Regular Articles

Highly Efficient Gate Controllabilityof Rashba Spin-orbit Interaction in a
Gate-all-around InAs Nanowire
MOSFET
Keiko Takase, Guoqiang Zhang, Kouta Tateno,
and Satoshi Sasaki
Abstract

Indium arsenide (InAs) is a III-V semiconductor with high mobility that has attracted much attention
as a next-generation semiconductor to replace the current silicon technology. At the same time, InAs is
known to have a large spin-orbit interaction that causes electron spin precession even without an external
magnetic field. Here, we report on an InAs nanowire metal-oxide-semiconductor field-effect transistor(MOSFET) with a gate-all-around structure, where the gate electrode is arranged coaxially with the InAs
channel. We not only show that our device has good FET properties such as a high ON/OFF ratio andhigh mobility but also demonstrate that the Rashba spin-orbit interaction is largely controlled with asmall gate voltage range, with its efficiency being ten times larger than those obtained for previously
reported III-V semiconductor MOS and Schottky FETs. This indicates that our FET is promising as aprototype of a low-power-consumption spin FET, in which transistor ON/OFF is controlled as aconsequence of electron spin flips.
Keywords: nanowire, spin-orbit interaction, spin FET

1. Introduction
Group III-V semiconductors such as indium arsenide (InAs) and InGaAs (indium gallium arsenide)
have been extensively studied as next-generation
high-mobility semiconductors that can replace thecurrent silicon (Si) technology. Another direction ofresearch on III-V semiconductors that has been offundamental interest for decades is their large spinorbit interaction, which acts as an effective magnetic
field and thus can rotate electron spin even in theabsence of an external magnetic field.
As these two research directions merged, a conceptof the spin field-effect transistor (FET) was proposedby Datta et al. in 1990 [1]. In contrast to a normaltransistor, in which transistor ON/OFF is defined by
NTT Technical Review

Vol. 17 No. 2 Feb. 2019the changing electrical current using gate voltage, aswitch for a spin FET relies on reversed spin directionin the channel, which is controlled via the spin-orbit
interaction using gate voltage. The required key effectis the Rashba spin-orbit interaction, which is proportional to the applied electric field. With this interaction, the spin precession can be accessed by the gate
voltage, and the device can consequently be used as aspin transistor.
A spin FET is conceptually shown in Fig. 1. The
spin FET is composed of ferromagnetic electrodes
and an electron channel formed in a semiconductorthat has a strong spin-orbit interaction. When spin is
injected from one of the ferromagnetic electrodes androtated by π via the spin-orbit interaction, the spin
becomes antiparallel to the spin on the other side of56

Regular Articles

Ferromagnetic
electrode

Gate metal rotation

Large
spin-orbit
interaction

2 rotation

Antiparallel[off]

Parallel[on]Fig. 1. Concept of a spin FET. The spin flip in the channel is controlled by the gate voltage via spin-orbit interaction.

the ferromagnetic electrodes, indicating that electrical current is no longer flowing.
In contrast, when the electron spin rotates up to 2π
due to the varied spin-orbit interaction, the spin turns
so it is parallel to the spin in the other electrode, indicating that electron current has started to flow. Thistransistor operation resulting from spin precession
can be realized by changing the gate voltage if we canuse the Rashba spin-orbit interaction.
This type of spin FET is expected to provide many
benefits in engineering as well as to continue to be offundamental interest. For instance, we can expect toachieve a lower-power-consumption transistor when
the spin flip voltage is lower than the pinch-off voltage in normal transistors. In addition, when the spinorbit interaction is large enough, we can integrate thespin FETs using the present Si technology.
Based on this background, various III-V semiconductors have been investigated to examine whether
spin-orbit interaction is indeed controlled by the gate
voltage. Nitta et al. demonstrated for the first timethat the spin-orbit interaction is varied by the gate
voltage using two-dimensional electron gas formed
in a III-V semiconductor quantum well [2]. Later,
Koo et al. demonstrated that spin injection and gate
control of the spin-orbit interaction occurred simultaneously for the same device [3]. Moreover, muchsmaller devices have been developed from quantum
wire [4] and bottom-up nanowires [5–9] with the aimof achieving lower power consumption and high integration. However, the Schottky FETs and metalNTT Technical Review

