Article

A New Route toward Semiconductor Nanospintronics: Highly Mn-Doped GaAs Nanowires Realized by Ion-Implantation under Dynamic Annealing Conditions

Institute for Solid State Physics, Jena University, Max-Wien-Platz 1, 07743 Jena, Germany.
Nano Letters (Impact Factor: 13.59). 08/2011; 11(9):3935-40. DOI: 10.1021/nl2021653
Source: PubMed
ABSTRACT
We report on highly Mn-doped GaAs nanowires (NWs) of high crystalline quality fabricated by ion beam implantation, a technique that allows doping concentrations beyond the equilibrium solubility limit. We studied two approaches for the preparation of Mn-doped GaAs NWs: First, ion implantation at room temperature with subsequent annealing resulted in polycrystalline NWs and phase segregation of MnAs and GaAs. The second approach was ion implantation at elevated temperatures. In this case, the single-crystallinity of the GaAs NWs was maintained, and crystalline, highly Mn-doped GaAs NWs were obtained. The electrical resistance of such NWs dropped with increasing temperature (activation energy about 70 meV). Corresponding magnetoresistance measurements showed a decrease at low temperatures, indicating paramagnetism. Our findings suggest possibilities for future applications where dense arrays of GaMnAs nanowires may be used as a new kind of magnetic material system.

Full-text

Available from: Sandeep Kumar
Published: August 17, 2011
r
2011 American Chemical Society
3935 dx.doi.org/10.1021/nl2021653
|
Nano Lett. 2011, 11, 39353940
LETTER
pubs.acs.org/NanoLett
A New Route toward Semiconductor Nanospintronics: Highly
Mn-Doped GaAs Nanowires Realized by Ion-Implantation under
Dynamic Annealing Conditions
Christian Borschel,*
,
Maria E. Messing,
Magnus T. Borgstrom,
Waldomiro Paschoal, Jr.,
Jesper Wallentin,
Sandeep Kumar,
Kilian Mergenthaler,
Knut Deppert,
Carlo M. Canali,
§
H
akan Pettersson,
,
||
Lars Samuelson,
and Carsten Ronning
Institute for Solid State Physics, Jena University, Max-Wien-Platz 1, 07743 Jena, Germany
Solid State Physics/The Nanometer Structure Consortium, Lund University, Box 118, SE-221 00 Lund, Sweden
§
Division of Physics, School of Computer Science, Physics and Mathematics, Linnæus University, 39233 Kalmar-Sweden
)
Department of Mathematics, Physics, and Electrical Engineering, Halmstad University, Box 823, SE-301 18, Halmstad, Sweden
b
S Supporting Information
F
erromagnetic ordering is observed in highly Mn-doped GaAs,
in bulk or as thin lms, where Mn provides the uncom-
pensated spins as well as p-doping, allowing hole-mediated
ferromagnetism. Transition-metal solubility limits in IIIV
semiconductors are low, but the necessary Mn fractions for
ferromagnetic dilute magnetic semiconductors (DMS) can be
achieved, for example, by nonequilibrium growth using low
temperature MBE at 250 C.
1
Higher growth temperatures or
subsequent annealing typically lead to higher doping levels in the
semiconductor, but also segregation into MnAs clusters.
2,3
Self-assembled semiconductor nanowires (NWs) have proven
to be of interest as versatile building blocks of high functionality
for various devices, such as eld-eect transistors,
4
sensors,
5
and
solar cells.
6
Moreover, the small footprint of semiconductor NWs
facilitates direct growth on silicon wafers. Dilute magnetic
semiconductor NWs would therefore oer a seamless integra tion
of future spintronic devices with mainstream silicon technology,
which is one of the main goals of More than Moore.
Self-organized growth of epitaxial GaAs NWs of high-crystal-
line quality is possible via metal organic vapor phase epitaxy
(MOVPE).
79
The relatively high growth temperatures for
nanowire growth, typically 400500 C, inhibit the homoge-
neous incorporation of high Mn-concentrations during growth
due to the low solubility limit.
Over the years, several attempts have been made to fabricate
Ga
1x
Mn
x
As NWs. Already in 2002, Sadowski et al. reported the
successful growth of InGaMnAs NWs by migration enhanced
epitaxy.
10
Later, they established growth of Ga
1x
Mn
x
As NWs
using MnAs nanoclusters as growth seeds.
11
The obtained NWs
contained up to 7% of Mn and were strongly tapered with
irregular side facets. The high Mn content led to a branching of
the NWs.
12
Martelli et al. performed doping of GaAs NWs using
Mn-assisted growth.
13
Recently, Kim et al. synthesized gold-
seeded Ga
1x
Mn
x
As NWs using a vapor transport method with
low Mn content (<5%) exhibiting room-temperature ferro-
magnetism;
14
however, the magnetic properties are unclear
due to lack of any mechanism which could explain the high T
C
values.
