Graphene has been attracting great interest because of its distinctive band structure and physical properties. Today, graphene is limited to small sizes because it is produced mostly by exfoliating graphite. We grew large-area graphene films of the order of centimeters on copper substrates by chemical vapor deposition using methane. The films are predominantly single-layer graphene, with a small percentage (less than 5%) of the area having few layers, and are continuous across copper surface steps and grain boundaries. The low solubility of carbon in copper appears to help make this growth process self-limiting. We also developed graphene film transfer processes to arbitrary substrates, and dual-gated field-effect transistors fabricated on silicon/silicon dioxide substrates showed electron mobilities as high as 4050 square centimeters per volt per second at room temperature.

We fabricate and characterize dual-gated graphene field-effect transistors (FETs) using Al 2 O 3 as top-gate dielectric. We use a thin Al film as a nucleation layer to enable the atomic layer deposition of Al 2 O 3 . Our devices show mobility values of over 8,000 cm 2

Using a novel structure, consisting of two, independently contacted graphene single layers separated by an ultra-thin dielectric, we experimentally measure the Coulomb drag of massless fermions in graphene. At temperatures higher than 50 K, the Coulomb drag follows a temperature and carrier density dependence consistent with the Fermi liquid regime. As the temperature is reduced, the Coulomb drag exhibits giant fluctuations with an increasing amplitude, thanks to the interplay between coherent transport in the graphene layer and interaction between the two layers.PACS numbers: 73.22.Gk Bilayer systems formed by two layers of carriers in close proximity are a fascinating testground for electron physics. In particular, the prospect of electron-hole pair (indirect exciton) formation, and dipolar superfluidity 1 has fueled the research of electron-hole bilayers in GaAs/AlGaAs heterostructures 2,3 . Graphene 4,5 is a particularly interesting material to explore interacting bilayers. The symmetric conduction and valence bands, and the large Fermi energy favor correlated electron states at elevated temperatures 6,7 . The zero energy band-gap allows a seamless transition between electrons and holes in each layer, and obviates the large inter-layer electric field required to simultaneously induce electrons and holes in GaAs bilayers 3 . Coulomb drag, a direct measurement of inter-layer electron-electron scattering 8 can provide insight into the ground state of two-9 and one-10 electron systems, as well as correlated bilayer states 11,12 . Here we demonstrate a novel, independently contacted graphene bilayer, and investigate the Coulomb drag in this system. Two main ingredients render the realization of independently contacted graphene bilayers challenging. First, an ultra-thin yet highly insulating dielectric is required to separate the two layers. Second, a method to position another graphene layer on a pre-existing device with minimum or no degradation is needed to create the second layer of the structure investigated here. The fabrication of our independently contacted graphene bilayers is described in Fig. 1. First, the bottom graphene layer is mechanically exfoliated onto a 280 nm thick SiO 2 dielectric, thermally grown on a highly doped Si substrate. E-beam lithography, metal lift-off, and etching are used to define a Hall bar on the bottom layer [ Fig. 1(a)]. A 7 nm thick Al 2 O 3 is then deposited on the bottom layer using a 2 nm oxidized Al interfacial layer, followed by 5 nm of Al 2 O 3 atomic layer deposition 13 . The second, top graphene layer is also mechanically exfoliated on a similar SiO 2 /Si substrate. A poly methyl metacrylate (PMMA) film is applied on the top layer and cured. Using an NaOH etch 14 , the PMMA film along with the graphene layer, and the alignment marks are detached from the host substrate, forming a free standing membrane. The membrane is placed face down on the substrate containing the bottom graphene layer [ Fig. 1(b)], and aligned with it. A Hall bar is subsequently defined on th...

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We investigate the scaling of Al2O3 dielectric on graphene by atomic layer deposition (ALD) using ultra-thin, oxidized Ti and Al films as nucleation layers. We show that the nucleation layer significantly impacts the dielectric constant (k) and morphology of the ALD Al2O3, yielding k = 5.5 and k = 12.7 for Al and Ti nucleation layers, respectively. Transmission electron microscopy shows that Al2O3 grown using the Ti interface is partially crystalline, while Al2O3 grown on Al is amorphous. Using a spatially uniform 0.6 nm-thick Ti nucleation layer, we demonstrate graphene field-effect transistors with top dielectric stacks as thin as 2.6 nm.

