Anomalous Eu layer doping in Eu, Si co-doped aluminium nitride based phosphor and its direct observation

Eu, Si co-doped AlN shows blue luminescence by UV and electron excitation. However, it is not clear how Eu is located in a wurtzite AlN lattice as there is not enough space for a large Eu cation. In the present study, Eu, Si co-doped AlN is analyzed to elucidate the luminescent Eu center's location and the role of co-dopant Si. The XRD shows that the lattice parameters decrease compared to Eu solely doped AlN and undoped AlN due to Si doping in the AlN lattice. The Eu solely doped product shows impurity phase contamination with a reddish body color. Si co-doping is essential for Eu incorporation into AlN. TEM-EDS and HAADF-STEM measurements clarify that Eu forms a single layer structure with the Si condensation between the AlN wurtzite blocks. Another Si layer condensation is observed at the half position between Eu layers. Electron diffraction shows that there is no apparent long range ordering of Eu layer. The XAFS study shows that all the Eu in the product has a divalent valence and that the bond distance to the first neighboring anion is long at 0.315(8) nm. This is in good agreement with the model structure and the blue emission property. This result indicates that layer type doping can be a used to design new phosphors.