(Invited) XAFS of Rare Earth Elements in Amorphous Silicon Alloys: What Do We Know about the Local Structure?

• Published in: ECS transactions 2016-07, Vol.72 (34), p.27-34
• Main Authors: Tessler, Leandro R., Bosco, Giacomo Ferreira
• Format: Article
• Language: English
• ISSN: 1938-5862; 1938-6737
• DOI: 10.1149/07234.0027ecst

X-ray absorption spectroscopy (both XANES, X-ray Absorption Near-Edge Structure and EXAFS, Extended X-ray Absorption Fine Structure) is a powerful tool to determine the chemical environment of selected elements in solids. In this paper we present results obtained for the rare earth elements Eu and Tb in hydrogenated amorphous silicon alloys. Whenever possible, rare earth atoms coordinate to oxygen present in environments that depend on the host and can be very distorted. Annealing at high enough temperatures 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 can be used to determine the relative concentration of each. When the concentration is above 4 at% it is possible to completely oxidize the Eu2+ ions by annealing in an oxygen atmosphere. Efficient luminescence of the 3+ ions is associated with low coordinated oxygen environments. This environment does not change significantly at the relatively low annealing temperatures (up to 500°C) that maximize photoluminescence. The main effects of the annealing are not related to the rare earth chemical neighborhood.