Very Gradual and Anomalous Oxidation at the Interface of Hydrogen

NTT Basic Research Laboratories, Nippon Telegraph and Telephone Corporation, 3-1 Morinosato-Wakamiya, Atsugi-shi, Kanagawa 243-0198, Japan...
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Cite This: J. Phys. Chem. C 2017, 121, 26389-26396

Very Gradual and Anomalous Oxidation at the Interface of Hydrogen-Intercalated Graphene/4H-SiC(0001) Fumihiko Maeda,*,†,‡ Makoto Takamura,‡ and Hiroki Hibino‡,§ †

Department of Information Electronics, Faculty of Engineering, Fukuoka Institute of Technology, 3-30-1 Wajiro-higashi, Higashi-ku, Fukuoka 811-0295 Japan ‡ NTT Basic Research Laboratories, Nippon Telegraph and Telephone Corporation, 3-1 Morinosato-Wakamiya, Atsugi-shi, Kanagawa 243-0198, Japan § Department of Nanotechnology for Sustainable Energy, School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda, Hyogo 669-1337 Japan S Supporting Information *

ABSTRACT: We analyzed the surfaces of hydrogen-intercalated graphene on SiC(0001) by photoelectron spectroscopy and Raman spectroscopy after applying electric currents to the graphene for carrier mobility measurements while the sample was exposed to air. We found that Si at the interface between graphene and SiC oxidizes very gradually, whereas the graphene does not. The Si of this Si oxide is mainly divalent, and the oxide is attributed to SiO molecules. These molecules are volatile, and they have not been observed either on the oxidized surface of SiC(0001) or on oxygen-intercalated graphene/SiC(0001) formed by annealing in air or oxygen. This anomalous oxidation, which is triggered by the application of current to the graphene overlayer, could be observed because of the presence of the graphene overlayer and the oxidation at room temperature.

1. INTRODUCTION A quasi-free-standing (QFS) state of epitaxial graphene has been achieved by breaking bonds between the C atoms of the buffer layer and Si atoms of the SiC substrate and by terminating Si dangling bonds at the interface with hydrogen.1 However, the expected performance of QFS graphene has not been achieved. The main factor contributing to this problem is Coulomb scattering caused by charged impurities,2 which originate from Si dangling bonds at the interface caused by incomplete hydrogenation.3 In order to show high mobility in graphene, the application of current in atmospheric ambient is crucial. However, the effects of the current flow on graphene and interface, such as possible structure deformation and chemical reactions, have not been examined. Although the graphene overlayer protects the SiC surface against oxidation after hydrogen intercalation,1,3 even if exposed to air, there is no certain evidence that this structure will be preserved after mobility measurements. For instance, it has been suggested that a 2D monolayer of adsorbed atmospheric water forms on epitaxial graphene.4 It is not abnormal that a water layer forms on hydrogen-intercalated graphene. In some situations, an extremely small amount of H2O induces severe damage in graphene; in cases where the graphene is transferred to another substrate, residual H2O or O2 causes structural deformation of the graphene when heated.5 Although this deformation is induced by heating, current applied while the graphene is exposed to air can also cause © 2017 American Chemical Society

unexpected effects because of the formation of a water layer, even if the current is sufficiently small. Thus, for the sake of examining the change in the surface and interface structure, we analyzed the surface of the prepared graphene/SiC samples using X-ray photoelectron spectroscopy (XPS) after mobility measurements. We found that Si at the interface was oxidized while the graphene overlayer remained scarcely changed. Since we noticed that it takes a long time for this interface oxidation, long-term time-dependence of the interface oxidation after the mobility measurements was investigated in order to show the reproducibility of the Si oxidation at the interface under the graphene. Furthermore, we examined the chemical state of the oxidized Si and found that its Si is mainly in the divalent chemical state, which has not been observed either on the oxidized surface of SiC(0001) or in the case of oxygen-intercalated graphene/ SiC(0001) formed by annealing in air 6,7 or oxygen. 8 Furthermore, we found that the divalent Si could be attributed to SiO. However, SiO is ordinarily difficult to observe by XPS because it is volatile and usually desorbs from the surface under ultrahigh vacuum (UHV), which is the normal measurement condition for XPS. We were able to observe this anomalous phenomenon because of the special oxidation conditions under Received: August 30, 2017 Revised: November 7, 2017 Published: November 11, 2017 26389

DOI: 10.1021/acs.jpcc.7b08631 J. Phys. Chem. C 2017, 121, 26389−26396

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The Journal of Physical Chemistry C

VDP measurements are identical except that, for the third sample, the XPS measurement just after the VDP measurements was omitted.

the graphene overlayer and the fact that the oxidation occurs at room temperature.

