c-Axis-Tilted ScAlN Film on Silicon Substrate for Surface Acoustic Wave Device

ScAlN films exhibit higher piezoelectricity than the AlN films. Furthermore, recent studies have shown that the appearance of ferroelectricity in the ScAlN films. Therefore, ScAlN films are currently being investigated to explore their potential for use in elastic wave devices for next-generation mobile networks. Surface acoustic wave (SAW) devices with a high frequency (gigahertz-range), a high electromechanical coupling coefficient $\boldsymbol{K}^{\boldsymbol{2}}$ and a high $\mathbf{Q}$ factor are required for filters and duplexers in the smartphones. Recently, ScAlN film/high BAW velocity substrate (e.g. diamond and SiC) structures were reported to have a high $\boldsymbol{K}^{\boldsymbol{2}}$ , whereas their substrates are very expensive. Conversely, silicon is known as inexpensive substrates and is suitable for device integration. We have previously shown that $\boldsymbol{K}^{\boldsymbol{2}}$ values of crystal class (6mm) including ZnO and AlN increases with their c-axis tilt angle. In this study, we theoretically demonstrated that the electromechanical coupling coefficient $\boldsymbol{K}^{\boldsymbol{2}}$ of SAW propagating in ScAlN film/Si substrate layered structure increases with c-axis tilt angle. In addition, a c-axis tilted ScAlN films were prepared on Si substrates by a sputtering method.