JOURNAL OF APPLIED PHYSICS 99, 08D501
2006 DOI: 10.1063/1.2150809

1. Width dependence of annealing effects in „Ga,Mn…As nanowires
B. L. Sheu, K. F. Eid, O. Maksimov, N. Samarth, and P. Schiffera͒
Department of Physics and Materials Research Institute, Pennsylvania State University, University Park,
Pennsylvania 16802
͑Presented on 1 November 2005; published online 17 April 2006͒
We study the time dependence of annealing on a series of GaAs-capped ͑Ga,Mn͒As nanowires of
varying widths. For different annealing times, our measurements indicate that decreasing the wire
width monotonically increases the Curie temperature enhancement associated with annealing, as
well as the drop in resistivity. These results are consistent with the lateral diffusion of interstitial Mn
ions, which constitute an important source of defects in these materials. Furthermore, the thinner
wires show a higher rate of change of conductivity with annealing time, suggesting a more efficient
removal of Mn interstitials in thinner wires. © 2006 American Institute of Physics.
͓DOI: 10.1063/1.2150809͔
An ongoing interest in spin-based devices has motivated
research into magnetoelectronic materials over the past
decade.1,2
Among them, III–V magnetic semiconductors such
as ͑Ga,Mn͒As are extremely attractive due to their compat-
ibility with current semiconductor technology. These materi-
als display “carrier-mediated ferromagnetism,” so that the
magnetic properties of the system can be manipulated by
precisely controlling the electronic properties.3–5
Previous
studies have shown that the Curie temperature ͑TC͒ of
͑Ga,Mn͒As can be increased substantially by performing
post-growth annealing.6,7
Recent work has shown that TC can
reach above 150 K,8,9
and a recent report of TCϳ250 K in
͑Ga,Mn͒As-based heterostructures10
even hints at the possi-
bility of room temperature ferromagnetism.
In situ resistance monitoring during annealing,9
as well
as studies of annealing effects on ͑Ga,Mn͒As under different
conditions11,12
or with GaAs or As capping,13,14
all suggest
that the removal of Mn interstitials ͑MnI͒ from ͑Ga,Mn͒As
plays an important role in the enhancement of TC upon an-
nealing. This can be easily understood, since the MnI both
introduce disorder and compensate the holes which mediate
ferromagnetic exchange. Understanding the rate-limiting fac-
tors of the MnI removal in ͑Ga,Mn͒As will be particularly
important for designing heterostructure devices, since MnI
cannot diffuse to a free surface if the ͑Ga,Mn͒As layer is
buried within a heterostructure. One route to effective an-
nealing of ͑Ga,Mn͒As in heterostructures is to lithographi-
cally reduce the lateral dimensions of the heterostructure and
thus allow MnI to diffuse out laterally.15
Here, we report a
detailed study of time-dependent annealing effects in a series
of wires of different widths fabricated from GaAs-capped
͑Ga,Mn͒As. The annealing effects on Curie temperature and
on resistivity increase substantially as the wire width is de-
creased, although we are not able to obtain a clear agreement
with a diffusion model as was possible for epilayers of dif-
ferent thicknesses.9
Our samples were fabricated by electron beam lithogra-
phy and dry etching of a GaAs/͑Ga,Mn͒As͑50 nm͒/
GaAs͑10 nm͒ heterostructure grown by molecular beam ep-
itaxy ͑MBE͒ on semi-insulating GaAs ͑001͒ wafers; details
of the MBE growth and post-growth processing are de-
scribed elsewhere.7,8,13,15
Figure 1͑a͒ shows a field emission
scanning electron microscopy image of the designed layout
for four probe resistance measurement of a 70 nm wide wire.
Figure 1͑b͒ shows the more detailed features of a 220 nm
wide single wire under higher magnification. All other wires
were patterned on the same wafer along ͓110͔ direction of
the GaAs substrate for consistency, although no effect of
crystalline anisotropy in resistivity is expected based upon
earlier studies.15
Magnetization versus temperature measurements for as-
grown and annealed ͑190 °C, 5 h, N2 gas flowϳ1.5 scfh͒
macroscopic pieces of our sample were performed in a com-
mercial superconducting quantum interference device
͑SQUID͒ magnetometer. Samples were precooled in 1 T
magnetic field and measured while warming in a 0.005 T
a͒
Electronic mail: schiffer@phys.psu.edu
FIG. 1. FESEM images of the nanowires. ͑a͒ The layout of a four probe
resistance measurement for a 70 nm wide single wire patterned on
GaAs/͑Ga,Mn͒As͑50 nm͒/GaAs͑10 nm͒. ͑b͒ A 220 nm wide single wire
under higher magnification. All the wires in the series of study are patterned
along ͓110͔ direction of the same GaAs wafer.
