Change in the Structure and Enhancement of Rare-Earth Emission in amorphous GaN and AlN Thin Films

We report strong enhancement of Er$^{3+}$, Ce$^{3+}$ , Tb$^{3+}$, Eu$^{3+ }$and Ho$^{3+}$ emission with annealing in mostly amorphous GaN and AlN thin films prepared by DC magnetron co-sputtering in different laboratories. We observe sharp characteristic emission peaks of intra-4f-shell transitions of Er $^{3+}$, Tb$^{3+}$, Eu$^{3+}$ and Ho$^{3+}$ions and a strong but broad peak of 5d-4f emission from Ce$^{3+}$ ions over the temperature range 2-300K. During annealing small crystallites form in the amorphous matrix. The crystallite diameters are between 4 and 7 nm as analyzed by high resolution transmission electron microscopy. We relate strong enhancement of the rare-earth emission with the occurrence of these small crystallites. Different mechanisms of energy transfer from absorbing states in the nanostructured wide band gap GaN and AlN matrix to the rare-earth ions are discussed