with a minimum thickness of ~10nm followed by sputtering deposition of a thick (above 2µm) piezoelectric Ni-doped MgxZn1-xO films. This novel deposition technology improves piezoelectric properties of a-MgxZn1-xO (0 ≤ x ≤ 0.3) films on r-Al2O3 for ZnO based tunable SAW device. Piezoelectric ZnO/SiC-6H structure offers high velocities and high coupling coefficients for high frequency SAW devices, which can be used in high-temperature and harsh environments. Epitaxial ZnO films grown on c-plane [0001] oriented SiC-6H substrates by MOCVD form a rectifying heterojunction. Thin film bulk acoustic resonators (TFBAR) are demonstrated using piezoelectric MgxZn1-xO films on Si substrates with an acoustic mirror consisting of alternating quarter-wavelength SiO2 and W layers. The bulk acoustic velocity of MgxZn1-xO increases with increasing Mg composition. A mass sensitivity higher than 103 Hzcm2/ng is obtained. ZnO nanostructures possess unique advantages for the biosensor applications, such as giant surface areas, high sensitivity, biological compatibility, and integratability with Si-based electronics. A prototype of biosensors integrating TFBAR with functionalized ZnO nanotips is demonstrated with improved sensitivity.Ph.D.Includes bibliographical references (p. 126-135)