University of Groningen
Spin transport in graphene-based van der Waals heterostructures
Ingla Aynés, Josep
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Publication date: 2018
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Ingla Aynés, J. (2018). Spin transport in graphene-based van der Waals heterostructures. Rijksuniversiteit Groningen.
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Spin transport in graphene-based van der Waals heterostructures
Spino
graph
Spintronics in Graphene
Zernike Institute PhD thesis series 2018-33 ISSN: 1570-1530
ISBN: 978-94-034-1178-1
ISBN: 978-94-034-1177-4 (electronic version)
The work described in this thesis was performed in the research group Physics of Nanodevices of the Zernike Institute for Advanced Materials at the University of Groningen, the Nether-lands.
Description of the cover: Spins float down a river, the process is analogous to spin drift. Be-tween the valleys, spins scatter with the trees. The spins which point out-of-plane keep their direction while the in-plane ones flip after scattering. The sketch is a simplified picture of the spin-valley coupling mechanism described in the thesis.
Spin Transport in graphene-based van
der Waals heterostructures
Proefschrift
ter verkrijging van de graad van doctor aan de Rijksuniversiteit Groningen
op gezag van de
Rector Magnificus Prof. Dr. E. Sterken en volgens besluit van het College voor Promoties.
De openbare verdediging zal plaatsvinden op
dinsdag 13 november 2018 om 11:00 uur
door
Josep Ingla Ayn´es
geboren op 29 juni 1990 te Ribera d’Ondara, Spanje
Promotor
Prof. dr. ir. B. J. van Wees
Copromotor Dr. I. J. Vera-Marun Beoordelingscommissie Prof. dr. G. E. W. Bauer Prof. dr. F. Casanova Prof. dr. I. Grigorieva
Contents
1 Introduction 1 1.1 Semiconductor electronics . . . 1 1.2 Spin electronics . . . 1 1.3 Graphene spintronics . . . 2 1.4 Thesis outline . . . 3 References . . . 42 Electronic properties of two-dimensional materials 7 2.1 Monolayer graphene . . . 7
2.2 Bilayer graphene . . . 8
2.3 Charge diffusion coefficient in monolayer and bilayer graphene . . . . 9
2.4 Bilayer graphene field effect transistors . . . 10
2.5 Hexagonal boron nitride . . . 12
2.6 Transition metal dichalcogenides . . . 12
References . . . 13
3 Graphene spintronics 17 3.1 Spin and charge currents . . . 17
3.2 Minority carrier drift in semiconductors . . . 18
3.3 Drift-diffusion equations . . . 20
3.4 Two-channel model and the nonlocal measurement configuration . . . 21
3.4.1 Two-channel model . . . 21
3.4.2 Spin injection . . . 23
3.4.3 Spin detection . . . 23
3.4.4 Contact-induced spin relaxation and the conductivity mismatch problem . . . 24
3.5 Nonlocal spin valve . . . 24
3.7 Spin precession in anisotropic systems . . . 29
3.8 Modelling of spin transport in complex device geometries . . . 30
3.9 Spin relaxation in graphene . . . 31
3.9.1 Theoretical considerations about spin relaxation in graphene . 32 3.9.2 Spin transport measurements in graphene . . . 32
3.9.3 Spin transport in graphene/TMD heterostructures . . . 34
References . . . 35
4 Methods 39 4.1 Sample fabrication . . . 39
4.1.1 Exfoliation of two-dimensional materials . . . 39
4.1.2 Dry pick-up technique . . . 40
4.1.3 Contact preparation . . . 41
4.1.4 Etching of graphene/hBN stacks using a hard mask . . . 43
4.2 Electrical measurements . . . 44
References . . . 46
5 24 micrometer spin relaxation length in boron nitride-encapsulated bilayer graphene 47 5.1 Introduction . . . 47
