Hafnium oxide thin films have been deposited on Si(001) substrates by atomic layer deposition from the tetrakis-dimethylamino hafnium precursor using both conventional thermal and plasma-enhanced methods. The structural, compositional and morphological film characterization has been carried out by transmission electron microscopy, X-ray photoelectron spectroscopy and atomic force microscopy. All the data indicate that some reactive phenomena occur at the film/substrate interface forming a hafnium silicate layer. The electrical characterization of the two deposited layers has been carried out in order to evaluate its potential implementation as an alternative dielectric. Their dielectric constant values have been evaluated to be 7.5 and 5.5 for films deposited by the plasma-enhanced and thermal ALD processes, respectively.