Electronic Structure, Bandgap, and Thermal Quenching of Sr[Mg3SiN4]:Eu2+ in Comparison to Sr[LiAl3N4]:Eu2+

In this study the band gap and electronic structure of the next-generation, red phosphor Sr[Mg3SiN4]:Eu2+ (SMS) are explored through a combination of soft X-ray spectroscopy, density functional theory calculations, and thermal quenching data for the Eu2+ 5d -> 4f emissions. The results will be compared to those for the high-efficiency phosphor Sr[LiAl3N4]:Eu2+ (SLA), which shows different, yet exceptional emission characteristics in the red-spectral region. It is found that SMS has an indirect band gap of 3.28 +/- 0.20 eV, strong uniformity in the density of states of its nonequivalent nitrogen sites, and an estimated energetic separation between the lowest Eu2+ 5d state and the conduction band of approximate to 0.13 eV. The Eu2+ 5d-conduction band separation in SLA is found to be approximate to 0.28 eV, which points to why the visible emissions of SLA, and not SMS, show outstanding thermal stability. A bonding scheme explaining the band gap difference of SMS and SLA is proposed based on the density of states of SMS. Modifications to each lattice are put forward for achieving optimized phosphor characteristics for use in pc-LEDs.