Structural and luminescence properties of re doped fluoride and silicate phosphors
Debelo, Nebiyu Gemechu
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This work covers several aspects of rare earth activated silicate and fluoride commercial phosphor powders and thin films. All the films were synthesized by pulsed laser deposition technique using Nd- YAG laser and characterized by different techniques with the sole aim of studying their structural and luminescence properties for possible applications in dosimetry and display devices. The Thermoluminescence (TL) properties of Y2SiO5 ∶ Ce3+ phosphor powder and thin films were reported. For the phosphor powder, the TL intensity increases with an increase in UV dose for up to 20 minutes and then decreases. The TL intensity peak shifts slightly to higher temperature region at relatively high heating rates, but with reduced peak intensity. Important TL kinetic parameters, such as the activation energy (E) and the frequency factor (s) were calculated from the glow curves using a variable heating rate (VHR) method and it was found that the glow peaks obey first order kinetics. For the films, broad TL emissions over a wide temperature range with low intensity as compared to that of the powder were observed. The maxima of the TL glow peaks of the films deposited in oxygen ambient and vacuum shift towards higher temperature relative to the TL peak position of the film deposited in an argon environment. Vacuum environment resulted in the formation of a deep trap as compared to oxygen and argon environments. Furthermore, the structure of Y2SiO5 ∶ Ce3+ phosphor powder transformed from x2-monoclinic polycrystalline phase to x1-monoclinic polycrystalline phase at low substrate temperature deposition. TL and photoluminescence (PL) properties of KY3F10: Ho3+ phosphor powder is also reported. The TL measurements were done for different heating rates and for various duration of UV exposure. The TL intensity increases with duration of UV exposure up to 20 minutes and then decreases. Decrease of the glow peak height was observed for the glow curves with increasing heating rate. The area under TL-time plot is calculated for each heating rate at constant UV dose and it is found to be constant and independent of the heating rate. It is therefore, the observed decrement in intensity of each glow curve following increment in heating rate is not attributed to the thermal quenching effect. Important TL kinetic parameters namely, the activation energy (E) and the frequency factor (s) were calculated using variable heating rate (VHR) method. The glow peaks obey first order kinetics. KY3F10: Ho3+ thin films were deposited by a pulsed-laser deposition technique with Nd-YAG laser radiation (λ= 266 nm) on (100) silicon substrate. The influence of background gas pressure, target to substrate distance, and substrate temperature on structural, morphological and luminescence properties of the films have been investigated. For the film grown under different background gas pressure, the XRD and FE-SEM results show improved crystalline structure for the film deposited at a pressure of 1 Torr. The AFM results show that the RMS roughness of the films increases with rise in argon gas pressure. The EDS elemental mapping shows Y-excess for all the films deposited under all pressures and this is attributed to its higher mass and low volatility as compared to K and F. XPS analysis further confirmed Y-excess in the deposited films. XRD analysis of the films deposited under various target to substrate distances in the range of 4-7 cm shows that high crystalline quality film with largest grain size is obtained for target to substrate distance of 4 cm. Decrease in the thickness of the films is observed at larger target to substrate distances. This is attributed to the increased hemispherical expansion of the laser induced plasma plume at larger distances reducing the particle flux of the target species over a substrate area. Moreover, all the films are characterized by low reflectance and high absorption in the visible region. Furthermore, for the films deposited under various substrate temperatures, the crystallinity is improved following increment in deposition temperature and the calculated average crystallite size is in the range of 39-74 nm. For all the KY3F10: Ho3+ commercial phosphor powder and thin films, PL emission spectra were also investigated at four main excitation wavelengths; namely, 362, 416, 454 and 486 nm. Green emission at 540 nm and faint red emission at 750 nm were observed for all the excitations. The green emission at 540 nm is ascribed to the 5F4−5I8 and 5S2−5I8 transitions and the faint red emission at 750 nm is due to the 5F4 −5I7 and 5S2−5I7 transitions. In addition to the sharp green emission at 540 nm, a broad emission centered at 600 nm was observed for excitation wavelength of 362 nm for the powder. The highest PL intensity occurs at excitation of 454 nm for all samples of this material. The Cathodoluminescence (CL) images of the films deposited under various background gas pressures show non uniform distribution of luminescent centers in the deposited films. Moreover, the CL emission spectra are similar to those of the PL with the main peak at 540 nm, suggesting that the electron beam did not change the electron energy level configuration or transitions of the activator ion in the film.