The effects of Al2O3 passivation, formed by atomic layer deposition (ALD) at the interface of HfO2/GaAs, were investigated by high resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and capacitance-voltage (C-V) measurements. The results indicate that the incorporation of Ga by diffusion into the HfO2 layer is reduced by the Al2O3 passivation at the HfO2/GaAs interface. The Ga and As contents of the HfO2 films decreased with increasing amount of interfacial Al2O3 passivation, while the capacitance value decreased. The Al2O3 phase optimized at five ALD cycles effectively suppressed the formation of interfacial oxide and subsequently improved the C-V electrical properties. (c) 2010 The Electrochemical Society. [DOI: 10.1149/1.3516615] All rights reserved.