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Inhomogeneous work-function hysteresis in chemical vapor deposition-grown graphene field-effect devices
Carbon 173 (2021) 594e599
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Inhomogeneous work-function hysteresis in chemical vapor deposition-grown graphene field-effect devices Hwi Je Woo a, Seongchan Kim a, Young-Jin Choi a, Jeong Ho Cho b, Seong Heon Kim c, *, Young Jae Song a, d, e, f, ** a
SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University (SKKU), Suwon, 16419, South Korea Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul, 03722, South Korea Department of Physics, Myongji University, Yongin, 17058, South Korea d Department of Nano Engineering, Sungkyunkwan University (SKKU), Suwon, 16419, South Korea e Department of Physics, Sungkyunkwan University (SKKU), Suwon, 440-746, Korea f Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS), Sungkyunkwan University (SKKU), Suwon, 440-746, Korea b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 September 2020 Received in revised form 14 November 2020 Accepted 17 November 2020 Available online 23 November 2020
The work function of graphene is known to be tuned by gate voltage on SiO2 substrates. For a graphene field-effect transistor (FET), chemical species such as oxygen and water molecules induce electrostatic interactions between the graphene and SiO2 substrate, and consequently, the conductance of graphene can be shifted with respect to the applied field effect. Furthermore, these shifts cause an inevitable hysteresis in the IeV characteristics of the graphene device, which degrades its performance. Herein, we study the work-function of graphene devices on SiO2 substrates with chemical vapor deposition-grown graphene using Kelvin probe force microscopy and report the gate voltage-dependent work-function hysteresis, which is analogous to the hysteresis in the electronic transport of graphene FETs. The degree of work-function hysteresis is inhomogeneous depending on the positions on the graphene, and it originates from the inhomogeneous distribution of the chemical species such as H2O and O2 molecules at the interface of a graphene/SiO2 substrate. This inhomogeneity of chemical species seems to be moderated by the gate voltage sweeping during the gate-voltage-dependent work-function measurements. The proposed study provides an advanced understanding of hysteresis phenomena in graphene devices and the guidance for developing controlled graphene devices with minimal influence from ambient conditions. © 2020 Elsevier Ltd. All rights reserved.
Keywords: Graphene Kelvin probe force microscopy Work-function Hysteresis
1. Introduction The exfoliation of a single graphene layer from highly oriented pyrolytic graphite, which was achieved for the first time in 2004 by the Manchester group, opened the era of graphene research in various science and engineering fields, including physics, chemistry, and materials science [1e3]. For decades, graphene (a monolayer of the honeycomb carbon structure) has been one of the most promising materials for applications in several types of nextgeneration devices, especially electronic devices such as fieldeffect transistors (FETs) owing to their outstanding optical,
* Corresponding author. ** Corresponding author. SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University (SKKU), Suwon, 16419, South Korea. E-mail addresses: [email protected] (S.H. Kim), [email protected] (Y.J. Song). https://doi.org/10.1016/j.carbon.2020.11.056 0008-6223/© 2020 Elsevier Ltd. All rights reserved.
electrical, and mechanical properties [4e7]. In particular, the development of a chemical vapor deposition (CVD) method for the reliable synthesis of large-scale graphene sheets drastically increased the expectation for the mass production of graphene devices in the semiconductor industry [8e15]. Several high performances graphene electronic devices have been fabricated using the CVD-grown graphene sheets, although several issues limit its industrial production [16e20]. In most cases, graphene FET devices are fabricated on SiO2 substrates, which are abundant and provide a simple insulator (SiO2) and back-gate (Si) structure. However, several studies have reported that SiO2 substrates are detrimental for graphene devices under ambient conditions [21e25]. The critical disadvantage of a SiO2 substrate is the gate voltage-dependent hysteresis in the electronic transport properties of graphene FETs in ambient air [23e25]. Indeed, a similar hysteresis of conductance has been
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Carbon 173 (2021) 594e599
Fig. 1. (a) Topography of the graphene FET device surface and its variation in height less than 6 nm. The scale bar represents 2.5 mm. (b) Raman spectra of the graphene FET device surface as a function of tip-source electrode distance; black line for 0 mm, red line for 160 mm away from the source electrode. (c) Schematic cross-sectional device structure of a typical bottom-gate graphene transistor. Each deposited Au electrode was 20 nm thick. (d) Transfer characteristics of the graphene transistor measured through an IeV curve indicated a p-type. The curves were obtained from 20 mm (blue) and 50 mm (red) channel length. (A colour version of this figure can be viewed online.)
