Session B11: Focus Session: Graphene/BN

Tunneling spectroscopy of graphene boron nitride heterostructures

We report on the fabrication and measurement of a graphene tunnel junction using hexagonal boron nitride as a tunnel barrier between graphene and a metal gate. The tunneling behavior into graphene is altered by the interactions with phonons and the presence of disorder. We extract properties of graphene and observe multiple phonon-enhanced tunneling thresholds. Finally, differences in the measured properties of two devices are used to shed light on mutually-contrasting previous results of scanning tunneling microscopy in graphene.   

Spin and valley quantum Hall ferromagnetism in graphene on hexa-Boron nitride substrates

In graphene subjected to a quantizing magnetic field, the strong Coulomb interactions and fourfold combined spin/valley degeneracy lead to an approximate SU(4) isospin symmetry within individual Landau levels). At partial filling, exchange interactions can drive the ground state to polarize ferromagnetically within this expanded isospin space, manifesting experimentally as additional integer quantum Hall plateaus outside the normal sequence. Here we report the observation of a wide number of these quantum Hall isospin ferromagnetic states. Using tilted field magnetotransport, we classify the states appearing at different Landau Level filling factors by their real spin structure. We find evidence for real spin polarized states supporting Skyrmionic excitations, charge- or spin- density order, and valley textured excitations at different filling factors. We also observe unexpected reentrant behavior in tilted field in the higher Landau levels. Our results confirm graphene as a highly isotropic SU(4) ferromagnet, in which symmetry breaking is dictated by the interplay between the Zeeman effect, lattice scale interactions, and disorder.   

Atomic Scale Observation of Electron-Electron Interactions in Single-Layer Graphene Devices on Boron Nitride Dielectrics

We have performed scanning tunneling spectroscopy measurements on gated-graphene devices in the quantum Hall regime under varying disorder potential landscapes. Relatively thin hexagonal boron-nitride (h-BN) crystals are mechanically exfoliated on SiO2/Si substrates and single-layer graphene films are later transferred on pre-located h-BN crystals. In this device scheme, we can investigate the interactions of Dirac particles with local impurities ranging from strongly disordered to weakly perturbed environments by adjusting the thickness of h-BN crystals, while varying both the Fermi-energy with respect to a Dirac point and magnetic field. In the h-BN devices, we have observed that the electron-hole puddles are larger in lateral size than those observed on SiO2 devices, and resonance scatterings are significantly reduced due to weakened disorder potentials. Accordingly, we start observing well-defined Landau levels (LLs) as early as 0.5 T and the width of individual LLs, broadened by the scattering of charged carriers, is much narrower than those from graphene on SiO2. In high magnetic fields, we observe the electronic structure of graphene devices is significantly altered by the electron-electron interactions and the formation of large interaction energy gaps. We will discuss the spatial, orbital quantum number, and magnetic field dependence of the observed interaction gaps.   

Landau Level Lifetimes and Residual Disorder in Single Layer Graphene on Boron-Nitride Substrates

Realization of the intrinsic electronic properties of graphene devices has been limited by charge scattering and surface roughness found when graphene is placed on SiO2 substrates. Recently, graphene devices fabricated on hexagonal boron-nitride (h-BN) dielectrics have shown superior device performance compared with graphene placed directly on SiO2 substrates. We have performed scanning tunneling microscopy / spectroscopy (STM / STS) measurements to investigate the local electronic structure of graphene devices on h-BN substrates as a function of charge density and magnetic field. The disorder potential is significantly reduced compared with graphene in direct contact with SiO2. Correspondingly, the widths of Landau levels (LLs) are much narrower becoming comparable to those measured in epitaxial graphene on SiC. The energy and the spatial dispersion of LLs is used to analyze the Fermi velocity of the Dirac particles at different charge densities, an electron-hole asymmetry, and discrete splittings of LLs due to residual spatially varying disorder potential.   

STM of graphene on boron nitride

Graphene on hexagonal boron nitride (hBN) has been shown to have significantly improved mobility and charge inhomogeneity based on electrical transport measurements. Using scanning tunneling microscopy, we have observed that the surface roughness is reduced by one order of magnitude as compared to graphene on SiO$_2$ devices. Near the Dirac point, graphene breaks up into a series of electron and hole puddles due to potential fluctuations. Using scanning tunneling spectroscopy, we have shown that the potential fluctuations are also reduced by an order of magnitude on hBN. The ultraflat and clean nature of graphene on hBN devices allows for the observation of scattering from buried step edges. The energy and spatial dependence of the scattering gives information about the dispersion relation of graphene and the chiral nature of the quasiparticles. In this talk, I will also discuss our recent spectroscopy measurements on hBN.   

