At adsorbate terminated surfaces as well as on clean surfaces the atomic
structure varies considerable from the bulk structure of a material.
As one consequence of this the vibrational Eigen modes of the surface
layer differ from "normal" bulk phonons. They have usualy a lower
energy and the vibration is limited to the first few atomic layers.
Since the number of modes and their symmetry depend on the atomic
positions, the measurement of surface phonons can be a useful tool
for testing models of a certain surface.
One drawback is the amplitude of the Raman signal. Since the
scattering volume is limited to the region where the surface phonon
is located (first few atomic layers) it is much smaller than the
scattering volume for Raman measurements at bulk modes (penetration depth
of the light). The amplitude of the surface phonon modes is therefore
much smaller (10-1000 times). The amplitude can be enhanced if
one uses incident laser light with an energy close to that of a
transition in the surface band structure (resonant Raman scattering).
Example Sb/GaAs(110)
Some of the first systems where surface phonons where observed
with Raman spectroscopy are
antimony covered III-V cleaved surfaces such as the GaAs(110) surface.
In Fig. 1 Raman spectra of that surface are shown and in Fig. 2
a graphic illustration of a surface vibration is given.
| Notation | measured energy | calculated energy |
| 1A' | 9.2 meV | 9.8 meV |
| 2A' | 11.0 meV | 11.1 meV |
| 3A' | 22.3 meV | 23.2 meV |
| 1A'' | - | 7.8 meV |
| 2A'' | 20.6 meV | 19.9 meV |
from: W.G. Schmidt et al. Surf. Sci. Rep. 25 p. 141-223 (1996)
|
Raman spectrum of Sb/GaAs(110)
from: Esser et al. Phys. Rev. B 66, p. 75330 (2002)
|
In order to visualize the surface vibrations we used the atomic positions
and displacement vectors provided in
W.G. Schmidt et al. Surf. Sci. Rep. 25 p. 141-223 (1996).
We assume a harmonic oscillation of the atoms around their minimum
energy position. So far in the animation is using positions derived from
the schematic images in the paper. In a next step the actual calculated
positions will be used.
Antimony is depicted green, arsenic dark grey and gallium light grey.
One views along the Sb dimer rows in the [1-10] direction.
