The simultaneous self-cleaning and passivation of an as-received GaAs substrate using trimethylaluminium (TMA) pretreatment before the atomic layer deposition (ALD) of HfO2 was systematically investigated. The change of the interfacial characteristics was probed as a function of the number of treatment cycles and the post-deposition annealing temperature and related with various electrical properties of the HfO2 film. The TMA pretreatment removed the Ga- and As-oxides more effectively than the ALD-HfO2 process, which reduced the amount of frequency dispersion. In addition, compared to the As case, it showed more predominant suppression of both Ga out-diffusion and Ga-O bond formation, which is believed to have delayed the thermal degradation of the capacitance equivalent thickness (CET). The optimal number of TMA treatment exhibited the best thermal stability without increasing the as-deposited CET. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.052201jes]