Effect of titanium ion irradiation on the surface and defect centre formation in sapphire

Swift titanium ion irradiation was carried out on sapphire (Al2O3: Fe, Ti, Cr) to study optical properties of defect centers and surface topography of modified sapphire using UV–visible absorption spectroscopy, photoluminescence (PL) and atomic force microscopy (AFM) techniques. Single crystals of sapphire were irradiated at room temperature with different fluence of 100 MeV Ti ions. The PL spectra showed three emission bands centred at 2.56 (480 nm), 2.45 (504 nm) and 2.31 eV (535 nm) assigned to F+ centre, F2 centre and F22+ defect centre, respectively. The intensity of these bands was found to change with fluence. R lines of Cr3+ ion impurity, though weak in pristine sample, grew tremendously on ion irradiation. The changes in the intensity of R lines are correlated with the Cr3+-defect centre pair formation. Both bands and lines showed downward trend at higher fluence but at a different fluence value. Results are interpreted in terms of creation of defect centres, aggregation/cluster formation, diffusion or radiation induced annihilation of the defects. The defect centres (F2+ and F22+) and R-line of Cr3+ ions were observed in optical spectra and their behaviour upon irradiation is also discussed in terms of dense ionization. The behaviour of bands corresponding to the electronic transitions of Fe2+/Ti4+ ions in the presence of oxygen vacancies, observed in optical spectra, are discussed in terms of dense ionization. The hillocks like structures were developed on the surface of irradiated sapphires. Their concentration, height and area are correlated with the ion beam fluence.