We have studied the lattice sites of Er in CZ Si single crystals by using conversion electron emission channeling from the isotope $^{167m}$Er (2.28 s) which is the decay product of radioactive $^{167}$Tm (9.25 d). Following 60 keV implantation of $^{167}$Tm at a dose of 4 $\times 10^{13}$ cm$^{-2}$ and annealing at 600°C, more than 90% of $^{167m}$Er is found close to tetrahedral insterstitial (T) sites. The tetrahedral fraction of $^{167m}$Er decreases considerably after 10 min annealing at 800°C and above. We attribute this to the onset of diffusion of the parent $^{167}$Tm and its trapping at other defects, presumably oxygen atoms or clusters of Tm/Er.
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