© 2014 Elsevier B.V. All rights reserved. The effects of post-annealing conditions on solid-phase crystallization of atomic-layer-deposited HfO2 films grown on GaAs were investigated. Film properties, including crystallinity (preferential crystal orientation and crystallite size), thickness, density, permittivity, optical band gap and chemical composition were monitored as a function of annealing conditions. Following rapid thermal processing (RTP) in nitrogen ambient at temperatures from 325 °C to 625 °C, the initially produced amorphous/partially crystallized HfO2 films changed into a well-ordered crystalline structure with no detectable interfacial layer between the film and the GaAs substrate. Though HfO2 properties were predictable and similar to those of HfO2 on Si for low temperature processing, in the case of annealing at ≥475 °C, the thickness of the film was relatively increased compared to that of an as-grown film and displayed unexpected film characteristics. Changes after annealing in the depth profile data related to stoichiometry indicated that As oxide is formed within the HfO2 film during the RTP. The formation of As oxide in the HfO2 film, resulting from the underlying substrate, is dissimilar to previously published results which reported the presence of Ga oxide.