[en] The Metal Organic Chemical Vapour Deposition (MOCVD) grown n-type Gallium nitride (GaN) layers on sapphire (0 0 0 1) substrates have been irradiated at low and room temperatures with 40 MeV Li³⁺ ions at the fluence of 1 x 10¹³ ions cm^-2. Irradiated samples were characterised by using X-ray diffraction (XRD), photoluminescence (PL), Raman spectroscopy and atomic force microscopy (AFM). XRD results show that the formation of Ga₂O₃ has been observed upon irradiation. This is due to interface mixing of GaN/Al₂O₃, at both temperatures. Also the GaN (0 0 0 2) peak splits into two at low temperature irradiation. PL measurements show a yellow emission band shift towards blue band side upon irradiation at 77 K. Raman studies indicate that the lattice disorder is high at room temperature irradiation compared to low temperature irradiation. AFM images indicate the increasing surface roughness after ion irradiation at room temperature when compared to pristine GaN and low temperature irradiated GaN. These observations are discussed in detail with the use of complementary techniques