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Electronic Transport in Novel Graphene Nanostructures UC Berkeley UC Berkeley Electronic Theses and Dissertations Title Electronic Transport in Novel Graphene Nanostructures Permalink https://escholarship.org/uc/item/8rk3r79k Author Gannett, William Joy Publication Date 2012 Peer reviewed|Thesis/dissertation eScholarship.org Powered by the California Digital Library University of California Electronic Transport in Novel Graphene Nanostructures by William Joy Gannett A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Physics in the Graduate Division of the University of California, Berkeley Committee in charge: Professor Alex Zettl, Chair Professor Feng Wang Professor Oscar Dubon Fall 2012 Electronic Transport in Novel Graphene Nanostructures Copyright 2012 by William Joy Gannett 1 Abstract Electronic Transport in Novel Graphene Nanostructures by William Joy Gannett Doctor of Philosophy in Physics University of California, Berkeley Professor Alex Zettl, Chair Graphene, a single sheet of sp2-bonded carbon atoms, is a two-dimensional ma- terial with an array of unique electronic, chemical, and mechanical properties. Ap- plications including high performance transistors, chemical sensors, and composite materials have already been demonstrated. The introduction of chemical vapor de- position growth of monolayer graphene was an important step towards scalability of such devices. In addition to scalability, the exploration and application of these properties require the fabrication of high quality devices with low carrier scattering. They also require the development of unique geometries and materials combinations to exploit the highly tunable nature of graphene. This dissertation presents the synthesis of materials, fabrication of devices, and measurement of those devices for three previously unexplored types of graphene de- vices. The first type of device is a field effect transistor made from chemical vapor deposited (CVD) graphene on hexagonal boron nitride (hBN) substrates. We demon- strate a significant improvement in carrier mobility from hBN substrates and are able to explore the sources of scattering in CVD graphene. The second type of device, fluorinated graphene transistors, allows us to examine doping and disorder effects from fluorination of the graphene crystal as well as electronic transport through un- fluorinated folds in the graphene. With the third type of device we demonstrate a new route to graphene nanoribbon devices using both hBN flakes and BN nanotubes that may reduce disorder and allow precise measurements of quantum phenomena in graphene nanoribbons. i To all the great teachers I've had, My parents first among them ii Contents Table of Contents iv List of Figuresv List of Tables viii 1 Introduction1 1.1 Graphene history.............................2 1.2 Unique properties.............................2 1.3 Graphene research outline........................3 2 Graphene theory4 2.1 Crystal structure.............................4 2.2 Band structure..............................5 2.2.1 Density of states.........................8 2.2.2 Carrier mobility..........................8 2.3 Scattering.................................9 2.3.1 Coulomb scattering........................ 10 2.3.2 Short-range scattering...................... 10 2.4 DC transport............................... 11 2.4.1 Scattering signatures....................... 11 2.4.2 Temperature dependence of scattering............. 11 2.5 Extracting parameters from transport data............... 12 3 Graphene synthesis 15 3.1 Exfoliation of graphite.......................... 15 3.2 CVD graphene growth.......................... 17 3.2.1 Foil growth recipe......................... 17 3.2.2 Post-growth transfer....................... 18 3.3 CVD growth on thin films........................ 21 3.4 Thin film copper on BN flakes...................... 22 3.5 Other graphene production techniques................. 26 iii 4 Graphene on boron nitride 27 4.1 Motivation for substrate improvements................. 27 4.1.1 Boron nitride........................... 28 4.2 Device fabrication............................. 30 4.2.1 Fabricating graphene/hBN heterostructures.......... 30 4.2.2 Lithography............................ 32 4.3 Resulting devices............................. 34 4.4 Transport measurement setup...................... 35 4.5 Preliminary transport measurements.................. 36 4.6 Additional sample processing...................... 37 4.6.1 Contamination observation.................... 37 4.6.2 Annealing to remove contamination............... 38 4.7 Post-anneal transport measurements.................. 39 4.8 Data analysis............................... 41 4.8.1 Unannealed samples....................... 41 4.8.2 Annealed samples......................... 