15
An additional complication is that the Mn may not
incorporate homogeneously in the NW s. In fact, it might be that
the Mn accumulates in the form of a shel l, as indicated by
tomographic atom probe measurements for Ge NWs doped
during VLS growth.
16
Jeon et al. reported on room-temperature
ferromagnetism of GaMnAs NWs with a Mn content of 20%,
17
which is a metallic alloy rather than diluted Mn in a GaAs matrix.
Received: June 27, 2011
Revised: August 12, 2011
ABSTRACT: We report on highly Mn-doped GaAs nanowires (NWs) of high
crystalline qua lity fabricated by ion beam implantation, a technique that allows
doping concentrations beyond the equilibrium solubility limit. We studied two
approaches for the preparation of Mn-doped GaAs NWs: First, ion implanta-
tion at room temperature with subsequent annealing resulted in polycrystalline
NWs and phase segregation of MnAs and GaAs. The second approach was ion
implantation at elevated temperatures. In this case, the single-crystallinity of the
GaAs NWs was maintained, and crystalline, highly Mn-doped GaAs NWs were
obtained. The electrical resistance of such NWs dropped with increasing temperature (activation energy about 70 meV).
Corresponding magnetoresistance measurements showed a decrease at low temperatures, indicating paramagnetism. Our ndings
suggest possibilities for future applications where dense arrays of GaMnAs nanowires may be used as a new kind of magnetic
material system.
KEYWORDS: Nanowires, GaAs, doping, DMS, ion implantation, dynamic annealing
Page 1
3936 dx.doi.org/10.1021/nl2021653 |Nano Lett. 2011, 11, 3935–3940
Nano Letters
LETTER
Rudolph et al. exploited the possibility to change growth mode
from axial to radial nanowire growth and fabricated ferromag-
netic GaAs/GaMnAs core/shell NW s.
15
However, the growth of
the GaMnAs shell leads to a morphological change from the
perfect hexagonal GaAs core NWs toward roughened core/shell
GaAs/GaMnAs NWs. A completely dierent approach was
performed by Wol et al.
18
who decorated GaAs NWs with
Mn nanoparticles, which were then transformed into MnAs
nanoparticles by thermal annealing in hydrogen and arsine.
Most of the reported attempts suer from segregation of the
MnAs phase during growth leading to nonideal nanow ire mor-
phologies. This can easily be exemplied by the morphological
change from the perfect hexagona l GaAs core NWs toward
totally roughened core/shell NWs after growth of the Ga
1x
-
Mn
x
As shells around them.
15
Furthermore, the detection of
segregated MnAs nanoclusters in GaAs with X-ray diraction
(XRD) or high-resolution transmission electron microscopy
(HR-TEM) is reported to be rather challenging
19
and these
clusters can easily be mistaken for a DMS. In conclusion, in situ
Mn-doping of GaAs NWs turns out to be rather dicult to
achieve by established growth methods. This observation is well
in agreement with the phase diagram determined by Ohno for
Ga
1x
Mn
x
As layer growth via MBE:
20
For x in the order of
0.56%, MnAs forms above about 300 C.
Postgrowth ion beam implantation is a nonequilibrium doping
method that allows concentrations beyond solubility limits and is
independent from the growth method of the crystal. It is a
standard method for doping of large scale wafers in industry, but
it has also been successfully applied for doping of semiconductor
NWs.
21
The successful p-doping of GaAs NWs via ion beam
implantation of Zn and subsequent annealing at 800 C under
tertiarybutylarsine atmosphere has been reported.
22
Ion beam
implantation introduces defects in the target, making post im-
plantation annealing a necessity. For Mn-implanted GaAs thin
lms, standard thermal annealing (650 C) has been reported to
lead to MnAs cluster formation.
23
Only pulsed laser melting
24,25
and ion beam-indu ced epitaxial crystallization annealing
26
have
shown promising results regarding the incorporation of Mn in
GaAs to create DMS systems.
The annealing methods from bulk cannot directly be trans-
ferred to nanosized objects, because, due to their large surface-to-
volume ratio, the equilibrium phases strongly dier from those
for bulk material. For example, NWs exhibit lower melting points
compared to their bulk counterparts.
In this Letter, we compare two approaches for preparation of
Mn-doped GaAs NWs by (a) ion implantation at room tem-
perature with subsequent annealing and (b) ion implantation at
elevated temperatures making use of in situ dynamic annealing.
Furthermore, we report on magnetotransport measurements
that were done in order to study the magnetic properties of
the implanted Ga
1x
Mn
x
As NWs.
Single crystalline, epitaxial GaAs NWs were grown via
MOVPE using monodisperse Au particles as catalyst. Details
have been published elsewhere.
