Studies of field and secondary electron emission from nanocomposite carbon films.

摘要

The Electron Field Emission (EFE) and Secondary Electron Emission (SEE) properties of sulfur-incorporated nanocomposite carbon films (n-C:S) grown by hot filament CVD were studied. First, as a foundation for the experimental EFE studies, the electrostatic field gradients present in measuring configurations were numerically studied using the finite element method. Especially, the generally assumed validity of the V/dCA approximation for the cathode surface electric field (ES) under commonly employed electron field emission configurations was investigated. Results indicate that the V/d CA approximation is far from being universally applicable to all the field emission measuring configurations, and that only one configuration (the flat cylindrical probe) gives a sufficiently uniform ES, which nearly equals V/dCA over most of the cathode area under the probe.;Second, the effect of adsorbates on EFE was investigated by inducing adsorption on a set of n-C:S films with similar EFE properties by liquid treatment at standard conditions. Adsorbates caused an increase in the turn-on field that was found to depend on the polarity of the liquid used: the larger [smaller] the polarity, the smaller [larger] the increase in turn-on field. The analysis of the data indicates that the increase in turn-on field is due to an increase in work function caused by adsorbates. Also, the hysteresis behavior, present in the field emission measurements, changes from clockwise to counterclockwise due to the adsorbates. This is due to the adsorption-desorption process occurring on the films' surface during emission.;Third, the role of Mo2C (present between the Mo substrate and the carbon film) in the EFE properties of nanocomposite carbon films was studied. A relation between the relative thickness of Mo2C (002) planes, obtained using weighed intensities, and the field emission turn-on fields was found. In general, the relation is direct: the turn-on field increases as the thickness of the Mo2C (002) planes increases. A model was successfully used to explain both the relation between the thickness of the Mo2C (002) planes and the turn-on field, and the shape of the FN plots. This model assumes that Mo2C anisotropy results in a semiconducting or insulating interfacial layer, and that electrons tunnel through multiple barriers.;Finally, SEE from n-C:S was studied. SEE results, analyzed using the Ascarelli theoretical model, indicate that the maximum SEE yield (delta max) is mainly determined by the surface oxygen concentration of the films. Obtained deltamax values confirm the composite nature of the material, as their values are between those for graphite and high purity diamond films. Qualitatively, an inverse relation between surface oxygen concentration and deltamax was found: the higher the oxygen concentration, the lower the value for delta max. The edge effect was also found to be a determinant factor for films that are not continuous. Values for the mean escape depth, lambda s, were also obtained for the films and discussed in terms of the structural nature of the films, as revealed by Raman spectroscopy.;The new knowledge developed through the systematic study of the electron emission phenomena from n-C:S paves the way to employing this material in technological applications of high society value.