1. APPLIED PHYSICS LETTERS 86, 152505 ͑2005͒
Nanoengineered Curie temperature in laterally patterned ferromagnetic
semiconductor heterostructures
K. F. Eid, B. L. Sheu, O. Maksimov, M. B. Stone,a͒ P. Schiffer, and N. Samarthb͒
Department of Physics and Materials Research Institute, Pennsylvania State University, University Park,
Pennsylvania 16802
͑Received 20 December 2004; accepted 1 March 2005; published online 5 April 2005͒
We demonstrate the manipulation of the Curie temperature of buried layers of the ferromagnetic
semiconductor ͑Ga,Mn͒As using nanolithography to enhance the effect of annealing. Patterning the
GaAs-capped ferromagnetic layers into nanowires exposes free surfaces at the sidewalls of the
patterned ͑Ga,Mn͒As layers and thus allows the removal of Mn interstitials using annealing. This
leads to an enhanced Curie temperature and reduced resistivity compared to unpatterned samples.
For a fixed annealing time, the enhancement of the Curie temperature is larger for narrower
nanowires. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1900938͔
Heterostructures derived from the ferromagnetic semi- dry etching. For the patterning process, the samples are first
conductor ͑Ga,Mn͒As are important for studying spin- spin coated with a bilayer of about 400-nm-thick electron-
dependent device concepts1–5 relevant to semiconductor beam resist P͑MMA-MAA͒ copolymer/PMMA with molecu-
spintronics.6–8 In contrast to metallic ferromagnets where lar weight of 950. The desired patterns are defined on the
crystalline defects rarely affect the magnetic ordering tem- sample using direct-write electron-beam lithography at elec-
perature, it is now well established that Mn interstitial tron energy of 100 keV. After development of the resist in
defects—double donors that compensate holes—play a key MIBK: IPA 1 : 1 solution, a metallic layer of either 45 nm Al
role in limiting the hole-mediated Curie temperature ͑TC͒ of or Al/ Au is deposited on the sample using thermal evapora-
͑Ga,Mn͒As to below 110 K in as-grown samples.9 In thin tion. Using standard liftoff techniques, we then obtain metal
͑Ga,Mn͒As epitaxial layers, post-growth annealing10,11 wires that serve as a hard mask for the subsequent chlorine-
drives Mn interstitials to benign regions at the free surface based dry etching process. After dry etching, for samples
where they are passivated, resulting in significant increases with an Al mask, the metal layer is dissolved in CD-26 pho-
in both the hole density ͑p͒ and TC ͑which can be as high as toresist developer, leaving just the semiconductor nanowire;
160 K͒.12–14 Such values of TC should in principle allow the for samples with an Al/ Au mask, the metal is retained on
fabrication of proof-of-concept spintronic devices operating portions of the sample. We note that in the latter case the
above liquid nitrogen temperatures. Unfortunately, the undoped GaAs capping layer serves as an insulator that pre-
vents the shunting of current through the metal during the
annealing-induced enhancement of TC is almost completely
measurement.17 Figures 1͑a͒ and 1͑b͒ show plan view SEM
suppressed in heterostructure devices containing buried
images of patterned nanowires. Three identical rows of wires
͑Ga,Mn͒As layers.15,16 In this letter, we describe a nanoengi-
are patterned on each wafer: one set is measured as-grown
neered solution to this problem: lithographic patterning of
͑Ga,Mn͒As heterostructures into nanowires. Such patterning
opens up new pathways for defect diffusion to free surfaces
at the sidewalls, and thus restores the annealing-enhanced TC
to buried ͑Ga,Mn͒As layers. These results suggest routes to-
wards the flexible fabrication of ͑Ga,Mn͒As-based spintronic
devices with relatively high TC.
