Epitaxial ferroelectric all-oxide varactors for application in reconfigurable RF front ends

Modern wireless communication systems require low-cost, compact, and highly integrated tunable components such as filters, phase shifters, frequency-agile antennas, and adaptive impedance matching networks. One basic element of these components is the tunable capacitor (varactor). Among other technologies, ferroelectric thin-film varactors exhibit high power handling, low tuning voltage, quick response, high capacitance density, and good adaptability into the microwave frequency range (DC to 20 GHz). In particular, BST-based thin-film varactors have an excellent potential to reduce size, weight, cost, and to improve the functionality of wireless communication systems. Electric losses of thin-film ferroelectric varactors depend highly on the loss of the dielectric layer. In general, the choice of the bottom electrode material has a big influence on the dielectric properties and the total losses. For this reason, efforts were made to use conducting oxide materials as bottom electrodes, enabling the deposition of a dielectric layer with low defect density and misfit strain. However, due to the high electrical resistivity of the so far used electrode materials, it was concluded that oxide electrodes are too resistive for integration into microwave varactors. In this work, the SrMoO3 as a highly conducting perovskite material was integrated for the first time as the bottom electrode into thin-film BST-based varactors. Epitaxial SrTiO3/SrMoO3/SrTiO3/Ba0.4Sr0.6TiO3 heterostructure was grown onto (110) GdScO3 substrate using Pulsed Laser Deposition (PLD). The microstructure of the deposited layers was characterized by X-ray diffraction techniques and Scanning Transmission Electron Microscopy (STEM). It was shown that epitaxial growth of the SrTiO3/SrMoO3/SrTiO3/Ba0.4Sr0.6TiO3 stack, locked commensurately to the GdScO3 substrate, is possible. Moreover, optimization of the deposition conditions resulted in homogeneous, single-phase growth of all the layers with low mosaicity and excellent crystal quality. Using lift-off lithography, Pt/Au top electrodes were deposited by Magnetron Sputtering to enable the electrical characterization of the MIM-structured varactors. It was shown that a tunability of 50% is achievable by applying small biasing voltage of 8V. Furthermore, a quality factor of 180 at 30 MHz suggests a very low dielectric loss at the Ba0.4Sr0.6TiO3. A commutation Quality Factor (CQF) of 10000 at 100 MHz shows the potential of SrMoO3 for integration into devices for microwave applications. As an initial step towards industrialization of this technology, a selective etching procedure for processing and fabrication of SrMoO3-based electronic devices was established