| 1. |
- Chua, C., et al.
(författare)
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Quantum Hall Effect and Quantum Point Contact in Bilayer-Patched Epitaxial Graphene
- 2014
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 14:6, s. 3369-3373
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Tidskriftsartikel (refereegranskat)abstract
- We study an epitaxial graphene monolayer with bilayer inclusions via magnetotransport measurements and scanning gate microscopy at low temperatures. We find that bilayer inclusions can be metallic or insulating depending on the initial and gated carrier density. The metallic bilayers act as equipotential shorts for edge currents, while closely spaced insulating bilayers guide the flow of electrons in the monolayer constriction, which was locally gated using a scanning gate probe.
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| 2. |
- Lartsev, Arseniy, 1987, et al.
(författare)
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Tuning carrier density across Dirac point in epitaxial graphene on SiC by corona discharge
- 2014
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 105:6
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate reversible carrier density control across the Dirac point (Δ n∼ 1013cm-2) in epitaxial graphene on SiC (SiC/G) via high electrostatic potential gating with ions produced by corona discharge. The method is attractive for applications where graphene with a fixed carrier density is needed, such as quantum metrology, and more generally as a simple method of gating 2DEGs formed at semiconductor interfaces and in topological insulators.
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| 3. |
- Yager, Thomas, 1987, et al.
(författare)
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Express Optical Analysis of Epitaxial Graphene on SiC: Impact of Morphology on Quantum Transport
- 2013
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 13:9, s. 4217-4223
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Tidskriftsartikel (refereegranskat)abstract
- We show that inspection with an optical microscope allows surprisingly simple and accurate identification of single and multilayer graphene domains in epitaxial graphene on silicon carbide (SiC/G) and is informative about nanoscopic details of the SiC topography, making it ideal for rapid and noninvasive quality control of as-grown SiC/G. As an illustration of the power of the method, we apply it to demonstrate the correlations between graphene morphology and its electronic properties by quantum magneto-transport.
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| 4. |
- Yager, Thomas, 1987, et al.
(författare)
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Low contact resistance in epitaxial graphene devices for quantum metrology
- 2015
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Ingår i: AIP Advances. - : AIP Publishing. - 2158-3226 .- 2158-3226. ; 5:8, s. 087134-
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Tidskriftsartikel (refereegranskat)abstract
- We investigate Ti/Au contacts to monolayer epitaxial graphene on SiC (0001) for applications in quantum resistance metrology. Using three-terminal measurements in the quantum Hall regime we observed variations in contact resistances ranging from a minimal value of 0.6 Ω up to 11 kΩ. We identify a major source of high-resistance contacts to be due bilayer graphene interruptions to the quantum Hall current, whilst discarding the effects of interface cleanliness and contact geometry for our fabricated devices. Moreover, we experimentally demonstrate methods to improve the reproducibility of low resistance contacts (
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| 5. |
- Yager, Tom, et al.
(författare)
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Wafer-scale homogeneity of transport properties in epitaxial graphene on SiC
- 2015
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Ingår i: Carbon. - : Elsevier. - 0008-6223 .- 1873-3891. ; 87, s. 409-414
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Tidskriftsartikel (refereegranskat)abstract
- Magnetotransport measurements on Hall bar devices fabricated on purely monolayer epitaxial graphene on silicon carbide (SiC/G) show a very tight spread in carrier concentration and mobility across wafer-size dimensions. In contrast, SiC/G devices containing bilayer graphene domains display variations in their electronic properties linked to the amount of bilayer content. The spread in properties among devices patterned on the same SiC/G wafer can thus be understood by considering the inhomogeneous number of layers often grown on the surface of epitaxial graphene on SiC. (C) 2015 Elsevier Ltd. All rights reserved.
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| 6. |
- Yager, Tom, et al.
(författare)
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High mobility epitaxial graphene devices via aqueous-ozone processing
- 2015
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Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 106:6, s. 063503-
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Tidskriftsartikel (refereegranskat)abstract
- We find that monolayer epitaxial graphene devices exposed to aggressive aqueous-ozone processing and annealing became cleaner from post-fabrication organic resist residuals and, significantly, maintain their high carrier mobility. Additionally, we observe a decrease in carrier density from inherent strong n-type doping to extremely low p-type doping after processing. This transition is explained to be a consequence of the cleaning effect of aqueous-ozone processing and annealing, since the observed removal of resist residuals from SiC/G enables the exposure of the bare graphene to dopants present in ambient conditions. The resulting combination of charge neutrality, high mobility, large area clean surfaces, and susceptibility to environmental species suggest this processed graphene system as an ideal candidate for gas sensing applications.
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| 7. |
- Alexander-Webber, J. A., et al.
(författare)
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Giant quantum Hall plateaus generated by charge transfer in epitaxial graphene
- 2016
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322 .- 2045-2322. ; 6
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Tidskriftsartikel (refereegranskat)abstract
- Epitaxial graphene has proven itself to be the best candidate for quantum electrical resistance standards due to its wide quantum Hall plateaus with exceptionally high breakdown currents. However one key underlying mechanism, a magnetic field dependent charge transfer process, is yet to be fully understood. Here we report measurements of the quantum Hall effect in epitaxial graphene showing the widest quantum Hall plateau observed to date extending over 50 T, attributed to an almost linear increase in carrier density with magnetic field. This behaviour is strong evidence for field dependent charge transfer from charge reservoirs with exceptionally high densities of states in close proximity to the graphene. Using a realistic framework of broadened Landau levels we model the densities of donor states and predict the field dependence of charge transfer in excellent agreement with experimental results, thus providing a guide towards engineering epitaxial graphene for applications such as quantum metrology.
