Search

This work presents the excess noise and thermoelectric (Seebeck) measurements on polycrystalline vanadium dioxide (VO2) thin films. Noise spectral power density (SPD) of current fluctuations in the semiconducting (SC) phase had a typical flicker noise (f−γ) characteristic with an average slope parameter γ of 1.13. Normalized SPD (Sn) values obtained in the SC-phase indicate that the noise originates in the bulk of the film. On the contrary, in the metallic (M)-phase, γ values were greater than unity, and the observed Sn values indicated that the origin of the noise is most likely from the contacts or surface rather than the bulk. A general decrease was observed in Sn by a factor of 4–5 from the SC- to M-phase. Moreover, Sn in the SC-phase showed no temperature dependence. An interpretation based on the number of charge carrier fluctuations in Hooge's model led to an unrealistically high Hooge parameter and had to be ruled out. We propose that the fluctuations are related to the mobility fluctuations of carriers arising primarily from grain-boundary scattering which explains the observed characteristics well. The Seebeck coefficients (S) obtained under both heating and cooling schedules showed the n-type nature of magnetron-sputtered VO2 films in the SC-phase. Differently, in the M-phase, the S value was positive. The S values obtained from the cooling schedule signified the low percolation threshold of the metal-to-insulator transition already demonstrated for VO2 thin films grown on r-cut sapphire using the Efros–Shklovskii percolation model. [ABSTRACT FROM AUTHOR]

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 (Δ T ˜ lum) and solar modulation ( Δ T ˜ 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. [ABSTRACT FROM AUTHOR]

Self-heating (SH)-induced electrical and optical switching in high quality VO2 films grown by magnetron sputtering on a c-cut sapphire substrate has been investigated under various applied constant current pulses (ID). The effect of SH on the behavior of electrical conductivity (σ), optical transmittance ( T ~) , and film temperature (T) examined by applying a constant current pulse of various magnitudes with a pulse duration of five seconds (Δt = 5 s) in VO2 films which were pre-heated to and stabilized at 57 °C, at the brink of insulator-to-metal transition (IMT). The SH effect that arose from the application of constant ID pulses led to a significant increase in T and substantial changes in σ and T ~ . Observations showed that, depending on the magnitude of ID, the σ and T ~ demonstrate strikingly different temporal behavior not only during the SH-induced IMT but also after the removal of ID. The observed phenomena could be explained using a simple model based on percolation theory previously proposed for the present system. The outcome of the model is confirmed by the results of the structural IMT obtained by Raman micromapping. [ABSTRACT FROM AUTHOR]

NbOx thin films have been deposited on silicon (100) and quartz substrates by magnetron sputtering using a metallic Nb target in an optimized argon–oxygen atmosphere. This work investigates the dependence of structure–property relations on the key deposition parameters towards establishing optimum deposition conditions for the growth of NbOx polycrystalline films. It is found that a sputtering condition corresponding to DC power density 9.87 W cm−2, a substrate temperature of 720 °C, low gas pressures of 8 mtorr, a target to substrate distance of 45 mm gives thin films with good homogeneity and a high degree of crystallinity in the case of both NbO2 and Nb2O5. X-ray diffraction (XRD) and Raman spectroscopy confirmed the tetragonal phase of NbO2 and orthorhombic phase of Nb2O5 for similar deposition temperatures. Scanning electron microscopy (SEM) observations indicate that NbO2 has a unique nanoslice structure while Nb2O5 has a flake-like structure. The optical transmittance of the films has been investigated and found to be dependent on the oxygen gas content during deposition; the optical transmittance decreases with increasing O2 gas content. Optical constants of the films were calculated by fitting a suitable thin film transmittance model to experimental transmittance spectra using a modified Swanepoel technique. The nanohardness and stress in the films were measured by nanoindentation and an optical profilometer respectively. Nanohardness and stress in the film show no large dependence on the oxygen gas content except at high oxygen gas content. The nanohardness value of NbO2 films is approximately 6 GPa, and the Young's modulus is 150 GPa. The Nb2O5 films exhibit a nanohardness of 5.8–13 GPa and a Young's modulus of 137–161 GPa. [ABSTRACT FROM AUTHOR]