Abstract
Ruthenium dioxide (RuO2) is an ideal buffer layer for vanadium dioxide (VO2) heterostructures due to its high electrical conductivity and matching crystal structure with metallic VO2. VO2 thin films were deposited on single crystal TiO2 (001) substrates with RuO2 buffer layers via pulsed laser deposition. The metal-insulator transition temperature (TMIT) in VO2 films can be controlled by the epitaxial strain between the VO2 film and RuO2 buffer layer by adjusting the buffer layer thickness (10 - 50 nm). We observed a decrease in the TMIT of VO2 films from 59 °C to 24 °C as the RuO2 thickness decreased from 50 nm to 10 nm. Additionally, we show that the RuO2 buffer layer can sustain an intermediate strain state in VO2 films up to 100 nm in thickness with a subsequently lower TMIT (30 °C). The 10 nm thick RuO2 buffer layer can reduce the TMIT in VO2 films by providing a pathway to relieve the strain through grain boundaries.
Highlights
Vanadium dioxide (VO2) undergoes a sharp metalinsulator transition (MIT) above room temperature at ∼67 ◦C, which is associated with a structural phase transformation (SPT) between a low-temperature insulating monoclinic phase and a high-temperature metallic tetragonal phase
In order to investigate the effect of the RuO2 film thickness on the epitaxial strain of VO2/RuO2 heterostructures, RuO2 buffer layers (10 - 50 nm) were prepared on TiO2 (001) substrates while holding the VO2 film thickness constant at 50 nm
With increasing RuO2 buffer layer thickness, the RuO2 (002) peak moves to lower 2θ angles (approaching the bulk RuO2 (002) angle) and the VO2 (002) peak moves to lower 2θ angles (approaching the bulk VO2 (002) angle). This result suggests that the c-axis lattice parameter of both the RuO2 buffer layers and the VO2 thin films increases as the RuO2 thickness increases, meaning that the epitaxial strain can be adjusted using different thicknesses of the RuO2 buffer layer
Summary
Vanadium dioxide (VO2) undergoes a sharp metalinsulator transition (MIT) above room temperature at ∼67 ◦C, which is associated with a structural phase transformation (SPT) between a low-temperature insulating monoclinic phase and a high-temperature metallic tetragonal phase. The MIT and SPT can be controlled by external parameters such as temperature, electric field, or photo-excitation, and the switching time of the transition can be on ultrafast timescales (∼100 fs) when the transition is induced optically. As the temperature of the VO2 increases above 67 ◦C, the electrical resistivity decreases by several orders of magnitude and the infrared transmittance decreases by ∼60 %.7 These unique properties have made VO2 an attractive candidate in many promising applications such as ultrafast switches, thermooptical modulators, field effect transistors, bolometric photodetection, plasmonic metamaterials, thermal actuators, and smart radiators for spacecraft.16The nature of the MIT and SPT in VO2 has been a longstanding debate. Vanadium dioxide (VO2) undergoes a sharp metalinsulator transition (MIT) above room temperature at ∼67 ◦C, which is associated with a structural phase transformation (SPT) between a low-temperature insulating monoclinic phase and a high-temperature metallic tetragonal phase.. As the temperature of the VO2 increases above 67 ◦C, the electrical resistivity decreases by several orders of magnitude and the infrared transmittance decreases by ∼60 %.7 These unique properties have made VO2 an attractive candidate in many promising applications such as ultrafast switches, thermooptical modulators, field effect transistors, bolometric photodetection, plasmonic metamaterials, thermal actuators, and smart radiators for spacecraft.. Recent reports have revealed that ultrathin VO2 films deposited on lattice matched TiO2 substrates show no monoclinic phase at room temperature, suggesting that the VO2 films are tetragonal rutile in both the insulating and metallic states, i.e., the films undergo an electronic phase transition without the structural phase transition.
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