Electromechanical Coupling Coefficient kt2 of Cr doped AlN Films Grown by Sputtering Deposition

Electromechanical coupling coefficients of ScAlN films are higher than those of other wurtzite piezoelectric films. Therefore, ScAlN films have attracted attention as a new piezoelectric layer in BAW resonators. However, Sc target for ScAlN film sputtering growth are very expensive because scandium is rare earth material. Recently, enhancement of d33 in Cr doped AlN film was found theoretically and experimentally. In this study, we investigated kt2 and longitudinal wave velocity for GHz range in c-axis oriented Cr0<x<0.16Al1-xN films. c-Axis oriented CrAlN films were grown by Cr ingot RF sputtering deposition. Cr concentration in CrAlN film were controlled by varying the amount of Cr ingots on Al target. kt2 and longitudinal wave velocity were estimated from a longitudinal wave conversion loss in CrAlN film HBARs with Cu/CrAlN/Ti/Silica glass substrate. The kt2 of 5.5 % in Cr0.01Al0.99N film was approximately 1.2 times higher than that in pure AlN film. On the other hand, the kt2 significantly decreased with increasing Cr concentration from 0.04 to 0.16. This decrease of kt2 may be due to the low crystalline orientation or the phase transition from a piezoelectric to a non-piezoelectric crystal phase. In addition, the longitudinal wave velocity decreased with increasing Cr concentration.