Cathodoluminescence and Photoluminescence Properties of CaS Thin Film Codoped with Pb and Cu

Abstract

Luminescent properties of CaS thin films doped with Pb and Cu have been investigated. Under UV and low-voltage electron-beam excitation, the CaS:Pb, Cu thin films exhibited highly saturated blue photoluminescence centered at 430 nm and cathodoluminescence peaked at 415-425 nm at room temperature. Spectroscopic study on CaS:Pb, Cu revealed that the emission characteristics were identical to CaS:Cu, although the excitation spectra resembled that of CaS:Pb film. These results demonstrated the efficient energy transfer from Pb to Cu, confirming successful sensitization. However, the efficient energy-transfer process from Pb to Cu could not be explained by the standard energy-transfer theory. Therefore, instead of a Dexter-type energy transfer, a phononassisted process was proposed as a possible coupling mechanism between Cu and Pb ions in CaS.The use of flat panel displays is increasing rapidly in various applications, including mobile, desktop, and home appliances, etc. The luminescent properties and improvement of phosphor materials have also gained renewed attention for flat panel display applications such as electroluminescent ͑EL͒ and field emission displays. 1,2 Among the numerous compounds of phosphors available, luminescent center ion-doped alkaline-earth sulfides have been extensively investigated due to their high luminescent yields. [3][4][5] Thin films of phosphor materials offer significant advantages over conventional bulk powders for display applications, such as higher resolution, more uniform density, and increased thermal stability, etc. However, polycrystalline thin-film phosphors are known to exhibit noticeably lower luminescent output than their bulk powder. 6 It appears that the greatest challenge in the fabrication of sulfide phosphor thin films is the incorporation of the luminescent ions and/or codopants with the correct charge state uniformly within the host lattice. Deficient stoichiometry and poor crystallinity of the host lattice have also been found to affect the luminescent properties of thin-film phosphors. 5 Calcium sulfide ͑CaS͒ and their solid solutions in polycrystalline form have been the subject of much interest for large-area displays. The introduction of rare earth and/or other dopants in these largebandgap host matrices permits them to be tuned as narrow or broadband visible light-emitting phosphors. Although CaS, a wide-gap semiconductor material, has been commercially used as a host material of phosphor and the photoluminescent ͑PL͒ and EL properties of Cu-doped CaS ͑CaS:Cu͒ powders have already been investigated, little work on CaS:Cu thin films has been carried out. [7][8][9] Although the correct mechanism of the luminescent transition of CaS:Cu has not been determined, the fact that CaS:Cu exhibited efficient PL and cathodoluminescence ͑CL͒ indicates that the main reason for poor EL performance of CaS:Cu could be poor excitation efficiency under EL excitation conditions. 3,7,8,10 Therefore, improvement of EL performance was attempted by changing the structure of a CaS:Pb ...