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Scanning Gate Microscopy on Graphene: Charge Inhomogeneity and Extrinsic Doping Romaneh Jalilian Purdue University

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Scanning Gate Microscopy on Graphene: Charge Inhomogeneity and Extrinsic Doping Romaneh Jalilian Purdue University Purdue University Purdue e-Pubs Birck and NCN Publications Birck Nanotechnology Center 7-22-2011 Scanning gate microscopy on graphene: charge inhomogeneity and extrinsic doping Romaneh Jalilian Purdue University Luis A. Jauregui Purdue University - Main Campus Gabriel Lopez Purdue University, [email protected] Jifa Tian Purdue University, [email protected] Caleb Roecker Purdue University, [email protected] See next page for additional authors Follow this and additional works at: http://docs.lib.purdue.edu/nanopub Part of the Nanoscience and Nanotechnology Commons Jalilian, Romaneh; Jauregui, Luis A.; Lopez, Gabriel; Tian, Jifa; Roecker, Caleb; Yazdanpanah, Mehdi; Cohn, Robert; Jovanovic, I.; and Chen, Yong P., "Scanning gate microscopy on graphene: charge inhomogeneity and extrinsic doping" (2011). Birck and NCN Publications. Paper 749. http://docs.lib.purdue.edu/nanopub/749 This document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact [email protected] for additional information. Authors Romaneh Jalilian, Luis A. Jauregui, Gabriel Lopez, Jifa Tian, Caleb Roecker, Mehdi Yazdanpanah, Robert Cohn, I. Jovanovic, and Yong P. Chen This article is available at Purdue e-Pubs: http://docs.lib.purdue.edu/nanopub/749 Home Search Collections Journals About Contact us My IOPscience Scanning gate microscopy on graphene: charge inhomogeneity and extrinsic doping This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2011 Nanotechnology 22 295705 (http://iopscience.iop.org/0957-4484/22/29/295705) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 128.46.220.6 The article was downloaded on 20/07/2011 at 21:14 Please note that terms and conditions apply. IOP PUBLISHING NANOTECHNOLOGY Nanotechnology 22 (2011) 295705 (9pp) doi:10.1088/0957-4484/22/29/295705 Scanning gate microscopy on graphene: charge inhomogeneity and extrinsic doping Romaneh Jalilian1,2,7, Luis A Jauregui2,3, Gabriel Lopez2,3,8, Jifa Tian1,2, Caleb Roecker4, Mehdi M Yazdanpanah5,6, Robert W Cohn6, Igor Jovanovic4,9 and Yong P Chen1,2,3 1 Department of Physics, Purdue University, West Lafayette, IN 47907, USA 2 Birck Nanotechnology Center, Purdue University, West Lafayette, IN 47907, USA 3 School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN 47907, USA 4 School of Nuclear Engineering, Purdue University, West Lafayette, IN 47907, USA 5 NaugaNeedles LLC, Louisville, KY 40299, USA 6 Department of Electrical Engineering, University of Louisville, Louisville, KY 40292, USA E-mail: [email protected] Received 11 March 2011, in final form 2 May 2011 Published 16 June 2011 Online at stacks.iop.org/Nano/22/295705 Abstract We have performed scanning gate microscopy (SGM) on graphene field effect transistors (GFET) using a biased metallic nanowire coated with a dielectric layer as a contact mode tip and local top gate. Electrical transport through graphene at various back gate voltages is monitored as a function of tip voltage and tip position. Near the Dirac point, the response of graphene resistance to the tip voltage shows significant variation with tip position, and SGM imaging displays mesoscopic domains of electron-doped and hole-doped regions. Our measurements reveal substantial spatial fluctuation in the carrier density in graphene due to extrinsic local doping from sources such as metal contacts, graphene edges, structural defects and resist residues. Our scanning gate measurements also demonstrate graphene’s excellent capability to sense the local electric field and charges. (Some figures in this article are in colour only in the electronic version) 1. Introduction graphene field effect transistor (GFET), a voltage applied to a gate (capacitively coupled to graphene) can tune the charge The past few years have witnessed intensive research carriers (effectively tuning the Fermi energy relative to the DP) on graphene (2D carbon), a single layer graphite with from p-type (holes) to n-type (electrons), with the graphene unique electronic properties [1–3] and exciting promise for resistance peaking at the charge-neutral DP [1, 3]. Such an applications ranging from nanoelectronics to sensors. As a ambipolar electric field effect, which can exhibit high mobility zero-gap semiconductor (semimetal), graphene’s conduction even at room temperature, underlies the operation of most and valence bands touch at the charge-neutral Dirac point graphene devices. The finite minimum conductivity (which (DP) with relativistic linear energy–momentum dispersion [1]. varies from sample to sample and shows discrepancies with Intrinsically charge-neutral, graphene