We present the role of nitrogen content in tantalum nitride ultra-thin buffers, on the carbon nanotubes (CNTs) growth by chemical vapour deposition at 850 degrees C, assisted by ferrocene as catalyst source. Tantalum nitride (TaNx) films with a very large range of concentration x = [0, 1.8] and various nanostructures, from amorphous Ta(N) to Ta3N5, were deposited by Highly Pulsed Plasma Magnetron Sputtering. The buffer films are characterized after heat treatment at 850 degrees C, and after the CNT growth, by wide angle X-ray scattering in grazing incidence and scanning electron microscopy. The CNT diameter explored by transition electron microscopy shows an all-out value for under stoichiometric thin films (Ta-1-N1-delta,Ta-3-N5-delta) and a minimum value just above the stoichiometric phases (Ta-1-N1+delta, Ta-3-N5+delta). Firstly one shows that the buffer films under the heat treatment present surface modification highly dependent on their initial state, which influences the catalyst particles diffusion. Secondly at the stoichiometric TaN phase we show that a specific ternary phase FeTa2O6 is formed at the interface CNT/buffer, not present in the other cases, leading to a special CNT growth condition. (C) 2014 Published by Elsevier B.V.