Analysis of SAW properties in ZnO/Al/sub x/Ga/sub 1-x/N/c-Al/sub 2/O/sub 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/sub x/Ga/sub 1-x/N/c-Al/sub 2/O/sub 3/ (0 /spl les/ x /spl les/ 1) substrates using the radio frequency (RF) sputtering technique. In comparison with a single Al/sub x/Ga/sub 1-x/N layer deposited on c-Al/sub 2/O/sub 3/ with the same total film thickness, a ZnO/Al/sub x/Ga/sub 1-x/N/c-Al/sub 2/O/sub 3/ multilayer structure provides several advantages, including higher order wave modes with higher velocity and larger electromechanical coupling coefficient (K/sup 2/). The surface acoustic wave (SAW) velocities and coupling coefficients of the ZnO/Al/sub x/Ga/sub 1-x/N/c-Al/sub 2/O/sub 3/ structure are tailored as a function of the Al mole percentage in Al/sub x/Ga/sub 1-x/N films, and as a function of the ZnO (h/sub 1/) to Al/sub x/Ga/sub 1-x/N (h/sub 2/) thickness ratio. It is found that a wide thickness-frequency product (hf) region in which coupling is close to its maximum value, K/sub max//sup 2/, can be obtained. The K/sub max//sup 2/ of the second order wave mode (h/sub 1/ = h/sub 2/) is estimated to be 4.3% for ZnO/GaN/c-Al/sub 2/O/sub 3/, and 3.8% for ZnO/AlN/c-Al/sub 2/O/sub 3/. The bandwidth of second and third order wave modes, in which the coupling coefficient is within /spl plusmn/0.3% of K/sub max//sup 2/, is calculated to be 820 hf for ZnO/GaN/c-Al/sub 2/O/sub 3/, and 3620 hf for ZnO/AlN/c-Al/sub 2/O/sub 3/. Thus, the hf region in which the coupling coefficient is close to the maximum value broadens with increasing Al content, while K/sub max//sup 2/ decreases slightly. When the thickness ratio of AlN to ZnO increases, the K/sub max//sup 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/sub x/Ga/sub 1-x/N/c-Al/sub 2/O/sub 3/ structures are found to be well matched.