Studies on the room temperature growth of nanoanatase phase TiO_2 thin films by pulsed dc magnetron with oxygen as sputter gas

摘要

The anatase phase titanium dioxide (TiO_2) 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 X 10~(-2) mbar are nanocrystalline (grain size of 28-43 nm) with anatase phase. The films deposited at the chamber pressure of 7.2 X 10~(-2) mbar are found to be highly crystalline with a direct optical band gap of 3.40 eV, refractive index of 2.54 (at λ=400 nm), and work function of 4.77 eV (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 TiO_2 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~+, TiO_2~+, and O~(2+) 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 X 10~(-2) mbar was studied for its electrochromic properties by protonic intercalation. It showed good electrochromic behavior with an optical modulation of ～45%, coloration efficiency of 14.7 cm~2 C~(-1), and switching time (t_c) of 50 s for a 2 X 2 cm~2 device at λ= 633 nm.