DC-Switchable and Tunable Piezoelectricity in RF Thin-Film BST Capacitors

Ferroelectric materials such as (Ba_x Sr_1-x) TiO₃ (BST) exhibit a non-zero piezoelectric coefficient under a DC field. The piezoelectric coefficient increases from 0 as the applied DC bias increases from 0V. This gives rise to electrostrictive resonance behaviour in capacitors fabricated with these materials. This paper presents the results of high frequency measurements and simulations of induced piezoelectricity for multilayer BST thin film structures under a 10V DC bias. The Mason model and transmission line theory is used as the basis of the simulation and our work extends it to include the effects of other layers in the passivated capacitor device. High-frequency capacitors with 100 nm thick (Ba_0.5Sr_0.5) TiO₃ and Pt electrodes were fabricated on polycrystalline Al₂O₃ ceramic and Si substrates with an amorphous oxide buffer layer. Correlation between measurements and simulations allowed us to evaluate the piezoelectric coefficient, its switchability and tunability as well as the contribution of the multilayer film structure to the electrostrictive resonance behaviour. The model fits very well with the measurements and can be used to adjust the resonance peaks to the desired frequencies.