Arrangement of Au-Si alloy islands at atomic steps

Hiroki Hibino and Yoshio Watanabe*
Materials Science Laboratory

@Approaches to fabricating semiconductor nanostructures can be roughly classified into gtop-downh lithography and gbottom-uph self-assembly. Self-assembly has potential advantages of low cost, large scale, high quality, and so on, but there still remain lots of problems to be solved, especially in the controllability of the size and position of nanostructures. Our group has developed novel self-organized nanostructure fabrication methods through atomic scale control of semiconductor surface structures. Here, we demonstrate that Au-Si alloy islands are self-assembled in a controlled manner using an atomic step array on Si(111) as templates.
@We formed three-dimensional (3D) islands on Si(111) by Au deposition at high (`700) and then at low (`400) temperatures. Figure 1 shows an atomic force microscopy (AFM) image of the islands and their height distribution. 3D islands with a relative deviation of 10% are arranged at atomic steps. Real-time observations of the island formation using low-energy electron microscopy (LEEM) revealed that the islands formed within a narrow Au coverage window, resulting in the narrow size distribution. The islands were allowed to coarsen by interrupting the Au deposition. When the Au deposition was resumed, the islands moved into upper terraces leaving trenches behind. This indicates that the islands are Au-Si alloy droplets. Additionally, the islands moved on the terraces almost at constant velocities, but when they approached the upper-side steps, they jumped to the steps (Fig. 2). The atomic steps provide stable positions for Au-Si alloy islands, which helps them to arrange at the steps.
@We showed that the position and size of Au-Si alloy islands can be controlled by using atomic steps as templates for island formation. Our method is very simple because it does not include any lithographic techniques, and it is useful for preparing substrates for semiconductor nanowire growth and attaching functional molecules.
@*Present address: NTT advanced technology Corporation

Fig. 1. AFM image of 3D islands and their height distribution.
Fig. 2. The moving distance of the island indicated by the dotted circle in LEEM images (a)-(c) as a function of time.