(A) study on fabrication and characterization of microwave tunable devices using $(Ba,Sr)TiO_3$ thin films

Abstract

The prospects of using ferroelectric thin films for the fabrication and development of frequency and phase agile microwave devices have increased in the past few years due to improvements in ferroelectric thin film processing techniques as well as innovative circuit designs. In microwave devices, ferroelectric thin film is used as a tuning layer through the nonlinear electric field dependence of the relative dielectric constant. In this thesis, firstly we report on an investigation of the effect of film stress as a significant factor affecting the dielectric properties and discuss the dependence of tunability on the texture of the BST films. To clarify the relationship between stress effect and film orientation, two types of capacitors such as Pt/BST/Pt and Pt/BST/LSCO/Pt were compared. Compared with the crystallinity of BST film on Pt electrode, BST film grown on LSCO/Pt showed an improved crystallinity. LSCO buffer layer can promote the growth of BST films with (100) preferred orientation due to the structural match between BST and LSCO. By using laser reflection method, we have observed that BST film with LSCO buffer layer had higher tension (5.138 GPa) than that (3.773 GPa) of BST thin film grown on Pt electrode. It may be inferred from this observation that the as-deposited BST film on LSCO/Pt is under more tension, which promotes the polarization of electric dipoles. This fact is substantiated by the capacitance-voltage curves of BST thin films. The BST film grown on LSCO/Pt shows a dielectric constant of 560 as its maximum value. It is noted that the dielectric constant of the BST film grown on LSCO buffer layer is considerably higher than that (380) obtained at the same deposition condition by using Pt electrode. At the applied voltage of 7 V, tunability values of BST films grown on Pt and LSCO/Pt were 2.2 : 1 and 2.8 : 1, respectively. Higher tunability has been attributed to the (100) preferred orientation in BST film grown on LSCO/Pt electrode. Name...