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Walt A. de Heer

Publications and source records attributed to Walt A. de Heer.

31 records · Page 2Linked to original sources
arXiv2010

Epitaxial Graphenes on Silicon Carbide

The materials science of graphene grown epitaxially on the hexagonal basal planes of SiC crystals is reviewed. We show that the growth of epitaxial graphene on Si-terminated SiC(0001) is much different than growth on the C-terminated SiC(000 -1) surface, and discuss the physical structure of these graphenes. The unique electronic structure and transport properties of each type of epitaxial graphene is described, as well as progress toward the development of epitaxial graphene devices. This materials system is rich in subtleties, and graphene grown on the two polar faces differs in important ways, but all of the salient features of ideal graphene are found in these epitaxial graphenes, and wafer-scale fabrication of multi-GHz devices already has been achieved.

cond-mat.mtrl-sci↗
arXiv2010

Structural Determination of Multilayer Graphene via Atomic Moiré Interferometry

Rotational misalignment of two stacked honeycomb lattices produces a moiré pattern that is observable in scanning tunneling microscopy as a small modulation of the apparent surface height. This is known from experiments on highly-oriented pyrolytic graphite. Here, we observe the combined effect of three-layer moiré patterns in multilayer graphene grown on SiC ($000\bar{1}$). Small-angle rotations between the first and third layer are shown to produce a "double-moiré" pattern, resulting from the interference of moiré patterns from the first three layers. These patterns are strongly affected by relative lattice strain between the layers. We model the moiré patterns as a beat-period of the mismatched reciprocal lattice vectors and show how these patterns can be used to determine the relative strain between lattices, in analogy to strain measurement by optical moiré interferometry.

cond-mat.mtrl-sci↗
arXiv2009

Half integer quantum Hall effect in high mobility single layer epitaxial graphene

The quantum Hall effect, with a Berry's phase of $π$ is demonstrated here on a single graphene layer grown on the C-face of 4H silicon carbide. The mobility is $\sim$ 20,000 cm$^2$/V$\cdot$s at 4 K and ~15,000 cm$^2$/V$\cdot$s at 300 K despite contamination and substrate steps. This is comparable to the best exfoliated graphene flakes on SiO$_2$ and an order of magnitude larger than Si-face epitaxial graphene monolayers. These and other properties indicate that C-face epitaxial graphene is a viable platform for graphene-based electronics.

cond-mat.mes-hall↗
arXiv2009

Anomalous quantum Hall effect in epitaxial graphene

The observation of the anomalous quantum Hall effect in exfoliated graphene flakes triggered an explosion of interest in graphene. It was however not observed in high quality epitaxial graphene multilayers grown on silicon carbide substrates. The quantum Hall effect is shown on epitaxial graphene monolayers that were deliberately grown over substrate steps and subjected to harsh processing procedures, demonstrating the robustness of the epitaxial graphene monolayers and the immunity of their transport properties to temperature, contamination and substrate imperfections. The mobility of the monolayer C-face sample is 19,000 cm^2/Vs. This is an important step towards the realization of epitaxial graphene based electronics.

cond-mat.mes-hall↗
arXiv2009

Top and side gated epitaxial graphene field effect transistors

Three types of first generation epitaxial graphene field effect transistors (FET) are presented and their relative merits are discussed. Graphene is epitaxially grown on both the carbon and silicon faces of hexagonal silicon carbide and patterned with electron beam lithography. The channels have a Hall bar geometry to facilitate magnetoresistance measurements. FETs patterned on the Si-face exhibit off-to-on channel resistance ratios that exceed 30. C-face FETs have lower off-to-on resistance ratios, but their mobilities (up to 5000 cm2/Vs) are much larger than that for Si-face transistors. Initial investigations into all-graphene side gate FET structures are promising.

cond-mat.mtrl-sci↗
arXiv2008

The epitaxial-graphene/graphene-oxide junction, an essential step towards epitaxial graphene electronics

Graphene oxide (GO) flakes have been deposited to bridge the gap between two epitaxial graphene electrodes to produce all-graphene devices. Electrical measurements indicate the presence of Schottky barriers (SB) at the graphene/graphene oxide junctions, as a consequence of the band-gap in GO. The barrier height is found to be about 0.7 eV, and is reduced after annealing at 180 $^\circ$C, implying that the gap can be tuned by changing the degree of oxidation. A lower limit of the GO mobility was found to be 850 cm$^2$/Vs, rivaling silicon. {\it In situ} local oxidation of patterned epitaxial graphene has been achieved.

cond-mat.mes-hall↗
arXiv2008

Hot Dirac Fermions in Epitaxial Graphene

We investigate the ultrafast relaxation dynamics of hot Dirac fermionic quasiparticles in multilayer epitaxial graphene using ultrafast optical differential transmission spectroscopy. We observe DT spectra which are well described by interband transitions with no electron-hole interaction. Following the initial thermalization and emission of high-energy phonons, the electron cooling is determined by electron-acoustic phonon scattering, found to occur on the time scale of 1 ps for highly doped layers, and 4-11 ps in undoped layers. The spectra also provide strong evidence for the multilayer stucture and doping profile of thermally grown epitaxial graphene on SiC.

