Structure and energetics of Ga-rich GaAs(001) surfaces - Institut für ...

K. Seino et al. / Surface Science 507–510 (2002) 406–410 409
increases the surface energy by 0.3 eV at Ga-rich
preparation conditions. The large energy difference
indicates that this ad-structure is only a transient
state.
The PES allows furthermore to predict the diffusion
characteristics of adatoms. We find the
diffusion for Ga and As atoms to be rather different.
Ga atoms will preferably migrate in the
trenches along the [1 1 0] direction, where energy
barriers of only 0.2 eV need to be overcome. The
minimum energy barrier for diffusion along the
[1 1 0] direction is 0.6 eV. The motion of As adatoms
will be somewhat less anisotropically. It will
preferably occur along the [1 1 0] direction. The
minimum energy barrier along [1 1 0] is 0.5 eV
compared to about 0.7 eV in [1 1 0] direction. We
mention that the diffusion behaviour predicted
here for As and Ga adatoms is very different from
earlier findings [16], where the formerly accepted
b2ð4 2Þ structure has been used as starting point
for the calculation.
The calculated phase diagram in Fig. 2 predicts
the formation of a (2 4) mixed-dimer structure
for surfaces which are even more cation-rich than
the fð4 2Þ structure. While such mixed-dimer
structures occur for cation-rich InP and GaP surfaces
[17], they are not observed for GaAs. Rather
the formation of (4 6) symmetries is reported
for extreme Ga-rich GaAs surfaces [1]. Xue et al.
discriminate between a genuine (4 6) reconstruction
which is more Ga-rich than the Ga-rich
(4 2) reconstruction, and a pseudo (4 6) phase,
which actually consists of a mixture of the (2 6),
the (4 2), and the genuine (4 6) phase. More
recent STM works indicate, however, that also the
genuine (4 6) symmetries occur only locally and
have no long-range order [12,13].
The PES for Ga adatoms may be used as a
starting point to explore possible surface structures
explaining the locally occurring GaAs-(0 0 1)
(4 6) surface symmetries. We start from the AC
Fig. 4. Top view of the investigated GaAs(0 0 1)(4 6) surface
structures described in the text.
adsorption site, but consider now the (4 6) surface
unit cell shown in Fig. 4. The resulting C1
adsorption geometry is lower in energy than the
AC position (cf. Table 2). From that we can conclude
on a repulsive interaction between the Ga
adatoms. However, the C1 geometry is still higher
in energy than the clean GaAs-(0 0 1)fð4 2Þ surface.
In order to find favourable surface structures
we therefore investigated further models. These
include the formation of Ga dimers (C2), Ga
clusters of various shapes ranging in size from
four to eight atoms (C4 ...C8) as well as exchange
geometries, where one (S1) or four (S4) of the
surface anions marked gray in Fig. 4 are replaced
by Ga. The energies for the most favourable configuration
of each structure class are compiled in
Table 2. None of the structures investigated is
stable. It has been suggested that Ga clusters on
GaAs may be particularly stable, if they contain a
‘‘magic’’ number of electrons [18]. The trend of
the energies given in Table 2 does not support
this hypothesis. Instead, we find that the surface
energy increases nearly linearly with the cluster
size.
Given the fact that single Ga adatoms lead still
to the smallest increase in energy, they may be the
Table 2
Relative surface energies for the (4 6) and (4 2) surface structures described in the text
Structure C1 C2 C4 C5 C6 C8 S1 S4 AC AA
DE (meV) 28 48 119 161 153 225 29 121 104 295
The energies refer to a (1 1) surface unit cell and are given with respect the Ga-rich GaAs(0 0 1)fð4 2Þ surface under extreme
Ga-rich conditions (l(Ga)¼ l(Ga) bulk ).