Co-doped BST thin films for tunable microwave applications

Ba0.5Sr0.5Ti1.0O3.0 ferroelectric thin films with different concentrations of Co ion dopant (undoped, 0.25 mol%, 0.5 mol% and 1.0 mol%) were deposited on sapphire < 0001> substrates by the chemical solution deposition (CSD) method. The scanning electron microscopy (SEM) showed that increasing Co doping concentration would cause a decrease in grain size while the thin film surface remains crack-free. According to the X-ray diffraction results, all sample films were composed of cubic perovskite structures. In order to characterize the dielectric properties of the films, coplanar waveguides (CPW) were fabricated on top of the thin films using the photolithography process. The dielectric properties of the Co-doped BST films were found to be strongly dependent on the Co concentration. As the Co content increases, the dielectric loss tangent (tan δ) of the BST films at 12 GHz decreases from 0.17 to 0.038. The improvement of dielectric loss could be related to the decrease in the concentrations of intrinsic oxygen vacancies, thus lowering the electron concentration. However, the increase of Co concentration also resulted in the decrease of BSTʹs dielectric constant (εr) and tunability, leading to a larger space consumption of BST thin film based microwave devices.