Color Point Tuning for (Sr,Ca,Ba)Si₂O₂N₂:Eu²⁺ for White Light LEDs

Abstract

Color point tuning is an important challenge for improving white light LEDs. In this paper, the possibilities of color tuning with the efficient LED phosphor Sr 1− x − y − z Ca x Ba y Si 2 O 2 N 2:Eu z 2+ (0 ≤ x, y ≤ 1; 0.005 ≤ z ≤ 0.16) are investigated. The emission color can be tuned in two ways: by changing Eu 2+ concentration and by substitution of the host lattice cation Sr 2+ by either Ca 2+ or Ba 2+ . The variation in the Eu 2+ concentration shows a red shift of the emission upon increasing the Eu concentration above 2%. The red shift is explained by energy migration and energy transfer to Eu 2+ ions emitting at longer wavelengths. Along with this (desired) red shift there is an (undesired) lowering of the quantum efficiency and the thermal quenching temperature due to concentration quenching. Partial substitution of Sr 2+ by either Ca 2+ or Ba 2+ also results in a red-shifted Eu 2+ emission. For Ca 2+ this is expected and the red shift is explained by an increased crystal field splitting for Eu 2+ on the (smaller) Ca 2+ cation site. For Ba 2+, the red shift is surprising. Often, a blue shift of the fd emission is observed in case of substitution of Sr 2+ by the larger Ba 2+ cation. The Eu 2+ emission in the pure BaSi 2 O 2 N 2 host lattice is indeed blue-shifted. Temperature dependent luminescence measurements show that the quenching temperature drops upon substitution of Sr by Ca, whereas for Ba substitution, the quenching temperature remains high. Color tuning by partial substitution of Sr 2+ by Ba 2+ is therefore the most promising way to shift the color point of LEDs while retaining the high quantum yield and high luminescence quenching temperature.