Abstract
In our previous research we already demonstrated micro acoustic devices, such as membrane based thin film bulk acoustic shear wave resonators and surface acoustic shear wave resonators, based on Metal-Organic-Vapour-Phase-Epitaxial (MOVPE) grown highly oriented a-plane piezoelectric material. Although 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 as also design and simulation of micro acoustic devices is nowadays a well established knowledge, the linkage between both is quite a technological challenge. Using an adapted MOVPE growth process for a-plane GaN on r-plane sapphire with a process linked improved surface quality; the challenge to build up high-overtone bulk acoustic wave resonators (HBAR) with a shear polarization of the acoustic wave was risen within this research. Different designs of MEMS-based prototypes of HBARs were processed on a-plane GaN after intensive simulations, their acoustic electrical behaviour analyzed and the temperature coefficient of frequency determined.
Highlights
Using an adapted MOVPE growth process for a-plane GaN on r-plane sapphire with a process linked improved surface quality; the challenge to build up high-overtone bulk acoustic wave resonators (HBAR) with a shear polarization of the acoustic wave was risen within this research
Different designs of MEMS-based prototypes of HBARs were processed on a-plane GaN after intensive simulations, their acoustic electrical behaviour analyzed and the temperature coefficient of frequency determined
High-overtone bulk acoustic wave resonators (HBARs) devices are based on the effect of an almost total wave reflection at the substrates bottom interface to air at the backside of the substrate
Summary
High-overtone bulk acoustic wave resonators (HBARs) devices are based on the effect of an almost total wave reflection at the substrates bottom interface to air at the backside of the substrate. Instead of using a sputter deposition process for the piezoelectric resonator layer on top of a thin film metal layer, a MOVPE process was used to guarantee highly and homogenous oriented a-plane GaN. Crystal growth of group-III-nitride layers is a currently well established process for light emitting devices and high-power transistors and recently high-frequency application as surface or bulk acoustic wave devices [1],[2],[3]. These works are mainly focused on c-plane oriented material like c-plane AlN or GaN most on silicone or some on c-plane sapphire substrates. In LEDs grown with c-axis orientation, the piezoelectric polarization fields along the growth axis lead to a reduction in quantum efficiency, especially for longer wavelength LEDs
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