Calculated influence of work function on SE escape probability and Secondary Electron Emission yield

The influence of changes of the work function, ϕ, or electron affinity, χ, on the escape probability, A, of Secondary Electrons, SE, is derived from their angular and energy distributions, respectively ∂δ/∂α and ∂δ/∂Ek. Based on the evaluation of the spectral distribution of inner SEs, the present approach quantifies the dominant role of potential barrier on the SE emission and its change with surface treatments or thin film deposits. For instance it is shown that a 1 eV-increase of ϕ for Au leads to a decrease of A, and then of SE emission yield, δ, of about 50% while a 0.4 eV-increase of χ for potassium chloride induces a decrease of a factor 4 for A and then for the SEE yield δ. These results are summarized by empirical expressions of form A/A° = (ϕ/ϕ°)−3 for Au and A/A° =(χ/χ°)−3/2 for KCl. Applied here to an insulating sample and to a metal, the present approach may be easily transposed to any kind of material of known Fermi energy and work function, metals, or known affinity, semiconducting and insulating samples. The large SEE yield values of inorganic insulators relative to that of metals are explained by larger values of their escape probability A - KCl: A° ∼ 25% for χ° = 0.6 eV; Au: A° ∼ 4% for ϕ° = 3.5 eV - combined to larger SE attenuation lengths and despite a less SE generation factor. This approach underlines the significant role of A on the large deviations between SEEY data as reported in literature and a strategy combining in situ δ and ϕ measurements is suggested to partly compensate the corresponding dispersion of experimental results. The present approach may be transposed to other energetic projectiles such as X-rays or ions and some practical consequences related to Scanning Electron Microscopy, mechanisms of contamination and crystalline contrasts, are pointed out.