Thin-film interface engineering of low-voltage tunable ferroelectric varactors with oxide electrodes

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Ferroelectric varactors with tunable dielectric (Ba,Sr)TiO3 and metallic electrodes are considered for applications in tunable electric devices at gigahertz frequencies. Their highly tunable permittivity, as well as endurance, fast tuning speed, and low power consumption have innovated various integrated microwave components. In our work on the previous project, the concept of all-oxide thin-film epitaxial ferroelectric varactors has been proven based on thin films of low-resistive SrMoO3 electrodes and (Ba,Sr)TiO3 tunable dielectric. The concept has two advantages that can only be achieved using extremely thin films as tunable dielectric: tunability at low (battery) voltages and operation at high frequencies. Several fundamental scientific questions regarding the defect chemistry of the involved oxide interfaces, which are the key to varactor functionality, must be addressed to be able to realize full potential of the all-oxide epitaxial ferroelectric varactors.This follow-up project aims at an engineering on the atomic level of the thermodynamically and kinetically stable interfaces between epitaxial perovskite oxides in ferroelectric varactors, to allow the growth of functional materials with incompatible stability regions of their thermodynamic phase diagrams. The interfaces will be stabilized using thin-film interlayers as oxygen diffusion barriers and fine adjustments of the materials growth kinetics. The materials parameters of the engineered multilayer structure (crystal and electronic structure, permittivity, stoichiometry, morphology) will be correlated with the varactor electrical performance parameters such as tunability, leakage current and microwave losses. The varactor electric characterization at gigahertz frequencies serves not only as a device characterization method but at the same time as a highly sensitive measure of the materials properties, and demonstrates the applicability of the multilayer structure in a model device. Dielectric modelling of thin-film varactor heterostructures allows extraction of the dielectric parameters of the interfaces, which are not directly accessible in the electric measurements. The interdisciplinary approach between Materials Science and Electrical Engineering is essential for the project as high performance of the ferroelectric varactors is achieved using thin film electrodes of the novel highly conducting oxide electrode material SrMoO3.