Photoluminescent properties of yellow emitting Ca_1-xEu _xSi₂O_2-δN_2+2δ/3 phosphors for white light-emitting diodes

Ca_1-xEu_xSi₂O_2-δN _2+2δ/3 phosphors with Eu²⁺ concentration (x) in the range of 0-0.25 are synthesized by solid-state reaction. With increasing x, the energy dispersive spectra show that the O/N ratio increases from 0.54 to 1.43 and the emission and excitation spectra show a redshift due to increased splitting of the low-lying 4f⁶5d¹ state of Eu ²⁺. An additional excitation band appears at around 460 nm for high doping concentrations of Eu²⁺, perfectly matching the emission wavelength of blue light-emitting diodes (LEDs). The phosphors can be excited in the near-UV to blue region to emit a broad yellow band in the range 543-562 nm, which depends on x. A white LED is fabricated by combining the Ca _0.85Eu_0.15Si₂O_2-δN _2+2δ/3 phosphor with a blue GaN chip. Under 20 mA forward-bias current, a CIE chromaticity coordinate of (0.3396, 0.3474), the corresponding colour temperature (T_c) of 5223 K, the colour rendering index of 74 and luminous efficiency of 20 lm W^-1 are obtained.