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C60-based hot-electron magnetic tunnel transistor
M. Gobbi,1,a) A. Bedoya-Pinto,1 F. Golmar,1,2 R. Llopis,1 F. Casanova,1,3 and L. E. Hueso1,3
1CIC nanoGUNE Consolider, Tolosa Hiribidea 76, 20018 Donostia - San Sebastian, Spain
2I.N.T.I.-CONICET, Av. Gral. Paz 5445, Ed. 42, B1650JKA, San Mart!ın, Bs As, Argentina
3IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain
(Received 19 July 2012; accepted 20 August 2012; published online 4 September 2012)
A C60-based magnetic tunnel transistor is presented. The device is based on the collection of
spin-filtered hot-electrons at a metal/C60 interface, and it allows an accurate measurement of the
energy level alignment at such interface. A 89% change in the collected current under the
application of a magnetic field demonstrates that these devices can be used as sensitive magnetic
field sensors compatible with soft electronics.VC 2012 American Institute of Physics.
[http://dx.doi.org/10.1063/1.4751030]
Carbon-based materials, characterized by long spin
coherence times, are promising candidates for next genera-
tion spintronics. On one hand, graphene and carbon nano-
tubes have typically high carrier mobilities and long mean
free paths which translate into extremely large spin diffusion
lengths. Therefore, they are template materials for applica-
tions in which the spin signal needs to be conserved over
long distances.1–4 On the other hand, the combination
between ferromagnetic (FM) metals and molecules offers the
possibility to design spintronic devices with additional func-
tionalities.5 Indeed, diverse organic semiconductors have
been used as spacers between ferromagnetic layers, consti-
tuting the building blocks of spin-valve structures with very
high figures of merit.6–11 Such spin-valve devices can be
used as highly sensitive sensors and can be integrated in soft
electronics circuitry, finding applications in the emerging
field of flexible spintronics.12 In these spintronic devices, the
interface between the ferromagnetic metal and the carbon-
based material plays a fundamental role.13,14 In particular,
it has been shown that the energy level alignment at the
metal/molecule interface affects not only the electrical con-
duction but also the spin polarization of the current.7
In this letter, we report a hot-electron magnetic tunnel
transistor (MTT) which employs C60 as semiconducting
channel, and we show two very different applications of
such device. First, the device structure allows the direct mea-
surement of the metal/molecule energy level alignment in a
ballistic electron emission spectroscopy experiment. Second,
the output electrical current is extremely sensitive to small
magnetic fields, making it suitable for sensing applications.
MTTs are 3-terminal devices with the same scheme of a
metal base transistor, in which a hot-electron current is
injected into the device by an emitter, and a spin-valve base
modulates the amount of current reaching the semiconduct-
ing collector (Fig. 1).15,16 The device is based on the spin fil-
tering effect of a thin FM layer on hot electron currents. The
energy attenuation length of hot-electrons in a FM layer is
spin dependent, so that minority electrons lose their energy
much faster than majority electrons.15 The spin polarization
of the hot-electron current can thus exceed 90% for FM
layers thicker than 3 nm.17 The Schottky barrier at the spin-
valve/semiconductor interface is used to collect only those
electrons that have retained their energy upon travelling
through the base, i.e., the spin filtered electrons. When the
spin-valve base is in parallel (P) state, a fraction of the ma-
jority electrons can travel through both layers without losing
their energy, therefore getting collected as electrical current
at the semiconducting terminal (Fig. 1(a)). When the spin-
valve base is in anti-parallel (AP) state, electrons are filtered
in either one or the other FM layer, leaving ideally a negligi-
ble current in the collector (Fig. 1(b)).15 Under these condi-
tions, the current change in the collector due to the magnetic
state of the spin valve is called magnetocurrent (MC) and is
defined as
MCð%Þ ¼ 100$ ðIp % IapÞ=Iap: (1)
So far, only MTTs based on conventional bulk inorganic
semiconductors such as Si or GaAs have been experimen-
tally demonstrated.16–22 We produced and characterized
MTTs employing C60 as a semiconducting collector, taking
advantage of the properties which make it ideal both for
organic spintronic devices10,11,23 and for metal-base transis-
tors.24 In this study, we demonstrate that the C60-based MTT
perform as state-of-the-art inorganic MTT, with a room tem-
perature MC reaching 89% at zero base-collector bias. More-
over, we show that this MC value can be enhanced by the
FIG. 1. The energy diagram of the device when the spin valve is in the paral-
lel state (a) and in the antiparallel state (b). The emitter-base voltage VEB
determines the alignment of the Fermi Energy at the tunnel junction termi-
nals, while at the metal/C60 interface an energy barrier naturally forms.
