Growth of surface films on silver

Abstract

Ellipsometry was used to determine growth rates for the Ag 2S corrosion film which forms on silver in room air. This system is a particularly unfavorable one for an ellipsometric investigation since the Ag 2S is strongly absorbing and grows as a discontinuous film. The calculational problem of determining the thickness of a very thin absorbing film was solved by using the refractive index we obtained from artificially grown, thick Ag 2S and showing that probable variations in this index will not greatly affect the calculated thickness values. When freshly evaporated silver films were held in vacuum no appreciable growth could be detected until the air admitted to the vacuum system reached a pressure of about 70 Torr. An adsorbed layer, probably water vapor, was observed beginning at a pressure of about 6 Torr. At atmospheric pressure the Ag 2S grew initially at a rate of roughly 0.6 Å per hour but the rate decreased with time and was found to be quite sensitive to atmospheric conditions in the laboratory. In a week films were typically about 25 Å thick. Samples of Ag 2S on silver were also examined using transmission electron microscopy, and the film thicknesses determined from the micrographs correlated well with those measured ellipsometrically. These results demonstrate that ellipsometry does give reliable average film thicknesses even for films which are discontinuous and strongly absorbing.