High Dielectric Tunability of (Ba, Sr)TiO3 Thin Films and Their Coplanar Waveguide Phase Shifter Applications

Abstract

In the present work, we report the epitaxial growth and dielectric properties of barium strontium titanate (BST) thin film which can be used to make frequency and phase agile tunable microwave (MW) components with a high performance, such as tunable filters, tunable oscillators, and phase shifters for application in phased array antennas. Epitaxial BST thin films were deposited on a MgO substrate by the insertion of a thin BaTiO3 (BT) seed layer between the BST films and the MgO substrate using pulsed laser ablation. The crystalline structure, dielectric tunability and microwave properties of the epitaxial BST thin films depended strongly on the thickness of the BT seed layer. The dielectric tunability and microwave phase tuning increased with increasing thickness of the BT seed layer. An interdigitated (IDT) capacitor based on the epitaxial BST/BT(12 nm) film on the MgO substrate exhibited a large capacitance tunability of 78% at a frequency of 100 kHz and an applied electric field of 133 kV/cm. Also, a coplanar waveguide (CPW)-type phase shifter fabricated on the BST/BT(12 nm) film showed a phase shift of 153° at 10 GHz and 40 V. These results indicate that the thin BT layer with an intermediate lattice constant between those of the BST film and MgO substrate can be used as an excellent seed layer for improving the crystallization and dielectric properties of BST films, and the high microwave performance of ferroelectric MW devices.