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.