Vol. 17 No. 2 Feb. 2019oxide-semiconductor field-effect transistors (MOSFETs) used in the previous reports have not greatly
improved the gate controllability of spin-orbit interaction.
In this article, we report that a gate-all-around(GAA) InAs nanowire MOSFET that we recently
developed has superior gate controllability of thespin-orbit interaction, which is ten times higher thanthat obtained for MOSFET or Schottky FETs [10].
Our demonstration will open the way toward developing low-power-consumption spin FETs.
2. Experimental
The GAA InAs nanowire is schematically shown in
Fig. 2(a). The InAs nanowire was grown using thevapor-liquid-solid method [11]. The diameter of the
nanowire is about 100 nm, and the gate length isabout 3.3 µm. The GAA structure was prepared asfollows. We first grow Al 2O 3/HfO 2 (aluminium
oxide: 2 nm/hafnium oxide: 4 nm) as a gate insulator
around the nanowire and then put it on a pre-patterned
SiO2 (silicon dioxide)/Si substrate and deposit thegate metal by an electron beam. A coaxial gate structure using a thin high-k gate insulator enables us toapply a strong electric field to the nanowire channel.
The details of the fabrication procedure are describedin other reports [10, 12, 13].
A TEM (transmission electron microscope) imageof our device is shown in Fig. 2(c). We can see that athin gate insulator and surrounding gate metal are57

Regular Articles(a)Gate insulator

(b)

(c)

InAs
NW

D

InAs

Metal

G
InAs
nanowire

SiO2

Vg

S
Vsd

(d)

Au
Ti
HfO2
Al2O3

(e)

50 nm

(f)

(g)

(h)

(i)

50 nm
Al: aluminium
Al2O3: aluminium oxide
As: arsenide
Au: gold
Hf: hafnium

HfO2: hafnium oxide
In: Indium
InAs: Indium arsenide
NW: nanowire
SiO2: silicon dioxide

Ti: titanium
Vg: gate voltage
Vsd: source-drain voltage

Fig. 2. (a) Schematic illustration of our GAA InAs nanowire MOSFET. (b) Schematic cross section of our device. (c) TEM(transmission electron microscope) image of the cross section of our device. (d)–(i) EDS (energy dispersive X-ray
spectroscopy) images taken at the same time as image shown in (c). These images correspond to each element in
the device.

formed around the nanowire. These geometries wereconfirmed by energy dispersive X-ray spectroscopy,
which also indicates that there are no notable impurities or migration. The measurements were performedat temperatures ranging from room temperature to
1.5 K.
3. Results and discussion
In this section, we report the results of our experiments and explain their significance.
3.1 FET characteristics
The transfer and output characteristics obtained atroom temperature and 1.5 K are presented inFig. 3(a)–(d). As shown in Fig. 3(a) and (c), the electrical current increases rapidly as the gate voltage
increases, indicating that the device is acting as atransistor. The ON/OFF ratios extracted from thesedata are 104 and 105 at room temperature and 1.5 K.
The subthreshold swing defined as SS ≡ dVg/dlog(Isd)is 350 mV/dec and 25 meV/dec at room temperatureand 1.5 K. The mobility ranges from 400 to 1200
cm2/Vs. These values are comparable to those reported for back-gated InAs nanowires fabricated by other
NTT Technical Review

