Selected Papers

Semiconductor Spintronics
Junsaku Nitta†
Abstract
In semiconductor spintronics, electron spin rather than charge is the key property. This paper describes
several spin-related devices using spin-orbit interaction. We have experimentally confirmed that the
spin-orbit interaction in a semiconductor two-dimensional electron gas channel can be controlled by agate voltage. This is the first step towards the creation of functional spin devices. The operating principles of a spin field effect transistor, a spin filter, and a spin interference device are discussed.

1. Introduction
Semiconductor and ferromagnetic materials areindispensable for present electronics technologies.
Semiconductor spintronics (or spin electronics) combines semiconductor microelectronics with spindependent effects that arise from the interaction
between the spin of a charge carrier and the magnetic properties of the materials. Ferromagnetic metals
show a magnetization hysteresis loop in which thecooperative motion of a large number of electron
spins plays a role. This hysteresis loop has been usedin the development of nonvolatile memory devices,such as hard disk drives. Recently, much attention has
been focused on the development of magnetic random access memory (MRAM), whose nonvolatilememory function is expected to reduce power consumption because no current (refresh) is required tomaintain it, unlike dynamic random access memory(DRAM). The MRAM is based on the tunnelingmagnetoresistance effect arising from the dependence of device resistance on the spin orientations
between two ferromagnetic electrodes [1]. However,
it is very difficult to make an active device, such as alogic circuit from only ferromagnetic metals.
Semiconductor materials enable logic functions to
be made because channel properties can be very easily controlled by an electric field, which has been themain reason for the tremendous progress of semiconductor electronics. An example is three-terminal
operation using gate electrodes in field effect transistors (FETs). This FET operation is the foundation ofmany functional devices. Downscaling of semiconductor devices has mainly been responsible for the
progress of integrated circuits (ICs). Downscaling
reduces the capacitance of components and allows us
to increase the number of components. The lower
capacitance increases the operating speed of ICs anddecreases their power consumption. However, it ispredicted that semiconductor fabrication technology
will reach the downscaling limit in a few years.
Overcoming the downscaling limit requires a newparadigm in electronics. In semiconductor electronics, electron charge plays the key role while electron
spin plays no role at all. Electronics based on the spin
degrees of freedom of the electron represents a newparadigm, which will require a way of controllingelectron spins in semiconductor channels by using
the gate voltage.
In this paper, we describe several spin-related
devices that use a semiconductor two-dimensional
electron gas (2DEG) channel. We have experimentally confirmed that the spin-orbit interaction can becontrolled by the gate voltage, which is the first step
towards the development of functional spin devices.
The operating principles of a spin-FET, a spin filter,
and a spin interference device are discussed.

† NTT Basic Research Laboratories
Atsugi-shi, 243-0198 Japan
E-mail: nitta@will.brl.ntt.co.jp

Vol. 2 No. 6 June 200431

Selected Papers

2. Operating principle of spin-FET
The discovery of giant magnetoresistance (GMR)[2], [3] showed that there is an intimate connection
between the charge and spin degrees of freedom of
the electron. In a GMR device, the electrical resistance is small when the magnetization orientation offerromagnetic thin layers is aligned, but very large
when it is anti-parallel. This suggests that information can be encoded not only in the electron’s charge
but also in its spin state, i.e., through alignment of thespin (either “spin-up” or “spin-down”) relative to themagnetization orientation of ferromagnetic film. In
GMR devices, the magnetization orientation is controlled by an external magnetic field. The basic ideaof the spin-FET, as proposed by Datta and Das [4], isto control the spin orientation by applying a gate voltage.
A spin-FET consists of ferromagnetic electrodes
and a semiconductor 2DEG channel with a gate electrode, as shown in Fig. 1. It is basically similar in
structure to a conventional FET. The source and drainelectrodes are ferromagnetic metals, and the 2DEG
channel should have a strong spin-orbit interaction.

