Spin-valve effect in nanoscale
Silicon spin-valve devices

Duong Dinh HiepPhD Dissertation

Dissertation supervisor
Associate Professor Pham Nam Hai

Department of Physical Electronics
Tokyo Institute of Technology

January, 2019

Doctoral Committee:
Associate Professor Pham Nam Hai, supervisor
Professor Shigeki Nakagawa
Professor Takaaki Manaka
Professor Akira Yamada
Associate Professor Shinsuke Miyajima
Dr. Hidekazu Saito

Copyright 2019 by Duong Dinh HiepContents
CHAPTER 1: Introduction .................................................................... 1
1.1.

Spintronics ....................................................................................... 1
1.1.1. Spin valve devices .................................................................. 4
1.1.2. Spin FETs ................................................................................ 5

1.2.

Silicon spintronics .......................................................................... 9

1.3.

Motivation ........................................................................................ 9

1.4.

Thesis outline ................................................................................. 11

1.5.

Summary ........................................................................................ 13
References

CHAPTER 2: Fundamental Physics of Spin transport...................... 17
2.1. Spin transport in diffusive regime.................................................. 18
2.1.1. Two current model ............................................................... 18
2.1.2. Spin injection and relaxation ............................................... 20
2.1.3. Conductivity mismatch problem ......................................... 23
2.2. Spin transport in ballistic regime ................................................... 25
2.3. Electrical spin injection and transport measurements.................... 28
2.3.1. Two terminal local spin-valve .................................................. 29
2.3.2. Four terminal non-local spin-valve ........................................... 31
2.3.3. Three-terminal Hanle precession measurements ...................... 32i

2.4. Summary ........................................................................................ 34
References
CHAPTER 3: Device Fabrication and Characterization................... 37
3.1. Techniques ..................................................................................... 38
3.2. Fabrication process ........................................................................ 39
3.3. Substrate preparation...................................................................... 42
3.4. FM electrodes and tunnel barriers deposition ................................ 42
3.5. Fabrication of nanoscale Si channel .............................................. 44
3.6. Connecting the spin-valve device to the microscale contacts ....... 46
3.7. Wire bonding.................................................................................. 48
3.8. Characterization ............................................................................. 48
3.9. Summary ........................................................................................ 50
References
CHAPTER 4: Investigation of spin-valve devices fabricated by Electron
Beam Evaporation ....................................................................................... 52
4.1. Spin-valve device structure ............................................................ 53
4.2.

Conductance characteristics........................................................... 55

4.3. Spin-dependent transport characteristics ....................................... 58
4.3.1. Local spin-valve effect........................................................... 59ii

4.3.2. Dependence of the spin-valve signal on the bias voltage andtemperature.......................................................................................... 62
4.3.3.

Dependence of the spin-valve signal on the magnetic-field

direction .............................................................................................. 66
4.4.

Spin-dependent output voltage ...................................................... 69

4.5. Summary ........................................................................................ 70
References
CHAPTER 5: Investigation of spin-valve devices fabricated by Molecular
Beam Epitaxy ............................................................................................... 73
5.1.

Spin-valve device structure............................................................ 74

5.2.

Structure characterization .............................................................. 75

5.3.

Conductance characteristics........................................................... 77

5.4.

Spin-dependent transport characteristics ....................................... 79

5.4.1. Local spin-valve effect ............................................................. 80
5.4.2. Dependence of spin-valve signal on magnetic-field direction . 83
5.4.3. Dependence of the spin-valve signal on temperature – The
inverse spin-valve effect ..................................................................... 85
5.5.

Spin-dependent output voltage ...................................................... 94

5.6.

Summary ........................................................................................ 96
Referencesiii

CHAPTER 6: Investigation of the role of ballistic transport in thenanoscale Si channel.................................................................................... 98
6.1.

Spin-valve device structure............................................................ 99

6.2.

Spin-valve signal in the diffusive regime .................................... 100

6.3.

Role of ballistic transport in the nanoscale Si channel ................ 102

6.3.1. Generation of the spin-valve effect without tunnel barrier ... 103
6.3.2. Suppression of spin-flip scattering to achieve a higher spinvalve ratio at shorter channel length ................................................. 106
6.4.

Summary ...................................................................................... 108

CHAPTER 7: Optimization of the MgO tunnel barrier thickness ...... 111
7.1.

Spin-valve device structure.......................................................... 112

7.2.

Conductance characteristics......................................................... 113

7.3.

Spin-dependent transport characteristics ..................................... 116

7.3.1. Local spin-valve effect ........................................................... 116
7.3.2. Dependence of the spin-valve signal on the bias voltage in
different MgO thickness ................................................................... 118
7.4.

Spin-dependent output voltage .................................................... 124

7.5.

Summary ...................................................................................... 125
Referencesiv

CHAPTER 8: Improvement of the spin-valve ratio by using an Mg buffer
layer between the Fe and MgO layer....................................................... 127
8.1.

Spin-valve device structure.......................................................... 128

8.2.

Optimization of the Mg buffer layer thickness............................ 129

8.3.

Optimization of the MgO tunnel barrier thickness in Mg-inserted

devices .................................................................................................... 131
8.3.1. Conductance characteristics ................................................... 131
8.3.2. Local spin-valve effect ........................................................... 133
8.4.

Summary ...................................................................................... 136
References

CHAPTER 9: Conclusions and Future Perspectives ............................. 137
Acknowledgement .................................................................................... 144
List of publications .................................................................................... 146
List of conferences ..................................................................................... 147v

1|Page

Chapter 1

CHAPTER 1
INTRODUCTION
1.1. Spintronics
Every electron has an intrinsic form of angular momentum, called “Spin”. Understanding
the electron spin property and its interaction with surrounding solid-state environments gives
us an opportunity to control the state of not only the spin of individual electron but also awhole spin system. Spintronics, brief of spin-based electronics, is the study in which the spin
degree freedom of electrons are employed to create new forms of information storage, as wellas the foundation for new paradigm of information processing and low-power nonvolatileelectronics.
The beginning of spintronics could be considered starting from the researches of Nevill
Francis Mott, an English physicist who won the Nobel Prize for Physics in 1977. By
investigating an unusual behavior of resistance in ferromagnetic metals, he outlined someprimary paths for our understanding of spin-polarized transport. In his researches, when thetemperature is low enough, he recognized that magnon scattering becomes very small, thenthe majority and minority spin which the magnetic moment are parallel and antiparallel tothe magnetization of a ferromagnet do not mix in the scattering processes. As a result, thecurrent in ferromagnets is polarized into majority and minority spin, thus the total

2|Page

Chapter 1
conductivity can be identified by the sum of two independent and unequal parts for two
different spin projections [1,2].
Then, the first work to impressively bring the spin information from pure physics into
real applications belongs to the groups of Albert Fert and Peter Grunberg, who demonstrated
the giant magnetoresistance (GMR) in the end of the 1980s [3,4]. Until that time, people only
employed the electrical characteristic of electron in electronic devices. Because of its impact
in establishing the new field of spintronics, the discovery of the GMR brought to Albert Fert
and Peter Grunberg the Physics Nobel award in 2007 [5]. The GMR devices detect
information by utilizing the interaction of spin of electron with magnetic materials. The
application of GMR could be listed here such as spin-valve sensors, hard disk drives or
MRAM. In the GMR devices, there are two or more ferromagnetic (FM) layers with a few
nanometers thick each layer separated by non-magnetic materials. In the operation process,the device shows a low resistance when the status of the FM layer magnetizations are paralleland high resistance in the antiparallel status. This simple concept works very successfully inthe read heads of standard computer hard disk drives nowadays. Additionally, the parallel –antiparallel concept is employed to develop the so-call novel non-volatile magnetic random
access memory (MRAM) as well. The spin transfer torque (STT) effect is also anotherpromising direction in the memory technology. Based on the STT effect, people
demonstrated the possibility of exciting the magnetization of ferromagnet and switching its
direction by a spin current instead of using an external magnetic field [6,7,8,9], brightening
the concept of STT-MRAM.

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Chapter 1
Recently, semiconductor based lateral spin devices have received great attention for
employing the spin nature of electrons in potential applications for the ultra-dense, highspeed, low-power operation memory devices and reconfigurable logic [10,11,12]. Spin
MOSFET research is directed in this point of view in the emerging research devices in
Beyond CMOS era [13]. The understandings of spin injection, detection and manipulation
are vital for purpose of fully functional spin transistors realizing.
Semiconductor based lateral spin devices
In lateral spin devices, generally the electrodes are arranged horizontally, which is easy
for that fabrication process. Based on the operating method as well as geometry structure,people divide the lateral spin based devices into two types: a two-terminal spin valve device
and a three-terminal spin transistor. In two-terminal spin valve devices, there are twoferromagnetic electrodes. The resistance between the two ferromagnetic electrodes becomeshigh when the two FMs are aligned in the antiparallel configuration and low when the two
FMs are in parallel configuration. The operation concept is similar to GMR or TMR devices.
Even though experimental results show the switching behavior of spin-dependent resistancein all metal-based spin valve devices, it is challenging to observe such response in
semiconductor-based devices because of the conductivity mismatch between the
ferromagnetic electrodes and the semiconductor channel. This issue is the main obstacle on
the way of bringing the semiconductor-based spin valve devices into practical applications.

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Chapter 1
1.1.1. Spin valve devices
The origin of spin valve devices came from the idea of the spin injection fromferromagnetic electrodes into a non-magnetic medium which is proposed firstly by Aronov,then experimentally demonstrated by Johnson and Silsbee [14] in a metallic channel. For
semiconductor channel, the spin valves were experimentally demonstrated [15,16] in a GaAs
channel device in 2005. The operation concept of a spin valve device is similar to a diodelike device, in which there are two resistance status: one is high when the two ferromagnetic
electrodes are in antiparallel configuration, and another is low when they are in parallel
configuration. Base on the geometry structure, the semiconductor spin valve devices are
divided into two types: the 1st is the lateral structure, and the 2nd is the vertical structure, as
shown in Fig. 1.1.

Figure 1.1: Schematic illustration of a (left) lateral spin valve and (right) vertical spin
valve geometry.
In the lateral spin valve (LSV) devices, the spin injection and transport directions are
orthogonal to each other. This type of device is based on planar technologies which iscommonly used in conventional microelectronics technology. Hence, it has an advantage ofbeing easily integrated into process flows without a harsh change. Furthermore, LSVs have

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Chapter 1
the possibility of demonstrating electric current switching, by using a third gate electrode tocontrol the spin of the electrical current.
In the vertical spin valve (VSV) devices, the spin injection and transport directions areparallel to each other. The VSV device has similar operating mechanism to a LSV device,
but shows higher magnetoresistance response. The reason is in this type of device, people
can precisely control the channel length and conducting area by using epitaxial growthtechniques. The short channel length could preserve the spin information of electrons, thenallowing more spin-polarized electrons reach to detector before the spin-relaxation process
finish. However, manipulating spin in the VSV device is more difficult than the LSV, becauseof the problem in fabricating an all-around gate electrode around the thin channel layer.
1.1.2. Spin FETs
Spin-based logic devices (spin transistors) are generally viewed as the promising
candidate for ultra-dense, high-speed, low-power operation memory devices andreconfigurable logic [10,11,17]. The Spin-based logic devices have been classified as the“Non-Conventional Charge-based Extended CMOS Devices” by ITRS. It means that, thesedevices show transistor-like behaviors with the operating mechanism of a spintronic device.
The spintronic logic devices are defined as possessing the abilities of control the current or
voltage by the magnetization configuration of the ferromagnetic contacts with respect to themajority spin orientation, and non-volatile information storage using the magnetoresistanceand spin torque effects. Those abilities are very advanced functionalities for energy efficient,high-performance circuit architectures but are being inaccessible to conventional CMOS

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Chapter 1
circuits. Several spin-based logic devices have been proposed. The first one is known as the
Datta-Das transistor, in which the spin of electron is controlled by a gate voltage through thespin-orbit coupling [12]. However, there is no Datta-Das transistor demonstrated so far, anda convincing experimental realization of the device has shown the impractical possibility ofthis concept. Nevertheless, it still provides the basic idea and platform for future spin-based
logic devices to improve upon by using the spin-orbit interaction to systematically control aspin system.

Figure 1.2: Structure of the spintronic analog of the electro-optic modulator proposed by
Datta and Das in 1990 [12]. Reprinted from Appl. Phys. Lett. 56, 665 (1990), with thepermission of AIP Publishing.
Besides the Datta-Das spin transistor concept, Fabian and Zutic also proposed anotherpotential spin-logic device – the magnetic bipolar transistor (MBT) [18]. The operation
concept of this device is similar to the conventional bipolar junction transistor (BJT) withoffers the new function of controlling current amplification by spin property. In order to7|Page

Chapter 1
maximize the gain in a conventional BJT, it is necessary to minimize the relative contributionof holes in the emitter current or prevent the e-h (electron-hole) recombination in the baseregion. This condition is realized by the ability of spin control in the MBTs [19].

Figure 1.3: Illustration of the magnetic n-p-n transistor in the amplification mode inwhich the B-E junction is forward biased (VBE > 0) to lower the barrier and reduce thedepletion layer width, while the B-C junction is reverse biased (VBC < 0) to raise the barrier
and expand the depletion layer width. It is possible to control the current amplification β =IC/IB by the spin polarization in the base as well as by the nonequilibrium spin in the emitter[18].
The most practical spin-logic device is the concept of spin metal-oxide-semiconductor
field-effect transistor (spin-MOSFET), which has been proposed by Tanaka and Sugahara[13]. The silicon (Si) based spin MOSFET is considered to be one of the most promising
candidates because of its high compatibility with CMOS technology and long spin lifetime

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Chapter 1
in Si [17,13,20,11,21]. However, there is no Si-based spin MOSFET demonstrated so far
because the issue of conductivity mismatch has been not solved yet.

Figure 1.4: (a) Schematic device structure and band diagrams of the spin MOSFET in (b)parallel and (c) antiparallel magnetization configurations. Solid arrows in the HMF
source/drain show up-spin and down-spin electrons at the Fermi energy of the metallic spinband and at the valence band edge of the insulating spin band. Open arrows represent the
conduction band edge of the insulating spin band of the HMF source/drain [13]. Reprinted
from Appl. Phys. Lett. 84, 2307 (2004), with the permission of AIP Publishing.

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Chapter 1

1.2. Silicon spintronics
Recently, there is a huge attention in combining the semiconductors and magnetic
materials, aiming to create a revolutionary and energy-efficient information technology inwhich digital data are encoded in the spin of electrons. Magnetic materials show advantages
in non-volatile memory such as rapidly switching, unlimited endurance, and high resistanceto noise. On the other hand, semiconductor is in the heart of logic technology. Therefore, acombination of memory and logic computing is promising for next generation electronic
devices. In that circumstance, implementing spin functionality in silicon, the mainstream
semiconductor, is essential to establish spin-based electronics with potential to change
information technology beyond imagination. Si-based spin-MOSFET is the most realistic
concept to do so. In fact, Si shows key advantages such as it is the de-facto material ofelectronics; it has small spin-orbit interaction, thus long spin lifetime of electron in Si is
expected; and the spin transport over µm in Si has been demonstrated. Therefore, it is thevery promising channel material for Spin-MOSFET.

1.3. Motivation
Conventional CMOS (complementary metal oxide semiconductor) device is based on
manipulation of the electron charge current. The main innovation trend of CMOS is primarily
focused on miniaturization, roughly doubling the number of transistors in one unit every 18
months, known as Moore’s law. Leading semiconductor corporations have been able to keep

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Chapter 1
Moore’s law strong by improving lithographic techniques and fabricating smaller devices, as
shown in Fig. 1.5. However, the enormous enhancement in lithography and innovations indevice engineering cannot change the fact that, sooner or later, conventional CMOS devices
will reach a physical limit where further miniaturization of the device will be fundamentally
impossible. Moreover, conventional CMOS technology is facing serious problems of highidling power consumption and heat generation due to leakage off-current when the device
size is miniaturized to nanoscale, which restricts the device operating speed. Hence, there isa demand for alternative low-power solutions to overcome these problems in the beyondCMOS era. The silicon (Si) based spin-MOSFET is a promising solution because of its highcompatibility with the well-established CMOS technology and long spin lifetime in Si[13,17,11,20,21]. For that reason, there has been a great interest in demonstrating spin
injection and detection of spin transport in Si by ferromagnetic electrodes. In fact, spin
injection into microns of Si channels by using the three terminal Hanle effect [22,23,24] orthe four-terminal spin-valve effect [25,26,27] has been demonstrated. However, previous
researches only studied the spin transport in micron-scale Si channels and the typical spindependent output voltages were less than 1 mV which is not enough for realistic spinMOSFET.

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Chapter 1

Figure 1.5: R&D pipeline of Intel for the scaling down process in IC technology [source:Intel].
To improve the spin-dependent output voltage, I propose the spin-valve effect utilizing
ballistic electron transport in nanoscale Si channel. It is expected that the ballistic transportof electrons in such nanoscale channels may overcome the conductivity mismatch problem
which arises at the interface between a ferromagnetic (FM) electrode and a diffusive
semiconductor (SC) channel [28,29], resulting in a higher spin-dependent output voltage.

1.4. Thesis outline
The purpose of this thesis is systematically investigating the spin-dependent transportcharacteristics in nanoscale Si-based 2 terminal spin-valve devices with a Si channel length
as short as 20 nm.

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Chapter 1
The thesis is divided into 9 chapters.
Chapter 1 is an introduction to the topic of spintronics in general, and silicon spintronics
in particular, and the motivation for this study.
Chapter 2 introduces the physics of spin injection and relaxation in diffusive regime andballistic regime, then briefly discusses the electrical spin injection and transportmeasurements.
Chapter 3 describes in detail the fabrication and measurement process of nanoscale Sibased 2 terminal spin-valve devices in this study.
Chapter 4 focuses on the investigation of spin-valve devices fabricated by Electron Beam
Evaporation.
Chapter 5 focuses on the investigation of spin-valve devices fabricated by Molecular
Beam Epitaxy.
Chapter 6 describes the important role of ballistic transport in nanoscale spin-valve
devices.
Chapter 7 presents the optimization of the MgO tunnel barrier thickness in spin-valve
devices to achieve a higher spin-dependent output voltage.
Chapter 8 describes the effect of inserting a Mg buffer layer in between the Fe and MgOlayer to improve the spin-injection in spin-valve devices.
Chapter 9 summarizes the main results of this thesis and discusses on future respects.

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Chapter 1

1.5. Summary
Spintronics is the technology which aims to utilize the spin degree freedom of electrons
for many applications in nowadays and future technology such as new forms of information
storage, as well as the foundation of new paradigm for information processing and low-power
nonvolatile green electronics. In spintronics, the spin-MOSFET is considered to be thebuilding block of low-power-consumption electronics devices. In the respect of asemiconductor material, Si shows key advantages such as it is the de-facto material ofelectronics; it has small spin-orbit interaction, thus long spin lifetime of electron in Si is
expected; and the spin transport over µm in Si has been demonstrated. Therefore, Si is thevery promising channel material for Spin-MOSFET. One obstacle staying on the developing
road is the low performance of spin-valve device in the diffusive regime, which is replacedby ballistic regime in this study. This chapter gave a quick view of silicon spintronics
researches, especially in spin injection and transport.

References
1.

Mott N. F. and Wills H. H. The Electrical Conductivity of the Transition Metals.
Proc. R. Soc. A Math. Phys. Eng. Sci. 153, 699–717 (1936).

2.

Mott N. F. The Resistance and Thermoelectric Properties of the Transition Metals.
Proc. R. Soc. A Math. Phys. Eng. Sci. 156, 368–382 (1936).

3.

Baibich, M. N. et al. Giant Magnetoresistance of (001)Fe/(001)Cr Magnetic

14 | P a g e

Chapter 1
Superlattices. Phys. Rev. Lett. 61, 2472–2475 (1988).
4.

Binasch, G., Grünberg, P., Saurenbach, F. & Zinn, W. Enhanced magnetoresistancein layered magnetic structures with antiferromagnetic interlayer exchange. Phys.
Rev. B 39, 4828–4830 (1989).

5.

Wolf, S. A. Spintronics: A Spin-Based Electronics Vision for the Future. Science
294, 1488–1495 (2001).

6.

Slonczewski, J. C. Current-driven excitation of magnetic multilayers. J. Magn.
Magn. Mater. 159, L1–L7 (1996).

7.

Berger, L. Emission of spin waves by a magnetic multilayer traversed by a current.
Phys. Rev. B 54, 9353–9358 (1996).

8.

Tsoi, M. et al. Excitation of a Magnetic Multilayer by an Electric Current. Phys. Rev.
Lett. 80, 4281–4284 (1998).

