Abstract Tantalum oxides have been prepared by thermal oxidation of sputtered tantalum films on silicon and silicon dioxide substrates at temperature between 450°C and 700°C. The composition of such oxide is tantalum pentoxide (Ta 2O 5) and the amount of silicon diffused into the tantalum oxide from the silicon substrate increases as the oxidation temperature increases. The temperature dependence of the dielectric constant of Ta 2O 5 on silicon can be explained in terms of its silicon concentration. CV curves of Al/Ta 2O 5/Si capacitors reveal the presence of donor-type interface states whose density decreases as the oxidation temperature increases. The flat band voltage of Al/Ta 2O 5/SiO 2/Si capacitors become more negative as the oxidation or annealing temperature increases regardless of the annealing ambient.