Increase of electromechanical coupling coefficient k t 2 in (0001)-oriented AlN films by chromium doping

Piezoelectric AlN films possess high bulk acoustic wave velocity, low acoustic attenuation, and good temperature characteristics. However, they have a relatively small electromechanical coupling coefficient kt2 of 6%. It was recently reported that Cr doping in AlN films increased the piezoelectric constant d33. The kt2 of AlN film bulk acoustic wave resonators may thus be enhanced by Cr doping. In this study, we investigated the effect of Cr doping in (0001)-oriented AlN films on kt2 from the frequency characteristics of high overtone bulk acoustic wave resonators. The kt2 of the Cr-doped AlN films was increased for Cr contents of less than 3%. The maximum kt2 observed for the Cr0.012Al0.988N film was 5.9%, which was approximately 1.4 times higher than that of pure AlN film.