The formation of rare earth silicide nanoclusters on Si surface attracted much attention as a prototype of self-organized nanostructure. The surface structures of ErSi2 nanocluster have been investigated by various methods such as scanning tunneling microscopy (STM).[1] However, the surface structure and the electrical property have not been fully elucidated at the sidewall of the nanocluster. The conventional metal tip for STM cannot trace the sidewall precisely because radius of the tip is larger than size of nanocluster. To overcome such a problem, we have recently developed a metal-coated carbon nanotube (CNT) tip, which has 10-nm-scale radius and high aspect ratio.[2] In this study, we applied this tip to observe the sidewall surfaces of the nanocluster. The nanoclusters were formed by thermal deposition of Er onto the Si(100) surface and annealing at 900°C in ultrahigh vacuum. The STM image obtained by a metal-coated CNT tip exhibited a real shape of the nanocluster. The nanocluster was about 200 nm wide and 40 nm high. The sidewall of the nanocluster was truncated by four plane facets. From the 3D profile, it was found that each plane facet has different angle with respect to the basal plane. On each sidewall surface, periodic rows that run parallel to the edge of the cluster were observed. The vertical period of periodic rows was estimated to be about 1.0 nm, which corresponds to 1.5 times the unit length in the [1-100] direction of hexagonal ErSi2 bulk crystal. From the vertical period and the angle between each sidewall and basal plane, sidewall faces were found to be (1-430), (1-103) and (1-101) planes, respectively. On the (1-430) sidewall surface, the atomic steps of (0-110) and (1-210) orientation were observed. The atomic step on sidewall surface is an energetically favorable nucleation site in the crystal growth. On the basis of these results, the growth mechanism of the nanocluster will be discussed. This work was partly supported by the SENTAN Program of the Japan Science and Technology Agency, and a Grant-in-Aids for Scientific Research from the Japan Society for the Promotion of Science.
[1] Y. Chen et al., Appl. Phys. Lett. 76 (2000) 4004.
[2] T. Ikuno et al., Jpn. J. Appl. Phys. 43 (2004) L644. |