Luminescence and afterglow due to defects in β-SiAlON crystal powder

Abstract

Si6−zAlzOzN8−z (β-SiAlON):Eu2+ is among the most suitable green phosphors for white LEDs. To further improve the performance of this phosphor, it is important to investigate the properties of the nitride host crystal, β-SiAlON, which is considered to contain nitrogen vacancy defects (VN) to some extent. From time-resolved fluorescence (TR-F) measurements of β-SiAlON without luminescent activators, we observed blue, yellow, and ultraviolet emissions. We focused on the blue emission typically centered at 460 nm, which was quenched at room temperature, but showed a continued afterglow for several milliseconds. The electron spin resonance signal of Si dangling bonds due to VN in β-SiAlON was observed to have a high concentration of 5.8 × 1016 g−1. We have previously reported the blue emission in nitride phosphor CaAlSiN3:Eu2+, for which the origin has been identified as VN. Therefore, we predicted that the blue afterglow might cause the green emission of the Eu2+ ions to continue for several milliseconds in β-SiAlON:Eu2+. In this study, we proved that spectra decay characteristics measured by TR-F are useful for clarifying the phosphor characteristics interacting with the defects of β-SiAlON, such as VN.

Effects of Eu2+ on the luminescence and afterglow that arise from defects in β-SiAlON:Eu2+

“…This Eu concentration was a number ratio per unit cell of the host material crystal. For non-doped β-SiAlON (Eu = 0), we used the same sample as our previous study [14]. Samples were fired at 2050 • C for 120 min under N 2 at a pressure of 0.9 MPa [1][2][3][4] and milled to a fine powder.…”

“…This was the same as the absorption spectrum shown in figure 2(b). The broad absorption at 300 nm arose from the Al level of the host crystal [14,19,20]. The fine structure at 400-500 nm corresponded to the 7 F J energy of Eu 3+ including a phonon shift [3,4], and Eu 2+ was directly excited in this wavelength region.…”

“…The glow curve measures the depth of trap levels corresponding to the activation energy from 50 to 600 K. Trap levels below room temperature immediately release electrons owing to thermal vibration, but the trap level itself does not disappear; therefore it is necessary to investigate this effect. We compared the experimental data with reference to the light emission from defects of non-doped (Eu = 0) β-SiAlON [14]. We considered the occurrence of afterglow using the interaction picture of the defect and Eu 2+ .…”

Dependence of the Electronic Structure of β‐Si6−zAlzOzN8−z on the (Al,O) Concentration z and on the Temperature

“…A lot of attention was paid to the structure of β ‐Si 6− z Al z O z N 8− z . The studies performed up to now indicate that the structure is neither crystalline nor totally random; probably there is some degree of ordering [5–14] . If we describe the structure of parental β ‐Si 3 N 4 via the space group 176 ( P 6 3 /m ), the Si atoms are all equivalent occupying 6 h Wyckoff positions, whereas the N atoms are split into two groups, occupying 6 h positions and 2 c positions.…”

“…The studies performed up to now indicate that the structure is neither crystalline nor totally random; probably there is some degree of ordering. [5][6][7][8][9][10][11][12][13][14] If we describe the structure of parental β-Si 3 N 4 via the space group 176 (P6 3 /m), the Si atoms are all equivalent occupying 6h Wyckoff positions, whereas the N atoms are split into two groups, occupying 6h positions and 2c positions. In β-Si 6 À z Al z O z N 8 À z , Al atoms substitute Si atoms and O atoms substitute N atoms.…”