  This paper presents a thorough experimental investigation of erbium-doped
aluminium nitride thin films prepared by R.F. magnetronsputtering, coupling
Scanning Transmission Electron Microscopy X-ray-mapping imagery, conventional
Transmission Electron Microscopy and X-ray diffraction. The study is an attempt
of precise localisation of the rare earth atoms inside the films and in the
hexagonal w{\"u}rtzite unit cell.The study shows that AlN:Erx is a solid
solution even when x reaches 6 at.%, and does not lead to the precipitation of
erbium rich phases. The X-ray diffraction measurements completed by simulation
show that the main location of erbium in the AlN w{\"u}rtzite is the metal
substitution site on the whole range. They also show that octahedral and
tetrahedral sites of the w{\"u}rtzite do welcome Er ions over the [1.6--6%]
range. The XRD deductions allow some interpretations on the theoretical
mechanisms of the photoluminescence mechanisms and more specifically on their
concentration quenching.
