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Cite this: Nanoscale, 2016, 8, 12118
Received 16th March 2016,
Accepted 23rd May 2016
DOI: 10.1039/c6nr02190ewww.rsc.org/nanoscale

Attractive interaction between Mn atoms on theGaAs(110) surface observed by scanning tunnelingmicroscopy
Atsushi Taninaka, Shoji Yoshida, Ken Kanazawa, Eiko Hayaki, Osamu Takeuchi and
Hidemi Shigekawa*

Scanning tunneling microscopy/spectroscopy (STM/STS) was
carried out to investigate the structures of Mn atoms deposited on
a GaAs(110) surface at room temperature to directly observe thecharacteristics of interactions between Mn atoms in GaAs.
Mn atoms were paired with a probability higher than the random
distribution, indicating an attractive interaction between them. In
fact, re-pairing of unpaired Mn atoms was observed during STS
measurement. The pair initially had a new structure, which wastransformed during STS measurement into one of those formed byatom manipulation at 4 K. Mn atoms in pairs and trimers werealigned in the <110> direction, which is theoretically predicted to
produce a high Curie temperature.

Over the past few decades, diluted magnetic semiconductors(DMSs), in which some atomic sites in a host semiconductor
are substituted by transition metal atoms, have been energetically studied to provide prospective materials for application to
spintronics, because they are compatible with general semiconductor devices.1,2 Control of the spatial distribution oftransition-metal atoms in a DMS is a key factor in realizing
desirable magnetic characteristics. Several papers havereported such control by carrier doping for DMSs such as(Zn, Cr)Te and GeMn.1–3 In these studies, the spatial distributions of transition-metal atoms were varied by changing thepolarity of the host semiconductors; for example, for (Zn, Cr)
Te,3 Cr atoms were aggregated by donor doping and stronglyinteracted with each other by ferromagnetic coupling, which
may result in a ferromagnetic transition temperature higher
than room temperature (RT). For further advances, however, itis necessary to understand such interactions in detail and thecharacteristics of their atomic structures.
(Ga, Mn)As is one of the fundamental and well-studied
ferromagnetic DMSs.2,4 In (Ga, Mn)As, Mn dopants, which
substitute Ga sites, act as not only magnetic impurities but
also acceptors generating holes. Since a (Ga, Mn)As sample

Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 3058573, Japan. E-mail: hidemi@ims.tsukuba.ac.jp; http://dora.bk.tsukuba.ac.jp;
Tel: +81-29-853-5276

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with a high hole concentration realizes a high ferromagnetictransition temperature (Curie temperature: Tc), the model ofcarrier-mediated ferromagnetism has been proposed andwidely accepted as the origin of the ferromagnetic character
observed for (Ga, Mn)As.5 However, the highest Tc experimentally achieved so far for (Ga, Mn)As is about 200 K, whichis still low for practical applications.6–8 According to theoretical and experimental studies,9–12 the ferromagnetic interaction between two transition metal atoms changes with theirordering direction and the distance of the neighbouring
atoms. The ordering of the nearest-neighbouring Mn atoms ina pair along the <110> direction is expected to achieve thestrongest ferromagnetic interaction.9,13 Hence, if we cancontrol Mn atoms to align along the <110> direction, we may
realize a (Ga, Mn)As sample with a higher Tc.
Such magnetic interactions between Mn atoms, which
depend on the atom arrangements, were experimentallystudied by Kitchen et al. by scanning tunneling microscopy
and spectroscopy (STM/STS).10,11 They fabricated structures ofMn pairs by STM atom manipulation at 4 K and investigatedtheir ferromagnetic interactions by observing the splitting of
Mn acceptor levels which reflects the degree of bonding andantibonding state formation. And thus, they reported results
similar to that predicted by theoretical study. However, sincethese structures were artificially fabricated at 4 K, understanding of the characteristics of natural structures in the interaction between transition metals is necessary.
Here, we present the STM/STS results obtained for the Mn
structures deposited on a GaAs(110) surface at room temperature (RT) showing attractive interaction between Mn atomsaligned in the <110> direction; this suggests the possibility ofself-arrangement of Mn atoms to produce a higher Tc.
A clean GaAs(110) surface was prepared by cleaving a GaAs
sample in an ultrahigh vacuum of 1 × 10−6 Pa. Mn atoms were
deposited on the GaAs(110) surface at RT by using an e-beam
evaporator. All STM/STS measurements were carried out at RT
using an Omicron variable-temperature STM system. I–V
characteristics were probed by changing the bias voltage from
−1.8 V to +1.5 V at 27 × 27 mesh points in each scan area

