Development of Nitride and Carbide Cathodes for Realization of Highly Stable Vacuum Micro Electron Sources

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We have investigated the potential of transition metal nitride and carbide thin films as a cathode material for highly stable vacuum micro electron source. Firstly, thin films of HfN, VN, TaN, ZrC, HfC, VC, NbC, TaC, Cr_3C_2, WC were deposited on flat silicon substrates. Composition of nitrogen and carbon were examined using backscattering of proton and helium ion beam with high sensitivity. Properties of crystallinity, orientation, work function, electric resistivity, which are important parameters for cathodes, were investigated as a function of composition. As a result, it was found that the work function of nitride films (except VN) were mostly 4.7eV-4.8eV, while that of carbides were relatively higher 438eV-5.2eV. Electron emission properties from flat sample were investigated in ultra high vacuum with the gap spacing of 1.5μm. Current-voltage characteristics and short term stability were acquired. With an aid of our original evaluating method of S-K chart, we judged that some materials of HfN, TaN, VC, Cr_3C_2 were nominated as a candidate for the next examination. Secondly, we investigated the relationship between the orientation and the current stability, and found that (111) oriented films had the properties superior to (100) oriented ones. Also resistance against oxidation, which is closely related to the life of the cathode, was examined and it was found that nitrides are more resistive than the carbides. Taking all the results described above into consideration, one of the possible candidates was found to be (111) oriented HfN film. To give final judge, we fabricated silicon field emission array, on which we deposited the (111) oriented HfN film and examined its electron emission properties. The test could not show better performance as compared to NbN field emission array, which has already measured by the present researchers. The results showed the best material at present is NbN.