Abstract
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).
Highlights
The use of the perovskite BaxSr1−xTiO3 (BST) in frequencyagile microwave applications is based on the possibility to tune its permittivity, ε, by cation displacement through a quasistatic electric field.1–3 The most suitable device design for energy-efficient low-voltage applications in mobile communication consists of a metal-insulator-metal (MIM) varactor where the tunable BST is sandwiched between two electrode layers each having a thickness exceeding the skin depth.2 Due to the lack of structurally matching oxide electrode materials, existing devices combine a metal bottom electrode such as Pt with the subsequently grown BST layer on top, resulting in a defect-rich, strained, polycrystalline BST with so-called size effects associated with local polarized nanoregions and moveable charge defects.1,4 In today’s technology, the choice of metal electrode materials structurally incompatible with BST has to be compensated by rather thick (typically 200–300 nm) BST layers which, in turn, require rather large tuning voltages in the range of several tens of volts
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. Some relevant devices such as tunable ferroelectric microwave capacitors based on BaxSr1−xTiO3 are stymied by the absence of suited compatible, very low resistive oxide electrode materials on the micrometer scale
We start with the epitaxial growth of the exceptionally highly conducting isostructural perovskite SrMoO3 having a higher room-temperature conductivity than Pt
Summary
The use of the perovskite BaxSr1−xTiO3 (BST) in frequencyagile microwave applications is based on the possibility to tune its permittivity, ε, by cation displacement through a quasistatic electric field.1–3 The most suitable device design for energy-efficient low-voltage applications in mobile communication consists of a metal-insulator-metal (MIM) varactor where the tunable BST is sandwiched between two electrode layers each having a thickness exceeding the skin depth.2 Due to the lack of structurally matching oxide electrode materials, existing devices combine a metal bottom electrode such as Pt with the subsequently grown BST layer on top, resulting in a defect-rich, strained, polycrystalline BST with so-called size effects associated with local polarized nanoregions and moveable charge defects.1,4 In today’s technology, the choice of metal electrode materials structurally incompatible with BST has to be compensated by rather thick (typically 200–300 nm) BST layers which, in turn, require rather large tuning voltages in the range of several tens of volts. 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.
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