Electrical and optical properties and deep level spectra of undoped n-AlGaN films with Al contents of about 40% grown on sapphire by hydride vapor phase epitaxy were studied before and after hydrogenation. The electrical properties prior to hydrogenation are shown to be dominated by deep donors with an activation energy of 0.25 eV and concentration of similar to 10(18) cm(-3). Other deep traps include centers with activation energy of 0.12, 0.3, and 0.45 eV detected by photoinduced current transient spectroscopy and defects giving rise to defect bands near 2.3 and 3.7 eV in microcathodoluminescence spectra. Hydrogen plasma treatment at 250 degreesC for 0.5 h allows introduction of high concentrations of hydrogen down to a depth of about 0.1 mum and leads to strong suppression of all defects in that region. (C) 2004 American Vacuum Society.