AVS 50th International Symposium
    Surface Science Tuesday Sessions
       Session SS2-TuA

Paper SS2-TuA9
Buckling of Si and Ge(111)2x1 Surfaces

Tuesday, November 4, 2003, 4:40 pm, Room 327

Session: Surfaces and Interfaces of Semiconductors and Compound Materials
Presenter: S. Nie, Carnegie Mellon University
Authors: S. Nie, Carnegie Mellon University
R.M. Feenstra, Carnegie Mellon University
J.Y. Lee, Pohang University of Science and Technology, Korea
M.H. Kang, Pohang University of Science and Technology, Korea
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The structure of cleaved Si or Ge (111)2x1 surfaces is well established as consisting of @pi@-bonded chains.@footnote 1@ However, the buckling (i.e. tilt) of these chains has been the subject of recent interest, primarily because a determination of buckling will enable a better comparison between optical absorption data and recent quasi-particle calculations of the surface bandgap and optical response.@footnote 2@ Based on such a comparison it has been suggested that the sign of the buckling may be opposite on Si and Ge surfaces.@footnote 2@ Recent voltage-dependent scanning tunneling microscopy results support that interpretation,@footnote 3@ although that data were restricted to a narrow range of voltages and were performed only on Ge (not Si). In the present work we have performed detailed voltage-dependent STM imaging of both Si and Ge (111)2x1 surfaces, over a wide range of sample-tip voltages. The results are compared with first-principles theoretical predictions, for both positive and negative values of the buckling angle. We find that the wide range of voltages used in the study enables a definitive determination of buckling. Our results demonstrate that the sign of the buckling is indeed opposite on Si and Ge surface, in agreement with the prior work.@footnote 2@ We furthermore deduce from a comparison of the experimental and theoretical STM images a comprehensive view of the image contrast mechanism, incorporating energy-dependent contributions from each atoms in the unit cell. In particular, for empty states, the contrast shifts as the sample-tip voltage is increased from the lower atoms to the upper ones. @FootnoteText@@footnote 1@ K. C. Pandey, Phys. Rev. Lett. 47, 1913 (1981); 49, 223 (1982).@footnote 2@ M. Rohlfing, M. Palummo, G. Onida, and R. Del Sole, Phys. Rev. Lett. 85, 5440 (2000).@footnote 3@ R. M. Feenstra, G. Meyer, F. Moresco, and K. H. Rieder, Phys. Rev. B 64, 081306 (2001).