Structural and optical studies of Co and Ti implanted sapphire

Single crystals of α-Al₂O₃ with different orientations were implanted with several fluences of Ti and Co ions. For low fluences both Ti and Co ions are fully incorporated in Al lattice sites and remain stable up to annealing temperatures of 1000 °C. For fluences of 5 × 10¹⁶ cm^-2 the implanted region becomes completely disordered (amorphous) for samples implanted with Ti while for Co the same condition is achieved only for higher fluences (2 × 10¹⁷ Co⁺/cm²). The recovery of the implantation damage is almost complete after annealing at 1000 °C in either oxidizing or reducing atmospheres for fluences below 5 × 10¹⁶ cm^-2. For higher fluences annealing in a reducing ambient promotes the precipitation of crystalline metallic Co and Ti particles, as revealed by TEM and RBS. These precipitates retard the damage recovery. The presence of oxygen during annealing leads to the formation of mixed Co and Al oxides through the entire implanted region. Annealing at 1000 °C promotes the formation of a spinel phase (Al₂CoO₄) and the blue or green coloration of sapphire, depending on the Co fluence. Moreover, narrow red emission lines were observed. For Ti the oxides concentrate at the surface and optical absorption measurements show the presence of a broad absorption band centered at 325 nm. This band is absent in the samples annealed in a reducing atmosphere. Photoluminescence measurements reveal the presence of an emission band centered near 840 nm.