Luminescence investigations of CaS:Eu2+ powder and pulsed laser deposited thin films for application in light emitting diodes
Nyenge, Raphael Lavu
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The main objective of this thesis was to investigate the luminescent properties of commercial CaS:Eu2+ powder and pulsed laser deposited thin films for application in light emitting diodes. X-ray diffraction (XRD), X-ray photoelectron spectroscopy, and photoluminescence (PL) spectroscopy data suggest that the CaS:Eu2+ phosphors contain secondary phases that were possibly formed during the preparation or due to unintended contamination. An intense red PL broad band with a maximum at 650 nm was observed when the powder was excited at 484 nm using a monochromatized xenon lamp. When the powder was excited using a 325 nm He-Cd laser an additional PL emission peak was observed at 384 nm. The origin of this emission is discussed. Auger electron spectroscopy and Cathodoluminescence (CL) spectroscopy were used to monitor the changes in the surface chemical composition and CL intensity when the phosphor was irradiated with a 2 keV electron beam in vacuum. Possible mechanism for the degradation of CL intensity is presented. Thermal quenching in CaS:Eu2+ occurred at a relatively low temperature of 304 K. The kinetic parameters, namely activation energy and order of kinetics of γ-irradiated CaS:Eu2+ were determined using initial rise and peak shape methods, respectively. An Edinburgh Instruments FS920 fluorescence spectrometer equipped with a Xe lamp as the excitation source was used to collect emission and excitation spectra at low temperature. The samples were exposed to γ-radiation ranging from 10 to 50 Gy for thermoluminescence studies, from a 60Co source. The thermoluminescence data were obtained using a Harshaw thermoluminescence Reader (Harshaw 3500 TLD Reader). The possible mechanism leading to the decay of luminescence is explored. Pulsed laser deposited thin films of CaS:Eu2+ phosphor were grown on Si (100) or Si (111) substrates using the Q-switched Nd: YAG laser. For the purpose of this work, the deposition parameters which were varied during the film deposition are: laser wavelength, working atmosphere, number of laser pulses, deposition pressure, and substrate temperature. The film thickness, crystalline structure, surface morphology, and the photoluminescent properties of the thin films were found to be a function of the laser wavelength. The results from XRD showed that the as-deposited CaS:Eu2+ thin films were amorphous, except for the (200) diffraction peak observed from the films deposited at the wavelengths of 266 and 355 nm. The Rutherford backscattering (RBS) results indicate that film thickness depends on the laser wavelength used during deposition. Atomic force microscopy and scanning electron microscopy results show that the roughness of the samples is determined by the laser wavelength. The interaction of laser with matter is discussed, and the best wavelength for ablating this material is proposed. With RBS, it was possible to look at the variation of composition with depth as well as to determine the thickness of the thin films. Compositional analysis carried out using the energy dispersive X-ray spectroscopy showed that the films contained oxygen as an impurity. The films prepared in an oxygen atmosphere were amorphous while those prepared in a vacuum and argon atmosphere showed a degree of crystallinity. The roughness of the films has a strong influence on the PL intensity. The PL intensity was better for films in the argon atmosphere; showing bigger surface structures with respect to the other films. The emission detected at around 650 nm for all the films was attributed to 6 1 7 4 f 5d 4 f transitions of the Eu2+ ion. An emission at around 618 nm was observed, and was attributed to 2 7 0 5 D F transitions in Eu3+, suggesting that Eu2+ was unintentionally oxidized to Eu3+. Results from time-of-flight secondary ion mass spectroscopy study show that all the films contain oxygen although the film prepared in oxygen contain more oxygen. The PL intensity of the CaS:Eu2+ films was found to depend on the pulse rate, with PL intensity increasing as the number of pulses is increased. XRD studies showed that there was an improvement in crystallinity of CaS:Eu2+ thin films upon post-deposition annealing, and subsequently an improvement on the PL intensity . PL intensity also improved significantly at a substrate temperature of 650oC. The best PL intensity as a function of deposition pressure was obtained at an argon pressure of 80 mTorr.