2. EXPERIMENTAL METHOD We used semi-insulating 4H-SiC(0001) substrates, on which QFS bilayer (BL) graphene was formed by hydrogen intercalation of epitaxial monolayer (ML) graphene on SiC(0001) with a buffer layer. First, the SiC substrates were annealed at 1500 °C in H2 ambient at 25 Torr to clean the surface and subsequently annealed at 1630 °C in Ar ambient at 100 Torr, resulting in the formation of the epitaxial ML graphene. For the formation of BL graphene, these substrates were heated to 1000 °C in molecular hydrogen at atmospheric pressures for the hydrogen intercalation. The heating time at 1000 °C in hydrogen ambient was 1 h. For the mobility measurements, which were done using the Van der Pauw (VDP) method in air, a current was applied, and the voltage was measured using four probes attached to the corners of the samples, which were in the shape of 1 cm squares. Before the measurements, we scribed these sample surfaces and drew lines on four sides to avoid electrical contact of graphene on the (0001) surfaces with that on the sides and backside of the samples. A current of approximately 0.6 mA was applied for each of several VDP measurements for each sample. For the analysis of the sample surface and interface, the substrates were placed in an XPS measurement system equipped with a monochromatized Al Kα source (1486.6 eV) and a photoelectron analyzer. In this XPS system, whose base pressure was less than 1.0 × 10−9 Torr, photoelectron spectra were captured just after the samples were introduced into the UHV. The takeoff angle of the photoelectrons was set at 25° from the surface. To calibrate binding energy (BE), Fermi-edge spectra were measured on a gold plate, and the energy resolution of photoelectron measurements was estimated to be 0.5 eV. We prepared three samples. The experimental procedures are as follows. First, we prepared the hydrogen-intercalated graphene/SiC(0001), and then the sample surface was analyzed by XPS. Next, the current was applied to this sample for the VDP measurement in air. Then, the sample was placed in a chip tray and kept in a semiclean room in ambient atmospheric conditions. After the VDP measurements, the sample surfaces were analyzed by XPS at three time intervals, which were 4 h, 5 days, and 80 days. Raman spectra were also recorded to check the damage in the graphene overlayer. For reference, another hydrogen-intercalated graphene/ SiC(0001) sample was prepared, also with the surface scribed and four lines drawn on each side, but not subjected to the VDP measurement. The surface of this reference sample was analyzed by XPS immediately after the hydrogen intercalation procedure and then again after 62 days. This sample was also kept in ambient atmospheric conditions. A third sample was prepared in order to analyze the effects of UHV annealing on the interface structure of the oxidized hydrogen-intercalated graphene/SiC(0001). The sample was prepared similarly to the first sample, and the interface oxidation was confirmed by XPS analysis. However, the storage time in air was shorter than 2 months, and the sample was not introduced into the UHV chamber until the surface was analyzed by XPS. Then, this sample was annealed at 550 °C in UHV, and the surface was analyzed in situ by XPS. For the first and third samples, procedures from the graphene formation up to the XPS measurements to confirm the oxidation after the

3. EXPERIMENTAL RESULTS 3.A. Long-Term Interface Oxidation. First, the surface changes before and after the VDP measurements will be described. Figure 1 shows the wide scan XPS spectra and the core level spectra of O 1s, C 1s, and Si 2p. In each panel, the spectra captured after hydrogenation (before the VDP

Figure 1. Wide scan XPS spectra (bottom panel) and O 1s (second panel from bottom), C 1s (third panel), and Si 2p (top panel) spectra captured from the QFS bilayer graphene on hydrogen-terminated SiC(0001) before (open circles) and after mobility was measured by the VDP method at three time intervals, which were 4 h (upsideinverted open triangles), 5 days (open triangles), and about 2 months (open squares). In the bottom panel, the conditions are indicated above the spectra. 26390

