Electrical and Optical Properties of Vanadium Dioxide and Vanadium Oxyfluoride Thermochromic Thin Filmsa and Their Applications to Energy Efficient Smart Windows

Abstract

This thesis reports on the rmochromic thin films of Vanadium dioxide and Vanadium oxyfluoride produced by reactive rf magnetron sputtering followed by annealing post-treatment. Such coatings have optical properites that make them interesting for application on energy-efficient Smart Windows. The VO2 coatings were characterized by transmission electron micro- scopy. They were polycrystalline with grains 50 nm to 100 nm in diameter. Thin films of VOXFy were made by rf magnetron sputtering of V in Ar + O2 + CF4. They were analysed by Rutherford Backscattering Spectrometry and transmission electron microscopy. Imaging in bright-field and dark-field modes showed that the crystallites were 50-90 nm in linear size. Electron diffractograms showed ring type patterns, characteristic of microcrystalline samples. Electrical measurements on VO films gave a metal-insulator transition temperature (t) at 68°C. Spectral measurements on these films in the whole solar and thermal wavelength range exhibit thermochromism. Electrical measurements as a function temperature in the range 20 <t< 100°C for VOXFy films indicate a depression of the transition temperature by roughly 15°C to 16°C. A detailed Investigation of the spectral character- istics of VOxFy films in the whole solar and thermal (0.3 - 50 μm) range were carried out at elevated temperature and the samples were found to exhibit thermochromism. This reports what we believe is the first study of thermochromic vanadium oxyfluoride coatings. Fluorination led to a significant boost in transmittance. The effect was most prominent at thickness t> 100 nm. At t = 200 nm, VOXFy has Tlum = 25 % while VO2 only has Tlum = 10 %, where Tlum refers to integrated luminous transmittance. Optical properties at the metal-insulator transition in thermochromic VOXFy thin films were measured in detail as a function of temperature. This study verified that the spectral transmittance as well as the reflectance were essentially wavelength independent at a temperature which corre- sponds to 65°C, i.e. at the metal-insulator transition. The complex dielectric function of VO2 and VO Fy coatings in the whole solar range (0.3 2.5 μm) were determined by spectrophotometric measurements. The author's results for VO2 are in overall agreement with earlier published dielectric functions. The optical constants for VOXFy, as far as author knows, have not been measured before.