Session Y11: Focus Session: Graphene Devices - Fabrication

Lattice-Nanotomy for Large-Scale Production of Transferrable and Dispersible Graphene-Nanostructures of Controlled Shape and Size

In this talk, we will present a novel graphite-lattice-nanotomy (nanoscale-cutting) process for high throughput production of monodispersed graphene nanostructures (GNs) with controlled shape (square, rectangle, ribbons and triangle), dimensions (sizes at 5 nm resolution with a range of 5--600 nm) and chemical-construct. We demonstrate that this versatile process enables the realization of unprecedented graphene-nanostructures, which exhibit the evolution of semiconductor-characteristics and electrical transport mechanism. Further, we will present in detail the size and shape-dependent electrical and optical properties of these GNs via various microscopic and spectroscopic techniques. This nanotomy process can provide access to virtually-infinite and unprecedented GNs for development of fundamental optical/electrical/structural correlations and novel applications.   

Direct Writing of Graphene-based Nanoelectronics via Atomic Force Microscopy

We use direct writing with an atomic force microscope (AFM) to fabricate simple, graphene-based electronic components to explore their electronic properties and the feasibility of manufacturing electronic devices via AFM. The process being studied, thermochemical nanolithography (TCNL), involves flowing current through an AFM cantilever to provide thermal energy to a chemically modified graphene (CMG) film, either graphene oxide or graphene fluoride. The heat reduces the insulating CMG film back into conductive graphene. Thus, these nanoribbons can be used to fabricate nano-scale electronic components such as resistors, capacitors, and transistors. The technique, as compared to other attempts to produce graphene-based devices, is simple, does not involve solvents or other complicated fabrication steps, and allows for the exact placement of the devices on the substrate. The electronic properties of the devices produced using the two materials, measured using current-voltage characteristics at various temperatures down to 2 K and in variable magnetic fields up to 9 T, will be discussed. This work was partially supported by the US Naval Academy Research Office and the Nanoscience Institute at NRL.   

A study on tapered graphene nanoribbons with controlled angle: Fabrication and conductivity studies

Graphene, the newest member in the nanocarbon family, is a perfect single atom thick 2D sheet made up of carbon with exceptional electrical and mechanical properties. It is well-known that the band-gap of graphene nanoribbons (GNRs) can be controlled via their width. Here we demonstrate that GNRs with tapered morphology have semiconducting-to-metallic continuum along its length, and thus exhibit unique electrical properties. The device is fabricated from a single layer graphene grown on a Cu foil \textit{via} the standard CVD process. Here, the graphene is transferred on a silica substrate and electron beam lithography etching is performed to produce a tapered graphene GNR device, followed by with Pd-Au electrode-deposition. We demonstrate the unique carrier transport properties, electrical rectification and carrier modulation in these novel devices.   

Toward graphene based electronics from homogeneous top-gated graphene devices

We report high quality top-gated graphene field effect devices integrated on 150mm substrate, which involves unprecedented homogeneous chemical vapor deposition growth of monolayer graphene on Ni/Cu. Statistics of maximum resistance, dirac voltage and also mobility over a thousand devices are shown for the first time, in which 5\% of measurable devices are in 3,000$\sim$5,000 cm2/V$\cdot$s of mobility. Furthermore, logic gates such as NOT and AND based on top-gated graphene devices were demonstrated. Our results imply a route toward graphene based electronics with complementary metal-oxide-semiconductor (CMOS) compatible process.   

Gate-Tunable Superconductor-Insulator Transition in Bilayer-Graphene Josephson Junctions

Bilayer graphene shows opening of electric-field-induced band gap, the size of which is proportional to the intensity of the electric field. We report electronic transport measurements on superconducting proximity effect in planar dual-gated bilayer-graphene Josephson junction with Pb$_{0.93}$In$_{0.07}$ (PbIn) electrodes ($\Delta_{PbIn}$ $\sim$ 1.1meV, $T_c$ = 7.0 K). The junction resistance along the charge-neutral point (CNP) increases as we modulate top- and back-gate voltages away from the zero-gap CNP. The resistive state near the CNP shows a variable-range-hopping-type insulating behavior in $R$-$T$ curve with lowering temperature crossing the superconducting transition of PbIn electrodes. However, a highly doped regime shows metallic $R$-$T$ behavior and junction becomes superconducting below $T_c$. Moreover, magnetic-field-induced Fraunhofer supercurrent modulation, microwave-induced Shapiro steps, and multiple Andreev reflection (MAR) are observed, which indicate the formation of genuine Josephson coupling across the planar junctions below $T_c$ with sufficiently transparent superconductor$-$bilayer graphene interface. The separatrix of the superconductor-insulator transition corresponds to the square junction conductance of $G_{sq}\sim$ 6$-$8$e^{2}/h$.   

