THE MECHANISMS OF EPITAXIAL GROWTH AND THE TRANSITION TO NON-EPITAXY IN ELECTRODEPOSITED NICKEL

For the first time the structure of the cross section of a deposited metal was extensively studied by transmission electron microscopy using thinned-foil samples. Foils parallel to the surfaces at various thicknesses supplemented the electron-microscopic study. The growth mechanisms of nickel electrodeposits in very highly and conventionally purified all-sulfate electrolytes were investigated. The effects of the additives saccharin and phenosafranine to the plating solutions were assessed. The orientation of the copper substrates also affected the structure.;The thickness of the epitaxial layer was determined by the appearance of non-uniform growths in the deposit surfaces in the form of hillocks. The hillocks were the first stage in the transition to non-epitaxial growth. Hillock formation occurred on relatively thin epitaxial layers when a non-close-packed plane such as {110} was parallel to the surface and also when saccharin or certain unknown foreign materials not removed by conventional purification were present in the plating solution.;The transition from epitaxial to non-epitaxial growth occurred by the development of twins on the hillocks. Twinning probably became possible because {111} planes became exposed on the hillocks because of their shape. Multiple twinning, which probably resulted from atomic displacements due to adsorbed foreign material, led to the development of new grain orientations, i.e., non-epitaxial deposit layers. The smallest observed twins were of the same size as the three-dimensional, epitaxial crystallites. The fact that twinning rather than simple three-dimensional nucleation led to non-epitaxial deposits indicates that the crystallites develop from two-dimensional layers and their subsequent bunching. The growth size in the non-epitaxial deposit depended on the size of twins which, in turn, was proportional to the size of the hillocks.;It was found that the growth mechanism of three-dimensional crystallite formation and coalescence previously found in the first 100 nm was prevalent throughout the epitaxial layer. This determination was based on three experimental observations. (1) A background structure on the surface of the deposits which was attributed to incomplete crystallite coalescence was observed at all thickness levels. (2) The defects caused by the ion milling thinning operation and found to be related to the as-plated structure were the same throughout the deposit thickness. (3) Small misorientations between regions the size of the crystallites observed in very thin sections by dark-field electron microscopy were noted throughout the cross sections.