Optical properties of magnesium aluminate spinel crystals implanted with helium ions

Measurements of the optical properties of helium-implanted spinel crystals have been used to obtain information on the formation of point defects and their evolution during thermal annealing. Optical absorption spectra showed that the main defects in helium-implanted spinel were the anion vacancies which were removed by annealing to 1000 K. Additional defects in implanted and annealed specimens were observed and are related to cation vacancies and anti-site defects. The increased cationic disorder in ion-implanted spinel crystals was demonstrated by measuring the radio-luminescence in these crystals. Optical micrographs show that, at low fluences, the heterogeneous nucleation of bubbles take place, but at a fluence of 1×1021 ion/m2 the homogeneous formation of small bubbles of higher density was obtained. Finally, at the highest fluence of 3×1021 ion/m2 the bubbles begin to grow by coalescence and become large enough to cause flaking of the implanted surface.