TOF SIMS analysis, structure and photoluminescence properties of pulsed laser deposited CaS:Eu²⁺ thin films

CaS:Eu²⁺ thin films were deposited on Si (100) substrates using the pulsed laser deposition technique to investigate the effect of argon (Ar), oxygen (O₂), and vacuum deposition atmospheres on the structural, morphological and photoluminescence properties of the thin films. The phosphor target was ablated using a 266 nm Nd: YAG laser. X-ray diffraction, atomic force microscopy, scanning electron microscopy, energy dispersive X-ray, fluorescence spectrophotometry and time-of-flight secondary ion mass spectrometry were used to characterize the thin films. The thin films prepared in oxygen were essentially amorphous, while those prepared in argon and vacuum showed some degree of crystallinity. The film deposited in Ar showed better photoluminescence intensity than those deposited in an O₂ and vacuum. The emission observed at around 650 nm for all the films is attributed to the transitions from the excited 4^f6[F07]5^d1(t2_g) state to the ground state 4^f7(S7/28) of the Eu²⁺ ions. The emission at 618 nm, which is more prominent in the film prepared in O₂, is ascribed to the D5→0F27transitions in Eu³⁺, suggesting that Eu²⁺ was unintentionally oxidized to Eu³⁺. The atomic force microscopy data showed that the film prepared in argon was rougher than those prepared in oxygen and vacuum.