CPM Seminar
Direct Imaging of Atomic Arrangement and Potential Profile
on Semiconductor Surfaces by Atomic Force Microscopy
Toyoaki Eguchi
The Institute for Solid State Physics
University of
Tokyo
Scanning tunneling microscopy (STM) has been utilized to determine surface
atomic structure with atomically resolved images. Probing surface electronic
states near the Fermi energy (EF), however, STM images do not necessarily
represent the atomic structure of surfaces. On the other hand, atomic force
microscopy (AFM) probes the force acting between the tip and the sample
surface. It is believed that AFM provides us with surface topographic images
without being disturbed by the electronic states. Compared with scanning
tunneling microscopy, however, its spatial resolution has been still
limited. One of the reasons is a site-insensitive background component due
to the van der Waals force. We demonstrate that high resolution in
non-contact (NC) AFM can be achieved by reducing the background force and
detecting a single chemical bonding force. A reduction of the van der Waals
force is made possible by using small amplitude of the cantilever oscillation
and an atomically sharp tip. Using a tip annealed at high temperature, we
successfully took atomically resolved images of the Si(111)-(7x7) surface,
with the second-layer rest-atoms clearly resolved [1].
Furthermore, A Ge(105)-(1x2) surface grown on the Si(105) substrate is used
to examine the performance of NC-AFM. The surface has been studied as a facet
surface of Ge "hut" cluster epitaxially grown on the Si(001) substrate.
Recent studies by STM and first-principles calculation found that the
electronic effect strongly affects STM imaging on this surface [2,3]. High-resolution AFM images were successfully taken on
this surface and revealed all dangling bonds of the surface regardless to
their electronic situation, surpassing the scanning tunneling microscopy,
whose images were strongly deviated from the atomic structure by the
electronic states involved. In addition, an atomically resolved electrostatic
potential profile by Kelvin probe force microscopy (KFM) directly shows
potential variations among the dangling bond states, directly confirming a
charge transfer between them. These results clearly demonstrate that
high-resolution NC-AFM with KFM is an ideal tool for analyses of atomic
structures and electronic properties of surfaces [4].
[1] T. Eguchi and Y. Hasegawa, Phys. Rev. Lett.
89, 266105 (2002).
[2] Y. Fujikawa et al., Phys. Rev. Lett. 88, 176101 (2002).
[3] T. Hashimoto et al., Surf. Sci. 513, L445 (2002).
[4] T. Eguchi et al., 7th International Conference
on NC-AFM, 12-15 September 2004, Seattle, USA
Monday, September 20th 2004, 15:30
Ernest Rutherford Physics Building, R.E. Bell Conference Room (room 103)
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