Schottky barrier height of ferromagnet/Si(001) junctions

Article abstract of DOI:10.1063/1.2337562

The Schottky barrier height of ferromagnetic metal/semiconductor junctions is one of the most important parameters for the performance of semiconductor-based spin-electronic devices. The authors investigated the Schottky barrier height φ_B of ferromagnet/Si(001) junctions with various ferromagnetic metals (CoFe, CoFeB, and MnAs), and found that φ_B of epitaxial MnAs/Si(001) junctions fabricated by molecular beam epitaxy was 0.16 eV for electrons, which is much lower than φ_B (～0.7 eV) of CoFe/Si(001) and CoFeB/Si(001) junctions. This implies that MnAs is a promising ferromagnetic material for Si-based spin-electronic devices, especially for the source and drain of spin metal-oxide-semiconductor field effect transistors.

Full text of DOI:10.1063/1.2337562

Schottky barrier height of ferromagnet/Si(001) junctions
Kuniaki Sugiura, Ryosho Nakane, Satoshi Sugahara, and Masaaki Tanaka
Citation: Applied Physics Letters 89, 072110 (2006); doi: 10.1063/1.2337562
View online: http://dx.doi.org/10.1063/1.2337562
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APPLIED PHYSICS LETTERS 89, 072110 ͑2006͒
Schottky barrier height of ferromagnet/Si„001… junctions
Kuniaki Sugiura
Department of Electronic Engineering, The University of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-8656, Japan
Ryosho Nakane
Department of Electronic Engineering, The University of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-8656, Japan and Japan Science and Technology Agency, 4-1-8 Honcho,
Kawaguchi-shi, Saitama 332-0012, Japan
Satoshi Sugahara
Department of Frontier Informatics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa-shi,
Chiba 277-8583, Japan
a͒
Masaaki Tanaka
Department of Electronic Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8656, Japan and Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi-shi,
Saitama 332-0012, Japan
͑
Received 24 May 2006; accepted 13 July 2006; published online 18 August 2006;
corrected 18 December 2006͒
The Schottky barrier height of ferromagnetic metal/semiconductor junctions is one of the most
important parameters for the performance of semiconductor-based spin-electronic devices. The
authors investigated the Schottky barrier height ␾ of ferromagnet/Si͑001͒ junctions with various
B
ferromagnetic metals ͑CoFe, CoFeB, and MnAs͒, and found that ␾ of epitaxial MnAs/Si͑001͒
B
junctions fabricated by molecular beam epitaxy was 0.16 eV for electrons, which is much lower
than ␾ ͑ϳ0.7 eV͒ of CoFe/Si͑001͒ and CoFeB/Si͑001͒ junctions. This implies that MnAs is a
B
promising ferromagnetic material for Si-based spin-electronic devices, especially for the source and
drain of spin metal-oxide-semiconductor field effect transistors. © 2006 American Institute of
Physics. ͓DOI: 10.1063/1.2337562͔
3
,8
Recently, Si-based spin electronics has attracted consid-
erable attention, because it can lead to a variety of spin-
electronic devices which utilize spin degrees of freedom and
well-established Si technologies. Moreover, due to its nature,
electrons in Si can have much longer spin lifetime compared
ferromagnet.
Furthermore, like the ordinary Schottky
MOSFETs, low Schottky barrier height is required for high-
performance electrical output characteristics of the spin
MOSFETs. From the viewpoints of both the spin-dependent
characteristics and electrical performance, thermally stable
ferromagnetic metal/Si junctions with low Schottky barrier
height are strongly needed.
