Field Emission Properties of Thin Molybdenum Carbide and Diamond Films Deposited by Dielectrophoresis
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molybdenum carbide system, therefore, we have studied the properties of molybdenum carbide
and diamond films deposited by dielectrophoresis [2] on flat surfaces and field emission tips as potential candidates for stable field emission tips. EXPERIMENTAL Flat samples were 99.95% molybdenum foil. Tips were 0.02" thick molybdenum wire electrochemically etched in a 2% mol KOH solution using a +10 V bias voltage on the wire and then rinsed in distilled water. Molybdenum carbide powder (99.5%) of 3-4 micron average size and synthetic diamond powder of 0.25 microns were mixed in an aqueous solution of 10% ethanol. This solution was used to deposit molybdenum carbide and diamond films via dielectrophoresis [2] on molybdenum foils and tips at room temperature by applying a +240 V bias to the sample for 10 minutes. Annealing was done in a processing chamber attached to a VG ESCALAB II system over a temperature range from 26 'C to 900 'C in 1 hr intervals at a average base pressure of 1 x 10-7 T. Flat surfaces were characterized using XPS data obtained before and after each annealing stage and SEM micrographs taken before the samples were introduced into the processing and analysis chambers and at the end of the analysis. UPS data was used to measure work functions of flat surfaces. UPS and XPS data was analyzed with a peakfit program using Gaussian peaks. Field emission current-voltage (I-V) characteristics and simultaneous field emission energy distributions (FEEDs) were obtained for field emission tips using a gated diode configuration set 117 Mat. Res. Soc. Symp. Proc. Vol. 558 02000 Materials Research Society
up in the analysis chamber of the VG system. The I-V data was analyzed using the FowlerNordheim equation 312 I = aV2 exp(-b (D /V)
(1)
In equation 1, (Dis the work function of the surface and the parameters a and b depend primarily on emission surface area and tip shape, respectively. Provided a and b remain constant as the voltage changes, In (IV 2 ) vs IN is a straight line with slope -b(' 2 . From this so-called I-V plot, the product of bY312 may be obtained and the work function cannot be separated from the parameter b. However, when a simultaneous measurement of an I-V plot and a field emission energy distribution (FEED) is made, this separation may be performed. The FEED data were analyzed with a peakfit program using exponentially modified Gaussian peaks. The typical pass energy used in this work for XPS and FEEDs in this work is 10 eV. This
resolution gives an energy scale error of about ± 0.2 eV. The UPS data was obtained with a pass energy of 5 eV which gives an error of about ± 0.1 eV. RESULTS Initial XPS of molybdenum carbide samples after deposition showed C, 0 and Mo. Deconvolution of the core electron binding energy position of the Mo3d doublet confirms the presence of MoO 3 and MOO 2 . These results are in reasonable agreement with previous measurements [3,4]. The presence of MoO also was determined. The C/Mo, O/Mo and O/C peak height intensity ratios were calculated from the Mo3d 5/2, C I s an
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