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Nanoscalealong with simultaneous STM imaging with the set point ofbias voltage and tunneling current of (Vs, It) = (−1.5–2.0 V,
0.05–0.20 nA).
According to Kitchen et al., Mn atoms deposited at 4 K
form pairs without manipulation for an n-type sample, andthus they carried out an experiment using a p-type sample that
did not exhibit any pairing.10,11 To clarify this point, we
carried out experiments on both p- and n-type samples.
Fig. 1(a) and (b) show STM images of p-type (Zn-doped,
1–5 × 1017 cm−3) and n-type (Si-doped, 1–5 × 1017 cm−3)
GaAs(110) sample surfaces with Mn atoms deposited on them,
respectively. Single, pair, and trimer Mn structures are labelled
S, P, and T in the figure, respectively, and their magnified
images are shown together on the right side of the wide-scan
images. Fig. 1(c) and (d) show the spectra obtained above abare GaAs surface and the structures of S and P for p- andn-type samples, respectively. There is a signal peak at ∼0.8 V
for S, and an earlier increasing characteristic was observed for
P in comparison with the spectrum obtained for a bare GaAs
surface. The apparent diﬀerence in the negative sample bias
voltage region observed between the spectra of p- and n-typesamples is due to the diﬀerence in the Fermi energy position,

Communicationas schematically shown in Fig. 1(e) (shown for a positive bias
voltage here),14 and the additional tunneling of electrons fromthe valence band through the Mn acceptor level for the case of
the p-type sample at negative bias voltages.
Fig. 1(f ) shows the histogram of the Mn structures obtained
from the total count of 300 structures for each of the p- andn-type surfaces. The ratios of Mn pairs to single Mn atoms were20% and 29% for p- and n-types, respectively, which are higher
than those expected from the random distribution, 0.8% and1.4% for p- and n-types, respectively. The random distribution
was estimated by the equation: 1 − (1 − x)n, where x is the Mnconcentration obtained from STM images, 0.4% for p-type and0.7% for n-type, and n is the number of the nearest neighbouring sites, 2 for this case. These results indicate that there exists
an attractive interaction between Mn atoms on the GaAs(110)
surface. The results were almost the same between the p- andn-type samples. The slight diﬀerence in the possibility ofpairing observed between the p- and n-type samples may berelated to the fact that pair structures without manipulation at
4 K were observed only for the n-type surface, as mentioned in
the work by Kitchen.11 Since a Mn atom placed at a Ga site acts
as an acceptor, such a replacement may be suppressed forFig. 1 Filled state STM images of Mn atoms deposited on (a) p-type (Zn-doped, 1–5 × 1017 cm−3, VS = −1.5 V, It = 0.05 nA) and (b) n-type (Si-doped,
1–5 × 1017 cm−3, VS = −1.5 V, It = 0.05 nA) GaAs(110) surfaces at room temperature. Single, pair, and trimer Mn images are labelled S, P, and T, andtheir magniﬁed images are shown on the right side of the wide-scan images. (c), (d) Spectra obtained above a bare GaAs (black) and the structuresof S (red) and P (blue) for p-type and n-type samples. The origin of the signal which appeared in Fig. 2 (f ) is shown in (c). (e) Schematic illustrationsof the band structures with Mn atoms. Sample was illuminated in the case of n-type to reduce the tip-induced band bending induced for positive
sample bias voltages (635 nm, 0.5 mW), which was necessary to observe the acceptor level. (f ) Histogram of ratio among structures labelled S, P and
T in (a) and (b).

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hole-rich p-type GaAs. On the other hand, such a replacement
may be easier for n-type GaAs because of the compensation of
the negative charge, which produces a positive potential aroundthe Mn atom and attracts another Mn atom to form a pair.
Such a diﬀerence may be reduced at RT.
Fig. 2(a)–(e) show an example of the direct observation of
the structural changes of a Mn pair during STS measurement
for a p-type sample. Schematic illustrations of their structuralmodels are shown together, where P2110(Ga), for example, indicates the pair structure with two Mn atoms located in thesecond-nearest-neighbouring sites in the <110> direction witha Ga atom, as shown in the schematic illustrations. STM tip
scanning was carried out from left to right and bottom to top.
Fig. 2(f ) and (g) show the current maps deduced from I–V
curves for Vs = +1.35 V and +0.8 V, respectively, which are considered to be related to the electronic structures of pair andsingle Mn structures, respectively. Since I–V curves were noisy
owing to the structural change during STS measurement, integrated current images are shown here instead of diﬀerential

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images. As expected from the I–V curve shown in Fig. 1(c), no
signal related to S (Vs = +0.8 V) was observed, whereas a spatial
distribution of the electronic structure related to P (Vs =+1.35 V), which is shown in Fig. 1(c), was observed. During thenext scan, P2110(Ga) changed into P2110, as shown in Fig. 2(b).
Fig. 2(c) shows the STM image of P2110, which was obtained
without spectroscopy because P2110 was very unstable. In fact,
when the next scan was started, it easily changed into P1110
shown in Fig. 2(d). Therefore, a clear current map over the total
structure could not be obtained for P2110. A partially obtained
current map obtained for the structure shown in Fig. 2(b) isshown in Fig. 2(h). Subsequent changes are shown in Fig. 2(d)and (e). Fig. 2(i) and ( j) show the current maps obtained forP1110 and P111 shown in Fig. 2(d) and (e), respectively.
The electronic structure at Vs = +0.8 V was located above theMn site for S, as shown in Fig. 2(k) and (l), whereas it was
located between two Mn atoms for pair structures. The I–V
curves averaged over the squares drawn in Fig. 2(h)–( j) and (l)are shown in Fig. 2(m). The diﬀerence between the spectra ofFig. 2 (a)–(e) STM images of structural changes observed during STS measurement for the p-type sample. (f ) and (g) Current images obtained for
Vs = +1.35 V and Vs = +0.8 V. (h) to ( j) Current images at Vs = +0.8 V obtained for structures shown in (b), (d), and (e), respectively. (k) and (l) STM
image and its current map at Vs = +0.8 V obtained for the structure of single Mn labelled S in Fig. 1. (m) I–V curves averaged in squares drawn in (h)to ( j) and (l).

