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Electronic Transport in Atomically Thin Layered Materials by MASSACHUSETTS INGTIME Britton William Herbert Baugher OF TECHNOLOGY B.A. Physics, Philosophy JUL 0 1 2014 University of California at Santa Barbara, 2006 LIBRARIES Submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY June 2014 @ Massachusetts Institute of Technology 2014. All rights reserved. Signature redacted Author ...... Department of Physics May 23, 2014 Signature redacted Certified by... Pablo Jarillo-Herrero Associate Professor / I Thesis Supervisor Signature redacted Accepted by .......... .................. Krishna Rajagopal Chairman, Associate Department Head for Education Electronic Transport in Atomically Thin Layered Materials by Britton William Herbert Baugher Submitted to the Department of Physics on May 23, 2014, in partial fulfillment of the requirements for the degree of Doctor of Philosophy Abstract Electronic transport in atomically thin layered materials has been a burgeoning field of study since the discovery of isolated single layer graphene in 2004. Graphene, a semi-metal, has a unique gapless Dirac-like band structure at low electronic energies, giving rise to novel physical phenomena and applications based on them. Graphene is also light, strong, transparent, highly conductive, and flexible, making it a promising candidate for next-generation electronics. Graphene's success has led to a rapid expansion of the world of 2D electronics, as researchers search for corollary materials that will also support stable, atomically thin, crystalline structures. The family of transition metal diclialcogenides represent some of the most exciting advances in that effort. Crucially, transition metal dichalco- genides add semiconducting elements to the world of 2D materials, enabling digital electronics and optoelectronics. Moreover, the single layer variants of these materials can posses a direct band gap, which greatly enhances their optical properties. This thesis is comprised of work performed on graphene and the dichalcogenides MoS 2 and WSe 2. Initially, we expand on the family of exciting graphene devices with new work in the fabrication and characterization of suspended graphene nanoelec- tromnechanical resonators. Here we will demonstrate novel suspension techniques for graphene devices, the ion beam etching of nanoscale patterns into suspended graphene systems, and characterization studies of high frequency graphene nanoelectromechan- ical resonators that approach the GHz regime. We will then describe pioneering work on the characterization of atomically thin transition metal dichalcogenides and the development of electronics and optoelectronics based on those materials. We will describe the intrinsic electronic transport properties of high quality monolayer and bilayer MoS 2 , performing Hall measurements and demonstrating the temperature de- pendence of the material's resistivity, mobility, and contact resistance. And we will present data on optoelectronic devices based on electrically tunable p-n diodes in monolayer WSe 2 , demonstrating a photodiode, solar cell, and light enmitting diode. Thesis Supervisor: Pablo Jarillo-Herrero Title: Associate Professor 3 4 Acknowledgments This thesis had a long, complex, and convoluted life, and its successful completion was only made possible by the ever-present guidance and help of my family, friends, and colleagues. I would like to thank my advisor, Pablo, for his guidance and patience, my thesis committee for their stewardship of my thesis, my group for all of their insight and assistance, my teammates, Hugh and Yafang, Tchefor and Kevin, and Max, for all their hard work and their direct contributions to this thesis, my family and friends for their encouragement, and my wife, Allie, for her untiring love and support. 5 6 Contents 1 Introduction 15 1.1 M otivation ... .... .... .. 15 1.2 Outline. ...... ...... .. 17 1.3 Graphene .... ...... ... 18 1.4 Dichalcogenides ..... ..... 20 2 Fabrication of Electronics Based on Atomically Thin Layered Mate- rials 23 2.1 Introduction . ... 23 2.2 Graphene Suspension 24 2.3 Current Annealing 28 2.4 Ion Beam Etching. 33 2.5 Acknowledgements 36 3 Etching of Graphene Devices with a Helium Ion Beam 37 3.1 Introduction ...... ...... ............. 37 3.2 Motivation ............. ............. 38 3.3 R esults ............... ............. 39 3.4 Conclusions ... ......... .. .......... 42 3.5 Methods .............. ............. 43 3.6 Acknowledgments ......... ....... ...... 43 4 Approaching GHz Resonant Frequencies in Suspended Graphene 7 NEMS Resonators 45 4.1 Introduction ... ... ... ... ... .. ... 45 4.2 Motivation . 46 4.3 Device Fabrication 46 4.4 Mixing Current 49 4.5 Resonant Modes. 