Anisotropic Strain for Controlled Growth of Nanostructures

Koji Sumitomo, Zhaohui Zhang, Hiroo Omi, and Toshio Ogino
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.

Fig. 1. Ge/Si(113)-2~2 structural model we proposed and STM images (a,b) experiment and (c,d) simulation.
Fig. 2. STM image for Ge nanowires.