The role of tantalum nitride (TaNx) thin films as buffer layers on the control of nucleation and growth of aligned carpet-like carbon nanotubes (CNTs) has been proved. TaNx thin films have been deposited on Si by controlled magnetron sputtering process. Multiwall CNTs have been synthesized at 850 degrees C using an aerosol of ferrocene diluted in toluene. Electron microscopy images show a strong correlation between the growth rate and morphology of the CNTs and the initial composition of the TaNx thin films. Multi-scale investigations reveal that both morphology and structure of the CNTs are determined by the properties of the TaNx films. Raman and X-ray photoelectron spectroscopy, high resolution TEM imaging at the submicrometric and atomic scales have been used to confirm these hypotheses.