9.

Katine, J. A., Albert, F. J., Buhrman, R. A., Myers, E. B. & Ralph, D. C. CurrentDriven Magnetization Reversal and Spin-Wave Excitations in Co / Cu / Co Pillars.
Phys. Rev. Lett. 84, 3149–3152 (2000).

10.

Fabian, J., Matos-Abiague, A., Ertler, C., Stano, P. & Zutic, I. Semiconductor
Spintronics. Acta Phys. Slovaca 57, 565–907 (2007).

11.

Žutić, I., Fabian, J. & Das Sarma, S. Spintronics: Fundamentals and applications.
Rev. Mod. Phys. 76, 323–410 (2004).

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Chapter 1
12.

Datta, S. & Das, B. Electronic analog of the electro‐optic modulator. Appl. Phys.
Lett. 56, 665–667 (1990).

13.

Sugahara, S. & Tanaka, M. A spin metal–oxide–semiconductor field-effect transistor
using half-metallic-ferromagnet contacts for the source and drain. Appl. Phys. Lett.
84, 2307–2309 (2004).

14.

Johnson, M. & Silsbee, R. H. Interfacial charge-spin coupling: Injection anddetection of spin magnetization in metals. Phys. Rev. Lett. 55, 1790–1793 (1985).

15.

Saha, D., Basu, D., Holub, M. & Bhattacharya, P. Two-dimensional spin diffusion in
multiterminal lateral spin valves. Appl. Phys. Lett. 92, 022507 (2008).

16.

Lou, X. et al. Electrical detection of spin transport in lateral ferromagnet–
semiconductor devices. Nat. Phys. 3, 197–202 (2007).

17.

ITRS. International Technology Roadmap for Semiconductors 2.0: Executive
Report. Int. Technol. roadmap Semicond. 79 (2015).

18.

Fabian, J. & Žutić, I. Spin-polarized current amplification and spin injection inmagnetic bipolar transistors. Phys. Rev. B 69, 115314 (2004).

19.

Flatté, M. E., Yu, Z. G., Johnston-Halperin, E. & Awschalom, D. D. Theory ofsemiconductor magnetic bipolar transistors. Appl. Phys. Lett. 82, 4740–4742 (2003).

20.

Tanaka, M. & Sugahara, S. MOS-Based Spin Devices for Reconfigurable Logic.
IEEE Trans. Electron Devices 54, 961–976 (2007).

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Chapter 1
21.

Zarifis, V. & Castner, T. G. ESR linewidth behavior for barely metallic n-typesilicon. Phys. Rev. B 36, 6198–6201 (1987).

22.

Dash, S. P., Sharma, S., Patel, R. S., de Jong, M. P. & Jansen, R. Electrical creationof spin polarization in silicon at room temperature. Nature 462, 491–494 (2009).

23.

Dash, S. P. et al. Spin precession and inverted Hanle effect in a semiconductor near afinite-roughness ferromagnetic interface. Phys. Rev. B 84, 054410 (2011).

24.

Li, C. H., van ’t Erve, O. M. J. & Jonker, B. T. Electrical injection and detection ofspin accumulation in silicon at 500 K with magnetic metal/silicon dioxide contacts.
Nat. Commun. 2, 245 (2011).

25.

Huang, B., Monsma, D. J. & Appelbaum, I. Coherent Spin Transport through a 350
Micron Thick Silicon Wafer. Phys. Rev. Lett. 99, 177209 (2007).

26.

Sasaki, T. et al. Electrical spin injection into silicon using MgO tunnel barrier. Appl.
Phys. Express 2, (2009).

27.

Aoki, Y. et al. Investigation of the inverted Hanle effect in highly doped Si. Phys.
Rev. B 86, 081201 (2012).

28.

Schmidt, G., Ferrand, D., Molenkamp, L. W., Filip, A. T. & van Wees, B. J.
Fundamental obstacle for electrical spin injection from a ferromagnetic metal into a
diffusive semiconductor. Phys. Rev. B 62, R4790–R4793 (2000).

29.

Fert, A. & Jaffrès, H. Conditions for efficient spin injection from a ferromagnetic
metal into a semiconductor. Phys. Rev. B 64, 184420 (2001).

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Chapter 2

CHAPTER 2
FUNDAMENTAL PHYSICS of SPIN TRANSPORTIn this chapter, overviews of the spin transport mechanisms in diffusive regime andballistic regime were presented. Additionally, electrical spin measurements techniques werediscussed. An understanding of the physical mechanisms that govern spin transport inside
semiconductors is essential for proper design and characterization of these devices, as well
as for overcoming the current issues.

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Chapter 2

2.1. Spin transport in the diffusive regime
2.1.1. Two-current model
A conventional approach to understand and explain spin transport effects is the twocurrent model which was firstly proposed by Valet and Fert in 1993 [1]. In this model,
electrons have 2 types of spin: up-spin and down-spin. Up-spin electrons have their spinsaligned with the magnetization of the ferromagnetic metal (FM), while down-spin electrons
have their spins aligned in the opposite direction. Other spin directions are mapped on thesetwo directions. That is the reason why it is called two-current model.
According to the Einstein relation, the conductivity of a material is described by theformula:𝜎 = 𝐷𝑒 2 𝜈(𝐸𝐹 )(2.1)

In the above formula, D is the diffusion coefficient, e is the electron charge and ν is thedensity of states (DOS) of electron at the Fermi level EF. In the FM, because the DOS of two
spin directions at EF are different, there is a difference in conductivity of up-spin and downspin 𝜎↑,↓ = 𝐷𝑒 2 𝜈↑,↓ (𝐸𝐹 ), then the total conductivity and total DOS can be calculated as 𝜎 =𝜎↑ + 𝜎↓ and 𝜈 = 𝜈↑ + 𝜈↓ , respectively. The symbol ↑ and ↓ represent the spin directions, spinup and spin-down.
By using this difference in conductivity of two spin directions, one can characterize therelative magnetization direction of two FM electrodes in a serial circuit. That is the idea ofthe local spin-valve effect. In the serial circuit with FM electrodes, the circuit response can

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Chapter 2
be illustrated as Fig. 2.1(a) and 2.1(b) by using the two-current model, disregarding theresistance of materials between the FM electrodes. If all FM electrodes have theirmagnetizations aligned in the same direction (parallel state), one type of spin exhibits a highconductance (low resistance) in all FM electrodes, while the other type of spin exhibits a low
conductance (high resistance) as described in Fig. 2.1(a). On the other hand, in the
antiparallel state in which each FM electrode aligned its magnetization in opposite directions,both spin types exhibit a low resistance in one FM electrode and a high resistance in the otherFM electrode as shown in Fig. 2.1 (b). Therefore, the antiparallel state usually appears in ahigher resistance than the parallel state. This model is used to explain the local spin-valve
signal as indicated in Fig. 2.1 (c). The magnetizations of two FM electrodes connected to asilicon channel are switched from parallel to antiparallel orientation and back, using amagnetic field [2]. The FM electrodes can be switched independently as they have different
coercive fields and the system shows a higher resistance when the electrodes are in
antiparallel alignment.
In this local spin-valve concept, two FM electrodes work as an injector/detector and haveto be separated by a non-magnetic material to enable the independent switching of theirmagnetizations. The spins are injected from the injector electrode into the non-magnetic
channel, transport through this channel without losing their spin orientation, then detected at
the detector electrode. If the spin orientation is maintained while the electrons travel between
the FM electrodes, the relative orientation of the magnetizations of the FM electrodes is theprimary factor influences the resistance of the device.

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Chapter 2

Figure 2.1: The two-current model for two FM electrodes in serial connection. Spin-down
electrons exhibit a high resistance R↓ while spin-up electrons show a low resistance R↑. The
illustration for parallel state (a) and antiparallel state (b) of the magnetizations demonstrates
the different resulting resistances. (c) Local spin-valve signal in the nanoscale Si channel [2].
2.1.2. Spin injection and relaxation
The Fig. 2.2 illustrates the concept of spin injection and relaxation with spin diffusion
length. The left of Fig. 2.2(a) describes the DOS of electron in the FM in equilibrium with ashifted DOS for spin-up and spin-down and an imbalance of the DOS 𝜈↑ > 𝜈↓ . The right of

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Chapter 2
Fig. 2.2 (a) describes the DOS of electron in a non-magnetic material (NM) in equilibriumwith a balance of the DOS at EF 𝜈↑ (𝐸) = 𝜈↓ (𝐸). When electrons are injected from the FM
into the NM, there is a spin accumulation close to the interface of FM and NM shown as adifference in DOS in the center of Fig. 2.2(a), because the spin imbalance of the FM istransferred into the NM. This is a non-equilibrium state at the interface of FM and NM, andthe spin accumulation will diffuse from the interface into the bulk of the NM where there isno spin accumulation. During diffusion, spins relaxation will occur.

Figure 2.2: (a) Concept of spin injection and relaxation illustrated by the spin-dependent
DOS of a FM and a NM in equilibrium (left and right, respectively) and the spin accumulationin the NM close to the interface when an electrons are injected from the FM into the NM(center). (b) The spin electrochemical potential 𝜇S in the NM decays as a function of theposition x with the spin diffusion length λ [3].

22 | P a g e

Chapter 2
Generally, the spin injection and transport from FM into NM can be characterized byapplying the two-current model to define the spin current density 𝐽S = 𝐽↑ − 𝐽↓ and the charge
current density 𝐽 = 𝐽↑ + 𝐽↓
𝐽↑,↓ = −

𝜎↑,↓
𝑒

∇𝜇↑,↓

(2.2)

With 𝜇↑,↓ is the spin-dependent electrochemical potentials of the electrons. The
1electrochemical potential 𝜇0 = 2 (𝜇↑ + 𝜇↓ ) and the spin electrochemical potential 𝜇𝑆 =12

(𝜇↑ − 𝜇↓ ) describes the spin accumulation [3].
In the bulk of the FM, we have shown that 𝜎↑ ≠ 𝜎↓ as discussed in the previous section,

the 𝐽↑ ≠ 𝐽↓ , while in the bulk of the NM we have 𝐽↑ = 𝐽↓ . Therefore, spin accumulation builds
up at the interface. The mechanism of this spin accumulation defined by the spinelectrochemical potential 𝜇S is described by the Bloch equation [3]⃗⃗⃗⃗⃗S𝑑𝜇𝑑𝑡= 𝐷∇2 ⃗⃗⃗⃗
𝜇S −

⃗⃗⃗⃗⃗
𝜇S
𝜏

+𝜔
⃗⃗⃗⃗⃗L × ⃗⃗⃗⃗
𝜇S

(2.3)

⃗ is thewhere D is the diffusion coefficient, τ is the spin relaxation time, 𝜔L = 𝑔𝜇B /ℏ𝐵⃗ is the magnetic field with the gyromagnetic factor g (g-factor, g = 2 forLarmor frequency. 𝐵
free electrons) and the Bohr magneton 𝜇B .
The spin chemical potential ⃗⃗⃗⃗
𝜇S in this equation is a vector quantity, representing not only
the spin chemical potential 𝜇↑ and 𝜇↓ with respect to the z-axis, but also the 3-dimensional⃗⃗⃗⃗⃗⃗ = 0, the system is in a steady state with 𝑑𝜇S = 0, and with thespin accumulation. For 𝐵
𝑑𝑡

23 | P a g e

Chapter 2boundary condition 𝜇S (𝑥 → ∞) = 0 the spin accumulation exponentially decays away from𝑥

the interface with 𝜇𝑆 ∝ exp(− 𝜆) where 𝜆 = √𝐷𝜏 is the spin diffusion length in the NM.
2.1.3. Conductivity mismatch problem
The conductivity mismatch problem is a crucial issue for attempts to efficiently inject
spins into a diffusive transport channel. The first discussion about this fundamental problem
in spin injection from FMs into semiconductors (SC) was raised by Schmidt et al. [4]. They
explained that the difference in resistivity of the FM injector and the SC transport channel
causes a vanishing polarization of the injected spins. Although it was shown that themismatch can be circumvented when using a ferromagnetic SC material for spin injector, thisinjection method is limited to low temperatures, due to the very low Curie temperatures ofthese ferromagnetic SC materials.
According to the theory of spin transport in the diffusive transport regime [4,5], whenspin polarized electrons are injected from FM into SC, without barrier between the FM layer
and the SC channel, the spin polarization of the current at the FM/SC interface is given by J  J  (SP)I =  .
 J   J   I 1  rN / rF

(2.4)

Here,  is the spin-polarization of the FM layer, rF and rN are characteristic resistances
defined by 𝑟𝐹 = 𝜌𝐹 𝜆𝐹 and 𝑟𝑁 = 𝜌𝑁 𝜆𝑁 , where  F ,  N are the resistivity of the FM and SC,
and 𝜆𝐹 , 𝜆𝑁 are the spin-diffusion length in FM and SC. The ratio rN / rF determines the spinpolarization at the interface. Using  F ~ 110-5 cm and 𝜆𝐹 ~ 2 nm for Fe,  N = 210-2 cm

24 | P a g e

Chapter 2
for n-Si (n =1018 cm-3) and assuming that 𝜆𝑁 ~10 m, the ratio rN / rF is as large as 1107.
This means that there is almost no spin polarization at the interface between Fe and n-Si. The
situation is even worse for the spin-valve ratio in the FM/SC/FM structure. Assuming thatthe length of the SC layer 𝑡N ≪ 𝜆N , the spin-valve ratio is given byNR
2 rF l sf 8 
Rrt
 N N

2

(2.5)

2

 rF lsfN  is of the order of 10-9,
For the Fe/n-Si interface, even when tN = 20 nm, the ratio rt N N
which means that there is almost no spin-valve effect.
The conductivity mismatch problem can be avoided by inserting a barrier between the
FM and SC [5,6], which has been widely used to obtain spin injection into SC. However, thismethod still has some problems. While spin injection from FM into SC through a barrier has
been definitely demonstrated, typical values of the spin-valve ratio reported so far in lateraldevices are as small as 0.01% ~ 0.1%. Furthermore, high-resistance barriers at thesource/drain electrodes are not desired from the viewpoint of current-driving capability ofspin-transistors. On the other hand, the conductivity mismatch problem can also be avoided
by using nano-scale SC channels with ballistic (or quasi-ballistic) transport. If the SC channel
length is comparable or shorter than the electron mean free path, the transport regime changes
from diffusive to (quasi) ballistic. In this case, the Ohm’s law and diffusion equations used
to derive the spin-polarized current are no longer valid. The transport in the SC channel may
be modeled using quantum mechanics rather than classical transport equations of electrons.

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Chapter 2
In such a case, the transfer matrix method may be used to calculate the spin-polarizedtransport in the SC channel. Such a method was used to calculate the magnetocurrent in spinMOSFETs in the original work by Tanaka and Sugahara [7].

2.2. Spin transport in the ballistic regime
The ballistic transport is the transport of electrons in the medium that can eliminate theelectrical resistivity by scattering. If the length of the channel (L) is smaller than the mean
free path (λ) of electrons, the ballistic transport could occur. In case of L ~ λ, we have thequasi-ballistic transport. In devices with submicron dimensions, this phenomena is especiallytrue because λ for momentum scattering can easily exceed device dimensions. Moreover, if
the conductor of such materials is narrow enough, it easily become quantum wires, where
current is carried by only a small number of transverse subbands. In a sufficient shortquantum wire, intersubband scattering becomes inconsequential and quantization ofelectrical conductance is manifested [8,9].

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Chapter 2

Figure 2.3: Carrier transport in channel depends on relative dimension of the device size
and the mean free path [10]. Reprinted from Appl. Surf. Sci. 254, 6194 (2008), with thepermission of Elsevier Publishing.
Bart van Wees and his colleagues discovered the quantization of the conductance in a
two-dimensional electron gas (2DEG) in 1998 [8]. In that experiment, they manipulated thewidth of a channel where electrons flowed by applying a gate voltage to 2DEG, and observed
a change of the conductance in a stepwise manner, with steps of (2e2/h). This quantized
conductance may be explained as follows. Restricted regions where a current flows may beconsidered to be pseudo-one-dimensional. The electronic state in such restricted regions iscontinuous along the current flow direction, but it is quantized perpendicular to the current

27 | P a g e

Chapter 2
flow, with the quantization of the electronic state is identified by the Fermi wave-length λF.
When the width of the restricted region is close to λF, only one state is available for electrons.
As a result, two electrons (having up and down spins) may contribute to a current, making
increase to a conductance of Γ0 = 2𝑒 2 /ℎ . If the width of the sample is expanded, the2𝑒 2conductance increases stepwise in steps of Γ = (ℎ) × 𝑛.

In the ballistic transport, each channel is characterized by the quantized conductance Γ0 =2𝑒 2 /ℎ. Therefore, the total conductance may be given byΓ = ∑𝑖 Γ0 𝜏𝑖(2.6)

where 𝜏𝑖 is the transmission coefficient of the ith channel. This conductance is independentof material conductance, thus it is possible to overcome the conductivity mismatch in theballistic regime.
In fact, there are some experiments showing the promising magnetoresistance ratio inballistic transport regime. An investigation of MR effect in Ni point contacts reported by
García et al. [11] has shown that the MR ratio becomes large when the conductanceapproaches Γ0 . In that experiment, the maximum MR ratio was 280% (for the pessimistic
definition). These results are shown in Fig. 2.4. The MR effect observed was termed asballistic MR. Other researches conducted by Chopra and Hua [12,13] have reported that the
MR ratio is 3000 – 10,000 % for usual point contacts of Ni or those produced byelectrodeposition. However, the resistance in their measurements is only in range of 0.1 kΩ,which is much smaller than the inverse of the quantization conductance Γ0−1 = 12.9 𝑘Ω.

28 | P a g e

Chapter 2
Therefore, the conductive properties of their samples might be different from those observed
by García et al. For the vertical spin-valve structure, Pham et al. reports the large MR ratioof 8 % in the MnAs vertical spin-valve devices with MnAs nanoparticles at the bottom
electrode. [14].

Figure 2.4: Experimental results of conductance change as a function of quantized
conductance in Ni point contacts [11]. Reprinted from Phys. Rev. Lett. 82, 2923 (1999), withthe permission of APS Publishing.

2.3. Electrical Spin Injection and Transport Measurements
This section describes the typical measurements used to investigate the spin transportphenomena. Basically, the mechanism of these measurements is investigation of

29 | P a g e

Chapter 2
magnetoresistance, which is manifested as a change in the resistance (or voltage) acrossterminals at a certain magnetic field range (antiparallel magnetization of the twoferromagnetic electrodes).
2.3.1. Two-terminal local spin-valve
The mechanism of two-terminal local spin valve measurement is analogous to the metal
based spin valves, in which spin polarized carriers are injected from a FM electrode (as an
injector) into a non-magnetic semiconductor channel and detected by a second FM electrode(as a detector) that is placed within the spin diffusion length associated with thesemiconductor material. The Fig. 2.5 sketches a typical two-terminal spin-valve
measurement setup. Although this measurement can produce large spin signals (e.i. Largechange in resistance or voltage) and is the simplest method to set up, it is also highly
influenced by spurious effects. Therefore, it is necessary for a systematical investigation forthe spin-valve device in case of using the two-terminal local spin-valve measurement.
According to Fert and Jaffers [5], it is possible to estimate the transverse spin relaxation
time with channel length dependent magnetoresistance data by using these equations:

30 | P a g e

Chapter 2
The ratio of ΔR/RP is defined as the magnetoresistance. In the above formulas, β and γ are thebulk spin polarization and a spin-dependent tunneling parameter, respectively. lN is the nonferromagnetic channel length, rF and rN are the resistivity of the ferromagnetic electrode andnon-ferromagnetic channel, respectively, and rb* is the interface resistance of the tunnel
barrier contact. While the four-terminal non-local measurements can provide conclusive
evidence of spin injection, the two-terminal local measurements can still provide themaximum magnitude of spin response which is more applicable in practical devices.

Figure 2.5: Two-terminal local spin-valve measurement scheme on a four-terminal lateralspin-valve. In this configuration, lN is the channel length (center-to-center distance between
the two ferromagnetic (FM) electrodes. For a constant current bias (or voltage bias) acrossthe two FM electrodes, the voltage change (or current change) is measured as an external
magnetic field is swept across the device. The outer two electrodes can either be a FM or
non-magnetic ohmic contact.

