Sr_xBa_(1−x)TiO₃ Thin Films for Active Microwave Applications

Abstract

The dielectric constant, loss tangent and Curie temperature for Sr_xBa_(1ȡx)TiO₃ (SBT) thin films with x = 0.2 − 0.8 have been investigated at microwave frequencies. SBT films (0.5 − 3 εm thick) were grown on (100) MgO and LaAlO₃ substrates by pulsed laser deposition at substrate temperatures from 850 – 900 °C in 0.35 Torr of oxygen. Deposited ferroelectric films were single phase, highly oriented, and characterized by x-ray rocking curve widths of ≤ 0.5°. Highly oriented SBT films with x-ray rocking curve widths of 72 arc seconds were observed. In general, the thin film dielectric constant at microwave frequencies is low (200–950) compared to the reported bulk value, but strongly dependent on the Sr/Ba ratio. Biasing of a ferroelectric interdigital capacitor (< 200 kV/cm) produces a change in the dielectric constant which resulted in a phase shift in the reflected signal (S₁₁) measured as a function of frequency from 100 MHz to 10 GHz. The dielectric loss tangent measurement, as measured at room temperature and 9.2 GHz, ranges from 0.1 to 1.2 × 10⁻³ and depends on the Sr/Ba ratio. These data show that SBT thin films are suitable for the development of frequency tunable microwave circuits and components.