Center for Functional Nanomaterials Seminar

The formation of SixGe1-x self-organized quantum dots (huts) on the Si(001)-2x1 surface [1] (Fig. 1) has attracted much attention because of its status as the premier model system for investigating strain-induced nanostructure growth. These huts have a highly ordered structure bounded by SiGe{105} facets, but little was known about the structure of the {105} facet itself. We have, therefore, investigated Ge(105)-1x2 surfaces formed on Si(105) substrates using high-resolution FI-STM. We have documented that atomically-resolved STM images of the Ge/Si(105)-1x2 surface exhibit significant bias dependence. With help of first-principles calculations, we proposed a new model which consists of rebonded steps (RS model) [2], different from the generally accepted one [1]. Simulated STM images of our structure model show excellent agreement with the experiment [2]. Additionally, a direct observation of the RS structure is achieved using a high-performance non-contact AFM, which could not be performed with STM [3].
Our model reveals importance of tensile surface strain coming from the rebonded steps. For example, hydrogen adsorption dramatically destabilizes the Ge/Si(105) surface, which can be understood by the fact that hydrogen adsorption increases the surface strain by blocking the optimization of the RS structure [4]. This result explains the previously observed phenomenon of hydrogen suppressing Ge hut formation on Si(001) (surfactant effect) [5]. Our findings on the strained atomic structure of Ge(105) implies that the structure and the strain of semiconductor nanostructures may be controlled through the complete understanding of structures and properties of their surfaces.
1) Y.-W. Mo et al.: Phys. Rev. Lett. 65 (1990) 1020.
2) Y. Fujikawa et al.: Phys. Rev. Lett. 88 (2002) 176101; T. Hashimoto et al.: Surf. Sci. 513 (2002) L445.
3) T. Eguchi et al.: Phys. Rev. Lett. 93 (2004) 266102.
4) Y. Fujikawa et al.: Phys. Rev. Lett. 94 (2005) 086105.
5) A. Sakai and T.