31 | P a g e

Chapter 2
2.3.2. Four-terminal non-local spin-valve
Different from the two-terminal local measurement which use the same injector / detector
for spin and charge currents, the four-terminal non-local spin valve measurement separates
the charge current path and the pure spin diffusion path, eliminating most of the spurious
effects mentioned previously. Conventionally, it consists of two laterally separated FM
electrodes placed on the SC channel as the injector and detector for spin current, and twoouter reference ohmic contacts (either FM or non-magnetic) as shown in Fig. 2.6. In thismeasurement, the charge current flows from one of the FM electrode to its closest outermost
reference contact, causing a non-equilibrium spin accumulation induced in the SC channel.
The spin accumulation from the injecting FM electrode diffuses away from the injection point
to both left and right directions and relaxes with a characteristics length known as the spin
diffusion length. An adjacent detector FM electrode which is within the spin diffusion length
will detect the diffused spin electrons outside of the electric current path. The spin signal
which is considered as the voltage change when the injector and detector are in parallel andanti-parallel configuration can be defined as:∆𝑅 =∆𝑉 𝑃1 𝑃2 𝜆𝑠 𝜌𝐿=
exp (− )𝐼𝐴𝜆𝑠

In the above formula, P1 and P2 are the spin polarization of the current across theinterfaces of the (FM electrode-injector)/NM channel and NM channel/(FM electrodedetector), respectively, I is the electron injection current, ρ is the resistivity of thesemiconductor, A is the cross junction area of the semiconductor, and L is the lateral distance
between the two FM electrodes.

32 | P a g e

Chapter 2

Figure 2.6: The configuration of a four-terminal non-local spin-valve measurement. The
charge current is passed through the outer region on the left-side of the channel, while theright-side circuit with the FM detector is used to detect the diffused spin accumulation outsideof the charge current path.
2.3.3 Three-terminal Hanle precession measurements
Different from both local and non-local spin-valve measurements, the three-terminal
Hanle use a single FM electrode to observe the spin accumulation and precession. Thismeasurement configuration consists of three terminals – two reference electrodes (either FMor non-magnetic) and one FM electrode. The injection of spin polarized carriers into a SC
channel generates a net spin accumulation, resulting in splitting of the spin-dependent
electrochemical potential: Δ𝜇 = 𝜇↑ − 𝜇↓ . This splitting is detected as a voltage ∆𝑉3T =𝛾∆𝜇/2𝑞, in which γ is the tunneling spin polarization of the FM tunnel contact. Under anout-of-plane magnetic field Bz applied with respect to the z axis, the spins precess at the33 | P a g e

Chapter 2
Larmor frequency ωL = gµBBz/ћ, resulting in precessional dephasing of the net spin
accumulation in the channel, where g is the Lande g-factor, µB is the Bohr magneton, and ћ
is the reduced Planck’s constant. This precessional dephasing manifests itself in a Lorentzian
shaped data which can be defined by
𝑉3T (𝐵𝑍 ) = ∆𝑉3T (0)/[1 + (𝜔L 𝜏s )2 ]in which τs is the spin relaxation time. This measurement technique allows us to focus on andprobe the characteristics of the spin system directly underneath the spin injecting contact.
However, there is a crucial issue that may prevent one from obtaining an accurate spinsignal. Because this measurement uses the same electrode for both injection and detection of
the spin accumulation and precession, the interface quality between the FM/SC becomessignificantly important. According to Tran et al. [15], the spin polarized electrons can betrapped at the interface states or defect sites near the FM/SC interface, leading to longer spin
lifetimes than what the actual value inside the SC channel and causing an overestimation of
the spin lifetime value. There are methods to verify that interface states are not affecting thethree-terminal signal. The first one is to change the doping of the SC channel and confirming
the change in spin lifetimes. Conventionally, a lower doping concentration will cause a longer
spin lifetimes due to decrease in scattering. Another method is to use several tunnel barriermaterials. If the interface states or defect sites are not dominant in the observed spin signal,
the spin lifetime should remain relatively constant across multiple tunnel barrier materials.

34 | P a g e

Chapter 2

Figure 2.7: The configuration of a three-terminal Hanle measurement. Spin injection anddetection is experienced by a single FM electrode. The outer two contacts are ohmic referencecontacts. The magnetic field is swept orthogonal to the sample surface.

2.4. Summary
This chapter introduces the overviews of the spin transport mechanisms in diffusive
regime with the conductivity mismatch issue and ballistic regime with advantages toovercome that issue. Electrical spin measurements techniques were discussed as well. An
understanding of the physical mechanisms that govern spin transport inside various
semiconductors is essential for proper design and characterization of these devices, as well
as for overcoming the issue of conductivity mismatch.

35 | P a g e

Chapter 2
References
1.

Valet, T. & Fert, A. Theory of the perpendicular magnetoresistance in magnetic
multilayers. Phys. Rev. B 48, 7099–7113 (1993).

2.

Hiep, D. D., Tanaka, M. & Hai, P. N. Spin transport in nanoscale Si-based spinvalve devices. Appl. Phys. Lett. 109, 232402 (2016).

3.

Fabian, J., Matos-Abiague, A., Ertler, C., Stano, P. & Zutic, I. Semiconductor
Spintronics. Acta Phys. Slovaca 57, 565–907 (2007).

4.

Schmidt, G., Ferrand, D., Molenkamp, L. W., Filip, A. T. & van Wees, B. J.
Fundamental obstacle for electrical spin injection from a ferromagnetic metal into a
diffusive semiconductor. Phys. Rev. B 62, R4790–R4793 (2000).

5.

Fert, A. & Jaffrès, H. Conditions for efficient spin injection from a ferromagnetic
metal into a semiconductor. Phys. Rev. B 64, 184420 (2001).

6.

Rashba, E. I. Theory of electrical spin injection: Tunnel contacts as a solution of the
conductivity mismatch problem. Phys. Rev. B 62, R16267–R16270 (2000).

7.

Sugahara, S. & Tanaka, M. A spin metal–oxide–semiconductor field-effect transistor
using half-metallic-ferromagnet contacts for the source and drain. Appl. Phys. Lett.
84, 2307–2309 (2004).

8.

van Wees, B. J. et al. Quantized conductance of point contacts in a two-dimensional
electron gas. Phys. Rev. Lett. 60, 848–850 (1988).

9.

Wharam, D. A. et al. One-dimensional transport and the quantisation of the ballistic
resistance. J. Phys. C Solid State Phys. 21, L209–L214 (1988).

10.

Natori, K. Ballistic/quasi-ballistic transport in nanoscale transistor. Appl. Surf. Sci.

36 | P a g e

Chapter 2254, 6194–6198 (2008).
11.

García, N., Muñoz, M. & Zhao, Y.-W. Magnetoresistance in excess of 200 % in
Ballistic Ni Nanocontacts at Room Temperature and 100 Oe. Phys. Rev. Lett. 82,
2923–2926 (1999).

12.

Chopra, H. D. & Hua, S. Z. Ballistic magnetoresistance over 3000% in Ni
nanocontacts at room temperature. Phys. Rev. B 66, 020403 (2002).

13.

Hua, S. Z. & Chopra, H. D. 100,000 % ballistic magnetoresistance in stable Ni
nanocontacts at room temperature. Phys. Rev. B 67, 060401 (2003).

14.

Hai, P. N., Sakata, Y., Yokoyama, M., Ohya, S. & Tanaka, M. Spin-valve effect byballistic transport in ferromagnetic metal (MnAs)/semiconductor (GaAs) hybrid
heterostructures. Phys. Rev. B - Condens. Matter Mater. Phys. 77, 1–6 (2008).

15.

Tran, M. et al. Enhancement of the spin accumulation at the interface between aspin-polarized tunnel junction and a semiconductor. Phys. Rev. Lett. 102, 1–4(2009).

37 | P a g e

Chapter 3

CHAPTER 3
DEVICE FABRICATION and
CHARACTERIZATIONThis chapter describes the fabrication process and characterizations of devices in thisstudy. Several techniques were used and a large number of samples were fabricated. This
chapter is aimed at providing an insight into the fabrication and characterization of Si-based
nanoscale spin-valve devices.

38 | P a g e

Chapter 3

3.1. Techniques
In this research, I employ 5 main techniques to fabricate the Si-based nanoscale spinvalve devices. They are Electron Beam (EB) Evaporation and Molecular Beam Epitaxy(MBE) for depositing ferromagnetic (FM) and tunnel barrier layers, Electron Beam
Lithography (EBL) for the nanoscale mask design, ion milling technique for patterning thenanoscale Si channel, and photolithography to fabricate the measurement electrodes.
Electron Beam Evaporation

EB Evaporation or Electron Beam Physical Vapor Deposition is a material deposition
method in which a target anode is bombarded with an electron beam emitted from acharged tungsten filament under high vacuum. The target material is heated by thiselectron beam and transformed into the gaseous phase, then evaporated in the vacuum
chamber, and coating the in-situ sample. In this study, I used the ULVAC EB evaporator.


Molecular-beam Epitaxy

MBE is an epitaxy method for deposition of high quality crystalized thin film, which
is normally operated in high or ultra-high vacuum of 10-6 – 10-10 Pa. In this research, I
use a custom-made MBE system with the based pressure of 10-8 Pa. In the MBE chamber,there are different material sources in the so-called effusion cells. From these cells, thematerials are heated and evaporated, then the molecules land on the surface of sample toform thin films. The most importance of MBE is the low deposition rate that allows thefilms to grow epitaxially and low impurity concentration.
Electron beam lithography

EBL is an important technique in nanotechnology. In this technique, a focused beamof electrons is used to pattern features down to sub-10 nm on a surface covered with anelectron-sensitive film called a resist. The electron beam changes the solubility of theresist, enabling selective removal of either the exposed or non-exposed regions of theresist by immersing it in a developer. In this research, the EBL technique was employed
to pattern the 20 nm Si spin-valve devices. In this study, I used the Hitachi S-4500 EBL
system which is both EBL and Scanning Electron Microscope (SEM) machine. In this39 | P a g e

Chapter 3
EBL process, the mixer of ZEP-520A and ZEP-A in 1:1 ratio was used as electron beam
resist.


Ion milling technique

Ion Milling is a physical etching technique whereby the beam of high-energy Ar ionsbombard into a substrate in vacuum in order to erode the surface to some desired depth
or under-layer. [VLSI electronics Microstructure Science, volume 8, chapter 11, 1984]

Photolithography

Photolithography technique is widely used in microfabrication to design the
microscale structures. In this technique, the ultra-violet light is used to project a geometricstructure onto a photoresist surface, which is a light-sensitive chemical, by a photomask.
The surface then is undergone some chemical treatments to remove the un-needed area,
to get the desired structure. In this study, I use OFPR-54CP as the photoresist.

3.2. Fabrication process
Si-based nanoscale 2 terminal spin valve device
A schematic drawing of the Si-based nanoscale 2 terminal spin-valve device is shown infigure 2.1.

Figure 2.1: A schematic drawing of the Si-based nanoscale 2 terminal spin-valve device

40 | P a g e

Chapter 3
The basic process flow for a local spin valve device is shown in figure 2.2. Firstly, the Si
substrate is prepared by standard cleaning process of acetone – H2SO4 – BHF to remove thenative SiO2 on surface.
Secondly, the samples were introduced into the ultra-high vacuum to deposit the FM
and tunnel barrier layers. In this step, I have fabricated 2 series of samples with similar
structure but by different deposition method: EB evaporation and MBE.
The third step is creating the 20 nm Si channel by EBL and ion milling techniques. In
this step, I have used EBL to pattern 30 nm-thick Au hard masks with a 20 nm-long and 100
nm-wide gap in between, followed by using Ar ion milling to etch the exposed Fe area anddefine a 20 nm-long Si channel.
Finally, I have used the photolithography, EB evaporation and liftoff techniques tofabricate the Au (40 nm) / Cr (5 nm) pad as the measurement electrodes.

41 | P a g e

Chapter 3

1. The Si substrate after the cleaning process of acetone –H2SO4 – BHF.
2. Deposition of the Ge/MgO double tunnel barrier and Fe
electrode layers by EB evaporation or MBE.

3. ZEP-520A resist is spin coated on the sample and baked
at 1800C in 4 minutes to prepare for the EBL process.

4.
The EBL process, hard mask deposition and lift-off
process is conducted for the hard mask fabrication.

5. The ion milling process is conducted to etch the sample
surface to define the nanoscale Si channel. The exposed Fe
area is eroded to form the 20 nm-long Si channel. The area
covered by hard mask is protected to become the FM
electrodes.
6. Finally, the photolithography, EB evaporation and liftoff
techniques is conducted to fabricate the Au/Cr (40nm/5nm)
pad as the measurement electrodes.

Figure 2.2: The fabrication process for a local spin valve device

42 | P a g e

Chapter 3

3.3. Substrate preparation
In this research, the highly doped n-type Si (100) substrates with electron density n = 1
×1018 cm-3 were used. In order to get a clean sample for the fabrication process, 3 kinds ofcleaning process were employed: standard cleaning process (with acetone, isopropanol (IPA),
and deionized water (DI water)) and Piranha cleaning process (with H2O2 and H2SO4) toremove dust and organic element. Finally, the BHF etching process was used to remove thenative oxide layer on the surface of sample before doing the fabrication process. The sample
preparation is as follow.


Si substrates were cut into appropriate size, e.g. square 20 mm on a side.

Standard cleaning of acetone – isopropyl alcohol (IPA) – DI water with ultrasonicin 2 minutes in each.

Piranha cleaning process (H2SO4:H2O2 in 1:1 ratio). Samples are dipped in
Piranha solution in 10 minutes, then rinsed in DI water in 10 minutes, followedby drying with air gun.



BHF etching process to remove the native oxide layer on surface of sample.
Samples are dipped in BHF in 90 seconds, then rinsed in DI water, followed bydrying with air gun. Samples are ready for the deposition step.

3.4. FM electrodes and tunnel barriers deposition
After the cleaning process, the samples were introduced into ultra-high vacuum todeposit the FM and tunnel barrier layers. We have fabricated 2 series of sample with similar
structure but by different deposition method: Electron-Beam (EB) evaporation and
Molecular-Beam-Epitaxy (MBE).

43 | P a g e

Chapter 3
3.4.1. EB evaporation of the FM electrodes and tunnel barriers
The 1st series were fabricated by the EB evaporation method. After the cleaning process,the samples were introduced into an ultra-high-vacuum EB evaporation chamber with thebase pressure of 8×10-6 Pa to deposit the 10 nm Fe layer and finally capped with a 3 nm Au
thin film. In order to enhance the spin injection efficiency from Fe to Si, I inserted an
MgO/Ge double layer between the Fe electrodes and Si substrates. According to literature,
MgO is a promising spin-dependent tunnel barrier for efficient spin injection fromferromagnetic electrode into semiconductors (SCs) [1,2]. However, it is difficult to grow avery thin MgO layer with a thickness of 1~2 nm on Si at room temperature for using as atunnel barrier. On the other hand, epitaxial growth of MgO on Ge has been reported [3,4].
Moreover, deposition of smooth thin film of Ge on Si has been demonstrated at low
deposition temperature [5]. Therefore, in these studies I decided to employ an ultra-thin filmof Ge as a buffer layer between MgO and Si to improve the quality of MgO layer grown atroom temperature. In order to investigate the role of the MgO/Ge double layer comparing
with conventional MgO single barrier, I have prepared 3 spin-valve device structures; Device
1 with Fe electrodes deposited directly on the Si channel; Device 2 with 2 nm MgO inserted
between Fe electrodes and Si channel; and Device 3 with an 2 nm-thick MgO / 1 nm-thick
Ge double layer as the tunnel barrier. Figure 1(a) shows the schematic structure of device 3
with (2 nm MgO / 1 nm Ge) double tunnel barriers. All of the deposition processes have been
conducted in the same EB evaporation chamber without breaking the vacuum.
3.4.2. MBE growth of the FM electrodes and tunnel barriers
In the second deposition method, I grew the tunnel barrier and Fe electrode layers by
MBE in order to improve the crystal quality of Fe electrode and tunnel barrier layers. Afterthe cleaning process, Si substrates were introduced into a MBE chamber with a base pressureof 1×10-8 Pa for growing the multilayers stack of Fe electrode and tunnel barrier. In thisresearch, I have grown 2 series of sample by MBE method. In the first one, in order toinvestigate the spin transport in nanoscale Si channel at different thickness of MgO tunnel
barrier layer, I grew the stack of 1 nm Ge / (1.5 – 3.5 nm) MgO tunnel barrier / 10 nm Fe

44 | P a g e

Chapter 3
layer, and finally capping by 1.1 nm MgO layer. In the second one, in order to enhance thespin injection in our spin-valve devices by prevent the magnetically-dead layer arise between
Fe and MgO layers, I have inserted an Mg buffer layer in between them. I grew the stack of
1 nm Ge / (1.5 – 3.5 nm) MgO tunnel barrier / 1 nm Mg / 10 nm Fe / 1.1 nm MgO layer.
Knudsen cells were used for thermal evaporation of Ge and Fe, while a low-power electronbeam evaporator was used to deposit MgO with a slow rate of 0.03 Å/s. The X-ray diffraction
has been employed to investigate the crystal quality of the tunnel barrier, and has confirmed
the crystal orientation of Fe(110) || MgO(001) || Ge(001) in the MBE grown samples.
Therefore, I expect that the crystal quality of the MBE-grown MgO/Ge double layer is muchbetter than that grown by EB evaporation.

3.5. Fabrication of nanoscale Si channel
After deposit the tunnel barrier and FM layers, I used the EBL, EB evaporation and ion
milling techniques in order to create the 20 nm Si channel. In this step, the EBL and EB
evaporation processes are used to make the hard mask on the prepared sample. The e-beamresist (mixer of ZEP-520A and ZEP-A in 1:1 ratio) is spin coated onto the sample with the
thickness of 200 nm. Then, the sample is baked at 1800C in 4 minutes by a hot plate, followed
by transferring into the EBL chamber for EB exposure. Next, the sample is developed in thedeveloper ZED-N50 in 4 minutes and rinsed in ZMD-B to dissolve the exposed resist. Afterdeveloping, the sample is transferred into the high vacuum EB evaporation chamber todeposit hard mask. For samples capped by 3 nm Au, the hard mask is 30 nm Au; for samples
capped by 1 nm MgO, the hard mask is (5 nm Cr / 35 nm Au). The lift-off process was carriedout in N,N-Dimethylformamide at 800C in 30 minutes, followed with aceton, IPA and DI
water. Finally, I employed the Ar ion milling method to etch away the exposed Fe area anddefine a 20 nm-long Si channel. Figure 2.3 shows the spin-valve device with Fe electrodes
as the spin injector / detector and the 20 nm Si channel in between.

45 | P a g e

Chapter 3

Figure 2.3: a) The spin-valve device with Fe electrodes as the spin injector / detector
and a 20 nm Si channel in between. b) The scanning electron microscope (SEM) image ofthe 20 nm Si channel.

46 | P a g e

Chapter 3

3.6. Connecting the spin-valve device to the microscale contacts
For characterization, it is necessary to connect the spin-valve device to micro contacts forspin-valve measurement. In order to fabricate the contacts, the photolithography, EB
evaporation and lift-off techniques were used.
Firstly, the photoresist (OFPR-54CP) is spin coated onto the sample at 500 rpm – 5 sec
and 5000 rpm – 30 sec. Then, the sample is soft baked at 1100C in 3 minutes by a hot plate,followed by transferring into the photolithography chamber for exposure. Next, the sample
is developed in the developer NMD-3 in 90 sec and rinsed in DI water in 2 minutes to dissolve
the exposed resist. After developing, the sample is transferred into the high vacuum EB
evaporation chamber to deposit the Au (40 nm) / Cr (5 nm) pad electrodes. The lift-off
process was carried out in N,N-Dimethylformamide at 800C in 30 minutes, followed withaceton, IPA, DI water and drying by air gun to obtain the final Si-based spin-valve device asshown in Fig. 2.4. The sample is ready for measurements.

47 | P a g e

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Figure 2.4: The final 2 terminal spin-valve devices after fabrication process.

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Chapter 3

3.7. Wire bonding
Because the spin-valve effect measurement is conducted in the cryostat at low
temperature, the final step before measurement is the wire-bonding of sample onto thecryostat sample holder, and wire bonding is used for this purpose. In this step, instead ofusing a wire bonder whose ultrasonic wave has high possibility to break the nanoscale device,
the portable soldering iron and indium were used to weld the gold wire connecting the devices
and cryostat sample holder. This bonding method has 100% of success, and does not breakany device.

