Gallium Nitride High-order Mode Lamb-wave Resonators and Delay-lines

This work reports on theoretical and experimental study of zero-order as well as high-order symmetric and asymmetric modes of Lamb-wave resonators and delay line structures realized in single-crystalline GaN thin films. We investigate the phase velocity, dispersion characteristics, electromechanical coupling (kt), motional impedance (Rm) and Quality factor (Q) of different Lamb-wave modes (e.g. A0, S0, A1, S1) propagating in GaN thin films. The dispersion characteristics of these modes in GaN-based thin films are simulated and devices with different pitch sizes are fabricated and tested to compare their performance against simulation. High frequency×Q value of 3.6×10 is realized for devices with a pitch size of 4.6 μm, showing a very low motional impedance of Rm= 284 Ω. Depending on the propagation and dispersion characteristics of resonance modes, certain modes are shown to be dominant experimentally (S0 and A1). Furthermore, we utilize the resonance modes in a one-port resonator to build two-port delay-line structures. Such delay-line topologies operate based on travelling acoustic waves, since the free boundary (trenches) do not exist in the direction of wave propagation, reflections from the free edges are minimized, thus reducing spurious modes [1]. This work marks the first steps towards building acoustic diodes in GaN based on the “Acousto-electric Effect” [2]. Upon application of DC electric field to the piezoelectric semiconductor transmission media, acoustic wave can be amplified or attenuated depending on the direction of the electric field.