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

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).

Molecular beam epitaxy growth of the highly conductive oxide SrMoO3

“…3(a)). A broad satellite was also observed next to the sharp peaks, which is likely Mo 4+ 3d emission of unscreened final states 9,10,28,29 . We also collected a spectrum at a lower photon energy (600 eV), which is more sensitive for the film near the surface, and observed additional features at 233.1 eV and 236.4 eV.…”

“…For the growth of high quality SrMoO 3 films, we used a buffer layer of STO with 4-8 unit cells, between the SrMoO 3 film and the STO (or KTO) substrate. Such buffer layer has been used for the growth of SrMoO 3 and EuMoO 3 by PLD 9,10,22 . In this study, we firstly deposited SrTiO 3 using Sr and Ti under O 2 gas flow, and then we started the growth of SrMoO 3 films at the peak top of oscillation in RHEED for SrTiO 3 (Figs.…”

“…The room temperature (RT) resistivity ρ of singlecrystalline SrMoO 3 is as low as 5 µΩ cm, which is much lower than typical oxide materials 4 and is rather close to those of nearly free electron systems such as sodium and copper. The utmost feature of this oxide has stimulated intensive studies to grow epitaxial films [5][6][7][8][9] for applications, for example, electrodes between oxide interfaces 5,10 and transparent conductors 6,11 . Unfortunately, all the films, so far prepared by pulsed laser deposition (PLD) [7][8][9][10][11][12] or sputtering 5,6 , show rather poor resistivity (27-150 µΩ cm) than that of the bulk single crystal, possibly due to the presence of defects or inclusion of impurity phases.…”

“…Unfortunately, all the films, so far prepared by pulsed laser deposition (PLD) [7][8][9][10][11][12] or sputtering 5,6 , show rather poor resistivity (27-150 µΩ cm) than that of the bulk single crystal, possibly due to the presence of defects or inclusion of impurity phases. It is notable that an insulating Mo 6+ phase is often found in the surface state [10][11][12] .…”

Optical Properties of Highly Conductive SrMoO3 Oxide Thin Films in the THz Band and Beyond

“…The epitaxial nature of such heterostructures allows lower defect densities and therefore enhanced functional properties in comparison with polycrystalline thin films. Interface engineering at the atomic level and precise stoichiometry tuning of the layers in MIM varactor heterostructures enable improved functional properties of the materials such as high electric conductivity of oxide electrodes and high tunability of the dielectrics [1,2]. Moreover, all-oxide epitaxial MIM heterostructures, including structurally compatible high-temperature superconductors, can be used for realization of novel thermally tuned THz metamaterials [3,4].…”