Laterally patterned wires are fabricated from
GaAs/ ͑Ga, Mn͒As/ GaAs heterostructures grown by molecu-
lar beam epitaxy on epiready semi-insulating GaAs ͑100͒
substrates. Crystal growth conditions are identical to those
described in an earlier publication.15 The samples consist of
a 5 nm thick low temperature GaAs buffer layer, followed by
a Ga1−xMnxAs layer ͑Mn content ϳ6%, thickness of 15 or 50
nm͒ and finally a low temperature grown 10 nm GaAs cap-
ping layer. The data shown are for 50 nm thick magnetic
layers, but the results were qualitatively the same for the 15
nm thick samples. We pattern these samples into wires that
are ϳ4 ␮m long and with two different widths ͑1 ␮m and 70
nm͒ using electron beam lithography followed by liftoff and
a͒
Currently at the Condensed Matter Sciences Division of the Oak Ridge
National Laboratory. FIG. 1. SEM images of nanowires patterned along two different crystallo-
b͒
Electronic mail: nsamarth@phys.psu.edu graphic directions: ͑a͒ ͓110͔ and ͑b͒ ͓010͔.
0003-6951/2005/86͑15͒/152505/3/$22.50 86, 152505-1 © 2005 American Institute of Physics
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2. 152505-2 Eid et al. Appl. Phys. Lett. 86, 152505 ͑2005͒
FIG. 2. ͑a͒ Magnetization vs temperature for macroscopic pieces ͑area of
ϳ5 mm2͒ of as-grown and annealed GaAs/ ͑Ga, Mn͒As͑50 nm͒ /
͑10 nm͒GaAs heterostructure. The data are measured in a magnetic field of
50 Oe in-plane upon warming after field cooling at 10 kOe. The sample is
annealed at 190 °C in ultrahigh purity nitrogen gas ͑5N͒ for 5 h. The data
show that TC is not affected by annealing. ͑b͒ Resistance vs temperature
͑measured in van der Pauw geometry͒ for a 1 mmϫ 2 mm mesa patterned
from the same wafer as in ͑a͒. FIG. 3. Temperature dependence of the ͑four-probe͒ resistivity for as-grown
and annealed single wires of ͑a͒ 1 ␮m width and ͑b͒ 70 nm width. Both sets
of wires are patterned along the ͓110͔ direction. Note that the data for the
and patterned, while the other two sets are annealed after annealed 70 nm wire in ͑b͒ are plotted using a different scale ͑axis on right
hand side͒. Plot ͑c͒ shows the same measurements for four individual 70 nm
patterning at 190 °C for 5 h. wires patterned along different crystalline orientations. All wires are pat-
We probe the ferromagnetic phase transition in single terned from the same wafer used in Fig. 2, and the annealing conditions are
wires using four probe measurements of the temperature- identical to those used in Fig. 2.
dependent resistance R͑T͒; it is well-established7,8 that R͑T͒
shows a well-defined peak close to TC in ͑Ga,Mn͒As, espe-
cially for samples with TC Ͻ 100 K. For higher TC, the peak 190 °C for 5 h. The data indicate that annealing results in a
is less well-defined but still yields a reasonable estimate of slight increase in TC, accompanied by a slight increase in the
the ordering temperature ͑typically an overestimate of resistivity of the sample. These observations suggest that an-
ϳ10 K͒.7,8 In addition, we measure the temperature depen- nealing induces modest diffusion of Mn interstitials to the
dence of both the magnetization ͓using a commercial super- free surfaces at the sidewalls of the wire. The diffusion co-
conducting quantum interference device ͑SQUID͒ magneto- efficient ͑estimated to be D ϳ 100 nm2 / h at ϳ190 ° C14͒ is
meter͔ and the resistivity in macroscopic pieces of the parent simply not large enough to allow significant removal of Mn
wafers ͑using the van der Pauw method͒. interstitials from the bulk of the 1 ␮m wide wire within the
Figure 2͑a͒ shows the magnetization as a function of 5 h annealing time. There is however some alteration of the
temperature for both as-grown and annealed ͑ϳ5 mm2͒ defect states, as indicated by the small increase in the high
pieces of a GaAs/ 50 nm͑Ga, Mn͒As/ 10 nm GaAs hetero- temperature resistivity; we do not have an explanation for
structure. As found in earlier studies,15 the cap layer com- this observation but note that a similar effect has been seen
