Observation of high out-of-plane tunability and low loss in Ruddlesden-Popper films

Microwave circuits like tunable filters and phase shifters rely on high tunability and low loss dielectrics. About a decade ago, scientists observed a record-breaking combination of high in-plane tunability and low loss in strained Ruddlesden-Popper phase Ba_xSr_1-xTiO₃ thin films at microwave frequencies. However, most voltage-tunable devices employ out-of-plane geometries, hindering the implementation of Ruddlesden-Popper films in microwave circuits. Still, theory predicts that engineering strainless Ruddlesden-Popper phase Ba_xSr_1-xTiO₃ with a high barium content should result in high out-of-plane tunability. Here, we report on measuring this high out-of-plane tunability in Ruddlesden-Popper phase Ba_xSr_1-xTiO₃ thin films and show that we can engineer the Ruddlesden-Popper phase to result in low dielectric loss at microwave frequencies. This result shows that Ruddlesden-Popper phase dielectrics can indeed be tunable out-of-plane while retaining their low dielectric loss. Our observations set a new benchmark for low loss out-of-plane tunable microwave dielectrics.