Field emission (FE) is traditionally discussed in the framework of Fowler-Nordheim (F-N) theory, where FE characteristics are determined by the work function and the radius of the tip apex. We have however observed that the applied onset voltage for electron emission from Si field emitter arrays (FEA) is drastically lowered by coating the tip apex with the hydrogen-free carbon film. Since the tip apex gets blunt upon coating and the work function is similar, this cannot be explained by the classical FE model. In this paper, we have examined the local work function of the carbon film by using scanning tunneling microscopy (STM), and have found that the local tunneling barrier height (LBH) is largely lowered by applying the electric field on the carbon film surface.
The hydrogen-free carbon film is prepared by the arc-discharge of carbon rod in vacuum and has a smooth morphology in a nanometer scale according to electron microscopy. STM observations and Raman spectroscopy reveal that this film consists of graphite nano-grains surrounded by amorphous carbon and that the amorphous carbon has lower barrier heights under the electric field during STM observation.
It has been established that the HfC coating onto the Si FEA tip apex improves the emission characteristics [1]. The carbon coating is more effective than the HfC coating. Although the work function of the carbon film measured with Kelvin probe is similar to that of Si, the slope of the F-N plot indicates that the effective work function for emission is very low for the carbon-coated emitters, which is consistent with the STM observation.
According to the theoretical calculation [2], the hydrogen-termination suppresses the electron emission from the graphite ribbon. The absence of the hydrogen in the carbon film examined here is possibly responsible for the observed peculiar feature of the electron emission.
[1] T. Sato et al., J. Vac. Sci. Technol. B21 (2003) 1589.
[2] M. Araidai et al., Phys. Rev. B 70 (2004) 245410.
|