Electronic spin transport in bilayer and single layer graphene [Elektronische Ressource] / Tsung-Yeh Yang

Sujets

• Physik

Informations

• Publié par: rheinisch-westfalischen_technischen_hochschule_-rwth-_aachen
• Publié le: 01 janvier 2011
• Nombre de lectures: 25
• Langue: English
• Poids de l'ouvrage: 14 Mo

Extrait

Electronic Spin Transport in Bilayer
and Single Layer Graphene
Von der Fakultat fur Mathematik, Informatik und Naturwissenschaften
der RWTH Aachen University zur Erlangung des akademischen Grades
eines Doktors der Naturwissenschaften genehmigte Dissertation
vorgelegt von
M. Sc. Tsung-Yeh Yang
aus Hsinchu city, Taiwan
Berichter: Universitatsprofessor Dr. sc. nat. Gernot Gun therodt
Universit Dr. rer. nat. Markus Morgenstern
Tag der mundlic hen Prufung: 06. Juli. 2011
Diese Dissertation ist auf den Internetseiten der Hochschulbibliothek
online verfugbar.Abstract
Graphene has drawn plenty of attention since its discovery in 2004. Due to
its excellent properties, such as long spin relaxation length and gate-tunable
spin transport, graphene is expected to be a potential candidate for spintron-
ics applications. In this thesis, the systematic study of the spin relaxation
mechanisms in bilayer and single layer graphene is presented. Graphene-
based spin valve devices in four-terminal non-local geometry are fabricated
for the investigation of the charge and spin transport properties. From the
correlation between the charge carrier mobility and spin relaxation time, the
major role of the D’yakonov-Perel’ type of spin relaxation in bilayer graphene
is discovered. And the Elliott-Yafet mechanism could dominate in single layer
graphene. The rst observation of long spin relaxation times of 2 ns in bi-
layer graphene is presented in this thesis, which is longer than that in single
layer and few layer graphene.
Next the spin valve devices with CVD synthesized single layer and bilayer
graphene are demonstrated. Both the charge and spin transport properties of
CVD SLG and BLG show very comparable performances, including the carrier
mobility, spin relaxation time, and spin relaxation length, to the exfoliated
natural graphene. The results suggest that the CVD synthesized graphene
could be promising for spintronics applications and possible to be integrated
into wafer-scale semiconductor manufacturing.CONTENTS
Contents
1 Introduction 1
1.1 Spintronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2 Theoretical aspects 7
2.1 Spin injection and spin precession . . . . . . . . . . . . . . . . 7
2.1.1 Electrical spin injection and detection . . . . . . . . . . 7
2.1.2 Non-local spin valve geometry . . . . . . . . . . . . . . 11
2.1.3 Electron spin precession . . . . . . . . . . . . . . . . . 14
2.2 Conductivity mismatch . . . . . . . . . . . . . . . . . . . . . . 17
2.3 Electronic properties of graphene . . . . . . . . . . . . . . . . 23
2.3.1 Electronic properties of single layer graphene . . . . . . 23
2.3.2 El properties of bilayer graphene . . . . . . . . 28
2.4 Spintronic properties of graphene . . . . . . . . . . . . . . . . 32
2.4.1 Spin relaxation . . . . . . . . . . . . . . . . . . . . . . 33
2.4.2 D’yakonov-Perel’ mechanism . . . . . . . . . . . . . . . 33
2.4.3 Elliott-Yafet mechanism . . . . . . . . . . . . . . . . . 34
2.4.4 Spin-orbit coupling . . . . . . . . . . . . . . . . . . . . 36
3 Experimental techniques 41
3.1 Graphene preparation . . . . . . . . . . . . . . . . . . . . . . 41
3.1.1 Graphene from mechanical exfoliation . . . . . . . . . . 42
3.1.2 G from chemical vapor depostion . . . . . . . . 47
3.2 Spin valve device fabrication . . . . . . . . . . . . . . . . . . . 48
3.2.1 MgO deposition . . . . . . . . . . . . . . . . . . . . . . 50
3.2.2 Device layout design . . . . . . . . . . . . . . . . . . . 52
iCONTENTS
3.3 Electrical measurement set up . . . . . . . . . . . . . . . . . . 55
14 Electricspininjectionandspinrelaxationinbilayergraphene 59
4.1 Spin injection into bilayer graphene at room temperature . . . 59
4.1.1 Bilayer graphene resistance . . . . . . . . . . . . . . . . 61
4.1.2 Bilayer spin valve device with global MgO bar-
riers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
4.1.3 Graphene spin valve device with local MgO barriers . . 72
4.1.4 MgO barrier . . . . . . . . . . . . . . . . . . . . . . . . 80
4.2 Spin transport in bilayer graphene at low temperature . . . . . 84
4.3 Summary of the chapter . . . . . . . . . . . . . . . . . . . . . 90
5 Spin transport in single layer graphene 91
5.1 Spin transport in single layer . . . . . . . . . . . . . 91
5.2 Summary of the chapter . . . . . . . . . . . . . . . . . . . . . 96
