Dielectric Response Properties of Nanostructured Mott Transition Vanadium Dioxide (VO2) in the Terahertz (THz) Regime

Abstract

A reversible first-order insulator-to- metal transition in VO2 at a transition temperature 340 K (68 C), was reported by Morin (1959). This reversible change in the structure (from the monoclinic to tetragonal phase) of the crystalline sample of VO2, is accompanied by reversible changes in its electrical and optical properties. We investigated some optical and dielectric properties of VO2 thin films across the insulator-metal transition temperature. In our study, we used VO2 thin films on a 300 mm thick c-cut sapphire (Al2O3) substrate synthesized by pulsed laser deposition (PLD) technique. Their properties and composition were determined by X-ray diffraction (XRD); and scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) to confirm high purity VO2 layers. The samples were characterized using XRD and exhibited monoclinic phase VO2 (M). Scanning electron microscope (SEM) images showed that the deposited films consist of crystalline nano-grains. Transmission spectroscopy measurements of the VO2 thin films were performed across the metal-insulator transition temperature. Optical properties such as transmission and absorption coefficient, were determined at temperatures below and above the phase transition temperature in both the visible and the near-infrared ranges of the electromagnetic spectrum. Dielectric response properties of vanadium dioxide (VO2) thin films were investigated in Terahertz (THz) frequency range. To investigate the dielectric properties of a series of VO2 thin films with different thickness, i.e. 10, 50, 100 and 200 nm, synthesized by PLD technique on a 300 mm thick c-cut sapphire (Al2O3) substrate were used. The samples were measured in a high-field THz time-domain spectroscopy (THz-TDS) setup, for temperatures below and above the critical temperature (68 C) of the metal-insulator transition of VO2. The data obtained was analyzed in time and frequency domains and the dielectric properties were determined. THz radiation was detected using free-space electro-optic sampling (FS-EOS)

in a < 110 > ZeTe crystal. The obtained data was used to determine the charge carrier density (N) of VO2 thin films on sapphire substrates, for the temperature range of 25 C to 100 C. We observed a big jump in the change of the charge carrier density at the phase transition temperature. Below the phase transition temperature (T < 68 C), the change in the charge carrier density is approximately zero. However, above the phase transition temperature (T > 68 C), the change in the charge carrier density gently increases.