Structure, composition, morphology, photoluminescence and cathodoluminescence properties of ZnGeN2 and ZnGeN2:Mn2+ for field emission displays

Yellowish-orange-emitting ZnGeN2 and orange-red-emitting ZnGeN2:Mn were synthesized by a facile and mild gas-reduction–nitridation reaction at 1153 K under NH3 flow with air-stable raw materials ZnO, GeO2 and MnCO3. The structure, composition, morphology, photoluminescence and cathodoluminescence properties of ZnGeN2 doped with or without Mn2+ were systematically investigated. Rietveld refinements show that the as-synthesized samples are obtained as single-phase compounds and crystallize as an orthorhombic structure with a space group of Pna21. The actual chemical composition of the as-prepared ZnGeN2 determined by energy dispersive X-ray spectroscopy suggests that the Ge vacancy defects probably exist in the host. The SEM image reveals that the Zn0.99Mn0.01GeN2 particles form aggregates ∼500–600 nm in size. The diffuse reflection spectrum and photoluminescence excitation spectrum confirm that the band edge absorption of ZnGeN2 at low energy is 3.3 eV (∼376 nm). Upon UV light excitation and electron beam excitation, ZnGeN2 gives an intense yellowish-orange emission around 580–600 nm, associated with a deep defect level due to the Ge vacancy defects, and ZnGeN2:Mn shows an intense red emission at 610 nm due to the 4T1g(4G) → 6A1g(6S) of Mn2+. The unusual red emission of Mn2+ in tetrahedral Zn2+ sites is attributed to the strong nephelauxetic effect between Mn2+ and the surrounding tetrahedrally coordinated nitrogen. The photoluminescence and cathodoluminescence emission colors of ZnGeN2:Mn have a high color purity of ∼93–98%. These results demonstrate that ZnGeN2:Mn is a novel, promising red-emitting nitride, potentially applicable to field emission displays with brilliant color-rendering properties and a large color gamut.