The spin-polarized electrons injected from the ferromagnetic source electrode (FM1) start to rotate
because an effective magnetic field is created by the
spin-orbit interaction in the 2DEG channel. The origin of the spin-orbit interaction is the relativistic
effect, and an electron feels the effective magnetic
field Beff perpendicular to its motion in the electricfield E and propagation velocity direction v, such that
Beff ∝ v × E. If the spin orientation in the 2DEG channel is aligned to the ferromagnetic drain electrode, theelectrons can flow into the ferromagnetic drain electrode. However, when the spin orientation is flippedin the 2DEG as shown in Fig. 1, the electrons cannot
enter the drain electrode (FM2) because there is nostate for the flipped spin in FM2. The spin-orbit interaction is modified by the gate voltage (the effective
magnetic field is changed). Therefore, the spin precession angle can be controlled by the gate voltage. Ina spin-FET, current flow is modified by the spin precession angle.
A spin-FET would have several advantages over a
conventional FET: It could be used not only as a logic
gate, but also as a nonvolatile memory component.
Less energy would be necessary to flip an electron’sGate electrode
Electric fieldy

Ferromagnetic
electrode(FM1)

Ferromagnetic
electrode(FM2)x
Velocity direction
Semiconductor 2DEGz
Effective magnetic field

E

E
EF

EF
Spin precession
in 2DEGN (E )
0Density of states in FM1

N (E )
0Density of states in FM2

Fig. 1. Schematic of a spin-FET. Spin-polarized electrons are injected into the semiconductor 2DEG from the ferromagnetic
source electrode (FM1). Injected electrons start to rotate because of an effective magnetic field created by a spinorbit interaction. Spin-flipped electrons cannot enter the ferromagnetic drain electrode (FM2) because there is nostate for the flipped spin at the Fermi energy in FM2. A gate voltage that can tune the spin-orbit interaction is used
to control the spin precession angle.

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spin, and the spin could be flipped much faster thanelectrons can be pushed away from their channel by agate voltage. It may also be possible to change themagnetization direction of the electrodes by applying
a magnetic field, which would allow us to change thefunction anytime by changing the magnetization orientation of the electrodes.
For spin-FETs, there are two ongoing issues: howmuch can the spin-orbit interaction be controlled by
the gate voltage and how efficiently can spin-polarized electrons be injected into the 2DEG from the ferromagnetic electrode?
3. Gate control of spin-orbit interaction
It is well known that an external magnetic field lifts
spin degeneracy. Even without an external magnetic
field, an electric field perpendicular to the 2DEG
yields an effective magnetic field for moving electrons. Two important alternative mechanisms for spin
splitting are i) the host crystal electric field, the socalled Dresselhaus spin-orbit interaction [5] and ii)
the interface electric field, the so-called Rashba spinorbit interaction [6]. The possibility of gate control ofthe spin-orbit interaction was discussed in the early
1990s [4]. However, there has been no clear experi-mental evidence of such controllability until weexperimentally verified gate-voltage control of spinorbit interaction in InGaAs/InAlAs heterostructures[7]. The spin-orbit interaction parameter has since
been obtained from the beating pattern in the Shubnikov-de Haas (SdH) oscillations [8]-[10]. The dominant mechanism governing the change in the spinorbit interaction parameter is the Rashba spin-orbit
interaction.
Figure 2(a) shows the gate voltage dependence of
SdH oscillations at 0.4 K. The magnetic field wasapplied perpendicular to the 2DEG. Beating patterns
are observed in the SdH oscillations because of theexistence of two closely spaced SdH frequency components with similar amplitudes. These observed
beating patterns are attributed to the spin splitting.
When the negative gate voltage is increased, the node
position shifts to a higher magnetic field. The spinorbit interaction parameters were obtained from theanalysis of the beating pattern observed in the SdH
oscillations. In Fig. 2(a), a comparison between themeasured SdH oscillations and numerical simulations shows good agreement. The dependence of carrier concentration Ns of the spin-orbit interaction α isplotted in Fig. 2(b). The spin-orbit interaction parameter increases as Vg becomes more negative. A
1.1

(a)

(b)

160
Vg = –0.5 V

1.0

α (× 10–11 eVm)

Rxx (arb. units)

140

120
Vg = –0.3 V
100

0.9

0.8

Vg = 0 V80
0.7
60

Exp. (red)Sim. (blue)
0.6
1.2

1.6

2.0

2.4

B (T)

Fig. 2(a). Gate voltage dependence of SdH oscillations.
Beating patterns in the SdH oscillations appear
due to a spin-orbit interaction. By comparing theoscillations with the numerical simulation basedon Rashba spin-orbit interaction, we can obtainspin-orbit interaction parameter α.