53 4.6 Mode Fitting . 55 4.7 Conclusions ... 58 4.8 Acknowledgments 58 5 Intrinsic Electronic Transport Properties of High Quality Monolayer and Bilayer MoS 2 59 5.1 Introduction ... ...... 59 5.2 Motivation .... ...... 60 5.3 Device Fabrication ..... 61 5.4 Resistivity Measurements. 62 5.5 Hall Measurements .. ... 65 5.6 Conclusions ... ...... 68 6 Optoelectronic Devices Based on Electrically Tunable p-n Diodes in a Monolayer Dichalcogenide 69 6.1 Introduction .... ......... .......... 69 6.2 M otivation ..... ......... .......... 70 6.3 Device Fabrication ....... .......... ... 72 6.4 Transport in Gate Controlled p-n Junctions ...... 72 6.5 Optoelectronics . ................... .. 76 6.6 Conclusions ........................ 80 6.7 M ethods .......................... 81 A MoS 2 Supplementary Information 83 A. 1 Fabrication ....... ................................ 83 8 A.2 Metal Insulator Transition ... ...... ...... ...... ... 86 A.3 Hall Measurements . ..... ..... ..... ..... ..... .. 86 A.4 Mobility . .... .... .... .... ..... .... .... .... 87 A.5 Acknowledgments .. ..... ..... ..... ..... ..... .. 89 B WSe 2 Supplementary Information 91 B.1 Device Fabricatioi ........ ......... ........ ... 91 B.2 M id-Gap Current ... ........ ........ ......... 94 B.3 Schottky Barriers ..... ........ ........ ........ 95 B.4 Acknowledgements . ..... ..... ..... ..... ..... .. 99 9 10 List of Figures 1-1 Atomically layered materials with diverse electronic and optoelectronic prop erties ......... ............ ........... 17 1-2 Schematic and electronic band structure of graphene ...... ... 18 1-3 Overview of a TMD crystal structure and band structure . ...... 20 2-1 Suspended graphene structures ........... .......... 24 2-2 Current annealing of suspended graphene devices ..... ...... 30 2-3 Etching suspended graphene with a gallium based focused ion beam (FIB) and a helium ion beam .. ...... ...... ...... .. 33 2-4 Etching suspended graphene by electron beam assisted water etching 36 3-1 Schematic of a graphene device . ......... ........ ... 38 3-2 Etching suspended graphene with a helium ion bean ...... ... 40 3-3 Electrically isolating suspended graphene with a helium ion beam . 41 3-4 Etching graphene on SiO 2 with a helium ion beam ..... ...... 42 4-1 Graphene NEMS device schematics, image, and electrical readout .. 48 4-2 Resonance maps, avoided crossings, quality factor, and high frequency m o d es ...... ........ ........ ....... ...... 53 4-3 Fitting resonance modes and mode softening ... ........ ... 57 5-1 MOS2 device schematics, images, and two-ternminmal transport measure- m en ts ......... ........... ............ ... 60 5-2 Contact resistance and four-terminal resistivity of monolayer and bi- layer M oS 2.......... ....................... 64 11 5-3 Field-effect and Hall mobilities as a function of back-gate voltage and temperature for monolayer and bilayer MoS 2 . .... ..... .... 66 6-1 Gate-controlled monolayer WSe 2 p-n junction diodes ......... 71 6-2 Current through the device as a function of doping configuration .. 75 6-3 Photodetection in monolayer WSe 2 ..... ...... ...... .. 77 6-4 Photovoltaic response and light emission ... ....... ...... 79 A-i MoS 2 flake AFM and step heights .. ............ ...... 84 A-2 Effect of annealing on two-terminal resistance of MoS 2 devices ... 85 A-3 Hall measurements of MoS 2 ....... ........ ........ 87 A-4 Contact resistance and four-terminal resistivity of monolayer device Mi 88 A-5 Room temperature mobilities and leakage current ..... ...... 88 B-1 Optical micrograph and AFM with step height of the main device . 92 B-2 Optical micrograph the EL device ..... ...... ...... ... 93 B-3 Temperature dependence of mid-gap current . ........ ..... 94 B-4 Photocurrent and reflected light image .. ..... .... ..... 97 B-5 Dependence of photovoltaic power generation on laser power ... .. 98 B-6 Reflected and emitted light image ..... ..... ..... ..... 98 12 List of Tables A. 1 Device Fabrication Parameters ..... ..... ..... ..... .. 84 A.2 Metal Insulator Transition . ....... ...... ...... .... 86 13 14 Chapter 1 Introduction 1.1 Motivation Electronic transport in atomically thin layered materials has been a burgeoning field of study since the discovery of isolated single layer graphene in 2004 [1]. Single layer graphene is a sheet of carbon atoms bonded together in a flat, hexagonal, honey- comb lattice, exactly one atom thick. The inherently two dimensional (2D) nature of graphene, as a one atom thick material, and its highly symmetric honeycomb lattice combine to give it a myriad of new and interesting properties. Graphene is light [2], strong [3], transparent [4], highly conductive [5], and flexible [3]. It has a unique gapless
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