Thermal hysteresis measurement of the VO2 dielectric function for its metal-insulator transition by visible-IR ellipsometry

Abstract

The real and imaginary parts of the dielectric function of VO2 thin films, deposited on r-plane sapphire via pulsed laser deposition, are measured by means of visible-infrared ellipsometry for wavelengths ranging from 0.4 to 15 μm and temperatures within its phase transition. For both the insulator-to-metal (heating) and metal-to-insulator (cooling) transitions, it is shown that the two ellipsometric signals exhibit three temperature-driven behaviors, which are well described by appropriate combinations of the Tauc-Lorentz, Gaussian, and Drude oscillator models. By fitting Bruggeman's effective medium model for the dielectric function to the corresponding measured experimental values, using the volumetric fraction of the VO2 metallic domains as a fitting parameter for different temperatures within the VO2 phase transition, we have found that this model is suitable for describing the dielectric function in visible and near-infrared wavelengths (∼0.4 to ∼3.0 μm), but it generally fails for longer infrared ones. Furthermore, the hysteresis loop of the VO2 emissivity averaged over a relevant interval of wavelengths is determined and shown to vary from ∼0.49, in the insulator phase, to ∼0.16, in the metallic one. These values, based on the VO2 dielectric function, are consistent with previous measurements reported in the literature, and therefore, our measured data are expected to be useful for describing the behavior of VO2 films involved in optical and radiative applications.

On the Optical Properties of Thin‐Film Vanadium Dioxide from the Visible to the Far Infrared

“…For wavelengths below ≈2 µm, which approximately corresponds to the energy region of interband transitions of VO 2 in its insulating phase (>0.6 eV), all of our films show substantial differences in their insulating‐phase optical properties. Such large differences of optical properties in the visible and near‐infrared ranges are also regularly found in the literature for different VO 2 samples ( Figure ). The band structure of the insulating state can be sensitive to a variety of factors, including strain, stoichiometry, grain size, and defects .…”

“…Our data also agrees reasonably with the very recent ref. [], which covers wavelengths up to 15 µm, though the metal‐phase VO 2 from this work appears to be less metallic compared to our films.…”

“…For a more‐comprehensive comparison, we plot the complex refractive index of Film 1, together with data from a selection of the literature, in Figure . Most existing characterization works focus on the visible and near‐infrared region, in which the optical properties of VO 2 differ substantially, as discussed previously.…”

“…Knowledge of the complex refractive index of VO 2 across its IMT can enable computational design of various optical and optoelectronic devices incorporating this material. There is a large literature base of experiments that have been used to extract such optical properties; however, many of these investigations focus almost entirely on the visible and near‐infrared ranges, with one recent study extending into the mid infrared …”

Correlation between in situ structural and optical characterization of the semiconductor-to-metal phase transition of VO2 thin films on sapphire

“…Upon the SMT, the material switches from the semiconductor state (with a monoclinic crystallographic structure) to the metallic state (tetragonal, rutile structure), changing drastically its electrical properties (electrical conductivity may change up to The change of physical properties in vanadium dioxide during the phase transition is also characterized by an hysteresis loop whose temperature width varies from few degrees (2-3 • C) in single crystals, up to few tens of degrees (10-40 • C) in thin films containing small nanocrystals with different sizes. [26][27][28][29][30][31] The possibility to control the hysteresis parameters and the SMT properties by choosing specific technologies for the thin film deposition is very important for many applications, as for example for heat management 32,33 or thermal camouflage, [34][35][36][37][38][39] and thus much work has been devoted to the study of VO 2 thin films realized with different methods, such as reactive evaporation, 40 sputtering, [41][42][43][44] metal-organic-chemical-vapor deposition (MOCVD), 45,46 , sol-gel deposition 47 and pulsed laser deposition (PLD). 48,49 In spite of the wide investigation performed in the last years on the optical and electrical properties of VO 2 thin films upon the phase transition and the structural characterization of the initial and final states of the STM, much less is known about the kinetics of the structural processes occurring during the SMT.…”