The emission and excitation spectra of single crystals of CaGa 2 S 4 doped with Ce 3ϩ , Pr 3ϩ , Tb 3ϩ , Er 3ϩ , and Tm 3ϩ were measured with the aim to determine the location of the lanthanide energy levels relative to the valence and conduction band of the host crystal. By combining energies known for charge transfer bands involving trivalent lanthanides with systematic behavior in 4f-5d transition energies of divalent lanthanides, the level positions of 4f and 5d states for each divalent lanthanide can be drawn relative to the host levels. With similar techniques combined with results on the thermal quenching behavior of Ce 3ϩ emission, the levels for the trivalent lanthanides were constructed. We demonstrate consistency between the proposed level scheme and the spectroscopic information presented here and elsewhere. The scintillation properties of Ce 3ϩ -doped CaGa 2 S 4 are addressed also.Thin film electroluminescence ͑TFEL͒ technology in flat panel displays combines the advantages of good visual angle, luminescence without heat, and the panels being made flexible. The wellknown phosphor ZnS:Mn has been widely used in monochrome displays as it presents an intense and broadband yellow luminescence which allows the visualization of both yellow and red using appropriate filters. The lack of a bright blue phosphor to produce the third primary color was a key issue in the realization of full color thin film electroluminescent ͑FCTFE͒ displays until the breakthrough discovery of alkaline earth thiogallate thin films. In the 1990s, a saturated green electroluminescence was obtained with thin sputtered films of Eu 2ϩ -doped SrGa 2 S 4 1 and a deep blue one was achieved with Ce 3ϩ -doped SrGa 2 S 4 and CaGa 2 S 4 thin films. 2,3 Since then various other preparation techniques like molecular beam epitaxy 4 or flash evaporation 5 have been reported. Higher brightness and better color purity were obtained by optimizing the Sr/Ca ratio and doping by oxygen in mixed (Sr, Ca͒Ga 2 S 4 :Ce 3ϩ thin films. 6 TFEL is based on the passage of charge at very high electric field through a thin film of a phosphor material inducing the excitation of light-emitting centers. Electrons transported at high electric field gain kinetic energy that enable them to impact-excite luminescence centers. Two distinct mechanisms of excitation known as impact excitation and impact ionization are distinguished. 7 Impact excitation promotes an electron in the luminescence center to a higher energy state followed by photoemission. Impact ionization ionizes the luminescence center and supplies new carriers to the conduction band. The ionized centers create space charges that affect the excitation yield and therefore the luminescence yield. The actual mechanism depends on the location of the excited state of the luminescence center relative to the bottom of the conduction band. 8 For synthesis reasons, the thioaluminates are more appropriate for obtaining flexible panels than the thiogallates. Nevertheless, the knowledge of the energy location of Ce 3ϩ a...

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