The linear dispersion relation in graphene gives rise to a surprising prediction: the resistivity due to isotropic scatterers, such as white-noise disorder or phonons, is independent of carrier density, n. Here we show that electron-acoustic phonon scattering is indeed independent of n, and contributes only 30 Omega to graphene's room-temperature resistivity. At a technologically relevant carrier density of 1 x1012 cm-2, we infer a mean free path for electron-acoustic phonon scattering of >2 microm and an intrinsic mobility limit of 2 x 105 cm2 V-1 s-1. If realized, this mobility would exceed that of InSb, the inorganic semiconductor with the highest known mobility ( approximately 7.7 x 104 cm2 V-1 s-1; ref. 9) and that of semiconducting carbon nanotubes ( approximately 1 x 105 cm2 V-1 s-1; ref. 10). A strongly temperature-dependent resistivity contribution is observed above approximately 200 K (ref. 8); its magnitude, temperature dependence and carrier-density dependence are consistent with extrinsic scattering by surface phonons at the SiO2 substrate and limit the room-temperature mobility to approximately 4 x 104 cm2 V-1 s-1, indicating the importance of substrate choice for graphene devices.

Esaki's discovery of NDR in heavily doped semiconducting germanium p-n junctions in 1958 was the first experimental evidence of quantum mechanical tunneling transport of electrons in all-condensedmatter systems 3,4 . This discovery motivated Giaever's tunneling experiments that proved the existence of the superconductive energy gap predicted by the then-newly formulated Bardeen-Cooper Schrieffer (BCS) theory of superconductivity 5 . After these initial breakthroughs, tunneling in various classes of crystalline matter has been observed, and forms the basis for several practical applications. For example,Josephson junctions exploit tunneling in superconductors for exquisitely sensitive magnetic flux detectors in superconducting quantum interference devices (SQUIDs) 6 , and are now being investigated as the building blocks of quantum computers 7 . Electron tunneling forms the basis for low-resistance ohmic contacts to heavily doped semiconductors for energy-efficient transistors, as low-loss cascade elements in multi-junction solar cells, and for coherent emission of long-wavelength photons in quantum-cascade lasers 8,9 . In addition to such practical applications, the extreme sensitivity of tunneling currents to various electronic, vibrational, and photonic excitations of solids makes tunneling spectroscopy one of the most sensitive probes for such phenomena 10 .Recently, interband tunneling in semiconductors has been proposed as the enabler for a new class of semiconductor transistors called tunnel field-effect transistors (TFETs) that promise very low-power operation. The heart of such devices is an Esaki tunnel diode, with preferably a near broken-gap band alignment at the source-channel heterojunction 11,12 . As these heterojunction TFETs are scaled down to the nanometer regime, the increase in bandgap barrier due to quantum confinement may significantly prohibit the desired tunneling currents, because tunneling current decreases exponentially with the barrier height. Layered semiconductors with a sizable bandgap and a wide range of band alignments can potentially avoid such degradation, and have been proposed as ideally suited for such applications 13,14 .This class of devices distinguishes themselves from the graphene-based SymFET by offering a desired low off-current 15,16 . Compared to traditional 3D heterojunctions, such structures are expected to form high-quality heterointerfaces due to the absence of dangling bonds [1][2][3]20 . The weak vdW bonding in principle does not suffer from lattice mismatch requirements and makes strain-free integration possible.Among the previous reports on vdW solids, heterojunctions of type-I (straddling) and type-II (staggered) band alignments have been demonstrated [17][18][19] . The Esaki diode device structure is schematically shown in Fig. 1a. The devices are fabricated using a dry transfer process with flake thicknesses of ~50-100 nm for both BP and SnSe 2 24 . BP is the p-type semiconductor, and SnSe 2 the n-type semiconductor of the vdW Esaki diode. A detailed de...

1We review our recent work on the physical mechanisms limiting the mobility of graphene on SiO 2 . We have used intentional addition of charged scattering impurities and systematic variation of the dielectric environment to differentiate the effects of charged impurities and short-range scatterers. The results show that charged impurities indeed lead to a conductivity linear in density (σ(n) ∝ n) in graphene, with a scattering magnitude that agrees quantitatively with theoretical estimates [1]; increased dielectric screening reduces scattering from charged impurities, but increases scattering from short-range scatterers [2]. We evaluate the effects of the corrugations (ripples) of graphene on SiO 2 on transport by measuring the height-height correlation function. The results show that the corrugations cannot mimic long-range (charged impurity) scattering effects, and have too small an amplitude-to-wavelength ratio to significantly affect the observed mobility via short-range scattering [3, 4]. Temperature-dependent measurements show that longitudinal acoustic phonons in graphene produce a resistivity linear in temperature and independent of carrier density [5]; at higher temperatures, polar optical phonons of the SiO 2 substrate give rise to an activated, carrier density-dependent resistivity [5]..Together the results paint a complete picture of charge carrier transport in graphene on SiO 2 in the diffusive regime.2

We have examined the impact of charged impurity scattering on charge carrier transport in bilayer graphene (BLG) by deposition of potassium in ultra-high vacuum at low temperature. Charged impurity scattering gives a conductivity which is supra-linear in carrier density, with a magnitude similar to single-layer graphene for the measured range of carrier densities of 2-4 x 10^12 cm^-2. Upon addition of charged impurities of concentration n_imp, the minimum conductivity Sigma_min decreases proportional to n_imp^-1/2, while the electron and hole puddle carrier density increases proportional to n_imp^1/2. These results for the intentional deposition of potassium on BLG are in good agreement with theoretical predictions for charged impurity scattering. However, our results also suggest that charged impurity scattering alone cannot explain the observed transport properties of pristine BLG on SiO2 before potassium doping

Graphene decorated with 5d transitional metal atoms is predicted to exhibit many intriguing properties; for example iridium adatoms are proposed to induce a substantial topological gap in graphene. We extensively investigated the conductivity of single-layer graphene decorated with iridium deposited in ultra-high vacuum at low temperature (7 K) as a function of Ir concentration, carrier density, temperature, and annealing conditions. Our results are consistent with the formation of Ir clusters of ~100 atoms at low temperature, with each cluster donating a single electronic charge to graphene. Annealing graphene increases the cluster size, reducing the doping and increasing the mobility. We do not observe any sign of an energy gap induced by spin-orbit coupling, possibly due to the clustering of Ir.

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The temperature-dependent conductivity of bilayer graphene with adsorbed layers of the halocarbon molecule CF3Cl was studied under ultra high vacuum conditions. Upon warming CF3Cl sub-monolayer from 25 K, the electrical conductivity drops abruptly at 47 K and exhibits additional inflection points at 60 K and 69 K. CF3Cl multi-layers exhibit an abrupt conductivity gain at 54 K. These conductivity features correspond to known temperature-coverage phase boundaries for CF3Cl films measured on graphite. The changes in conductivity reflect changes in dielectric screening and disorder potential of the CF3Cl adlayer. The chemical specificity of phase transitions presents a basis for sensor selectivity.