The conduction mechanism and the nature of the filament formed in the low-resistive state of TiN/HfO2/TiN resistive random access memory devices with and without the insertion of an Hf anodic interlayer were studied from the temperature dependence of their current-voltage characteristics. These characteristics were analyzed using the percolation theory of conductor networks, allowing to extract the effective resistance of the filament. In the samples without the Hf interlayer the effective resistance decreased with temperature, suggesting a semiconducting nature of the filament. Conversely, the introduction of an Hf interlayer in the structure, likely acting as an oxygen scavenging layer, yielded a filament showing a metallic-like behaviour, i.e. with a significantly reduced temperature dependence of its effective resistance. These results can be explained by taking into account the role of oxygen vacancies created in the HfO2 layer during the filament formation, as these cause a transition from the original insulating material to a semiconducting or metallic sub-stoichiometric composition, depending on their local density. (c) 2013 Published by Elsevier B.V.