| 1. |
- Armakavicius, Nerijus, et al.
(författare)
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Cavity-enhanced optical Hall effect in epitaxial graphene detected at terahertz frequencies
- 2017
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Ingår i: Applied Surface Science. - : Elsevier. - 0169-4332 .- 1873-5584. ; 421, s. 357-360
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Tidskriftsartikel (refereegranskat)abstract
- Cavity-enhanced optical Hall effect at terahertz (THz) frequencies is employed to determine the free charge carrier properties in epitaxial graphene (EG) with different number of layers grown by high-temperature sublimation on 4H-SiC(0001). We find that one monolayer (ML) EG possesses p-type conductivity with a free hole concentration in the low 1012 cmￜￜￜ2 range and a free hole mobility parameter as high as 1550 cm2/Vs. We also find that 6 ML EG shows n-type doping behavior with a much lower free electron mobility parameter of 470 cm2/Vs and an order of magnitude higher free electron density in the low 1013 cmￜￜￜ2 range. The observed differences are discussed. The cavity-enhanced THz optical Hall effect is demonstrated to be an excellent tool for contactless access to the type of free charge carriers and their properties in two-dimensional materials such as EG.
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| 2. |
- Armakavicius, Nerijus, et al.
(författare)
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Resolving mobility anisotropy in quasi-free-standing epitaxial graphene by terahertz optical Hall effect
- 2021
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Ingår i: Carbon. - : Elsevier BV. - 0008-6223 .- 1873-3891. ; 172, s. 248-259
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Tidskriftsartikel (refereegranskat)abstract
- In this work, we demonstrate the application of terahertz-optical Hall effect (THz-OHE) to determine directionally dependent free charge carrier properties of ambient-doped monolayer and quasi-free-standing-bilayer epitaxial graphene on 4H–SiC(0001). Directionally independent free hole mobility parameters are found for the monolayer graphene. In contrast, anisotropic hole mobility parameters with a lower mobility in direction perpendicular to the SiC surface steps and higher along the steps in quasi-free-standing-bilayer graphene are determined for the first time. A combination of THz-OHE, nanoscale microscopy and optical spectroscopy techniques are used to investigate the origin of the anisotropy. Different defect densities and different number of graphene layers on the step edges and terraces are ruled out as possible causes. Scattering mechanisms related to doping variations at the step edges and terraces as a result of different interaction with the substrate and environment are discussed and also excluded. It is suggested that the step edges introduce intrinsic scattering in quasi-free-standing-bilayer graphene, that is manifested as a result of the higher ratio between mean free path and average terrace width parameters. The suggested scenario allows to reconcile existing differences in the literature regarding the anisotropic electrical transport in epitaxial graphene.
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| 3. |
- Bouhafs, Chamseddine, et al.
(författare)
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Decoupling and ordering of multilayer graphene on C-face 3C-SiC(111)
- 2016
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Ingår i: Applied Physics Letters. - : AMER INST PHYSICS. - 0003-6951 .- 1077-3118. ; 109:20
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Tidskriftsartikel (refereegranskat)abstract
- We show experimentally that few layer graphene (FLG) grown on the carbon terminated surface (C-face) of 3C-SiC(111) is composed of decoupled graphene sheets. Landau level spectroscopy on FLG graphene is performed using the infrared optical Hall effect. We find that Landau level transitions in the FLG exhibit polarization preserving selection rules and the transition energies obey a square-root dependence on the magnetic field strength. These results show that FLG on C-face 3C-SiC(111) behave effectively as a single layer graphene with linearly dispersing bands (Dirac cones) at the graphene K point. We estimate from the Landau level spectroscopy an upper limit of the Fermi energy of about 60 meV in the FLG, which corresponds to a carrier density below 2.5 x 10(11) cm(-2). Low-energy electron diffraction mu-LEED) reveals the presence of azimuthally rotated graphene domains with a typical size of amp;lt;= 200 nm.mu-LEED mapping suggests that the azimuth rotation occurs between adjacent domains within the same sheet rather than vertically in the stack. Published by AIP Publishing.
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| 4. |
- Bouhafs, Chamseddine, et al.
