Zinc oxide and magnesium zinc oxide-based multilayer structures for tunable surface acoustic wave devices

This dissertation addresses the research and development of a new piezoelectric semiconductor material, the ternary compound Mg_xZn_1−x O, development of simulation tools to analyze Mg_xZn _1−xO based piezoelectric multilayer structures, the application of Mg_xZn_1−xO/ZnO multilayer structures to piezoelectric property tailoring, and design and analysis of tunable surface acoustic wave (SAW) devices built on Mg_xZn_1−xO/ZnO heterostructures.; Mg_xZn_1−xO is a newly developed wide bandgap ternary semiconductor, formed by alloying MgO with ZnO. MgO, which has a rock-salt crystal structure, is non-piezoelectric, whereas wurtzite ZnO is piezoelectric. MgO has higher bulk acoustic wave velocities compared to ZnO. It is shown for the first time that wurtzite Mg_xZn_1−xO, with Mg mole concentrations less than 35%, is piezoelectric. Furthermore, for piezoelectric Mg_xZn_1−xO, the acoustic velocities increase and piezoelectric coupling coefficients decrease as the Mg composition increases. The Mg_xZn_1−xO material constants are calculated and used in SAW simulations. It is demonstrated that piezoelectric properties can be tailored in two ways; (i) by adjusting the composition of the ternary, and (ii) by using Mg_xZn_1−xO/ZnO multilayer structures. Piezoelectric Mg_xZn_1−xO films are grown on r-plane sapphire (r-Al₂O₃) substrates using the MOCVD technique, and SAW test devices are fabricated on these films. Measurement results from these devices are in agreement with theoretical predictions.; A novel monolithically integrated tunable SAW (MITSAW) device is proposed, designed and analyzed. The device consists of Mg_xZn_1−x O/ZnO multilayer structures grown on r-Al₂O₃ substrates. It integrates piezoelectric and semiconducting properties in one material system. The ZnO/r-Al₂O₃ system is chosen for its high acoustic velocities and large piezoelectric coupling coefficients, which result in high operating frequencies and low loss. The SAW velocity can be tuned using the interaction between electrons at the Mg_xZn_1−x O/ZnO interface and the electric fields accompanying the propagating SAW. The piezoelectric properties of the MITSAW are analyzed using the transfer matrix method. A large tunability range with high acoustic velocities is predicted for MITSAW devices using the Sezawa wave mode, leading to better performance compared to GaAs and hybrid GaAs/LiNbO₃ tunable SAW devices. These integrated tunable SAW devices are expected to have broad applications in communications, and in chemical and biochemical sensing.