Analysis of SAW properties in ZnO/Al _xGa _1-xN/c-Al ₂O ₃ structures

Piezoelectric thin films on high acoustic velocity nonpiezoelectric substrates, such as ZnO, AlN, or GaN deposited on diamond or sapphire substrates, are attractive for high frequency and low-loss surface acoustic wave devices. In this work, ZnO films are deposited on Al _xGa _1-xN/c-Al ₂O ₃ (0 ≤ x ≤ 1) substrates using the radio frequency (RF) sputtering technique. In comparison with a single Al _xGa _1-xN layer deposited on c-Al ₂O ₃ with the same total film thickness, a ZnO/Al _xGa _1-xN/c- Al ₂O ₃ multilayer structure provides several advantages, including higher order wave modes with higher velocity and larger electromechanical coupling coefficient (K ²). The surface acoustic wave (SAW) velocities and coupling coefficients of the ZnO/Al _xGa _1-xN/c-Al ₂O ₃ structure are tailored as a function of the Al mole percentage in Al _xGa _1-xN films, and as a function of the ZnO (h ₁) to Al _xGa _1-xN (h ₂) thickness ratio. It is found that a wide thickness-frequency product (hf) region in which coupling is close to its maximum value, K _max ², can be obtained. The K _max ² of the second order wave mode (h ₁ = h ₂) is estimated to be 4.3% for ZnO/GaN/cAl ₂O ₃, and 3.8% for ZnO/AlN/c-Al ₂O ₃. The bandwidth of second and third order wave modes, in which the coupling coefficient is within ±0.3% of K _max ², is calculated to be 820 hf for ZnO/GaN/c-Al ₂O ₃, and 3620 hf for ZnO/AlN/cAl ₂O ₃. Thus, the hf region in which the coupling coefficient is close to the maximum value broadens with increasing Al content, while K _max ² decreases slightly. When the thickness ratio of AlN to ZnO increases, the K _max ² and hf bandwidth of the second and third higher wave modes increases. The SAW test devices are fabricated and tested. The theoretical and experimental results of velocity dispersion in the ZnO/Al _xGa _1-xN/c-Al ₂O ₃ structures are found to be well matched.