27
In order to optimi ze the
implantation angle, the NWs were grown on GaAs(001) sub-
strates leading to an inclined growth direction of 35 toward the
substrate.
28
NWs were implanted with Mn ions using a general
purpose implanter (High Voltage Engineering Europa). Implan-
tation energies of typically 4060 keV were selected. Results
from computer simulation s of the ion beam implantation show
that these energies lead to a reasonably homogeneous concen-
tration of Mn in the GaAs NWs. Frequently, these simulations
are carried out with the widely used SRIM program.
29
However,
SRIM assumes at sample geometry and cannot accurately
describe the distribution of implanted ions in NWs. Therefore,
we used the new in-house developed code iradina,
30
which
correctly takes into account the nanowire geometry.
NWs were implanted with Mn doses of 2 10
15
to 1 10
16
ions/cm
2
resulting in total Mn concentrations from 0.5 to 2.9%
(corresponding to a stoichiometry of Ga
1x
Mn
x
As with x from
x = 0.01 to x = 0.058) as calculated with the iradina code.
Detailed simulation results comparing SRIM and iradina for the
given situation are shown in the Supporting Information Figure 1.
The rst set of samples (a) was implanted at room temperature
and subsequently annealed for 30 min either under vacuum at
temperatures up to 500 C, or under AsH
3
atmosphere (molar
fraction of 2.20 10
3
) from 350 to 650 C. The second set of
samples (b) was implanted at elevated temperatures ranging
from 100 to 350 C. The NWs were characterized by transmis-
sion electron microscopy (TEM) using a Jeol 3010 and a Jeol
3000F microscope. The Mn concentration was monitored via
X-ray energy dispersive spectroscopy (XEDS).
For magnetotransport measurements, NWs were mechani-
cally transferred onto a silicon substrate covered by a 210 nm
thick silicon dioxide layer on whi ch reference markers and
macroscopic meta l pads were predened. Prior to transferring
of the NWs, trenches were etched in the SiO
2
layer to align the
wires for magnetotransport studies. Electron beam lithography
was used to de ne contacts conne cting individual NWs to the
macroscopic contact pads. The samples were treated in HCl/
H
2
O solution for 15 s followed by a 2 min surface passivation in a
heated (40 C) NH
4
S
x
/H
2
O solution. To investigate the inu-
ence of the contact resistance, 4 Pd(10 nm)/Zn(10 nm)/Pd-
(35 nm) contacts were evaporated on each NW after passi-
vation.
31
The sample processing was nalized by a lift-o process.
The magnetotransport measurements were performed in a
Janis VariTemp superconducting cryomagnet system (Model
8T-SVM).
Figure 1a shows an HR-TEM micrograph of a typical non-
implanted, as-grown, NW for comparison. The single-crystalline
NWs of zinc blende (ZB) structure we re grown in the Æ111æ
direction and exhibit twin planes perpendicular to the growth
direction, as commonly observed for IIIV semiconductor
NWs.
32
Resulting from the implantation parameters at room
temperature, the NWs were mostly amorphized, which is illu-
strated in Figure 1b. Depending on the Mn dose, the ion energy,
and the exact diameter of the NWs, some parts that face away
Figure 1. (a) HR-TEM image of an as-grown GaAs nanowire, with the
FFT shown in the inset. (b) TEM micrograph of an implanted,
nonannealed nanowire (with approx 2.5% Mn), that is partly amor-
phized and partly crystalline. The two insets show FFTs of the
amorphous and crystalline parts, respectively.
Page 2
3937 dx.doi.org/10.1021/nl2021653 |Nano Lett. 2011, 11, 3935–3940
Nano Letters
LETTER
from the ion beam however remained crystalline as illustrated by
the fast Fourier transform (FFT) diraction pattern (inset in
Figure 1b).
The NWs recrystallized under annealing in vacuum up to
400 C, however not to single crystals. A typical polycrystalline
nanowire is described by the HR-TEM image and its FFT pattern
in Figure 2a. After annealing in vacuum at 500 C or higher
temperatures, we observed that the NWs disappeared, which we
interpret as a temperature-induced decomposition of the GaAs
NWs, mainly due to the high equilibrium As vapor press ure over
GaAs. Therefore, annealing in AsH
3
atmosphere, as previously
used to recrystallize Zn-implanted GaAs NWs,
22
was attempted.
Within the investigated parameter space, the present Mn-doped
NWs survived the treatment but although some recrystallization
occurred, they did not become single crystalline (see Figure 2b).
Additionally, they featured bumps on the sides, as illustrated in
Figure 2c. Analysis of the FFT patterns and XEDS shows that
these bumps consist of crystalline GaAs without measurable Mn
content; the postimp lantation annealing with arsine background
pressure seems to initiate unwanted regrowth of crystalline GaAs
on the amorphized NW.
