Elemental characterization of electroluminescent SrS:Ce thin films

Abstract

Impurities in SrS:Ce and SrS:Ce,Na thin films prepared by atomic layer epitaxy (ALE) and SrS:Ce,Mn,Cl prepared by reactive evaporation have been studied by combining various ion beam techniques, viz. Rutherford backscattering spectroscopy, nuclear resonance broadening, proton induced x-ray emission, particle induced gamma-ray emission, deuteron induced reactions and the two variants of elastic recoil detection analysis (ERDA), i.e., ERDA with an absorber and time-of-flight (TOF)-ERDA. In both ALE and reactively evaporated films the SrS or (Sr+Mn)/S ratios were close to unity and Ce concentrations varied from 0.1–0.4 at. %. The major impurities in the SrS bulk were found to be H, C, and O. The concentrations of these impurities in the state-of-the-art ALE SrS:Ce films were ∼0.3 at. % for H, <0.2 at. % for C and ∼0.1 at. % for O, whereas 0.6–1.8 at. % H, 0.2–0.7 at. % C and 0.5–1.8 at. % O were found from the films prepared by the reactive evaporation. In the ALE SrS:Ce samples high electroluminescence (EL) brightness was found to correlate with low overall impurity content. However, though reactively evaporated SrS:Ce,Mn,Cl films contain higher level of impurities, they usually have better EL brightness than the ALE made SrS:Ce films. In many ALE SrS:Ce,Na samples Na was found to concentrate at the interface between upper insulator and the SrS:Ce,Na film. The EL performances of the corresponding thin-film electroluminescent stacks were poor.