Interfacial and electrical properties of atomic-layer-deposited HfO2 gate dielectric on epitaxial GaAs (epi-GaAs)/Ge and bulk GaAs substrates have been investigated. Atomic layer deposition provides a unique opportunity to integrate high quality gate dielectrics on epi-GaAs. The cross-sectional transmission electron microscopy of a HfO2/III-V gate stack shows a similar interfacial layer thickness for HfO2 on bulk p-GaAs and epi-GaAs substrates. However, X-ray photoelectron spectroscopy shows a Ga oxide-rich interfacial layer after postdeposition annealing at 500 degrees C for films grown on epi-GaAs. Although the epi-GaAs surface is rough with nanoscale features, the electrical properties of the HfO2 gate dielectric deposited on epi-GaAs are comparable with bulk p-GaAs-based devices. The Au/HfO2/epi-GaAs gate stack shows a low frequency dispersion (13%), hysteresis voltage (0.72 V), and a leakage current density of 2.1 x 10(-3) A cm(-2) at V-FB + 1 V (where FB is flatband) for an equivalent oxide thickness of 1.4 nm. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3453935] All rights reserved.