University of Groningen
Charge and spin transport in two-dimensional materials and their heterostructures
Bettadahalli Nandishaiah, Madhushankar
DOI:
10.33612/diss.135800814
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below.
Document Version
Publisher's PDF, also known as Version of record
Publication date: 2020
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Bettadahalli Nandishaiah, M. (2020). Charge and spin transport in two-dimensional materials and their heterostructures. University of Groningen. https://doi.org/10.33612/diss.135800814
Copyright
Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
Take-down policy
If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.
Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum.
Charge and spin transport in two-dimensional materials
and their heterostructures
Zernike Institute PhD thesis series 2020-17 ISSN: 1570-1530
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 Netherlands. This work has received funding from Dieptestrategie funding from the Zernike Institute for Advanced Materials, Core Project 1 and 2 of the European Union Horizon 2020 research and innovation programme and NWO Spinoza prize awarded to Prof. B. J. van Wees by the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO).
Cover page design: Bhoomika Patel Bettadahalli Gangadharaiah
Cover art: The spinning top represents the electron spin travelling in a two-dimensional graphene (hexagon pattern) reported in this PhD thesis as spin transport studies in single and bi-layer graphene.
to obtain the degree of PhD at the University of Groningen
on the authority of the Rector Magnificus Prof. C. Wijmenga
and in accordance with the decision by the College of Deans.
Charge and spin transport in
two-dimensional materials and their
heterostructures
PhD Thesis
This thesis will be defended in public on Friday 15 May 2020 at 14.30 hours
Madhushankar Bettadahalli Nandishaiah
born on 9 June 1989 in Bengaluru, India
Friday 23 October 2020 at 11.00 hours
Supervisor
Prof. B.J. van Wees
Co-supervisor
Prof. C.H. van der Wal
Assessment committee
Prof. S.P. Dash Prof. M.A. Stöhr Prof. J. Ye
Contents
1 Introduction ... 1
1.1 Scaling of field-effect transistors ... 2
1.2 Two-dimensional materials ... 2
1.2.1 Graphene ... 3
1.2.2 Germanane ... 3
1.3 Spintronics ... 3
1.3.1 Graphene spintronics ... 4
1.3.2 Spintronics with graphene-TMD heterostructures ... 5
1.4 Outline of the thesis ... 6
2 Two-dimensional materials ... 13
2.1 Germanane ... 14
2.2 Graphene ... 14
2.3 Transition metal di-chalcogenides ... 15
2.4 Graphene-TMDheterostructure ... 16
3 Theory of electronic and spin transport in two-dimensional materials ... 19
3.1 Electronic transport in 2-dimensional materials ... 20
3.2 Charge and spin current ... 20
3.3 Metal oxide semiconductor field-effect transistor ... 21
3.3.1 Graphene FET ... 22
3.4 Spin injection into non-magnetic materials ... 23
3.4.1 Two-channel model ... 24
3.5 Spin transport in graphene ... 25
3.5.1 Spin diffusion equation ... 26
3.5.2 Non-local spin-valve measurement ... 26
3.5.3 Hanle spin precession measurement ... 30
3.6 Spin relaxation in graphene ... 35
3.6.1 Elliot-Yafet (EY) mechanism ... 35
3.6.2 D’yakonov-perel mechanism ... 35
3.6.3 Hyperfine interaction ... 35
3.6.4 Rashba spin-orbit coupling ... 36
3.7 Spin-orbit interaction in single and bilayer graphene in the proximity of TMD 36 3.7.1 Spin-orbit interaction in single-layer graphene ... 36
3.7.2 Spin-orbit coupling in single-layer graphene in the proximity of TMD .... 37
3.7.3 Spin-orbit coupling in bi-layer graphene ... 39
3.7.4 Spin-orbit coupling in BLG in the proximity of TMD ... 40
3.8 Spin absorption at the TMD/graphene interface... 40
4 Experimental methods ... 45
4.1 Sample preparation ... 46
4.1.1 Substrate preparation ... 46
4.1.2 Mechanical exfoliation ... 48
4.1.3 Optical image ... 49
4.1.4 Atomic force microscopy ... 50
4.1.5 Pickup and transfer technique ... 50
4.1.6 Electron beam lithography... 53
4.1.8 Wire bonding ... 55
4.2 Electrical measurements ... 55
4.2.1 Charge and spin transport ... 57
5 Electronic properties of germanane field-effect transistors ... 61
5.1 Introduction ... 62
5.2 Device fabrication and characterization ... 62
5.3 Measurement ... 63
5.4 Conclusion ... 68
5.5 Supplementary information ... 69
6 Study of proximity-induced SOC and anisotropic spin relaxation in single layer graphene − multi-layer WSe2 van der Waals heterostructures ... 79
6.1 Introduction ... 80
6.2 Device fabrication and characterisation ... 81
6.3 Measurements ... 82
6.3.1 In-plane spin transport in the graphene region under and near the WSe2 covered region ... 84
6.3.2 Spin relaxation anisotropy in the graphene region under and near the WSe2 covered region ... 89
6.3.3 Nonhomogeneous Bloch-diffusion model with spin lifetime anisotropy .. 94
6.4 Conclusion ... 96
6.5 Supplementary information ... 97
7 Large spin-relaxation anisotropy in bilayer-graphene/ WS2 heterostructures ... 113
7.1 Introduction ... 114
7.2 Device fabrication and measurements ... 115
7.4 Conclusion ... 120
7.5 Supplementary information ... 122
8 Study of proximity induced SOC in WSe2/bi-layer graphene heterostructures ... 139
8.1 Introduction ... 140
8.2 Device fabrication and characterisation ... 141
8.3 Results ... 142 8.4 Conclusion ... 150 8.5 Supplementary information ... 151 9 Conclusion ... 157 9.1 Conclusion ... 158 Summary ... 161 Samenvatting ... 165