At this point, we can conclude that room-temperature im-
plantation and subsequent annealing is not suitable to create
single crystalline GaAs NWs with a high Mn concentration under
the parameter conditions investigated, neither in vacuu m, nor in
an arsine atmosphere.
During ion beam implantation, so-called dynamic annealing
can occur.
33
Each ion induces a collision cascade in the material,
producing a large number of point defects (at the order of 10
3
for
the parameters used in this study). Additionally, a high amount of
energy is deposited by the incident ion as lattice vibrations in the
cascade region. Within this so-called thermal spike, immediate
annihilation of defects can occur. When collision cascades from
subsequent ions overlap, defects that were created in the earlier
cascade can be annihilated by the following ones. If this dynamic
annealing is weak, the damage created by subsequent collision
cascades is accumulated and ultimately amorphization of the
material will occur, if a large uence of ions is implanted, as
observed in this study. Actually, dynamic annealing is enhanced
in NWs compared to bulk
34,35
due to the conned geometry
resulting into a slower dissipation of the impact energy. However,
this annealing eect is not strong enough at room-temperature
implantation of Mn into GaAs NWs, and is thus not able to
prevent amorphization: the NWs became amorphous when
implanted with x = 1% of Mn. Thus, the crystal orientation
was lost in the amorphous regions, and subsequent annealing
lead to polycrystalline recrystallization, as was described above.
Dynamic annealing can further be enhanced by heating the
target during ion implantation
33
due to increased phononpho-
non scattering resulting also into a slower dissipation of the
impact energy. In order to make use of this enhanced dynamic
annealing, samples were implanted at elevated temperatures. A
maximum of 400 C was used, because decomposition of the
GaAs NWs was observed in vacuum at higher temperatures.
Figure 3a,b shows TEM micrographs of a nanowire implanted
with Mn at a temperature of 100 C. A large part of the nanowire
remained crystalline during implantation, and the crystalline
structure appears similar to the nonimplanted nanowire
(Figure 1a). This is in contrast to room-temperature implanta-
tion at which the NWs were mostly amorphized. How ever, an
amorphous neck close to the catalyst particle appeared after
implantation at 100 C, as illustrated in Figure 3b. This latter
observation can be attributed to dierent e ciency of the
dynamic annealing in the proximity of the catalyst particle. For
most part of the nanowire, the energy within the thermal spike is
sucient to cause in situ removal of most defects, the crystal
orientation is never lost and the material remains single-crystal-
line in ZB phase and oriented in the (111)B direction. However,
when an ion hits the nanowire close to the Au particle, the energy
of the thermal spike dissipates much faster, because the thermal
conductivity of Au is about 56 times larger than of GaAs. Thus,
less time is available for annihilation of defects, and amorphization
of the material occurs in this area. Such a partial amorphization
Figure 2. TEM micrographs of implanted and annealed NWs. (a) After annealing in vacuum at 400 C, the FFT in the inset shows that the nanowire is
not single crystalline. (b,c) After annealing in As atmosphere at 550 C. (b) Nanowire is polycrystalline. (c) Bumps appear on nanowire, consisting
of GaAs.
Figure 3. (a) GaAs nanowire implanted with x = 1% Mn at 100 C.
(b) HR image of the same wire showing the amorphous gap between
crystalline nanowire core and Au tip. (c) GaAs nanowire implanted with
x = 1% Mn at 200 C.
Page 3
3938 dx.doi.org/10.1021/nl2021653 |Nano Lett. 2011, 11, 3935–3940
Nano Letters
LETTER
could be avoided by further increasing the implantation tem-
perature to above 100 C. Then, the dynamic annealing was
suciently enhanced to maintain crystallinity of the complete
nanowire, as illustrated for a nanowire implanted at 200 Cin
Figure 3c. It should be noted that apart from dynamic annealing
caused by the ion beam, normal thermal healing may also occur
at the higher temperatures in between the ion impacts. However,
it is not possible to separate the contributions from ion beam and
temperature in the annealing.
From thin lm growth of Ga
1x
Mn
x
As it is known that phase
separation of MnAs occurs above 250300 C depending on the
Mn concentration.
20
However, due to their small size this limit
may be lower for NWs. Therefore, we also examined implanta-
tion with high doses of Mn at 250 C; the results are illustrated in
Figure 4a, showing an HR-TEM image of a nanowire implanted
with 10
16
ions/cm
2
, corresponding to a total Mn concentration
of 2.9% (x = 0.058). Even at these high concentrations, the NWs
were observed to remain single-crystalline and no secondary
phases were observed. XEDS measurements were performed on
several areas on these NWs (Supporting Informat ion Figure 2).
They conrmed a Mn concentration of about 23%, showing
that Mn has not diused out during implantation at elevated
temperatures.
