Emitting Carbon Thin Films and Their Characterizations
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emission cathode depends entirely on the properties of the film. Understanding the nature of emission sites, film structure and their correlation to the process parameters is, therefore, the key step in improving the film emission properties. We report here the emission characteristic of our films, UV- Raman spectroscopy studies of these films and investigation of individual emission sites by atomic force microscopy (AFM). Based on these results, we hypothesize on the nature of the emission site. EXPERIMENTS AND RESULTS The films are grown by hot filament assisted chemical vapor deposition on ceramic substrates. A hydrogen and methane gaseous mixture is introduced into a reactor chamber and activated by hot filaments. The activated species are transported to and deposited on the substrate surface. Process parameters such as filament and substrate temperature, gas flow rate, gas composition, and chamber pressure are carefully controlled during the growth to obtain films with good electron emission properties. The emission site density of a cathode is measured by assembling the cathode with a phosphor coated anode plate in a diode configuration with 43 jim gap. Negative voltage pulses are applied to the cathode to provide the electric field needed to extract electrons from the cathode. As the emitted electrons strike the anode plate, an image is formed on the phosphor 47 Mat. Res. Soc. Symp. Proc. Vol. 509 © 1998 Materials Research Society
screen. This image is magnified and further transferred to a computer by a microscope and a CCD camera. The emission site density is then determined by counting the number of sites on the magnified image and dividing the number by the total area of the image. Figure 1 shows a schematic of this test. To evaluate the emission uniformity over a larger area, the cathodes and
phosphor anode are assembled with a larger gap, 0.75 mm, and the images on the phosphor screen are sent directly to computer.
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Computer Camera Microscop.P Diplay Negative of emission sites on phosphor screen Figure 1. Measurement of emission site density Emission Characteristic Figure 2 (a) below shows the emission site density as a function of electric field. The upper curve records changes as the electric field increases and the lower one represents changes as it decreases. The differences of these two curves are caused by the initial burning out of some of "hot spots" as the electric field ramps up. As the field increases, the ESD increases exponentially and rapidly until about 1.2 x 10'/cm2. Further increases in the field result in a slower increase in ESD. Figure 2 (b) demonstrates ESD as a function of current density from the same cathode. (The lower curve records changes as the current density increases and the
upper one represents changes as it decreases). Again, the curve is divided into two regions. One region shows a fast increase of ESD with current density that corresponds to the first rapid increase region in Figure 2(a). The slope of this segment is crucial. A steep curve impl
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