Breaking symmetry yields a low-loss out-of-plane tunable microwave dielectric

Voltage-tunable capacitors (varactors) are key components in microwave circuits. Tunable dielectric varactors can outperform competing technologies but typically suffer from high dielectric loss. Ruddlesden–Popper dielectric thin films can, by contrast, offer low microwave loss. Unfortunately, their crystallographic symmetry is usually not compatible with an out-of-plane parallel-plate varactor design, which can minimize size and maximize the electric field in the tunable dielectric compared with an in-plane device design. Here we show that a low-loss and tunable Ruddlesden–Popper dielectric thin film that is compatible with the parallel-plate varactor design can be created by breaking this crystallographic symmetry. We study films that are similar to the widely studied tunable microwave dielectric Ba_0.45Sr_0.55TiO₃ but have a Ba_0.45Sr_0.55O rock-salt layer for every n-perovskite unit cells. The film with n = 8 exhibit an optimum combination of tunability and loss, with a material quality factor of around 200 and a relative tunability of 51% at an applied electric field of 250 kV cm⁻¹, which results in a dielectric tuning figure of merit of 100 at 10 GHz.