(författare)
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Multi-scale investigation of interface properties, stacking order and decoupling of few layer graphene on C-face 4H-SiC
- 2017
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Ingår i: Carbon. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0008-6223 .- 1873-3891. ; 116, s. 722-732
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Tidskriftsartikel (refereegranskat)abstract
- In this work, we report a multi-scale investigation using several nano-, micro and macro-scale techniques of few layer graphene (FLG) sample consisting of large monolayer (ML) and bilayer (BL) areas grown on C-face 4H-SiC (000-1) by high-temperature sublimation. Single 1 x 1 diffraction patterns are observed by micro-low-energy electron diffraction for ML, BL and trilayer graphene with no indication of out-of-plane rotational disorder. A SiOx layer is identified between graphene and SiC by X-ray photoelectron emission spectroscopy and reflectance measurements. The chemical composition of the interface layer changes towards SiO2 and its thickness increases with aging in normal ambient conditions. The formation mechanism of the interface layer is discussed. It is shown by torsion resonance conductive atomic force microscopy that the interface layer causes the formation of non-ideal Schottky contact between ML graphene and SiC. This is attributed to the presence of a large density of interface states. Mid-infrared optical Hall effect measurements revealed Landau-level transitions in FLG that have a square-root dependence on magnetic field, which evidences a stack of decoupled graphene sheets. Contrary to previous works on decoupled C-face graphene, our BL and FLG are composed of ordered decoupled graphene layers without out-of-plane rotation. (C) 2017 Elsevier Ltd. All rights reserved.
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| 5. |
- Bouhafs, Chamseddine, 1984-
(författare)
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Structural and Electronic Properties of Graphene on 4H- and 3C-SiC
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Graphene is a one-atom-tick carbon layer arranged in a honeycomb lattice. Graphene was first experimentally demonstrated by Andre Geim and Konstantin Novoselov in 2004 using mechanical exfoliation of highly oriented pyrolytic graphite (exfoliated graphene flakes), for which they received the Nobel Prize in Physics in 2010. Exfoliated graphene flakes show outstanding electronic properties, e.g., very high free charge carrier mobility parameters and ballistic transport at room temperature. This makes graphene a suitable material for next generation radio-frequency and terahertz electronic devices. Such applications require fabrication methods of large-area graphene compatible with electronic industry. Graphene grown by sublimation on silicon carbide (SiC) offers a viable route towards production of large-area, electronic-grade material on semi-insulating substrate without the need of transfer. Despite the intense investigations in the field, uniform wafer-scale graphene with very high-quality that matches the properties of exfoliated graphene has not been achieved yet. The key point is to identify and control how the substrate affects graphene uniformity, thickness, layer stacking, structural and electronic properties. Of particular interest is to understand the effects of SiC surface polarity and polytype on graphene properties in order to achieve large-area material with tailored properties for electronic applications. The main objectives of this thesis are to address these issues by investigating the structural and electronic properties of epitaxial graphene grown on 4HSiC and 3C-SiC substrates with different surface polarities. The first part of the thesis includes a general description of the properties of graphene, bilayer graphene and graphite. Then, the properties of epitaxial graphene on SiC by sublimation are detailed. The experimental techniques used to characterize graphene are described. A summary of all papers and contribution to the field is presented at the end of Part I. Part II consists of seven papers.
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| 6. |
- Bouhafs, Chamseddine, et al.
(författare)
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Structural properties and dielectric function of graphene grown by high-temperature sublimation on 4H-SiC(000-1)
- 2015
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Ingår i: Journal of Applied Physics. - : American Institute of Physics (AIP). - 0021-8979 .- 1089-7550. ; 117:8, s. 085701-
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Tidskriftsartikel (refereegranskat)abstract
- Understanding and controlling growth of graphene on the carbon face (C-face) of SiC presents a significant challenge. In this work, we study the structural, vibrational, and dielectric function properties of graphene grown on the C-face of 4H-SiC by high-temperature sublimation in an argon atmosphere. The effect of growth temperature on the graphene number of layers and crystallite size is investigated and discussed in relation to graphene coverage and thickness homogeneity. An amorphous carbon layer at the interface between SiC and the graphene is identified, and its evolution with growth temperature is established. Atomic force microscopy, micro-Raman scattering spectroscopy, spectroscopic ellipsometry, and high-resolution cross-sectional transmission electron microscopy are combined to determine and correlate thickness, stacking order, dielectric function, and interface properties of graphene. The role of surface defects and growth temperature on the graphene growth mechanism and stacking is discussed, and a conclusion about the critical factors to achieve decoupled graphene layers is drawn. (C) 2015 AIP Publishing LLC.