We investigate the carrier mobility in mono-and bi-layer graphene with a top HfO 2 dielectric, as a function of the HfO 2 film thickness and temperature. The results show that the carrier mobility decreases during the deposition of the first 2-4 nm of top dielectric and remains constant for thicker layers. The carrier mobility shows a relatively weak dependence on temperature indicating that phonon scattering does not play a dominant role in controlling the carrier mobility. The data strongly suggest that fixed charged impurities located in close proximity to the graphene are responsible for the mobility degradation.

We investigate the transverse electric field (E) dependence of the ν=0 quantum Hall state (QHS) in dual-gated graphene bilayers in high magnetic fields. The longitudinal resistivity (ρxx) measured at ν=0 shows an insulating behavior which is strongest in the vicinity of E=0, and at large E-fields. At a fixed perpendicular magnetic field (B), the ν=0 QHS undergoes a transition as a function of E, marked by a minimum, temperature-independent ρxx. This observation is explained by a transition from a spin polarized ν=0 QHS at small E-fields, to a valley (layer) polarized ν=0 QHS at large E-fields. The E-field value at which the transition occurs has a linear dependence on B. We investigate dual-gated graphene bilayers, a device geometry which allows independent control of the total density and transverse electric field. At a fixed perpendicular magnetic field (B), we observe the emergence of a quantum Hall state (QHS) at filling factor ν = 0 in the presence of a transverse electric field (E), evinced by a large longitudinal resistivity (ρ xx ) with an insulating behavior, consistent with the opening of a gap between the electron and hole bands. Interestingly, as the B-field is increased we observe a developing ν = 0 QHS at E=0, explained by the Zeeman splitting of the Landau levels at zero energy. As a function of E, the ν = 0 QHS undergoes a transition from spin polarized at small E-fields, to valley (layer) polarized at large E-fields.Our samples consist of natural graphite mechanically exfoliated on a 300 nm SiO 2 dielectric layer, thermally grown on a highly doped n-type Si substrate, with an As doping concentration of ∼ 10 20 cm −3 . Optical inspection and Raman spectroscopy are used to identify graphene bilayer flakes for device fabrication. We define metal contacts using electron beam (e-beam) lithography followed by 50 nm Ni deposition and lift-off [ Fig. 1(a)].A second e-beam lithography step followed by O 2 plasma etching are used to pattern a Hall bar on the graphene bilayer flake. To deposit the top gate dielectric, we first deposit a ∼ 20Å thin Al layer as a nucleation layer for the atomic layer deposition (ALD) of Al 2 O 3 . The sample is then transferred ex-situ to an ALD chamber. X-ray photoelectron spectroscopy and electrical measurements confirm the Al layer is fully oxidized in the presence of residual O 2 during evaporation, and the exposure to ambient O 2 [10]. Next, a 15 nm-thick Al 2 O 3 film is deposited using trimethyl aluminum as the Al source and H 2 O as oxidizer [11], followed by the Ni top gate deposition [ Fig. 1(a)]. Longitudinal (ρ xx ) and Hall (ρ xy ) resistivity measurements are performed down to a temperature of T = 0.3 K, and using standard low-current, low-frequency lock-in techniques. Three samples, labeled as A, B, and C, with mobilities of 1500 − 2400 cm 2 /Vs were investigated in this study, all with similar results.We use Hall measurements to determine the total carrier density (n tot ) as a function of top (V T G ) and back (V BG ) gate voltages, and the corresponding cap...

On the basis of scanning thermal microscopy (SThM) measurements in contact and lift modes, the low-frequency acoustic phonon temperature in electrically biased, 6.7-9.7 μm long graphene channels is found to be in equilibrium with the anharmonic scattering temperature determined from the Raman 2D peak position. With ∼100 nm scale spatial resolution, the SThM reveals the shifting of local hot spots corresponding to low-carrier concentration regions with the bias and gate voltages in these much shorter samples than those exhibiting similar behaviors in the infrared emission maps.