Electronic structure of Eu-doped Ba-Si oxynitrides for w-LED application

Poncé, Samuel [UCL] Giantomassi, Matteo [UCL] Mikami, Masayoshi [Mitsubishi Chemicals] Gonze, Xavier [UCL]

Phosphors with optimized luminescence properties for white-LEDs applications are the subject of very active research [1]. Still, a detailed theoretical understanding of their electronic and temperature-dependent properties is lacking. A striking example is given by the two chemically close oxynitride materials, Ba3Si6O12N2:Eu2+ and Ba3Si6O9N4:Eu2+: the host materials have very similar electronic and structural properties [2] but Eu doping leads to different luminescence properties and temperature behaviour. The green phosphor Ba3Si6O12N2:Eu2+ [3] has weak thermal quenching at working temperature (around 100 °C), while Ba3Si6O9N4:Eu2+ is a bluish-green phosphor that exhibits an unfavourable decrease of the luminescence intensity with temperature. Luminescent properties of such rare-earth doped compounds are based upon radiative transition between 5d and 4f electronic states of the dopant. We propose a scheme for the computation of the excited states in such phosphors that allows to identify and study active luminescent centres and to shed light on their temperature-dependent behaviour. This scheme is based on the Δ-SCF methodology, coupled to a stabilization of the created hole in the 4f electronic state. We find that in both cases there is only one active luminescent centre, despite the existence of several doping sites. This explains the narrow and well-resolved emission peaks for the two BSON compounds (with a full width at half maximum of 50 nm or less) [4]. The relaxation of the Eu doped excited state is of crucial importance as there are major changes in the electronic bandstructure due to Stokes shifts. We also confirm the Dorenbos energy transfer model [5]: the gap between 5d state and conduction band is 0.2 eV larger in Ba3Si6O12N2:Eu2+ than in Ba3Si6O9N4:Eu2+. [1] W. M. Yen, S. Shionoya, H. Yamamoto (Eds. 2) Phosphor Handbook, CRC Press, 2007. [2] B. Bertrand, S. Poncé, D. Waroquiers, M. Stankovski, M. Giantomassi, M. Mikami, X. Gonze, Phys. Rev. B 88 (2013) 075136. [3] M. Mikami, S. Shimooka, K. Uheda, H. Imura, N. Kijima, Key Eng. Mat. 403 (2009) 11. [4] M. Mikami, Opt. Mater. 35 (2013) 1958. [5] E. van der Kolk, P. Dorenbos, C. W. E. van Eijk, Opt. Commun. 197 (2001) 317.