Applied Science and Convergence Technology 2020; 29(6): 180-182
Published online November 30, 2020
Copyright © The Korean Vacuum Society.
Oh Hun Gwona, Jong Yun Kimb, Seok-Ju Kangb, and Young-Jun Yua,b,*
aDepartment of Physics, Chungnam National University, Daejeon 34134, Republic of Korea
bInstitute of Quantum Systems, Chungnam National University, Daejeon 34134, Republic of Korea
Correspondence to:E-mail: email@example.com
In this work, we investigated the variation in surface conditions of graphene by employing atomic force microscopy and optical images. We inspected the resistance and doping at selected areas of graphene by transport measurements. Especially, the area with a dominant contribution from the edges, which have more disorders and poly (methyl methacrylate) residues compared to the central surface of graphene, showed large resistance as well as large doping concentration. Our study facilitated the understanding of the electric conditions on graphene surface and edge areas.
Keywords: Graphene, Carrier density, Resistance, Transport characterization
Graphene, a two-dimensional carbon atom network, has been studied for applications in transparent and flexible electrodes. In particular, the formation of high-quality and large-size graphene with perfect carbon crystal is one of the major goals in research involving graphene [1–7]. Although the electric property of a graphene flake is characterized by measuring resistance and accumulated carrier density, these values are averaged over areas with varying condition such as disorders, and residues on graphene flake. Thus, we should understand the role of disordered areas on graphene flake. Several studies have carried out the spatial investigation of graphene surface by employing a scanning probe microscope (SPM) [8,9,12]. However, since the characterization using SPM has limitations for inspecting the direct conductance of graphene, electron transport characterization is preferred for direct measurement of the conductance of graphene [8–12]. In this work, we study the electric properties of partial areas of graphene, primarily focusing on either the center or the edge, by electron transport characterization. We observe that the graphene channel with a large contribution from the edge area shows relatively high resistance and increased p-doping level, due to more disorders and Poly(methyl methacrylate) (PMMA) residues.
For measuring the resistance, graphene flakes were mechanically exfoliated on a 280-nm-thick SiO2 substrate and Cr/Au (5 nm/30 nm thickness) electrodes were used to make electric contacts, as shown in Fig. 1(a) [4–7]. For making the contacts with Cr/Au electrodes, PMMA was employed as a mask in the e-beam lithography process and this PMMA was removed by acetone solution after evaporating Cr/Au metal. Each thickness of graphene was confirmed by inspection using a commercial atomic force microscope (AFM, XE-100, Park systems Corp.) [8,9,12]. The resistance of graphene as a function of back-gate voltage (
Figures 1(b) and 1(c) show the AFM image and height profile between SiO2 and graphene, respectively. Here, the graphene is confirmed as a bilayer with a height difference of ~ 0.6 nm between graphene and SiO2 [see Fig. 1(c)]. On this clean surface of bilayer graphene, the charge-neutral position (
As a result, we observed various transport curves with different resistance (
By comparing the optical image in Fig. 3 and resistance values in Fig. 4, we can speculate the electric conditions in each region of the graphene flake. For the area between electrodes 1 and 2, we obtained,
This manifests that although PMMA residues at the edge contribute equally to
In this work, we showed that the resistance and doping deviation in graphene depending on the position between the center area of the flake and the edge could be diagnosed by transport characterization. In particular, we observed that the edge leads to suppression of conductance and increased doping due to increasing disorders and PMMA residues, respectively. This result will help understand the origin of the difference in resistance and properties in different areas of graphene.
This work was supported by the research fund of Chungnam National University.