Dielectric properties of SrBi2Ta2O9 films in the low-temperature range

Abstract

Ferroelectric materials, which show various interesting properties, such as high-dielectric constants, large spontaneous polarization, and optical nonlinearities, have attracted great attention for device application in nonvolatile memory, uncooled infrared detectors, and optical sensor protection [1–3]. In recent years, ferroelectric thin films are being used to develop a new class of tunable microwave devices. These devices are based on the large electric field dependent dielectric constant observed in ferroelectric materials, which are desirable to have a large capacitance change ratio [tunability(η) = (Cmax − Cmin)/Cmax] under a certain electric-field range accompanied by a small dielectric loss. The devices employing ferroelectric films have high-tuning speed, high-radiation resistance, low power consumption, and low cost [4–9]. In this letter, we report on the dielectric properties and the voltage tunability of SrBi2Ta2O9 thin films in low temperature range of 10–300 K. Origin for the temperature effect of the dielectric properties and the voltage tunability of SBT films, which may arise from switchable polarization that is frozen at lower temperature, is discussed. These results provide additional insight into their use for ferroelectric microwave device applications. The SBT thin films were fabricated on platinized silicon (Pt/Ti/SiO2/Si) using pulsed laser deposition (PLD) assisted by a dc glow discharge plasma. The laser used for SBT thin film deposition was an ArF (Lambda Physik LPX220icc, wavelength 193 nm) excimer laser with 5 Hz repetition frequency, 17 ns pulse duration, and an energy of 160 mJ/pulse. The output laser beam was focused onto a rotating SBT ceramic target at an angle of 45◦ by a UV lens with a focal length of 50 cm. The platinized silicon (Pt/Ti/SiO2/Si) substrates were mounted onto a heated substrate holder and placed parallel to the target at a distance of 4–5 cm. The stability of the incoming beam was monitored using an energy meter. In order to minimize the chemical reaction between the film and substrate and control stoichiometry and structure, a low oxygen pressure dc glow discharge was used during laser deposition. A copper ring was placed halfway between the target and substrate. The substrate and target were electrically grounded while the ring was held at +700 V.