Under these conditions, and assuming a perfect spin filtering effect, the cur-
rent enters the C60 collector only when the spin valve is in the parallel state.
a)Author to whom correspondence should be addressed. Electronic mail:
m.gobbi@nanogune.eu.
0003-6951/2012/101(10)/102404/4/$30.00 VC 2012 American Institute of Physics101, 102404-1
APPLIED PHYSICS LETTERS 101, 102404 (2012)
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application of a proper voltage at the collector, reaching in
principle an infinite value due to a negligible current in the
off-state.
The devices were grown in an ultra-high vacuum dual
chamber evaporator, with one chamber dedicated to the
metal e-beam evaporation and the other one to the C60 ther-
mal evaporation. In every chip, six devices were patterned
by deposition through shadow masks on 10$ 10mm2 SiO2
(150 nm)/Si substrates. The emitter is composed by a 10-nm-
thick Al layer, which is in situ plasma-oxidized to form an
insulating AlOx barrier at the interface with the metal base.
The voltage VEB applied at the tunnel junction (TJ) terminals
defines the energy of the injected electrons (Fig. 1). This
energy has to be set well above the Fermi energy of the base
to generate a hot electron current. The base is a metallic spin
valve, composed by a Co(4 nm)/Cu(4 nm)/Ni80Fe20(4 nm)
trilayer grown on top of the Al/AlOx emitter (Fig. 1). The
electrical resistance of the trilayer changes by a few percent
(in our devices typically <1%) depending on the relative
alignment of the magnetization of the two FM layers (either
parallel or antiparallel).25,26 The collector is a 200-nm-thick
C60 layer with an Al top electrode for the actual electric con-
tact placed above.
The current-voltage (I-V) characteristics of our devices
are shown in Fig. 2. Fig. 2(a) shows the emitter-base current
IEB flowing through the TJ when the voltage VEB is swept
between the emitter and the spin-valve base. This non-linear
I-V trace is typical of TJs. Fig. 2(b) shows the current IBC
flowing across the C60 layer when the voltage VBC is applied
directly between the base and the C60 top terminal. The
200-nm-thick C60 layer is highly resistive (R> 20MX at low
bias voltage), and the I-V trace is again highly non-linear
due to the energy barriers at the metal/organic interfaces.
Finally, Fig. 2(c) shows the base-collector current IBC when
the voltage VEB is swept at the emitter-base terminals (with
zero set voltage across the C60, VBC¼ 0). In this case, IBC is
due to those hot electrons that have retained enough energy
to overcome the barrier at the metal/C60 interface. At low
VEB, IBC is extremely small whereas it rises abruptly when
the voltage VEB is higher than the metal/semiconducting bar-
rier. We highlight two important aspects: (1) the I-V trace of
Fig. 2(c) is very different from both I-V traces measured ei-
ther across the TJ and the C60 due to its different physical or-
igin, namely the injection of hot electrons into the C60. (2)
The I-V trace is highly asymmetric and in particular we mea-
sure higher current when the emitter is negatively biased,
i.e., it is injecting electrons and not holes. This is in good
agreement with the well-accepted n-type nature of C60,
meaning that the majority carriers are electrons.27 Further-
more, the attenuation length of hot holes in FM layers is
from 2 to 5 times shorter than the electron attenuation
length.21 This MTT device configuration allows a direct
measurement of the energy level alignment at the C60/base.28
From the threshold voltage after which IBC begins to rise
(electrons with enough energy to enter the collector), we
estimate the NiFe/C60 energy barrier to be around 1.0 eV.
We move now to the magnetic properties of the MTTs.
First, we analyze the case in which a bias voltage is applied
at the emitter-base terminals, with the collector kept at the
same potential of the base. In this situation, the current meas-
ured at the collector displays the typical magnetoresistive
behavior shown in Fig. 3(a). IBC is at its maximum when the
external magnetic field is strong enough to keep the FM
layers in the base parallelly aligned. A minimum is reached
FIG. 2. I-V traces measured across the
tunnel junction (a) and the C60 layer (b).
(c) Hot-electron current IBC measured at
the collector terminal when the emitter-
base voltage VEB is swept and the base-
collector voltage VBC is kept at 0V.
Insets: energy diagrams to illustrate how
the voltages are applied at the different
terminals.
