(Ba,Sr)TiO3 thin films grown by pulsed laser deposition with low dielectric loss at microwave frequencies

We report a method for producing BST films with consistently high figures of merit for tunable microwave applications. (Ba_1-x,Sr_x)TiO₃ (x=0.4, target doped with 1% W) thin films have been deposited using pulsed laser deposition onto (100)MgO substrates. Films were deposited at low partial pressures of oxygen (50 mTorr) at a substrate temperature of 730 °C. An analysis of the X-ray diffraction data indicates that the film has a nearly cubic structure, with the overall lattice parameter enlarged relative to the bulk material due to the presence of oxygen vacancies. A post-deposition anneal of the film in flowing oxygen (1000 °C for 6 h) resulted in a decrease in the lattice parameter while remaining nearly cubic. An analysis of the microwave dielectric properties (1–20 GHz) showed that the annealed film exhibited about 10% tunability for an applied bias field of 67 kV/cm with a dielectric Q(1/tan)>600. Investigation of the films by time-resolved confocal scanning optical microscopy (CSOM) has revealed that there is an out-of-plane polarization at zero applied field (E_DC=0). The results show that the paraelectric response is relatively insensitive to applied field, while the ferroelectric response is correlated with the growth of in-plane nanodomains. We find these results to be consistent with a large number of studies that show that strain-relief is of paramount importance if ferroelectric films are to be developed as microwave circuit components. PACS 81.15.Fg; 85.50.-n