It should be noted at this point, that computer simulations of
the ion beam implantation require that the correct nanowire
geometry is taken into account. Simulations using SRIM
29
over-
estimate the implanted concentration of Mn in the NWs by about
a factor of 1.7, while simulations using the iradina code
30
predict
the Mn concentration more accurately to about 2.9%. The reason
is that SRIM can only simulate at targets and therefore cannot
take into account ions leaving the nanowire from the sides.
A recovery stage for the electronic properties of GaAs around
250 C has been reported by Thommen for high-energy electron-
beam irradiated GaAs and subsequent annealing.
36
Thus, it seems
useful to use at least 250 C during implantation, although single-
crystallinity was already observed at 150 C. We further increased
the implantation temperature to 350 C and still obtained single
crystalline NWs; however, the optimum temperature has to be
balanced, since, as mentioned in the introduction, at higher
temperatures the precipitation of MnAs nanoclusters becomes
more likely.
37
We did not observe MnAs precipitates in the
implanted NWs up to 350 C, but nanometric inclusions of this
phase are not easily detectable
19
and cannot be excluded.
Considering the upper and lower limitations, 250 C was
identied as the suitable temperature for creating GaMnAs
nanowire with high Mn content via ion beam implantation. This
is consistent with the temperature at which Ohno et al. have
grown LT-MBE magnetic GaMnAs thin lms.
1
Furthermore,
250 C is also the same temperature Chen et al. used for ion-
beam annealing of Mn implanted GaAs thin lms.
26
Transport measurements were carried out between 1.6 and
300 K. To investigate possible inuence of contact resistance,
2-point and 4-point measurements were compa red down to
about 70 K where 4-point measurements became dicult to
carry out due to a very high resistance. From these meas ure-
ments, we conclude that the contact resistance is neg ligible
compared to the NW resistance. The IV-characteristics are in
general fairly linear down to about 100 K (Supporting Informa-
tion Figure 3). At lower temperatures, a symmetrical nonlinear
behavior becomes increasingly dominant. Figure 5 displays the
resistance of a typical implanted NW versus reciprocal tempera-
ture from which we deduce an activation energy of about 70 meV.
We have investigated several NWs that all show the same
behavior. A similar behavior has previously been reported for
planar GaMnAs lms and attributed to thermal emission of holes
from a Mn impurity band to the valence band.
38
The activation
energy is smaller than the typical 100110 meV observed
for weakly doped GaAs:Mn, where the emission originates from
isolated Mn impurities.
39
Interestingly, the room-temperature
resistance is in the MΩ regime, which is unexpectedly high,
considering the nominal x = 5.8% Mn incorporation. A
plausible explanation for the apparent low hole concentration
(10
17
cm
3
assuming a reduced mobility of 60 cm
2
/(V s)
38
and 40 nm wire diameter) is a Fermi-level pinning at the
GaAs surface, which eectively creates a radially depleted semi-
conductor wire. Also, most likely compensation occurs due to
incorporation of interstitial Mn and substitutional As
Ga
antisites,
both acting as double donors. The resulting hole concentration is
too low to create the necessary eective ferromagnetic coupling
between Mn spins, which results in a paramagnetic state rather
than in ferromagne tic ordering. The absence of the onset of any
ferromagnetic order in our sample is also evident from the lack of
any singularity in the derivative of the resistance as a function of
temperature in the range of temperatures (40190 K), which is
expected to occur at the critical temperature.
40
Further under-
standing of the magnetic state of our implanted wires is obtained
from magnetoresistance (MR) studies. In Figure 6, we plot the
MR as a function of an external magnetic eld applied in the
direction parallel and perpendicular to the wire. The data readily
shows a negative MR signal up to about 50 K. No evident trace of
Figure 4. (a) HR-TEM micrograph of GaAs nanowire implanted with
x = 5% Mn at 250 C and FFT of the complete image as inset
demonstrating excellent crystalline quality of the Mn-implanted wires.
(b) HR-TEM micrograph with higher magnication.
Figure 5. Plot of resistance versus reciprocal temperature with indi-
cated activation energy. The inset shows an SEM micrograph of a
nanowire supplied with contacts for 4-point measurements.
Page 4
3939 dx.doi.org/10.1021/nl2021653 |Nano Lett. 2011, 11, 3935–3940
Nano Letters
LETTER
hysteresis eects was found in our measurements and addition-
ally, the MR does not seem to saturate (up to 5 T). There is
also no clear indication of any anisotropic MR changes at small
magnetic elds that typically occur in ferromagnetic DMS due to
reorientation of the magnetization relative to the current direc-
tion. While these MR results support the conclusion that our
samples are not ferromagnetic, they are also a strong indication of
a paramagnetic state of the system due to the presence of Mn
impurities. Indeed, nonimplanted NWs display dramatically
dierent electrical properties with very high resistance (200
GΩ) already at room temperature, which makes magnetotran-
sport measurements virtually impossible. Negative MR traces are
normally attributed to spin-disorder scattering in metallic
samples
20
and to suppression of Anderson localization of holes
in insulating or semiconducting samples.