Generally, many difficulties arise in
semiconductor junction consisting of materials with different
crystal structures. In many cases, there exists midgap Fermi-
7
1
with those in compound semiconductors, resulting in unique
and highly spin-dependent transport performances. To estab-
lish Si-based spin electronics, active devices will be indis-
pensable as its block element, such as ordinary metal-oxide-
semiconductor field effect transistors ͑MOSFETs͒ in very
large scale integration technologies. In this respect, spin
a
metal/
9
level pinning at the interface, and the formation of intermix-
2
,3
MOSFETs, which consist of a MOS gate capacitor and
ferromagnetic source and drain, are promising. It was shown
that the spin MOSFETs can be potentially used for reconfig-
ing layer proceeds easily. These phenomena are frequently
observed in Co/Si and Fe/Si junctions, indicating that they
would not be suitable for Si-based spin-electronic devices, in
spite of their spin polarization of ϳ0.4. Ferromagnetic
MnAs is a good candidate, because it has relatively high spin
polarization of ϳ0.5, and epitaxial MnAs/Si͑001͒ junc-
tions with an atomically flat interface can be fabricated by
molecular beam epitaxy ͑MBE͒. Since the crystal structure
of MnAs is hexagonal NiAs-type, midgap Fermi-level pin-
ning in MnAs/Si͑001͒ is also anticipated. However, recent
reports showed that the Schottky barrier of a metal/Si junc-
tion can be lowered through the interface modification using
Se passivation and As segragation. Thus, we can expect
such low Schottky barrier height in the MnAs/Si junction
due to As surface modification at the initial stage of the MBE
growth. In this letter we investigate the Schottky barrier
height of epitaxial MnAs/Si͑001͒ junctions and compare
4
5
urable logic gates and nonvolatile memory, since the output
characteristics can be controlled by both the gate voltage and
magnetization of the source and drain.
10
11
The proposed spin MOSFETs have various types, which
3
are categorized by the structure. One of the most practical
1
2
devices is a spin MOSFET with ferromagnetic metals for the
source and drain, which can be simply realized by substitut-
ing ferromagnetic metals for the source and drain of an or-
6
,7
dinary Schottky MOSFET. It is well recognized that spin
injection from a ferromagnetic metal into a semiconductor is
the most important issue for such spin-electronic devices,
and that the spin injection efficiency is considerably affected
by the ferromagnet/semiconductor interface quality, its
Schottky barrier height, and the spin polarization of the
1
3
14
1
5
a͒Author to whom correspondence should be addressed; electronic mail:
with that of Co Fe /Si͑001͒ and ͑Co Fe ͒ B /Si͑001͒
90 10 90 10 70 30
masaaki@ee.t.u-tokyo.ac.jp
junctions.
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003-6951/2006/89͑7͒/072110/3/$23.00 89, 072110-1 © 2006 American Institute of Physics
29.105.215.146 On: Tue, 23 Dec 2014 11:11:19
0
1

072110-2
Sugiura et al.
Appl. Phys. Lett. 89, 072110 ͑2006͒
FIG. 1. Room temperature I-V characteristics of Co Fe10/Si ͑dotted line͒,
90
͑
Co Fe ͒ B30/Si ͑dashed line͒, and MnAs/Si ͑solid line͒ junctions, with
90 10 70
2
FIG. 2. Arrhenius plots ln͑I /T ͒ vs 1/T of Co Fe10/Si ͑solid circles and
R
90
round shaped structures 200 ␮m in diameter.
dotted line͒, ͑Co Fe ͒ B30/Si ͑open circles and dashed line͒, and
9
0
10 70
MnAs/Si ͑open square and solid line͒ junctions, where the applied bias
voltage was −0.3 V. The junction shapes were circles 200 ␮m in diameter
Schottky barrier height of a metal/Si junction can be
estimated by I-V-T measurements. In an ideal Schottky di-
ode, the current consists of a thermionic emission current
2
for Co Fe and ͑Co Fe ͒ B samples, and a square of 500ϫ500 nm
9
0
10
90 10 70 30
for the MnAs sample.
only. Then the reverse bias current I of the Schottky diode
can be described as
R
ϳ1015 cm ͒ Si substrates in the temperature range higher
than room temperature.