Vol. 2 No. 6 June 2004

1.6

1.8

2.0

2.2

2.4

Ns (× 1012 cm–2)

Fig. 2(b). Spin-orbit interaction parameter α is plotted as afunction of carrier concentration Ns, which isrelated to gate voltage Vg. Spin-orbit interaction
can be controlled by the gate voltage.

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weak anti-localization analysis has also shown thatthe spin-orbit interaction can be controlled by thegate voltage [11].
In relating this result to the spin-FET, the spin precession angle θ is given by
2αm* L,(1)h2
where L is the distance between source and drain ferromagnetic electrodes. Equation (1) implies that the
spin precession angle, which is proportional to thespin-orbit interaction, can be controlled by the gate
voltage Vg. If we assume L = 0.5 µm, the spin precession angle can be controlled as θ = 1.2π at Vg = 0
V (Ns = 2.45 × 1012 cm–2) to θ = 1.8π at Vg = –0.5 V(Ns = 1.82 × 1012 cm–2). In this experiment, α did notsaturate in the above gate voltage range, which suggests that it will be possible to obtain larger α modulation.

θ=and the ability to eliminate the stray field that may
have undesirable effects on the spin-filtered electrons.
We have proposed a spin filter that uses a triple barrier resonant tunneling diode and can generate a spinpolarized current without using magnetic materials[19]. We utilized the Rashba spin-orbit interaction to
induce spin split resonant tunneling levels in the proposed device even in the absence of a magnetic field.
The device combines spin splitting with level matching between the spin-dependent resonant tunnelinglevels. The level matching procedure is analogous to
the proposed spin-blockade concept. Detailed calculations for the InAlAs/InGaAs material system predict that a splitting of a peak should be observed in
the I-V curve of this device as a result of the spin-filtering effect. The filtering efficiency exceeds 99.9%
at the peak position in the I-V curve.
5. Spin Interference device

4. Spin injection from ferromagnetic electrodeinto 2DEG
To explore the roles of the spin degrees of freedom
in a semiconductor, we must first create a spin-polarized current source. Much effort has been dedicatedto demonstrating spin injection from a ferromagnetic
contact (FM) into semiconductor (SM) because spin
injection will allow us to exploit many new phenomena and applications in the field of spin-polarizedtransport. It has been pointed out that spin injection isvery difficult due to the large conductivity mismatch
between ferromagnetic contact and diffusive semiconductor [12]. In cases where we used FM/SM/FM
structures, the resistance change due to the spin injection was less than 1% even at low temperature [13].
However, Rashba suggested that inserting a tunnel
barrier (I) between the FM and SM can solve the conductivity mismatch problem [14]. We have shown,
based on the free electron approximation model, that
more than a 10% resistance change can be expectedin FM/I/SM/I/FM junctions [15], [16].
A 2% spin injection efficiency at room temperature
has been reported for epitaxially grown Fe on GaAs
[17]. More recently, electron spin polarizations of
32% at 4.5 K have been obtained in a GaAs quantum
well via electrical injection through an Fe/AlGaAs
Schottky contact [18]. Despite the success of theseapproaches, it is also important to develop a spinpolarized current source or a spin filter that uses only
nonmagnetic semiconductors from the viewpoints ofboth the attainability of high-quality heterostructures
34

We have also proposed a spin-interference device
that works without any ferromagnetic electrodes andwithout any external magnetic field [20]. The phases
acquired by the spin wave functions during a cyclic
evolution are calculated in an Aharonov-Bohm (AB)ring in the presence of the Rashba spin-orbit interaction. The conductance modulation in a one-dimensional ring can be expressed by
G=

