Analyses of Substrate-Dependent Broadband Microwave (1–40 GHz) Dielectric Properties of Pulsed Laser Deposited Ba_0.5Sr_0.5TiO₃ Films

Abstract

Ba0.5Sr0.5TiO3 (BST-0.5) thin films (600 nm) were deposited on single crystal MgO, SrTiO3 (STO), and LaAlO3 (LAO) substrates by pulsed laser deposition at an oxygen partial pressure of 80 mTorr and temperature of 720 °C. X-ray diffraction and in situ reflection high-energy electron diffraction routinely ascertained the epitaxial quality of the (100)-oriented nanocrystalline films. The broadband microwave (1–40 GHz) dielectric properties were measured using coplanar waveguide transmission line test structures. The out-of-plane relative permittivity (ε⏊/) exhibited strong substrate-dependent dielectric (relaxation) dispersions with their attendant peaks in loss tangent (tanδ), with the former dropping sharply from tens of thousands to ~1000 by 10 GHz. Although homogeneous in-plane strain (ϵǁ), enhances ε⏊/ with εMgOBST−0.5⏊/>εSTOBST−0.5⏊/>εLAOBST−0.5⏊/ at lower frequencies, two crossover points at 8.6 GHz and 18 GHz eventually change the trend to: εSTOBST−0.5⏊/>εLAOBST−0.5⏊/>εMgOBST−0.5⏊/. The dispersions are qualitatively interpreted using (a) theoretically calculated (T)−(ϵǁ) phase diagram for single crystal and single domain BST-0.5 film, (b) theoretically predicted ϵǁ-dependent, ε⏊/ anomaly that does not account for frequency dependence, and (c) literature reports on intrinsic and extrinsic microstructural effects, including defects-induced inhomogeneous strain and strain gradients. From the Vendik and Zubko model, the defect parameter metric, ξs, was estimated to be 0.51 at 40 GHz for BST-0.5 film on STO.

Keywords

• dielectric permittivity
• loss tangent
• epitaxial BST thin films
• microwave characterization
• homogeneous in-plane strain engineering
• inhomogeneous strain