Titanium dioxide thin films were grown by atomic layer deposition (ALD) at 100-250 degrees C with tetrakis-dimethyl-amido titanium, using H2O2 as a counter-reactant. We have explored the effects of deposition temperature and the reactant pulse and purge times on the film growth rate to optimize the ALD process of TiO2. The film growth rate decreased with growth temperature below 175 degrees C, but was saturated to 0.28 angstrom/cycle at 175-250 degrees C. All the as-deposited films were highly pure, and the root-mean-square roughness was less than 2.5% of the film thickness. The films deposited at 150 degrees C and above were polycrystalline with an anatase structure, whereas those deposited at 125 degrees C were amorphous. There was no change of crystal structure after annealing the crystalline films at temperatures of 450-650 degrees C. All the crystalline films were shown to have a photocatalytic activity in decomposing methylene blue in an aqueous solution. The films prepared at higher deposition temperatures were shown to have a better photocatalytic activity in decomposing methylene blue. (c) 2006 American Vacuum Society.