modulated by the physical or chemical adsorption of various chemical species [35e39]. Therefore, it is possible that the chemical species that induce gate-dependent conductance hysteresis in graphene FETs can influence the characteristics of the graphene work function. In this study, we investigated the work-function characteristics in a graphene-based field-effect device using Kelvin probe force microscopy (KPFM), which is an atomic force microscopy (AFM)-based powerful technique to study the surface potential, and consequently, the work-function at the nanoscale. In addition, we demonstrated that the work-function also has gate voltage-dependent hysteresis characteristics in graphene fieldeffect devices. Furthermore, the degree of work-function hysteresis is spatially inhomogeneous because we could find specific positions that show prominent work-function hysteresis on the graphene. This inhomogeneity originates from the inhomogeneous distribution of the doping species at the interface of a graphene/ SiO2 substrate, and it seemed to be moderated by the gate voltage sweeping during the gate voltage-dependent work-function measurements. This study provides an advanced understanding of hysteresis phenomena in graphene-based field-effect devices and guidance for developing controlled graphene devices with a minimal influence from ambient conditions.
reported in studies based on carbon nanotube (CNT) FETs [26,27]. Research has shown that the hysteresis of graphene FET conductance can be effectively suppressed by applying additional treatments to the graphene FET devices, including vacuum treatment, annealing, introduction of hydrophobic polymer layers between graphene and SiO2, and encapsulation using various protection layers [23,28e32]. However, most solutions for resolving the conductance hysteresis still require an additional fabrication process, which can increase the fabrication time and cost. Several studies have reported possible mechanisms to explain the conductance hysteresis in graphene FET devices [23,24,30,33,34]. Among them, a highly plausible mechanism is charge transfer by doping species such as H2O and O2 from ambient air [24,25,31,33,34]. In particular, the controlled Raman spectroscopy experiments reported by Xu et al. found that the exposure of graphene FETs to a mixture of O2 and H2O brings a heavier doping and larger hysteresis feature than the exposure to O2 or H2O alone. They also experimentally demonstrated that the SiO2 substrate is involved in the hysteresis feature of graphene FETs in ambient air [25]. Based on these findings, they concluded that the doping species (H2O and O2) are bonded at the interface of a graphene/SiO2 substrate by hydrogen bonding with SieOH groups on the SiO2 surface, and the electron transfer from graphene to the O2/H2O redox couple through electrochemical redox reactions induces pdoping and gate-dependent conductance hysteresis in graphene FETs [25]. The work function of graphene is an important factor in graphene-based devices, since it affects the device performance. Research has shown that the work function of graphene can be
2. Material and methods Graphene samples were synthesized through low-pressure chemical vapor deposition on a Cu foil (Alpha Aesar) with a gas mixture of CH4 and H2 under 1000 C. To transfer the synthesized graphene, a Cu foil was etched with a 0.2 M aqueous solution of 595
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Fig. 2. (a) Schematic of AFM/KPFM measurement on the graphene FET. (b) KPFM line scan 2D map images at positions of 0 mm (left) and 160 mm (right) away from the source electrode, which represents different gate voltages. Fast scan direction is þx, slow scan was conducted from bottom to top. The scale bar is 2.5 mm and 170 pixels in vertical. (A colour version of this figure can be viewed online.)
at least one conductive component. To measure field effect on a graphene surface through SiO2, a Si substrate was used as a backgate connected to a Keithley 4310 unit for sweeping the gate voltage. All AFM-based measurements were conducted with a commercial atomic force microscope (NX-10, Park Systems Corp.) under ambient conditions.
