MOCVD-SrTiO3 thin film microwave coplanar tunable devices: modelling of varactors

Abstract

SrTiO3 (STO) ferroelectric (FE) thin films associated with High Temperature Superconductor (HTSC) is a good compromise to realize electronically tunable microwave devices combining tunable dielectric properties of FE films with low loss microwave conductivity in HTSC. STO, which exhibits a perovskite structure, is suitable for epitaxial growth of YBaCuO films and has been widely studied to realize tunable components around 77 K. Up to now, STO thin films were essentially deposited by sputtering and pulsed laser deposition. In the framework of this study, we have explored the feasibility of microwave devices made from STO thin films prepared by metalorganic chemical vapour deposition (MOCVD). We have characterized Au/STO(250 nm thick)/MgO coplanar waveguide transmission lines and microwave variable capacitors (varactors) from 45 MHz up to 40 GHz, in the 300 K to 60 K temperature range. Dielectric characteristics of the STO films were extracted from measurements and studied as a function of frequency, electric field and temperature. The geometry of the interdigital capacitors (IDC) was chosen after evaluating their capacitances using an analytical theoretical model. We have used a de-embedding method to extract, from electromagnetic simulations using Sonnet® Software, the capacitance of the IDC alone.