Verlag des Forschungszentrums Jülich
JUEL-2914
Rosenfeld, Georg
Manipulation von Wachstumsmodi in der Homoepitaxie am Beispiel einer Ag(111)-Fläche
82 S., 1994
Abstract
Scattering of thermal energy Helium atoms has been used to study the homoepitaxial
growth of Ag(111) and to test possibilities of growth mode manipulation.
During conventional growth (i.e., continuous deposition at a constant rate onto a
substrate at fixed temperature), Ag(111) grows three-dimensionally rough below the
transition temperature to step fiow ('" 450 K). Down to the lowest temperature studied
(100 K), no reentrant layer-by-Iayer growth is found. This behaviour is attributed to a
high activation barrier for downward diffusion of adatoms at island edges, which effectively
hinders the interlayer mass transport.
It is shown, however, that even for this system two-dimensional layer growth can be
achieved by artificially enhancing the island density without decreasing the mobility
of adatoms on top of islands.. With the help of this "trick", the island size at a given
coverage is reduced so that atoms deposited on top of an island visit the island edge
more frequently, and the mean time between two deposition events on top of an island
is decreased Both factors lead to an enhanced interlayer mass transport. As the
island density can be enhanced by lowering the mobility of adatoms during
nucieation, this generai method for growth mode manipulation is termed "the concept
of two mobilities". This concept is generaliy formulated and illustrated using rate
equations and Monte Carlo simulations..
In an experimentai test of this concept for the system Ag/Ag(111). the enhanced island
density is created by reducing the substrate temperature during nucleation in
the first layer. Growth is con1inued at a higher temperature which is chosen subject
to the condition that the adatom islands have a compact shape but do not decompose
due to Ostwald ripening .. By decreasing the temperature during nucleation, a transition
from three-dimensional to two-diruenslonal growth of the first layer is indeed
observed.
An enhanced density of islands can also be induced by bombarding the surface with
low energy Argon ions prior to growth or during the first stages of growth. This
method is applied to grow several layers in a layer-by-Iayer mode by using a pulsed
ion beam which is turned on for a short period of time during the nucleation in each
layer.
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