Film Preparation Conditions and Characterization of co-Deposited Tungsten Doped Zinc Oxide Phosphor
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room temperature, exhibits light emission in the visible range. Various dopants have been introduced in zinc oxide to produce red and yellow luminescence [2][3]. Green luminescence from zinc oxide is believed to be due to a self-activated charge transfer in zinc oxide. Presence of native defects in zinc oxide is believed to be responsible for this self-activated transition [4]. The ability of zinc oxide to luminesce at red, yellow and green wavelengths has made it an attractive candidate as a thin film phosphor for applications such as flat panel displays. Research at Texas A&M University (TAMU) was initiated to find a suitable dopant for zinc oxide to obtain blue luminescence. From this research, it was discovered that blue light emission is observed when tungsten is diffused into zinc oxide [5]. Tantalum has been reported as being responsible for blue luminescence near 400 nm from zinc oxide [6]. Since the discovery of tungsten as a blue light emitting dopant in zinc oxide, the phosphor has been used in the fabrication of a field emission device at TAMU [7]. Following the discovery of tungsten as a blue light emitting dopant inzinc oxide the next logical direction for research after this discovery is to understand the chemical composition of the film and identify the source of luminescence from the phosphor. This required a method of phosphor preparation enabling a quantitative and fundamental analysis of the film. This paper presents the research conducted to isolate the optimum tungsten concentrations in zinc oxide and annealing conditions to fabricate the most efficient blue light 9 Mat. Res. Soc. Symp. Proc. Vol. 558 © 2000 Materials Research Society
emitting phosphor. Results from material characterization to identify the composition of the phosphor and an understanding of mechanism leading to blue light emission will also be reported. EXPERIMENT Following the preparation of phosphor samples, study of samples requires various measurements. Characterization of luminescence properties of the samples includes measurement of peak emission wavelength, efficiency of the blue light emitted and the color space coordinates for cathodoluminescence. Material characterization of the samples was carried out using X-ray diffraction analysis. Infrared spectroscopy was also employed to confirm material identification as determined by x-ray diffraction. Phosphor Preparation Ion mill sputtering, a well-established technique has been used in preparing the thin films
with varying tungsten and zinc oxide concentrations. With this method tungsten and zinc oxide were deposited simultaneously and the deposition rates of tungsten and zinc oxide controlled independently to vary the relative amounts of tungsten and zinc oxide in the films. The films were prepared under high vacuum with a chamber base pressure of 5x1 06 Torr. Substrates during the deposition were held at room temperature. The substrates are 2-inch silicon wafers with 3000 A of thermally grown silicon dioxide. The purpose of Si0 2 on the bare silicon wafer is to prev
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