Relationship of Field Emission Characteristics on Process Gas Nitrogen Content in Nitrogen Doped Diamond Films
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EXPERIMENT Nitrogen doped diamond films were deposited in a commercially available ASTeX HPMS stainless steel microwave (2.45GHz) plasma CVD deposition chamber. In situ growth rate and film thickness information was monitored using laser reflectance interferometry (LRI). The conventional gas mixtures of hydrogen and methane were used as the growth precursors. Two sources of nitrogen were used depending on the desired nitrogen concentration in the process gas. For low nitrogen process concentrations a mixture of nitrogen (2.11%) diluted in hydrogen was used. For high nitrogen concentrations zero-grade nitrogen (99.998% minimum purity) was directly admitted to the process gas. With these two sources, nitrogen could be added as an impurity to the process gas with gas phase [N]/[C] ratios spanning from 0 to 80. Polycrystalline diamond films containing nitrogen were deposited on 25mm diameter n-type (1.cm) silicon substrates. In order to enhance nucleation, the substrates were hand polished for 10 minutes using 1-2[im diamond grit applied to a nylon polishing cloth. Following diamond grit polishing, the substrates were cleaned ultrasonically in acetone and methanol. Diamond nucleation was achieved at -760°C surface temperature, 600W microwave power, 20Torr chamber pressure, and at a flow rate of 400sccm using process gases consisting of 2 vol.% methane in hydrogen. Nucleation time was determined by monitoring the LRI signal for an initial drop in intensity. For most samples the nucleation time was 21 minutes. Following the nucleation step the substrate temperature, microwave power, and chamber pressure were increased to the growth conditions. Nitrogen doped diamond films were grown at substrate temperatures of -900'C, 1300W microwave power, and 50Torr chamber pressure. The growth process gases consisted of 0.5 vol.% methane and 0-12 vol.% nitrogen in hydrogen at a total flow rate of 500sccm. Nitrogen was only added to the process gas during the growth step. Following deposition, the nitrogen doped diamond films were characterized by micro-Raman spectroscopy, photoluminescence, optical microscopy, scanning electron microscopy (SEM), and field emission measurements. The micro-Raman and PL spectra were recorded at room temperature with an ISA U-1000 scanning double monochromator using the 514.5nm line of an argon ion laser as the excitation source. The laser beam was focused on the samples to a spot size of -3g.tm diameter using an Olympus BH-2 microscope. The samples were examined using a Olympus BX60 microscope with magnifications up to 500x to identify large surface defects and/or damage both before and after field emission measurements. To evaluate the diamond film morphology and to distinguish smaller damage resulting from field emission measurements, the diamond thin films were imaged with a JEOL 6400 field emission SEM. Field emission measurements were obtained in an ultra-high vacuum (UHV) environment with pressures typically < Ix10 Torr. A cylinder of molybdenum (3mm or 1mm in diameter) was chosen as the anode for
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