State-of-the-art SrS based blue thin film electroluminescent (TFEL) phosphors, namely, SrS:Ce, SrS:Cu, and SrS:Ag,Cu,Ga, were characterized by combined ion beam analysis techniques and photoluminescence (PL) and electroluminescence (EL) measurements. A selection of different elements were ion implanted into SrS:Ce and SrS:Cu thin films and their effects on the luminescence properties of the phosphor materials were examined.
Impurities in thin films of SrS:Ce made by Atomic Layer Epitaxy (ALE) and SrS:Ce,Mn,Cl made by reactive evaporation were analyzed by various ion beam techniques, viz. Rutherford backscattering spectroscopy, elastic recoil detection analysis (ERDA), time-of-flight (TOF)-ERDA, nuclear resonance broadening, proton induced x-ray emission, particle induced g-ray emission, and deuteron induced reactions. All samples were of high purity, with Sr/S or (Sr+Mn)/S ratio close to unity. The major impurities in the thin film bulk were H, C, and O. In ALE SrS:Ce, good EL performance correlated with an overall low impurity content, in particular low C content. In intentionally codoped SrS:Ce,Na samples, Na was found to concentrate at the phosphor-insulator interface. The EL performances of the corresponding TFEL devices were poor. For the reactively evaporated SrS:Ce,Mn,Cl samples, the EL performances were better than the ALE SrS:Ce devices despite their higher levels of H, C, and O impurities.
Ion implantation of ALE SrS:Ce thin films with Na, K, Ag, P, Ga, F, and Cl showed that positive ions may be more favorable as codopants than negative ions. Implantation of F resulted in about 10 nm blue shift of the emission, but annealing above 500 °C quenched the PL intensity. K implantation enhanced the PL intensity by a factor of two when annealed at 800 °C, and even greater enhancement was achieved with Ag implantation under the same annealing conditions. Blue shift of about 10 nm was also present in these samples as a result of high temperature annealing. Implanted SrS:Ce,Ag exhibited the best decay value (SN=22 ns) ever reported for ALE SrS:Ce thin films and the EL results are encouraging. Implantation of Cl and Na did not improve the PL of SrS:Ce thin films, while P and Ga quenched the luminescence.
Negative ions seem to be more favorable as codopants for SrS:Cu, perhaps as a result of the presence of S vacancies. Implantation of F, Cl, and O enhanced the PL emission. SrS:Cu,Cl also exhibited a pronounced blue shift of the green emission band. Implantation of B also improved the emission intensity, but the oxidation state of B is yet undetermined. Implantation of Ag, Al, and Ga did not improve the PL intensity. Nevertheless, a few SrS:Cu,Ag films showed the blue band due to Ag emission.
It was verified that both blue and green emission may be observed in SrS:Cu at room temperature. The color gamut is determined by the intensity ratio of two emission bands located at 460 (H band) and 520 nm (L band). The L band is attributed to the emission of isolated Cu+ ion substitutes octahedral coordinated Sr but at off-center position and the H band to Cu+ at a different site symmetry. Luminescence of SrS:Cu is likely to follow a three-level mechanism as characterized by the increased decay time with decreasing temperature. The completely green luminescence of SrS:Cu with a single band located above 520 nm possibly originates from Cu pairs and aggregated Cu centers. The blue luminescence of SrS:Ag,Cu,Ga may be related to Ag+-Ag+ pairs or Ag+-(Cu+) associated centers, or both.