Lattice sites of implanted Na in GaN and AlN in comparison to other light alkalis and alkaline earths

The lattice location of ion implanted radioactive 24Na (t1/2 = 14.96 h) in GaN and AlN was determined using the emission channeling technique at the ISOLDE/CERN facility. In the room temperature as-implanted state in both GaN and AlN, the majority of the sodium atoms are found on interstitial sites near the octahedral position, with a minority on cation Ga or Al substitutional sites. Following annealing at 800–900 °C, the interstitial fraction is reduced while the substitutional incorporation increases. Our results thus further establish the amphoteric character of Na in GaN and AlN, in analogy to other light alkali Li and alkaline earths Be and Mg. The site changes upon annealing are attributed to the onset of migration of interstitial Na, for which an activation energy of 2.2–3.4 eV is estimated in GaN and 2.0–3.1 eV in AlN, and its subsequent capture by cation vacancies resulting from the implantation. A comparison of the lattice site change behavior of Li, Be, Na, and Mg shows that the onset of interstitial migration correlates with the ionic radii of these elements.