3.8. Characterization
3.8.1. Local magneto-transport measurement
In conventional two-terminal spin-valve effect measurements, the spin-injecting andspin-detecting terminals are the same. Thus, they suffer from parasitic local
magnetoresistance effects, such as the anisotropic magnetoresistance (AMR) effect of the
FM electrodes and the tunneling anisotropic magnetoresistance (TAMR) effect at the FM /SC interfaces. To avoid such local effects, two well-known methods have been used to detect
intrinsic spin transport in mm-long SC channels. One method is to observe the non-local
spin-valve effect in a four terminal configuration, where the spin-injecting terminals and thespin-detecting terminals are different. If the distance l between the spin-injecting and spindetecting terminals is far enough compared with the depth d of the SC channel where theelectric current flows, then the spin-detecting terminal voltage will not be affected by local
effects and can detect a pure spin current [6]. However, if l is comparable or shorter than d,the non-local voltage is also affected by local effects as shown by careful experiments anddevice simulations [7]. As a result, the four-terminal configuration has no advantage over the
two-terminal configuration in our case of nano-scale SC channels, because both methods
suffer from local effects. Another method is to observe spin precession when the spinpolarized electrons diffuse through the SC channel under an applied magnetic field (the Hanle

49 | P a g e

Chapter 3
effect) [8]. In this method, however, the SC channels have to be long enough so thatconsiderable spin precession can occur. Therefore, the Hanle effect cannot be used in my 20
nm-long Si channels, where the spin precession is negligible. Finally, I note that any realistic
nano-scale spin MOSFETs should operate only with source and drain electrodes,
corresponding to the two terminal configuration. Taking the above into consideration, I
conclude that the two-terminal spin-valve effect measurement is a realistic way to detect the
spin transport in our devices, even though this method suffers from local effects. To
distinguish the intrinsic spin-valve effect from the parasitic local effects in the two-terminal
configuration, I systematically investigated the bias voltage dependence, temperaturedependence, and magnetic-field direction dependence of MR in our devices. The figure 2.5
shows the 2 terminal local spin-valve measurement configuration. In this measurement, I
used a JANIS liquid He cryostat system for low temperature measurement and a Keithley
2400 SourceMeter as the DC voltage source and current meter.
3.8.2. Cryostat system
In this study, a cryogenic magneto resistance measurement system is used. In the liquid
helium cryostat, there are a sample chamber inside and vacuum chamber outside whichisolated to each other. The liquid helium can be injected into the sample chamber inside to
decrease the sample temperature to maximum of 4.3 K. The outside vacuum chamber is used
to isolate the sample chamber to from the outside world.

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Chapter 3

Figure 2.5: The 2 terminal local spin-valve measurement configuration.

3.9. Summary
This chapter describes the fabrication process and characterization of device used in thisstudy. After introducing the general concept of techniques mainly used in this research suchas EB evaporation, MBE, EBL, ion milling, and photolithography, I discuss about the process
flow which is used for device fabrication, from the beginning of substrate preparation, FM
electrodes and tunnel barriers deposition, fabrication of nanoscale Si channel, connecting thespin-valve device to the microscale contacts, to the last step is wire bonding on chip carrier.
In the end of the chapter, the setups and techniques used to measure our devices are briefly
discussed.

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Chapter 3
References
1.

Jiang, X. et al. Highly Spin-Polarized Room-Temperature Tunnel Injector for
Semiconductor Spintronics using MgO(100). Phys. Rev. Lett. 94, 056601 (2005).

2.

Martı́nez Boubeta, C. et al. Epitaxial Fe/MgO heterostructures on GaAs(001). J.
Cryst. Growth 226, 223–230 (2001).

3.

Jeon, K.-R., Park, C.-Y. & Shin, S.-C. Epitaxial Growth of MgO and CoFe/MgO on
Ge(001) Substrates by Molecular Beam Epitaxy. Cryst. Growth Des. 10, 1346–1350(2010).

4.

Han, W. et al. Growth of single-crystalline, atomically smooth MgO films on
Ge(001) by molecular beam epitaxy. J. Cryst. Growth 312, 44–47 (2009).

5.

Schöllhorn, C., Oehme, M., Bauer, M. & Kasper, E. Coalescence of germanium
islands on silicon. Thin Solid Films 336, 109–111 (1998).

6.

Lou, X. et al. Electrical detection of spin transport in lateral ferromagnet–
semiconductor devices. Nat. Phys. 3, 197–202 (2007).

7.

Nakane, R., Sato, S., Kokutani, S. & Tanaka, M. Appearance of anisotropic
magnetoresistance and electric potential distribution in Si-based multiterminal
devices with Fe electrodes. IEEE Magn. Lett. 3, (2012).

8.

Johnson, M. & Silsbee, R. H. Interfacial charge-spin coupling: Injection anddetection of spin magnetization in metals. Phys. Rev. Lett. 55, 1790–1793 (1985).

52 | P a g e

Chapter 4

CHAPTER 4
INVESTIGATION of SPIN-VALVE DEVICESFABRICATED by ELECTRON BEAM
EVAPORATIONIn this chapter, the investigation of the spin transport in nanoscale silicon (Si)-based spinvalve devices with Fe electrodes, MgO/Ge tunnel barriers, and a 20 nm-long Si channel was
conducted. I observed a clear spin-valve effect when a magnetic field was applied in the film
plane along and perpendicular to the Si channel transport direction. Systematic investigationsof the bias voltage dependence, temperature dependence, and magnetic-field direction
dependence of the magnetoresistance indicate that the observed spin-valve effect is governedby the spin transport through the nanoscale Si channel. The spin-valve effect remains
observable up to 200 K. For the device with MgO/Ge tunnel barriers, with a bias voltage of
1.7 V at 50 K, the spin-dependent output voltage is 13 mV, which is among the highest values
reported so far. The results in this chapter have been published on APL 109, 232402 (2016).

53 | P a g e

Chapter 4

4.1. Spin-valve device structures
The nanoscale spin-valve devices were fabricated on a highly doped n-type Si (100)substrate with an electron density n = 1×1018 cm-3. The Si substrates were cleaned by acetone– H2SO4/H2O2 – BHF, then rinsed in DI water. After that, the samples were introduced into
an ultra-high-vacuum electron-beam (EB) evaporation chamber to deposit a 10 nm Fe layer,
and finally capped with a 3 nm Au layer.
To enhance the spin injection efficiency from Fe to Si, I inserted an MgO/Ge double layer
between the Fe electrodes and Si substrates. Recently, a number of studies have shown that
MgO is a promising tunnel barrier for efficient spin injection to semiconductors (SCs) [1,2].
While it is known that one can grow a relatively thick (~10 nm) MgO layer of good quality
on Si at high growth temperature (~ 300 oC) [3], it is difficult to grow a very thin MgO layer
with a thickness of 1~2 nm on Si at room temperature for use as a tunnel barrier. On the other
hand, epitaxial growth of MgO on Ge has been reported [4,5]. Moreover, deposition ofsmooth thin film of Ge on Si has been demonstrated at low deposition temperature [6]. In
this work, to improve the quality of MgO layer at room temperature, I employed an ultrathin film of Ge as a buffer layer between MgO and Si. In order to investigate the role of the
MgO/Ge double layer comparing with conventional MgO single barrier, 3 spin-valve device
structures have been prepared; Device 1 with Fe electrodes deposited directly on the Sichannel; Device 2 with 2 nm MgO inserted between Fe electrodes and Si channel; and Device
3 with an 2 nm-thick MgO / 1 nm-thick Ge double layer as the tunnel barrier. Figure 4.1
shows the schematic structure of 3 devices with different tunnel barriers.

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Chapter 4

Figure 4.1: The schematic spin-valve device structures used in this chapter. (a) Device 1
with Fe electrodes deposited directly on the Si channel; (b) Device 2 with 2 nm MgO tunnel
barrier; (c) Device 3 with a 2 nm-thick MgO / 1 nm-thick Ge double tunnel barrier; and (d)SEM image of Device 3.

55 | P a g e

Chapter 4
After depositing the tunnel barrier and FM layer, e-beam lithography (EBL) and ionmilling techniques were used to fabricate nanoscale Si spin-valve devices. Finally, I
fabricated Au (40 nm) / Cr (5 nm) pad electrodes by EB evaporation and standard
photolithography. Figure 4.1(d) shows a top-view image of Device 3 taken by scanningelectron microscopy (SEM). A Si channel with a length of 20 nm and a width of 100 nm wasformed between the Fe electrodes. Note that although the shape of the Fe electrodes looks
symmetric, their coercive forces can be different. The coercive forces of Fe electrodes are
determined by the domain wall pinning potential at grain boundaries or defects. In reality,
details of such grain boundaries or defects are not exactly the same in the left and rightelectrodes, which results in different coercive forces. Indeed, I found that the coercive force
differs by 200 G, as shown later. This small difference establishes a narrow range of magnetic
field where the anti-parallel magnetization configuration is realized, thus the spin-valve effectis observed.

4.2. Conductance characteristics
In this section, the conductance characteristics of the Si-based nanoscale 2 terminal spinvalve device fabricated by EB evaporation method is investigated.
Firstly, I measure the current – voltage characteristics (I-V curves) of the devices. Figure
4.2 shows the I-V curves and the resistances of Device 3 at different temperatures. In the
figure 4.2(a), the I-V curves clearly show non-linear dependence of the current on the bias

56 | P a g e

Chapter 4
voltage, indicating that the current is governed by the tunneling process at the Fe/MgO/Ge/Si
interfaces. Furthermore, the strong temperature dependence of the I-V curves was also
observed. At low temperatures, thermionic emission of electrons from the Fe electrodes to
the Si over the tunnel barrier is suppressed. In addition, free carriers in the Si channel are
partly quenched. Therefore, the current becomes smaller at low temperatures. This suggeststhat the resistance of the device is dominated by the transport through the spin-valve structure
consisting of Fe(interface)/(MgO/Ge)/Si/(Ge/MgO)/Fe(interface). In reality, however, there
is always contribution of the parasitic resistance of the Fe electrodes, especially at low bias
voltages. In general, the resistance R can be decomposed into two components: R = RFe + Rsv,
where RFe is the parasitic resistance of the Fe electrodes and Rsv is the resistance of theFe(interface)/(MgO/Ge)/Si/(Ge/MgO)/Fe(interface) spin-valve structure. In order toquantitatively estimate the contribution of RFe, I have measured the dependence of R on
temperature from 300 K to 4.3 K which is plotted in the Fig. 4.2(b). Here, R is measured at
V = 20 mV. As temperature decreases, R increases rapidly. From this data, the ratio RFe/R
could be estimated as follows. Since the parasitic RFe(T) decreases as temperature decreases,
RFe(T)  RFe(300K) < R(300K). Therefore, RFe(T)/R(T) should be smaller than R(300K)/R(T).
From this consideration, it is reasonable to estimate that RFe/R is smaller than 4% and 9% at
4.3 K and 200 K, respectively. This estimation will be used to distinguish the intrinsic spinvalve effect from parasitic local magnetoresistance (MR) effects of the Fe electrodes, as
discussed later.

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Chapter 4

Figure 4.2: Conductance characteristics of the Si-based nanoscale spin-valve devices
fabricated by EB evaporation (Device 3 with MgO/Ge double layer). a) I-V curves of Device
3 at different temperatures; b) The dependence of resistance on temperature of Device 3
measured at 20 mV. Reprinted from Appl. Phys. Lett. 109, 232402 (2016), with thepermission of AIP Publishing.

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Chapter 4

4.3. Spin-dependent transport characteristics
In this section, the systematic investigation of the spin-dependent transportcharacteristics of the Si-based nanoscale 2 terminal spin-valve device fabricated by EB
evaporation method in the aspect of bias voltage dependence, temperature dependence andmagnetic-field direction dependence was conducted.
In conventional two-terminal spin-valve effect measurements, the spin-injecting andspin-detecting terminals are the same. Thus, they suffer from parasitic local
magnetoresistance effects, such as the anisotropic magnetoresistance (AMR) effect of the FM
electrodes and the tunneling anisotropic magnetoresistance (TAMR) effect at the FM / SCinterfaces. To avoid such local effects, two well-known methods have been used to detect
intrinsic spin transport in µm-long SC channels. The first is the non-local spin-valve effect ina four terminal configuration, where the spin-injecting terminals and the spin-detecting
terminals are different. If the distance l between the spin-injecting and spin-detecting
terminals is far enough compared with the depth d of the SC channel where the electric
current flows, then the spin-detecting terminal voltage will not be affected by local effects
and can detect a pure spin current [7]. However, if l is comparable or shorter than d, the nonlocal voltage is also affected by local effects as shown by careful experiments and device
simulations [8]. As a result, the four-terminal configuration has no advantage over the twoterminal configuration in our case of nanoscale SC channels, because both methods sufferfrom local effects. The second is to observe spin precession when the spin-polarized electrons
diffuse through the SC channel under an applied magnetic field (the Hanle effect) [9]. In this59 | P a g e

Chapter 4
method, however, the SC channels have to be long enough so that considerable spin
precession can occur. Therefore, the Hanle effect cannot be used in our 20 nm-long Si
channels, where the spin precession is negligible. Finally, it is noticed that any realistic
nanoscale spin MOSFETs should operate only with source and drain electrodes,
corresponding to the two terminal configuration. Taking the above into consideration, it could
be concluded that the two-terminal spin-valve effect measurement is a realistic way to detect
the spin transport in our devices, even though this method suffers from local effects. To
distinguish the intrinsic spin-valve effect from the parasitic local effects in the two-terminal
configuration, I systematically investigated the bias voltage dependence, temperaturedependence, and magnetic-field direction dependence of MR in our devices.
4.3.1. Local spin-valve effectFirstly, the local spin-valve effect is measured. Figure 4.3 shows the resistance-magnetic
field (magnetoresistance – MR) characteristics of Device 1 – 2 – 3, respectively, measured at
50 K with a magnetic field applied along the Si channel (along the x-direction in Fig. 4.1(a)),
and a bias voltage of 100 mV applied between the two FM electrodes. In Fig. 4.3, blue dots
are data taken when the magnetic field was swept from +5 kG to -5 kG, while red dots aredata taken when the magnetic field was swept from -5 kG to +5 kG. In Device 1 with Feelectrodes deposited directly on the Si channel, I have observed the small local signal ∆R of
0.6 Ω, corresponding to MR ratio of 0.03% as shown in fig. 4.3(a). In Device 2, by using 2
nm MgO conventional tunnel barrier between Fe electrodes and Si channel as previous
studies, the local signal is increased to ∆R of 3 Ω – MR of 0.25% (Fig. 4.3(b)). In Device 3

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Chapter 4
with a 2 nm-thick MgO / 1 nm-thick Ge double layer as the new tunnel barrier, I achieved astrong local signal up to 12 Ω – corresponding to MR ratio of 0.8% in the device with
MgO/Ge tunnel barrier as depicted in figure 4.3(c). This is also the highest local spin-valve
signal in lateral Si-based spin-valve device at that time. It strongly demonstrates theadvantage of my new double tunnel barrier compare to the conventional single MgO tunnel
barrier. It could be understood that the spin coherence of tunneling electrons through thetunnel barrier has been improved by using the MgO/Ge as the tunnel barrier. Therefore, inthe following sections, I focus on the spin-valve effect of the Device 3 with MgO/Ge tunnel
barrier.

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Chapter 4

Figure 4.3: Resistance – magnetic field (magnetoresistance, MR) characteristics of (a)Device 1 with Fe electrodes deposited directly on the Si channel, (b) Device 2 with 2 nm
MgO conventional tunnel barrier, and (c) Device 3 with a 2 nm-thick MgO / 1 nm-thick Ge
double tunnel barrier, respectively, measured at 4.3 K with a bias voltage of 100 mV. Here,a magnetic field was applied along the Si channel transport direction (along the x-direction).
Blue dots are data taken when the magnetic field was swept from +5 kG to -5 kG, while red
dots are data taken when the magnetic field was swept from -5 kG to +5 kG.

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Chapter 4
4.3.2. Dependence of spin-valve signal on bias voltage and temperature
In order to verify that the observed MR originates from the spin-valve effect rather thanthe AMR effect of the Fe electrodes, the dependence of local spin-valve signal R on bias
voltages V was measured. Since the parasitic RFe of the metallic Fe electrodes does not dependon the bias voltage, the resistance change RFe due to the AMR effect of Fe should not depend
on V. In contrast, the spin-valve signal Rsv due to the spin-valve effect varies with V due tothe bias dependence of the spin-injection / detection efficiency [7]. Figures 4.4 shows thedependence of R on bias voltage V of device 1 – 2 – 3, respectively. I found that R
decreases as V increases in all three devices, contrary to the AMR effect theory. It indicatesthat the AMR effect of the Fe electrodes is not the origin of the observed MR.

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Chapter 4

Figure 4.4: Bias voltage dependence of the magnetoresistance ∆R of Device 1 – 2 – 3,
respectively. a) Relationship of ∆R – Vbias of Device 1 measured at 4.3 K. b) Relationship of∆R – Vbias of Device 2 measured at 50 K. c) Relationship of ∆R – Vbias of Device 3 measured
at 4.3 K. All devices show the decrease of ∆R as bias voltage increases. Reprinted from Appl.
Phys. Lett. 109, 232402 (2016), with the permission of AIP Publishing.

64 | P a g e

Chapter 4
It is also possible to distinguish the intrinsic spin-valve effect from the parasitic AMR
effect by measuring the temperature dependence of R. Since the parasitic RFe increases withincreasing temperature, RFe also increases with increasing temperature. In contrast, theresistance change Rsv due to the spin-valve effect is expected to decrease with increasing
temperature because the spin-life time of electrons in the Si channel becomes shorter as
temperature increases [10]. Figures 4.5 shows the dependence of R on temperature T of
device 1 – 2 – 3, respectively. I have observed the R decreases with increasing temperaturein all devices, which is conflicted with the AMR effect theory, demonstrating that theobserved MR is not related to the AMR effect.

65 | P a g e

Chapter 4

Figure 4.5: The temperature dependence of local spin-valve signal ∆R of (a) Device 1 measured
at a bias voltage of 100 mV, (b) Device 2 measured at a bias voltage of 50 mV, and (c) Device 3
measured at a bias voltage of 100 mV. All devices show the R decreases with increasing

temperature. Reprinted from Appl. Phys. Lett. 109, 232402 (2016), with the permission of
AIP Publishing.

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Chapter 4
Furthermore, since RFe/R is at most 4% at 4.3 K for Device 2 and the AMR ratio RFe/RFeof Fe is in the order of 0.1%, the contribution of the AMR effect to the MR ratio RFe/Rshould be at most (RFe/RFe)×(RFe/R) ~ 10-5, which is too small to account for the observed
MR ratio of 0.8%. From the above qualitative and quantitative considerations, it could beconcluded that the contribution of the parasitic AMR effect is negligible.
4.3.3. Dependence of spin-valve signal on magnetic-field direction
There is another local effect that should be addressed; the TAMR effect at theFe(interface)/MgO/Ge/Si. The TAMR effect originates from the dependence of the tunnelingdensity of states (DOS) in the FM electrodes on the magnetization direction. The contributionof TAMR to the spin valve signals was investigated by measuring the dependence of themagnetoresistance on the magnetic-field direction ϕ with respect to the x-direction, as shownin Fig. 4.6(a). Figures 4.6(b)-(e) show the MR curves of Device 3 measured at 50 K when themagnetic field was applied along ϕ = 0o, 30o, 60o, and 90o, respectively. In these measurement,
if the observed MR originated from the TAMR effect, the MR signal would be reversed when
ϕ changes from 0o to 90o; that is, there would be two drops in the MR curve at ϕ = 0o, but two
jumps at ϕ = 90o [11,12,13]. However, the same shape and polarity of the MR curves for all
ϕ have been observed. Furthermore, I observed clear minor loops (green curves) in all cases.
(Note the difference of about 200 G of the coercive force between the major and minor loops.)
These results indicate that the observed resistance jumps depend only on the relative angle
between the magnetization directions of the two FM electrodes, consistent with the behaviorof the spin-valve effect. It could be concluded that, while there might be TAMR in my devices,

67 | P a g e

Chapter 4
its contribution should be smaller than the experimental noise level (<0.1%), and cannot be
observed in our experiments. The absence of the TAMR effect in my devices can be explainedby the fact that the Fe electrodes are polycrystalline, thus the change of tunneling DOS withthe direction of magnetization in different crystal grains is canceled and becomes zero. It isalso noted that the observed MR effect remains observable up to 200 K, which is in good
agreement with previous reports on the temperature dependence of the spin-life time ofelectrons in Si [10,14]. All of these results strongly suggest that the observed MR is governedby spin transport through the nanoscale Si channel in our devices.

68 | P a g e

Chapter 4

Figure 4.6: (a) Experimental setup for measurement of magnetic-field direction
dependence of the magnetoresistance. Here, the magnetic field is applied in the x-y plane
along the angle ϕ with respect to the Si channel (the x-direction). (b)-(e) Magnetoresistanceof Device 3 measured at 50 K with a bias voltage of 20 mV, when ϕ = 0o, 30o, 60o, and 90o,
respectively. Minor loop magnetoresistance curves (green) are also shown. Reprinted from
Appl. Phys. Lett. 109, 232402 (2016), with the permission of AIP Publishing.

