The structure and photoluminescence of Ca1-xLaxS (x=0-0.3) were investigated. The samples were prepared by sulfurizing the mixture of CaCO3and La2O3in the flux of Na2CO3-S or K2CO3-S, and by gas reaction with CS2. By using different methods of preparation, defect structure and concentration could be controlled chemically. Ca1-xLaxS prepared in the Na2CO3-S flux has a wider solid solution range than that prepared in the K2CO3-S flux, and such a feature seems to be due to similar ionic size of Na+and Ca2+. As the substitution of La3+increases, the band gap of the host material decreases due to the increase of the lattice parameters, and the photoluminescence spectra of the Ca1-xLaxS shift to longer wavelengths. Since La3+ion itself is transparent to ultraviolet radiation, vacancies (VCa2+, VS2+) and substituted ions (Na+Ca2+, La3+Ca2+) seem to be associated with luminescence centers. The acceptor levels of Na+Ca2+and VCa2+are estimated to be about 1.1 and 0.5 eV above the valence band, respectively, and the donor levels of La3+Ca2+and VS2-to be about 1.8 eV below the conduction band. The emission bands observed at 500-580 nm suggest the recombination processes of donors with acceptors.
Bibliographical noteFunding Information:
S.-J. Kim acknowledges financial support from the Korean Science and Engineering Foundation (961-0306-063-1).