41
Possibly, in the
regime of low conductance of our sample, both mechanisms
are contrib uting to some extent in reducing the resistance. The
observed MR eect may reect a decrease in resistance as a result
of a Zeeman shift of the Fermi energy toward the Anderson
mobility edge by the applied magnetic eld, whi ch further-
more reduces spin-disorder scattering by successively aligning
the randomly oriented individual Mn spins along its direction.
The Zeeman energy for a spin 5/2 is of the order of 20 K at 5 T.
If we assume that the magnetization of the sample at a given
temperature and magnetic eld is proportional to the Brillouin
function, the magnetization and consequently the MR should
reach saturation at elds on the order of 8 T when the tem-
perature is 1.6 K. On the other hand, as shown in Figure 6, at
temperatures above 70 K and at a magnetic eld of 5 T, the
thermal energy totally randomizes the spin orientation which
consequently quenches the MR.
The MR curves further indicate that the Mn is mainly diluted
in the GaAs host and not incorporated in the form of ferromag-
netic MnAs clusters; Michel et al. have theoretically studied the
inuence of ferromagnetic MnAs clusters in a paramagnetic
GaMnAs host matrix on the MR behavior.
42
They nd that a
high density of MnAs clusters leads to a strong negative MR
below 30 K and a very strong positive MR of several hundred
percent between 30 and 100 K. The Mn-doped nanowires only
show a weak negative MR below 70 K and no MR at higher
temperatures, indicating that there are either no MnAs clusters or
at least a very low density of them.
In conclusion, we have realized high-crystalline quality
Mn-doped GaAs NWs via ion beam implantation, which allows
incorporation of dopants into the target material far beyond the
solubility limit. In order to minimize ion beam-induced defects
dierent annealing routes were investigated. Postannealing was
not successful for NWs, because the NWs either decomposed or
became polycrystalline within the investigated parameter space.
However, heating the sample to higher temperatures during
implantation (250 C) enabled increased dynamic annealing in
addition to thermal healing, resulting in single-crystalline
Ga
1x
Mn
x
As NWs with high Mn content.
We also report on magnetotransport measurements on the
Mn-implanted NWs, where a strong-temperature dependence of
the resistance was observed in addition to a clear negative MR at
low temperatures. These results indicate dilute Mn incorporation
and, in combination with the observed high resistance of the
NWs, support the hypothesis that the implanted NWs are
paramagnetic due to a strong reduction in free hole concentra-
tion that stems from surface pinning eects and, possibly, from
Mn interstitials and As
Ga
antisites.
This study provides the groundwork to pur sue further ion-
implantation experiments, for example, in thicker NWs, and to
develop novel technology for tuning the hole concentration in
the NWs to facilitate an electrically controlled hole concentration
and thus ferromagnetism. We believe that the work presented
here demonstrates a promising route for future fabrication of
complex integrated DMS nanostructures, where arrays of Mn-
doped NWs may be functioning as a novel kind of material
system for science as well as for applicatio ns.
ASSOCIATED CONTENT
b
S
Supporting Information. Additional information and
gures. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: christian.borschel@uni-jena.de.
ACKNOWLEDGMENT
We acknowledge support by the Swedish Research Council
under Grant Number: 621-2010-3761.
REFERENCES
(1) Ohno, H.; Shen, A.; Matsukura, F.; Oiwa, A.; Endo, A.; Katsumoto,
S.; Iye, Y. Appl. Phys. Lett. 1996, 69,363.
(2) De Boeck, J.; Oesterholt, R.; Bender, H.; Van Esch, A.;
Bruynseraede, C.; Van Hoof, C.; Borghs, G. J. Magn. Magn. Mater.
1996, 156, 148.
(3) Wellmann, P. J.; Garcia, J. M.; Feng, J.-L.; Petro,P.M.Appl.
Phys. Lett. 1997, 71, 2532.
(4) Cui, Y.; Zhong, Z.; Wang, D.; Wang, W. U.; Lieber, C. M. Nano
Lett. 2003, 3, 149.
(5) Yeh, P.-H.; Li, Z.; Wang, Z. L. Adv. Mater. 2009, 21, 4975.
(6) Hochbaum, A. I.; Yang, P. Chem. Rev. 2010, 110, 527.
(7) Borgstrom, M.; Deppert, K.; Samuelson, L.; Seifert, W. J. Cryst.
Growth 2004, 260, 18.
(8) Wagner, R. S.; Ellis, W. C. Appl. Phys. Lett. 1964, 4, 89.
(9) Wacaser, B. A.; Dick, K. A.; Johansson, J.; Borgstrom, M. T.;
Deppert, K.; Samuelson, L. Adv. Mater. 2009, 21, 153165.