26 Spin transport in large-scale graphene spin valve devices 97
6.1 Spin transport in CVD synthesized single layer graphene . . . 98
6.1.1 CVD SLG-based spin valve devices . . . . . . . . . . . 98
6.1.2 Structural defect-induced spin-orbit coupling . . . . . . 104
6.2 Spin transport in CVD synthesized bilayer graphene . . . . . . 105
6.3 Summary of the chapter . . . . . . . . . . . . . . . . . . . . . 109
7 Summary and outlook 111
7.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
7.2 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
References 115
Acknowledgements 129
Curriculum Vitae 131
1T.-Y. Yang et al., Phys. Rev. Lett. 107, 047206 (2011).
2 A. Avsar , T.-Y. Yang , S. Bae et al., Nano Lett. 11, 2363 (2011).
iiFIGURES
Figures
1.1 GMR spin valve structure . . . . . . . . . . . . . . . . . . . . 3
1.2 Optical microscopic image of graphene akes . . . . . . . . . . 4
2.1 Resistor models of GMR spin valves . . . . . . . . . . . . . . . 9
2.2 Experiment of GMR spin valve . . . . . . . . . . . . . . . . . 10
2.3 Spin valve experiment in four-terminal non-local geometry . . 12
2.4 Spin precession under ballistic transport condition . . . . . . . 15
2.5 Hanle spin precession experiment . . . . . . . . . . . . . . . . 16
2.6 Spin injection from a ferromagnet into a semiconductor . . . . 19
2.7 HRTEM of MgO barrier . . . . . . . . . . . . . . . . . . . . . 22
2.8 Raman spectrum of SLG after MgO deposition . . . . . . . . . 22
2.9 Graphene lattice structure . . . . . . . . . . . . . . . . . . . . 23
2.10 SLG band structure and eld-e ect resistance . . . . . . . . . 24
2.11 Momentum scattering time as a function of carrier density in
SLG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.12 Bilayer graphene band structure . . . . . . . . . . . . . . . . . 29
2.13 BLG carrier density dependence of charge and spin di usion
constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.14 Temperature dependence of BLG conductivity . . . . . . . . . 30
2.15 Momentum scattering time as a function of carrier density in
BLG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.16 The D’yakonov-Perel’ and Elliott-Yafet spin relaxation mecha-
nisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
2.17 Spin relaxation length as a function of spin di usion constant
at room temperature . . . . . . . . . . . . . . . . . . . . . . . 36
iiiFIGURES
2.18 Monte Carlo simulations of the momentum relaxation rate and
the spin relaxation time in SLG . . . . . . . . . . . . . . . . . 38
3.1 Optical contrast spectrum and images of single layer graphene 42
3.2 contrast spectrum as a function of number of graphene
layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
3.3 Raman spectra of SLG and BLG . . . . . . . . . . . . . . . . 45
3.4 AFM images of graphenes . . . . . . . . . . . . . . . . . . . . 46
3.5 Process ow of graphene-based spin valve device fabrication . 48
3.6 Multichamber UHV-MBE system . . . . . . . . . . . . . . . . 49
3.7 MgO deposition optimization test on SiO /Si substrates . . . 512
3.8 AFM image of a 1nm thick MgO lm deposited on BLG . . . 52
3.9 AFM images of a 1.5 nm thick MgO lm deposited on SLG . . 53
3.10 Layout of graphene spin valve device . . . . . . . . . . . . . . 53
3.11 AMR e ect experiments of Co wires . . . . . . . . . . . . . . . 54
3.12 Electrical measurement equipments . . . . . . . . . . . . . . . 56
3.13 Schematic drawing of electronic set up connection . . . . . . . 56
4.1 Schematic drawings of graphene spin valve devices . . . . . . . 60
4.2 Experiment of gate voltage dependence of bilayer graphene re-
sistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
4.3 Carrier density dependence of BLG conductivities . . . . . . . 63
4.4 Gate voltage dependence of BLG spin valves and Hanle spin
precessions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
4.5 Summary of gate voltage dependence of spin transport quanti-
ties in a BLG device with a global MgO barrier at room tem-
perature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
4.6 Carrier density dependences of momentum scattering time and
spin relaxation time . . . . . . . . . . . . . . . . . . . . . . . . 69
4.7 Room temperature carrier density dependences of di usion con-
stant and spin relaxation time of BLG devices with global MgO
barriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
4.8 Experiments of spin valve and Hanle spin precession of a BLG
device with local MgO barriers . . . . . . . . . . . . . . . . . . 72
ivFIGURES
4.9 Summary of carrier density dependence of spin transport quan-
tities of a BLG device with local MgO barriers at room tem-
perature . . . . . . . . . . . . . . . . . . . . . . . . .