Degradation mechanism of phosphors by vacuum ultraviolet excitation B. Moine * , G. Bizarri Laboatoire de Physico-Chimie des Mate ´riaux Luminescents, UMR 5620 du CNRS Universite ´ Claude-Bernard LYON1 Ba ˆt. A. Kastler, 10 rue A.M. Ampe `re, 69622 Villeurbanne, France Available online 11 November 2005 Abstract The mechanism of luminance decrease of phosphors excited by a Xe plasma discharge has been studied. It is shown experimentally that the aging process is mainly due to the vacuum ultraviolet excitation (VUV). It is demonstrated that the degradation mechanism can be accelerated by using 193 nm laser excitation. Based on excitation, reflectance, thermoluminescence spectra and ageing or bleaching processes by laser excitation, the main causes of the degradation are demonstrated. The aging process can be separated in two different processes depending on the temperature: a first one, at low temperature, corresponding to the autoionisation of luminescent centers; and a second one, at high temperature, linked to the formation of traps in the phosphor. These traps induce a perturbation of the energy migration in the phosphor. The relevant parameters of the creation of traps are highlighted: density of the VUV excitation, temperature, atmosphere and pressure surrounding the phosphor. Ó 2005 Elsevier B.V. All rights reserved. 1. Introduction Barium magnesium aluminate BaMgAl 10 O 17 (BAM) activated by divalent europium ions Eu 2+ which is widely used as the blue-light-emitting commercial phosphor for the PDPs because of its high quantum efficiency (around 80%), its good chromaticity, corresponding to a broad band around 450 nm due to the transition from 4f 6 5d 1 to 4f 7 of divalent europium ions and its strong absorption of vacuum ultraviolet excitation. However the stability of BAM is not good. Its luminous efficiency decreases during panel manufacture [1,2] and also during the operation of the panel [3,4]. Many attempts were made to clarify the whole complex aging process during the operation of panels without explaining the real degradation of BAM. Accordingly, sev- eral candidates have been proposed to explain the aging process—the bombardment of ions, interaction between the different compounds of a plasma cell and the phosphor [5–7] but the most likely hypothesis is linked to the vacuum ultra violet excitation of the plasma discharge. The purpose of this study is to clarify the mechanism of luminance decrease in BaMgAl 10 O 17 phosphor during the operation of PDPs and is based, in addition to conventional spectro- scopic measurements, on the evolution of the fluorescence intensity excited with a 193 nm laser. 2. Experimental The VUV–UV fluorescence experimental set-up allows an excitation from 120 nm to 350 nm with two different excitation sources: • A 150 W Deuterium lamp with MgF 2 output window from Hamamatsu is coupled with a vacuum monochro- mator (0.8 m focal length) allowing a wavelength excita- tion from 120 nm to 240 nm. With this set-up it is possible to record simultaneously excitation and reflec- tance spectra. The detection part consists of a photo- multiplier tube EMI 9789 (excitation spectrum) and a calibrated solar blind Hamamatsu photomultiplier R1220 placed inside the experimental chamber (reflec- tance spectrum). In both cases spectra are recorded by using the photon counting technique (Ortec counter 974 and Ortec amplifier/discriminator 9302). The excitation 0925-3467/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.09.067 * Corresponding author. Tel.: +33 4 72 44 83 30; fax: +33 4 72 43 11 30. E-mail address: moine@pcml.univ-lyon1.fr (B. Moine). www.elsevier.com/locate/optmat Optical Materials 28 (2006) 587–591