Atomically interface engineered micrometer-thick SrMoO3 oxide electrodes for thin-film BaxSr1-xTiO3 ferroelectric varactors tunable at low voltages

• Main Authors: P. Salg, D. Walk, L. Zeinar, A. Radetinac, L. Molina-Luna, A. Zintler, R. Jakoby, H. Maune, P. Komissinskiy, L. Alff
• Format: Article
• Language: English
• Published: AIP Publishing LLC 2019-05-01
• Series: APL Materials
• Online Access: http://dx.doi.org/10.1063/1.5094855

In the field of oxide electronics, there has been tremendous progress in the recent years in atomic engineering of functional oxide thin films with controlled interfaces at the unit cell level. However, some relevant devices such as tunable ferroelectric microwave capacitors (varactors) based on BaxSr1−xTiO3 are stymied by the absence of suited compatible, very low resistive oxide electrode materials on the micrometer scale. Therefore, we start with the epitaxial growth of the exceptionally highly conducting isostructural perovskite SrMoO3 having a higher room-temperature conductivity than Pt. In high-frequency applications such as tunable filters and antennas, the desired electrode thickness is determined by the electromagnetic skin depth, which is of the order of several micrometers in the frequency range of a few gigahertz. Here, we report the pulsed laser deposition of a fully layer-by-layer grown epitaxial device stack, combining a several micrometers thick electrode of SrMoO3 with atomically engineered sharp interfaces to the substrate and to the subsequently grown functional dielectric layer. The difficult to achieve epitaxial thick film growth makes use of the extraordinary ability of perovskites to accommodate strain well beyond the critical thickness limit by adjusting their lattice constant with small shifts in the cation ratio, tuned by deposition parameters. We show that our approach, encompassing several orders of magnitude in film thickness scale whilst maintaining atomic layer control, enables the fabrication of metal-insulator-metal (MIM) varactors based on 50–100 nm thin BaxSr1−xTiO3 layers with high tunability above three at the Li-ion battery voltage level (3.7 V).