69 | P a g e

Chapter 44.4. Spin-dependent output voltage
Finally, it is the investigation of the spin-dependent output performance of my devices.
In realistic applications, a spin-dependent output ∆𝑉 = ∆𝑅 × 𝐼 of the order of 100 mV is
required for correct read-out. However, previous studies on spin injection to Si reported aread-out voltage of only a few µV in 4 terminal measurements, and about 1 mV in 3 terminal
measurements. In the voltage-biased spin-valve devices of mine, the corresponding spin𝑉∆𝑅

dependent output is calculated by ∆𝑉 = ∆𝑅 × (𝑅) = ( 𝑅 ) × 𝑉. Figure 4.7 shows the plot of
the spin-dependent output V = (R/R)×V of Device 3 as a function of the bias voltage V at
50 K. In this device, I achieved V = 13 mV at V = 1.7 V. Although still far behind the target
value of ~ 100 mV, this value is among the highest values reported so far at that time.
According to the first principles calculation taking into account electron-phonon andelectron-impurity scattering, the mean free path of electrons in n-type silicon with n = 1018
cm-3 is between 20 ~ 40 nm [15]. In reality, there are many other scattering mechanisms, suchas interface scattering, that can make the real mean free path shorter than the calculated value.
The spin transport in our Si channel may be quasi-ballistic rather than fully ballistic.
Nevertheless, it is able to observe the clear spin-valve effect and large spin-signal up to 13mV.

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Chapter 4

Figure 4.7: Bias voltage dependence of the spin-dependent output voltage (R/R)V of
device 3 at 50 K. The highest output voltage ∆V of 13 mV was achieved at the bias voltageof 1.7 V. Reprinted from Appl. Phys. Lett. 109, 232402 (2016), with the permission of AIP
Publishing.

4.5. Summary
In conclusion, this chapter presents the investigation of the spin transport in nanoscale Si
spin-valve devices with the FM and tunnel barrier prepared by the EB evaporation method.
Systematic investigations of the bias voltage dependence, temperature dependence, andmagnetic-field direction dependence of the magnetoresistance indicate that the observed spin
valve effect is governed by the spin transport through the nanoscale Si channel. The highest
magnetoresistance of 12 Ω corresponding to MR ratio of 0.8 % and the highest spindependent output voltage of 13 mV were achieved for a device with the MgO/Ge tunnel layer

71 | P a g e

Chapter 4
at a bias voltage of 1.7 V at 50 K, which are among the highest values reported so far. The
spin-valve effect decreases with increasing temperature but remains observable up to 200 K.
It is expected that if the Si channel can be made shorter so that ballistic transport can occur,larger spin-valve effect can be realized. The results in this chapter have been published on
APL 109, 232402 (2016).

References
1.

Jiang, X. et al. Highly Spin-Polarized Room-Temperature Tunnel Injector for
Semiconductor Spintronics using MgO(100). Phys. Rev. Lett. 94, 056601 (2005).

2.

Martı́nez Boubeta, C. et al. Epitaxial Fe/MgO heterostructures on GaAs(001). J.
Cryst. Growth 226, 223–230 (2001).

3.

Miao, G. X. et al. Disturbance of tunneling coherence by oxygen vacancy inepitaxial Fe/MgO/Fe magnetic tunnel junctions. Phys. Rev. Lett. 100, 1–4 (2008).

4.

Jeon, K.-R., Park, C.-Y. & Shin, S.-C. Epitaxial Growth of MgO and CoFe/MgO on
Ge(001) Substrates by Molecular Beam Epitaxy. Cryst. Growth Des. 10, 1346–1350(2010).

5.

Han, W. et al. Growth of single-crystalline, atomically smooth MgO films on
Ge(001) by molecular beam epitaxy. J. Cryst. Growth 312, 44–47 (2009).

6.

Schöllhorn, C., Oehme, M., Bauer, M. & Kasper, E. Coalescence of germanium
islands on silicon. Thin Solid Films 336, 109–111 (1998).

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Chapter 4
7.

Lou, X. et al. Electrical detection of spin transport in lateral ferromagnet–
semiconductor devices. Nat. Phys. 3, 197–202 (2007).

8.

Nakane, R., Sato, S., Kokutani, S. & Tanaka, M. Appearance of anisotropic
magnetoresistance and electric potential distribution in Si-based multiterminal
devices with Fe electrodes. IEEE Magn. Lett. 3, (2012).

9.

Johnson, M. & Silsbee, R. H. Interfacial charge-spin coupling: Injection anddetection of spin magnetization in metals. Phys. Rev. Lett. 55, 1790–1793 (1985).

10.

Huang, B., Monsma, D. J. & Appelbaum, I. Coherent Spin Transport through a 350
Micron Thick Silicon Wafer. Phys. Rev. Lett. 99, 177209 (2007).

11.

Gould, C. et al. Tunneling anisotropic magnetoresistance: A spin-valve-like tunnel
magnetoresistance using a single magnetic layer. Phys. Rev. Lett. 93, 1–4 (2004).

12.

Rüster, C. et al. Very Large Tunneling Anisotropic Magnetoresistance of a(Ga,Mn)As/GaAs/(Ga,Mn)As Stack. Phys. Rev. Lett. 94, 027203 (2005).

13.

Moser, J. et al. Tunneling anisotropic magnetoresistance and spin-orbit coupling in
Fe/GaAs/Au tunnel junctions. Phys. Rev. Lett. 99, 1–4 (2007).

14.

Sasaki, T. et al. Electrical spin injection into silicon using MgO tunnel barrier. Appl.
Phys. Express 2, (2009).

15.

Qiu, B. et al. First-principles simulation of electron mean-free-path spectra andthermoelectric properties in silicon. EPL (Europhysics Lett. 109, 57006 (2015).

73 | P a g e

Chapter 5

CHAPTER 5
INVESTIGATION of SPIN-VALVE DEVICESFABRICATED by MOLECULAR BEAM EPIXATY

In this chapter, the investigation of the spin-valve effect in nano-scale silicon (Si)-based
spin-valve devices using a Fe/MgO/Ge spin injector / detector grown on Si by molecularbeam epitaxy was presented. For a device with a 20 nm Si channel, a clear magnetoresistance(MR) up to 3 % at low temperature was achieved when a magnetic field was applied in thefilm plane along the Si channel transport direction. A large spin-dependent output voltage of
20 mV was observed at a bias voltage of 0.9 V at 15 K, which is among the highest values inlateral spin-valve devices reported so far. Furthermore, I observed that the sign of the spinvalve effect is reversed at low temperatures, suggesting the possibility of a spin-blockade
effect of defect states in the MgO/Ge tunneling barrier. The results in this chapter have been
published in JAP 122, 223904 (2017) and has been selected as the AIP Science Highlight
paper and JAP Featured paper.

74 | P a g e

Chapter 55.1. Spin-valve device structures
All of the spin-valve devices were fabricated on n-type Si (100) substrates with anelectron density n = 1×1018 cm-3. The Si substrates were cleaned by H2SO4/H2O2 solution,
followed by diluted hydrofluoric acid solution to remove the native oxide layer, and thenrinsed in de-ionized water. After that, the substrates were introduced into a molecular-beamepitaxy (MBE) chamber with a base pressure of 1×10-8 Pa for deposition of a 1 nm-thick Ge
spacer, a 2 nm-thick MgO tunnel barrier, a 10 nm Fe layer, and a 1 nm-thick MgO capping
layer. Knudsen cells were used for thermal evaporation of Ge and Fe, while a low-powerelectron-beam evaporator was used to deposit MgO with a slow rate of 0.03 Å/s. In-situ
reflection high energy electron diffraction confirmed that MgO grown on the Ge buffer layer
became crystalline when the MgO thickness exceeded 1 nm.
After depositing the MgO/Ge tunnel barrier and the Fe electrodes by MBE, e-beamlithography and Ar ion-milling techniques were used to fabricate nanoscale spin-valve
devices. Details of the fabrication process are described in chapter 3. Figure 5.1(a) illustrates
the device structure examined in this work. Figure 5.1(b) shows a scanning electron
microscopy image of a spin-valve device. A Si channel with a length of 20 nm was formed
between the Fe electrodes. The coercive forces of the Fe electrodes are determined by thedomain wall pinning potentials at grain boundaries or defects of the Fe electrodes near thenanoscale Si channel, which are not exactly the same but slightly different between the left
and right electrodes. Indeed, it is found that the coercive force of the left and the rightelectrode differs by 120 G. This small difference establishes a narrow range of magnetic field

75 | P a g e

Chapter 5
where the anti-parallel magnetization configuration is realized, thus the spin-valve effect canbe observed.

Figure 5.1: (a) Schematic spin-valve device structure with a Fe/MgO/Ge spin injector /detector and a 20 nm-long Si channel. (b) Scanning electron microscopy image (top view) of
a device. A silicon channel with a length of 20 nm was formed between the Fe electrodes.
Reprinted from J. Appl. Phys. 122, 223904 (2017), with the permission of AIP Publishing.

5.2 Structural characterization
In this section, the crystal quality of Ge, MgO, and Fe structure in my spin-valve devices
was investigated.

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Chapter 5
In order to investigate the crystal quality of the MgO, Fe, and Ge layers in my devices,
several reference samples were prepared; a 40 nm-thick Ge layer, MgO(40 nm)/Ge(1nm) andFe(40 nm)/MgO(2 nm)/Ge(1 nm) multilayers [denoted as Ge, MgO, and Fe samples,
respectively] on Si substrates prepared both by EB evaporation and by MBE. Then X-ray
diffraction (XRD) measurements were conducted on these reference samples. Figure 5.2
shows the XRD spectra of the reference samples deposited by EB evaporation and MBE,
respectively. In Fig. 5.2(a) of the EB evaporation samples, the XRD spectrum of the Ge
sample shows no peak. Meanwhile, the XRD spectrum of the MgO sample shows a weakMgO(200) peak, and that of the Fe sample shows both Fe(110) and Fe(211) peaks. These
results indicate that the Ge layer was amorphous, while the crystal quality of the MgO layer
was not good, and the Fe layer was poly-crystalline with dominant (110) and (211)orientations. On the contrary, all of the MBE-grown samples show clear peaks corresponding
to Ge(400), MgO(200) and (400), and Fe(110) and (220) as shown in Fig. 5.2(b), indicatingmuch improved crystal quality. Based on these results, it is expected that the crystal qualityof spin injectors/detectors grown by MBE is much better than those deposited by EB
evaporation.

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Chapter 5

Figure 5.2: (a) – (b) X-ray diffraction (XRD) spectra of reference samples fabricated by
EB evaporation and MBE, respectively.

5.3. Conductance characteristics
This section describes the investigation of the conductance characteristics of the Sibased nanoscale 2 terminal spin-valve devices fabricated by the MBE method.
Figure 5.3(a) shows representative current – voltage characteristics (I-V curves) of aspin-valve device with a 2 nm MgO / 1 nm Ge tunnel barrier at various temperatures, which78 | P a g e

Chapter 5clearly show non-linear dependence of the current on the bias voltage, suggesting thattunneling transport occurs at the Fe/MgO/Ge/Si junctions. Furthermore, a strong dependenceof I-V curves on temperature also was observed. Figure 5.3(b) shows the device resistance asa function of temperature, measured at V = 150 mV. I have observed that the resistance
rapidly increases with temperature, from the order of ~ 101 Ω at 300 K to ~ 108 Ω at 4.3 K.
The origin of this behavior will be discussed in the later section. From this data, it is able toestimate the contribution of the parasitic anisotropic magnetoresistance (AMR) of the Feelectrodes to the total spin-valve effect as follows. The total device resistance R can be
decomposed into two components: R = RFe + Rsv, where RFe is the parasitic resistance of the
Fe electrodes and Rsv is the intrinsic resistance of the Fe(interface)/(MgO/Ge)/Si/(Ge/MgO)/Fe(interface) spin-valve structure. Since the parasitic RFe(T) decreases astemperature decreases, RFe(T)  RFe(300K) < R(300K) = 30 . The AMR effect |RFe/RFe| of
Fe is in the order of 0.1%, thus |RFe| is in the order of 0.3 . From this consideration, it ispossible to estimate the contribution of the parasitic AMR effect of |RFe/R(T)| at various
temperatures. For example, |RFe/R(T)| at 15 K should be in the order of 10-8. Therefore, it
could be concluded that the contribution of the parasitic AMR effect of the Fe electrodes isnegligible at low temperatures.

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Chapter 5

Figure 5.3: (a) Current–voltage characteristics (I-V curves) of a device with a 2 nm MgO/ 1 nm Ge tunnel barrier grown by MBE at various temperatures. (b) Temperature dependenceof the resistance measured at 150 mV. Reprinted from J. Appl. Phys. 122, 223904 (2017),
with the permission of AIP Publishing.

5.4.

Spin-dependent transport characteristics

In this section, the systematic investigation of the spin-dependent transport characteristicsof the Si-based nanoscale 2 terminal spin-valve devices fabricated by the MBE method at
various bias voltages, temperatures and magnetic-field directions was conducted.

80 | P a g e

Chapter 55.4.1. Local spin-valve effectTraditionally, the spin transport in Si has been studied by using the Hanle effect. In thismethod, however, the semiconductor channels have to be long enough so that considerable
spin precession can occur. In the 20 nm-long Si channels, the spin precession is negligible.
Therefore, the Hanle effect cannot be used. In this work, the local spin-valve effect was used
to characterize the spin transport. Although this method suffers from parasitic local effects,
such as the AMR effect of the FM electrodes and the tunneling anisotropic magnetoresistance(TAMR) effect at the FM/SC interfaces, it was shown in Chapter 4 that it is possible todistinguish the intrinsic spin-valve effect from parasitic local effects by systematic
measurements of the bias voltage dependence, temperature dependence, and magnetic-field
direction dependence of magnetoresistance (MR) [1]. Figure 5.4(a) shows the MR
characteristic of our spin-valve device measured at 15 K with a bias voltage of 300 mV anda magnetic field applied along the Si channel (x-direction in Fig. 5.1(a)). In Fig. 5.4(a), blue
dots are the resistance data taken when the magnetic field was swept from +3 kG to -3 kG,
while red dots are taken when the magnetic field was swept from -3 kG to +3 kG. The inset
shows the minor loop MR, when the magnetic field was swept from 3 kG to -0.08 kG, thenback to 3 kG. An enormous drop of resistance |R| ~ 57 kΩ, corresponding to |R/R| = 3%
was observed. This |R| and |R/R| values are five orders of magnitude larger than that of theparasitic AMR effect of the Fe electrodes at 15 K (|RFe| ~ 0.3  and |RFe/R|=10-8).
Furthermore, I measured |R| at various bias voltage V, and found that |R| also strongly
depends on V, as shown in Fig. 5.4(b). This cannot be explained by the AMR effect, because

81 | P a g e

Chapter 5|RFe| does not depend on the bias voltage. From the above quantitative and qualitative
considerations, it is concluded that the contribution of the parasitic AMR effect is negligible.
The |R|-V relationship closely follows R-V as shown in Fig. 5.4(b), indicating that theobserved MR effect originates from the spin-dependent tunneling process of electrons
between the Fe electrodes and the Si channel through the MgO/Ge barrier.

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Chapter 5

Figure 5.4: (a) Magnetoresistance characteristics of the device measured at 15 K with abias voltage of 300 mV. Blue dots are the resistance data taken when the magnetic field was
swept from +3 kG to -3 kG, while red dots are taken when the magnetic field was swept from
-3 kG to +3 kG. Inset shows the minor loop of the device. (b) Magnetoresistance |∆R| anddevice resistance R as a function of bias voltage at 15 K. Reprinted from J. Appl. Phys. 122,
223904 (2017), with the permission of AIP Publishing.

83 | P a g e

Chapter 55.4.2. Dependence of the spin-valve signal on the magnetic-field direction
In order to distinguish the intrinsic spin-valve effect from the TAMR effect at theFe(interface)/MgO/Ge/Si interface, the dependence of the MR curve on the magnetic-field
direction ϕ with respect to the x-direction was measured, as shown in Fig. 5.5(a). Here, theTAMR effect originates from the dependence of the tunneling density of states (DOS) in the
FM electrodes on the magnetization direction. Figures 5.5(b)-(f) show the MR curves
measured at 100 K when the magnetic field was applied along ϕ = 0o, 30o, 45o, 60o, and 90o,
respectively. In these measurement, if the spin-valve signals are caused by the TAMR effect,
the MR curve would be reversed or changed in shape when ϕ changes from 0o to 90o [2,3,4].
However, the same shape and polarity of the MR curves for all the ϕ values were observed.
These results indicate that the observed MR depends only on the relative angle between themagnetic moments of the two Fe electrodes, thus it is caused by the spin-valve effect. Notethat observed MR is negative at 100 K, as shown in Figs. 5.5(b)-(f), and the sign of MR will
be discussed in the next sub section.

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Chapter 5

Figure 5.5: (a) Measurement configuration of magnetic-field direction dependence of themagnetoresistance. Here, the magnetic field is applied in the x-y plane along the angle ϕ withrespect to the Si channel (the x-direction). (b)-(f) Magnetoresistance curves measured at 100
K with a bias voltage of 100 mV, when ϕ = 0o, 30o, 45o, 60o, and 90o, respectively. Reprinted
from J. Appl. Phys. 122, 223904 (2017), with the permission of AIP Publishing.

85 | P a g e

Chapter 5
5.4.3. Dependence of the spin-valve signal on temperature – The inverse spin-valve

effectFigure 5.6(a) shows the temperature dependence of the spin-valve MR ratio (R/R). Here,the negative (positive) spin-valve MR ratio corresponds to the resistance drop (jump) in the
MR curves. It was found that the spin-valve MR ratio is negative at temperatures lower than
200 K, and becomes positive at temperatures higher than 200 K. A representative positive
MR curve at 250 K is shown in the inset of Fig. 4(a). Such behavior is unusual and has notbeen observed before in Si-based spin-valve devices.