DOI: 10.1021/acs.jpcc.7b08631 J. Phys. Chem. C 2017, 121, 26389−26396

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The Journal of Physical Chemistry C measurements) and after the VDP measurements at three time intervals, which were 4 h, 5 days, and about 2 months, are shown. In the spectra ranging from 0 to 1350 eV, no photoelectron or Auger peaks other than those of C, Si, and O were detected except for faint peaks attributed to F 1s and KLL Auger. The origin of this fluorine is not clear at present. These faint peaks were always observed, but their intensities varied. They disappeared when the sample was annealed at relatively low temperature (around 250 °C) in UHV. We speculate that the fluorine is included in the surface adsorbates on the graphene surface. These adsorbates are due to exposure to air in the clean room or to the vacuum environment evacuated by turbo-molecular pumps that include fluorinebased oil. In any case, there is no major influence on our experimental result because the fluorine−based adsorbates are physisorbed species and their amount is small. In the wide scan XPS spectra, we found that the intensity of the O 1s peak increased slightly after 5 days and drastically after 2 months. We found in the high-resolution spectra of O 1s that this intensity increase is attributed to a peak at the binding energy (BE) of 532.6 eV. Incidentally, on the hydrogenintercalated graphene surface, a faint photoelectron peak of O 1s is always observed, even though the oxide-related peak is not observed in either the photoelectron spectra of Si or C corelevels. Because this peak always disappears after annealing in the UHV around 250 °C,3 this spectral feature is attributed to physisorbed species arising from exposure to air, although the chemical component is not clear. In the Si 2p spectra after 2 months, a peak at BE higher than that of the bulk peak emerged. While the C 1s spectra scarcely changed, this peak intensity simultaneously increased with the increase of the O 1s peak. This indicates that the newly emerged peaks in the O 1s and Si 2p spectra are attributed to formation of Si−O bonds and that Si was oxidized. In this case, since Si is located under the graphene overlayer, this Si oxide should be located at the interface between graphene and the SiC substrate. These oxide-related peaks in the O 1s and Si 2p spectra were not observed in the spectra recorded after 4 h. For the sample not subjected to the VDP measurements, the wide scan XPS spectra together with the O 1s, C 1s, and Si 2p spectra are shown in Figure 2. These spectra for the ashydrogenated sample and for the sample successively exposed to air for 2 months are essentially identical. This indicates that the interfacial Si is not oxidized if current is not applied. Therefore, the above-mentioned results indicate that the oxidation of the interfacial Si is induced by the current applied during the VDP measurement. However, it is not directly induced by the applied current, and it progresses very gradually. Next, we will describe the chemical component analysis of the core-level spectra. By fitting the spectrum of O 1s that was captured 2 months after the VDP measurements, two peaks were resolved as shown in Figure 3. Because the peak at higher BE (533.8 eV) is similar to the peak in the spectrum of O 1s captured after the hydrogenation in Figure 1, this peak is attributed to the contribution of species adsorbed on the graphene because of air-exposure. These results indicate that the peak at lower BE (532.6 eV) appeared because 2 months had passed after the current application. Figure 4 shows the Si 2p spectra with the fitting results for the sample of the QFS bilayer graphene on hydrogenterminated SiC(0001) before (bottom panel) and 2 months after (top panel) current application in air. In the bottom panel of Figure 4, only two peaks are resolved, and they are attributed

Figure 2. Wide scan XPS spectra (bottom panel) and O 1s (second panel from bottom), C 1s (third panel), and Si 2p (top panel) spectra captured captured from the QFS bilayer graphene on hydrogenterminated SiC(0001): as-hydrogenated (open circles) and about 2 months after the hydrogenation (open squares). In the bottom panel, the conditions are indicated above the spectra.

Figure 3. O 1s spectrum (open circles) captured from the QFS bilayer graphene on hydrogen-terminated SiC(0001) after mobility measurement in air by VDP method. Lines indicate fitting results.

to bulk SiC (B) and Si−H bonding states (Si−H), which are consistent with previous studies1,3 and indicate that Si atoms at the surface of SiC are terminated by hydrogen. In the spectrum from the sample surface captured 2 months after the current application, a shoulder at the higher BE side is observed. Since the fit using only two oxide-related peaks labeled by Si+ and Si 4+, which were observed on the oxygen-intercalated graphene/SiC(0001),8 did not reproduce the spectrum well, another peak was required, and the result shown in the top panel of Figure 4 was obtained. We can identify the chemical state as labeled in the figure because these three peaks are related to oxides. In our fitting, the shift of the Si1+ peak with respect to the bulk peak was 0.5 eV, which is in good agreement with previous core-level 26391

