X-ray absorption spectroscopy (both XANES, X-ray Absorption Near-Edge Spectroscopy and EXAFS, Extended X-ray Absorption Fine Structure) is a powerful tool to determine the chemical environment of selected elements in solids. It is particularly well suited to study rare earth elements in amorphous silicon alloys. In this paper we review the technique and results obtained for different rare earth elements, especially Er, Eu and Tb.

Whenever possible, the rare earth elements coordinate to any oxygen present in subcoordinated envitronments that vary with the host and can be very distorted. The details of the environment depend on the technique used to prepare the samples. Annealling above 400C makes the coordination evolve towards oxide-like 6-fold environments.

Europium is a special case. It may assume either 2+ or 3+ oxidation states.  The X-ray absorption peaks associated with the core electrons binding energy can be used to estimate the relative concentration of each. Annealling in oxidizing atmospheres can modify the Eu3+/Eu2+ ratio, affect the atomic surroundings and change the luminescence spectrum.

In general, efficient luminescence of the 3+ ions is associated with low coordination oxygen environments. This environment does not change significantly at the  relatively low annealling temperatures (up to 300C) that maximize the photoluminescence. The reason for the increase of the luminescence intensity with annealling not a modification of the local environment of the rare earth ions.