Sandwich-type composite multilayer films of strontium titanate and barium strontium titanate and their controllable dielectric properties

• ST/KBST/ST multilayer film with various ST and KBST layers has been designed originally. • Each KBST layer coated by two ST layers is to better match adjacent structures. • The loose structure of the KBST layer makes it fully compound with the ST layer. • Preheating each layer at 600 °C improves the sandwich-type films in structure and growth. • ST/KBST/ST film shows excellent comprehensive dielectric properties. It is a challenge to reduce the dielectric loss and increase the tunability of pure barium strontium titanate (BST) films for microwave tunable application because these two properties change simultaneously. Herein, a novel composite of strontium titanate (ST) and potassium-doped BST (KBST) has been designed as ST/KBST/ST sandwich-type film with various ST and KBST layers. X-ray diffraction patterns show that the film exhibits cubic perovskite polycrystalline structure composed of BST and ST phase, mainly grow along (110) crystal plane with average grain size of less than 20 nm and decreasing BST phase/ST phase ratio with increasing film thickness. Scanning electron microscope shows that no interfacial layer can be observed, indicating that ST and KBST are fully compounded. Low dielectric loss and high tunability at -10 – 10 V and stable and excellent dielectric properties at 1 GHz are achieved, meeting the needs of microwave tunable application at high frequency. The surface structures are also studied by other analysis methods, and ST/MgBST/ST sandwich-type film is compared.