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very small peak, denoted as A in the figure, appeared. The BE of this peak is 1.0 eV higher than that of the SiC bulk peak. A similar peak (1.2 eV higher than the bulk peak) was observed in studies of the oxidation of 4H-SiC.13,14 The authors of those studies considered the origin of the peak to be carbon clusters on the surface, possibly at steps and domain boundaries. Another candidate is carbon atoms in the buffer layer that bond to one underlying Si atom, whose peak was observed at 0.7 eV higher than the bulk peak in C 1s on a nanomesh of SiC(0001).16 Furthermore, in the topmost SiC bilayer on oxidized SiC(0001̅), carbon atoms with a dangling bonds gave rise to an XPS peak 0.8 eV higher than the bulk peak, and these atoms also do not have bonds with oxygen.17 We cannot specifically identify peak A as one of these peaks at present because the differences in BE between this peak and the SiC bulk peak are similar but not identical to differences seen in those studies. In any case, the peaks were attributed to carbon atoms not bonded to oxygen, and peak A of our study is also attributed to those carbon atoms. Thus, no oxygen-related peak was observed in the C 1s spectrum, indicating that oxygen does not bond with carbon. Hence, oxygen does not break the bonds between the Si and C atoms, such as the back-bond of the topmost Si atom of SiC, as discussed in section 4. 3.B. UHV Annealing for Interface-Oxidized Hydrogenated-Graphene/SiC. To obtain insight into the interface structure after the gradual oxidation induced by current application for hydrogenated graphene/SiC, we annealed the sample in UHV. We prepared another sample by the identical procedure, including VDP measurements, as mentioned in the section 2. Before annealing, we analyzed the surface of the sample and confirmed the interface oxidation. The photoelectron spectra captured immediately after introduction into the UHV chamber are shown in the bottom panels of Figure 6. In the Si 2p spectrum, the Si2+ peak was clearly observed. After annealing at 550 °C, the photoelectron spectra in the top panels of Figure 6 were obtained. Comparing the spectra before and after annealing, we found that the intensity of the peak corresponding to Si2+ in the Si 2p spectrum reduced and the intensity ratio (before and after annealing) was 0.48. Similarly, the intensity of the Si−O peak at the lower BE in the O 1s spectra reduced, and the corresponding ratio is 0.54. These values reveal a correlation between the reductions (desorption) of O and the Si2+. However, we did not observe a significantly large difference in the C 1s spectra, such as broadening in the SiC-bulk peak, even though it has been observed previously when the Si2+ peak is observed.18 Therefore, in our case, the Si2+ peak is not attributed to a Si−O−C mixed oxide10,18 but to a Si oxide like SiO. Deriving peak intensities from the areas of the Si2+, hydrogen-Si bond (Si−H), and bulk-SiC peaks, we estimated the nominal coverage of the Si2+ and Si−H bonds to be about 0.79 and 0.57 MLs, respectively, by using the formula described in ref 19 with λ1 = 13 Å20 for the electron mean-free path and A = 1.3 Since the sum of these values is more than 1 ML, this result is consistent with the idea that excess molecules corresponding to Si2+ are confined at the interface. Furthermore, the thermal desorption of an oxide monolayer from the SiC surface occurs at around 1000 °C,21 which is much higher than the oxide-desorption temperature in our case. The desorption temperature can be lowered by about 200 °C by deposition of Si, which causes the formation of SiO.22 Therefore, considering that the Si2+ peak is significantly reduced by the 550 °C annealing, the identification of the

Figure 4. Si 2p spectra captured from QFS bilayer graphene on hydrogen-terminated SiC(0001) before (bottom panel) and 2 months after (top panel) applying current in air. Open circles are experimental data, and lines indicate fitting results.

photoelectron spectroscopy studies9−11 of the oxidized SiC (0001) surface; the shift values range from 0.25 to 0.60 eV. For the Si4+ peak, the shift value is 2.1 eV in our case. This value is also reasonable, because the chemical shift of SiO2 varies from 2.1 to 2.6 eV in accordance with the thickness,11−13 and the chemical shift of SiO2 (Si4+) in our case corresponds to that of the fairly small thickness. For Si2+, the peak shift was found to be 1.5 eV, which is consistent with previous studies for Si2+ observed on SiC(0001̅)10,14, (101̅0),10 and (112̅0),10 but not for (0001). From these arguments, we found that this shoulder occurred by emerging three peaks as shown in the top panel of Figure 4. In contrast, in the C 1s spectra shown in Figure 5, the difference is not nearly as pronounced. In these C 1s spectra,

