Applied Science and Convergence Technology 2024; 33(1): 7-12
Published online January 30, 2024
Copyright © The Korean Vacuum Society.
Department of Materials Science and Engineering, Chungnam National University, Daejeon 34134, Republic of Korea
†These authors equally contributed to this work.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (https://creativecommons.org/licenses/by-nc-nd/4.0/) which permits non-commercial use, distribution and reproduction in any medium without alteration, provided that the original work is properly cited.
This study investigated the variations in the structural and electrical characteristics of graphene under different O2 plasma treatment conditions and durations, employing Raman spectroscopy and Id−Vg and Id−Vd measurements. Initially, we examined the Raman spectra and the Id−Vg and Id−Vd curves of graphene following remote O2 plasma treatments ranging from 1 to 4 s. We observed the p-doping effects on the electrical properties of graphene. Subsequently, we transitioned to a reactive ion etching plasma treatment mode, followed by a comparative analysis to determine the most suitable plasma mode for enhancing the graphene properties without compromising its bonding network integrity. Notably, a shift in the Raman peak positions and intensities owing to the plasma treatment duration revealed that the high-energy plasma ions disrupted the symmetry of sp2 orbital bonds of graphene, leading to the formation of sp3 hybridization orbital bonds. Furthermore, we confirmed the restorative effects of the heat treatment by observing changes in the electrical characteristics when the plasma-treated graphene was annealed in a furnace.
Keywords: Two-dimensional material, Graphene, Transfer process, O2 plasma, Raman spectroscopy, Electrical properties, Dirac point, p-type doping
Two-dimensional (2D) materials, including graphene , WSe2, ReS2 , and MoS2, have attracted considerable attention owing to their distinctive electrical, optical, and mechanical characteristics. The- se properties make 2D materials, such as graphene valuable for applications in radio frequency (RF) devices, field-effect transistors (FETs), and photodetectors [3–5]. Moreover, the chemical vapor deposition (CVD) growth method facilitates the large-scale synthesis of 2D materials , providing controlled modulation of thickness and electronic characteristics [7–9]. Nevertheless, achieving precise control of these properties remains a persistent challenge. Suitable doping methods are required to control the electrical properties of graphene. However, conventional techniques often face obstacles as they may cause substantial damage to the graphene lattice structure during highenergy processes .
The O2 plasma treatments disrupt the C–C bonds in graphene, leading to the formation of various functional groups such as epoxy (-O-) and carboxyl (-COOH). These functional groups can transform the graphene surface from hydrophobic to hydrophilic because of their stronger interactions with water molecules (hydrogen bonding) than with carbon atoms (van der Waals bonding) . Consequently, the pristine
The primary objective of this study was to elucidate the correlation between this transition and the electrical properties of graphene. We employed low-energy O2 plasma treatment to facilitate the doping of graphene, enabling control over its electrical properties [14,15]. We aimed to tailor the lattice structure and electrical characteristics of graphene using Raman spectroscopy and current-voltage (
Our results indicate that the lattice structure of graphene undergoes a gradual transition from
A Cu foil (Alfa Aesar, 99.8 %) was subjected to pre-treatment with an acetic acid solution at 60 °C for 10 min followed by washing with deionized (DI) water. Then, the foil was heated to a graphene synthesis temperature of 1,030 °C and annealed for 3 h at a rate of 100 sccm (standard cubic centimeters per minute) of mixed gases, including 10 % H2 and 90 % Ar to remove native oxides from the Cu foil and enlarge its grains. For graphene growth, 1 sccm of methane (CH4) gas and 100 sccm of a mixed gas (10 % H2 and 90 % Ar) were introduced into a hotwall tubular inductive coupled plasma-CVD reaction chamber. The reactor pressure was maintained at 1 Torr for 20 min at an RF power of 200 W. The graphene synthesized on Cu was transferred to the target substrate using the conventional wet-transfer method, as described in the following section.
(1) The transfer process employed polymethyl methacrylate (PMM-A)  as the transfer film, which is a widely used method for transferring 2D materials  onto SiO2/Si substrates, as shown in Fig. 1(a). (2) Initially, PMMA was applied to the CVD-grown graphene on a Cu foil using a spin coater. The spin coating process involved two steps: 400 rpm for 10 s, followed by 6,000 rpm for 1 min. Subsequently, the PMMA-coated graphene/Cu foil underwent heating at 90°C for 1 min, after which it was placed on an etchant (ammonium persulfate) to selectively remove the Cu foil. Throughout this transfer process, the PMMA protected the graphene from physical damage. (3) Following the removal of the Cu foil, the PMMA/graphene film was rinsed four times with DI water to eliminate the residual etchant. The film floating on DI water was then carefully transferred onto a target substrate (SiO2/Si). Subsequently, the substrate with the transferred film was air dried for more than 12 h. (4) Subsequently, the PMMA/graphene/SiO2 substrate was immersed in a solvent (acetone) to dissolve and remove the PMMA layer, followed by rinsing with isopropyl alcohol (IPA) and DI water. (5) Finally, the graphene was successfully transferred onto the SiO2/Si substrate, followed by the formation of silver electrodes at the corners, as shown in Fig. 1(b).
