Studying GaN films exposed to Ar plasma by spectroscopic ellipsometry and reflectance, we found an "anomalous" pseudodielectric function (PDF) for which the imaginary part is significantly higher as compared to GaN, while the real part of the PDF remains close to the value for GaN. In addition, a higher reflectance at low angles of incidence was observed. The data are explained in terms of a thin highly absorbing surface layer arising due to non-stoichiometry in the near-surface region. Comparison to samples grown by molecular beam epitaxy shows that similar mechanisms are responsible for optical properties of the surfaces of films obtained under Ga-rich conditions. (c) 2006 Elsevier B.V. All rights reserved.