Characterization of Gd2O2S: Tb³+ phosphor powder and thin films
Under Ultra violet (UV), cathode-ray and X-ray excitation, terbium activated rare earth oxysulphide (Gd2O2S:Tb3+) phosphors shows bright green luminescence. Due to its superior luminescent performance, Gd2O2S:Tb3+ phosphor is used in the manufacturing of TV screens. The degradation of commercially available Gd2O2S:Tb3+ phosphor powder and pulsed laser deposited (PLD) thin films were studied with Auger Electron Spectroscopy (AES) and Cathodoluminescence (CL). The surface reactions were monitored with AES while the light output was measured with a PC2000-UV spectrometer. The CL of the Gd2O2S:Tb3+ was excited with a 2 keV energy electron beam with a beam current density of 26 mA/cm2. The CL and AES were measured simultaneously while the sample was bombarded with the electrons in an oxygen atmosphere. A comparison between the low energy peaks of the AES spectra before and after degradation showed significant differences in the shape of the peaks. A linear least squares (LLS) method was applied to resolve the peaks. Elemental standards from Goodfellow were used in conjunction with the measured data to subtract the S and Gd peaks. A direct correlation between the surface reactions and the CL output was found for both the thin films and the powder. The adventitious C was removed from the surface as volatile gas species, which is consistent with the electron stimulated surface chemical reactions (ESSCR) model. The CL decreased while the S was removed from the surface during electron bombardment. A new non-luminescent surface layer that formed during electron bombardment was responsible for the degradation in light intensity. X-ray photoelectron (XPS) indicated that Gd2O3 and Gd2S3 thin films are formed on the surfaces of the Gd2O2S:Tb3+ powder and thin films during prolonged electron bombardment. Luminescent Gd2O2S:Tb3+ thin film phosphors were successfully grown by the PLD technique. The effects of oxygen pressure and substrate temperature on the morphology and the PL emission intensity were investigated. The films grown in a higher O2 ambient consist of smaller but more densely packet particles relative to the films grown at a lower O2 ambient. The PL intensity of the films increased relatively with an increase in deposition O2 pressure. The PL of the films grown at a higher substrate temperature was generally also more intense than those grown at a lower substrate temperature. It was clear from the Atomic Force Microscopy (AFM) images that spherical nanoparticles were deposited during the deposition process. X-ray diffraction (XRD) indicated that the broadening of the XRD peaks is reduced with an increase in annealing temperature.