The Effect of Aspect Ratio and SP 2 /SP 3 Content on the Field Emission Properties of Carbon Films Grown by N 2 -Spiked
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nd create reactive dangling bonds. However, the exact effect of the positive bias to the growth mechanism is not clear at this point. The temperature of the film during this stage of the growth is estimated to be about 800'C. We observed that reducing growth temperature by more than -100°C resulted in no deposition. Okano [2] reported diamond films grown with urea precursors have significantly improved emission properties attributed to the incorporation of nitrogen in the film. The simple reason behind this is that nitrogen is a donor in carbon and would thus raise the Fermi level of the film and lower the escape barrier for electrons. We were motivated by these results to incorporate nitrogen to our films by using a plasma of the following pre-ionized proportions: 10% CH 4 , X% N 2, (90X)% H2, where X=0, 20, 50, 70, and 90%. However, no nitrogen was observed in these films by any of our analytical methods (Auger, energy-dispersed x-ray). Nevertheless, some of them were found to have enhanced field emission, and its origin is the object of this investigation. The emission properties of our films were measured with our "lamp" system, which consisted of an indium-tin oxide coated anode placed 122 gim from the sample being tested. The area of the anode is 0.4 cm 2. This allows the spatial distribution of the emission sites to be videotaped while the field emission current vs. field curve was being recorded. The scanning electron microscopy in this investigation was carried out with a JEOL 6400FV field-emission source microscope. It has a quoted resolution of 1.5 nm, which is an order of magnitude smaller than any feature we saw in our films. Typical electron kinetic energy was 4 keV, but lower kinetic energies were also used in some samples to reduce the graphitization/damage to the sample during the imaging process. Near-Edge Extended X-ray Absorption Fine Structures (NEXAFS) measurements were made at the Advanced Light Source at Lawrence Berkeley National Laboratory. The spectra were taken in the total yield mode, by recording the secondary electron emission from the sample with a channeltron or with a picoammeter. RESULTS FieldEmission The Current vs. Field curve of these films as measured by the "lamp" setup is plotted in Fig. 1.
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