Session V37: Focus Session: Graphene Growth, Characterization, and Devices: Transport

Electronic Transport Properties of Graphene on Aluminum Nitride

We have fabricated graphene field-effect transistors on aluminum nitride (AlN) gate dielectric over silicon back gates. AlN thin films are prepared on Si by pulsed laser deposition, and exfoliated graphene on SiO2 is transferred onto the AlN/Si surface by using thermal tape as a transfer medium. After transfer, Raman spectra and AFM measurement have been performed to confirm the quality of graphene on AlN. Electron transport measurements will be reported.   

Exploring Transport Effects in Nanoscale Graphene Devices

Graphene, the single- to few-atomic layers cousin to graphite, has become a very interesting topic of research owing to its unique mechanical, optical, thermal and electrical properties. Many of the properties of graphene can be traced to its structural uniformity, allowing both electrons and holes to travel long distances (up to several microns) before scattering. However, studying graphene on the micron level can mask its true nanoscale behavior. Using very short length scales allows for the investigation of the behavior of charge impurities, contact effects and ballistic transport. In this work, we fabricate sub-30 nanometer suspended graphene 3-terminal devices on gold and platinum electrodes. We present data from electrical measurements on charge impurities that are apparent at this length scale and the effect of electrode work function on contact resistance. We compare this to mechanically exfoliated graphene on a silicon/SiO2 substrate with gold electrodes.   

Electrical transport study of suspended graphene nanoribbons near the Dirac point

We have fabricated graphene nanoribbon Field-effect transistors from high-quality graphene nanoribbons produced by sonicating multiwall carbon nanotubes in an organic solvent. To minimize the influence of the underlying substrate, individual nanoribbons in the devices were suspended by removing the underneath silicon oxide using a wet etching method. Subsequently, in situ current annealing was carried out in high vacuum to further reduce the impurities adsorbed to the ribbon surfaces. The electrical transport properties of the devices were measured for a wide range of temperatures, revealing a range of unusual phenomena pertinent to the competing effects of improved overall charge homogeneity and reduced charge puddle sizes when the graphene nanoribbons are tuned close to the Dirac point. The electrical transport results on suspended graphene nanoribbon with varying disorder will be presented and discussed.   

Effects of disorder on the transport properties of chemically derived graphene

Transport properties of chemically derived graphene (CDG) are strongly influenced by the concentration of defects that are introduced during synthesis. We present a comprehensive transport study on a range of CDG films with varying degrees of disorder. The electric properties of CDG were found to be tunable over several orders of magnitude via controlled oxidation and reduction. The structural properties of CDG were monitored by analyzing the defect-related features in the Raman spectra and correlated with transport. The temperature dependence of the resistivity of these samples indicate that the conduction mechanism evolves from tunneling to hopping for strongly disordered samples and to activated transport for weakly disordered samples. Strong disorder causes localization of carriers and field-dependent modulation of hopping conduction. We discuss the temperature- and gate-bias-dependence of the resistivity of weakly disordered samples in terms of scattering dominated by midgap states, as is the case in ion irradiated graphene [2]. \\[4pt] [1] G. Eda et al., J. Phys. Chem. C, 113, 15768 (2009). \\[0pt] [2] J.-H. Chen et al. Phys. Rev. Lett. 102, 236805 (2009)   

Influence of Strain on Quantum Transport in Graphene

Strain is unavoidable in graphene, either suspended or supported on a dielectric substrate such as SiO2. It is therefore crucial to identify the role of strain on transport properties in graphene. Experimentally, it is shown that local strain in graphene on a SiO2 substrate can modify graphene's conductance near the Fermi energy. The modification is attributed to the coupling of strain and phonon-mediated inelastic tunneling effects. However, conductance on the dielectric substrate is not ballistic and isolating the influence of strain is a difficult task. In this study, strain effects on ballistic conductance, an experimentally attainable transport property for suspended graphene, is studied using a combination of density functional theory and the Landauer-Buttiker formalism. It is found that, unlike in a CNT, regardless of the applied strain graphene's conductance at the Fermi energy is 0.21G0. Furthermore, for conducting electrons with energies higher or lower than the Fermi energy of the system, tensile hydrostatic strain is found to increase conductance but compressive hydrostatic strain decreases conductance. For an 8{\%} compressive hydrostatic strain, conductance increases by as large as 30{\%}. Surprisingly, for uni-axial strain, if the energy of the conducting electrons is higher than the Fermi energy, conductance remains approximately unchanged, whereas conductance by electrons less than the Fermi energy decreases (increases) with compressive (tensile) strain along the transport direction.   