can be easily doped earlier theories [1]) experimentally observed in graphene at the electrically or chemically [3–6]. For example, in a typical DP has been a subject of much discussion. It is now realized that the values of minimum conductivity measured in realistic 7 Present address: NaugaNeedles LLC, Louisville, KY 40299, USA. 8 Present address: Sandia National Laboratories, Livermore, CA 94550, USA. samples can be largely related to charge inhomogeneity [7, 8] 9 Present address: Department of Mechanical and Nuclear Engineering, in graphene, where the local charge density remains finite The Pennsylvania State University, State College, PA 16802, USA. in the form of electron and hole puddles even at zero 0957-4484/11/295705+09$33.00 1 © 2011 IOP Publishing Ltd Printed in the UK & the USA Nanotechnology 22 (2011) 295705 R Jalilian et al Figure 1. (a) Schematic of the experimental set up for contact mode scanning gate microscopy (SGM) on graphene. (b) SEM image of a custom-made contact mode SGM tip. (c) Magnified view of the end of the tip, showing a conductive Ag2Ga nanowire surrounded by parylene (dielectric) coating. (d) AFM image (tapping mode) of a graphene covered by residues from the device fabrication process. (e) Image of the same device in (d) after ‘nano-broom’ cleaning by contact mode AFM. (f) The field effect (resistance versus back gate voltage) of the GFET device before (d) and after (e) AFM cleaning. average charge carrier density at the (global) charge-neutral (SGM) to probe the local electronic properties and charge DP. Various sources, such as topographic corrugations inhomogeneity in graphene (exfoliated and supported on (e.g. ripples of graphene) [9], charged impurities [10, 11], SiO2). In SGM [21–29], a charged tip is used as a movable adsorbed molecules [5], surface contaminants [12]andmetal local top gate to modulate the carrier density underneath contacts [6, 13] have been suggested that could cause local the tip in a device whose resistance (or conductance) is doping and thus inhomogeneous charge density in graphene. measured. Scanning the tip (top gate) over the device The length scales of the resulting charge puddles and doping generates a map showing how the electrical resistance domains can range from nanometers (e.g. in the case of (or conductance) through the device depends on the tip- charged impurities [11]) to microns (e.g. in the case of metal induced local density modulation (local potential) at various contact [13]). locations. SGM has been previously applied to study While transport experiments can explore the signatures of the local electronic properties and defects in semiconductor inhomogeneous charge density and doping in graphene [14–16], nanostructures [21–24], nanowires [25] and carbon nanotubes scanning probe microscopy (SPM) measurements provide the (CNT) [26–29]. Similar techniques have recently been most direct probe of local electronic properties. Martin applied to graphene for studying the effects of local scattering et al demonstrated the formation of sub-micron (resolution potential on the quantum interference of carriers [30], imaging limited) electron and hole charge puddles near the DP localized states in graphene nanostructures [31], studying in graphene using scanning single-electron transistor (SET) effects of current annealing on electronic uniformity [32], and microscopy, and inferred the intrinsic size of the puddles to be ∼30 nm from measurements in the quantum Hall performing various other analyses of graphene layers [33]. regime [8]. High-resolution scanning tunneling microscopy Previous SGM measurements were typically performed in the (STM) experiments have directly imaged charge puddles ‘lift mode’, where a biased conductive AFM tip is kept at a of ∼10 nm in size, with the suggestion that they constant height (with possibly a small modulation [34]) above originate from individual charged impurities underneath the device surface and the corresponding sample conductance graphene [11, 17]. Scanning photocurrent microscopy (SPCM) response is measured. Sensitivity, spatial resolution (nonlocal revealed modulations in the electrical potential across the effects of the tip), measurement synchronization, tip drift over graphene (particularly near electrodes) and gave evidence time and potential tip–sample shortage (due to electrostatic for metal-induced doping [18, 19], although alternative attraction) are some of the common challenges [22, 34]. interpretations of SPCM data in terms of photothermal electric In our work, we used an alternative method to perform effects (PTE) have also been suggested [20]. SGM based on contact mode AFM with a dielectric-coated In this paper, we report our study using atomic metallic nanowire as the tip (figures 1(a)–(c)). This scheme force microscope (AFM) based scanning
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