(10) Sadowski, J.; Deppert, K.; Kanski, V.; Ohlsson, J.; Persson, A.;
Samuelson, L. Proceedings of the 7th International Conference on Nan-
ometer-scale Science and Technology, June 2428, Malmo, Sweden, 2002.
Figure 6. Plot of magnetoresistance versus temperature for parallel (
)
)
and perpendicular (^) orientation of magnetic eld relative to nanowire
(only one sweep direction is shown).
Page 5
3940 dx.doi.org/10.1021/nl2021653 |Nano Lett. 2011, 11, 3935–3940
Nano Letters
LETTER
(11) Sadowski, J.; Dzu_zewski, P.; Kret, S.; Janik, E.; yusakowska, E.;
Kanski, J.; Presz, A.; Terki, F.; Charar, S.; Tang, D. Nano Lett. 2007,
7, 2724.
(12) Dluzewski, P.; Sadowski, J.; Kret, S.; Dabrowski, J.; Sobczak, K.
J. Microsc. 2009, 236, 115.
(13) Martelli, F.; Rubini, S.; Piccin, M.; Bais, G.; Jabeen, F.;
Franceschi, S. D.; Grillo, V.; Carlino, E.; DAcapito, F.; Boscherini, F.;
Cabrini, S.; Lazzarino, M.; Businaro, L.; Romanato, F.; Franciosi, A.
Nano Lett. 2006, 6, 2130.
(14) Kim, H. S.; Cho, Y. J.; Kong, K. J.; Kim, C. H.; Chung, G. B.;
Park, J. Chem. Mater. 2009, 21, 1137.
(15) Rudolph, A.; Soda, M.; Kiessling, M.; Wojtowicz, T.; Schuh, D.;
Wegscheider, W.; Zweck, J.; Back, C.; Reiger, E. Nano Lett. 2009,
9, 3860.
(16) Perea, D. E.; Hemesath, E. R.; Schwalbach, E. J.; Lensch-Falk,
J. L.; Voorhees, P. W.; Lauhon, L. J. Nat. Nanotechnol. 2009, 4, 315.
(17) Jeon, H. C.; Kang, T. W.; Kim, T. W.; Yu, Y.-J.; Jhe, W.; Song,
S. A. J. Appl. Phys. 2007, 101, 023508.
(18) Wol, M. F. H.; Gorlitz, D.; Nielsch, K.; Messing, M. E.;
Deppert, K. Nanotechnology 2011, 22, 055602.
(19) Seo, S. S. A.; Noh, T. W.; Kim, Y.-W.; Lim, J. D.; Park, Y. D.;
Kim, Y. S.; Khim, Z. G.; Jeon, H. C.; Kang, T. W.; Pearton, S. J. J. Appl.
Phys. 2004, 95, 8172.
(20) Ohno, H. J. Magn. Magn. Mater. 1999, 200, 110129.
(21) Ronning, C.; Borschel, C.; Geburt, S.; Niepelt, R. Mater. Sci.
Eng., R 2010, 70, 30.
(22) Stichtenoth, D.; Wegener, K.; Gutsche, C.; Regolin, I.;
Tegude, F. J.; Prost, W.; Seibt, M.; Ronning, C. Appl. Phys. Lett. 2008,
92, 163107.
(23) Burger, D.; Zhou, S.; Grenzer, J.; Reuther, H.; Anwand, W.;
Gottschalch, V.; Helm, M.; Schmidt, H.
Nucl. Instrum. Methods Phys. Res.,
Sect. B 2009, 267, 1626.
(24) Scarpulla, M. A.; Dubon, O. D.; Yu, K. M.; Monteiro, O.; Pillai,
M. R.; Aziz, M. J.; Ridgway, M. C. Appl. Phys. Lett. 2003, 82, 1251.
(25) Scarpulla, M. A.; Farshchi, R.; Stone, P. R.; Chopdekar, R. V.;
Yu, K. M.; Suzuki, Y.; Dubon, O. D. J. Appl. Phys. 2008, 103, 073913.
(26) Chen, C. H.; Niu, H.; Hsieh, H. H.; Cheng, C. Y.; Yan, D. C.;
Chi, C. C.; Kai, J. J.; Wu, S. C. J. Magn. Magn. Mater. 2009 , 321, 1130.
(27) Mikkelsen, A.; Skold, N.; Ouattara, L.; Borgstrom, M.; Andersen,
J. N.; Samuelson, L.; Seifert, W.; Lundgren, E. Nat. Mater. 2004, 3,
519523.
(28) Hiruma, K.; Yazawa, M.; Katsuyama, T.; Ogawa, K.; Haraguchi,
K.; Koguchi, M.; Kakibayashi, H. J. Appl. Phys. 1995, 77, 447.
