Investigations of a-plane and c-plane GaN-based Synchronous Surface Acoustic Wave Resonators

Metal-Organic-Vapor-Phase-Epitaxy (MOVPE) is a well established process for compound semiconductor layer growth especially of III-V semiconductors as InP, GaAs, and the nitrides GaN or AlN. In the case of the nitrides one can obtain highly oriented a-plane piezoelectric material, well suited for shear-microwave applications, by tilting the c-axis of the piezoelectric unit cell by 90deg to result in a-plane oriented crystallites. The high Q and insertion loss of filters is currently limited by the actual minimum of achievable layer thickness, where the surface is still smooth and pits-free. A lower temperature coefficient of the frequency as for longitudinal polarized waves is expected. We present results of fabrication and measurement of MOVPE grown a-plane and c-plane gallium nitride based surface acoustic wave resonators on r-plane sapphire substrates. Both types of materials were used to build up resonators and their S-parameters, temperature coefficients up to 200degC and wave velocities were measured. Also the wave characteristics under periodic metal grating were computed for the considered substrate configuration, allowing for the simulation of the experimental device using a mixed matrix approach. Both theoretical and experimental admittance of the SAW test devices are subsequently compared.