Formation of Multiply-Twinned Particles in the Nucleation Stage of Film Growth

Abstract

During electron diffraction and electron-microscopic studies on the epitaxy of fcc metals condensed on cleavage face of NaCl crystals by evaporation in ultrahigh vacuum, particles with unusual structures and giving rise to anomalous 111 diffraction spots have been found at early stages of film formation, besides normal particles with orientations such as (001) and (111). Structures of the anomalous particles have been studied in detail chiefly with Au on NaCl, using the dark-field technique. They have been identified as regular pile-ups of twins and so named “multiply-twinned particles.” The following three types of these particles have been observed; the first type is composed of five tetrahedral twins of equal size of which the nucleus tetrahedron has the (111) orientation to the substrate face, the second is composed of 20 tetrahedral twins of equal size of which the nucleus tetrahedron has also the (111) orientation, and the third is composed of five twins of which the nucleus crystal has the (001) orientation. A simple pile-up of twins, however, results in each case in the formation of one or many gaps between twins that have never been observed on electron micrographs. A later study of lattice image revealed that the lattices of the particles are uniformly strained. The existence of multiply-twinned particles of very small size (20–30 A) suggests that they form already at the nucleation stage. In the anomalous particles atomic distances different from those of the normal lattice occur. With increasing size their structures become unstable owing to the spontaneous straining of the lattices.