cond-mat.mtrl-sci↗
arXiv2007

Epitaxial graphene

Graphene multilayers are grown epitaxially on single crystal silicon carbide. This system is composed of several graphene layers of which the first layer is electron doped due to the built-in electric field and the other layers are essentially undoped. Unlike graphite the charge carriers show Dirac particle properties (i.e. an anomalous Berry's phase, weak anti-localization and square root field dependence of the Landau level energies). Epitaxial graphene shows quasi-ballistic transport and long coherence lengths; properties which may persists above cryogenic temperatures. Paradoxically, in contrast to exfoliated graphene, the quantum Hall effect is not observed in high mobility epitaxial graphene. It appears that the effect is suppressed due to absence of localized states in the bulk of the material.Epitaxial graphene can be patterned using standard lithography methods and characterized using a wide array of techniques. These favorable features indicate that interconnected room temperature ballistic devices may be feasible for low dissipation high-speed nanoelectronics.

cond-mat.mes-hall↗
arXiv2007

Weak antilocalization in epitaxial graphene: evidence for chiral electrons

Transport in ultrathin graphite grown on silicon carbide is dominated by the electron-doped epitaxial layer at the interface. Weak anti-localization in 2D samples manifests itself as a broad cusp-like depression in the longitudinal resistance for magnetic fields 10 mT$< B <$ 5 T. An extremely sharp weak-localization resistance peak at B=0 is also observed. These features quantitatively agree with graphene weak-(anti)localization theory implying the chiral electronic character of the samples. Scattering contributions from the trapped charges in the substrate and from trigonal warping due to the graphite layer on top are tentatively identified. The Shubnikov-de Haas oscillations are remarkably small and show an anomalous Berry's phase.

cond-mat.mes-hall↗
arXiv2006

Non-classical dipoles in cold niobium clusters

Electric deflections of niobium clusters in molecular beams show that they have permanent electric dipole moments at cryogenic temperatures but not higher temperatures, indicating that they are ferroelectric. Detailed analysis shows that the deflections cannot be explained in terms of a rotating classical dipole, as claimed by Anderson et al. The shapes of the deflected beam profiles and their field and temperature dependences indicates that the clusters can exist in two states, one with a dipole and the other without. Cluster with dipoles occupy lower energy states. Excitations from the lower states to the higher states can be induced by low fluence laser excitation. This causes the dipole to vanish.

cond-mat.str-el↗
arXiv2004

Ultrathin epitaxial graphite: 2D electron gas properties and a route toward graphene-based nanoelectronics

We have produced ultrathin epitaxial graphite films which show remarkable 2D electron gas (2DEG) behavior. The films, composed of typically 3 graphene sheets, were grown by thermal decomposition on the (0001) surface of 6H-SiC, and characterized by surface-science techniques. The low-temperature conductance spans a range of localization regimes according to the structural state (square resistance 1.5 kOhm to 225 kOhm at 4 K, with positive magnetoconductance). Low resistance samples show characteristics of weak-localization in two dimensions, from which we estimate elastic and inelastic mean free paths. At low field, the Hall resistance is linear up to 4.5 T, which is well-explained by n-type carriers of density 10^{12} cm^{-2} per graphene sheet. The most highly-ordered sample exhibits Shubnikov - de Haas oscillations which correspond to nonlinearities observed in the Hall resistance, indicating a potential new quantum Hall system. We show that the high-mobility films can be patterned via conventional lithographic techniques, and we demonstrate modulation of the film conductance using a top-gate electrode. These key elements suggest electronic device applications based on nano-patterned epitaxial graphene (NPEG), with the potential for large-scale integration.

cond-mat.mtrl-sci↗
arXiv2002

Room Temperature Ballistic Conduction in Carbon Nanotubes

Multiwalled carbon nanotubes are shown to be ballistic conductors at room temperature, with mean free paths of the order of tens of microns. These experiments follow and extend the original experiments by Frank et al (Science, 280 1744 1998) including in-situ electron microscopy experiments and a detailed analysis of the length dependence of the resistance. The per unit length resistance r < 100 Ohm/micron, indicating free paths l > 65 microns, unambiguously demonstrate ballistic conduction at room temperature up to macroscopic distances. The nanotube-metal contact resistances are in the range 0.1-1 kOhm micron. Contact scattering can explain why the measured conductances are about half the expected theoretical value of 2 G0 . For V>0.1V the conductance rises linearly (dG/dV~0.3 G0 /V) reflecting the linear increase in the density-of-states in a metallic nanotube above the energy gap. Increased resistances (r =2- 10 k Ohm/micron) and anomalous I-V dependences result from impurities and surfactants on the tubes.Evidence is presented that ballistic transport occurs in undoped and undamaged tubed for which the top layer is metallic and the next layer is semiconducting. The diffusive properties of lithographically contacted multiwalled nanotubes most likely result from purification and other processing steps that damage and dope the nanotubes thereby making them structurally and electronically different than the pristine nanotubes investigated here.

cond-mat.mtrl-sci↗