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Bruce
Harmon, Cai-Zhuang Wang, James Morris, Kai-Ming Ho, and Dave Turner, Ames
Laboratory
Research Objectives
We are examining the roles of surfaces and interfaces in determining
the properties of materials through calculations of semiconductor surfaces,
metal-semiconductor interfaces, and bulk interfaces such as twin boundaries
and grain boundaries.
Computational
Approach
We use a classical molecular dynamics code to obtain a simple description
of materials at the atomic level. Due to its simplicity, it is the fastest
of our approaches, simulating tens of millions of atoms, but it is often
inaccurate in calculating material-specific properties. We therefore use
tight-binding molecular dynamics—an approach based on empirical data
of the electronic structure—to calculate interatomic forces and,
simultaneously, to obtain a simple description of the electronic structure.
Finally, our first-principles codes provide efficient non-empirical electronic
structure calculations, producing more accurate calculations of energies
and structures of atomic interfaces, and allowing us to test and develop
better tight-binding parameters.
Accomplishments
We have carried out a comparative study of the energetics and dynamics
of Si-Si, Ge-Ge, and mixed Ge-Si addimers on top of a dimer row in the
Si(001) surface, using first-principles calculations. Results show that
the buckling in the addimer observed in scanning tunneling microscopy
(STM) experiments is a signature of the mixed Ge-Si addimer. This distinctively
different dynamic appearance of a Ge-Si dimer, compared to a Si-Si or
Ge-Ge dimer, provides a unique way to identify it using STM.
A recent experiment discovered an interesting reversible intermixing
process involving the exchange of the Ge atom in an adsorbed SiGe dimer
on the Si(001) surface with a substrate Si atom. We have performed first-principles
total energy calculations to study the atomistic mechanisms of diffusion
and intermixing in this system. Our calculation suggests that intermixing
is triggered by the diffusion of the addimer on the surface. The energy
barriers for the diffusion and intermixing events obtained from our calculations
are in good agreement with experiment.
We have performed tight-binding molecular dynamics simulations to study
the atomic dynamics of diamond surfaces under laser irradiation. Our simulation
results suggest that the quality of the laser-treated diamond surfaces
is dependent on the length of laser pulse being used. Under nanosecond
or longer laser pulses, the diamond (111) surface is found to graphitize
via formation of graphite-diamond interfaces, leading to a dirty surface
after the laser treatment. By contrast, with femtosecond laser pulses,
graphitization of the surface is found to occur layer by layer, resulting
in a clean surface after the process. This atomistic picture provides
an explanation of recent experimental observations.
We have performed tight-binding calculations to study the atomic relaxation
and electronic properties of a stepped Si(111)-(7 x 7) surface. Our studies
reveal several new surface bands induced by the step.
Significance
The atomic geometry of transition-metal-silicon structures at silicon
surfaces and interfaces is a subject of interest both for its technological
applications and fundamental theoretical importance. Silicon nanowires,
for example, are expected to play important roles both as active components
and interconnects in future nanodevices.
Publications
M. Hupalo, B. J. Min, C. Z. Wang, K. M. Ho, and M. C. Tringides,
"Correlation between the STM imaged configurations and the electronic
structure on stepped Si(111)-(7 x 7) surfaces," Phys. Rev. Lett.
84, 2877 (2000).
V. Yeh, L. Berbil-Bautista, C. Z. Wang, K. M. Ho, and M. C. Tringides,
"Role of the metal/semiconductor interface in quantum size effects:
Pb/Si(111)," Phys. Rev. Lett. 85, 5158 (2000).
Z. Y. Lu, C. Z. Wang, and K. M. Ho, "Mixed SiGe addimer on Si(001):
Diffusion triggers intermixing," Phys. Rev. Lett. (submitted).
http://cmp.ameslab.gov/
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