Ultra-thin tantalum pentoxide (Ta2O5) layers of thicknesses ranging from 6 to 10 nm were deposited by low pressure chemical vapor deposition from Ta(OC2 H5)5 on rapid thermal nitrided silicon substrates. Films were annealed in UV-O3 at 450°C, in dry O2 at 750°C or by using a combination of these two treatments. The physico-chemical properties were studied by TEM and SIMS. Results showed an oxidation of the interfacial region during annealing. The C and H contaminants are reduced during the O2 post-deposition treatment, and this step may also lead to a modification of the chemical state of C in Ta2O5. Both capacitance-voltage and current-voltage measurements were performed on Al/Ta2O5/Si structures. Excellent electrical properties were recorded: high dielectric constant (between 14.8 and 19 in the case of the double step annealing, which corresponds to an equivalent SiO2 thickness ranging from 2.0 to 2.3 nm), low leakage current densities (close to 5×10−9 A.cm−2 @ 1MV.cm−1) and promising long-term reliability.