The loss of local dielectric integrity in ultrathin Al2O3 films grown by atomic layer deposition is investigated using conducting atomic force microscopy. I-V spectra acquired at different regions of the samples by constant and ramping voltage stress are analyzed for their pre- and post-breakdown signatures. Based on these observations, the thickness dependent dielectric reliability and failure mechanism are discussed. Our results show that remarkable enhancement in breakdown electric field as high as 130 MV/cm is observed for ultrathin films of thickness less than 1 nm. (C) 2013 Elsevier B. V. All rights reserved.