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 experiment- - al 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.