Thermal and Electronic Transport Properties of Graphene Nanoribbons with Defects

The interplay between graphene nanoribbon (GNR) structure and conductivity, both thermal and electrical, is probed with molecular dynamics and tight binding models. A variety of randomly oriented defects, vacancies and Stone-Wales, as well as edge terminations, zig-zag, armchair, and roughened, are studied in experimental sized systems ($>$100 nm long and $>$15 nm wide). It is found that GNR thermal conductivity responds similarly to edge roughness and moderate defect concentrations (0.0023) with a drastic reduction (81\%) in lattice thermal conductivity, compared to pristine GNR value. Conversely, the presence of randomly oriented defects completely erodes the ballistic nature of the electrons, reducing conductance by two orders of magnitude, while edge roughened structures leave the electrical conductance intact.   

First principles study of transport properties of pristine and passivated bilayer graphene nanoribbons

Transport properties of pristine and hydrogen passivated bilayers of zigzag-edged graphene nanoribbons (ZGNRs) coupled with gold electrodes are investigated using first-principles methods based on density-functional theory. The calculated ground state of the passivated bilayer 6-ZGNRs is non-magnetic and the antiferromagnetic coupling is energetically preferred for the pristine counterpart. The results of the bias and spin-dependent electron transmission and current calculated using the nonequilibrium Green's function formalism will be presented. The role of interlayer interaction in determining the I-V characteristics of bilayer graphene nanoribbons will also be discussed.   

1/f noise as a probe to investigate the band structure of graphene

The flicker noise or low frequency resistance fluctuations in graphene depend explicitly on its ability to screen external potential fluctuations and more sensitive compared to the conventional time average transport. Here we show that the flicker noise is a powerful probe to the band structure of graphene that vary differently with the carrier density for the linear and parabolic bands. We have used different types of graphene field effect devices in our experiments which include exfoliated single and multilayer graphene on oxide substrate, freely suspended single layer graphene, and chemical vapor deposition (CVD)-grown graphene on SiO$_{2. }$We find this difference to be robust against disorder or existence of a substrate. Also, an analytical model has been developed to understand the mechanism of graphene field effect transistors. Our results reveal the microscopic mechanism of noise in Graphene Field Effect Transistors (GraFET), and outline a simple portable method to separate the single from multi layered graphene devices. References A. N. Pal and A Ghosh, Phys Rev. Lett~102, 126805 (2009). A. N. Pal and A.~Ghosh, Appl. Phys. Lett$.,$ 95, 082105 (2009). A. N. Pal, A. A. Bol, and A. Ghosh, Appl. Phys. Lett$.$ 97, 133504 (2010). A. N. Pal et al., arXiv: 1009.5832v2.   

Charge Trapping and Transport in Epitaxial Graphene

A thorough characterization of the electronic transport behavior of charge carriers in graphene that is epitaxially grown on the silicon face of 6H(0001) SiC is presented. A nonlinear temperature dependence of the carrier density is observed, and is attributed to the presence of charge traps in the material. Observation of this trapping effect has been previously unidentified, and gives critical information about the material properties of epitaxially grown graphene. The nature of the electrostatic screening associated with these traps is evaluated using zero screening, full screening, and RPA screening approximations, and it is found that the zero screening approximation best describes the measurements. Electrostatic homogeneity of this material allows for exceptionally low carrier densities to be attained, where the carrier mobility sharply increases. The entire mobility profile can be phenomenologically simulated assuming Coulomb and short-range scattering as the dominant scattering mechanisms at low temperatures. Based on this result, the temperature independent residual impurity concentration of this material can be directly extrapolated.   

Transport Through Andreev Bound States in a Graphene Quantum Dot

We have performed transport measurements on a graphene-insulator-superconductor junction, and report the direct observation of sharp, gate-tunable Andreev bound states (ABS) in a graphene quantum dot (QD)[1]. The quantum dot is formed underneath the superconducting lead by local gating due to a work-function mismatch. We show that the ABS form when the discrete QD levels are proximity coupled to the superconducting contact. We find subgap resonant features which are remarkably narrow, can be tuned to zero energy by gating, and show a striking pattern as a function of applied bias and gate voltage. \\[4pt] [1] T. Dirks et al.,arXiv:1005.2749 (2010)   

Transport Properties of Graphene on Strontium Titanate

We have investigated transport properties of mechanically exfoliated graphene on strontium titanate(STO) in ultra high vacuum at cryogenic temperatures. Field- and temperature-dependent dielectric constant of STO is used to measure the impact of substrate-induced screening on transport properties of graphene.