Device Physics Laboratory
Nanostructure self-assembly is expected to be one of the key techniques in the bottom-up approach to realize futuristic semiconductor nanotechnology. In order to utilize the self-assembly process in Si integrated systems, however, it is important to control the fluctuations in size, shape, and position of such nanostructures. Here, we report that the anisotropic surface stress and strain relaxation of the 3D islands lead to elongation of the islands.
Si(113) covered with 2-ML Ge forms the stable 2×2 reconstructed surface.
Figure 1 shows the 2×2 structural model we proposed and scanning tunneling
(STM) images (experimental and simulations) [1]. The rows indicated an A-type
are attributed to rebonded atoms due to removal of the topmost Ge atoms and
the B-type row to tilted pentamers of five Ge atoms surrounding an interstitial
atom at the subsurface. The simulated STM images based on an optimized structure
from the first-principles calculations are in good agreement with the experiments.
We also found that anisotropic surface stress resulting in such unique surface
reconstruction plays a crucial role in nanostructure formation after further
Ge deposition. The elongation of 3D islands along [332(-)] is energetically
favorable because of tensile stress in spite of the 4% larger lattice constant
of Ge than Si. Once the nanowires begin to form (Fig. 2), the strain inside
the nanowire is relaxed across the wire and leading to stability [2]. Strain
field generates long-range repulsive interaction between the Ge nanowires
and results in uniform spacing between the wires. These results suggest the
possibility of atomically precise control of the shape, size, and position
of nanostructures by controlling anisotropic stress and strain.
[1] Z. Zhang et al., Phys. Rev. Lett. 88 (2002) 61011.
[2] K. Sumitomo et al., Phys. Rev. B 67 (2003) 35319.
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