FIG. 3. (a) Base-collector current IBC as a function of the applied magnetic
field (measured at VEB¼%1.4V, VBC¼ 0 V). Red and blue traces corre-
spond to negative to positive and positive to negative field sweep, respec-
tively. On the right axis, the corresponding magnetocurrent. (b)
Magnetocurrent as a function of the emitter-base voltage VEB. Inset: Energy
diagram showing how the voltages are applied for the measurement in (a)
and (b).
102404-2 Gobbi et al. Appl. Phys. Lett. 101, 102404 (2012)
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when the alignment is antiparallel, giving rise to MC values
up to 89% (Fig. 3(a)) at room temperature. As pointed out
above, a naive analysis would expect the current in the AP
state to be exactly zero (Fig. 1(b)), but this ideal case can
hardly be realized in actual devices due to the leakage cur-
rent Iap which flows also in the AP state. For this reason, a
MC value of 89% at room temperature is especially remark-
able, as in many cases the leakage current is too high to even
permit any sizable magnetic effect at room tempera-
ture.16,17,19–22 We highlight here that the injection of hot
electrons through ferromagnetic layers is one of the most ef-
ficient methods to inject highly spin-polarized current into
semiconductors.29,30 Under the assumption of a perfect spin
filtering17 and no leakage current, the hot-electron current
entering the semiconductor is 100% spin-polarized (see Fig.
1(a)). In our devices, considering the relatively low value of
Iap, we expect a high spin polarization of IBC entering the
C60 layer. However, in order to verify whether the spin
polarization is maintained across the C60 layer, a different
device geometry would be necessary.30
The dependence of MC with the emitter-base bias volt-
age is shown in Fig. 3(b). In agreement with the I-V meas-
urements (Fig. 2(c)), no hot-electron MC is recorded in the
C60 collector for VEB> 0. At negative voltages, the MC rises
for VEB<%1V, being VEB¼%1V the minimum bias
needed to inject hot-electrons into the C60. The MC bias de-
pendence is non-monotonic and the maximum value of 89%
is reached at VEB¼%1.5V, while for more negative vol-
tages the MC decreases. This behavior has been already
observed in fully inorganic MTTs and has been explained
using a model based on spin-dependent inelastic scattering in
the FM layers of the base.19
Finally, we demonstrate that MC can be modulated by
the application of a proper voltage between the base and the
collector. The highest reported values of MC are measured at
low temperatures when the leakage current flowing into the
collector in the AP state is minimized.15–22 We show that a
similar outcome can be obtained at room temperature by
applying a base-collector voltage VBC. Fig. 4(a) shows the
MC change by varying VBC while keeping VEB constant. In
the device shown in Fig. 4(a) we measured 50% MC with
VBC¼ 0. By setting VBC 6¼ 0, an additional current contribu-
tion flows between the base and the collector. In the case of
VBC< 0 (i.e., accelerating the electrons in the C60 layer), the
current reaching the collector increases because some elec-
trons enter the C60 directly from the base. However, since
these electrons do not have a hot origin, they do not actually
improve the MC and they just add to the leakage current,
effectively lowering the MC ratio (Fig. 4(b), negative vol-
tages). In the case VBC> 0, the current in the AP state (Iap)
shifts towards zero (Fig. 4(b)). As a consequence, the MC
increases to values higher than 50% (Fig. 4(a), positive volt-
age). In that way, the MC curve can be arbitrarily displaced
choosing the right VBC value. In Fig. 4(b), we show how the
MC curves evolve for three different selected VBC; the red
curve corresponding to VBC¼ 0.135V has very low current
in the AP state, giving rise to extremely high MC (8550%).
In conclusion, the realization of a magnetic tunnel tran-
sistor employing C60 as semiconducting layer has been dem-
onstrated, with performances comparable to conventional
inorganic MTTs. We have recorded a zero-collector-bias
magnetocurrent of 89% at room temperature, which can be
increased to any arbitrary value by applying a voltage at the
collector terminal. The device geometry allowed us to deter-
mine the energy barrier at the NiFe/C60 interface, which is
found to be 1 eV. We expect our results to be reproduced by
other molecular semiconductors, opening additional path-
ways for the development of organic spintronics.
This work is supported by the European Union 7th
Framework Programme (NMP3-SL-2011-263104-HINTS
and PIRG06-GA-2009-256470), by the European Research
Council (Grant No. 257654-SPINTROS), by the Spanish
Ministry of Science and Education under Project No.
MAT2009-08494, and by the Basque Government through
Project No. PI2011-1.
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