THIN-FILM GROWTH AND THE SCATTERING OF ATOMS FROM SURFACE ISLAND DEFECTS

We investigate the potential energy surface and dynamics of atoms impi nging upon pyramidal defects on fcc (100) and (111) surfaces. Illustra tive molecular dynamics trajectory results are used to highlight the s ubtleties of the growth processes. Homoatomic systems of both Cu and P d are considered in order to determine the influence of extreme differ ences in bonding coordination variation, i.e. the binding energy per a tom decreases by a factor of 3.3 from Cu(bulk) to Cu2 but more dramati cally by a factor of 7.5 for Pd(bulk) to Pd2. We find that stable adso rption sites on pyramidal facets only exist for three layer high (and higher) facets, but even these sites are not as energetically favorabl e as those at the pyramid's base. This single energetic effect can pro mote a Stranski-Krastanov type growth pattern, with a critical roughne ss of no less than three layers being required before a 3D growth mech anism can propagate in a surface temperature regime where post impact diffusion is small compared to the deposition rate. The transition to 3D growth at the minimum possible roughness would be found for Pd on t he (100) face where sticking to the sides of even the three layer pyra mid is a high probability event, at a low enough temperature to make p ost-impact diffusion negligible. However, for the Cu system, we find t hat the ''downward funneling'' mechanism smoothed the growth for the t hree layer pyramid and is still important for the five layer pyramid, indicating that Cu will exhibit layer-by-layer like growth even past t hree layers. A comparison of scattering simulations from defects show that the growth mode is diffusion limited on the fcc (100) crystal fac e, but becomes dominated by impact induced disruption of the original structure for a (111) surface. This implies that the (111) surface wil l grow more smoothly as a new atom displaces pre-adsorbed atoms more e asily, an effect that is not included in the ''downward funneling'' mo del. The ratio of the diatomic molecule's binding energy to the bulk c ohesive energy is shown to provide good insight into the preferred gro wth mechanism. As the ratio increases, at least in the range spanned b y typical metals, the film grows more layer-by-layer like on the (100) surface and less layer-by-layer like on the (111) surface. On the (10 0) surface, stronger low coordination bonding makes the defect structu res less able to adsorb the impacting atom on the sloped faces, increa sing the effectiveness of ''downward funneling.'' On the (111) surface , stronger low coordination bonding inhibits impact disruption of the defects, decreasing this important smoothing mechanism.