Epitaxial Growth of Ferroelectric Thin Films by Combustion Chemical Vapor Deposition and Their Electrical Properties

Combustion chemical vapor deposition (CCVD) has been used to grow a wide variety of ferroelectric thin films such as Ba_1-xSr_xTiO₃ (BST) and PbZr_xTi_1-xO₃ (PZT) on single crystal substrates. This paper presents the growth and characterization of epitaxial BST films on sapphire. Surface morphology, phase, and composition of the as -grown films were characterized. Planar gap capacitors were fabricated, and their capacitance, quality factor (Q) and tunability were investigated as a function of film thickness and DC bias. A 2:1 tunability was achieved for BST thin films under a DC bias of 10 V, and low-frequency Q can be as high as 100. Coplanar waveguide (CPW) structures were also designed and fabricated onto BST coated sapphire substrates. S-parameters of the CPW were tested using a vector network analyzer, and dielectric constant and loss tangent were then derived by comparing the measured data with electromagnetic (EM) simulation results. The dielectric constant was found to be in the range of 300-800, and a loss tangent of 0.05 at 40 GHz was achieved for a 300 nm thick BST film. The epitaxial BST films have been used to fabricate low-loss tunable filters and phase shifters with operation frequencies up to 40 GHz.