No abstract

No abstract

Electrical conductivity (σ) and optical transmittance of high quality VO2 thin films deposited by DC reactive magnetron sputtering on r-cut sapphire substrates (at 650 °C) have been measured simultaneously as a function of temperature by heating and cooling scans through the phase transition region. The partial concentration of the metallic phase (Xm) has been calculated from the optical transmittance, and the σ(Xm) dependence has been analyzed through an insulator-to-metal transition (IMT) during heating and through a metal-to-insulator transition (MIT) during cooling. The results have shown to be consistent with the Efros–Shklovskii percolation theory, predicting the formation of two-dimensional infinite conductive cluster (ICC) during IMT and the preservation of three-dimensional ICC during MIT. The critical concentrations (Xc) corresponding to the appearance of ICC at IMT and the disappearance of ICC at MIT were found to be very different, 0.57 and 0.06, respectively. A mathematical model explaining very small Xc at MIT was developed. The dissimilarity of the ICC topology during IMT and MIT is connected with the appearance and disappearance of local mechanical stresses imminent in VO2 phase transitions.

No abstract

In this work, VO2 thin films were deposited on Si wafers (onto (100) surface) by DC magnetron sputtering under different cathode bias voltages. The effects of substrate biasing on the structural and optical properties were investigated. The results show that the metal–insulator transition (MIT) temperature of VO2 thin films can be increased up to 14 K by applying a cathode bias voltage, compared to deposition conditions without any bias. The decrease in the transition efficiency and increase in the transition temperature are attributed to the enlarged grain size, increased defects, and the residual stress in the VO2 thin films induced by biasing. The optical transmittance measurements for different thickness films indicate an attenuation coefficient of 3.1 × 107 m−1 at 2000 nm or an extinction coefficient of 4.9 in the metal phase. The optical transmittance vs wavelength characteristics point to an indirect bandgap of 0.6 ± 0.5 eV and significant scattering in the bulk and/or at the interface.

No abstract

No abstract

This study focuses on the effect of the substrate temperature (TS) on the quality of VO2 thin films prepared by DC magnetron sputtering. TS was varied from 350 to 600 °C and the effects on the surface morphology, microstructure, optical and electrical properties of the films were investigated. The results show that TS below 500 °C favors the growth of V2O5 phase, whereas higher TS (≥500 °C) facilitates the formation of the VO2 phase. Optical characterization of the as-prepared VO2 films displayed a reduced optical transmittance (T˜) across the near-infrared region (NIR), reduced phase transition temperature (Tt), and broadened hysteresis width (ΔH) through the phase transition region. In addition, a decline of the luminous modulation (ΔT˜lum) and solar modulation (ΔT˜sol) efficiencies of the as-prepared films have been determined. Furthermore, compared with the high-quality films reported previously, the electrical conductivity (σ) as a function of temperature (T) reveals reduced conductivity contrast (Δσ) between the insulating and metallic phases of the VO2 films, which was of the order of 2. These outcomes indicated the presence of defects and unrelaxed lattice strain in the films. Further, the comparison of present results with those in the literature from similar works show that the preparation of high-quality films at TS lower than 650 °C presents significant challenges.