Figure 5. C 1s spectra, captured from QFS bilayer graphene on hydrogen-terminated SiC(0001) before (bottom panel) and 2 months after (top panel) applying current in air. Open circles are experimental data and lines indicate fitting results.

two major peaks [graphene (G) and SiC-bulk (B)] are observed, and a small peak component [hydrocarbon15 (CxHy)] is resolved, which indicates that hydrocarbons remain at the interface.3 Two months after the application of current, a slight difference was observed compared to the hydrogenintercalated graphene not subjected to current application; a 26392

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Figure 6. O 1s (left panels), C 1s (center panels), and Si 2p (right panels) spectra captured before (bottom panels) and after (top panels) annealing at 550 °C in UHV from the interface-oxidized QFS bilayer graphene on hydrogen-terminated SiC(0001) after mobility measurements. Open circles are experimental data and lines indicate fitting results. Photoelectron spectra are indicated after normalizing the intensity for each core-level.

Si2+ with SiO is reasonable, although its structure from the viewpoint of atomic bonding is not clear at present. We think that the species corresponding to the Si2+ peak were observed owing to the special oxidation condition of this work, which may suppress desorption because of the graphene overlayer and the low temperature (room temperature). Thus, SiO formation is the most plausible interpretation in this study. Further study is needed to investigate the detail of the interface structure. Meanwhile, the small peak A observed in the C 1s spectrum before annealing is shown in the top panel of Figure 5. This peak disappeared after the annealing. On the basis of the above discussion, peak A is attributed to C atoms not bonded with hydrogen or oxygen, such as C clusters, carbon atoms with a dangling bond, and C atoms at the topmost layer of SiC. This spectrum change indicates that C atoms corresponding to such a state were transformed into another stable state by the UHV annealing. Before annealing, the integrated intensity of peak A in the C 1s spectrum is more than four hundred times smaller than that of the SiC-bulk peak. This value corresponds to a coverage of 0.5% of the interface based on the abovementioned formula with the electron mean-free path value λ1 = 12 Å20 and under the assumption that the species corresponding to peak A covers the SiC surface. Thus, this change is minor and does not influence the discussion about the interface. No other differences between the spectra before and after the annealing in Figure 6 were observed. 3.C. Graphene Overlayer after Oxidation. Because a fairly small change, which is speculated to occur at the interface, was observed in the C 1s spectra when we compare spectra before and after long-term oxidation, no change is expected in the overlayer graphene even though interface oxidation occurred. To check any possible change in the graphene overlayer, we acquired Raman spectra before and after current application from two areas close to the center of the sample. The area size was approximately 10-μm square, and 180 spectra were acquired from one area. In the original spectrum, the only clear graphene-related peak is the 2D-band peak because the peaks attributed to the SiC substrate are more intense than those related to graphene. Hence, to clarify the contribution of the graphene, we corrected the spectrum for emission from the substrate by subtracting the spectrum of the SiC substrate. Figure 7 shows the corrected Raman spectra from the sample. In addition to G- and 2D-band peaks, faint D-band peaks, similar to those reported previously for QFS BL graphene,23 are observed. For the spectra acquired before and

Figure 7. Typical Raman spectra of QFS bilayer graphene on hydrogen-terminated SiC(0001) captured (a) after and (b) before mobility measurements in air by the VDP method. These spectra are the results of the summation of 180 spectra.

after the application of current, we compare their G/D ratios, full-width at half-maximum (FWHM) of the G band, and FWHM of the 2D band, which are indicative, respectively, of the domain size, crystallinity,24 and number of layers. Their average values obtained from the two different areas before current application are virtually the same if we take account of the errors of position dependence. Therefore, from the Raman spectra, we can conclude that no structural change was induced in the graphene overlayer by the current application from the viewpoint of the number of defects and the crystallinity.