ammonium persulfate with a poly-methyl-methacrylate (PMMA) supporting layer. After removing the Cu foil, the graphene with PMMA was then transferred onto a SiO2/Si wafer. The PMMA supporting layer was removed using acetone. The graphene was then patterned via typical photolithography with a reactive ion etching (RIE) process for 5 s at 50 W. To fabricate the graphene field-effect devices shown in Figs. 1c and 2a, Au electrodes were fabricated on the transferred graphene using a thermal evaporator. The position dependent quality of the graphene was inspected by Raman spectroscopy (XperRam 200, Nanobase) excited with a 532 nm laser. We measured the electronic transport properties of our graphene FET devices. The electrical properties of the graphene transistors were measured using a Keithley 4200A under ambient conditions. The 20 nm thick Au patterns (source and drain electrodes) were deposited on a SiO2 wafer by a thermal evaporator through a shadow mask. The channel lengths between the Au patterns were 20 or 50 mm, and the width was 1000 mm. Furthermore, to spatially map the work-function characteristics on graphene-based field-effect devices, we performed KPFM, which can acquire work-function information by measuring the contact potential difference (CPD), i.e., the relative surface potential of an AFM tip to that of a sample, for every pixel on the graphene surface. CPD map is simultaneously obtained with a topographic image of the sample surface via a tapping mode at 115 kHz. Metallic or metal-coated AFM tips (NSC36_B, MicroMasch) are generally used for KPFM measurements because CPD measurement requires
3. Results and discussion We employed a graphene-based field-effect device with a single top (source) electrode to investigate the graphene work-function characteristics as a function of the distance from the source electrode in a large-scale area and on the gate voltages. Fig. 1a shows the typical AFM topographic image of the CVD-grown graphene with RMS roughness of 1.237 nm, and its quality was inspected by Raman spectra collected at 532 nm (Fig. 1b) as a function of distance from the source electrode. In the Raman spectra, the characteristic G and 2D peaks of graphene are clearly measured at 1575 and 2673 cm1, respectively. A commonly used method to estimate the layer number of graphene is to calculate the ratio of G and 2D peak intensities (I2D/IG) or the full width at half maximum (FWHM) of the 2D peak in the Raman spectrum [40]. The I2D/IG is ~1 and FWHM of the 2D peaks ranges from ~50 to ~75 cm1 in Fig. 1b, which corresponds to bilayer or trilayer graphene [40]. In addition, to inspect the transfer characteristics of our graphene devices, we fabricated typical graphene FET devices with different channel 596
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transfer from the source electrode induce a band bending on graphene. To precisely investigate the CPD and consequent graphene work-function as a function of both the gate voltage and the distance from the source electrode, we performed KPFM measurements on the graphene sheet by sweeping the gate voltages in the range of 40 to 40 V and changing the tip-source distance from 0 to 160 mm at intervals of 20 mm; the results are shown in Fig. 3a. The CPD variation tendency with sweeping gate voltages, previously discussed in Fig. 2b, is clearly shown at all measurement positions in Fig. 3a. Note the distance-dependent inhomogeneity of CPD at the initial 40 V gate voltage. In the bottom line (red color) on the left-side panel of Fig. 3a, the CPD at the initial 40 V does not increase monotonically as the tip-source distance increases, and three indents are distinctly observed at the positions of 20, 60, and 120 mm. This inhomogeneity implies an inhomogeneous spatial distribution of the doping species within the graphene sheet. It is also interesting that the spatial inhomogeneity of CPD at the initial 40 V seems to be moderated by the gate voltage sweeping during the gate voltage-dependent work-function measurements using KPFM. The dents in the bottom line of the left-side panel of Fig. 3a fade away by sweeping the gate voltage, and eventually the CPD at the final 40 V on the right-side panel of Fig. 3a almost linearly increases as the tip-source distance increases. Another important feature of the CPD characteristics related to the tip-source distance and the gate voltage is that the CPD did not completely recover after the gate voltage sweeping, as clearly represented by the thick blue and red arrows in Fig. 3a. The increasing CPD after completion of the gate voltage sweeping indicates that the work-function decreased, as depicted in Fig. 3b. Furthermore, the irreversibility of CPD or work-function implies the presence of hysteresis characteristics. The CPD data in Fig. 3a were replotted in Fig. 4a, wherein the x-axis was changed to the gate voltage. All curves at different tip-source distances obviously show hysteresis characteristics that are highly analogous to the previously reported conductance hysteresis, while the curve tends to shift upward with increasing tip-source distance. The workfunction of bi- or trilayer graphene shows linear behavior depending on gate voltages, which is well defined in a previous study [41]. Fig. 4b shows the 2D map of CPD hysteresis gaps, wherein the CPD values in the forward gate voltage sweeping (from 40 to 40 V) were subtracted from the corresponding CPD values in the backward sweeping (from 40 to 40 V). The CPD difference (DCPD) values were plotted against the tip-source distance (x-axis) and gate voltage (y-axis). This map (Fig. 4a) represents the spatial distribution of the degree of CPD (or workfunction) hysteresis on the graphene device. The degree of the work-function hysteresis was spatially inhomogeneous on the graphene device, as shown by the specific positions with prominent CPD hysteresis indicated by three green arrows in Fig. 4b. These arrows coincide with the positions of three dents (20, 60, and 120 mm) in Fig. 3b (left) because the origin of this spatially inhomogeneous work-function hysteresis was the previously discussed nonmonotonic feature of spatial CPD variation at an initial gate voltage of 40 V. Therefore, the inhomogeneous distribution of the work-function hysteresis within the graphene device originates from the inhomogeneous distribution of the doping species between the graphene and the SiO2 substrate, which can induce the conductance hysteresis. As depicted in Fig. 4c, the higher density of doping species between the graphene and SiO2 substrate induces higher electron transfer and doping and vice versa. In addition, as previously discussed in Fig. 3a, the inhomogeneous distribution of the doping species tends to be moderated by the gate voltage sweeping with the KPFM measurements. When back gate voltage increases from 40 V to 40 V, the charge is injected to adsorbates.