5.2 Results . . . 48
5.2.1 Charge transport characterization . . . 48
5.2.2 Nonlocal spin transport . . . 50
5.2.3 Device simulations . . . 52
5.3 Conclusions . . . 54
5.4 Supplementary information . . . 54
5.4.1 Sample fabrication . . . 54
5.4.2 Transport measurements . . . 55
5.4.3 Determination of the mobility at 4 K . . . 55
5.4.4 Spin and charge transport at room temperature . . . 56
5.4.5 Spin transport at 4 K . . . 57
References . . . 58
6 88% directional guiding of spin currents with 90 micrometer relaxation length in bilayer graphene using carrier drift 61 6.1 Introduction . . . 61
6.2 Results and discussion . . . 62
6.3 Conclusions . . . 68
6.4 Supplementary information . . . 69
6.4.1 Modelling parameters . . . 69
6.4.3 Effect of τencin the modelling . . . 75
References . . . 75
7 Drift control of spin currents in graphene-based spin current demultiplex-ers 79 7.1 Spin drift model . . . 80
7.2 Results . . . 82 7.2.1 Geometry I . . . 82 7.2.2 Geometry II . . . 83 7.2.3 Demultiplexing operation . . . 84 7.2.4 Geometry III . . . 85 7.2.5 Geometry IV . . . 86 7.3 Discussion . . . 87 7.4 Conclusions . . . 88 References . . . 88
8 Large proximity-induced spin lifetime anisotropy in TMD/graphene het-erostructures 91 8.1 Introduction . . . 91
8.2 Results and discussion . . . 92
8.2.1 Hanle precession with Bz . . . 92
8.2.2 Hanle precession with Bx . . . 95
8.3 Conclusions . . . 97
8.4 Supplementary information . . . 98
8.4.1 Device fabrication . . . 98
8.4.2 Electrical characterization . . . 98
8.4.3 Contact resistances . . . 100
8.4.4 Local magnetoresistance measurements . . . 100
8.4.5 Models used for extraction of spin transport parameters . . . . 101
8.4.6 Hanle precession in pristine graphene with Bz . . . 103
8.4.7 Hanle precession in pristine graphene with Bx . . . 106
8.4.8 Hanle precession across the TMD/graphene region with Bx . . 106
8.4.9 Gate dependence of the spin signal . . . 108
8.4.10 Spin lifetime anisotropy in a WSe2/graphene heterostructure . 109 References . . . 110
9 Observation of spin-valley coupling induced large spin lifetime anisotropy in bilayer graphene 113 9.1 Introduction . . . 113
9.2 Results and discussion . . . 114
9.4 Supplementary Information . . . 120
9.4.1 Fabrication details . . . 120
9.4.2 Charge and spin transport characterization . . . 122
9.4.3 Spin lifetime anisotropy at zero DC bias . . . 124
9.4.4 Measurements using different injector-detector spacings . . . . 124
9.4.5 Low temperature anisotropy measurements . . . 125
9.4.6 Carrier concentration dependence of the in-plane spin lifetime 125 9.4.7 Spin precession measurements with in-plane magnetic fields . . 126
9.4.8 Carrier density dependence of the magnetoresistance . . . 128
9.4.9 Modeling of the spin lifetime anisotropy . . . 128
9.4.10 Effect of the contact resistance on the anisotropy . . . 130
9.4.11 Measurement of the contact resistances . . . 131
9.4.12 Estimation of the electric field . . . 132
9.4.13 Measurements on a second BLG device . . . 132
9.5 Electric field control of spin relaxation . . . 134
References . . . 135
10 Conclusions and outlook 139 10.1 High-quality graphene for long distance spin transport . . . 139
10.2 Spin guiding using drift currents . . . 140
10.3 Proximity induced spin orbit coupling in TMD/graphene heterostruc-tures . . . 141
10.4 Anisotropic spin transport in bilayer graphene . . . 141
References . . . 142 Summary 145 Samenvatting 149 Acknowledgements 153 List of publications 157 Curriculum Vitae 159