−3
−
q␾
B eff
*
*
2
Ferromagnetic metal/Si Schottky diodes were fabricated
on phosphorus-doped n-type Si͑001͒ substrates with a resis-
tivity of 10 ⍀ cm ͑with a doping concentration of 4.45
I = SA T expͩ ͪ
,
͑1͒
R
kT
where S is the area of the Schottky junction, A** is the modi-
1
5
−3
ϫ10 cm ͒. Before the deposition of the metallic films, the
Si substrates were chemically cleaned with acetone, followed
by H SO +H O solution to remove metallic and organic
fied Richardson constant, k is the Boltzmann constant, q is
the electronic charge, and ␾B eff is the effective Schottky bar-
rier height. Since the effective Schottky barrier height con-
tains the image-charge-induced barrier height lowering ⌬␾,
2
4
2
2
contaminations, and then the resulting chemical oxide was
etched with buffered HF solution. Co Fe and
Co Fe ͒ B films with 40 nm thickness were deposited
9
0
10
the Schottky barrier height ␾ can be calculated as
B
͑
9
0
10 70 30
+
␾B = ␾B eff + ⌬␾,
͑2͒
on the Si͑001͒ substrates by an Ar rf magnetron sputtering
−6
system with a background pressure of 5ϫ10 Torr. During
the deposition, the Ar pressure was 7.6ϫ10− Torr, and the
substrate temperature was kept at room temperature. The
compositions of Co, Fe, and B were estimated by energy
dispersive x-ray spectroscopy and x-ray photoelectron spec-
troscopy. Also, we grew a 40 nm thick MnAs thin film on the
Si͑001͒ substrate in our standard III-V based MBE chamber
at 3ϫ10− Torr, equipped with a Mn effusion cell and a
reflective high energy electron diffraction ͑RHEED͒ system.
After thermal treatment at 600 °C for 10 min, the substrate
3
q3N
1/4
kT
q
⌬
␾ =
ͫ
ͩ
␾ − V − V −
ͪ
ͬ
,
͑3͒
␲2␧3
B
n
8
1
6
by assuming ␾
ӷ⌬␾ and ␾ Ϸ␾
.
Here N is the dop-
B eff
B
B eff
ing concentration, ␧ is the dielectric constant of Si, V is the
applied reverse bias, and V is the position of the Fermi level
relative to the bottom of the conduction band of Si. Equation
n
10
͑
1͒ can be transformed into
I
q␾B eff 1
R
2
**
was cooled to 275 °C and then exposed to an As flux to
4
lnͩ ͪ= ln͑SA ͒ −
.
͑4͒
12,18
T
k
T
prevent the interfacial MnSi formation.
At that time, the
Si surface showed a ͑2ϫ2͒ reconstruction. Then, a Mn flux
was supplied to start MnAs growth with a rate of 40 nm/h.
The MnAs surface exhibited fourfold symmetric streaky
RHEED patterns, indicating that the epitaxial relationship
Therefore, the effective Schottky barrier height can be
estimated from the linear slope in the Arrhenius plot
2
ln͑I /T ͒ vs 1/T.
R
In the case of an actual Schottky diode, the reverse bias
current also contains a thermionic-field emission current and
a field emission current. Equation ͑4͒ can be used when the
thermionic emission current is dominant compared with the
thermionic-field emission current and the field emission cur-
rent. The parameter kT/E00 gives a measure of the dominant
19
between MnAs and Si was type II.
After the deposition of the magnetic films on the Si sub-
strates, these samples were processed into round-shaped
Schottky diode structures 200 ␮m in diameter by standard
photolithography and wet chemical etching. For the MnAs
sample, Schottky diode structures with squares of 500
1
7
current in the Schottky diode. E00 is given by
2
ϫ500 nm in size were also prepared by electron beam li-
+
thography and Ar ion milling.