Fig. 3. (a) CPD between tip and graphene surface plotted against distance from the source electrode, depending on the gate voltage from 40 to 40 V (left) and from 40 to 40 V (right). Each line is the averaged value for 10 pixels in y axis of Fig. 2b. (b) Consequently, CPD increased around 200 meV before and after the gate voltage was induced, thereby decreasing the work function. (A colour version of this figure can be viewed online.)
lengths (20 and 50 mm), as depicted in Fig. 1c. The graphene was patterned using conventional photolithography and RIE. The transport characteristics (drain current versus gate voltage) exhibited p-type behavior with large hysteresis and a positive Dirac point (approximately þ 150 V), as shown in Fig. 1d. The large hysteresis can be explained by the highly doped species between SiO2 and the bilayer graphene [24,25,31,33,34]. In addition, as the channel length increased, the hysteresis characteristic tended to become intense because of the doping species, as mentioned previously. Fig. 2a shows a schematic of the KPFM measurements on the graphene FET with a single top electrode. The back-gate voltage was swept in the range of 40 to 40 V. Fig. 2b shows 2D maps of gate voltage-dependent CPD acquired at two positions on graphene in the vicinity of and 160 mm away from the source electrode. The x and y axes in Fig. 2b correspond to the line-scanned CPD and the swept-gate voltages, respectively. In Fig. 2b, with the forward sweeping of gate voltages from 40 to 40 V followed by backward sweeping from 40 to 40 V, the CPD values increased and reached the maximum point between 40 V and 0 V, and then decreased. This CPD variation tendency is almost the same in the two maps in Fig. 2b, although the map at the 160 mm position shows relatively higher CPD values (lower work function) than that at the 0 mm position. This position-dependent CPD change would be originated from Fermi-level alignment and charge transfer between graphene and electrode. Our experimental study, therefore, shows that the capacitive coupling with the back-gate electrode and the charge 597
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Fig. 4. (a) CPD is replotted versus gate voltage to show the clear hysteresis of work function. Despite different tip positions, all curves show obvious hysteresis in CPD. The maximum hysteresis gap occurs at a gate voltage of 10 V. (b) The spatial distribution of CPD differences. Three indents at 20, 60, and 120 mm in Fig. 3b indicates harsh variance adjacent to 10 V. (c) Schematic of doping species between graphene and SiO2 boundary that can be transferred between electrons or dopants. (A colour version of this figure can be viewed online.)
review & editing. Young Jae Song: Supervision, Investigation, Writing - original draft, Writing - review & editing.
As a result, there is no space for charge to be injected to adsorbates which means the work function of graphene tuned by back gate is saturated. On the other hand, when back gate voltage decrease from 40 V to 40 V, under enough fast back gate sweeping speed, adsorbates cannot have sufficient time to charge relaxation [42].
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
4. Conclusions In this study, we investigated the work-function characteristics of graphene-based field-effect devices. It was demonstrated that the work-function has gate voltage-dependent hysteresis characteristics in graphene field-effect devices, analogous to the hysteresis in the electronic transport of graphene FETs. Furthermore, the spatial inhomogeneity in the degree of work-function hysteresis is observed, which originates from the inhomogeneous distribution of the doping species at the interface of a graphene/SiO2 substrate. The inhomogeneous feature tends to be moderated by gate voltage sweeping during the gate-voltage-dependent work-function measurements using KPFM. This study provides an advanced understanding of hysteresis characteristics in graphene-based field-effect devices and guidance for developing controlled graphene devices with minimal influence from ambient conditions.
Acknowledgments SHK was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (Grant No. 2020R1A2C1005299). YJS was supported by the Basic Science Research Program (Grant No. 2015M3A7B4050455), by Institute for Basic Science (Grant No. IBS-R011-D1) through the National Research Foundation (NRF) funded by the Ministry of Science and ICT (MSIT) in South Korea and the SRC Center for Topological Matter (Grant No. 2018R1A5A6075964) through the National Research Foundation (NRF) funded by the Ministry of Science and ICT (MSIT) in South Korea, by Industrial Strategic Technology Development Program (Grant No. 10085617) funded by the Ministry of Trade Industry & Energy (MOTIE) in South Korea.
