The distribution of deep-level traps at atomic-layer-deposited Al2O3/n-GaN interface is studied by photo-assisted high-frequency capacitance-voltage (C-V) method. The dark C-V curve of the fabricated metal-oxide-semiconductor (MOS) capacitors shows positive-shifted ideal MOS characteristics with deep-depletion behavior. The electrons trapped at deep states can be neutralized by holes generated by ultra-violet (UV) light illumination. An interface-trap-related voltage stretch-out of the C-V curve is then obtained by sweeping the capacitor from depletion back to accumulation after UV illumination, at which electrons gradually inject back into the emptied donor-like trap states. By comparing the voltage shift of the post-UV curve with an "ideal" trap-free C-V curve obtained by a parallel-shift of the dark C-V curve, a decayed interface trap profile towards the mid-bandgap is derived, yielding a peak value of similar to(1-2) x 10(12) eV(-1) cm(-2) at similar to 0.3 eV below the conduction band edge of GaN. (c) 2012 Elsevier Ltd. All rights reserved.