The electroforming and unipolar resistive switching characteristics of as-deposited and annealed HfO2 and Hf0.6Si0.4O2 films are reported. The reliability of HfO2 devices is shown to be significantly degraded by annealing at 600 degrees C, during which the film is observed to crystallize. In contrast, the characteristics of Hf0.6Si0.4O2 devices subjected to the same annealing conditions are found to be unchanged, consistent with the fact that the films remain amorphous. These differences are attributed to the presence of grain boundaries and can have important implications for the use of HfO2 in ReRAM applications. (C) 2013 Elsevier B.V. All rights reserved.