Ⅱa-Ⅵa compounds doped with luminescent centers have been traditionally used as phosphor materials for display devices. Thin film phosphors adopting alkaline-earth sulfides, such as calcium sulfide (CaS) and strontium sulfide (SrS), as host matrix have shown promising results as color phosphors for full color flat panel displays. For practical use of these phosphors, however, there are many problems to be solved. Especially, the preparation of a CaS-based phosphor is more difficult compared to zinc sulfide (ZnS), because of their chemical instability and hygroscopic nature resulting in non-stoichiometric composition and poor crystallinity at a preparation temperature lower than 800℃.
The aim of this work was to investigate the effect of additive ions on the structural and luminescent properties of blue-emitting CaS:Cu thin film phosphor.
The CaS film was grown by atomic layer deposition (ALD) process. The precursors of Ca and S were bis(2,2,6,6-tetramethyl-3,5-heptanedionato)-calcium, $Ca(thd)_2$, and $H_2S$, respectively. Luminescent center and additive ions were introduced into un-doped CaS film via diffusion during a post annealing process. Cu doping was accomplished by chemical vapor deposition (CVD) of very thin Cu film using copper(I) hexafluoroacetonate trimethylvinylsilane, Cu(hfac)(TMVS), as the precursor. After the deposition of Cu, the samples were annealed using rapid thermal annealing (RTA) process at temperatures of 600 and 650℃ for 2 min in $N_2$ atmosphere. Luminescent characteristics of Pb co-doped CaS:Cu thin film phosphor were also investigated. CaS:Pb phosphor thin films, host material of Pb co-doped samples, were also deposited using ALD and the precursors of Ca, S, and Pb were $Ca(thd)_2$, $H_2S$, and $Pb(C_2H_5)_4$.
The CaS film thickness was well consistent with the layer thickness estimated on the basis of the number of the ALD film growth cycles and the deposition rate per cycle. Rutherford Backscattering (RBS) spectrum also showed Cu was uniformly distributed in CaS film and implied that we could easily prepare a low concentration (below 1 at. %) Cu -doped phosphor film using CVD method followed by post annealing. X-ray photoelectron spectroscopy (XPS) measurement of the samples indicated that F was incorporated in the Cu film due to the F-containing Cu-precursor.
The microstructural changes of thin films during post annealing were investigated closely. Significant surface smoothening and grain growth were observed in Cu doped CaS thin film during high temperature annealing. From these results, it can be concluded that F was incorporated in Cu film as residue of Cu precursor during CVD process and possibly acted as flux agent.
The chemical state of Cu in CaS host was characterized using XPS and transmission electron microscopy-energy dispersive spectroscopy (TEM-EDS) to verify emission characteristics. The binding energy state of Cu 2p photoelectron and the Cu concentration profile in 650℃-annealed samples revealed that the luminescent center Cu was properly incorporated into CaS host in the octahedrally coordinated $Cu^+$ ionic state.
The PL and CL studies of CaS:Cu thin film showed blue emission peaked at the range of 405~425nm. Maximum PL intensity was appeared in the range of 0.25~0.35at.% Cu concentration and emission spectra shifted toward longer wavelengths as increasing Cu content. Contrarily, the CL spectrum showed mainly dependent on the surface morphology. In excitation spectrum, two excitation bands at 265 and 293nm associated with $Cu^+$ and the excitation band at 243nm absorption by host were observed. Blue emission would be originated from the inner shell transition of $^3E_g \rightarrow^1A_{1g}$ in the $Cu^+$ ion under $O_h$ symmetry. Emission intensity increased as increasing annealing temperature owing to enhanced excitation efficiency of the two $Cu^+$ bands. From the time-resolved spectroscopy result, CaS:Cu phosphor represented fluorescent decay characteristics. However, EL emission was very weak and detailed studies indicated that the poor emission efficiency was due to the lack of charge injection in the alternating current driven thin film electroluminescent (ACTFEL) device.
The PL and CL emission characteristics of Pb co-doped CaS:Cu thin film phosphor were similar to that of CaS:Cu. All samples showed that the luminescent peaks at the wavelength of 410~430nm regardless of Cu concentrations. A detailed spectroscopic study on CaS:Pb,Cu revealed that the emission characteristics of CaS:Pb,Cu were identical to CaS:Cu, whereas the excitation spectra resembled CaS:Pb. These results demonstrated the efficient energy transfer from Pb ion to Cu luminescent center. The EL emission from CaS:Pb,Cu was blue with an emission peak around 460nm. The chromaticity was improved using Pb as a sensitiser, resulting in blue emission with the CIE color coordinates x=0.18 and y=0.16.