Characterization of graphene quantum dot hybrid structures

We report electrical transport, photo-electric response and Raman spectroscopy measurements in macroscopic samples of graphene decorated with inorganic quantum dots (CdSe QDs). QDs are deposited on chemical vapor deposition (CVD) graphene by spin-coating. Raman measurements of graphene decorated with QDs on Si wafer show very similar spectra with clear G and 2D peaks that reveal no degradation of graphene during the QDs deposition process. Furthermore, two types of device architectures (QDs-graphene and graphene-QDs-graphene) are fabricated with graphene as a transparent electrode and QD as a light absorbent for electrical photoresponse characterization. Upon application of either a broadband light source or a 532-nm monochromatic laser source, graphene-QDs-graphene devices demonstrate photoconducting response, but not in the case of QDs-graphene devices.   

Fabrication of nanometer-scale suspended graphene transistors

We present a method to fabricate suspended ultra-short graphene transistors. We define narrow bowtie gold junctions on exfoliated graphene, and then use an oxygen plasma to etch away the graphene except under the gold junctions. The next step is to wet etch the SiO$_{2}$ under the junctions to suspend the devices. Finally, we use a feedback-control electromigration procedure to break the gold junctions and expose sections of graphene which are 100 to 300 nm wide and as short as $\approx$ 10 nm. Using electron transport, we show that these suspended graphene nanocrystals form ballistic two-dimensional Dirac electron gas systems. We study them as a function of temperature and charge carrier density. These ultra-short NEMS transistors offer the prospect of exploring the coupling between flexural vibrons and charge carriers in graphene.   

Implementation of embedded-gate graphene field effect transistors on flexible substrates

In this work we present embedded-gate graphene field effect transistors (GFETs) on flexible polyimide films for high frequency RF and sensor applications. Graphene transistors with gate lengths from 1um to 4um and width of 8um have been realized by e-beam lithography. Chemical vapor deposited monolayer graphene with negligible defects is transferred to flexible polyimide substrates for device fabrication. The electrostatic measurement reveals that fabricated device shows a mobility of over 1000cm$^{2}$/Vs at room temperature, the highest reported among graphene FETs implemented on flexible films. Surface roughness and impurity doping are of crucial importance in fabricating flexible graphene devices since its charge transport is limited by mobility degradation due to surface roughness and doping related with the chemical transfer process. The stack of atomic layer deposited Al$_{2}$O$_{3}$ as gate dielectric and gate pads underneath provides smooth surface for graphene to sit on as confirmed by atomic force microscopy. This results in a field effect mobility of an embedded gate GFET twice that of a top gated GFET on polyimide substrates. The minimum conduction point is close to zero, indicting the impurity induced doping for the device is negligible.   

Molecular Layer-seeded Ultra-thin Top-gate Dielectrics for High Transconductance Graphene Transistors

The potential of graphene in integrated analog and digital circuits can only be fully realized through incorporation of ultra-thin gate dielectrics to enable large-scale small-channel graphene field-effect transistors (GFETs). Atomic-layer deposition (ALD) is a viable technique to fabricate gate-dielectrics, however, it requires a seeding layer on otherwise inert graphene. Here, we demonstrate a single molecule thick perylene-3,4,9,10-tetracarboxylic dianhydride overlayer as an effective seeding layer to grow high-$\kappa $ Al$_{2}$O$_{3}$ on mechanically exfoliated graphene for high-performance GFETs. Using an ultra-thin ($<$ 1nm) seeding layer, in contrast to polymer films (5-10 nm), we demonstrate fabrication of the thinnest ALD-grown gate-dielectric (4 nm) reported to date in top-gated GFETs. This yields high performance GFETs with the intrinsic transconductance parameter approaching 2.4 mS and the field-effect mobility $\sim $3000 cm$^{2}$/Vs. We also demonstrate generalization of this molecular layer seeded-ALD growth method to higher- $\kappa $ gate dielectrics, yielding further enhanced GFET transconductance for possible application to radio-frequency circuits.   

