Microwave characterization of two Ba _0.6Sr _0.4TiO ₃ dielectric thin films with out-of-plane and in-plane electrode structures

Ferroelectric thin films have recently attracted renewed interest in research due to their great potential for designing novel tunable electromagnetic devices such as large intelligent surfaces (LIS). However, the mechanism of how the polar structure in the ferroelectric thin films contributes to the desired tunable performance, especially at the microwave frequency range which is the most widely used frequency range of electromagnetics, has not been illustrated clearly. In this paper, we described several straightforward and cost-effective methods to fabricate and characterize Ba_0.6Sr_0.4TiO₃ (BST) thin films at microwave frequencies. The prepared BST thin films here exhibit homogenous structures and great tunability in a wide frequency and temperature range when the applied field is in the out-of-plane direction. The high tunability can be attributed to the high concentration of polar nanoclusters. Their response to the applied DC field was directly visualized using a novel non-destructive near field scanning microwave microscope (NSMM) technique. Our results have provided some intriguing insight into the application of ferroelectric thin films for future programmable high-frequency devices and systems.