Effect of nitrogen doping concentration on the properties of TiO2 films grown by atomic layer deposition

N-doped TiO2 films were grown on n+-silicon substrates by atomic layer deposition using titanium chloride and vapor mixture of ammonia and water as the reactants. The effects of doping concentration on the microstructure and photocurrent response of as-deposited films were investigated. The results show that the doping levels were 0.2, 0.7, 1.2, 1.5, and 4.3 at% for films grown at NH3-to-H2O injection volume ratios of 350, 380, 440, 520, and 550, respectively. The off-plane lattice constant of TiO2 films increased with the increase of doping level, and the transformation of anatase to rutile was inhibited by the doping as the doping concentration reached 1.2 at%. The wavelength-dependent photocurrents suggest an optimal N doping concentration lying between 0.7 and 1.2 at% for the visible light active TiO2 films. Doping with a too-low or a too-high nitrogen level resulted in an inefficient visible light generation or a serious carrier recombination, respectively.