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Chapter 5

Figure 5.6: (a) Temperature dependence of the spin-valve MR ratio (defined as ∆R/R)
measured at a bias voltage of 300 mV. The spin-valve MR ratio is positive at temperatureshigher than 200 K, but become negative at temperatures lower than 200 K, (b) the local spinvalve signal of device measured at 250 K with a bias voltage of 120 mV. Reprinted from J.
Appl. Phys. 122, 223904 (2017), with the permission of AIP Publishing.
In order to understand the origin of such “inverse” spin-valve effect, I focus on theelectron tunneling process between the Fe electrodes and Si channel. It is firstly remindedthat the device resistance increases rapidly as temperature decreases, from the order of 101 Ω

87 | P a g e

Chapter 5
at 300 K to 108 Ω at 4.3 K, as shown in Fig. 5.2(b). This fact immediately indicates that direct
tunneling of electrons between the Fe electrodes and Si channel through the MgO tunnel
barrier is not dominant, because the resistance due to direct tunneling does not or very weakly
depends on temperature. Instead, the strong temperature dependence of the device resistanceR suggests that the electron transport through the barrier is governed by thermally activatedtunneling through some defect states in the barrier, whose energy level is much smaller than
the intrinsic barrier height between Fe an MgO. Such defect states in MgO barriers are wellknown. For example, in Fe/MgO/Fe magnetic tunnel junctions, oxygen vacancy defects in
the MgO barrier form gap states at about 1.2 eV below the conduction band bottom of MgO,
which reduces the MgO barrier height to just 0.39 eV [5]. Therefore, it is reasonable toassume such defect-induced gap states inside the MgO barrier of our spin-valve devices.
Figures 5.7 shows my model to explain the “inverse” spin-valve effect. In this model, the two
Fe electrodes work as a spin injector and detector. Spin-polarized electrons tunnel from the
Fe spin injector to the Si channel through defects-induced gap states inside the MgO barrier.
The electrons then transport in the Si channel without or with little scattering (ballistic or
quasi ballistic transport), and reach the opposite MgO barrier with higher kinetic energy.
Finally, the electrons tunnel through gap states inside another MgO barrier to the Fe spin
detector. In this picture, the relevant gap states are those with energy levels higher than the
Fermi level of the Fe spin injector. At high temperatures, electrons in the Fe spin injector
with high enough thermal energy can tunnel to the gap states (thermally activated tunneling),
and the device resistance is low (~ 101 Ω at 300 K). At low temperatures, however, thenumber of thermal electrons drastically decreases, resulting in much higher resistance (~ 108

88 | P a g e

Chapter 5Ω at 4.3 K). Note that at low temperatures, only a limited number of the gap states whoseenergy levels are very close to the Fermi level of the Fe spin injector can allow such tunneling.
If these gap states are filled with majority spins whose residence time is long enough at low
temperatures, only minority spins from the spin injector can go through. Such a “spinblockade” phenomenon, originated from the Pauli exclusion principle, is well known and has
been observed in many quantum dot systems as well as defect states in semiconductors [6,7].
In contrast, the electrons that arrive at the opposite MgO barrier can have higher kinetic
energy, thus they can tunnel through many available gap states whose energy levels are higher
than the Fermi level of the Fe spin detector. Thus, spin-blockade strongly occurs only at the
spin injector side, and not at the spin detector side. In such a situation, the device resistancebecomes high (low) at parallel (antiparallel) magnetization configuration, as shown in Figs.
5.7(a) and 5.7(b). This explains the observed inverse spin-valve effect at low temperatures.
As temperature increases, there are more available gap states with higher energy levels forthermally activated tunneling from the spin injector. Furthermore, the residence time ofmajority spins at the defect states can be much shorter at high temperatures. As a result, thespin-blockade is not effective anymore, thus the normal spin-valve effect is dominant at
higher temperature as shown in Fig. 5.7(c) and 5.7(d). My model is also consistent with the
impurity assisted tunneling model proposed for explanation of the three-terminal Hanle effectobserved in some FM/SC systems [8].
In fact, the negative MR has been reported in Co nanoparticle systems by Yakushiji etal. [9]. In their experiment, Yakushiji has investigated the magnetotransport properties of the
Co nanoparticles with the particle size ~ 5 nm sandwiched between a Co and Al electrode,89 | P a g e

Chapter 5and observed an oscillation of MR behavior in the range from -10 % to +15 % as shown in
Fig. 5.8(a). Fig. 5.8(b) shows a negative MR signal observed at the bias voltage of 0.05 V.
Here, the MR ratio is defined as =𝑅AP −𝑅P
𝑅AP , where RP (RAP) is the resistance of the system

in parallel (anti-parallel) alignment. This negative MR arises due to the long spin-relaxation
time τs of electrons in the Co nanoparticle (~ 150 ns). In the nanoscale confinement particles,
the spin-relaxation is predicted to be strongly suppressed because of the quantum-size effect.
If this spin-laxation time τs is smaller than tunneling time τ of electrons for jumping in/out
the nanoparticles (τs << τ), there is no spin accumulation in the nanoparticles. In that case, thespin transport is governed simply by the Jullier model of tunneling magnetoresistance (TMR)
effect for which there is no negative MR. On the contrary, in case of τs >> τ, there is a strong
spin accumulation inside the nanoparticles, leading to the difference between the Fermi levelsof majority and minority spin of the nanoparticles. This spin accumulation causes theoscillation and the negative MR. This phenomenon could be understood as in the model
shown in Fig. 5.8(c)-(d). At a certain bias voltage close to a Coulomb staircase (discreteenergy level of Coulomb blockade), the majority spin is blocked due to the higher discrete
charging level inside the nanoparticle, thus only the minority can pass through as shown in
Fig.5.8(c). Thus, the current is small in the parallel state. On the other hand, in the antiparallel state, the majority spin is not blocked in the nanoparticle as shown in Fig. 5.8(d). In
such a situation, the device resistance becomes high (low) at parallel (antiparallel)magnetization configuration. This is the reason for the negative MR in the nanoparticle
system. In my devices, the similar negative MR, which is originated from the spin blockadeof majority spins in the parallel state, has been observed. There is clear analogy between my90 | P a g e

Chapter 5
inverse MR and that by Yakushuji et al., even though the physics is different. The inverse
MR in my devices is due to spin-blockage at defect states in the MgO tunnel barrier, which
originates from the Pauli exclusion principle. Meanwhile, the negative MR in Yakushuji’s
devices is due to Coulomb blockade and spin accumulation.

91 | P a g e

Chapter 5

Figure 5.7: Proposed spin-dependent transport model through defect-induced gap statesin the MgO barriers for parallel and anti-parallel magnetization configuration, respectively.
(a)-(b) At low temperatures, defect-induced gap states in the MgO barrier at the spin injector

92 | P a g e

Chapter 5side are occupied with majority spins, thus only minority spins can tunnel through the MgObarrier because of the Pauli exclusion principle (spin-blockade effect). This leads to theinverse spin-valve effect, where the resistance is high at (a) parallel and low at (b) antiparallel
magnetization configuration. (c)-(d) At high temperatures, there are more available gap stateswith higher energy levels for thermally activated tunneling from the spin injector. Moreover,
the residence time of majority spins at the defect states can be much shorter at hightemperatures. Therefore, the spin-blockade is not effective, thus the normal spin-valve effectis dominant at higher temperature with low resistance at (c) parallel and high resistance at (d)antiparallel magnetization configuration. Reprinted from J. Appl. Phys. 122, 223904 (2017),
with the permission of AIP Publishing.

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Chapter 5

eVSpin
accumulation
Coulomb
energy U

Figure 5.8: (a) Oscillation with negative MR in the Co nanoparticle system. Inset shows
the spin transport model of the nanoparticle system: paramagnet / ferromagnetic nanoparticle/ ferromagnet double tunnel junction. (b) The negative MR signal of the Co nanoparticle
system observed at a bias voltage V = 0.05 V [9]. (c)-(d) Model for negative MR in the parallel
and anti-parallel state at a certain bias voltage close to a Coulomb staircase (discrete energy
level of Coulomb blockade). Reprinted from Nature Mater. 4, 57-61 (2005), with thepermission of Nature Publishing Group.

94 | P a g e

Chapter 5

5.5.

Spin-dependent output voltage

In this section, I investigate the spin-dependent output performance of our devices.
Previous studies on the spin injection into Si channels reported a low read-out voltage of only
a few µV in 4 terminal measurements, and about 1 mV in 3 terminal measurements. In
Chapter 4, it is succeeded in increasing the spin-dependent output voltage up to 13 mV innanoscale spin-valve devices with EB evaporated Fe/MgO/Ge junctions [1]. In this chapter,I have significantly improved the crystal quality of the Fe/MgO/Ge junctions by using MBE,
as evidenced by the much higher device resistance at 4.3 K (~ 108 Ω compared to ~ 103 Ω ofmy previous devices fabricated by EB evaporation). Therefore, it is reasonable to expect
higher spin-dependent output voltage. In Fig. 5.9(a), the spin-dependent output |V| =|R/R|V was plotted as a function of the bias voltage V at 15 K. The Fig. 5.9(b) shows thedependence of MR ratio on bias voltage. In this device, I achieved |V| = 20 mV at V = 0.9
V, which is among the highest values reported so far in lateral Si-based spin-valve devices.

95 | P a g e

Chapter 5

Figure 5.9: (a) Bias voltage dependence of the spin-dependent output voltage |∆V| =(|R|/R)V of the device at 15 K. The highest output voltage |∆V| of 20 mV was achieved at
the bias voltage of 0.9 V. (b) The bias voltage dependence of |R|/R at 15 K. Reprinted from
J. Appl. Phys. 122, 223904 (2017), with the permission of AIP Publishing.

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Chapter 5

5.6.

Summary

In this chapter, the investigation of the spin transport in the nano-scale Si spin-valve
devices prepared by MBE was presented. A large spin-valve effect with |R| up to 57 kΩ,
corresponding to |R/R| = 3%, has been clearly observed. The sign of the spin-valve effect isreversed at low temperatures, suggesting the possibility of the spin-blockade effect of defectstates in the MgO tunnel barrier. The highest spin-dependent output voltage is 20 mV at thebias voltage of 0.9 V at 15 K, which is among the highest values reported so far in lateral Sibased spin-valve devices. My result is an important step towards the realization of nano-scale
spin-MOSFETs. The results in this chapter have been published in JAP 122, 223904 (2017)and has been selected as the AIP Science Highlight paper and JAP Featured paper.

References

1.

Hiep, D. D., Tanaka, M. & Hai, P. N. Spin transport in nanoscale Si-based spinvalve devices. Appl. Phys. Lett. 109, 232402 (2016).

2.

Gould, C. et al. Tunneling anisotropic magnetoresistance: A spin-valve-like tunnel
magnetoresistance using a single magnetic layer. Phys. Rev. Lett. 93, 1–4 (2004).

3.

Rüster, C. et al. Very Large Tunneling Anisotropic Magnetoresistance of a(Ga,Mn)As/GaAs/(Ga,Mn)As Stack. Phys. Rev. Lett. 94, 027203 (2005).

4.

Moser, J. et al. Tunneling anisotropic magnetoresistance and spin-orbit coupling in
Fe/GaAs/Au tunnel junctions. Phys. Rev. Lett. 99, 1–4 (2007).

97 | P a g e

Chapter 55.

Yuasa, S., Nagahama, T., Fukushima, A., Suzuki, Y. & Ando, K. Giant roomtemperature magnetoresistance in single-crystal Fe/MgO/Fe magnetic tunnel
junctions. Nat. Mater. 3, 868–871 (2004).

6.

Weber, B. et al. Spin blockade and exchange in Coulomb-confined silicon double
quantum dots. Nat. Nanotechnol. 9, 430–435 (2014).

7.

Ono, K., Austing, D. G., Tokura, Y. & Tarucha, S. Current Rectification by Pauli
Exclusion in a Weakly Coupled Double Quantum Dot System. Science (80-. ). 297,
1313–1317 (2002).

8.

Txoperena, O. et al. Impurity-Assisted Tunneling Magnetoresistance under a Weak
Magnetic Field. Phys. Rev. Lett. 113, 1–6 (2014).

9.

Yakushiji, K. et al. Enhanced spin accumulation and novel magnetotransport innanoparticles. Nat. Mater. 4, 57–61 (2005).

98 | P a g e

Chapter 6

CHAPTER 6
INVESTIGATION of THE ROLE of BALLISTIC
TRANSPORT IN NANOSCALE Si CHANNELIn this chapter, the investigation of the role of ballistic transport in my nanoscale Si spinvalve device was described. I have prepared the Si devices with the long channel lengths of
L = 500 nm, 1 µm, and 6 µm, and measured the spin-valve effect in these devices, to compare
to the 20 nm Si channel device. Two important roles of (quasi) ballistic transport in thenanoscale Si channel have been demonstrated: (i) generation of spin-valve effect even when
there is no barrier at room temperature, and (ii) suppress spin-flip scattering to achieve ahigher spin-valve ratio at low temperature. The results in this chapter have been published in
AAPPS Bulletin 28, 7-15 (2018) – Featured paper.

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Chapter 6

6.1. Spin-valve device structures
All of the spin-valve devices were fabricated on n-type Si (100) substrates with anelectron density n = 1×1018 cm-3. The Si substrates were cleaned by H2SO4/H2O2 solution,
followed by diluted hydrofluoric acid solution to remove the native oxide layer, and thenrinsed in de-ionized water. After that, the substrates were introduced into a molecular-beamepitaxy (MBE) chamber with a base pressure of 1×10-8 Pa for deposition of a 1 nm-thick Ge
spacer, a 2 nm-thick MgO tunnel barrier, a 10 nm Fe layer, and a 1 nm-thick MgO capping
layer. Knudsen cells were used for thermal evaporation of Ge and Fe, while a low-powerelectron-beam evaporator was used to deposit MgO with a slow rate of 0.03 Å/s. In-situ
reflection high energy electron diffraction confirmed that MgO grown on the Ge buffer layer
became crystalline when the MgO thickness exceeded 1 nm.
After depositing the MgO/Ge tunnel barrier and the Fe electrodes by MBE, e-beamlithography and Ar ion-milling techniques were used to fabricate the long Si channel length
spin-valve devices. Details of the fabrication process are described in Chapter 3. Figure 6.1
illustrates the device structure examined in this work. In this chapter, I have fabricateddevices with different Si channel length from 500 nm to 6 µm, to compare with the 20 nm Si
channel spin-valve device, in order to investigate the role of ballistic transport in thenanoscale Si channel spin-valve device.

100 | P a g e

Chapter 6

Figure 6.1: Schematic spin-valve device structure with a Fe/MgO/Ge spin injector /detector and the variable Si channel length. The Si channel length L is varied from 500 nm
to 6 µm to compare with the 20 nm Si channel length device in the similar growing condition.

6.2. Spin-valve signal in the diffusive regime
It is well-known that when spin polarized electrons are injected from FM into SC, thespin injection efficiency is strongly suppressed by the conductivity mismatch problem [1,2].
When there is no barrier between the FM layer and the SC channel, the spin polarization of J  J  
the current at the FM/SC interface is given by (SP)I =  
. Here,  is
 J   J   I 1  rN / rF
the spin-polarization of the FM layer, rF and rN are characteristic resistances defined byrF   F lsfF and rN   N lsfN , where  F ,  N are the resistivity of the FM and SC, and lsfF , lsfN are
the spin-diffusion length in FM and SC. The ratio rN / rF determines the spin polarization at theinterface. Using  F ~ 110-5 cm and lsfF ~ 2 nm for Fe,  N = 210-2 cm for n-Si (n =1018

101 | P a g e

Chapter 6cm-3) and assuming that lsfN ~10 m, the ratio rN / rF is as large as 1107. This means that there
is almost no spin polarization at the interface between Fe and n-Si. The situation is evenworse for the spin-valve ratio in the FM/SC/FM structure. Assuming that the length of the2

 r lN 
R
SC layer t N  l , the spin-valve ratio is given by 8 2  F sf  . For the Fe/n-SiR
 rN t N Nsf2

 r lN 
interface, even when tN = 20 nm, the ratio  F sf  is of the order of 10-9, which means that rN t N 
there is almost no spin-valve effect. The conductivity mismatch problem can be avoided byinserting a barrier between the FM and SC [2,3], which has been widely used to obtain spin
injection into SC. However, this method still has some problems. While spin injection fromFM into SC through a barrier has been definitely demonstrated, typical values of the spinvalve ratio reported so far in lateral devices are as small as 0.01% ~ 0.1%. Furthermore, highresistance barriers at the source/drain electrodes are not desired from the viewpoint of
current-driving capability of spin-transistors. On the other hand, the conductivity mismatch
problem can also be avoided by using nanoscale SC channels with ballistic (or quasi-ballistic)transport. If the SC channel length is comparable or shorter than the electron mean free path,the transport regime changes from diffusive to (quasi) ballistic. In this case, the Ohm’s law
and diffusion equations used to derive the spin-polarized current are no longer valid. The
transport in the SC channel may be modeled using quantum mechanics rather than classicaltransport equations of electrons. In such a case, the transfer matrix method may be used to
calculate the spin-polarized transport in the SC channel. Such a method was used to calculate
the magnetocurrent in spin-MOSFETs [4]. The advantages of using ballistic transport to102 | P a g e

Chapter 6
overcome the conductivity mismatch are (i) high spin-valve ratio, similar to tunnelingmagnetoresistance (TMR), can be achieved, and (ii) spin-valve effect can be observed even
if there is no tunnel barrier, allowing high current driving capability. Unfortunately, there
have been very few experimental studies on the FM/SC/FM structure with a nanoscale SC
channel. Exceptional one is MnAs / GaAs (10-30nm) / MnAs vertical spin-valve structures
grown by MBE [5], which demonstrates a spin-valve ratio as large as 8%.

6.3. Role of ballistic transport in the nanoscale Si channel
To demonstrate the role of ballistic transport in our nanoscale Si channel, the Si spinvalve devices with the long channel lengths of L = 500 nm, 1 m, and 6 m have been
prepared and measured. The channel of these devices is long enough so that electron transport
is diffusive, making it possible to compare them with the nanoscale Si spin-valve devices.
The table 6.1 shows the comparison of the spin-valve MR ratio between the long Si channel
lengths, where the electron transport is diffusive, and the 20 nm Si channel length in whichelectron transport is considered as ballistic.

103 | P a g e

Chapter 6

Temp

L = 20 nm

L = 500 nm

L = 1 µm

L = 6 µm

∆R/R (%)∆R/R (%)∆R/R (%)∆R/R (%)

15

-3.1

-2.2

-2

-1.8

50

-1.08

-0.86

-0.74

-0.7

100

-0.4

-0.33

-0.31

-0.28

150

-0.09

-0.08

-0.08

-0.057

200

0

0

0

0

250

0.033

0.02

0.018

0.011

300

0.042

0

0

0

Table 6.1: The comparison of the spin-valve MR ratio measured at different temperaturesof various devices with different Si channel length.

6.3.1. Generation of the spin-valve effect without a tunnel barrier
The first advantage of ballistic transport in the Si-based spin-valve channel is the abilityof generating the spin-valve effect even without a tunnel barrier. Figure 6.2(a) shows the MR
curves measured at room temperature for the nanoscale channel device with L = 20 nm, while
Figs. 6.2(b)-(d) show those of long-channel devices with L = 500 nm, 1 m, and 6 m,
respectively. Here, the MR curves in Figs. 6(a)-(d) are measured with a bias voltage of 80
mV. At room temperature, all the devices show low resistance values of the order of 10 .
The low resistance indicates that the tunnel barriers of those devices are no longer effective

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Chapter 6
due to various thermally activated transport processes through defect states in the barriers.
Thus, the barrier resistance is negligible, and no spin-valve effect can be expected for the
long-channel devices. Indeed, the data in Figs. 6.2(b)-(d) show no spin-valve effect for thedevices with L = 500 nm, 1 m, and 6 m, as expected from the diffusion theory. In contrast,
Fig. 6(a) shows a clear spin-valve effect of about 0.042% for the nanoscale device, eventhough there is no barrier for this device at room temperature. This demonstrates theimportant role of (quasi) ballistic transport in generating the spin-valve effect when there isno barrier.

105 | P a g e

Chapter 6

Figure 6.2: (a)-(d) Magnetoresistance of local spin-valve effect of several Si spin-valve
devices with different channel length of L = 20 nm, 500 nm, 1 µm, and 6 µm, respectively,
measured at 300 K with a bias voltage of 80 mV. All the devices show the low resistancevalues of the order of 10 , indicating that the tunnel barriers of those devices are no longer
effective. Reprinted from J. Appl. Phys. 122, 223904 (2017), with the permission of AIP
Publishing.

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Chapter 6
6.3.2. Suppression of spin-flip scattering to achieve a higher spin-valve ratio at
shorter channel length
The second advantage of ballistic transport in the Si-based spin-valve channel is theability of suppressing the spin-flip scattering to achieve a higher spin-valve ratio at shorter
channel length. Figures 6.3 shows the MR curves of devices at 15 K, measured with a bias
voltage of 300 mV. At this low temperature, all the devices show high resistance values ofM, indicating that thermally activated transport processes through defect states are strongly
suppressed and that the barriers are effective for suppressing the conductivity mismatch inlong-channel devices. As a result, all the devices show relatively large spin-valve ratios.
However, the data clearly show that the spin-valve ratio systematically decreases withincreasing channel length; the MR ratio decreases from ~ 3% for L = 20 nm to 2.2%, 2% and1.8% for L = 500 nm, 1 m, and 6 m, respectively. Figure 6.4 shows the dependence ofspin-valve ratio |∆R/R| on the channel length L in the long channel devices (L = 0.5 µm, 1µm and 6 µm), in which the electron transport is under diffusive regime. The drop of thespin-valve ratio is slow, consistent with the long spin-diffusion length in Si. Performing the−𝐿best fitting with |∆R/R| ~ 𝑒 𝜆𝑛 , where λN is the spin diffusion length in Si, I deduce the λN ~5.5 µm, which is consistent with data reported in literature.

107 | P a g e

Chapter 6

Figure 6.3: (a)-(d) Magnetoresistance of local spin-valve effect of several Si spin-valve
devices with different channel length of L = 20 nm, 500 nm, 1 µm, and 6 µm, respectively,
measured at 15 K with a bias voltage of 300 mV. All the devices show the high resistancevalues of the order of M, indicating effectiveness of the tunnel barriers. Reprinted from J.
Appl. Phys. 122, 223904 (2017), with the permission of AIP Publishing.

108 | P a g e

Chapter 6

Figure 6.4: The dependence of spin-valve ratio on the Si channel length in the long−𝐿

channel devices. The red line shows the best fitting curve |∆R/R| ~ 𝑒 𝜆𝑛 , with n ~ 5.5 m.

6.4. Summary
The results in this chapter demonstrate two important roles of (quasi) ballistic transport
in the nanoscale Si channel: (i) generation of spin-valve effect even when there is no barrierat room temperature, and (ii) suppress spin-flip scattering to achieve a higher spin-valve ratio
at low temperature. Indeed, a large spin-valve ratio of 3%, which is much larger than thatobserved in m-long Si channel devices reported before, has been obtained. Nevertheless,the electron transport in my nanoscale devices may be quasi ballistic rather than fully ballistic,
because the channel length (20 nm) is not much shorter than that of the mean free path of 20

109 | P a g e

Chapter 6~ 40 nm for n-type Si with n = 1018 cm-3 [6]. By using lightly-doped Si substrates and further
downsizing the Si channel length to sub-10 nm, it is possible to achieve fully ballistictransport and higher spin-valve signals. The results in this chapter have been published in
AAPPS Bulletin 28, 7-15 (2018) – Featured paper.

