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

Abstract

Silicon microcantilevers were coated by pulsed laser deposition with vanadium dioxide (VO2) (monoclinic M1 phase) and V1−xCrxO2 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 (2¯01)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 and 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 V1−xCrxO2-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 V1−xCrxO2 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.