ELLIPSOMETRIC INVESTIGATION OF CHEMISORPTION ON CLEAN SILICON (111) AND (100) SURFACES

Gas adsorption on silicon in the monoatomic layer range was studied by means of ellipsometry and the results were compared with volumetric adsorption measurements on powders in order to test several theoretical approaches relating the ellipsometric effect to surface coverage. Using literature values of atomic polarizabilities and diameters to calculate an effective index of refraction for the adsorbed layer, good agreement was obtained between observed and calculated values with the macroscopic ellipsometric theory. In the chemisorption experiments an anomalous change was observed in one of the ellipsometrically determined angles (ψ). This is explained by assuming that the clean silicon crystal has a very thin (10 Å) optically absorbing surface layer, which becomes less absorbing due to the chemisorption process. Surface specificity appeared to exist for some of the adsorptions investigated. Several gases adsorb twice as much on Si(100) as on Si(111), whereas the other gases adsorb in the same amounts on both surfaces. © 1969.