References
1.

Schmidt, G., Ferrand, D., Molenkamp, L. W., Filip, A. T. & van Wees, B. J.
Fundamental obstacle for electrical spin injection from a ferromagnetic metal into a
diffusive semiconductor. Phys. Rev. B 62, R4790–R4793 (2000).

2.

Fert, A. & Jaffrès, H. Conditions for efficient spin injection from a ferromagnetic
metal into a semiconductor. Phys. Rev. B 64, 184420 (2001).

3.

Rashba, E. I. Theory of electrical spin injection: Tunnel contacts as a solution of the
conductivity mismatch problem. Phys. Rev. B 62, R16267–R16270 (2000).

4.

Sugahara, S. & Tanaka, M. A spin metal–oxide–semiconductor field-effect transistor
using half-metallic-ferromagnet contacts for the source and drain. Appl. Phys. Lett.
84, 2307–2309 (2004).

5.

Hai, P. N., Sakata, Y., Yokoyama, M., Ohya, S. & Tanaka, M. Spin-valve effect byballistic transport in ferromagnetic metal (MnAs)/semiconductor (GaAs) hybrid
heterostructures. Phys. Rev. B - Condens. Matter Mater. Phys. 77, 1–6 (2008).

110 | P a g e

Chapter 6
6.

Qiu, B. et al. First-principles simulation of electron mean-free-path spectra andthermoelectric properties in silicon. EPL (Europhysics Lett. 109, 57006 (2015).

111 | P a g e

Chapter 7

CHAPTER 7
OPTIMIZATION of THE MgO TUNNEL BARRIER
THICKNESS
In this chapter, the investigation of the spin transport in nanoscale silicon (Si)-based spinvalve devices with a 20 nm Si channel and various stacks of Fe/MgO/Ge as the spin
injector/detector was conducted. By optimizing the MgO barrier thickness, a large spindependent output voltage of 25 mV at 15 K, which is the highest value reported so far, wasachieved.

112 | P a g e

Chapter 7

7.1. Spin-valve device structures
In this chapter, the spin-valve devices were fabricated with ferromagnetic electrodes anda tunnel barrier deposited on highly doped n-type Si (100) substrates with an electron density
n = 1×1018 cm-3. Before thin film deposition, the substrates were cleaned by acetone,
followed by H2SO4/H2O2 solution, then etched in diluted hydrofluoric acid solution toremove the native oxide layer, and rinsed in de-ionized water. After the cleaning process,stacking layers of Fe (electrode) / MgO (tunnel barrier) / Ge (buffer layer) (from the top to
the bottom) for the ferromagnetic electrodes and the tunnel barrier were deposited on Si.
These stacking layers were deposited by molecular-beam epitaxy (MBE). After thecleaning process, Si substrates were introduced into a MBE chamber with a base pressure of
1×10-8 Pa. In this study, in order to optimize the MgO tunnel barrier layer, the stack of 1.1
nm MgO capping layer / 10 nm Fe / (1.5 – 3.5 nm) MgO / 1 nm Ge on Si substrates wasgrown from the top to the bottom. In the MBE growth, Knudsen cells were used for thermal
evaporation of Ge and Fe, while a low-power electron-beam evaporator was used to deposit
MgO at a slow rate of 0.03 Å/s.
After depositing the tunnel barrier and the ferromagnetic layer, the electron-beamlithography (EBL), EB evaporation, lift-off, and ion-milling techniques were employed tofabricate nanoscale Si spin-valve devices. Finally, the Au (40 nm) / Cr (5 nm) pad electrodes
were formed by EB evaporation. Figure 7.1(a) shows the schematic structure of our devices.
A typical top-view scanning electron microscopy (SEM) image of our spin-valve devices
with a 20 nm Si channel is shown in the Fig. 7.1(b).

113 | P a g e

Chapter 7

Figure 7.1: (a) Schematic spin-valve device structure with a Fe/MgO/Ge spin injector /detector and a 20 nm Si channel. The MgO thickness tMgO is varied from 1.5 to 3.5 nm. (b)Scanning electron microscopy image (top view) of a device with a 20 nm Si channel between
the Fe electrodes.

7.2. Conductance characteristics
This section describes the investigation of the conductance characteristics of the Si-based
nanoscale 2 terminal spin-valve device with different thicknesses of the MgO tunnel barrier
layer.

114 | P a g e

Chapter 7
Figure 7.2(a) shows the current – voltage characteristic (I – V curves) at 50 – 300 K ofdevices grown by MBE with a 1.5 nm-thick MgO tunnel barrier. The non-linear I – V curves
were observed, suggesting that the resistances of the devices are dominated by the electrontransport through the spin-valve structure consisting of Fe/MgO/Ge – Si – Ge/MgO/Fe. The
strong dependence of the I – V curves on temperature was also found; the figure 7.2(b) shows
the resistance of this device as a function of temperature, measured with a bias voltage of
300 mV. I observed a rapid increase of the resistance as temperature decreases, from theorder of ~ 101 Ω at 300 K to ~ 107 Ω at 15 K. Such behavior indicates that the electrontransport through the barrier occurs not by direct tunneling, but by thermally activatedhopping through defect states in the MgO tunnel barrier [1]. From the temperaturedependence of the resistance, it is able to evaluate the contribution of the parasitic AMR of
the Fe electrodes to the total spin-valve effect at various temperatures as follows. The total
device resistance R can be decomposed into two components: R = RFe + Rsv, where RFe is theparasitic series resistance of the Fe electrodes and Rsv is the intrinsic tunneling resistance ofthe spin-valve structure. Since the parasitic RFe(T) decreases as temperature decreases, RFe(T) RFe(300K) < R(300K) ≈ 20 . The AMR effect |RFe/RFe| of Fe is in the order of 0.1%,
thus the magnetoresistance of Fe |RFe| is in the order of 0.2 . From this consideration, it ispossible to estimate the contribution of the parasitic AMR effect of |RFe/R(T)| at various
temperatures: For example, |RFe/R(T)| at 15 K should be in the order of 10-8 which isextremely small compared with the observed MR signal of 2% – 3%, as shown later.
Therefore, the contribution of the parasitic AMR effect of the Fe electrodes is negligible at
low temperatures.

115 | P a g e

Chapter 7

Figure 7.2: (a) Current – voltage characteristics (I – V curves) of a MBE-grown device
with an MgO thickness tMgO = 1.5 nm. (b) shows the temperature (T) dependence of theresistance R measured at 300 mV.

116 | P a g e

Chapter 7

7.3. Spin-dependent transport characteristics
In this section, the investigation of the spin-dependent transport characteristics of the Sibased nanoscale 2 terminal spin-valve devices fabricated by the MBE method with different
MgO barrier thicknesses was presented.
7.3.1 Local spin-valve effectConventionally, the spin transport in Si has been studied by the Hanle effect or 4-terminal
non-local spin-valve measurements rather than the 2-termimal local spin-valve measurement
to avoid the parasitic local MR effect, such as anisotropic magnetoresistance (AMR) or
tunneling AMR (TAMR) effects. However, in nanoscale channel devices, the spin transport
time in the Si channel is too short for spin precession induced by the external magnetic field,
thus the Hanle effect cannot be used. Furthermore, the 4-terminal non-local measurement has
no advantages compared with the local measurement in the nanoscale channels, as discussed
before [1,2]. Furthermore, I have shown that it is possible to distinguish the intrinsic spinvalve effect from parasitic local effects by systematic measurements of the bias voltage
dependence, temperature dependence, and magnetic-field direction dependence of the MR
signal in previous chapters [2,1]. Therefore, in this chapter, I employed the two-terminal local
spin-valve measurement to investigate the spin transport in the nanoscale spin-valve devices.
Figure 7.3(a) shows the MR characteristic of a MBE-grown spin-valve device with an
MgO tunnel barrier (tMgO = 1.5 nm) at 15 K, measured with a bias voltage of 500 mV and amagnetic field applied along the Si channel (x direction in Fig. 7.1(a)). The inset show theminor loop of MR signal. An inverse magnetoresistance effect with a large drop of resistance|R| ~ 30 kΩ, corresponding to |R/R| = 2.5 %, was observed. Note that the |R| and |R/R|

117 | P a g e

Chapter 7
values are five orders of magnitude larger than that of the parasitic AMR effect of the Feelectrodes at 15 K (|RFe| ~ 0.2  and |RFe/R|=10-8). Furthermore, I measured |R| at various
bias voltage V, and found that |R| strongly depends on V, as shown in Fig. 7.3(b). Thiscannot be explained by the AMR effect, because |RFe| does not depend on the bias voltage.
Moreover, the |R| – V characteristic closely follows the R – V characteristic as shown in Fig.
7.3 (b), indicating that the observed MR effect originates from the spin-dependent tunnelingprocess of electrons between the Fe electrodes and the Si channel through the MgO/Gebarrier. Note that the MR of the devices prepared by MBE is negative (inverse MR, as shownin Fig. 7.3(a)), contrasting to the positive MR in the conventional spin-valve effect. The
origin of the inverse MR in MBE devices has been explained in Chapter 5 as the result of thespin-blockade effect of defect states in the MgO barrier [1].

118 | P a g e

Chapter 7

Figure 7.3: (a) Local magnetoresistance (MR) of a MBE-grown device with a 1.5 nmthick MgO layer at 15 K, measured with a bias voltage of 500 mV. The inset shows the minor
loop MR. (b) Magnetoresistance |∆R| and device resistance R as a function of the bias voltage
at 15 K of the same device as shown in (a).
7.3.2. Dependence of spin-valve signal on bias voltage in different MgO thicknesses

This part describes the investigation of the dependence of spin-valve signal on the bias
voltage in different MgO thicknesses.

119 | P a g e

Chapter 7
Figure 7.4(a) shows the dependence of the MR ratio on the bias voltage for devices withdifferent MgO barrier thicknesses (tMgO = 1.5 – 3.5 nm) at 15 K. At low bias voltages (V 
300 mV), the device with the thinnest MgO layer (tMgO = 1.5 nm) shows lower MR ratio thanthe devices with thicker MgO layers (tMgO  2 nm). However, at higher bias voltages (V >300 mV), the devices with thin MgO layers (tMgO = 1.5 and 2 nm) show better performance
than the devices with thick MgO layers (tMgO = 2.5 and 3.5 nm). Especially, the device withtMgO = 1.5 nm maintains the high MR ratio over -2% at the bias voltage as high as 1.1 V.

Figure 7.4: (a) Dependence of the MR ratio |∆R|/R on the bias voltage in MBE-grown
devices with various MgO thicknesses (tMgO = 1.5 – 3.5 nm) at 15 K. (b) Dependence ofdevice resistance R on the bias voltage in devices with various MgO thicknesses.

120 | P a g e

Chapter 7
To understand the dependence of the MR ratio on the bias voltage and on the thicknessof the MgO tunnel barrier, it is important to understand the tunneling process of electrons inour devices. First, the strong temperature dependence of the tunnel resistance in my devices
indicates that the electron transport through the tunnel barrier is governed by thermally
activated hopping via defect states in the barrier. Such defect states, such as oxygen vacancy,
are well-known for MgO [3]. Second, the Fig. 7.4(b) shows that the device resistancedecreases with increasing the MgO barrier thickness, contradicting the expectation fromdirect tunneling. This can be explained as my proposed model in figure 7.5. During the MgOdeposition process, the thicker the MgO layer is, the longer time the MgO layer is exposed
to an ultrahigh vacuum, which allows more desorption of oxygen from the MgO layer when
the growth rate of MgO is very slow like our case (0.03 Å/s). As a result, the thicker MgOlayers can have higher defect-state density than the thinner MgO layers, which enhances the
hopping process of electrons via defect states and reduces the device resistance, as shown in
the Fig. 7.5.

121 | P a g e

Chapter 7

Figure 7.5: The electron transport model to explain the dependence of device resistance
R on the thickness of MgO tunnel barrier. (a) Electron transport model when MgO layer isthicker, (b) when MgO layer is thinner.
Based on this consideration, I propose a model to explain the bias voltage dependence
and the MgO barrier thickness dependence of the MR ratio, which are shown in Fig. 7.6.
From the Fe injector, spin-polarized electrons tunnel to the Si channel through defect states
inside the MgO barrier. If these defect states are filled with majority spins whose residence
time is long enough at low temperatures, only minority spins from the spin injector can go
through. Such a “spin-blockade” phenomenon, originated from the Pauli exclusion principle,
is well known and has been observed in semiconductor quantum dots and various defectstates in semiconductors [4,5]. In the devices grown by MBE, the spin blockade results in the
inverse spin-valve effect [1]. At low bias voltages, only defect states close to the Fermi level122 | P a g e

Chapter 7
are relevant for spin transport. At higher bias voltages, there are more available defect states
for spin transport. When the MgO layer is thin, the number of relevant defect states increases
slowly with bias voltage, and the spin blockade is effective at both the low and high bias
voltages, as shown in Fig. 7.6(a) and 7.6(b). Thus, the inverse MR ratio remains high at highbias voltages for the samples with a thin MgO layer. In contrast, in the devices with a thick
MgO layer, the available defect-states increases rapidly with increasing the bias voltage.
Thus, both minority and majority spins can transport through the barrier, resulting in theweaker spin-blockade effect, as shown in Fig. 7.6(c) and 7.6(d). Therefore, the inverse MR
ratio of devices with the thicker MgO layer decreases rapidly at high bias voltages.

123 | P a g e

Chapter 7

Figure 7.6: Spin-dependent transport model through defect states in the MgO barriers fordevices with (a)(b) thin and (c)(d) thick MgO layers, at (a)(c) low and (b)(d) high bias

124 | P a g e

Chapter 7
voltages, respectively. Here, the MgO barrier thickness is exaggerated for clarity. In thin
MgO barriers, available defect states for electron tunnelling increase slowly with increasing
the bias voltage, thus the spin blockade is effective at both low and high bias voltages. In
thick MgO barriers, the available defect states increases rapidly with increasing the bias
voltage, thus the spin blockade is weakened at high bias voltages, and the inverse MR ratio
drops quickly with increasing the bias voltage.

7.4. Spin-dependent output voltage
This section presents the investigation of the spin-dependent output performance of mydevices. Here, large V = R×I = R×(V/R) = (R/R)×V is important for correct read-out inrealistic applications. Previous studies on the spin injection into Si channels reported a low
read-out voltage of only a few µV in 4 terminal measurements, and about 1 mV in 3 terminal
measurements. Chapter 5 has shown an increase of the spin-dependent output voltage up to
20 mV in nanoscale spin-valve devices Fe/MgO/Ge junctions [1]. In this chapter, by
optimizing the MgO thickness, I have succeeded in improving V to the maximum value of
25 mV for the device with the 1.5 nm MgO barrier, which is the highest value reported so far
in lateral Si-based spin-valve devices. Figure 7.7 shows the spin-dependent output V as afunction of V. V tends to decrease as the MgO layer thickness increases, and significantly
drops when tMgO is over 2.5 nm due to the rapid drop of the MR ratio at high bias voltages.

125 | P a g e

Chapter 7

Figure 7.7: The bias voltage dependence of the spin-dependent output voltage of devices
with different MgO layer thicknesses. The highest output voltage |∆V| of 25 mV was achieved
at bias voltage of 1.1 V in the device with a 1.5 nm MgO layer.

7.5. Summary
This chapter shows the investigation of the spin transport in nanoscale Si spin-valve
devices prepared by MBE with various spin injectors/detectors. In the device with MgO (1.5nm) / Ge (1 nm) double layers grown by MBE, I obtained the highest spin-dependent output
voltage of 25 mV. The results in this chapter have been submitted and under review.

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Chapter 7
References
1.

Hiep, D. D., Tanaka, M. & Hai, P. N. Inverse spin-valve effect in nanoscale Si-based
spin-valve devices. J. Appl. Phys. 122, 223904 (2017).

2.

Hiep, D. D., Tanaka, M. & Hai, P. N. Spin transport in nanoscale Si-based spinvalve devices. Appl. Phys. Lett. 109, 232402 (2016).

3.

Yuasa, S., Nagahama, T., Fukushima, A., Suzuki, Y. & Ando, K. Giant roomtemperature magnetoresistance in single-crystal Fe/MgO/Fe magnetic tunnel
junctions. Nat. Mater. 3, 868–871 (2004).

4.

Ono, K., Austing, D. G., Tokura, Y. & Tarucha, S. Current Rectification by Pauli
Exclusion in a Weakly Coupled Double Quantum Dot System. Science (80-. ). 297,
1313–1317 (2002).

5.

Weber, B. et al. Spin blockade and exchange in Coulomb-confined silicon double
quantum dots. Nat. Nanotechnol. 9, 430–435 (2014).

127 | P a g e

Chapter 8

CHAPTER 8
IMPROVEMENT of THE SPIN VALVE RATIO by
USING AN Mg BUFFER LAYER between Fe andMgO LAYERS
In this chapter, the investigation of the spin transport in nanoscale silicon (Si)-based spinvalve devices with a 20 nm Si channel and a Fe/Mg/MgO/Ge stack as the spin
injector/detector was conducted. By inserting an ultrathin (1 nm) Mg layer in between thetunnel barrier MgO and the Fe electrode to prevent the formation of a magnetically-deadlayer, the spin-valve ratio has been increased up to –3.6% at 15 K. This is the highest values
reported in Si lateral spin-valve devices. The results in this chapter have been submitted andunder review.

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Chapter 88.1. Spin-valve device structures
The highly doped n-type Si (100) substrates with electron density n = 11018 cm-3 wereused for device fabrication. The Si substrates were first cleaned by H2SO4/H2O2 solution,then etched by diluted hydrofluoric acid solution to remove the native oxide layer, and rinsed
in de-ionized water. For evaporation of the tunneling barrier and ferromagnetic electrodes,
Molecular-Beam-Epitaxy (MBE) was employed.
After the cleaning process, the substrates were introduced into a MBE chamber with abase pressure of 1×10-8 Pa for growing the stack of Fe (electrode) / Mg (spacer) / MgO( tunnel barrier) / Ge (buffer layer) (from the top to the bottom). Knudsen cells were used forthermal evaporation of Ge, Mg and Fe, while a low-power electron-beam evaporator wasused to deposit MgO with a slow rate of 0.03 Å/s. In-situ reflection high energy electron
diffraction confirmed that MgO grown on the Ge buffer layer became crystalline when the
MgO thickness exceeded 1 nm. After thin film deposition, the electron-beam lithography(EBL) and ion-milling techniques were used to fabricate the 20 nm Si channel. Finally, theAu (40 nm) / Cr (5 nm) pad electrodes for wire bonding were deposited by EB evaporation.
Figure 8.1 shows the schematic structure of a device.

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Chapter 8

Figure 8.1: The schematic structure of a device with the Mg buffer layer to prevent theformation of a magnetically-dead layer at the interface of Fe and MgO.

8.2. Optimization of the Mg buffer layer thickness
In this section, I have varied the Mg buffer layer thickness and investigated the local spinvalve effect of the Si-based nanoscale spin-valve devices in order to optimize the Mg
thickness in this device structure. Figure 8.2(a) shows the device structure in this experiment.
The devices were fabricated with the fixed structure of 10 nm Fe / Mg / 1.5 nm MgO /1nm Ge but varying the Mg thickness tMg from 0.5 to 2 nm. Figure 8.2(b) shows thedependence of MR ratio on bias voltage in different Mg thicknesses. The device with a 0.5
nm Mg buffer layer shows the maximum MR ratio lower than those with thicker Mg.
However, the device with 2 nm Mg also shows the worse performance than the others. It is130 | P a g e

Chapter 8
found that the window for high MR ratio is between 1 nm and 1.5 nm Mg buffer layer.
Therefore, the optimized thickness of Mg at 1 nm was chosen.

Figure 8.2: (a) The schematic structure of devices which are used for the optimizationof Mg buffer layer thickness, (b) the dependence of local spin-valve signal MR on appliedbias voltage for various devices with different Mg thickness, and (c) the dependence of
local MR ratio on the Mg buffer layer thickness measured at 300 mV.

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Chapter 88.3. Optimization of the MgO tunnel barrier thickness in Mginserted device
In this section, a 1 nm Mg buffer layer has been inserted into the device and varied the
MgO thickness in order to optimize the MgO thickness in this new structure. Figure 8.3
describes the device structure in this experiment.
Devices were fabricated with the fixed structure of 10 nm Fe / 1 nm Mg / MgO / 1nm Ge
but varied the MgO thickness tMgO from 1.5 to 3.5 nm.

