In order to study formation energies, atomic geometry, electronic structures, and surface states of oxidized Al0.25Ga0.75N(001) (2 x 2) reconstruction surfaces, models of Al0.25Ga0.75N (0 0 1) (2 x 2) reconstruction surfaces with different coverages of oxygen atoms were built, and first-principles calculations were performed based on density functional theory (DFT). The results of these calculations show that formation energy of the oxidized Al0.25Ga0.75N(0 0 1) (2 x 2) reconstruction surfaces decreases as the increasing of O coverage under O-rich conditions. O atom prefers to locate on the top of Ga-Ga and Ga-Al dimers. After relaxation, the length values of Ga-O and Al-O bonds are 1.856 angstrom and 1.853 angstrom, respectively. With smaller electronegativity values, Ga and Al atoms donate electrons to O atom, and the dangling bonds of Ga and Al atoms on the surface were occupied. At Fermi level, the partial density of states (PDOS) of Ga and Al atoms on the topmost surface decrease significantly after Ga and Al atoms bond with O atoms, showing that surface states disappear. O atoms adsorption or replacement on Al0.25Ga0.75N(001) (2 x 2) reconstruction surface induces charge redistribution, which in turn forms Ga(Al)-O dipoles oriented to O atoms. With Ga(Al)-O dipoles, work functions of oxidized surfaces are larger than that of clean surface, and the work function increases as O coverage increases. (C) 2015 Elsevier B.V. All rights reserved.