Field penetration into amorphous‐carbon films: consequences for field‐induced electron emission

Abstract

AbstractThe phenomenon of low‐macroscopic‐field (LMF) electron emission is sometimes exhibited by thin films of non‐metallic materials. Both wide‐band‐gap semiconductors and hopping conductors such as amorphous carbon exhibit this phenomenon, and its origin has been under discussion for some years. This paper first uses a simple theory to discuss field penetration into hopping conductors with a high density of localised states near the charge‐neutrality level. This theory is physically similar to widely used existing theories, in that it assumes continuous charge distributions and uses the one‐dimensional Poisson's equation. The theory suggests that little field penetration occurs, and that models of LMF emission based on uniform barrier lowering are not viable. Unexpected self‐consistency difficulties are also created for some types of local‐field‐enhancement model. A re‐assessment of the validity of this simple field‐penetration theory suggests the following. Contrary to a basic assumption of this type of theory, the granularity of the screening charge distribution is a dominant feature of the physics associated with LMF emission from nanoscale‐sized emission sites on thin films of hopping conductors. The authors think that the screening is carried out by discrete charges in localised states, although details are not clear. This conclusion is potentially very far‐reaching. In particular, new forms of field‐penetration theory may be required before definitive decisions can be made about the self‐consistency of some models of local field enhancement. Copyright © 2007 John Wiley & Sons, Ltd.