Photoluminescence properties of Pb2+ centres in CaS:Pb thin films

The emission and excitation spectra of CaS:Pb thin films have been obtained at 10 K together with the emission decay curves. The CaS:Pb samples were prepared with the variation of the concentration and the spatial uniformity of active Pb2+ ions within the CaS host matrix using atomic layer deposition (ALD) and tetraethyl lead (TEL) as the Pb precursor. The deep blue emission at the wavelength of 400-450 nm was shown to be regulated by means of the homogeneous distribution of the luminescent centres rather than the number of Pb ions. At the same concentration and uniformity, the higher growth temperature of 380degreesC was found to be more advantageous for the deep blue emission than 350degreesC. These observations have been interpreted in terms of the selective incorporation of blue-emitting Pb2+ dimers achieved by the controlled ALD technique and the preferential reactivity of TEL. The excitation spectra indicated that the absorption of Pb2+ monomers and CaS matrix as well as the dimeric luminescent centres induced blue emission through efficient energy transfer to the dimer centres. In addition, the visible emission above 450 nm is believed to be attributable to the formation of aggregates larger than dimers in view of the measured decay times.