We have investigated the effects of the inductively coupled plasma (ICP) etching of AlGaN surface on the resulting interface properties of the Al2O3/AlGaN/GaN structures. The experimentally measured capacitance–voltage (C–V) characteristics were compared with those calculated taking into account the interface states density at the Al2O3/AlGaN interface. As a complementary method, photoassisted C–V method utilizing photons with energies less than the bandgap of GaN was also used to probe the interface state density located near AlGaN midgap. It was found that the ICP etching of the AlGaN surface significantly increased the interface state density at the Al2O3/AlGaN interface. It is likely that ICP etching induced the interface roughness, disorder of chemical bonds and formation of various type of defect complexes including nitrogen‐vacancy‐related defects at the AlGaN surface, leading to poor C–V curve due to higher interface state density at the Al2O3/AlGaN interface.