Vol. 17 No. 2 Feb. 2019groups [14–16]. This demonstrates that our GAA
InAs nanowire device shows good FET performance.
3.2 Gate controllability of spin-orbit interaction
We next investigate the strength of the spin-orbit
interaction and its gate controllability. For this aim,
we measure magnetoconductance to examine whether weak localization and weak antilocalization effects
are observed for our device. Weak localization (antilocalization) is a quantum correction effect, whichhappens in the absence (presence) of the spin-orbit
interaction. When the spin-orbit interaction is negligibly small, time-reversal symmetry is protected atzero magnetic field, and thus, electron waves propagating in the clockwise and anticlockwise directions
produce constructive interference, leading to a weak
localization. Then when the magnetic field is applied,
time-reversal symmetry is broken, and the conductance is increased with the increasing magnetic field.
In contrast, the opposite behavior occurs when the
spin-orbit interaction is strong. At zero magnetic
field, the spin flips due to the spin-orbit interaction,and thus, destructive interference occurs rather thanconstructive interference. This is known as a weak
antilocalization effect, which results in decreasing
58

Regular Articles

20

10

10

dec

10−1

SS

10−2
10−3
−2

0
−2

(c)

−1

0
1
Vg (V)

1.5 K

2.5

1.0

2

(d)

Vsd = 50 mV
40
30
20
10

2.0
1.5

2

0
1
−1
Gate voltage Vg (V)

0.5

1
10−2
10−4
10−6
−2

0.0
−2

−1

1
0
Vg (V)

−1

2

0

RT

Vg = 2 V
1.6
1.2
0.8

40
30
20
10
0
0.0

0.1
0.2
0.3
0.4
Source-drain voltage Vsd (V)0.5

Vg = 2 V
1.6
1.2
0.8

15 1.5 K

10102
lsd (µA)

Source-drain current lsd (µA)

1

Source-drain current lsd (µA)4030(b) 50Vsd = 500 mV
400
300
200
100

Source-drain current lsd (µA)

RT

lsd (µA)

Source-drain current lsd (µA)(a) 50

1

2

5

0
0.0

0.1

0.2

0.3

0.4

0.5

Source-drain voltage Vsd (V)Gate voltage Vg (V)RT: room temperature
SS: subthreshold swing

Fig. 3. (a), (c) Transfer characteristics and (b), (d) output characteristics, which were obtained at room temperature and
1.5 K.

magnetoconductance with the increasing magnetic
field.
Such weak localization and antilocalization effects
were first considered in two-dimensional electron
systems by Hikami et al. [17] and are now observedin various two-dimensional materials that possess
strong spin-orbit interaction. Moreover, these effects
are also observed with one-dimensional systems. In
disordered one-dimensional systems, magnetoconductance (defined here as the difference ∆G from thezero-field conductance) is described with the following equation:

[(
(

2
2
∆G = − 2e 3 12 + 4 2 + W 4
3lso
hLg 2 lφ
3lB2
− 1 12 + W 4
2 lφ
3lB

) ]
−1/2

,

)

−1/2

(1)

where h is Planck’s constant, Lg is the gate length, lφ
is the phase relaxation length, lso is the spin-orbit
length, W is the nanowire diameter, and lB is the magnetic length given by lB = h/(2πeB) (e: electron
charge, B: magnetic field).
The data are analyzed using this model. In Fig. 4(a),NTT Technical Review

Vol. 17 No. 2 Feb. 2019

∆G is shown as a function of B for various values ofgate voltage Vg. We can see that ∆G with respect to B
shows a dip-to-peak structure with increasing gate
voltage, corresponding to the transition from weak
localization to weak antilocalization. Moreover, themean free path obtained for our device is about 10 nm
and is much smaller than the nanowire diameter ofapproximately 100 nm, so the system indeed satisfies
the condition in which the model can be applied. Thelines in Fig. 4(a) indicate that our data fit nicely withthe one-dimensional model described by Eq. (1).
From this fitting, we can extract lso and lφ, which areplotted as a function of Vg as shown in Fig. 4(b). This
clearly shows that lso varies significantly with Vg andthat lso becomes smaller than lφ around the crossoverfrom weak localization to weak antilocalization.
3.3 Comparison with previous reports
Next, we compare our results with those obtainedfor previously reported III-V semiconductors withvarious sample geometries. The dependence of lso on
Vg measured for various InAs nanowires is summarized
in Fig. 5(a). Our results plotted in different colors(blue and orange) are obtained for the same sample
59Regular Articles(a) 1.5(b)
0.23 V

