RE.PUBLIC@POLIMI pubblicazioni di ricerca del Politecnico di Milano

Surface-sensitive secondary electron emission spectroscopy in low voltage scanning electron microscope for Valence Band analysis

Abbas Kosari Mehr;Mohamed Zaghloul;Wenzheng Cao;Erfan Afshar;Silvia Maria Pietralunga;Anjam Khursheed;Alberto Tagliaferri
2025-01-01
2025
IntroductionGaining insights into the valence band (VB) at the surface and near-surface level with nanoscale lateral resolution is vital for progress in surface science and related applications, and conventional spectroscopic methods fall short of delivering the necessary spatial resolution. Scanning Electron Microscopy (SEM), known for its high-resolution imaging, can be combined with Secondary Electron Emission Spectroscopy (SEES) in the sub-50 eV range. When used with low-voltage electron beams (<3 kV), the SEES technique provides strong signal output and fast data acquisition, making it well-suited for nanoscale mapping and VB characterization [1]. However, interpreting spectra at such low kinetic energies is challenging due to a significant background arising from secondary electron cascades. At present, there is no complete theoretical model that fully describes the fine structure and mechanisms of secondary electron (SE) emission. ObjectivesThe aim of this research is to establish an experimental method for probing the valence band electronic structure of materials using SEM integrated with a customized electron energy analyzer. The study investigates how fine structures in the spectra relate to the distribution of filled/unfilled electronic states in the VB. It also introduces a theoretical framework linking the valence band density of states (DOS) with the secondary electron signal observed in experiments. Materials & MethodsA scanning Auger microscope (SAM) operating in an ultra-high vacuum (UHV) environment was paired with a Cylindrical Mirror Analyzer (CMA) and enhanced with specially designed entrance optics to detect low-energy electrons. Tests were conducted on highly oriented pyrolytic graphite and various metallic samples at primary beam energies below 3 kV. Surface cleanliness was maintained by performing measurements in UHV and minimizing beam-induced contamination. Data interpretation relied on the Fermi Golden Rule and perturbation theory, using the density of states obtained from ab initio calculations. ResultsThe spectral data obtained were highly consistent across multiple trials. The fine structures within the spectra remained stable across different primary electron energies, suggesting a high level of robustness. The spectral features—both in terms of their energy positions and relative intensities—aligned well with the theoretical predictions, supporting the validity of the proposed model. ConclusionThis work demonstrates that SEES-based measurements can yield consistent and interpretable results under UHV conditions. The proposed approach provides a new route to exploring the valence band density of states in advanced materials and electronic systems, with the potential for widespread applications in surface and materials science. This site uses cookies for statistics and to improve your navigation and web applications. More information are avail
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11311/1322945
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