After graphene transfer, O2 plasma treatments were performed using two distinct modes, remote and RIE, both employing the same bias power of 20 W. The flow rate and processing duration were set at 10 sccm and between 1 and 4 s, respectively, as shown in Fig. 2. This structural alteration manifested in observable changes in the lattice structure and electrical properties of graphene, as discerned through
Three electrodes–the source, drain, and gate–are essential to assess the electrical properties of graphene FETs. The Dirac point of graphene was observed by applying a current between the source and drain electrodes while applying gate biases to the SiO2/Si substrate. Under highvacuum conditions, the Dirac point of graphene is at
Raman characterization was performed using a Raman spectrometer (UniRAM, Micro Raman-PL sample chamber, grating 1200/500). The Raman spectra were accumulated five times, and the shift center value of the Raman mapping was set to 2,099.77 cm−1. The properties of graphene can be confirmed through Raman spectra involving the analysis of three distinct Raman peaks−the G, 2D, and D bands−to evaluate the characteristics of the material . Alterations in the peak positions can be indicative of the effects of plasma treatments, such as doping. The experimental parameters for the Raman measurements were set as follows: laser power of 100 mW, wavelength of 532 nm, signal accumulation of 4, and exposure time of 1 s.Raman mapping
Raman mapping was performed to assess the uniformity of the plasma-treated graphene surfaces. This involved acquiring mapping data by specifying the measurement area and size for Raman signal collection while the stage gradually adjusted its position during laser irradiation. Additionally, 3D images representing the
Plasma-treated graphene films exhibited higher resistance states when subjected to a 4-s treatment duration, in contrast to pristine graphene, as shown in the
The electrical characteristics of graphene, both before and after various plasma treatments of different durations, were assessed and compared using
Furthermore, with an increase in the plasma treatment time, a gradual shift in the Dirac point toward higher
Raman spectra were obtained, as shown in Fig. 6, to investigate the material properties of graphene with respect to the plasma treatment duration. Before plasma treatment, the Raman peaks of pristine graphene were identified (black spectra). Subsequently, Raman peak measurements were conducted after the remote and RIE plasma treatments for 1–4 s. With increasing plasma treatment duration, both the remote and RIE plasma treatments led to an increase in the D peak and a decrease in the 2D peak. This phenomenon can be attributed to the bonding of oxygen and hydrogen atoms to dangling bonds resulting from the breaking of C–C bonds during the O2 plasma treatment. These defect-mediated Raman scattering events gave rise to enhanced D-bands. Moreover, the collapse of
Despite the fewer induced defects, the blue-shift in the G peak was more pronounced in the graphene treated with RIE plasma, with a shift of 10.8 cm−1, compared to that of the remote plasma-treated graphene (8.9 cm−1), indicating stronger doping effects in the case of RIE plasma treatment. Figure 7 shows 2D frequency graphs in relation to the G frequency, demonstrating a linear fit with respect to both doping and strain in graphene subjected to both remote and RIE plasma treatments. The graphs reveal the presence of doping effects along with a slight change in strain. The observed strain is likely attributed to functional groups or defects induced by the plasma treatments.
Raman mapping was employed to examine the alterations in the graphene surface following plasma treatment, as illustrated in Figs. 8 and 9. The mapping images depicting the ratio reveal notable changes in the C–C bond structure, which is characterized by a reduction in the
The use of RTA equipment can facilitate the defect healing process in graphene . The experimental results for Raman spectroscopy and the
In this study, we investigated the changes in the structural and electrical properties of graphene using various O2 plasma treatment methods and durations . The analysis of the overall characteristics of the material indicated that the remote-mode plasma treatment led to a more pronounced and sudden shift in the Dirac point and resistance, which was attributed to the stronger intensity of remote-mode plasma compared to that of RIE plasma. Furthermore, recovery was observed during the thermal treatment. We observed the electrical characteristics of graphene following O2 plasma treatment, anticipating that such recovery effects could contribute to the advancement of high-performance 2D FETs [6,12,29], fostering the exploration of alternative materials to Si.
This work was supported by the Sejong Science Fellowship (2022R1- C1C2005607), Regional Innovation Strategy (RIS, 2021RIS-004), and Basic Science Research Program (2021R1I1A1A01057416, 2021R1A6-A1A03043682) funded by the National Research Foundation of Korea (NRF).
The authors declare no conflicts of interest.