2 


 αm*  e2 
1 + cos2π 1 +  R 2   ,

h 
h






(2)

where R is the radius of the ring.
Figures 3(a) and (b) show the operating principle
and an SEM image of a fabricated spin-interference
device. The gate controls the spin-orbit interaction α
and covers the whole AB ring area. The origin of thespin interference is the spin precession angle difference between the left and right branches. The phases
acquired in the left and right branches are not thesame: they have opposite signs because the precession orientation is opposite. It is interesting that theobtained spin dynamical phase is very similar to thatof the spin-FET. In both cases, the origin of the phasedifference is related to the spin precession.
In the case of the spin-FET, the spin-polarized electrons injected from the ferromagnetic electrode contribute to conductance modulation. The spin polarization in the conventional ferromagnetic metals is not
100%; values of 30 to 40% have been reported for
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Selected Papers

(b)
(a)
Gate

Beff

V

V

Beff

Fig. 3(a). Schematic of the spin-interference device. The electron spinfeels an effective magnetic field perpendicular to its velocity
direction; therefore, the precession angles of the left and rightbranches are different. A ring made of an InGaAs/InAlAs
heterostructure is covered with a gate electrode, which
controls the spin-orbit interaction.

superconductor(S)/I/FM tunneling experiments [21].
On the other hand, all carriers can contribute to spin
interference in the proposed spin-interference device.
In the one-dimensional AB ring, the channel consists of a single mode. Here, we emphasize that thisspin-interference device would also work with multiple modes because the phase difference of the spinwaves is independent of the wave vector of the modesin the upper and lower branches. Generally, electron
quantum interference devices must be single-mode inorder to obtain sufficient modulation because the difference in the wave vector from one mode to anothercauses a difference in phase shift, which smears theinterference effect. An important advantage of our
spin-interference device is that the current modulation is not washed out even when multiple modes are
involved. Experimental measurements are underway.

ics, we must establish a way to control spin properties
in semiconductors by a gate because the electricalmanipulation of electron spins is a key technology. In
this paper, we focused on gate control of spin-orbit
interaction and its application in a spin-FET, spin filter, and a spin interference device. Recently, it has
been experimentally confirmed that the electron gfactor can be controlled by the gate voltage [22]-[24],and gate control of ferromagnetism has been demonstrated in a diluted semiconductor (InMnAs) [25]. In
addition, a ferromagnetic spin state has been predicted in a non-magnetic semiconductor quantum dot
array [26], where ferromagnetism can be controlled
by the gate voltage [27]. These electrical manipulations of the electron spin state will lead to a new paradigm of electronics based on spin degree of freedom.

6. Concluding remarks
There are three important parameters related toelectron spins in semiconductors: exchange interaction, spin-orbit interaction, and the electron g-factor.
The feature of semiconductors is that the channel
properties can be easily controlled by a gate voltage.
This gate controllability is the basis of the transistor,and since its invention, many device applications
have been established. However, the controllability ismainly limited to changing the number of carriers inthe semiconductor channel. For future spin electronVol. 2 No. 6 June 2004

Fig. 3(b). SEM image of a fabricatedspin-interference device.

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Junsaku Nitta
Group Leader, Spintronics Research Group,
Physical Science Research Laboratory, NTT
Basic Research Laboratories.
He received the B.E. degree from Yamaguchi
University, Yamaguchi in 1979 and the M.E. and
Ph.D. degrees from Kyushu University, Fukuoka
in 1981 and 1990, respectively. In 1981, hejoined the Electrical Communication Laboratories, Nippon Telegraph and Telephone Public
Corporation (now NTT), Musashino. He was aguest scientist in Groningen University from1994 to 1995. Since 1996, he has been working
on spin-related transport in semiconductor
nanostructures. From 2000 to 2003, he was theresearch coordinator of a NEDO international
joint research project. He has also been the coordinator of a CREST research team for semiconductor spin engineering since 2002. He is a member of the Physical Society of Japan, the Japan
Society of Applied Physics, and the American
Physical Society.

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NTT Technical Review