Figure 8.3: The schematic structure of devices which is used for the optimization of
MgO tunnel barrier thickness. The MgO thickness is varied from 1.5 to 3.5 nm.
8.3.1. Conductance characteristics
This part shows the investigation of the conductance characteristics of the Si-based
nanoscale 2 terminal spin-valve devices with the 1 nm Mg buffer layer.
Figure 8.4(a) shows the current – voltage characteristic (I – V curves) at 50 – 300 K ofthe device grown by MBE with a 3.5 nm-thick MgO tunnel barrier and a 1 nm-thick Mg
insertion layer. The non-linear I – V curves were observed, suggesting that the resistances of132 | P a g e

Chapter 8
the devices are dominated by the electron transport through the spin-valve structure
consisting of Fe/Mg/MgO/Ge – Si – Ge/MgO/Mg/Fe. The strong dependence of the I – V
curves on temperature was also found. Figure 8.4(b) shows the resistance of this device as afunction of temperature, measured with a bias voltage of 300 mV. Here, a rapid increase of
the resistance as temperature decreases, from the order of ~ 101 Ω at 300 K to ~ 106 Ω at 15
K, was observed. Such behavior indicates that the electron transport through the barrier
occurs not by direct tunneling, but by thermally activated hopping through defect states in
the MgO tunnel barrier [1]. From the temperature dependence of the resistance, it is possible
to evaluate the contribution of the parasitic AMR of the Fe electrodes to the total spin-valve
effect at various temperatures as follows. The total device resistance R can be decomposed
into two components: R = RFe + Rsv, where RFe is the parasitic series resistance of the Feelectrodes and Rsv is the intrinsic tunneling resistance of the spin-valve structure. Since theparasitic RFe(T) decreases as temperature decreases, RFe(T)  RFe(300K) < R(300K) ≈ 20 .
The AMR effect |RFe/RFe| of Fe is in the order of 0.1%, thus the magnetoresistance of Fe|RFe| is in the order of 0.2 . From this consideration, it could be estimated the contributionof the parasitic AMR effect of |RFe/R(T)| at 15 K should be in the order of 10-7 which isextremely small compared with the observed MR signal of 2% – 3%, as shown later.
Therefore, the contribution of the parasitic AMR effect of the Fe electrodes is negligible at
low temperatures.

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Chapter 8

Figure 8.4: (a) Current – voltage characteristics (I – V curves) of a MBE-grown device
with an MgO thickness tMgO = 3.5 nm and Mg thickness of 1 nm. (b) The temperature (T)dependence of the resistance R measured at 300 mV.
8.3.2. Local spin-valve effect
In this part, the investigation of the spin-valve effect in devices with the 1 nm Mg layer
inserted between the ferromagnetic Fe layer and the tunnel barrier MgO layer was presented.
This Mg insertion layer is used to protect the Fe layer from intermixing with other atoms in
the MgO barrier and from forming a magnetically-dead layer which reduces the spin

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Chapter 8
coherence of tunneling electrons [2,3]. Therefore, the Mg insertion layer is expected enhance
the spin injection efficiency in my devices. Figure 8.5(a) shows the MR characteristics of aspin-valve device with tMgO = 3.5 nm at 15 K, measured with a bias voltage of 200 mV anda magnetic field applied along the Si channel. In this device, by inserting the ultrathin (1 nm)
Mg layer, I have succeeded in improving the local MR ratio up to |R| ~ 75 kΩ and |MR| =3.6%. Figure 8.5(b) shows the MR ratio of the devices with various tMgO as a function of thebias voltage V. At low V, the best MR ratio is obtained for the sample with tMgO = 3.5 nm.
However, the MR ratio is nearly the same for all the devices at V  400 mV.
Finally, it is the discussion about the spin-dependent output voltage V of the devices inthis series. Figure 8.5(c) shows V of the devices with various tMgO as a function of V at 15
K. It is found that the maximum V in this series is limited to ~ 10 mV, which is lower thanin the highest value of 25 mV in the devices without an Mg insertion layer. The reason is thedecrease of the MR ratio of the devices with the Mg insertion layer at high bias voltages asshown in Fig. 8.5(b). I conclude that, while inserting the Mg layer between the Fe and the
MgO layer helps to increase the MR ratio at low bias voltages, it cannot generate a large spindependent output voltage at high bias voltages, which is important in the spin transistor
applications.

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Chapter 8

Figure 8.5: (a) Local magnetoresistance of the device grown by MBE with a 1 nm-thickMg insertion layer and 3.5 nm-thick MgO barrier layer, measured at 15 K with a bias voltage
of 200 mV. Inset shows the minor loop MR. (b) Bias voltage dependence of the MR ratio at
15 K for the devices with tMgO = 1.5 nm, 2.0 nm, and 3.5 nm. (c) Bias voltage dependence ofthe spin-dependent output voltage at 15 K for the devices with tMgO = 1.5 nm, 2.0 nm, and
3.5 nm.

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Chapter 88.4. Summary
This chapter describes the investigation of the spin transport in nanoscale Si spin-valve
devices prepared by MBE with various spin injectors/detectors. An ultrathin (1 nm) Mg layer
in between the tunnel barrier MgO and the Fe electrodes was utilized to prevent the formationof a magnetically-dead layer at this interface, in order to enhance the spin injection in this Sibased spin-valve structure. By optimizing the Mg buffer layer thickness and MgO tunnel
barrier thickness, I have succeeded in improving the MR ratio up to –3.6% in this Si-based
spin-valve structure. This value is the highest reported so far in Si lateral spin-valve devices.
These results give better understanding and control of the spin transport through the tunnel
barrier into Si and will be helpful for improving performance of Si-based spin-valve devices.
The results in this chapter have been submitted and under review.

References
1.

Hiep, D. D., Tanaka, M. & Hai, P. N. Inverse spin-valve effect in nanoscale Si-based
spin-valve devices. J. Appl. Phys. 122, 223904 (2017).

2.

Sato, S., Nakane, R., Hada, T. & Tanaka, M. Spin injection into silicon in threeterminal vertical and four-terminal lateral devices with Fe/Mg/MgO/Si tunnel
junctions having an ultrathin Mg insertion layer. Phys. Rev. B 96, 235204 (2017).

3.

Linn, T. & Mauri, D. Method of forming a barrier layer of a tunnelingmagnetoresistive sensor. US Patent, (2005).

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Chapter 9

CHAPTER 9
CONCLUSIONS and FUTURE PERSPECTIVES

Conclusions
The purpose of this study is to investigate the spin transport in the nanoscale Si-based
spin-valve device with Fe electrodes, (Mg)/MgO/Ge tunnel barriers, and a 20 nm-long Si
channel. Key results and conclusions are highlighted below.
In Chapter 1, I gave a brief review of spintronics, the technology which aims to utilize
the spin degree freedom of electrons for many applications in nowadays and futuretechnology such as new forms of information storage, foundation for new paradigm for
information processing to low-power nonvolatile green electronics. In spintronics, the spinMOSFET is considered to be the building block of low-power-consumption electronics
devices. And Si with its advantages is the promising channel material for Spin-MOSFET.
This chapter also figures out the current issues of Si spintronics in the diffusive regime, andthe motivation to solve them by ballistic transport.
In Chapter 2, overviews of the spin transport mechanisms in diffusive regime and ballistic
regime were presented. Additionally, electrical spin measurements techniques were138 | P a g e

Chapter 9
discussed. An understanding of the physical mechanisms that govern spin transport inside
semiconductors is essential for proper design and characterization of these devices, as well
as for overcoming the current issues.
In Chapter 3, the fabrication process and device characterization in this study were
described. At the beginning, I gave an introduction about the techniques mainly used in thisresearch, such as EB evaporation, MBE, EBL, ion milling, and photolithography. Then, I
discussed about the process flow which is used for device fabrication, from the substratepreparation, FM electrodes and tunnel barriers deposition, fabrication of nanoscale Sichannel, fabrication of microscale contacts, to the last step of wire bonding to chip carriers.
In the end of this chapter, the setups and techniques used to measure my devices are briefly
discussed.
In Chapter 4, investigation of spin-valve devices fabricated by electron beam evaporation
was presented. I have investigated the spin transport in nanoscale Si spin-valve devices with
the FM and tunnel barrier prepared by the EB evaporation method. Systematic investigationsof the bias voltage dependence, temperature dependence, and magnetic-field direction
dependence of the magnetoresistance indicate that the observed spin valve effect is governedby the spin transport through the nanoscale Si channel. A high magnetoresistance of 12 Ω
corresponding to MR ratio of 0.8 % and a large spin-dependent output voltage of 13 mV wereachieved for a device with the MgO/Ge tunnel layer at a bias voltage of 1.7 V at 50 K, which
are among the highest values reported at that time. The spin-valve effect decreases with

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Chapter 9
increasing temperature but remains observable up to 200 K. The results in this chapter havebeen published on APL 109, 232402 (2016).
In Chapter 5, investigation of spin transport in spin-valve devices fabricated by molecularbeam epitaxy was presented. An enormous spin-valve effect with |R| up to 57 kΩ,
corresponding to |R/R| = 3%, has been clearly observed. The sign of the spin-valve effect isreversed at low temperatures, suggesting the possibility of the spin-blockade effect of defectstates in the MgO tunnel barrier. A high spin-dependent output voltage is 20 mV at the bias
voltage of 0.9 V at 15 K, which is among the highest values reported in lateral Si-based spinvalve devices at that time. The results in this chapter have been published in JAP 122, 223904(2017), which is selected as the AIP Science Highlight paper and JAP Featured paper.
In Chapter 6, I demonstrated two important roles of (quasi) ballistic transport in thenanoscale Si channel: (i) generation of spin-valve effect even when there is no barrier at room
temperature, and (ii) suppress spin-flip scattering to achieve a higher spin-valve ratio at low
temperature. Indeed, I have obtained a large spin-valve ratio of 3%, which is much larger
than that observed in m-long Si channel devices reported before. Nevertheless, the electrontransport in my nanoscale devices may be quasi ballistic rather than fully ballistic, because
the channel length (20 nm) is not much shorter than that of the mean free path of 20 ~ 40 nm
for n-type Si with n = 1018 cm-3 [1]. By further downsizing the Si channel length to sub-10
nm, it is possible to achieve fully ballistic transport and higher spin-valve signals. The resultsin this chapter have been published in AAPPS Bulletin 28, 7-15 (2018) – Featured paper.

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Chapter 9
In Chapter 7, optimization of the MgO tunnel barrier thickness of spin-valve devices was
described. I have investigated the spin transport in nanoscale Si spin-valve devices preparedby MBE with different MgO layer thickness. In the device with MgO (1.5 nm) / Ge (1 nm)
double layers grown by MBE, I obtained the highest spin-dependent output voltage of 25
mV. The results in this chapter have been submitted and under review.
In Chapter 8, I presented improvement of the spin-valve ratio by using an Mg buffer layer
between Fe and MgO layers. In this chapter, an ultrathin Mg layer inserted between the tunnel
barrier MgO and the Fe electrodes was utilized to prevent the formation of a magneticallydead layer at this interface, in order to enhance the spin injection in this Si-based spin-valve
structure. By optimizing the Mg buffer layer thickness (1 nm Mg) and MgO tunnel barrier
thickness (3.5 nm MgO), I have succeeded in improving the MR ratio up to -3.6 % in this Sibased spin-valve structure. This value is the highest reported so far in lateral spin-valve
devices. These results give better understanding and control of the spin transport through thetunnel barrier into Si and will be helpful for improving performance of Si-based spin-valve
devices. The results in this chapter have been submitted and under review.

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Chapter 9

Future Perspectives
For practical applications of Si-based spin-MOSFET, it is necessary to obtain the highspin-dependent output voltages (~ 100 mV) and high MR ratio (~ 20 %) in devices using
spin-valve effect. However, previous works on spin-valve devices with µm-long Si channels
showed poor spin output voltage (∆V ~ 1 mV) and low MR ratio (~ 0.1 %). In this study, I
have demonstrated the advantages of ballistic transport by using the nanoscale Si-based spin
valve device. In the ballistic regime, significant improvements of the MR ratio (MR = 3.6 %)and the spin output voltage change (∆V = 25 mV) have been shown.
However, the ballistic transport itself would be not enough to realize the large spin output
voltage and MR ratio for practical applications of spin-MOSFET. Another element whichplays a crucial role in spin injection process is the MgO spin filter layer. In this study, I foundthat the quality of MgO layer is not as good as expected. It is known that the task of growing
a good quality but thin MgO layer directly on Si is challenging. Therefore, I used Ge as abuffer layer for growing MgO on Si. Indeed, the MgO/Ge double layer tunnel barrier has
shown good results in improvement of the ∆V and MR ratio, comparing to a MgO single
layer, as shown in Chapter 4. However, this MgO/Ge double layer has been not perfected yet.
I found that the device resistance strongly depends on the temperature (~ 101 Ω at room
temperature, but ~ 107 Ω at 15K), indicates that electron transport through the tunnel barrieris governed not by the direct tunneling, but by thermally activated hopping via defect statesin barrier. This thermally hopping tunneling causes the inverse spin-valve effect reported inthis study, which could reduce the coherent tunneling of electron through the tunnel barrier,142 | P a g e

Chapter 9thus decrease the spin injection and transport in the spin-valve device. Another evidence forthe high defect-state density is the decrease of device resistance with increasing the MgO
thickness which has been explained in Chapter 7. All of those evidences manifest theunsatisfactory quality of the MgO layers with high defect density. Therefore, in the future,
improvement of the crystal quality of the MgO spin filter barrier is the key to improve thespin-valve signal in nanoscale Si spin-valve devices.
Here, several possible methods to improve the quality of the MgO layer are outlined. The
1st method could be growth of MgO at higher growing temperature. In this study, the MgOlayer has been only grown at low temperature (below 100oC). By optimizing the growing
temperature (e.g. growing MgO at higher temperature), we could improve the quality of thisMgO layer. The 2nd method could be using another buffer layer instead of Ge. In this study,
while Ge shows its role in improving the spin injection efficiency, it is still not enough for
growing a good MgO barrier with low defect density. Other materials, such as SiO2, Al2O3
or HfO2, as the buffer layer or spin filter layer, may help improve the ∆V and MR ratio. The
3rd method could be the complement of oxygen in the MgO growing process. Because MgOlayer is grown at very low rate (0.03 Å/s), there is the possibility of desorption of oxygen
atom from MgO during the growth process, which can create oxygen vacancy defects in MgO.
By introducing O2 into the growth chamber during the MgO growth process, we can reducethe defect density in the MgO layer.
In this study, I used the bulk Si substrates, in which it is difficult to control the depth d ofthe Si channel. If the ratio of L/d << 1, here L is the Si channel length, it is difficult to force

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Chapter 9
the polarized electrons move directly from injector to detector, instead of moving around in
the Si substrate. Decreasing the Si channel depth by using silicon-on-insulator (SOI)
substrate would be possible way to improve the signal in the Si-based nanoscale spin-valve
device.
Finally, realizing the Si-based spin MOSFET will be the ultimate goal.

References
1.

Qiu, B. et al. First-principles simulation of electron mean-free-path spectra andthermoelectric properties in silicon. EPL (Europhysics Lett. 109, 57006 (2015).

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Acknowledgements
At the end of 2014, a turn in life came to me to start researches on spin transport in silicon,
which is a promising research topic for future electronic technology. And in the spring of
2015, by great support from Prof. Pham Nam Hai, I had opportunity to start PhD research inTokyo Institute of Technology. 4-year is not a long time, but it is enough for me to receive
wonderful things, from academic knowledge to fun in life. I also received a lot of help frompeople, and I would like to name some of them.
First of all, I wish to express my gratitude and appreciation to my supervisor, professor
Pham Nam Hai. He is a physicist with a great understanding about the nature, especially in
spintronics. In research, when unidentified problems block my path, the discussions with him
clarify the problems and brighten my way. He taught me not only science, but also the attitude
toward science. I have learned from him how to keep an open mind to absorb new things, but
still employ the fundamental logic to evaluate the problems. I will not forget the atmospherein parties at his home, with the great food made from his wife.
I would like to express my sincere thankfulness to professor Masaaki Tanaka for his
collaboration and support in experimental facilities as well as valuable comments in mypapers.
I would like to thank my Japanese friends in Pham Lab such as Yoshida, Ueda, Yao,
Yamane. They helped me to do the Japanese procedures which are tough to me. From thefirst days I came to Pham Lab, together we have made our lab better day by day. I will notforget the “nomikai” in our lab with funny discussions, and the wonderful ski tour organizedby Ueda. I learned a lot of things about seriousness and responsibility from them.
I express my sincere thanks also to Dr. Bui Cong Tinh, the JSPS fellow in Pham Lab forthe valuable discussion with him. He is one of my close friend in Tokyo Tech with lot ofmemories.

145 | P a g e

I would like to thank Mr. Nguyen Huynh Duy Khanh, the PhD student in Pham Lab, for
his help in work and for being an honest friend of mine.
I would like to appreciate all the members of Pham Lab for creating a friendly atmospherein laboratory and supporting my research.
I also would like to appreciate members of Tanaka Lab in Tokyo University who support
me in doing experiment.
I would like to express my gratitude to Rotary Yoneyama Scholarship and Iwatani
Scholarship for the supports for my PhD study. Without their supports, it is tough for me to
study in Japan. Especially, I wish to express the thankfulness to my Rotary sponsor, Mr.
Egawa for his help. He taught me many things about Japan and life. And the food of his
restaurant is so delicious. Japan gave me a chance to grow up, to complete myself, and thosescholarships gave me the opportunities to be proud of being their scholarship students.
I would like to extend my sincerest thanks and appreciation to all of my friends, whogave me love and trust. You raise me up. Especially I wish to express the gratitude to mylovely girlfriend. Without her love, the weather is too cold.
Finally, I would like to say thank you to my beloved family. For all of your help andsupport, I cannot thank you enough. Without you, I am nothing.

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List of publications
This thesis is based on the following publications:•

Spin transport in nanoscale Si-based spin-valve devices
Duong Dinh Hiep, Masaaki Tanaka and Pham Nam Hai
Applied Physics Letters 109, 232402 (2016)•

Inverse spin-valve effect in nanoscale Si-based spin-valve devices
Duong Dinh Hiep, Masaaki Tanaka and Pham Nam Hai
Journal of Applied Physics 122, 223904 (2017) – Featured Paper of Journal ofApplied Physics and Science Highlight Paper of American Institute of Physics
(AIP)

•

Spin-valve effect in nanoscale Si-based devices
Duong Dinh Hiep, Masaaki Tanaka and Pham Nam Hai
Association of Asia Pacific Physical Societies Bulletin 28, 7-15 (2018) (Featured
Paper)•

Large spin-valve effect and spin-dependent output voltage in nanoscale Silicon spinvalve devices
Duong Dinh Hiep, Masaaki Tanaka and Pham Nam Hai
Submitted and under review

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List of conferences International Conference•

9th International Conference on Physics and Applications of Spin-Related
Phenomena in Solids (2016) – Spin transport in nanoscale silicon channels – Duong
Dinh Hiep, Masaaki Tanaka and Pham Nam Hai•

SSDM International Conference (2016) – Spin-valve effect in nanoscale Si-based
devices with ferromagnetic electrodes – Duong Dinh Hiep, Masaaki Tanaka andPham Nam Hai•

SSDM International Conference (2017) – Inverse spin-valve effect in MBE-grownnanoscale Si spin-valve devices – Duong Dinh Hiep, Masaaki Tanaka and Pham Nam
Hai•

Kanamori Memorial International Symposium (2017) – Inverse spin-valve effect inspin-valve devices with nanoscale Si channels – Duong Dinh Hiep, Masaaki Tanaka
and Pham Nam Hai•

IEEE International Magnetic Conference INTERMAG 2018 – Large spin-valve effectin Si nano spin-valve devices – Duong Dinh Hiep, Masaaki Tanaka and Pham Nam
Hai

148 | P a g e Domestic Conference•

第 64 回応用物理学会春季学術講演会 (2017) – Spin valve effect in Si-based spin
valve devices with a nano-scale Si channel – Duong Dinh Hiep, Masaaki Tanaka andPham Nam Hai•

第 65 回応用物理学会春季学術講演会 (2018) – Inverse spin valve effect in nanoscale Si-based spin-valve devices – Duong Dinh Hiep, Masaaki Tanaka and PhamNam Hai•

第 66 回 応 用 物 理 学 会 春 季 学 術 講 演 会 (2019) – Large spin-dependent
magnetoresistance and output voltage in the nanoscale Si spin-valve devices – Duong
Dinh Hiep, Masaaki Tanaka and Pham Nam Hai