JOURNAL OF APPLIED PHYSICS 99, 08D501 ͑2006͒
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2. in-plane field. The results are shown in the inset of Fig. 2͑a͒,
where the as-grown and annealed piece both show similar TC
of ϳ66 K, identifying suppressed effect of annealing due to
GaAs capping.13
An alternative method of determining TC of
͑Ga,Mn͒As is through the peak of the temperature-
dependent resistivity, where the peak temperature is usually
within 10 K of the estimated TC from SQUID
measurements.16,17
The equivalence is demonstrated in Fig.
2͑a͒ by resistivity versus temperature data from 1 mm
ϫ2 mm mesas patterned on the same as-grown and annealed
pieces.
We fabricated wires of widths 1 ␮m, 420, 220, and
120 nm for four probe resistivity measurement. We measured
the temperature-dependent resistivity, ␳͑T͒, of all the wires
and then annealed for an hour at 190 °C in flowing N2, and
then measured again, and then annealed again for another
hour. This cycle was repeated for 6 h of total annealing time,
but the samples degraded during a longer anneal which was
attempted after 6 h, and became unmeasurable. Typical data
are shown in Figs. 2͑b͒ and 2͑c͒ for the wires as-grown and
annealed for 6 h total.18
Compared to the results for the mac-
roscopic pieces of sample ͓Fig. 2͑a͔͒, there is a significant
effect of annealing on the nanowires. The increase in TC after
annealing is almost 45 K for the 120 nm wire, indicating the
effectiveness of outdiffusion of MnI towards the lateral free
surfaces. Though we have no explanation for the slightly
increased resistivity in our set of annealed wires compared to
the unpatterned as-grown sample, similar effects due to re-
construction of defect structures have been observed for con-
FIG. 3. ͑a͒ Curie temperature, ͑b͒ conductivity at T=300 K, and ͑c͒ rate of
change of conductivity ͑measured at T=300 K͒ for the different wires as a
function of total anneal time. Each of these wires is measured as-grown and
after every additional hour of annealing at 190 °C up to 6 h. The rate
change of conductivity was calculated based on the neighboring conductiv-
ity values of the corresponding annealing time.
FIG. 2. ͑a͒ Resistivity vs temperature ͑measured in van der Pauw geometry͒
of as-grown and annealed 1 mmϫ2 mm mesa patterned on
GaAs/͑Ga,Mn͒As͑50 nm͒/GaAs͑10 nm͒ heterostructures. The inset shows
magnetization vs temperature for the same sample measured in a magnetic
field of 50 Oe in-plane while warming after field cooling in a 1 T field. The
values of TC obtained from both measurements are consistent. ͑b͒ Tempera-
ture dependence of ͑four-probe͒ resistivity for unannealed wires, patterned
from the same sample. ͑c͒ The temperature dependence of the resistivity of
the same wires after six 1 h anneals.
08D501-2 Sheu et al. J. Appl. Phys. 99, 08D501 ͑2006͒
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3. ditions such as higher annealing temperatures and longer an-
nealing times.6,7
In Fig. 3, we show the annealing time dependence of TC
and resistivity at 300 K. We note that after 6 h of annealing,
the increase in TC for the 120 nm wire is three times of the
1 ␮m wire; at the same time, the resistivity at the ␳͑T͒ peak
is reduced to half that of the 1 ␮m wire, confirming the
strong correlation between the enhancement of TC and de-
crease of resistivity, which was observed in uncapped
epilayers.7
Figure 3͑c͒ shows the rate change of conductivity versus
annealing time, indicating the MnI diffuse out faster in a
thinner wire in the initial stages of annealing, and the rate of
MnI removal decreases with annealing time. This is some-
what different from the results of in situ resistance monitor-
ing in uncapped epilayers, in which the thinner layers had a
slower rate of change of resistivity.9
Due to the limited time
interval of our measurements, we cannot rigorously test our
data against the one-dimensional diffusion model used in that
study. On the other hand, the higher rate of change of the
conductivity in thinner wires, suggests that a more complex
model would be required, perhaps due to the more complex
geometry presented by the wires. Another possibility is that
diffusion is different in the thinner epilayers studied previ-
ously or different in the lateral as opposed to the growth
direction ͑although different lateral directions appear to be
equivalent͒.15
In summary, we studied lateral diffusion of Mn intersti-
tials in a post-growth annealed series of GaAs-capped
͑Ga,Mn͒As wires of different widths. For different anneal-
ing time periods, our measurements indicate that the nar-
rower wires are fabricated, the more significant are the ef-
fects of Curie temperature enhancement, accompanied by a
larger resistivity decrease and higher rate change of conduc-
tivity.
This research has been supported by Grant Nos. ONR
N0014-05-1-0107, DARPA/ONR N00014-99-1093, -00-1-
0951, University of California-Santa Barbara Subcontract
KK4131, and NSF DMR-0305238 and DMR-0401486. This
work was performed in part at the Penn State Nanofabrica-
tion Facility, a member of the NSF National Nanofabrication
Users Network.
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08D501-3 Sheu et al. J. Appl. Phys. 99, 08D501 ͑2006͒
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