  The electronic structures of Sn and Pb implanted SiO2 are studied using soft
X-ray absorption (XAS) and emission (XES) spectroscopy. We show, using
reference compounds and ab initio calculations, that the presence of Pb-O and
Sn-O interactions can be detected in the pre-edge region of the oxygen K-edge
XAS. Via analysis of this interaction-sensitive pre-edge region, we find that
Pb implantation results primarily in the clustering of Pb atoms. Conversely,
with Sn implantation using identical conditions, strong Sn-O interactions are
present, showing that Sn is coordinated with oxygen. The varying results
between the two ion types are explained using both ballistic considerations and
density functional theory calculations. We find that the substitution of Pb
into Si sites in SiO2 requires much more energy than substituting Sn in these
same sites, primarily due to the larger size of the Pb ions. From these
calculated formation energies it is evident that Pb requires far higher
temperatures than Sn to be soluble in SiO2. These results help explain the
complex processes which take place upon implantation and determine the final
products.
