Microstructure Evolution of Pulsed Laser-Deposited (Ba, Sr)TiO3 Films on MgO for Microwave Applications

To develop low-loss tunable microwave circuits, based on the field dependence of dielectric permittivity, phase pure (Ba 0.5 , Sr 0.5 )TiO 3 doped with 1% W (BST) thin films 0.3-μm thick were deposited on single crystal MgO wafers by pulsed laser deposition. The BST films were characterized by X-ray θ-2θ scans and pole figure analysis, field emission scanning electron microscopy (FESEM), and cross-sectional transmission electron microscopy (TEM), coupled with selected-area electron diffraction (SAED). Although, the X-ray θ-2θ scan indicated an epitaxial nature of BST with an out-of-plane orientation of (100), the pole figure analysis confirmed the presence (4-6%) of (111)-oriented grains in a matrix of (100) textured grains. The columnar grains exhibited an in-plane (i.e., along the plane perpendicular to the growth direction) grain size that was thickness-dependent. The cross-sectional TEM, coupled with SAED in the thickness direction, corroborated the pole figure analysis. Additionally, from X-ray analysis, it was observed that the textured films were under in-plane tension. The deposited film was characterized at microwave frequencies (1-20 GHz) using interdigitated electrodes deposited on top of the film. The film was characterized by a relatively low dielectric Q of 5-7. A 17% change in the capacitance was observed when applying a 40V bias. From the observed microstructure, a preliminary understanding of its evolution and its relationship with the microwave dielectric properties is discussed, and some ideas to obtain truly epitaxial BST films are presented.