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NanoscaleFig. 3 Schematic illustrations (a) and simulated STM image (b) obtained
for the structure of an Mn pair with a Ga atom shown in the schematic
illustration (VS = −1.0 V). Here, the Ga atom was placed at the brightest
point in the STM image with a distance of 0.24 nm from the nearest neighbouring Ga atom below. (c) STM image for comparison (VS = −1.5 V).

S and P1110 is clear and the spectral characteristics obtainedfor P111 and P2110 are between those for S and P1110, showing achange in the degree of attractive interaction between Mn
atoms. Spatial distributions for the structures of 2(b) to (e) for
Vs = +1.35 V (not shown) were not clearly discriminated fromthose for Vs = +0.8 V, possibly because of the fact that the integrated signals are used here and they are not sensitive to thespectral changes shown in Fig. 2(m).
Although the STM image and spectrum obtained for the
single Mn structure are similar to those obtained for the struc-

Communicationture formed by atom manipulation at 4 K in the previous work,
those obtained for the structure labelled P are diﬀerent fromthe results obtained for any of the pair structures formed in
the previous work,10,11 namely, the structure P is asymmetricwith respect to the <110> axis. However, since the structure P
was transformed into a symmetric structure, one of the structures artificially formed at 4 K during STS measurement as isshown in Fig. 2, it is considered to be an intermediate structure naturally formed at RT.
Fig. 3 shows the STM images simulated for the structuralmodel schematically shown for the structures labelled
P observed in Fig. 1, where, as a possible structure, anadditional Ga atom was placed as shown in the schematic
model. We carried out a calculation based on a simple density
functional theory (DFT) by constructing supercells with a slab
model consisting of five GaAs(110) atomic layers and a vacuum
region also with a thickness of five atomic layers.15,16 The
atoms in the topmost layer were displaced from the positionsof the ideal GaAs(110) surface following the surface reconstruction.17 The plane-wave cut-oﬀ energy for the calculation was
50 Ry. We used 2 × 2 × 1 Monkhorst–Pack k-point grids tosample the Brillouin zone. The Ga adatom is the most brightand the topmost As atoms in the neighbouring site of the Mn
atom are brighter than the other As atoms.
Although a more detailed calculation is necessary to accurately determine the structure, the simulated image is in good
agreement with the STM image, suggesting this to be an intermediate structure between the original one and that of the Mnpair without a Ga adatom artificially formed at 4 K.18
Similar structural and spectral changes during STS
measurement were also observed for the Mn atoms deposited

Fig. 4 STM images of structural changes observed during STS measurement for the n-type sample. The yellow triangles indicate the positionalchange of the Mn pair during STS measurement.

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on an n-type substrate. An example of such observations isshown in Fig. 4. Single Mn structures were stable and used asa marker to show the changes in the pair structures. The
yellow triangles in 4(a), (e) and (g) indicate the change in theposition of the Mn pair. The observed results for the re-pairingof unpaired Mn atoms for both the p- and n-type samples are
considered to support the existence of an attractive interaction
between Mn atoms on the GaAs(110) surface. Pairing of Mn
was not observed for a p-type sample when Mn atoms were deposited at 4 K, however, its appearance at RT suggests a rather
low barrier height for the replacement of Ga by Mn. In either
case, the arrangement of Mn atoms in a pair structure naturally formed at RT was in the <110> direction, which is
required to realize a high-Tc (Ga, Mn)As.

Conclusions
Mn atoms deposited on a GaAs(110) surface at room temperature were paired with a probability higher than the random
distribution. Re-pairing of unpaired Mn atoms was observed
during STS measurement, with the change in I–V characteristics, showing the interaction between the paired Mn atoms.
Mn atoms naturally formed in a pair and a trimer were alignedin the <110> direction, which was theoretically predicted to
produce a high Curie temperature. With further optimizationof the growth conditions, we may be able to realize a structureof Mn atoms aligned in the <110> direction, which will provide(Ga, Mn)As with a high Tc. Studies on the coverage dependent
evolution of pair or trimer formations and how the configurations change with thermal annealing, which is left for future
work, are expected to advance further understanding of themechanism, which is under consideration.

Acknowledgements
H. S. acknowledges the support from the Japan Society for the
Promotion of Science (Grant-in-Aid for Scientific Research,
15H05734).

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