Microstructures in co-sputtered Al–C thin films developing at elevated temperatures

The structure formation of carbon doped Al thin films has been investigated in samples prepared on carbon layers supported by air-cleaved NaCl crystals and on a-SiO2/Si single crystal substrates by co-deposition at 350 °C substrate temperature. At higher C content (> 15 at.%), sequentially grown layers of metallic Al and that of Al/Al4C3 microcrystalline composite structure are formed in thicker films (0.1–1 μm) instead of a homogenous globular composite structure. The Al4C3 phase has generally amorphous/nanocrystalline structure (tissue phase) but can be present also in the form of crystalline platelets grown epitaxially on the surface of larger Al crystals. Lamellar growth of Al crystals could be also identified as a result of carbon doping. The cross sectional TEM images clearly show the repeated nucleation of Al crystals on the surface of the Al4C3 tissue phase as well as that the Al crystals and the Al/Al4C3 composite structure or the epitaxial Al4C3 crystalline phases are growing side-by-side. The intensive mutual segregation of both excessive Al and C species, each with long range diffusion on the growing phases, and a strong decrease in the sticking probability of C species on pure Al crystal surfaces are proposed as fundamental atomic mechanisms of this self-organised structure formation.