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| 7. |
- Bouhafs, Chamseddine, et al.
(författare)
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Synthesis of large-area rhombohedral few-layer graphene by chemical vapor deposition on copper
- 2021
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Ingår i: Carbon. - : Elsevier BV. - 0008-6223. ; 177, s. 282-290
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Tidskriftsartikel (refereegranskat)abstract
- Rhombohedral-stacked few-layer graphene (FLG) displays peculiar electronic properties that could lead to phenomena such as high-temperature superconductivity and magnetic ordering. To date, experimental studies have been mainly limited by the difficulty in isolating rhombohedral FLG with thickness exceeding 3 layers and device-compatible size. In this work, we demonstrate the synthesis and transfer of rhombohedral graphene with thickness up to 9 layers and areas up to ∼50 μm2. The domains of rhombohedral FLG are identified by Raman spectroscopy and are found to alternate with Bernal regions within the same crystal in a stripe-like configuration. Near-field nano-imaging further confirms the structural integrity of the respective stacking orders. Combined spectroscopic and microscopic analyses indicate that rhombohedral-stacking formation is strongly correlated to the underlying copper step-bunching and emerges as a consequence of interlayer displacement along preferential crystallographic orientations. The growth and transfer of rhombohedral FLG with the reported thickness and size shall facilitate the observation of predicted unconventional physics and ultimately add to its technological relevance.
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| 8. |
- Knight, Sean, et al.
(författare)
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In-situ terahertz optical Hall effect measurements of ambient effects on free charge carrier properties of epitaxial graphene
- 2017
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Ingår i: Scientific Reports. - : NATURE PUBLISHING GROUP. - 2045-2322. ; 7
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Tidskriftsartikel (refereegranskat)abstract
- Unraveling the doping-related charge carrier scattering mechanisms in two-dimensional materials such as graphene is vital for limiting parasitic electrical conductivity losses in future electronic applications. While electric field doping is well understood, assessment of mobility and density as a function of chemical doping remained a challenge thus far. In this work, we investigate the effects of cyclically exposing epitaxial graphene to controlled inert gases and ambient humidity conditions, while measuring the Lorentz force-induced birefringence in graphene at Terahertz frequencies in magnetic fields. This technique, previously identified as the optical analogue of the electrical Hall effect, permits here measurement of charge carrier type, density, and mobility in epitaxial graphene on silicon-face silicon carbide. We observe a distinct, nearly linear relationship between mobility and electron charge density, similar to field-effect induced changes measured in electrical Hall bar devices previously. The observed doping process is completely reversible and independent of the type of inert gas exposure.
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| 9. |
- Persson, Ingemar, et al.
(författare)
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Origin of layer decoupling in ordered multilayer graphene grown by high-temperature sublimation on C-face 4H-SiC
- 2020
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Ingår i: APL Materials. - : AMER INST PHYSICS. - 2166-532X. ; 8:1
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Tidskriftsartikel (refereegranskat)abstract
- We study the origin of layer decoupling in ordered multilayer graphene grown by high temperature sublimation on C-face 4H-SiC. The mid-infrared optical Hall effect technique is used to determine the magnetic field dependence of the inter-Landau level transition energies and their optical polarization selection rules, which unambiguously show that the multilayer graphene consists of electronically decoupled layers. Transmission electron microscopy reveals no out-of-plane rotational disorder between layers in the stack, which is in contrast to what is typically observed for C-face graphene grown by low temperature sublimation. It is found that the multilayer graphene maintains AB-stacking order with increased interlayer spacing by 2.4%-8.4% as compared to highly oriented pyrolytic graphite. Electron energy loss spectroscopy mapping reveals Si atoms trapped in between layers, which are proposed to be the cause for the observed increased interlayer spacing leading to layer decoupling. Based on our results, we propose a defect-driven growth evolution mechanism for multilayer graphene on C-face SiC via high temperature sublimation.