0.28 V

0.0

0.36 V

−0.2

0.5

lso
l

400

0.2

0.59 V
0.88 V

0.00

0.03
B (T)

0.28 V

0.06

0.0
0.59 V
0.88 V−0.5−1.0

1.47 V−0.10 −0.05 0.00

0.05

0.10

Magnetic field B (T)Spin-orbit length lso
Phase relaxation length l (nm)Magnetoconductance G (µS)

1.0

G (µS)

0.4

300

200

100

0
0.0

0.5

1.0

1.5

Gate voltage Vg (V)Fig. 4. (a) Magnetoconductance with respect to magnetic field for different gate voltages. (b) Spin-orbit length and phaserelaxation length as a function of gate voltage.

with different cooling times, which confirmed therobustness of the sample as well as reproducibility
after thermal cycles. We can see that there is little
modulation in lso for InAs nanowires with conventional back gate or top gate structures [5–9], even if agate voltage over 10 V is applied.
In contrast, our GAA and the ion-gated device
developed by Liang et al. [18] show large modulationin lso within a much smaller Vg range. This is because
the GAA and ion-gated structures can change theelectric field over a wide range, indicating that the
Rashba spin-orbit interaction, which is proportional
to the electric field, varies widely with a small gate
voltage.
We then deduce the Rashba coupling parameter aRfrom lso. Here, aR is defined in the Hamiltonian representing the Rashba spin-orbit interaction,
H = aR σ · (p × ẑ) (h̄: Planck’s constant divided byh̄
2π, p: momentum, ẑ: direction parallel to the electricfield). We can intuitively understand that an effective
magnetic field, which occurs in the direction perpendicular to the momentum and electric field, interacts
with spin; consequently, the strength of the spin-orbit
interaction is determined by the size of aR. Here, aRh̄2(m*:can relate with lso in the formula, aR =2m*lsoeffective mass).
In Fig. 5(b), aR obtained for our device is comNTT Technical Review

Vol. 17 No. 2 Feb. 2019

pared with those reported for one-dimensional andtwo-dimensional FETs that have been investigated in
the context of spin FETs. The gate modulation efficiency of aR for our device is more than ten times
larger than those obtained for back-gated InAs
nanowire [5] and Schottky FETs fabricated fromvarious III-V semiconductor quantum wells [2, 3].
Although our GAA MOSFET is a bit less effective
than the ion-gated device, our device has great significance in that it demonstrates that such high controllability and large Rashba parameter are obtainedusing a MOSFET, which is much faster and morestable than ion-gated devices.
We finally note the electric field ER associated with
the Rashba spin-orbit interaction, which is given by
ER = aR/(a0e) (a0: Rashba coefficient, a0 = 1.17 nm2for InAs [19]). We obtain the ratio to the peak electricfield E0 that is expected from the gate geometry. The
results of ER vs. E0 obtained for our GAA device andthe ion-gated device are plotted in Fig. 5(c). With
both devices, ER is reduced more than E0 is. The ratio
stays at 5% for the entire Vg range (Fig. 5(d)). This
indicates that the screening effect due to the interface
states that may be included in the devices is nearly thesame for both devices.
4. Summary
We demonstrated that our GAA device has higher
60

Regular Articles(a) 400
Spin-orbit length lso (nm)Back gate MOS

GAA MOS

350

This work

Hansen et al.
Dhara et al.
Liang et al.
Roulleau et al.
Scherübl et al.

Ion gate
Liang et al.

