Investigations on the 100 MeV Au7+ ion irradiation of GaN

Metal organic chemical vapour deposition (MOCVD) grown n-type GaN on sapphire substrates was irradiated with 100 MeV Au7+ ions, varying the fluence as 1 x 10(12), 1 x 10(13) and 5 x 10(13) ions cm(-2) at room temperature. The irradiated samples were characterized by x-ray diffraction (XRD), atomic force microscopy (AFM), Raman scattering, photoluminescence and UV-visible optical absorption spectrum. XRD analysis reveals a huge lattice disorder for fluence beyond 1 x 10(13) ions cm(-2). This is observed from the increase in the FWHM and decrease in the intensity of the GaN (0 0 0 2) peak. The sapphire peak totally vanishes for higher fluence. Raman modes E-2 (high) and A(1) (LO) are observed at 568 cm(-1) and 749 cm(-1) respectively. For the fluence of 1 x 1012 ions cm(-2) the E2 (high) shifts to lower frequency and a further increase in fluence results in a broader less intense E2 mode. Additional modes around 100 cm(-1) and 220 cm(-1) were observed and the origin of those modes is discussed. We also observed the Boson peak, the intensity of which increases as the fluence increases. AFM images show the roughness of GaN increases with increasing ion fluences. At a fluence of 5 x 10(13) ions cm(-2), a new type of ditch and dike structure was observed. These structures were distributed over the irradiated GaN surface. UV-visible absorption spectrum shows that the band gap energy decreases with increasing ion fluences and also that the band edge is broadened. The results are discussed by the complementary use of these studies.