Evidence of chemical effects in niobium implanted oxides

We have investigated the influence of chemical bonding and structure of different oxides on niobium implantation effects. For this purpose, single crystals of α-Al2O3, TiO2, MgO and amorphous SiO2 have been implanted with 150 keV Nb+ ions and fluences ranging from 1016 up to 1017 ions cm-2. Two different behaviours have been observed depending on ionicity of the studied materials. For more covalent oxides such as α-Al2O3, TiO2 and SiO2 implanted with low fluences, the implantation profiles are in good agreement with theoretical predictions (TRIM). For high doses (>5x1016 ions cm-2) the formation of an amorphous layer has been observed in sapphire and rutile. For a typical ionic oxide such as MgO an anomalous depth distribution profile of implanted particles is observed and the major part of implanted niobium is in substitutional sites. The amorphization is not observed even for the highest implantation doses. These results suggest that different chemical effects occur and they could be correlated with the ability to deform octahedral sites in these oxides and space charge effects. (orig.)