First-principles study of phosphors for white LEDs applications and of the temperature dependence of the electronic structure

by Samuel Poncé

Institution: Université Catholique de Louvain
Department: Nanoscopic Physics
Year: 2015
Keywords: White LEDs; Electron-phonon; Phosphors; Allen-Heine-Cardona; Dorenbos model; First-principles; Temperature dependence of eigenenergies
Record ID: 1075359
Full text PDF: http://hdl.handle.net/2078.1/156658


Two europium-doped barium-silicate oxynitrides (BSON), which are used as blue-green/green coating for white LEDs, are investigated based on first-principles calculations to explain their puzzling behavior upon variation of temperature. The emission intensity of Ba3Si6O9N4:Eu2+ is strongly reduced at working temperature (thermal quenching) whereas that of Ba3Si6O12N2:Eu2+ stays almost constant. The traditional model based on the configurational diagram fails to describe this difference. An alternative model, called the Dorenbos auto-ionization model, directly links the thermal quenching to the gap between the Eu5d states and the conduction band minimum (CBM). For both BSON compounds, we identify a single luminescent center. The difference between the Eu5d states and CBM gaps in the two materials was computed to be 0.34 eV, enough to explain the experimental difference in thermal quenching. Another effect that is though to play an important role in such light emission is the evolution of the Eu5d to CBM gap with temperature. The decrease of such a gap with temperature is directly linked with the thermal quenching in the Dorenbos model. Thus, we also investigate the temperature dependence of the optical properties of semiconductors within the adiabatic perturbation-based Allen-Heine-Cardona (AHC) theory. The calculation of the temperature dependence of the electronic structure for the two undoped BSON compounds is our most advanced trial to bring the AHC formalism to the realm of phosphor materials. The zero-point motion renormalization of the lowest conduction band is shown to be about -120 meV in both compounds due to electron-phonon coupling. (FSA - Sciences de l)  – UCL, 2015