Photoluminescence and optical absorption in CaS : Eu 2+ : Sm 3+ thin films
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P. C. Eklund Department of Physics and Astronomy, University of Kentucky, Lexington, Kentucky 40511-8433
Z. L. Hua, L. G. Salamanca-Riba, and M. Wuttig Materials and Nuclear Engineering Department, University of Maryland, College Park, Maryland 20742-2115
P. K. Soltani and G. M. Storti Quantex Corporation, Rockville, Maryland (Received 5 April 1991; accepted 30 September 1991)
The optical absorption and photoluminescence of Eu2+ : Sm3+-doped CaS films have been investigated in this paper. The energy dependence of the refractive index and absorption coefficient of the film were obtained by analyzing transmission and reflection spectra of the thin film. The refractive index at low energies is very close to the bulk value for CaS (n — 2.03) and decreases with increasing energy in the range of 1.2 to 2.5 eV. The energy gap of CaS : Eu2+ : Sm3+ thin films is found to be a direct gap with value Ee = 4.48 eV. Photoluminescence studies on annealed CaS : Eu2+ : Sm3+ thin films showed a broad band at ~1.92 eV identified with emission from Eu ions, and a set of sharp lines corresponding to emission from Sm3+ ions. Most of the strong features presented in the room temperature spectra are found in good agreement with previous bulk studies of CaS : Eu2+ and CaS : Sm3+. The abnormal increase of the Sm3+ photoluminescence with increasing temperature is explained by the phonon-assisted energy transfer from Eu2+ ions to Sm3+ ions.
I. INTRODUCTION In this paper, we discuss the deposition, optical characterization, and photoluminescence of CaS films doped simultaneously with ~ 0 . 1 % Sm and Eu. Phononassisted energy transfer between Eu2+ and Sm3+ is reported. CaS hosts doped with rare earths (RE) appear particularly promising for applications as spatial light modulators because of their broad range of emission lines1"4 from the red to the ultraviolet. Broad bands in the photoluminescence spectra have been identified with electric dipole transitions between states of the 4/" and (4fn^)5d configurations.5"7 In a spatial light modulator, the emitted light intensity is proportional to the product of two excitation intensities, where the two excitation photons can have different energies. Present spatial light modulators, employing micro-channel plates, are quite expensive to fabricate. In contrast, the fabrication procedure for doped alkaline earth halide thin films is relatively simple, which makes them interesting candidates for this new technology. Extensive studies on both photoluminescence and absorption spectra of crystals of alkaline earth dihalides and chalcogenides doped with RE ions, such as Sm, Eu, etc., have been carried out.1-6"10 CaS doped with either Sm or Eu have, in powder form, been investigated J. Mater. Res., Vol. 7, No. 2, Feb 1992 http://journals.cambridge.org
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for applications as cathodo- and electro-luminescence devices11"15 and for infrared laser detectors.16'17 The paper is organized as follows. We first present optical absorption and dispersion data of the doped films. The results a
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