Mn enriched surface of annealed (GaMn)As layers annealed under arsenic capping
M. Adell, J. Adell, L. Ilver, and J. Kanski
Department of Applied Physics, Chalmers University, of Technology, SE-412 96 Göteborg, Sweden
J. Sadowski and J. Z. Domagala
Institute of Physics, Polish Academy of Sciences, al. Lotnikow 32/46, 02-668 Warszawa, Poland
Received 3 August 2006; published 22 February 2007
Using synchrotron radiation based spectroscopic methods we have investigated the surface modifications of
Ga
1-x
Mn
x
As layers due to post growth annealing under amorphous arsenic capping layers. It is established that
there is a clear increase of the Mn concentration at the surface after annealing. This is ascribed to a reaction
between diffusing Mn interstitials and the As capping. The reacted surface is smooth and well ordered with a
1 2 reconstruction. All data indicate that the annealed GaMnAs is terminated by a monolayer MnAs in
zinc-blende structure.
DOI: 10.1103/PhysRevB.75.054415 PACS numbers: 75.50.Pp, 79.60.Bm
I. INTRODUCTION
Since the discovery of ferromagnetism in GaMnAs,
1
this
system has become a test ground for development of deeper
insight into the mechanisms that control the magnetic prop-
erties of diluted magnetic semiconductors. In the perspective
of future spintronics applications GaMnAs has an obvious
advantage over other candidate systems: it is compatible
with the well-developed III-V technology and can therefore
be easily integrated into a whole range of optoelectronics. In
addition, already the as-grown GaMnAs has a remarkably
high ferromagnetic transition temperature 60–80 K
1–3
as
compared to only a few K, e.g., for II-VI based materials.
4
To be of practical use, however, the material must maintain
its ferromagnetic properties above room temperature. Much
of the ongoing research is thus aiming in this direction.
It has been shown by several groups that the Curie tem-
perarure T
c
of GaMnAs films can be considerably in-
creased by post growth thermal treatment,
5–8
from typically
60–80 K up to 160–170 K. However, these desired effects
were achieved only when the free surface was exposed to air
or nitrogen atmosphere. It was also found that protective
GaAs capping layers suppressed the annealing induced
modifications. These observations strongly suggested that the
annealing effects are associated with a surface reaction, and
it is therefore natural to assume that the process involves
out-diffusion and passivation of Mn interstitials Mn
I
. Due
to a tendency to compensate the increasing Mn
Ga
-induced
p-doping and also as a result of the low-temperature growth,
the density of n-type defects Mn
I
and As
Ga
is abundant in
GaMnAs. The Mn
I
are, however, highly mobile at tempera-
tures comparable with the GaMnAs growth temperature.
9,10
The removal of Mn
I
by annealing is reflected via a consid-
erably increased density of uncompensated carriers
8
and a
reduced lattice constant.
11
A serious limitation of the above-mentioned annealing
procedures is that the treated layers are not useful for further
processing. The reacted surfaces are disordered and cannot
be restored due to the metastable character of the material.
To get around this problem, we recently devised an alterna-
tive method of annealing.
12
The treatment is in this case car-
ried out in the MBE growth chamber, where the reactive
medium oxygen or nitrogen is replaced by a 1000 Å
thick surface layer of amorphous As deposited at a sample
temperature close to room temperature. The method is ex-
tremely efficient for removing the interstitial Mn, and the
resulting surface is well-ordered. Depending on the experi-
mental conditions, this preparation can result in either an
atomically smooth surface, suitable for further epitaxial
growth, or a surface covered with self-organized quantum
dots.
13
In the present paper the first type of surfaces have
been characterized with respect to their composition and
electronic properties. For the sake of completeness we also
discuss Hall effect, x-ray diffraction and magnetization data,
all obtained on samples annealed in air, but with an As cap.
Obviously, as long as the capping is sufficiently thick, it is
irrelevant whether the annealing is carried out in situ or ex
situ.
II. EXPERIMENT
The experiments were performed at the Swedish national
synchrotron radiation facility MAX-lab. The GaMnAs
films were prepared in a KRYOVAK MBE system, attached
to the photoelectron spectrometer at beamline 41 at the MAX
I storage ring. This growth system is equipped with five stan-
dard Knudsen effusion cells and one valved cracker source
for As
2
deposition. The substrates were 1 1 cm
2
pieces of
n-doped epiready 001 GaAs, indium glued to transferable
Mo substrate holders. During growth the sample temperature
was monitored by an IR pyrometer, covering the temperature
range 100 °C–700 °C, i.e., both the high temperatures
HT, 600 °C, used for regular GaAs growth and the low
temperatures LT, 230 °C, used for LT-GaAs and
GaMnAs growth. This pyrometer was also used to monitor
the sample temperature during the annealing treatments per-
formed in situ. The samples studied here were all annealed at
180 °C. A 10 keV RHEED system is used for monitoring
the sample surface during the growth. The growth rate and
concentration of substitutional Mn, Mn
Ga
, was determined
by means of RHEED oscillations, which persisted for ex-
tended time despite the extreme growth conditions.
3
In cases
PHYSICAL REVIEW B 75, 054415 2007
1098-0121/2007/755/0544156 ©2007 The American Physical Society 054415-1