**Ab initio Calculation of Structure and Dynamics of
Semiconductor Surfaces with Adatoms**

Structure and dynamics of adatom induced Si(111)-(1 1) and
Si(111)-( )R3 surfaces have been studied
by means of first principles methods. For the Arsenic and Hydrogen
covered 1 1 surfaces and for the Gallium and Boron covered
surfaces, the relaxation is determined
by minimizing the total energy within the local density
approximation and
the full phonon dispersions are calculated using
the density functional perturbation theory (DFPT).
The phonons of the Aluminum and Indium covered surfaces are
studied within a simple mass approximation. All results for the
surface localized modes are in good agreement with the experimental
data.
The observed anomalously large amplitudes of Ga vibrations at elevated
temperatures [1] are compared to the
mean square displacements calculated
within the harmonic approximation.

Anharmonic effects are scrutinized for the adsorbate vibrations of
the Si(111):H-(1 1) surface. The influence of the
zero-point motion and the substrate modes on the Si-H bond
stretching mode at the -point
are studied via cubic and quartic
anharmonic coupling coefficients
which are obtained by combining DFPT and the frozen
phonon method. The calculation of the temperature dependent
line shift in lowest order perturbation theory
includes also the surface thermal expansion.
The broadening of the Si-H stretch vibration is obtained
within higher order anharmonic perturbation theory using
an on-site potential approximation. The results compare well
with all available experimental data.

[1] R.E. Martinez, E. Fontes, J.A. Golovchenko, and J.R. Patel,
Phys. Rev. Lett. **69**, 1061 (1992).