Single crystalline colorless α-Al2O3 samples were
implanted with several fluences of Fe+ ions in the range of
1×1016 to 5×1017 Fe+ cm-2 at
room temperature. Optical absorption and luminescence measurements were
carried out before and after annealing in reducing and oxidizing
atmospheres. The structural changes were studied with RBS/channeling and
x-ray diffraction. After implantation, the damage induces a brownish
coloration in the samples for fluences below 2×1017
Fe+ cm-2. The optical spectra are characterized by
an absorption band centered at 200 nm. This band is strongly reduced after
annealing at 1100 °C in reducing atmosphere and a new well-defined band
develops around 350 nm. This new band shifts to lower values with the
implanted fluence, which is an indication of its correlation with the
dimensions of the iron precipitates formed in the implanted region. The
existence of these precipitates was confirmed by x-ray diffraction and TEM.
The samples implanted with fluences above 1×1016 Fe+
cm-2 annealed in oxidizing atmosphere display different
optical absorption spectra, with respect to those annealed in reducing
atmosphere, characterized by an increase in the intensity of the peak at 200
nm. Luminescence measurements show the presence of F and F+
centers in the samples. The existence of these defects can be explained by
the need of charge compensation and strain release due to the formation of
mixed iron oxide or metallic precipitates.