Dilution and clustering of Fe in the rutile phases ofTiO2andSnO2

Dilute magnetic semiconductors of Fe-doped ${\mathrm{SnO}}_{2}$ and ${\mathrm{TiO}}_{2}$ with the structure of rutile were prepared in forms of powder and thin films using the techniques of sol gel and pulsed-laser deposition. We present the results of measurement of vibrational density of states of Fe impurity dopants in these oxides and demonstrate the cases of dilution and clustering. The oxygen pressure during the film deposition was varied between ${10}^{\ensuremath{-}1}$ and ${10}^{\ensuremath{-}8}\phantom{\rule{0.3em}{0ex}}\mathrm{Torr}$. In ${\mathrm{TiO}}_{2}$ films made at ${10}^{\ensuremath{-}1}\phantom{\rule{0.3em}{0ex}}\mathrm{Torr}$, Fe is diluted, however, in films made at ${10}^{\ensuremath{-}8}\phantom{\rule{0.3em}{0ex}}\mathrm{Torr}$ Fe is clustered. The case of true Fe dilution in ${\mathrm{SnO}}_{2}$ is also shown. In spite of larger mass defect for Fe in ${\mathrm{SnO}}_{2}$ than that for Fe in ${\mathrm{TiO}}_{2}$ the dilute Fe species probe the phonon states in ${\mathrm{SnO}}_{2}$ more faithfully than in ${\mathrm{TiO}}_{2}$. This result is understood in terms of the combined effect of mass defect and nearest-neighbor force-constant changes. The impurity modes are more pronounced in ${\mathrm{TiO}}_{2}$ than in ${\mathrm{SnO}}_{2}$ due to ca. 10% difference of the lattice cell volumes between these two rutile oxides.