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| 10. |
- Stanishev, Vallery, et al.
(författare)
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Critical view on buffer layer formation and monolayer graphene properties in high-temperature sublimation
- 2021
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Ingår i: Applied Sciences (Switzerland). - : MDPI AG. - 2076-3417. ; 11:4, s. 1-16
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Tidskriftsartikel (refereegranskat)abstract
- In this work we have critically reviewed the processes in high-temperature sublimation growth of graphene in Ar atmosphere using closed graphite crucible. Special focus is put on buffer layer formation and free charge carrier properties of monolayer graphene and quasi-freestanding monolayer graphene on 4H–SiC. We show that by introducing Ar at higher temperatures, TAr, one can shift the formation of the buffer layer to higher temperatures for both n-type and semi-insulating substrates. A scenario explaining the observed suppressed formation of buffer layer at higher TAr is proposed and discussed. Increased TAr is also shown to reduce the sp3 hybridization content and defect densities in the buffer layer on n-type conductive substrates. Growth on semi-insulating substrates results in ordered buffer layer with significantly improved structural properties, for which TAr plays only a minor role. The free charge density and mobility parameters of monolayer graphene and quasi-freestanding monolayer graphene with different TAr and different environmental treatment conditions are determined by contactless terahertz optical Hall effect. An efficient annealing of donors on and near the SiC surface is suggested to take place for intrinsic monolayer graphene grown at 2000◦C, and which is found to be independent of TAr . Higher TAr leads to higher free charge carrier mobility parameters in both intrinsically n-type and ambient p-type doped monolayer graphene. TAr is also found to have a profound effect on the free hole parameters of quasi-freestanding monolayer graphene. These findings are discussed in view of interface and buffer layer properties in order to construct a comprehensive picture of high-temperature sublimation growth and provide guidance for growth parameters optimization depending on the targeted graphene application.
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| 11. |
- Yakimova, Rositsa, et al.
(författare)
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Morphological and electronic properties of epitaxial graphene on SiC
- 2014
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Ingår i: Physica B: Condensed Matter. - : Elsevier BV. - 0921-4526 .- 1873-2135. ; 439, s. 54-59
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Tidskriftsartikel (refereegranskat)abstract
- We report on the structural and electronic properties of graphene grown on SiC by high-temperature sublimation. We have studied thickness uniformity of graphene grown on 4H-SiC (0 0 0 1), 6H-SiC (0 0 0 1), and 3C-SiC (1 1 1) substrates and investigated in detail graphene surface morphology and electronic properties. Differences in the thickness uniformity of the graphene layers on different SiC polytypes is related mainly to the minimization of the terrace surface energy during the step bunching process. It is also shown that a lower substrate surface roughness results in more uniform step bunching and consequently better quality of the grown graphene. We have compared the three SiC polytypes with a clear conclusion in favor of 3C-SiC. Localized lateral variations in the Fermi energy of graphene are mapped by scanning Kelvin probe microscopy It is found that the overall single-layer graphene coverage depends strongly on the surface terrace width, where a more homogeneous coverage is favored by wider terraces, It is observed that the step distance is a dominating, factor in determining the unintentional doping of graphene from the SiC substrate. Microfocal spectroscopic ellipsometry mapping of the electronic properties and thickness of epitaxial graphene on 3C-SiC (1 1 1) is also reported. Growth of one monolayer graphene is demonstrated on both Si- and C-polarity of the 3C-SiC substrates and it is shown that large area homogeneous single monolayer graphene can be achieved on the Si-face substrates. Correlations between the number of graphene monolayers on one hand and the main transition associated with an exciton enhanced van Hove singularity at similar to 4.5 eV and the free-charge carrier scattering time, on the other are established It is shown that the interface structure on the Si- and C-polarity of the 3C-SiC (1 1 1) differs and has a determining role for the thickness and electronic properties homogeneity of the epitaxial graphene. (C) 2014 Elsevier B.V. All rights reserved
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