Material and resonator design dependant loss in langasite bulk acoustic wave resonators at high temperatures

Single crystalline langasite (La₃Ga₅SiO₁₄) resonators exhibit piezoelectrically exited bulk acoustic waves up to temperatures close to its melting point at 1470°C. The loss observed in bulk acoustic wave devices depends on the materials properties and the resonance frequency. Anticipated operation at extremely high temperatures requires the understanding of both influences and enables tailoring of both properties to reduce the loss. Electrical impedance spectroscopy and diffusion runs using stable isotopes are the key methods used to study the atomistic transport processes and the electromechanical properties of langasite. At elevated temperatures, electrical as well as mechanical loss contributions are found. In particular, oxygen vacancies are responsible for strong losses which can be, however, suppressed by light donor doping. Above 650°C, the impact of the conductivity related loss becomes pronounced. Further, the coupling of mechanical and electrical properties due to the piezoelectric effect causes a loss maximum at the dielectric relaxation frequency. Doping of langasite modifies the electrical conductivity and shifts, thereby, the dielectric relaxation frequency. Consequently, the choice of appropriate dopants and/or of the resonance frequency far off the latter frequency minimizes the loss. The concept is demonstrated and leads to an improved performance of resonant sensors at high temperatures.