Thickness Dependence of Electrical Breakdown in h-BN dielectric using C-AFM microscopy

For nano-scale graphene transistor applications, hexagonal boron nitride (h-BN) is a highly desirable dielectric material that is being investigated not only because of its intrinsic properties but also because of its low lattice mismatch with hexagonal graphene. Currently, SiO$_{2}$ limits the carrier mobility of graphene due to substrate phonon coupling. Therefore, h-BN can be employed for mobility enhancement beyond the values achievable on standard dielectric. Decreasing the device dimensionality however, requires a more detailed understanding of electrical breakdown at the nanoscale. We will present on the intrinsic breakdown electric field (E$_{BF})$ of h-BN thin films in a metal-insulator-metal (MIM) configuration. This nanoscaled MIM structure is measured using conductive-atomic-force microscopy (C-AFM). Here, C-AFM is used to extract breakdown voltage for various thicknesses of mechanically exfoliated h-BN flakes. We measure the dielectric properties of h-BN flakes that vary from 2nm to 25nm and determine the ultimate scalability of hBN dielectrics.   

Transport in graphene-boron nitride heterostructures

Transferring graphene on hexagonal boron nitride permits the fabrication of high mobility graphene devices. We report on in-plane transport measurements on dual gated graphene systems using boron nitride as a substrate. The low amount of disorder allows for ballistic effects to be probed, as we can gate-define regions narrower than the mean free path. This work is supported by the Center on Functional Engineered Nano-Architectonics.   

Tunable and Sizable Band Gap of Single Layer Graphene Sandwiched between Hexagonal Boron Nitride

It is a big challenge to open a tunable and sizable band gap of single layer graphene without big loss in structural integrity and carrier mobility. By using density functional theory calculations, we show that the band gap of single layer graphene can be opened to 0.16 (without electrical field) and 0.34 eV (with a strong electrical field) when sandwiched between two hexagonal boron nitride single layers in a proper way. The zero-field band gaps are increased by about 50{\%} when many-body effects are included. Ab initio quantum transport simulation of a dual-gated FET out of such a sandwich structure further confirms an electrical field-enhanced transport gap. The tunable and sizeable band gap and structural integrity render this sandwich structure a promising candidate for high-performance single layer graphene field effect transistors.   

High quality charge- and spin transport in graphene on commercially available boron nitride

In order to overcome the limitations that a silicon oxide substrate imposes on the electronic transport properties of graphene, hexagonal boron nitride (h-BN) has proven to be an excellent alternative. We present a fast, simple and accurate transfer technique of graphene, which yields atomically flat graphene flakes on h-BN that are almost completely free of bubbles or wrinkles. Using this transfer technique we prepared single- and bilayer graphene electronic devices on commercially available hexagonal boron nitride and extract mobilities as high as 125 000 cm$^{2}$/Vs at room temperature and 275 000 cm$^{2}$/Vs at 4.2 K. The high electronic quality is further confirmed by magnetotransport measurements, which show the development of the 1e$^{2}$/h Landau level already at 5T (P. J. Zomer et al. arXiv:1110.1045v1). Finally, we present very recent results of spin transport in high mobility h-BN supported graphene flakes (P. J. Zomer et al. in preparation). In conclusion, the potential of commercially available boron nitride combined with our transfer technique makes high mobility graphene devices more accessible.   

Examining CVD graphene quality using hexagonal boron nitride substrates

Chemical vapor deposition (CVD) of graphene has proven to be the most inexpensive and scalable synthesis technique for continuous graphene monolayers. However, CVD graphene typically has a lower mobility than that from exfoliation. This is likely due to a combination of intrinsic (defect and grain boundary) and extrinsic (substrate and contamination) effects. By fabricating CVD graphene transistors on hexagonal boron nitride (h-BN) substrates, we are able to reduce the extrinsic substrate interactions that otherwise occur with silicon dioxide layers. This greatly improves the mobility in our devices (up to 29000 cm$^2$/Vs). While such improvements from h-BN have been previously observed in exfoliated graphene devices, its success with CVD graphene is particularly notable because it shows that the low mobilities observed in CVD graphene are not from intrinsic effects, and that current synthesis techniques are more than sufficient to consistently produce graphene with $>$10000 cm$^2$/Vs. Furthermore, our research reveals that characterization of CVD growth recipes by measuring mobilities on silicon dioxide is insufficient, as scattering from the oxide will dominate, revealing little information about intrinsic graphene quality.