Phase transition behavior in microcantilevers coated with M1-phase VO2 and M2-phase VO2:Cr thin films

摘要

Silicon microcantilevers were coated by pulsed laser deposition with vanadium dioxide (VO₂) (monoclinic M₁ phase) and V_1-xCr_xO₂ with x near 0.024 (monoclinic M₂ phase), and their mechanical characteristics were studied as a function of temperature through the films’ insulator-to-metal transition (IMT). The undoped VO₂ films grew with (011)_M1 planes parallel to the substrate, while Cr-doped VO₂ films grew oriented with (201)_M2 and (201)_M2 planes parallel to the substrate. In both cases, the films transformed reversibly through the IMT to the tetragonal (rutile, R) phase, with film (110)_R planes oriented parallel to the substrate. The fundamental resonant frequencies of the cantilevers were measured as the temperature was cycled from ambient temperature, through the IMT, and up to 100  °C. Very high resonant frequency changes were observed through the transition for both types of samples, with increases during heating of over 11% and over 15% for the cantilevers coated with pure and Cr-doped VO₂, respectively. From the resonant frequencies measured at room temperature for the bare and coated cantilevers in each case, the effective Young’s moduli of the films were determined. The values obtained, assuming bulk densities for the films, are 156 ± 7.5 GPa for VO₂ (M₁ phase) and 102 ± 3 GPa for Cr-doped VO₂ (M₂ phase). Strong curvature changes during the transition to the R phase were also observed for cantilevers coated with both types of films, but these were significantly higher in the case for the Cr-doped film. Curvature changes for temperature ranges outside the IMT region were small and attributed to differential thermal expansion between film an- silicon substrate. From measured cantilever tip displacements in this post-transition range—for the undoped VO₂-coated microcantilevers—a rough estimate of 110 GPa was obtained for the effective Young’s modulus for R-phase VO₂. The substantially higher changes in resonant frequency and curvature for V_1-xCr_xO₂-coated cantilevers suggest that this material may be even more useful than M₁-phase VO₂ for prospective microelectromechanical or optomechanical device applications in which ample frequency tunability—in oscillators or filters—or large displacements—in actuators—within a small temperature range is desirable. Since M₂-phase V_1-xCr_xO₂ with Cr composition of a few atomic percent retains other desirable properties of VO₂, such as very high resistivity changes through the IMT and a transition temperature fairly close to ambient temperature, multifunctionality is not impaired and in fact may be enhanced for some applications.