Chemical state effects on the Auger transitions in Cr, Fe, and Cu compounds

As a sophisticated surface elemental analysis tool, electron-excited Auger electron spectroscopy offers excellent spatial resolution (< 10 nm), high surface sensitivity (similar to 2 nm in depth), and efficient detection (about 0.1 atm%) of all elements heavier than helium, which makes scanning Auger nanoprobe especially suitable for small feature analysis. However, elemental composition can generally only be done in a semi-quantitative way. In addition, the chemical-state identification based on peak position often fails due to the complex surface environment. In order to improve the accuracy of quantitative Auger analysis, we systematically investigated the influence of the oxidation states and primary beam on the sensitivity factors for Auger transitions in 3d transition metal Cr/Fe/Cu compounds. Relativistic Dirac-Fock-Slater calculations were performed to illustrate the electronic structure, electron-impacted ionization, and Auger transition dynamics at an atomistic and electronic level. It was found that a new set of chemical state-based sensitivity factors are required for highly accurate quantification, because the chemical states of an element directly affect all four major components in its sensitivity factor, ie., ionization cross section, backscattering factor, Auger yield, and mean free path. In this study, the chemical state and environment can be affected by the oxidation state, the number and types of coordinating atoms, the spin state, and magnetic environment; the focus here will be on the oxidation state. The electron configurations of different oxidation states directly affect the Coulomb and Breit interactions differently in Auger transitions. The intensity ratios and the energy gaps between different Auger transitions were found to change with oxidation state in these 3d transition metal compounds. These findings may provide a new guide to improve the accuracy of Auger quantification and help the oxidation state identification in complex materials.