CRediT authorship contribution statement References Hwi Je Woo: Investigation, Formal analysis, Writing - original draft, Writing - review & editing. Seongchan Kim: Formal analysis. Young-Jin Choi: Formal analysis. Jeong Ho Cho: Formal analysis. Seong Heon Kim: Supervision, Writing - original draft, Writing -
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Inhomogeneous work-function hysteresis in chemical vapor deposition-grown graphene field-effect devices Hwi Je Woo a, Seongchan Kim a, Young-Jin Choi a, Jeong Ho Cho b, Seong Heon Kim c, *, Young Jae Song a, d, e, f, ** a
SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University (SKKU), Suwon, 16419, South Korea Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul, 03722, South Korea Department of Physics, Myongji University, Yongin, 17058, South Korea d Department of Nano Engineering, Sungkyunkwan University (SKKU), Suwon, 16419, South Korea e Department of Physics, Sungkyunkwan University (SKKU), Suwon, 440-746, Korea f Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS), Sungkyunkwan University (SKKU), Suwon, 440-746, Korea b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 September 2020 Received in revised form 14 November 2020 Accepted 17 November 2020 Available online 23 November 2020
The work function of graphene is known to be tuned by gate voltage on SiO2 substrates. For a graphene field-effect transistor (FET), chemical species such as oxygen and water molecules induce electrostatic interactions between the graphene and SiO2 substrate, and consequently, the conductance of graphene can be shifted with respect to the applied field effect. Furthermore, these shifts cause an inevitable hysteresis in the IeV characteristics of the graphene device, which degrades its performance. Herein, we study the work-function of graphene devices on SiO2 substrates with chemical vapor deposition-grown graphene using Kelvin probe force microscopy and report the gate voltage-dependent work-function hysteresis, which is analogous to the hysteresis in the electronic transport of graphene FETs. The degree of work-function hysteresis is inhomogeneous depending on the positions on the graphene, and it originates from the inhomogeneous distribution of the chemical species such as H2O and O2 molecules at the interface of a graphene/SiO2 substrate. This inhomogeneity of chemical species seems to be moderated by the gate voltage sweeping during the gate-voltage-dependent work-function measurements. The proposed study provides an advanced understanding of hysteresis phenomena in graphene devices and the guidance for developing controlled graphene devices with minimal influence from ambient conditions. © 2020 Elsevier Ltd. All rights reserved.
Keywords: Graphene Kelvin probe force microscopy Work-function Hysteresis
1. Introduction The exfoliation of a single graphene layer from highly oriented pyrolytic graphite, which was achieved for the first time in 2004 by the Manchester group, opened the era of graphene research in various science and engineering fields, including physics, chemistry, and materials science [1e3]. For decades, graphene (a monolayer of the honeycomb carbon structure) has been one of the most promising materials for applications in several types of nextgeneration devices, especially electronic devices such as fieldeffect transistors (FETs) owing to their outstanding optical,
* Corresponding author. ** Corresponding author. SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University (SKKU), Suwon, 16419, South Korea. E-mail addresses: [email protected] (S.H. Kim), [email protected] (Y.J. Song). https://doi.org/10.1016/j.carbon.2020.11.056 0008-6223/© 2020 Elsevier Ltd. All rights reserved.
electrical, and mechanical properties [4e7]. In particular, the development of a chemical vapor deposition (CVD) method for the reliable synthesis of large-scale graphene sheets drastically increased the expectation for the mass production of graphene devices in the semiconductor industry [8e15]. Several high performances graphene electronic devices have been fabricated using the CVD-grown graphene sheets, although several issues limit its industrial production [16e20]. In most cases, graphene FET devices are fabricated on SiO2 substrates, which are abundant and provide a simple insulator (SiO2) and back-gate (Si) structure. However, several studies have reported that SiO2 substrates are detrimental for graphene devices under ambient conditions [21e25]. The critical disadvantage of a SiO2 substrate is the gate voltage-dependent hysteresis in the electronic transport properties of graphene FETs in ambient air [23e25]. Indeed, a similar hysteresis of conductance has been
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Fig. 1. (a) Topography of the graphene FET device surface and its variation in height less than 6 nm. The scale bar represents 2.5 mm. (b) Raman spectra of the graphene FET device surface as a function of tip-source electrode distance; black line for 0 mm, red line for 160 mm away from the source electrode. (c) Schematic cross-sectional device structure of a typical bottom-gate graphene transistor. Each deposited Au electrode was 20 nm thick. (d) Transfer characteristics of the graphene transistor measured through an IeV curve indicated a p-type. The curves were obtained from 20 mm (blue) and 50 mm (red) channel length. (A colour version of this figure can be viewed online.)
