Contact potential contrast in SEM images observed on sulfur-adsorbed Cu3Au (110) surfaces

A special kind of spot contrast is observed in the SEM images of the sulfur-adsorbed Cu3Au (110) surfaces. The spots are identified as segregating Au islands induced by the segregation of sulfur on the surface. This contrast cannot be explained with the conventional morphology contrast or the contrast formed by the difference in the secondary electron emission coefficients of the materials on the surface. A new kind of contrast mechanism, contact potential contrast, is proposed to explain the observed SEM images and their change with the surface sulfur coverage. Based on the proposed model, contact potential contrast of Cu3Au is calculated by using a calculated energy distribution curve of secondary electron emission. The result is compared with experimental data.