Conductance through two-terminal graphene junctions with wetting metal contacts

Metallic contacts become a relevant factor for the behavior of nanoscale graphene devices. A thin layer of a wetting metal --a metal that forms covalent bonds to graphene-- is customarily placed in between graphene and bulk leads. The most common choices for this wetting metal are Titanium, Chromium, and Palladium. We will present the equilibrium conductance through crystalline (defect- and impurity-free) two-terminal graphene junctions attached to normal, spin-unpolarized Titanium metal leads. In addition, we discuss the equilibrium potential profile across the junctions, and the presence of Fabry-Perot oscillations. The conductance shows pronounced noise, and the Fano factor near the Dirac point is seen to fluctuate, as in experiment [1]. The distribution of transmission eigenvalues is bimodal, indicating a disordered-metal-like charge transport through nanoscale two-terminal graphene junctions with wetting metals.\\[4pt] [1] L. DiCarlo, J. R. Williams, Y. Zhang, D. T. McClure, and C. M. Marcus. Phys. Rev. Lett. 100, 156801 (2008).\\[0pt] [2] S. Barraza-Lopez, M. Kindermann, and M.-Y. Chou. (Manuscript in preparation.)   

Fabrication of a tunable quantum point contact in bilayer graphene

The realization of a quantum point contact (QPC) in graphene is of interest for both physical and technological reasons. Fabricating a tunable QPC thus far, however, has been technologically challenging due to the inability to electrostatically deplete graphene. Recent advances have allowed for the creation of high mobility dual-gated bilayer samples sandwiched in hexagonal boron nitride. These samples display a robust, tunable band gap, which opens the door to electrostatically defining the conductance channel. In this presentation, we report the fabrication and characterization of point contact structures in high mobility dual-gated bilayer graphene samples employing hBN as both the top and bottom dielectric layer.   

Surperconducting Graphene Nanodevice in Ballistic Regime

Superconductivity carried by Dirac fermion can be realized through induced superconductivity in graphene. Observation of novel phenomena anticipated by theories requires superconducting graphene devices with low disorder so that the transport is ballistic. In this talk we present the fabrication and characterization of superconducting graphene nanodevices that are built on hexagonal Boron Nitride. The ultra flatness and lack of dangling bond in the boron nitride substrate reduces the disorder in graphene, opening the door to the study of ballistic Dirac fermion in superconducting regime.   

Scanning Tunneling Potentiometry Mapping of Electron Transport in Graphene on SiC

Single layers of graphene formed on SiC look to be a promising system for the realization of graphene electronics. To utilize the full potential of graphene on SiC a complete understanding of the physical and electronic properties of this system is needed. Scanning Tunneling Microscope (STM) images along with scanning tunneling spectroscopy is used to characterize the sample surface. STM images clearly show the distinction between 1 monolayer (ML) and 2ML regions and this transition is further confirmed by point spectroscopy and spectroscopic mapping across the boundary. Defects, grain boundaries, step edges and other potential scattering centers are thought to play a major role in the electronic properties, especially in transport, along the graphene sheets. Using a low temperature four-probe scanning tunneling microscope, potentiometry measurements are performed on the epitaxial graphene grown on 4H-SiC. Potentiometry maps spanning the transition from 1ML to 2ML graphene show a contrast change indicating a potential change at this interface. Preliminary results of the transport along this potentially revolutionary new electronic system will be presented.   

Atomic Force Microscopy-Based Local Tunable Oxidation of Graphene

We have fabricated graphene/graphene oxide/graphene (G/GO/G) junctions by local anodic oxidation lithography using atomic force microscopy (AFM). The conductance of the G/GO/G junction decreased with the bias voltage applied to the AFM cantilever $V_{tip}$. For G/GO/G junctions fabricated with large and small $\vert V_{tip}\vert $. GO was semi-insulating and semiconducting, respectively. AFM-based LAO lithography can be used to locally oxidize graphene with various oxidation levels and achieve tunability from semiconducting to semi-insulating GO [S. Masubuchi \textit{et al.}, Nano Lett. \textbf{11}, 4542 (2011).]   

High mobility Single Layer Epitaxial Graphene on 4H-SiC (000-1)

Multi-layer Epitaxial Graphene on 4H-SiC (000-1) has demonstrated very high mobility up to$\sim $27,000 cm$^{2}$/Vs [1]. Recently single layer graphene grown by the Confinement Control Growth method [2] exhibits mobility up to $\sim $ 25,000cm$^{2}$/V$\cdot $s at 4K and 13,000 cm$^{2}$/V$\cdot $s at 300K with p=3 x 10$^{12}$ cm$^{-2}$ The relation between Raman G peak features (FWHM and position) and carrier density of Epitaxial Graphene on carbon face is revealed. Quantum Hall Effect [3] is observed both for p and n type carriers on top gated sample. This indicates that top gated single layer graphene can be produced on the Carbon face with high quality and high carrier mobility. \\[4pt] [1] Science \textbf{312}, 1191 (2006) \\[0pt] [2] PNAS \textbf{108 }(41) 16900 (2011) \\[0pt] [3] APL \textbf{95}, 223108 (2009)