AbstractOptical absorption measurements were used to investigate deep defects in proton irradiated doped and undoped AlGaN thin films grown on sapphire substrates. Several samples were proton irradiated with energies ranging between 10 keV and 1 MeV. In certain samples, multiple-energy ion implantation was found necessary to produce a defect, which is responsible for the absorption band observed at 4.61 eV with a shoulder at around 4.10 eV in Al0.6Ga0.4N. Furnace thermal annealing of the irradiated samples show that this absorption band starts to anneal out at temperature as low as 200 oC. A combined isochronal and isothermal annealing in the temperature range of 200- 350°C shows that the activation energy (enthalpy associated with the migration process) of this defect is approximately 0.41 eV. This leads us to conclude that this absorption band is due to a N-vacancy related defect. It is observed that the peak position energy of the absorption band due to this defect is shifted depending on the Al mole fraction in good agreement with the theoretical predictions.