| 1. |
- Alexander-Webber, J. A., et al.
(författare)
-
Phase Space for the Breakdown of the Quantum Hall Effect in Epitaxial Graphene
- 2013
-
Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 111:9, s. e096601-
-
Tidskriftsartikel (refereegranskat)abstract
- We report the phase space defined by the quantum Hall effect breakdown in polymer gated epitaxial graphene on SiC (SiC/G) as a function of temperature, current, carrier density, and magnetic fields up to 30 T. At 2 K, breakdown currents (Ic) almost 2 orders of magnitude greater than in GaAs devices are observed. The phase boundary of the dissipationless state (ρxx=0) shows a [1-(T/Tc)2] dependence and persists up to Tc>45 K at 29 T. With magnetic field Ic was found to increase ∝B3/2 and Tc∝B2. As the Fermi energy approaches the Dirac point, the ν=2 quantized Hall plateau appears continuously from fields as low as 1 T up to at least 19 T due to a strong magnetic field dependence of the carrier density.
|
|
| 2. |
- Baker, A M R, et al.
(författare)
-
Energy loss rates of hot Dirac fermions in epitaxial, exfoliated, and CVD graphene
- 2013
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X .- 2469-9950 .- 2469-9969. ; 87:4, s. 045414-
-
Tidskriftsartikel (refereegranskat)abstract
- Energy loss rates for hot carriers in graphene have been measured using graphene produced by epitaxial growth on SiC, exfoliation, and chemical vapor deposition (CVD). It is shown that the temperature dependence of the energy loss rates measured with high-field damped Shubnikov-de Haas oscillations and the temperature dependence of the weak localization peak close to zero field correlate well, with the high-field measurements understating the energy loss rates by similar to 40% compared to the low-field results. The energy loss rates for all graphene samples follow a universal scaling of T-e(4) at low temperatures and depend weakly on carrier density proportional to n(-1/2), evidence for enhancement of the energy loss rate due to disorder in CVD samples.
|
|
| 3. |
- Baker, A M R, et al.
(författare)
-
Weak localization scattering lengths in epitaxial, and CVD graphene
- 2012
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X .- 2469-9950 .- 2469-9969. ; 86:23, s. 235441-
-
Tidskriftsartikel (refereegranskat)abstract
- Weak localization in graphene is studied as a function of carrier density in the range from 1 x 10(11) cm(-2) to 1.43 x 10(13) cm(-2) using devices produced by epitaxial growth onto SiC and CVD growth on thin metal film. The magnetic field dependent weak localization is found to be well fitted by theory, which is then used to analyze the dependence of the scattering lengths L-phi, L-i, and L-* on carrier density. We find no significant carrier dependence for L-phi, a weak decrease for L-i with increasing carrier density just beyond a large standard error, and a n(-1/4) dependence for L-*. We demonstrate that currents as low as 0.01 nA are required in smaller devices to avoid hot-electron artifacts in measurements of the quantum corrections to conductivity. DOI: 10.1103/PhysRevB.86.235441
|
|
| 4. |
- Beshkova, Milena, et al.
(författare)
-
Properties of 3C-SiC Grown by Sublimation Epitaxy on Different Type of Substrates
- 2010
-
Ingår i: Materials Science Forum, Vols. 645-648. - : Transtec Publications; 1999. ; , s. 183-186
-
Konferensbidrag (refereegranskat)abstract
- 3C-SiC layers have been grown by using sublimation epitaxy at a temperature of 2000 degrees C, on different types of on-axis 6H-SiC(0001) substrates. The influence of the type of substrate on the morphology of the layers investigated by Atomic Force Microscopy (AFM) is discussed. Stacking faults are studied by reciprocal space map (RSM) which shows that double positions domains exists.
|
|
| 5. |
- Beshkova, Milena, et al.
(författare)
-
Sublimation epitaxy of 3C-SiC grown at Si- and C-rich conditions
- 2012
-
Ingår i: Vacuum. - : Elsevier. - 0042-207X .- 1879-2715. ; 86:10, s. 1595-1599
-
Tidskriftsartikel (refereegranskat)abstract
- 3C-SiC layers have been grown by using sublimation epitaxy at a source temperature of 2000 degrees C, under vacuum conditions (andlt;10(-5) mbar) on well oriented (on-axis) 6H-SiC (0001) substrates. Close space sublimation growth geometry has been used in a RF-heated furnace employing high-purity graphite crucible with a possibility to change the growth environment from Si vapor-rich to C vapor-rich. The optical microscopy in transmission mode reveals continuous 3C-domains for 3C-SiC with less than 0.4% 6H-inclusions for the layer grown at Si-rich conditions, and separate 3C-SiC domains for the layer grown at C-rich conditions. The type of 6H-inclusions for layers with continuous domain structure investigated by Atomic Force Microscopy (AFM) is discussed. 2Theta-omega scan shows 0006 and 111 peaks coming from the substrate and the layer, respectively with a higher intensity of the 111 peak for 3C-SiC grown at Si-rich conditions which is related with the continuous character of the 3C-SiC domains.
|
|
| 6. |
- Boosalis, A., et al.
