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

Ba_0.5Sr_0.5TiO₃ (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 $({\epsilon }_{⏊}^{/})$ 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 $({ϵ}_{ǁ})$, enhances ${\epsilon }_{⏊}^{/}$ with $\epsilon _{MgO}^{BST-0.5}{}_{⏊}^{/}>\epsilon _{STO}^{BST-0.5}{}_{⏊}^{/}>\epsilon _{LAO}^{BST-0.5}{}_{⏊}^{/}$ at lower frequencies, two crossover points at 8.6 GHz and 18 GHz eventually change the trend to: $\epsilon _{STO}^{BST-0.5}{}_{⏊}^{/}>\epsilon _{LAO}^{BST-0.5}{}_{⏊}^{/}>\epsilon _{MgO}^{BST-0.5}{}_{⏊}^{/}$. The dispersions are qualitatively interpreted using (a) theoretically calculated (T)−$({ϵ}_{ǁ})$ phase diagram for single crystal and single domain BST-0.5 film, (b) theoretically predicted ${ϵ}_{ǁ}$-dependent, ${\epsilon }_{⏊}^{/}$ 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, ${\xi }_{\mathrm{s}}$, was estimated to be 0.51 at 40 GHz for BST-0.5 film on STO.