300
250
200
150
100
50
0

0

2

4

6
8
Gate voltage Vg (V)(c)

3

Ion gate
3

GAA MOS(This work)

2

Back gate

0

1

0.5

1.0

1.5

2.0

0
2.5

2.0
1.0
0.0

0

10 20 30 40 50 60
Peak electric field E0 (×107 V/m)
Ion gate
This work

0.06
0.03
0.00
0.0

Gate voltage Vg (V)14

Ion gate
This work

0.09

InGaAs QWInAs 2DEG

0.0

12

(d)

ER/E0

1

2

3.0

ER (×107 V/m)

4

ER (×107 V/m)

Rashba parameter R (×10−11 eVm)

(b)

10

0.4

0.8

1.2

1.6

Gate voltage Vg (V)QW: quantum well
2DEG: two-dimensional electron gas

Fig. 5. (a) Comparison of dependence of lso on Vg for our device and those reported previously [5–9]. (b) Comparison of the
Rashba parameter vs. gate voltage for our device and previously reported III-V semiconductor devices [2, 3, 5, 18].
(c) ER as a function of E0 for our device and the ion-gated device reported by Liang et al. [18]. (d) ER/E0 ratio as afunction of Vg for the devices in (c).

gate controllability of the spin-orbit interaction thanthose obtained for standard MOS or Schottky FETs
fabricated from one- and two-dimensional III-V
semiconductors. The biggest advantage is that highgate controllability and the large Rashba spin-orbit
interaction are obtained at a small gate voltage usinga MOSFET structure. This opens the way towardrealizing nanoscale spin FETs.

References
[1]
[2]
[3]
[4]
[5]
[6]

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Vol. 17 No. 2 Feb. 2019

S. Datta and B. Das, “Electronic Analog of the Electro-optic
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J. Nitta, T. Akazaki, H. Takayanagi, and T. Enoki, “Gate Control of
Spin-orbit Interaction in an Inverted In 0.53Ga0.47As/In0.52Al0.48As
Heterostructure,” Phys. Rev. Lett., Vol. 78, No. 7, 1335, 1997.
H. C. Koo, J. H. Kwon, J. Eom, J. Chang, S. H. Han, and M. Johnson,“Control of Spin Precession in a Spin-injected Field EffectTransistor,” Science, Vol. 325, 1515, 2009.
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Keiko Takase

Kouta Tateno

Senior Research Scientist, NTT Basic ResearchLaboratories.
She received a B.S., M.S., and Ph.D. in physics
from the University of Tokyo in 2004, 2006, and2009. During 2006–2009, she was a research
fellow (DC1) of the Japan Society for the Promotion of Science (JSPS). She joined NTT Basic
Research Laboratories in 2009. Since then, she
has been studying electrical transport in lowdimensional semiconductors. She is a member of
the Physical Society of Japan (JPS).

Guoqiang Zhang

Senior Research Scientist, NTT Basic ResearchLaboratories.
He received a B.S. in electronic ceramic materials science and an M.S. in semiconductor physics and chemistry from Zhejiang University, P. R.
China, in 1997 and 2000, and a Ph.D. in electronic materials science from Shizuoka University, Japan, in 2004. He joined NTT Basic
Research Laboratories in 2006. Since then, he
has been working on the growth and characterization of nanowire-based nanostructures. He is amember of the Japan Society of Applied Physics(JSAP) and the Japanese Association for Crystal
Growth.

NTT Technical Review

Vol. 17 No. 2 Feb. 2019

Senior Research Scientist, NTT Basic ResearchLaboratories.
He received a B.S., M.S., and Ph.D. in chemistry from the University of Tokyo in 1991, 1993,
and 2001. He joined NTT Opto-electronics Laboratories in 1993. His current research interests
include the fabrication and physics of nanostructures using semiconductor nanowires. He is amember of JSAP.

Satoshi Sasaki

Senior Research Scientist, NTT Basic ResearchLaboratories.
He received a B.S., M.S., and Ph.D. in applied
physics from the University of Tokyo in 1988,
1990, and 1993. He joined NTT Basic ResearchLaboratories in 1993. Since then, he has been
studying transport properties of mesoscopic
devices, especially quantum dots. He is a member of JPS and JSAP.

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