modulated by the physical or chemical adsorption of various chemical species [35e39]. Therefore, it is possible that the chemical species that induce gate-dependent conductance hysteresis in graphene FETs can influence the characteristics of the graphene work function. In this study, we investigated the work-function characteristics in a graphene-based field-effect device using Kelvin probe force microscopy (KPFM), which is an atomic force microscopy (AFM)-based powerful technique to study the surface potential, and consequently, the work-function at the nanoscale. In addition, we demonstrated that the work-function also has gate voltage-dependent hysteresis characteristics in graphene fieldeffect devices. Furthermore, the degree of work-function hysteresis is spatially inhomogeneous because we could find specific positions that show prominent work-function hysteresis on the graphene. This inhomogeneity originates from the inhomogeneous distribution of the doping species at the interface of a graphene/ SiO2 substrate, and it seemed to be moderated by the gate voltage sweeping during the gate voltage-dependent work-function measurements. This study provides an advanced understanding of hysteresis phenomena in graphene-based field-effect devices and guidance for developing controlled graphene devices with a minimal influence from ambient conditions.
reported in studies based on carbon nanotube (CNT) FETs [26,27]. Research has shown that the hysteresis of graphene FET conductance can be effectively suppressed by applying additional treatments to the graphene FET devices, including vacuum treatment, annealing, introduction of hydrophobic polymer layers between graphene and SiO2, and encapsulation using various protection layers [23,28e32]. However, most solutions for resolving the conductance hysteresis still require an additional fabrication process, which can increase the fabrication time and cost. Several studies have reported possible mechanisms to explain the conductance hysteresis in graphene FET devices [23,24,30,33,34]. Among them, a highly plausible mechanism is charge transfer by doping species such as H2O and O2 from ambient air [24,25,31,33,34]. In particular, the controlled Raman spectroscopy experiments reported by Xu et al. found that the exposure of graphene FETs to a mixture of O2 and H2O brings a heavier doping and larger hysteresis feature than the exposure to O2 or H2O alone. They also experimentally demonstrated that the SiO2 substrate is involved in the hysteresis feature of graphene FETs in ambient air [25]. Based on these findings, they concluded that the doping species (H2O and O2) are bonded at the interface of a graphene/SiO2 substrate by hydrogen bonding with SieOH groups on the SiO2 surface, and the electron transfer from graphene to the O2/H2O redox couple through electrochemical redox reactions induces pdoping and gate-dependent conductance hysteresis in graphene FETs [25]. The work function of graphene is an important factor in graphene-based devices, since it affects the device performance. Research has shown that the work function of graphene can be
2. Material and methods Graphene samples were synthesized through low-pressure chemical vapor deposition on a Cu foil (Alpha Aesar) with a gas mixture of CH4 and H2 under 1000 C. To transfer the synthesized graphene, a Cu foil was etched with a 0.2 M aqueous solution of 595
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Fig. 2. (a) Schematic of AFM/KPFM measurement on the graphene FET. (b) KPFM line scan 2D map images at positions of 0 mm (left) and 160 mm (right) away from the source electrode, which represents different gate voltages. Fast scan direction is þx, slow scan was conducted from bottom to top. The scale bar is 2.5 mm and 170 pixels in vertical. (A colour version of this figure can be viewed online.)
at least one conductive component. To measure field effect on a graphene surface through SiO2, a Si substrate was used as a backgate connected to a Keithley 4310 unit for sweeping the gate voltage. All AFM-based measurements were conducted with a commercial atomic force microscope (NX-10, Park Systems Corp.) under ambient conditions.
