Graphene nanoribbons display an imperfectly understood transport gap. We measure transport through nanoribbon devices of several lengths. In long (≥250 nm) nanoribbons we observe transport through multiple quantum dots in series, while shorter (≤60nm) constrictions display behavior characteristic of single and double quantum dots. New measurements indicate that dot size may scale with constriction width. We propose a model where transport occurs through quantum dots that are nucleated by background disorder potential in the presence of a confinement gap.Graphene is a two-dimensional conductor with remarkable properties ranging from long spin relaxation times 1 to very high mechanical strength 2 to the highest known room temperature mobilities. 3 Extended graphene sheets also display a non-zero minimum conductivity even at nominally zero carrier densities, which limits their applicability for some types of semiconductor devices such as transistors with high on-off ratios. When graphene is etched into narrow strips known as † Kathryn Todd et al. Quantum Dots in Graphene Nanoconstrictionsnanoribbons, however, conductance through the device is suppressed for a wide range of Fermi energies around the Dirac point. The observation of this conduction gap, 4,5,6 which scales inversely with ribbon width, 4 has led to the proposal of graphene nanoribbons as next-generation highfrequency transistors. However, the observed conduction gaps are larger than those expected from a single-particle confinement picture in the absence of lattice effects at the edges, while studies on nanoribbons of different orientations with respect to the graphene lattice 4 demonstrate that explanations depending on the presence of well-defined crystallographic edges 7,8,9 do not apply. Several alternative explanations have been put forward to explain the large energy scale of the conduction gap. One proposal involves a non-conductive "dead zone" at the ribbon edges due to atomic-scale disorder that causes the effective conducting ribbon width to be narrower than the physical width. 4Elaborations on this idea propose that atomic-scale disorder near the ribbon edge may give rise to localized states that extend into the ribbon body. 10 Conversely, random charged impurity centers in the body of the ribbon, in combination with the ribbon's confinement gap, have been proposed to cause a metal-insulator transition. 11 Others have suggested that lithographic line edge roughness may create a series of quantum dots defined by nanometer-scale variations in ribbon width. 12 Here we examine detailed conduction characteristics of long nanoribbons as well as short nanoconstrictions in an attempt to elucidate the origin of the large conduction gap in graphene nanoribbons.In light of our data, we propose that the random charged impurity centers present throughout the graphene sheet, in conjunction with a gap stemming from the constriction's confined geometry,give rise to isolated puddles of charge carriers acting as quantum dots both in long nanoribbons ...

High and low temperature behavior of Ohmic contacts to AlGaN/GaN heterostructures with a thin GaN capWe study the transport properties of quantum point contacts in a GaN / AlGaN heterostructure. The conductance of our devices shows well-quantized plateaus, which spin-split in high perpendicular magnetic field. The g factor is 2.55, as derived from the point contact subband splitting versus perpendicular magnetic field. In addition to the well-resolved plateaus, we also observe evidence of "0.7 structure" which has been mainly investigated in the GaAs system.

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High mobility two-dimensional electron systems in GaN/AlGaN heterostructures have been realized by plasma assisted molecular beam epitaxy on GaN templates. In the density range of 10 11 cm -2 to 10 12 cm -2 , mobility values exceeding 160 000 cm 2 /Vs have been achieved. Scattering mechanisms that presently limit the production of higher mobility samples are discussed. We present results of a systematic study of the weak localization and antilocalization corrections to the classical conductivity at very low magnetic fields. The unambiguous observation of a conductivity maximum at B = 0 suggests that spin -orbit scattering is not negligible in GaN heterostructures as one might expect for a wide-bandgap system. We have recently realized electron transport through GaN nanostructures. We report on the transport properties of the first quantum point contacts (QPCs) in GaN. These devices are used to study onedimensional transport in the Nitride system.