Electrically tunable dielectric materials and strategies to improve their performances

Electrically tunable dielectric materials have potential applications as various microwave devices, such as tunable oscillators, phase shifters and varactors. High dielectric tunability, low dielectric loss tangent and appropriate level of dielectric constant, are basic requirements for such applications. Ferroelectric materials are the most promising candidates. In general, strontium titanate (SrTiO₃ or ST) is used for devices operating at low temperatures, while the devices based on barium strontium titanate (Ba_1?xSr_xTiO₃ or BST) are operated at room temperatures. The modifications of parent ferroelectrics, such as Sr_1?xPb_xTiO₃, BaZr_xTi_1?xO₃ and BaTi_1?xSn_xO₃ etc., have also been widely investigated. In addition, there have been reports on electrically tunable dielectric materials, based on non-ferroelectric compounds, such as microwave dielectrics and carbon nanotube (CNT) composites. Specifically for ferroelectric materials, a critical issue is the reduction of the dielectric losses, because their dielectric loss tangents are relatively high for practical device applications. Recently, many efforts have been made in order to reduce the dielectric losses of BST based ferroelectrics. An efficient way is to dope oxides that have low dielectric losses, such as MgO, ZrO₂ and Al₂O₃, TiO₂, LaAlO₃, and Bi_1.5ZnNb_1.5O₇ etc., into the ferroelectric materials. In addition to the reduction in dielectric loss tangents, the introduction of oxides would also be able to modify the dielectric constant to be suitable for practical design of various devices. Meanwhile, dielectric and electrical properties of thin films can be improved by chemical doping, substrate adaptation, orientation and anisotropy optimization. This review provides an overall summary on the recent progress in developing electrically tunable dielectric materials, based on ferroelectrics and non-ferroelectrics, with a specific attention to the strategies employed to improve the performances of ferroelectric materials for microwave device applications.