Emission channeling experiments from the decay of $^{149}$Gd to $^{149}$Eu in GaN

The lattice site location of excited states of $^{149}$Eu was studied by means of the emission channeling technique. 60 keV implantation of $^{149}$Tb into a GaN thin film was performed at room temperature up to a dose of 2.0 $\times 10^{13}$ cm$^{-2}$. This radioactive isotope eventually decays into short-lived excited states of $^{149}$Eu. The conversion electrons emitted in the subsequent decay to the $^{149}$Eu ground state were detected with a position sensitive detector. We measured their angular distributions around the [0001], [1102], [1101] and [2113] axes in the as-implanted state and after 600°C and 900°C vacuum annealing. Already in the as-implanted state around 65% of $^{149^{*}}$Eu atoms were found on substitutional Ga sites. The root mean square (rms) displacements from the substitutional sites in the as-implanted state were found to be around 0.13-0.16 . Annealing up to 600$^\circ$C increased the substitutional fraction by a few percent and slightly reduced the rms displacements, most likely due to the removal of crystal defects in the vicinity of the Eu atoms, resulting in a better incorporation into substitutional sites.