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| 8. |
- Eless, V, et al.
(författare)
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Phase coherence and energy relaxation in epitaxial graphene under microwave radiation
- 2013
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Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 103:9
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Tidskriftsartikel (refereegranskat)abstract
- We have performed low-temperature magnetotransport measurements on monolayer epitaxial graphene under microwave radiation and extracted the radiation-induced effective temperatures, energy relaxation, and the dephasing times. We established that the response of the graphene sample is entirely bolometric at least up to 170 GHz. Dynamic dephasing, i.e., the time-reversal symmetry breaking effect of the ac electromagnetic field rather than mediated by heating, may become significant in the terahertz frequency range and in samples with longer phase coherence time.
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| 9. |
- Huang, J., et al.
(författare)
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Hot carrier relaxation of Dirac fermions in bilayer epitaxial graphene
- 2015
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Ingår i: Journal of Physics Condensed Matter. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 27:16
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Tidskriftsartikel (refereegranskat)abstract
- Energy relaxation of hot Dirac fermions in bilayer epitaxial graphene is experimentally investigated by magnetotransport measurements on Shubnikov-de Haas oscillations and weak localization. The hot-electron energy loss rate is found to follow the predicted Bloch-Gruneisen power-law behaviour of T-4 at carrier temperatures from 1.4K up to similar to 100 K, due to electron-acoustic phonon interactions with a deformation potential coupling constant of 22 eV. A carrier density dependence n(e)(-1.5) in the scaling of the T-4 power law is observed in bilayer graphene, in contrast to the n(e)(-0.5) dependence in monolayer graphene, leading to a crossover in the energy loss rate as a function of carrier density between these two systems. The electron-phonon relaxation time in bilayer graphene is also shown to be strongly carrier density dependent, while it remains constant for a wide range of carrier densities in monolayer graphene. Our results and comparisons between the bilayer and monolayer exhibit a more comprehensive picture of hot carrier dynamics in graphene systems.
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| 10. |
- Huang, J., et al.
(författare)
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Physics of a disordered Dirac point in epitaxial graphene from temperature-dependent magnetotransport measurements
- 2015
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - : American Physical Society. - 2469-9950 .- 2469-9969 .- 1098-0121 .- 1550-235X. ; 92:7
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Tidskriftsartikel (refereegranskat)abstract
- We report a study of disorder effects on epitaxial graphene in the vicinity of the Dirac point by magnetotransport. Hall effect measurements show that the carrier density increases quadratically with temperature, in good agreement with theoretical predictions which take into account intrinsic thermal excitation combined with electron-hole puddles induced by charged impurities. We deduce disorder strengths in the range 10.2-31.2 meV, depending on the sample treatment. We investigate the scattering mechanisms and estimate the impurity density to be 3.0-9.1x10(10) cm(-2) for our samples. A scattering asymmetry for electrons and holes is observed and is consistent with theoretical calculations for graphene on SiC substrates. We also show that the minimum conductivity increases with increasing disorder strength, in good agreement with quantum-mechanical numerical calculations.
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| 11. |
- Yager, Thomas, 1987
(författare)
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Epitaxial Graphene Technology for Quantum Metrology
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Graphene grown on silicon carbide by high-temperature annealing (SiC/G) is a strong contender in the race towards large-scale graphene electronics applications. The unique electronic properties of this system lead to a remarkably robust and accurate Hall resistance quantisation of 0.1 parts per billion, making SiC/G devices highly desirable for the endeavour of quantum resistance metrology. However, major challenges for this technology remain, such as control of the charge carrier density and reproducibility of material growth and device properties.The main aims of the work presented in this Thesis are to understand and improve the performance and reproducibility of SiC/G devices for quantum Hall resistance metrology. Moreover, in this work we developed further understanding of material issues such as the homogeneity of SiC/G for applications in quantum metrology, and ultimately wafer scale electronics.By correlating the nanoscopic growth features of SiC/G with electron transport measurements, we were able to reveal the wafer scale electronic homogeneity of monolayer graphene grown on SiC. We developed disparate gating methods to enable quantum Hall effect at lower magnetic fields (B = 2-5 T) and to study electron transport close to charge neutrality, useful for metrology. We investigated the role of the reconstructed SiC morphology and bilayer graphene inclusions on the reproducibility of nominally monolayer graphene quantum Hall devices, and reveal the important consequences for quantum resistance metrology.The above-mentioned studies were assisted by the development of optical microscopy methods to accurately identify single and multilayer domains of epitaxial graphene in addition to the nanoscopic details of the reconstructed SiC surface. This was found to be an important step of rapid and non-invasive quality control leading to improved device performance and reproducibility for quantum metrology applications.While the focus has been placed on quantum metrology devices, the findings and technologies developed in this Thesis work are readily applicable to investigate other two-dimensional materials and graphene systems, in both research and industrial environments.
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