Low Threshold Field Emission from Nanocluster Carbon Films
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Mat. Res. Soc. Symp. Proc. Vol. 593 @ 2000 Materials Research Society
nanoparticles and from nanotubes [17,18]. It is now recognised that a practical display re uires not only low threshold fields but also a high emission site density of order 10/cm2. None of the above films have been found to have sufficient site densities to be suitable for FED application, with the exception of nanocluster "coral-like" films [9], which exhibit high emission site densities at about 20 V/jim field. Thus, we have materials with low threshold field with low emission site density or material with high emission site density but also reasonably high threshold fields, grown using the arc process. This paper will review the work carried out in our laboratory on cathodic arc produced material in an effort to achieve low threshold emission with high site density. EXPERIMENTAL METHOD A typical vacuum arc system is shown in figure 1.
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Fig. I Schematic of Cathodic Arc Deposition System. The arc produces a highly ionised plasma stream with energetic ions from which carbon films can be grown. The sp3 content of the films can be altered by changing the incident ion energy by applying a negative bias to the substrate [12]. Films have been grown over the ion energy range from - 20 eV up to - 500 eV. Nitrogen can act as an ntype dopant in ta-C films. It has been observed that up to nearly - 1% nitrogen incorporation, some nitrogen seems to enter substitutionally, as the sp3 content and the band •ap does not vary but with higher nitrogen incorporation the bandgap decreases and the spq fraction reduces indicating the onset of graphitisation. A typical system has a base pressure of - 10-7 torr and the cathode is made of 99.999% pure graphite. The striker is usually made from a graphite rod of similar purity. Mirror smooth films of ta-C can be grown in the deposition chamber (region A) where macroparticles have been successfully
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filtered out. Nanocluster carbon films and fibrous type films on the other hand can be grown in region B which is within line of sight of the graphite cathode. Previously, as mentioned above we have shown that as grown ta-C films deposited in region A with ion energy of order 100 eV have threshold fields of order 10-20 V/jim [5,16] but that the emission site density was only of order 10-50/cm 2. When Nitrogen is added to the films the threshold field further reduces to -5 V/pjm and the emission site density improves but is still far below the 10 6/cm 2 required for FEDs. In an attempt to further explore emission behaviour we have recently grown films in region B. By varying both the helium and nitrogen partial pressure during growth a series of films of different morphology could be grown. In this instance the films were grown at floating potential with an ion energy of 20 eV and under two different conditions of nitrogen partial pressure: 1 0 4 torr and 10-3 torr . The aim was to study the effect of clu
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