Field and temperature dependent parameters of the dc field induced resonances in BaxSr1-xTiO3-based tunable thin film bulk acoustic resonators.

Electrically tunable solidly mounted thin film bulk acoustic resonators based on Ba0.25Sr0.75TiO3 and BaTiO3 films are fabricated and measured in wide dc bias voltage and temperature ranges. At room temperature, the tunability of the series and parallel resonances for the Ba0.25Sr0.75TiO3 resonator are 1.7% and 0.3%, respectively, for 15 V bias voltage applied over the 350 nm thick ferroelectric film (43 V/μm). The electromechanical coupling coefficient increases with dc bias up to 3.7% at 15 V. The measured tunability and coupling coefficient are limited partly by the quality of the used films. Potentially, they may be substantially increased for high quality films allowing application of higher dc fields. The resonator quality factor is approximately 100. The measured resonator response is in good agreement with available models based on the electromechanical equations describing the ferroelectric film under applied dc and ac electric fields. Measurements of the resonance frequencies of the Ba0.25Sr0.75TiO3 resonator as a function of temperature in the range 40–520 K reveal a sharp step at 150 K which is related to a structural phase transition of the ferroelectric material. The series resonance frequency is tuned to lower frequencies with increasing dc bias for all temperatures, while the parallel resonance frequency reveals a change in the sign of the tunability at approximately 150 K, from being tuned to lower frequencies at high temperatures to being tuned to higher frequencies at low temperatures. Measurements of BaTiO3 resonators in the temperature range 300–520 K are presented and compared to the Ba0.25Sr0.75TiO3 results. [ABSTRACT FROM AUTHOR]