Tungsten as a substitutional dopant and its effect on ultrafast switching of vanadium dioxide

VO$_{2}$ undergoes a metal-insulator transition (MIT) at 340K accompanied by a structural change from monoclinic (M1) to tetragonal (R). We have grown W-doped VO$_{2}$ films on glass and epitaxially on sapphire substrates and have characterized them by SEM, white light transmission, RBS, XRD, and Z-STEM. These provide direct experimental evidence that W acts as a substitutional dopant in the VO$_{2}$ lattice in addition to lowering the transition temperature. From GGA+U, DFT-based simulations we have also calculated the formation energy of substitutional W in VO$_{2}$, and relative stability of M1 and R phases before and after doping. Ultrafast pump-probe measurements at 800nm with varying pump fluences show that doped VO$_{2}$ switches at substantially lower fluences than undoped VO$_{2}$, indicating that the W dopant provides additional conduction-band electrons, thus altering the photo-induced dynamics of the phase transition.