Bulklike behavior of the optical anisotropy of cation-rich „001… surfaces of Ga
1 Àx
In
x
As alloys
V. L. Berkovits,
1
D. Paget,
2
A. V. Subashiev,
3
and O. E. Tereshchenko
4
1
A. F. Ioffe Physico-Technical Institute, 194021Saint Petersburg, Russia
2
Laboratoire de Physique de la Matie `re Condense ´e, Ecole Polytechnique, 91128 Palaiseau Cedex, France
3
Saint Petersburg Technical University, 194021 Saint Petersburg, Russia
4
Institute of Semiconductor Physics, Novosibirsk State University, 69000 Novosibirsk, Russia
Received 1 August 2003; published 30 January 2004
Using reflectance anisotropy spectroscopy, we have studied cation-rich 001 surfaces of Ga
1-x
In
x
As. All
the spectra contain a dominant negative line, for which the energy shifts between 1.8 eV for InAs and 2.3 eV
for GaAs. The dependence of this energy on In concentration is nonlinear and is perfectly interpreted using a
bowing parameter characteristic of bulk transitions. Using the present results, the amplitude of the energy shift,
and the fact that the transition lies far away from bulk critical points, we propose that the final state of the
optical transition penetrates deeply into the bulk, whereas the initial state of the transition keeps a more
pronounced surface character.
DOI: 10.1103/PhysRevB.69.033305 PACS numbers: 78.68.+m, 73.20.-r, 61.43.Dq, 68.35.Dv
Surface optical transitions on 001 surfaces of binary
semiconductors such as GaAs Ref. 1 and InAs Refs. 2 and
3 have been intensively investigated, using reflectance an-
isotropy RA spectroscopy. These studies have been per-
formed both for fundamental reasons and because of appli-
cations to epitaxial growth and to surface chemistry. One of
the key issues has been to determine the surface or bulk
origin of the transitions. For the arsenic-rich surface of
GaAs, such issue has been the subject of a longlasting de-
bate. In order to solve this issue from an experimental point
of view, the sensitivity to surface chemistry, i.e., to the ad-
sorption of foreign atoms,
2,4
and the effects of a quantum
well near the surface,
5
or of bulk composition in alloys
4
have
been studied. It has been concluded that both surface states
and surface-perturbed bulk states play a role in the main
transition near 3 eV.
For the cation-rich (4 2)/ c (8 2) surfaces of GaAs
Ref. 1 and InAs,
2,3
the well-known RA spectra are domi-
nated by a characteristic negative line, which lies in the 1.8 –
2.3-eV range. Despite of the fact that the two surfaces exhibit
known, similar, (4 2), unit cell structures,
6
the nature of
the observed optical transition and the properties of the cor-
responding electronic states are still unknown. It is generally
believed that, since the transitions lie far away from bulk
critical points, and as also suggested by experiments for
GaAs,
7
the anisotropy arises from the presence of Ga-Ga
dimer bonds lying in the 110 direction, but no microscopic
interpretation of this anisotropy is available. The only ab
initio calculations of the RA spectrum have been performed
for GaAs,
8,9
and have obtained a qualitative agreement. It
has been found that the signal near 2.3 eV is caused by a
superposition of numerous electronic transitions.
In the present work, we consider cation-rich 001 sur-
faces of ternary Ga
1 -x
In
x
As alloys and we investigate the
influence of indium concentration on the energy of the opti-
cal transition. Surprisingly, we find that this dependence has
a clear bulk character, and we propose an interpretation to
explain both the latter behavior and other results which
rather indicate a surface nature of the transition.
We have used epitaxial Ga
0.47
In
0.53
As layers grown on InP
by molecular beam epitaxy, as well as layers grown by mo-
lecular chemical vapor deposition MOCVD on GaAs, with
x being equal to 0.13 and 0.26. Undoped epilayers
( 10
16
cm
-3
) of large thickness 1 m have been chosen
in order to avoid effects in the spectrum of, respectively,
electric field,
10
and substrate-induced residual strains.
11
We
have used the same reflectance anisotropy spectroscopy
RAS setup
12
and ultrahigh vacuum UHV system
13
as de-
scribed elsewhere. The samples were treated by HCl-
isopropanol solutions and introduced into UHV in the ab-
sence of oxygen. This procedure has been shown both for
GaAs Ref. 14 and InAs Ref. 15 to yield, after annealing,
clean surfaces of improved quality and has recently been
applied to obtain As-rich surfaces of Ga
1 -x
In
x
As.
4
The
cation-rich surfaces of GaAs and InAs were obtained, as
characterized by RAS, after annealing at 810 and 670 K,
respectively. For the alloys, the corresponding surface was
prepared at a constant intermediate temperature near 740 K.
We have performed low energy electron diffraction experi-
ments in an independent setup, and have observed, at the
same annealing temperature, the same bright c (8 2) pattern
as for the cation-rich surfaces of GaAs and InAs. The RA
spectra were taken at 125 K low temperature LT in order
to obtain an improved resolution. Corresponding spectra,
also taken at 300 K room temperature RT for comparison
with energy positions found in the literature, exhibited the
same overall features.
Shown in Fig. 1 are the LT spectra, in the 1.5–3.5-eV
energy range. The signal is defined as equal to R / R
=( R
11
¯
0
-R
110
)/ R , where R
11
¯
0
and R
110
are the reflectivities
for light polarizations along the two corresponding axis of
the surface. The spectra of GaAs Ref. 1 and InAs Refs. 2
and 3 exhibit the known negative lines at 2.3 and 1.8 eV.
Also visible especially for curve a are a high-energy shoul-
der to the main line, as well as small but well-defined signals
indicated by arrows in the spectra at the energies of the E
1
and E
1
+
1
bulk critical points. Upon increasing x, all these
features progressively shift to low energy. It is seen from Fig.
1 that i the energy of the main line and the one of the E
1
PHYSICAL REVIEW B 69, 033305 2004
0163-1829/2004/693/0333054/$22.50 ©2004 The American Physical Society 69 033305-1