Luminescence of Ca-doped (Y1-x, Gdx)2O2S

Broad-band yellow luminescence peaking around 575–595 nm has been found in Ca-doped (Y_1-x, Gd_x)₂O₂S. The doping of Ca into Y₂O₂S with the concentration up to 1 mol% is possible. At the optimum concentration (about 0.2 mol%), the cathodoluminescence brightness of Ca-doped Y₂O₂S is 10% of that of yellow-emitting (Zn, Cd)S: Ag.The emission peak is 575 nm in Y₂O₂S: Ca and 595 nm in Gd₂O₂S: Ca. The phosphor exhibits strong thermoluminescence after UV excitation at 77 K. In (Y_1-x, Gd_x)₂O₂S, neither emission spectra nor the temperatures of thermal glow peak depend on x, indicating localized characters of the traps concerned. The photoluminescence is slow in buildup and persistent in decay. These results are explained by a model: the substitution of Y(Gd) by Ca creates acceptor levels in which holes are captured, giving rise to subsequent radiative recombination with excited electrons. Trapped electrons recombine with mobile holes nonradiatively, but exhibit thermoluminescence with high efficiency when they are thermally released.