Photoluminescence and optical absorption in CaS : Eu²⁺ : Sm³⁺ thin films

The optical absorption and photoluminescence of Eu²⁺ : Sm³⁺-doped CaS films have been investigated in this paper. The energy dependence of the refractive index and absorption coefficient of the film were obtained by analyzing transmission and reflection spectra of the thin film. The refractive index at low energies is very close to the bulk value for CaS (n = 2.03) and decreases with increasing energy in the range of 1.2 to 2.5 eV. The energy gap of CaS: Eu²⁺ : Sm³⁺ thin films is found to be a direct gap with value E_g = 4.48 eV. Photoluminescence studies on annealed CaS: Eu²⁺ : Sm³⁺ thin films showed a broad band at ∼1.92 eV identified with emission from Eu²⁺ ions, and a set of sharp lines corresponding to emission from Sm³⁺ ions. Most of the strong features presented in the room temperature spectra are found in good agreement with previous bulk studies of CaS: Eu²⁺ and CaS: Sm³⁺. The abnormal increase of the Sm³⁺ photoluminescence with increasing temperature is explained by the phonon-assisted energy transfer from Eu²⁺ ions to Sm³⁺ ions.