ammonium persulfate with a poly-methyl-methacrylate (PMMA) supporting layer. After removing the Cu foil, the graphene with PMMA was then transferred onto a SiO2/Si wafer. The PMMA supporting layer was removed using acetone. The graphene was then patterned via typical photolithography with a reactive ion etching (RIE) process for 5 s at 50 W. To fabricate the graphene field-effect devices shown in Figs. 1c and 2a, Au electrodes were fabricated on the transferred graphene using a thermal evaporator. The position dependent quality of the graphene was inspected by Raman spectroscopy (XperRam 200, Nanobase) excited with a 532 nm laser. We measured the electronic transport properties of our graphene FET devices. The electrical properties of the graphene transistors were measured using a Keithley 4200A under ambient conditions. The 20 nm thick Au patterns (source and drain electrodes) were deposited on a SiO2 wafer by a thermal evaporator through a shadow mask. The channel lengths between the Au patterns were 20 or 50 mm, and the width was 1000 mm. Furthermore, to spatially map the work-function characteristics on graphene-based field-effect devices, we performed KPFM, which can acquire work-function information by measuring the contact potential difference (CPD), i.e., the relative surface potential of an AFM tip to that of a sample, for every pixel on the graphene surface. CPD map is simultaneously obtained with a topographic image of the sample surface via a tapping mode at 115 kHz. Metallic or metal-coated AFM tips (NSC36_B, MicroMasch) are generally used for KPFM measurements because CPD measurement requires
3. Results and discussion We employed a graphene-based field-effect device with a single top (source) electrode to investigate the graphene work-function characteristics as a function of the distance from the source electrode in a large-scale area and on the gate voltages. Fig. 1a shows the typical AFM topographic image of the CVD-grown graphene with RMS roughness of 1.237 nm, and its quality was inspected by Raman spectra collected at 532 nm (Fig. 1b) as a function of distance from the source electrode. In the Raman spectra, the characteristic G and 2D peaks of graphene are clearly measured at 1575 and 2673 cm1, respectively. A commonly used method to estimate the layer number of graphene is to calculate the ratio of G and 2D peak intensities (I2D/IG) or the full width at half maximum (FWHM) of the 2D peak in the Raman spectrum [40]. The I2D/IG is ~1 and FWHM of the 2D peaks ranges from ~50 to ~75 cm1 in Fig. 1b, which corresponds to bilayer or trilayer graphene [40]. In addition, to inspect the transfer characteristics of our graphene devices, we fabricated typical graphene FET devices with different channel 596
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transfer from the source electrode induce a band bending on graphene. To precisely investigate the CPD and consequent graphene work-function as a function of both the gate voltage and the distance from the source electrode, we performed KPFM measurements on the graphene sheet by sweeping the gate voltages in the range of 40 to 40 V and changing the tip-source distance from 0 to 160 mm at intervals of 20 mm; the results are shown in Fig. 3a. The CPD variation tendency with sweeping gate voltages, previously discussed in Fig. 2b, is clearly shown at all measurement positions in Fig. 3a. Note the distance-dependent inhomogeneity of CPD at the initial 40 V gate voltage. In the bottom line (red color) on the left-side panel of Fig. 3a, the CPD at the initial 40 V does not increase monotonically as the tip-source distance increases, and three indents are distinctly observed at the positions of 20, 60, and 120 mm. This inhomogeneity implies an inhomogeneous spatial distribution of the doping species within the graphene sheet. It is also interesting that the spatial inhomogeneity of CPD at the initial 40 V seems to be moderated by the gate voltage sweeping during the gate voltage-dependent work-function measurements using KPFM. The dents in the bottom line of the left-side panel of Fig. 3a fade away by sweeping the gate voltage, and eventually the CPD at the final 40 V on the right-side panel of Fig. 3a almost linearly increases as the tip-source distance increases. Another important feature of the CPD characteristics related to the tip-source distance and the gate voltage is that the CPD did not completely recover after the gate voltage sweeping, as clearly represented by the thick blue and red arrows in Fig. 3a. The increasing CPD after completion of the gate voltage sweeping indicates that the work-function decreased, as depicted in Fig. 3b. Furthermore, the irreversibility of CPD or work-function implies the presence of hysteresis characteristics. The CPD data in Fig. 3a were replotted in Fig. 4a, wherein the x-axis was changed to the gate voltage. All curves at different tip-source distances obviously show hysteresis characteristics that are highly analogous to the previously reported conductance hysteresis, while the curve tends to shift upward with increasing tip-source distance. The workfunction of bi- or trilayer graphene shows linear behavior depending on gate voltages, which is well defined in a previous study [41]. Fig. 4b shows the 2D map of CPD hysteresis gaps, wherein the CPD values in the forward gate voltage sweeping (from 40 to 40 V) were subtracted from the corresponding CPD values in the backward sweeping (from 40 to 40 V). The CPD difference (DCPD) values were plotted against the tip-source distance (x-axis) and gate voltage (y-axis). This map (Fig. 4a) represents the spatial distribution of the degree of CPD (or workfunction) hysteresis on the graphene device. The degree of the work-function hysteresis was spatially inhomogeneous on the graphene device, as shown by the specific positions with prominent CPD hysteresis indicated by three green arrows in Fig. 4b. These arrows coincide with the positions of three dents (20, 60, and 120 mm) in Fig. 3b (left) because the origin of this spatially inhomogeneous work-function hysteresis was the previously discussed nonmonotonic feature of spatial CPD variation at an initial gate voltage of 40 V. Therefore, the inhomogeneous distribution of the work-function hysteresis within the graphene device originates from the inhomogeneous distribution of the doping species between the graphene and the SiO2 substrate, which can induce the conductance hysteresis. As depicted in Fig. 4c, the higher density of doping species between the graphene and SiO2 substrate induces higher electron transfer and doping and vice versa. In addition, as previously discussed in Fig. 3a, the inhomogeneous distribution of the doping species tends to be moderated by the gate voltage sweeping with the KPFM measurements. When back gate voltage increases from 40 V to 40 V, the charge is injected to adsorbates.
