HRXRD, AFM and optical study of damage created by swift heavy ion irradiation in GaN epitaxial layers

Epitaxial GaN layers grown by MOCVD on c-plane sapphire substrates are irradiated with 150 MeV Ag ions at a fluence of 5 x 10(12) ions/cm(2). Samples used in this study are 2 mu n thick GaN layers, with and without a thin AlN cap-layer. Surface morphology is studied using contact mode atomic force microscopy (AFM). Irradiated samples show qualitatively different morphologies as well as quantitative changes. Different kinds of morphology are attributed to specific type of dislocations using the existing models available in the literature. The residual strain and sample quality have been analysed before and after irradiation using high resolution X-ray diffraction (HRXRD). The Lorentzian shape analyses of the experimental scans complement the AFM results. Optical properties are studied by spectrophotometer used in the transmission mode. A sharp band-edge in the as grown samples was observed at similar to 3.4 eV. The band-edge absorption broadened due to irradiation and these results have been discussed in view of the damage created by the incident ions which compliment HRXRD results. In general the effect of irradiation induced-damages are analysed as a function of material properties. A possible mechanism responsible for the observations has been discussed. (c) 2006 Elsevier B.V. All rights reserved.