Morphology of Ni/GaN contact formed at room temperature (RT) by Ni vapor deposition onto the (0 0 0 1)-oriented n-type GaN surface under ultrahigh vacuum, and morphological changes introduced by annealing were studied. Measurements were carried out in situ using XPS, UPS, LEED and STM. The WF of the Ni film of the mean thickness 1 nm equaled 4.1 eV. For thicker layers (>= 2 nm), it increased to 5.1 eV. The Schottky barrier height of the Ni/GaN(0 0 0 1) contact formed at RT amounted to 1.20 eV. Annealing of the Ni/GaN contact at 650 degrees C resulted in Ga diffusion into the Ni film and Ni-Ga alloying. The dominating alloy phase was Ni3Ga. The alloying was accompanied by coalescence of Ni film grains into 3D islands of a Ni-Ga alloy. Annealing at 800 degrees C enriched the islands with Ga. The Ga-rich phases of NiGa and/or Ni3Ga2 were dominant in the alloy. (C) 2014 Elsevier B.V. All rights reserved.