SEY and low-energy SEY of conductive surfaces

Abstract

The study of Secondary Electron Yield (SEY) is widely performed to address important properties of materials to be used in a very wide spectrum of applications. It is, therefore, extremely important to understand the SEY dependence on material type, surface contaminants, structural quality and surface damage. We review here our recent studies of such items performed by looking at some representative conductive materials as noble metals and carbon based surfaces. Polycrystalline Ag, Au and Cu samples have been studied as introduced in the ultra-high vacuum chamber (therefore with an significant surface contamination) and after having been cleaned by ion sputtering. The comparison between the curves confirms that the SEY behavior is strongly influenced by the chemical state of the metal surfaces. We demonstrate the ability of SEY to determine work function values with high accuracy if the experimental system allows using very slow primary electrons. We also investigated, for the Cu sample, the effect on SEY of minimal amount of contaminants in the sub-monolayer regime showing that SEY is highly sensitive to the presence of adsorbates even at such very low coverages, specially for low energy primary electrons. In the case of C surfaces we summarize here the effect that the structural ordering of the C lattice has on the macroscopic SEY properties of ultrathin C layers. In particular we followed the SEY evolution during the thermal graphitization of thin amorphous carbon layers and during the amorphization of highly oriented pyrolytic graphite by means of Ar+ bombardment. In the first case the SEY decrease observed with the progressive conversion of sp3 hybrids into six-fold aromatic domains was related to the electronic structure of the C-films close to the Fermi level. We found that a moderate structural quality of the C layer, corresponding to aromatic clusters of limited size, is sufficient to obtain a SEY as low as ∼1. For the bombarded graphite, the strong lattice damage remains limited to the near surface layer, where the high density of defects reduces the transport of incoming and secondary electrons. Then, the SEY curves resulted differently modified in the low and high primary energy regions, but their maximal values remained favorably low. Our findings demonstrate that SEY, besides being an indispensable mean to qualify technical materials in many technological fields, can be also used as a flexible and advantageous diagnostics to probe surfaces and interfaces.