pletely suppresses any annealing-induced enhancement of in previous studies of long anneals, albeit at higher annealing
TC. This suppression of the annealing effect has been attrib- temperatures.11
uted to the formation of a p-n junction at each of the two Figure 3͑b͒ shows the effect of annealing for a 70 nm
GaAs/ ͑Ga, Mn͒As interfaces as some Mn interstitials wide nanowire ͑also patterned along ͓110͔͒. Here, annealing
͑double donors͒ initially diffuse across the boundaries into produces a striking increase in TC of almost 50 K, accompa-
GaAs.14,15 The resulting Coulomb barrier is expected to nied by a correspondingly significant decrease in the resis-
strongly inhibit the further diffusion of Mn interstitials from tivity of the wire. Both these observations strongly suggest
the bulk of the ͑Ga,Mn͒As layer towards the interfaces. The the successful removal of the Mn interstitials from the bulk
temperature dependence of the sample resistance is in com- of the ͑Ga,Mn͒As nanowire. Lateral diffusion of the Mn in-
plete agreement with the magnetization measurements. Fig- terstitials provides the most likely explanation for these ob-
ure 2͑b͒ shows van der Pauw measurements of R͑T͒ for a servations. We note that the time and length scales for diffu-
macroscopic mesa ͑1 mmϫ 2 mm͒; the peak of R͑T͒ does sion observed in our experiments ͑ϳ35 nm in 5 h͒ are
not change upon annealing. In addition, there is no signifi- consistent with earlier low-temperature annealing studies of
cant reduction in the resistivity of the sample with annealing, ͑Ga,Mn͒As epilayers.14
indicating that the hole density has not changed. It is important to examine whether defect-diffusion may
Lateral patterning produces distinctly different behavior. depend on the crystalline direction and/or the nature of the
Figure 3͑a͒ shows the temperature-dependent resistivity for a free surface where the interstitials eventually reside. We
1 ␮m wide wire patterned along the ͓110͔ direction. The data hence pattern four nanowires ͑each with 70 nm width͒, along
are shown for both an as-grown wire and one annealed at ¯
the principal cubic directions ͓͑110͔, ͓110͔, ͓100͔, and
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3. 152505-3 Eid et al. Appl. Phys. Lett. 86, 152505 ͑2005͒
͓010͔͒. All four nanowires are annealed under identical con-
2
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3
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8
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Our findings auger well for prospects of designing 11
S. J. Potashnik, K. C. Ku, S. H. Chun, J. J. Berry, N. Samarth, and P.
͑Ga,Mn͒As-based spintronic device heterostructures such as Schiffer, Appl. Phys. Lett. 79, 1495 ͑2001͒.
12
magnetic tunnel junctions with TC well above 77 K. In addi- K. W. Edmonds, K. Y. Wang, R. P. Campion, A. C. Neumann, C. T.
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K. C. Ku, S. J. Potashnik, R. F. Wang, S. H. Chun, P. Schiffer, N. Samarth,
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14
K. W. Edmonds, P. Boguslawski, K. Y. Wang, R. P. Campion, S. N. No-
This research has been supported by Grant Nos. ONR vikov, N. R. Farley, B. L. Gallagher, C. T. Foxon, M. Sawicki, T. Dietl, M.
N0014-05-1-0107, DARPA/ONR N00014-99-1093, -00-1- Buongiorno Nardelli, and J. Bernholc, Phys. Rev. Lett. 92, 037201
0951, University of California-Santa Barbara Subcontract ͑2004͒.
15
KK4131, and NSF DMR-0305238 and -0401486. This work M. B. Stone, K. C. Ku, S. J. Potashnik, B. L. Sheu, N. Samarth, and P.
Schiffer, Appl. Phys. Lett. 83, 4568 ͑2003͒.
was performed in part at the Penn State Nanofabrication Fa- 16
D. Chiba, K. Takamura, F. Matsukura, and H. Ohno, Appl. Phys. Lett. 82,
cility, a member of the NSF National Nanofabrication Users 3020 ͑2003͒.
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This is confirmed by comparing the resistivity of the wires with van der
Pauw resistivity measured on samples that had no metal.
1 18
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