(författare)
-
Visible to vacuum ultraviolet dielectric functions of epitaxial graphene on 3C and 4H SiC polytypes determined by spectroscopic ellipsometry
- 2012
-
Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 101:1
-
Tidskriftsartikel (refereegranskat)abstract
- Spectroscopic ellipsometry measurements in the visible to vacuum-ultraviolet spectra (3.5-9.5 eV) are performed to determine the dielectric function of epitaxial graphene on SiC polytypes, including 4H (C-face and Si-face) and 3C SiC (Si-face). The model dielectric function of graphene is composed of two harmonic oscillators and allows the determination of graphene quality, morphology, and strain. A characteristic van Hove singularity at 4.5 eV is present in the dielectric function of all samples, in agreement with observations on exfoliated as well as chemical vapor deposited graphene in the visible range. Model dielectric function analysis suggests that none of our graphene layers experience a significant degree of strain. Graphene grown on the Si-face of 4H SiC exhibits a dielectric function most similar to theoretical predictions for graphene. The carbon buffer layer common for graphene on Si-faces is found to increase polarizability of graphene in the investigated spectrum.
|
|
| 7. |
- Burnett, Tim L, et al.
(författare)
-
Identification of epitaxial graphene domains and adsorbed species in ambient conditions using quantified topography measurements
- 2012
-
Ingår i: Journal of Applied Physics. - : American Institute of Physics (AIP). - 0021-8979 .- 1089-7550. ; 112:5, s. 054308-
-
Tidskriftsartikel (refereegranskat)abstract
- We discuss general limitations of topographical studies of epitaxial graphene in ambient conditions, in particular, when an accurate determination of the layers thickness is required. We demonstrate that the histogram method is the most accurate for measurements of small vertical distances (andlt;0.5 nm) and generally should be applied to epitaxial graphene and similar types of samples in order to get the correct and reproducible values. Experimental determination of the step height between different domains of epitaxial graphene shows excellent agreement with the predicted values once the adsorption of a 2D monolayer is taken into account on top of the one layer graphene. In contrast to general limitations of AFM topography, electrostatic force microscopy imaging allows a straightforward identification of domains of epitaxial graphene of different thickness.
|
|
| 8. |
- Burnett, Tim, et al.
(författare)
-
Mapping of Local Electrical Properties in Epitaxial Graphene Using Electrostatic Force Microscopy
- 2011
-
Ingår i: NANO LETTERS. - : American Chemical Society. - 1530-6984 .- 1530-6992. ; 11:6, s. 2324-2328
-
Tidskriftsartikel (refereegranskat)abstract
- Local electrical characterization of epitaxial graphene grown on 4H-SiC (0001) using electrostatic force microscopy (EFM) in ambient conditions and at elevated temperatures is presented. EFM provides a straightforward identification of graphene with different numbers of layers on the substrate where topographical determination is hindered by adsorbates. Novel EFM spectroscopy has been developed measuring the EFM phase as a function of the electrical DC bias, establishing a rigorous way to distinguish graphene domains and facilitating optimization of EFM imaging.
|
|
| 9. |
- Darakchieva, Vanya, et al.
(författare)
-
Large-area microfocal spectroscopic ellipsometry mapping of thickness and electronic properties of epitaxial graphene on Si- and C-face of 3C-SiC(111)
- 2013
-
Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 102:21, s. 213116-
-
Tidskriftsartikel (refereegranskat)abstract
- Microfocal spectroscopic ellipsometry mapping of the electronic properties and thickness of epitaxial graphene grown by high-temperature sublimation on 3C-SiC (111) substrates is 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 ∼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(111) differs and has a determining role for the thickness and electronic properties homogeneity of the epitaxial graphene.
|
|
| 10. |
- Drexler, C, et al.
(författare)
-
Magnetic quantum ratchet effect in graphene
- 2013
-
Ingår i: Nature Nanotechnology. - : Nature Publishing Group. - 1748-3387 .- 1748-3395. ; 8:2, s. 104-107
-
Tidskriftsartikel (refereegranskat)abstract
- A periodically driven system with spatial asymmetry can exhibit a directed motion facilitated by thermal or quantum fluctuations(1). This so-called ratchet effect(2) has fascinating ramifications in engineering and natural sciences(3-18). Graphene(19) is nominally a symmetric system. Driven by a periodic electric field, no directed electric current should flow. However, if the graphene has lost its spatial symmetry due to its substrate or adatoms, an electronic ratchet motion can arise. We report an experimental demonstration of such an electronic ratchet in graphene layers, proving the underlying spatial asymmetry. The orbital asymmetry of the Dirac fermions is induced by an in-plane magnetic field, whereas the periodic driving comes from terahertz radiation. The resulting magnetic quantum ratchet transforms the a.c. power into a d.c. current, extracting work from the out-of-equilibrium electrons driven by undirected periodic forces. The observation of ratchet transport in this purest possible two-dimensional system indicates that the orbital effects may appear and be substantial in other two-dimensional crystals such as boron nitride, molybdenum dichalcogenides and related heterostructures. The measurable orbital effects in the presence of an in-plane magnetic field provide strong evidence for the existence of structure inversion asymmetry in graphene.
|
|
