Porous hot-wire deposited WO3 films with high optical transmission

Abstract

Tungsten oxide films were deposited on Si and fused silica substrates by heating metallic filaments at temperatures of 650, 750, and 800 °C at a pressure of 1 Torr of N2. During deposition the substrates remained at or near room temperature. These hot-wire (hwWO3) films were found to be composed by amorphous material and highly transparent within the range 350−1000 nm. Spectroscopic ellipsometry measurements have shown that the real part of refractive index (n) of hwWO3 films exhibited features similar to those of stoichiometric WO3 films indicating that hwWO3 films were also stoichiometric. The values of n were found to depend on deposition time (film thickness) and after 2 s, have fallen below 1.45 within the visible range, while the imaginary part (k) remained near zero. These low values of n and k were attributed to the porosity of hwWO3 films, which as shown by simulations based on the effective medium approximation, after 2 s of deposition saturated near 60%. As shown by scanning electron microscopy measurements, hwWO3 films were composed by grains with dimensions of the order of 15−20 nm increasing with thickness, and voids between these grains. Transmission electron microscopy (TEM) measurements have shown that these grains were composed by others with dimensions near 5 nm and voids between them. The above microscopy measurements explain the high porosity of hwWO3 films. Moreover, they indicate that hwWO3 films were grown by stoichiometric WO3 particles with dimensions of the order of 5 nm or clusters of such species, which evaporated from the filament and then condensed on the cold substrate.