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 (Ba0.5, Sr0.5)TiO3 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 40 V 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.