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Diffusion in thin films Johnson, Dale Bernard

Abstract

The nature of diffusion along thin evaporated films has been studied by optical and transmission electron microscopy. The thicknesses of the films were measured by multiple-beam interferometry. A preliminary survey of some 22 binary metal systems showed that only four - Ag-Se, Cu-Se, Cu-Te, and Ag-Te - diffused measurably at room temperature. In these four systems it was found possible to study only the diffusion of Cu or Ag into Se and Te; the reverse diffusion experiments failed, presumably because of extensive Kirkendall porosity which developed on the Se or Te side of the diffusion couples, impeding the motion of these atoms. The room temperature growth rates in each system were observed to be higher when the structure of the Se or Te consisted of isolated islands with a highly disordered inter-island network. This effect was attributed to a short circuit diffusion process analogous to grain boundary diffusion which took place in the inter-island channels. The effect was more pronounced in Cu-Te and Ag-Te where electron microscopy observations of the phase boundary interfaces showed a marked tendency for grain boundary diffusion to occur at all Se and Te thicknesses. For continuous films of Se and Te, the growth rates were found to be independent of the absolute thickness. Because of the evaporation geometry used in depositing the couples, there was a critical thickness ratio of Ag or Cu to Se or Te that had to be exceeded in order for diffusion to proceed. Theoretical treatment of the problem, based on the stoichiometry of the phases formed during diffusion, gave predictions of the critical ratio that were generally in good agreement with the experimental values obtained. In each system the critical ratio was found to be independent of the absolute Se or Te thickness. It was also possible to predict the composition of the phase formed during diffusion using the critical ratio. In every system but Cu-Te, the composition determined in this way was in agreement with that given by electron diffraction analysis of the diffusion zone. The activation energies for diffusion in Ag-Se, Cu-Te, and Ag-Te were fairly low suggesting that short circuit diffusion was the predominant mechanism in these systems. The activation energy in Cu-Se was quite large (23 kcal/mole), and it appears that the diffusion mechanism in this case is not consistent with that in the other systems. An interesting observation made during electron microscopy studies in Cu-Se was the formation of a second phase when high electron beam intensities were used. This phase (Cu₃Se₂), not observed in normal diffusion experiments up to 5 0°C, grew dendritically in the presence of the electron beam.

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