Lead films have been deposited onto room-temperature substrates having low thermal expansion coefficients and the residual mechanical stresses present in these films have been determined as a function of film thickness and film purity. At room temperature no measurable stress (<5×107 dyn/cm2) was present in any film. At 78° and 4.2°K the stress σ was given by σ=α+β/d, where α is a constant dependent on temperature and d is the film thickness. A model involving surface pinning of dislocations predicts an equation of this form, but the experimental values for the stress were found to be appreciably larger than values calculated on the basis of bulk data. The experimental values were substantiated by superconducting critical temperature measurements on films of various thicknesses. Localized crystalline growth was observed in lead films after cooling to 4°K. The film material around these micron-size crystals exhibiting five- and sixfold rotational symmetry was thinned to the point that physical holes were evident in the thinner films. In addition, with repeated temperature cycling, extensive cross slip was observed.
Tin films less than 800 Å in thickness deposited on 78°K substrates were found to be continuous at low temperatures but to agglomerate into discontinuous islands upon warming to room temperature. Any stage in the agglomeration process could be stabilized by admitting oxygen at a pressure of 10−4 Torr and warming the film to room temperature in the presence of oxygen. Electrical resistance data indicate the surface mobility of the tin is reduced by the formation of a surface oxide one to two monolayers in thickness and that there is no appreciable diffusion of oxygen into the bulk of the film material.
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