Anatase to rutile transition in TiO2 thin films: Role of tantalum and oxygen

The present study discusses the optimal solubility limit of tantalum in anatase TiO<inf>2</inf> thin films (dopant solubility limit) and its effect on crystal structure transformation to the rutile phase. The study also focuses on the role of lattice oxygen in this structural phase transition. The detailed chemical composition, constitution, and phase of these sputter-deposited thin films are correlated with their electrical and optical properties to understand the disparity between total impurity concentration in these films and actual impurity substitution into the host lattice sites (dopant content). Reduced oxygen vacancies from the oxygen lattice sites and/or Ta substitution in Ti lattice sites are found to hinder the anatase to rutile phase transformation in TiO<inf>2</inf> thin films. Our experimental data indicates the solubility limit of Ta being < 5 at% in TiO<inf>2</inf>. Similar behavior is seen when Ta is replaced with Nb in TiO<inf>2</inf> thin films. Moreover, an increased oxygen and/or impurity concentration is found to lead to impurity-rich-oxides in these films, degrading the electrical conductivity.