qh
N
E = ͩ ͪ
,
͑5͒
0
0
*
I-V-T measurements were performed by a standard
manual prober system with a hot chuck stage and a Hewlett-
Packard 4156A semiconductor parameter analyzer. The mea-
surement temperature was set from 300 to 450 K to enhance
the thermionic emission current. Figure 1 shows the I-V
characteristics of the Co90Fe10/Si, ͑Co90Fe10͒70B30/Si, and
MnAs/Si Schottky junctions with 200 ␮m in diameter at
4
␲ m ␧
*
where h is the Plank constant and m is the effective mass of
carriers. For kT/E ӷ1 the thermionic emission current is
dominant, while for kT/E ϳ1 the thermionic-field emission
current or the field emission current is dominant. In order to
0
0
0
0
meet the condition kT/E ӷ1, I-V-T measurements should
0
0
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be performed in samples with weakly doped ͑N
room temperature. The I-V characteristics of the Co Fe /Si
90 10
1
29.105.215.146 On: Tue, 23 Dec 2014 11:11:19

072110-3
Sugiura et al.
Appl. Phys. Lett. 89, 072110 ͑2006͒
FIG. 3. Schottky barrier heights of the Co Fe10/Si ͑solid circles and dotted
9
0
FIG. 4. SIMS depth profiles of the MnAs/Si junction, for Mn ͑dotted line͒,
As ͑solid line͒, and Si ͑dashed line͒. The dot-dashed circle represents the Mn
and As profiles at ϳ58 nm.
curve͒, the ͑Co Fe ͒ B30/Si ͑open circles with dashed curve͒, and the
9
0
10 70
MnAs/Si ͑open square and solid curve͒ junctions estimated from the
Arrhenius plots at each reverse bias.
and ͑Co Fe ͒ B /Si junctions exhibited clear rectification
files of Mn, As, and Si atoms in the MnAs/Si junction are
shown in Fig. 4. In order to eliminate the ion knock-on ef-
fect, the etching in the SIMS measurement was done from
the backside to the surface of the sample. Although both the
profiles of As and Mn atoms show the same exponentialde-
cay length near the interface of the junction ͑steep slope
region at 46–55 nm from the surface͒, the profile of the As
atoms has a gentle slope at around 58 nm compared to that
of the Mn atoms. This result indicates that a substantial
9
0
10 70 30
characteristics, and their ideality factors were 1.08 and 1.09.
On the other hand, the MnAs/Si junction showed very little
rectification behavior, suggesting that the MnAs/Si junction
has low Schottky barrier height for electrons. In evaluating
such low Schottky barrier height, the influence of a serial
parasitic resistance, which consists of a resistance of the Si
substrate and the measurement system, needs to be much
smaller than the resistance of the Schottky junction. Thus,
the MnAs/Si junction with a square shape of 500
1
8
19
−3
amount of As atoms ͑10 –10 cm ͒ near the interface is
likely to be present mainly on the Si side, and it is a possible
origin of the low Schottky barrier height.
2
ϫ500 nm was used in the following experiments so that the
resistance of the Schottky junction becomes higher.
2
The Arrhenius plots plots ͓ln͑I /T ͒ vs 1000/T͔ at a re-
R
This work was partly supported by Industrial Technol-
ogy Research Grant Program of NEDO, SORST of JST,
Grant-in-Aids for Scientific Research, IT Program of
RR2002 of MEXT, and Kurata-Memorial Hitachi Science &
Technology Foundation.
verse bias of −0.3 V of the Co Fe /Si, ͑Co Fe ͒ B /Si,
9
0
10
90 10 70 30
and MnAs/Si junctions are shown in Fig. 2, where each plot
is fitted with a linear function. In these measurement condi-
tions, kT/E00 were 34.9 for 300 K, 46.5 for 400 K, and 52.4
for 450 K, which were much larger than 1, indicating that the
dominant current was thermionic emission current and that
the present Schottky barrier height evaluation is valid.
1
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8
9
2
0
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23
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using 3 keV Cs ion beam incident at 60°. SIMS depth pro-
͑2004͒.
1
29.105.215.146 On: Tue, 23 Dec 2014 11:11:19