44 4.8.3 Coulombic vs short range scattering............... 47 4.9 Conclusions................................ 49 4.10 Graphene p-n junctions.......................... 50 4.11 Collaborations............................... 50 4.11.1 STM of graphene on hBN.................... 51 4.11.2 Scanning plasmon resonance................... 55 5 Folds in graphene 57 5.1 Microscopy on folds............................ 58 5.1.1 AFM of fold............................ 58 5.1.2 TEM analysis of folds...................... 60 5.2 Transport data.............................. 63 5.3 Fold ideas................................. 65 6 Graphene fluorination 67 6.1 Introduction to fluorination....................... 67 6.2 Grafold fluoride.............................. 68 6.3 Fluorination background......................... 69 6.3.1 Fluorination theory........................ 69 6.3.2 Previous fluorination experiments................ 70 6.4 Device fabrication............................. 71 6.5 Experimental design I: folds on SiO2 .................. 72 6.5.1 XeF2 system............................ 72 6.5.2 Fluorination of devices...................... 74 6.6 Data I................................... 75 6.6.1 Signature of bilayer fold isolation in B34............ 75 iv 6.6.2 Further fluorination of B44 and B45............... 78 6.7 Analysis I................................. 80 6.8 Experimental design II: in situ measurement and hBN substrates.. 80 6.8.1 Xetch electrical feedthrough................... 80 6.8.2 New sample holder design.................... 86 6.8.3 Graphene samples......................... 87 6.8.4 Fluorination and measurement.................. 87 6.9 Data II................................... 87 6.9.1 XeF2 desorption.......................... 88 6.9.2 Further fluorination........................ 89 6.10 Conclusions................................ 89 7 Graphene nanoribbons from BN nanotube masks 92 7.1 Graphene nanoribbons.......................... 92 7.1.1 Previous synthesis........................ 92 7.1.2 GNR transport.......................... 93 7.1.3 Improvements........................... 94 7.2 Experimental design........................... 95 7.2.1 Boron nitride nanotubes..................... 95 7.3 Fabrication I: Nanoribbons on SiO2 ................... 96 7.4 Measurement I.............................. 99 7.5 Analysis.................................. 99 7.5.1 Capacitance of nanoribbons................... 101 7.6 Fabrication II: Nanoribbons on hBN.................. 102 7.7 Measurement II.............................. 103 7.8 Conclusions................................ 104 Bibliography 121 A Graphene optical visibility 122 A.1 Constrast calculations.......................... 123 A.2 Mathematica code............................ 128 B Fabrication recipes 133 B.1 Lithography................................ 133 B.2 Graphene etching............................. 135 B.3 Other notes................................ 135 v List of Figures 1.1 Graphite crystal lattice, showing layered structure...........1 2.1 Graphene lattice.............................4 2.2 Graphene edge directions.........................5 2.3 Graphene band structure.........................6 2.4 3D rendering of band structure of graphene...............7 2.5 Magnified view of Dirac point......................7 2.6 Conductivity of different scattering densities.............. 12 2.7 Simulated variation in graphene doping................. 14 3.1 TEAM image of graphene........................ 15 3.2 Optical image of exfoliated graphene.................. 17 3.3 CVD graphene recipe diagram...................... 18 3.4 PMMA reflow technique......................... 21 3.5 SEM image of transferred CVD graphene................ 22 3.6 Schematic of thin film device fabrication................ 23 3.7 Collapsed CVD graphene device..................... 24 3.8 Suspended CVD graphene device.................... 24 3.9 Graphene grown on Cu on a BN flake.................. 25 3.10 Larger scale graphene grown on Cu/BN films.............. 25 4.1 Boron nitride lattice........................... 28 4.2 Exfoliated hBN flakes........................... 29 4.3 Raman spectrum of graphene on oxide and hBN............ 32 4.4 SEM map of graphene on hBN...................... 33 4.5 GFET fabrication schematic....................... 34 4.6 Optical image of final device....................... 35 4.7 Schematic of GFET measurement setup................ 36 4.8 Contamination between graphene and boron nitride.......... 38 4.9 Comparing conductivity of devices on hBN and oxide......... 40 4.10 Mobility vs T of graphene on hBN and oxide.............. 42 4.11 Conductivity minimum vs T of graphene on hBN and oxide..... 42 4.12 Dirac point vs T of graphene on hBN and oxide............ 43 vi 4.13 Scattering
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