Laser Spectroscopy of Eu Centres in MBE Grown CaF₂:Eu-CdF₂ Superlattices and CaF₂:Eu Thin Films

Molecular beam epitaxy (MBE) grown CaF₂-CdF₂ superlattices (SLs) and CaF₂ thin films doped with Eu ions were investigated by laser spectroscopic techniques. Eu ions were selectively doped into CaF₂ layers and were used as an optical probe to the SLs and thin films. Physical properties of the SLs and thin films were inferred from optical transitions of divalent and trivalent Eu centres. The 4ƒ⁶5d → 4ƒ⁷ transition of Eu²⁺ has shown strain dependent peak shifts of the zero phonon line. These shifts were thought to be a result of deformation in the crystal structure primarily due to the lattice mismatch with the Si substrate. Based on the amount of shifts, the strains associated with the MBE samples were calculated. Photoluminescence (PL) bleaching and its recovery of the same transition of Eu²⁺ in SLs were also explored. At low temperature the bleaching is best described as bi-exponential decay. Localisation of the liberated electrons from the 4ƒ⁶5d absorption band was considered for the bleaching effect. It was observed that at elevated temperatures the PL intensity of the 4ƒ⁶5d → 4ƒ⁷ transition was recovered. Combined excitation-emission spectroscopy (CEES) was employed to investigate trivalent Eu centres in SLs. The strong ⁷F₀ → ⁵D₁ excitation and the ⁵D₀ → ⁷F₁ emission of Eu³⁺ were studied. A novel centre, which is assigned as I, of Eu³⁺ in SL was observed and investigated in comparison with the cubic centre (O centre) of Eu³⁺. Relative to the O centre the I centre has shown a strong thickness dependent PL which can be demonstrated with a mono layer (1 ML = 3.15 Å) resolution. Possible transformation of the I centre to the O centre was also observed by additional thermal and UV excitations. The I and the O centres are proposed as the same centre except for having an electron in the vicinity of the I centre