In order to understand the mechanism of ohmic-contact formation by the annealing of Ti electrodes on n-type GaN, we have investigated the changes in the energy-band structure of Ti/n-type GaN depending on annealing temperature using photoemission spectroscopy. Valence-band spectrum for an as-deposited sample was explained by the simple summation of Ti and GaN spectra. Spectral line shapes significantly changed by annealing at 500 and 700 degrees C, suggesting the formation of a TiN layer. The peak shifts of Ga 3d and N 1s core levels are interpreted as the energy-band bending and interfacial reaction with the formation of the TiN layer. It is revealed that the ohmic-contact formation by the annealing is attributed to the formation of GaN with nitrogen vacancies, which is consistent with the current-voltage characteristics. (c) 2004 Published by Elsevier B.V.