Fig. 3. (a) CPD between tip and graphene surface plotted against distance from the source electrode, depending on the gate voltage from 40 to 40 V (left) and from 40 to 40 V (right). Each line is the averaged value for 10 pixels in y axis of Fig. 2b. (b) Consequently, CPD increased around 200 meV before and after the gate voltage was induced, thereby decreasing the work function. (A colour version of this figure can be viewed online.)
lengths (20 and 50 mm), as depicted in Fig. 1c. The graphene was patterned using conventional photolithography and RIE. The transport characteristics (drain current versus gate voltage) exhibited p-type behavior with large hysteresis and a positive Dirac point (approximately þ 150 V), as shown in Fig. 1d. The large hysteresis can be explained by the highly doped species between SiO2 and the bilayer graphene [24,25,31,33,34]. In addition, as the channel length increased, the hysteresis characteristic tended to become intense because of the doping species, as mentioned previously. Fig. 2a shows a schematic of the KPFM measurements on the graphene FET with a single top electrode. The back-gate voltage was swept in the range of 40 to 40 V. Fig. 2b shows 2D maps of gate voltage-dependent CPD acquired at two positions on graphene in the vicinity of and 160 mm away from the source electrode. The x and y axes in Fig. 2b correspond to the line-scanned CPD and the swept-gate voltages, respectively. In Fig. 2b, with the forward sweeping of gate voltages from 40 to 40 V followed by backward sweeping from 40 to 40 V, the CPD values increased and reached the maximum point between 40 V and 0 V, and then decreased. This CPD variation tendency is almost the same in the two maps in Fig. 2b, although the map at the 160 mm position shows relatively higher CPD values (lower work function) than that at the 0 mm position. This position-dependent CPD change would be originated from Fermi-level alignment and charge transfer between graphene and electrode. Our experimental study, therefore, shows that the capacitive coupling with the back-gate electrode and the charge 597
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Fig. 4. (a) CPD is replotted versus gate voltage to show the clear hysteresis of work function. Despite different tip positions, all curves show obvious hysteresis in CPD. The maximum hysteresis gap occurs at a gate voltage of 10 V. (b) The spatial distribution of CPD differences. Three indents at 20, 60, and 120 mm in Fig. 3b indicates harsh variance adjacent to 10 V. (c) Schematic of doping species between graphene and SiO2 boundary that can be transferred between electrons or dopants. (A colour version of this figure can be viewed online.)
review & editing. Young Jae Song: Supervision, Investigation, Writing - original draft, Writing - review & editing.
As a result, there is no space for charge to be injected to adsorbates which means the work function of graphene tuned by back gate is saturated. On the other hand, when back gate voltage decrease from 40 V to 40 V, under enough fast back gate sweeping speed, adsorbates cannot have sufficient time to charge relaxation [42].
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
4. Conclusions In this study, we investigated the work-function characteristics of graphene-based field-effect devices. It was demonstrated that the work-function has gate voltage-dependent hysteresis characteristics in graphene field-effect devices, analogous to the hysteresis in the electronic transport of graphene FETs. Furthermore, the spatial inhomogeneity in the degree of work-function hysteresis is observed, which originates from the inhomogeneous distribution of the doping species at the interface of a graphene/SiO2 substrate. The inhomogeneous feature tends to be moderated by gate voltage sweeping during the gate-voltage-dependent work-function measurements using KPFM. This study provides an advanced understanding of hysteresis characteristics in graphene-based field-effect devices and guidance for developing controlled graphene devices with minimal influence from ambient conditions.
Acknowledgments SHK was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (Grant No. 2020R1A2C1005299). YJS was supported by the Basic Science Research Program (Grant No. 2015M3A7B4050455), by Institute for Basic Science (Grant No. IBS-R011-D1) through the National Research Foundation (NRF) funded by the Ministry of Science and ICT (MSIT) in South Korea and the SRC Center for Topological Matter (Grant No. 2018R1A5A6075964) through the National Research Foundation (NRF) funded by the Ministry of Science and ICT (MSIT) in South Korea, by Industrial Strategic Technology Development Program (Grant No. 10085617) funded by the Ministry of Trade Industry & Energy (MOTIE) in South Korea.
CRediT authorship contribution statement References Hwi Je Woo: Investigation, Formal analysis, Writing - original draft, Writing - review & editing. Seongchan Kim: Formal analysis. Young-Jin Choi: Formal analysis. Jeong Ho Cho: Formal analysis. Seong Heon Kim: Supervision, Writing - original draft, Writing -
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