Modeling of Cathodoluminescence and Photoluminescence Properties of Pulsed Laser-Deposited Europium-Activated Yttrium Ox
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INTRODUCTION There has been significant interest in the development of thin film phosphors for field emission (FED) applications. In FED, an array of emission tips generate electrons which are accelerated toward the phosphor screen bringing about the phenomenon of cathodoluminescence (CL) [1-5]. Thin film phosphors have several advantages over powders, such as higher lateral resolution from smaller grains, reduced outgassing due to less porous structure, better adhesion to faceplate, and better electrical and thermal contact [6]. However, the most challenging part in the thin film phosphor is its low brightness and efficiency in comparison to those of bulk powder materials. The lower brightness and efficiency from the thin film phosphors are primarily associated with internal reflection through which the emitted light is channeled along transverse axes parallel to the surface instead of vertical emission from the surface [7]. One method to increase the brightness of the thin film phosphor is to increase the roughness of the film. This has been generally accomplished by changing the processing conditions, modifying the substrate surface, or adding a rough buffer layer [7-10]. In order to investigate the effects of film roughness on CL and PL brightness in this work, the substrate surfaces were roughened to various degrees. The effects of the film thickness on the CL and PL were also investigated. The roughness and the film thickness were found to play an important role in determining CL and PL brightness of europium-activated yttrium oxide (Eu:Y 20 3) thin film phosphors. Increasing the roughness or the film thickness significantly improved the PL brightness of the Eu:Y 20 3 phosphor, whereas the enhancement in CL brightness for each case was less significant. The improvement in brightness by increasing the roughness or by the film thickness is mainly attributed to reduced internal reflection or increased interaction volume, respectively. The relationship of brightness with roughness has been modeled.
21 Mat. Res. Soc. Symp. Proc. Vol. 558 ©2000 Materials Research Society
EXPERIMENTAL Europium-activated ( 4 wt.%) yttrium oxide (Eu:Y 20 3) powders (Osram Sylvania) h. were cold pressed without binder into a pellet J to prepare a bulk target for laser ablation. The final Eu:Y 20 3 target was obtained by sintering U(b) the cold pressed pellet at 14000 C for 24 hours in air. Eu:Y 20 3 thin films were deposited insitu on single crystalline (100) silicon or I (0001) sapphire substrates. The compacted Eu:Y 20 3 target was ablated using 248 nm KrF pulsed laser beam with an energy density of 1.2 or 1.6 J/cm 2. The laser was operated at (d I 'z -n Wc repetition rates of 5-30 Hz. The substrates were mounted on the faceplate of a substrate heater placed parallel to the target surface. The Figure 1 AFM micrographs taken from four distance between target and substrate was different Eu:Y 20 3 films showing an increase of 4.5cm. The vacuum chamber was first RMS roughness from (a) 25 nm (b) 75 nm (c) evacuated to 5 x 10. Torr and
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