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Surface Layer Analysis of a 28Si-Enriched Sphere Both in Vacuum and in Air by Ellipsometry

For a new definition of the kilogram, surface analysis of Si spheres is essential to determine the Avogadro constant using the X-ray crystal density method. In this paper, to investigate the surface characteristics of Si spheres both in vacuum and in air, a new spectroscopic ellipsometer having a vacuum chamber with an automatic sphere rotation system was developed, using which the thickness of the SiO2 layer on the surface of a 28Si-enriched sphere was determined based on a four-layer model. The obtained oxide layer (OL) thickness was in agreement with that obtained by X-ray photoelectron spectroscopy within their uncertainties. The OL thickness determined in vacuum was larger than that in air by 0.24 nm. This difference was examined by considering the physical properties of the constituents of the four-layer model.