Emission channeling experiments from the decay of 149Gd to 149Eu in GaN

The lattice site location of excited states of 149Eu was studied by means of the emission channeling technique. The 60 keV implantation of 149Tb into a GaN thin film was performed at room temperature up to a dose of 2.0×1013 cm−2. This radioactive isotope eventually decays into short-lived excited states of 149Eu. The conversion electrons emitted in the subsequent decay to the 149Eu ground state were detected with a position sensitive detector. We measured their angular distributions around the [0 0 0 1], [1̄ 1 0 2], [1̄ 1 0 1] and [2̄ 1 1 3] axes in the as-implanted state and after 600 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 Å. Annealing up to 600 °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.