Electrical properties of nanocrystalline anatase TiO2 thin films with different crystallite size

Nanocrystalline anatase TiO2 films with crystallites of three different sizes (approximately 6, 12, and 18 nm) were deposited on various substrates (glass, sapphire, and silicon) using colloidal suspensions. These films were thoroughly characterized with respect to their surface morphology, crystal structure, phase homogeneity, elemental composition, and the presence of impurities. A detailed study of the influence of sample temperature T and the ambient oxygen pressure p(O2) on the electrical conductivity σ of the films was performed. For all films a power-law dependence of σ on p(O2), σ∝p(O2)n, was observed. The values of the exponent n were found to exhibit a distinct dependence both on the crystallite size and on the specimen temperature; furthermore, an influence due to the presence of doping species was noted. For nanocrystalline films on sapphire substrates and for T=200 °C, n amounts to −1.31, −1.15 and −0.56 for the 6-, 12-, and 18-nm films, respectively. For reduced films, an exponential dependence of σ on T−1 was determined, yielding activation energies EA with values of 0.34, 0.38, and 0.51 eV for films with those crystallite sizes.