IMPROVEMENT OF Ba_0.5Sr_0.5TiO₃ THIN FILMS MICROWAVE PROPERTIES USING CODOPING WITH Mg-W AND Al-W

ABSTRACT

Codoped (Al-W and Mg-W) Ba_0.5Sr_0.5TiO₃ (BST-0.5) thin films are deposited on ceramic alumina (Al₂O₃) substrate by pulsed laser deposition (PLD). Al-W and Mg-W doped target are prepared by using a piece of material (Al₂O₃ and MgO respectively) placed on a 10%wt WO₃ doped BST-0.5 target, undoped BST-0.5 thin films are fabricated using a pure BST-0.5 target. After the deposition, crystallization of the films is carried out in a furnace. X-ray diffraction and Scanning Electron Microscope characterizations are used to accurately measure the lattice parameter and analyze the surface morphology of the films. Microwave properties (dielectric constant, loss tangent and tunability in the GHz range) of the films are characterized by two complementary diagnostics that use coplanar devices patterned on the samples: coplanar interdigital capacitors and thru-reflect-line analysis. The Al-W and Mg-W codoped BST-0.5 are polycrystalline and show respectively 0.03 tangent loss with 19% tunability and 0.03 tangent loss with 25% tunability (at 40 kV.cm⁻¹) compared to 0.055 tangent loss and 33% tunability for optimized undoped BST-0.5 films. These microwave properties demonstrate that codoping, especially with Mg-W, can improve the figure of merit of BST by more than 40%.

Keywords:

• BST
• tunable devices
• interdigital capacitors (IDC)
• pulsed laser deposition (PLD)
• codoping

ACKNOWLEDGMENTS

The authors wish to acknowledge the support of FQRNT (Quebec), NSERC (Canada) and the Canada Research Chairs Program. The authors would like to thank Étienne Charette (INRS-EMT) for his technical support and Martin Chicoine (from Université de Montreal) for ERD and RBS measurements.