Step Dynamics Observed on Ultra-Flat Si Surfaces
Paul Finnie and Yoshikazu Homma
Device Physics Laboratory
To the eye, a crystal surface appears smooth,
flat, and mirror-like, but on a microscopic
scale, it is covered in atomic steps, forming
the edges of perfectly flat terraces. Crystal
growth starts at these atomic steps, so they
play a pivotal role in atomically controlled
thin-film growth and nanostructure fabrication.
We have developed unique methods for both
observing and manipulating atomic steps,
and have analyzed the underlying mechanisms
behind their behavior.
The arena for these atomic-step motion studies
is an ultra-flat Si(111) surface, which is
atomically flat over macroscopic areas-areas
measuring 100 mm or more on a side. We discovered
that ultra-flat terraces form at the bottom
of craters in silicon surfaces when the surfaces
are heated to a high temperature. The terraces
form when the silicon is heated to sublimation,
causing the atomic steps to retreat [1].
On an ultra-flat terrace, steps can be controllably
introduced, one by one, and they can be tracked
with a scanning electron microscope, allowing
atomic phenomena to be seen on a macroscopic
scale. For example, studying the growth and
erosion of atom-high islands and craters
reveals the kinetics of attachment and detachment
of surface atoms. By creating two craters
or islands simultaneously, we can make atomic
steps collide, enabling us to probe the interaction
between steps [2]. One such collision is
shown in Figure 1 [3]. The series of snapshots
shows concentric atomic steps (crater edges)
expanding during sublimation. The graph shows
the time evolution of the step positions.
The innermost step collides with its neighbor,
and the resulting double step continues to
move.
These studies are contributing to our understanding
of the physics of crystal growth and sublimation.
We plan to use this knowledge to precisely
control step structures.
[1] Y. Homma et al., Phys. Rev. B 55 (1997) R10237.
[2] P. Finnie and Y. Homma, Phys. Rev. Lett. 82 (1999) 2737.
[3] P. Finnie and Y. Homma, J. Vac. Sci.
Technol. A 18 (2000) in press.
Fig. 1. The motion of atomic steps on an
ultra-flat terrace.
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