Studies on the room temperature growth of nanoanatase phase TiO2 thin films by pulsed dc magnetron with oxygen as sputter gas

The anatase phase titanium dioxide (TiO2) thin films were deposited at room temperature by pulsed dc magnetron sputtering using pure oxygen as sputter gas. The structural, optical, electrical, and electrochromic properties of the films have been studied as a function of oxygen pressure in the chamber. The x-ray diffraction results indicate that the films grown above 4.5×10−2mbar are nanocrystalline (grain size of 28–43nm) with anatase phase. The films deposited at the chamber pressure of 7.2×10−2mbar are found to be highly crystalline with a direct optical band gap of 3.40eV, refractive index of 2.54 (at λ=400nm), and work function of 4.77eV (determined by the Kelvin probe measurements). From the optical emission spectra of the plasma and transport of ions in matter calculations, we find that the crystallization of TiO2 at room temperature is due to the impingement of electrons and ions on the growing films. Particularly, the negative oxygen ions reflected from the target by “negative ion effects” and the enhanced density of TiO, TiO+, TiO2+, and O2+ particles in the plasma are found to improve the crystallization even at a relatively low temperature. From an application point of view, the film grown at 7.2×10−2mbar was studied for its electrochromic properties by protonic intercalation. It showed good electrochromic behavior with an optical modulation of ∼45%, coloration efficiency of 14.7cm2C−1, and switching time (tc) of 50s for a 2×2cm2 device at λ=633nm.