Emission and Processing Requirements for Carbide Films on Mo Field Emitters
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Mat. Res. Soc. Symp. Proc. Vol. 509 ©1998 Materials Research Society
(stability and low work function) in the deposited layer even after exposure to air or other processing necessities. EXPERIMENTAL The ZrC material used for film deposition consisted of crystalline targets prepared by arc float zone refinement. 10,11 These targets are electron beam heated and used as physical vapor deposition (PVD) sources. Bulk stoichiometries, or carbon to metal atom ratios, were checked by chemical analysis done in another laboratory. Through modifications in the zone-refining process, two stoichiometries were used in this study. Those ranged from sub-stoichiometric ZrC. (with x = 0.92) to approximately stoichiometric (i.e. x = 1). Auger spectroscopy was used to ensure bulk and film purity and determine minimum film thickness. Film thickness calibration was done by keeping the deposition parameters fixed and varying the deposition time.
When the low energy Auger signal of the substrate fell below 1%, we assumed the coverage was equal to approximately three monolayers. This was used as a guide for later ZrC deposition onto field emitter substrates. For ZrC coatings on Mo field emitter tips, we assumed a thickness of -5 nm based on the Auger study. Two UHV systems were used for field emission in these film coated Mo emitter experiments. A typical film system consisted of a ZrC evaporative source, a field emitter holder which would allow for heating for thermal cleaning of the tip, and a phosphor screen to view emission and obtain the necessary I(V) data. The single emitters used in this study were individually fabricated via electrochemical etching polycrystalline drawn Mo wire having a natural (110) orientation. The Mo emitters were etched in a 2N sodium hydroxide solution. Each single emitter was thermally cleaned, tested, and I(V) data compiled. Cleaning was accomplished by heating up to 1900 K for Mo. Field emission microscopy (FEM) was used to verify the crystal orientation and condition of the emitter apex. The cleaning temperature for Mo was chosen so the emitters could be cleaned without being blunted too much due to the effects of surface tension. It is interesting to note that we have never detected similar surface tension blunting in any solid carbide field emitter. After a clean Mo tip was obtained and I(V) data taken several processing scenarios were tried. ZrC film deposition followed immediately or in some cases after tip exposure to atmosphere. The film was either examined untreated or subjected to a variety of heating and/or plasma treatments described later. After deposition or treatment, FEM was used to examine the tip and I(V) data were again obtained. For the high current experiments, currents were generally increased in 2 steps with I(V) and I(t) data taken over typical times of 10-20 minutes. Short term noise and any current drop with time were noted. This process was usually continued until emitter failure. However, some emitters were held at 100 /A or 500 /A levels for additional time to better measure
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