Quasiparticle electronic structure of barium-silicon oxynitrides for white-LED application

The Ba-Si-O-N (BSON) system appear to be a promising host for white LED phosphors. We study two chemically close BSON compounds: Ba 3 Si 6 O 12 N 2 is an efficient green phosphor at working temperature (around 100 ◦ C), while Ba 3 Si 6 O 9 N 4 is a bluish-green phosphor whose luminescence decreases quite fast with temperature. In view of understanding these different behaviours, despite strikingly similar atomistic structures, we compute the quasi-particle electronic band structure of the two oxynitrides hosts, Ba 3 Si 6 O 12 N 2 and Ba 3 Si 6 O 9 N 4 , thanks to many-body perturbation theory in the G 0 W 0 approximation. The calculation shows a slightly narrower energy gap for Ba 3 Si 6 O 9 N 4 than for Ba 3 Si 6 O 12 N 2 , by about 0.43 eV, which might relate to the different luminescence behaviour. We analyse the character of the states at the top of valence band, at the bottom of the conduction band, and also the chemical shifts for the Ba site in the two compounds. The valence bands are directly impacted by the different stoechiometric ratio, but they are not thought to play a large role in the luminescence. Deceivingly, the dispersive bottom of valence band, more directly related to luminescent properties, is similar in both compounds. However, the spatial topology of the probability density of the bottom of the conduction bands differs, as well as the location of the 5d peak, with a much higher energy than the bottom of the conduction band in Ba 3 Si 6 O 12 N 2 than in Ba 3 Si 6 O 9 N 4.