Abstract

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.