(29) Ziegler, J. F.; Biersack, J. P.; Littmark, U. The stopping and ranges
of ions in solids; Pergamon Press: New York, 1985.
(30) Borschel, C.; Ronning, C. Nucl. Instrum. Methods Phys. Res., Sect.
B 2011, 269, 2133.
(31) Wallentin, J.; Persson, J. M.; Wagner, J. B.; Samuelson, L.;
Deppert, K.; Borgstrom, M. T. Nano Lett. 2010, 10, 974979.
(32) Yazawa, M.; Koguchi, M.; Hiruma, K. Appl. Phys. Lett. 1991,
58, 1080.
(33) Williams, J. S. Mate. Sci. Eng. 1998, 253,8.
(34) Dhara, S.; Datta, A.; Wu, C. T.; Lan, Z. H.; Chen, K. H.; Wang,
Y. L.; Chen, L. C.; Hsu, C. W.; Lin, H. M.; Chen, C. C. Appl. Phys. Lett.
2003, 82, 451.
(35) Colli, A.; Fasoli, A.; Ronning, C.; Pisana, S.; Piscanec, S.;
Ferrari, A. C. Nano Lett. 2008, 8, 2188.
(36) Thommen, K. Radiat. E. 1970, 2, 201.
(37) Ohno, H. Science 1998, 281, 951.
(38) Slupinski, T.; Caban, J.; Moskalik, K. Acta Phys. Pol., A 2007,
112, 325330.
(39) Ilegems, M.; Dingle, R.; Rupp, L. W., Jr. J. Appl. Phys. 1975,
46, 3059.
(40) Novak, V.; Olejník, K.; Wunderlich, J.; Cukr, M.; Vyborny, K.;
Rushforth, A. W.; Edmonds, K. W.; Campion, R. P.; Gallagher, B. L.;
Sinova, J.; Jungwirth, T. Phys. Rev. Lett. 2008, 101, 077201.
(41) Iye, Y.; Oiwa, A.; Endo, A.; Katsumoto, S.; Matsukura, F.; Shen,
A.; Ohno, H.; Munekata, H. Mater. Sci. Eng., B 1999, 63, 88.
(42) Michel, C.; Elm, M. T.; Goldlucke, B.; Baranovskii, S. D.;
Thomas, P.; Heimbrodt, W.; Klar, P. J. Appl. Phys. Lett. 2008, 92, 223119.
Page 6
  • Source
    • "In general, the annealing temperature ordinarily keeps at two thirds of the melting point of the implanted materials [18]. Lately, Borschel et al. [19] reported that GaAs nanowires implanted by Mn+ at 250°C remained as single crystalline. However, polycrystalline nanowires were acquired after implantation at room temperature with subsequent annealing. "
    [Show abstract] [Hide abstract] ABSTRACT: Nowadays, ion implantation is an extensively used technique for material modification. Using this method, we can tailor the properties of target materials, including morphological, mechanical, electronic, and optical properties. All of these modifications impel nanomaterials to be a more useful application to fabricate more high-performance nanomaterial-based devices. Ion implantation is an accurate and controlled doping method for one-dimensional nanomaterials. In this article, we review recent research on ion implantation-induced effects in one-dimensional nanostructure, such as nanowires, nanotubes, and nanobelts. In addition, the optical property of single cadmium sulfide nanobelt implanted by N+ ions has been researched.
    Full-text · Article · Apr 2013 · Nanoscale Research Letters
  • Source
    • "In this work, we concentrate on the higher doped wires. The NW growth and implantation techniques were discussed in detail previously [13]. "
    [Show abstract] [Hide abstract] ABSTRACT: Nanowires with magnetic doping centers are an exciting candidate for the study of spin physics and proof-of-principle spintronics devices. The required heavy doping can be expected to have a significant impact on the nanowires' electron transport properties. Here, we use thermopower and conductance measurements for transport characterization of Ga 0.95 Mn 0.05 As nanowires over a broad temperature range. We determine the carrier type (holes) and concentration and find a sharp increase of the thermopower below temperatures of 120 K that can be qualitatively described by a hopping conduction model. However, the unusually large thermopower suggests that additional mechanisms must be considered as well.
    Full-text · Article · Sep 2012 · Journal of Nanotechnology
  • Source
    [Show abstract] [Hide abstract] ABSTRACT: We report on temperature-dependent charge transport in heavily doped Mn(+)-implanted GaAs nanowires. The results clearly demonstrate that the transport is governed by temperature-dependent hopping processes, with a crossover between nearest neighbor hopping and Mott variable range hopping at about 180 K. From detailed analysis, we have extracted characteristic hopping energies and corresponding hopping lengths. At low temperatures, a strongly nonlinear conductivity is observed which reflects a modified hopping process driven by the high electric field at large bias.
    Full-text · Article · Aug 2012 · Nano Letters
Show more