Analysis of dual-mode thin film bulk acoustic resonators based on polar c-axis tilted wurtzite gallium nitride

Abstract

We present the analytical study of dual-mode thin film bulk acoustic resonators (FBARs) based on wurtzite gallium nitride (GaN) thin films with tilted c-axis orientation. The normal-plane and in-plane electric polarization vector components caused by the titled c-axis orientation induce the coexistence of the thickness longitudinal and thickness shear modes in the thin film resonator. Due to the c-axis orientation dependency of material properties including elastic stiffness, dielectric constants, and piezoelectric coefficients, bulk wave properties of FBARs including acoustic velocity and electromechanical coupling coefficient, and electrical impedance spectra are greatly affected by the tilt angle. It is found that the pure longitudinal mode occurs at θ=0° and 66.4°, and the pure shear mode occurs at θ=42.8° and 90°. In addition, the electromechanical coupling coefficient for thickness shear mode has a maximum value 3.18% at θ=34.2°, which is larger than the maximum value for pure thickness longitudinal mode 2.44% at θ=0°. The calculation results show that FBARs based on tilted c-axis wurtzite GaN thin films are promising for frequency control device design and applications.