4. DISCUSSION From the above-mentioned photoelectron and Raman spectroscopy results, we found that Si at the interface gradually oxidized while the sample was stored in air after applying current in the VDP measurement, although the graphene overlayer remains unchanged. Oxidation at the interface, caused by high-temperature (600 °C) annealing in air, has been reported for epitaxial graphene on SiC(0001) with a buffer layer, resulting in oxygen intercalation at the interface and transformation of the buffer layer to graphene.8 Furthermore, annealing at around 250 °C in in molecular oxygen at atmospheric pressures also induces similar decoupling of the buffer layer.6,7 However, our oxidation occurred at room temperature. In addition, the interfacial Si oxide in our case is different from that observed in other reported oxidation processes, suggesting that the oxidation mechanism is not identical. 26393

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formation of the holes, through which oxygen molecules or water molecules can penetrate. Here, the graphene thickness after the hydrogenation should be more than two MLs because the buffer layer transformed into a graphene layer because of hydrogen intercalation and the thickness before the hydrogenation was more than one ML. Thus, this penetration process of the molecules proceeded more than twice in average, and then they reached the interface. Finally, the oxygen-related gas diffuses through a very narrow space at the interface between the graphene overlayer and the hydrogen-terminated SiC surface. Very gradual oxidation at the interface is consistent with this scenario. Thus, this chemical reaction induced by the current flow can be considered to be the cause for desorption of the filling at the defects. The next process is the reaction of oxygen with Si atoms at the interface. We propose two possibilities for this reaction. When the terminating hydrogen is simply replaced with oxygen, the Si+ peak should emerge and be dominant.8 However, our result is different from this. Hence, when the oxidation reaction at the interface proceeds, oxygen should occupy one of the back bonds of the top Si. Because preferential oxidation of the Si back-bond was observed in a study of the initial stage of hydrogen-terminated Si(001) surface,29 this reaction is conceivable and is not an exceptional reaction. In this case, if the oxygen atom simply inserted into and remained at the backbond between top Si and C atoms, oxide-related carbon peaks should appear in the photoelectron spectra. However, the remarkable peak corresponding to the Si2+ peak in Si 2p was not observed in the C 1s spectrum in Figure 5. This indicates that oxygen only bonds with Si, meaning that Si2+ is in the form of SiO molecules. This further means that the oxygen atom was once inserted into the back-bond of a top Si atom, and then the bond with the C atom in the second layer was broken. This is one possible pathway to form SiO. Another conceivable pathway to form SiO is the reaction with Si atoms in an interstitial site of the SiC bulk near the surface. Its migration energy through the interstitial sites was theoretically calculated to be smaller than that of desorption through the graphene overlayer via pentagon-hexagonheptagon (H5,6,7) defects.30 Considering the growth conditions in our case under Ar ambient with much higher pressure than the UHV and under the graphene overlayer, staying at the interstitial sites could be more preferable for the Si atoms than passing and desorbing through the graphene overlayer. Therefore, even after the hydrogenation, the Si atoms could stay near the interface between the graphene and the SiC bulk and, then, react with the oxygen that penetrates the graphene overlayer from the air. When comparing these two pathways, we need to consider that the breaking of the C−O bond should occur in the case of the former one. Because it is unlikely to occur at room temperature, we think that the latter would be more favorable to form SiO. However, we do not have any theoretical and experimental evidence that room temperature is too low to break the C−O bond. On the other hand, the interstitial Si atoms have not experimentally been observed yet. Thus, the question of which pathway is the actual one is still open to discussion, and further study is needed to elucidate it. Meanwhile, SiO is volatile and normally desorbs from the surface under UHV. Thus, SiO on the surface is usually not detected by XPS. For instance, these characteristics make the discussion about the emission of SiO during the oxidation of clean Si surface controversial.31 In this case, SiO would desorb

First, we will discuss how the interfacial Si can be oxidized. In this case, the oxidation process is divided into at least two steps. The first step is penetration of oxygen through the graphene overlayer to the interface. The second is the reaction of oxygen with Si to form oxides mainly composed of Si2+. The details of these oxidation processes are not clear at present, but we speculate as follows. For the first process, we must consider that atomic-size holes, through which oxygen can penetrate to the interface, were introduced into the graphene overlayer when the current was applied to it because interface oxidation did not occur on the hydrogenated graphene/SiC that was not subjected the application of current. From this viewpoint, current application induced degradation in the graphene. The G/D ratio of the Raman spectra after the VDP measurements was approximately 32, which corresponds to an average distance of 57 nm between defects.25 Hence, the density of these holes was small, and the holes act as a sieve to limit the oxygen supply to the interface. However, the current application alone did not induce the holes in the graphene layer because we did not observe a significant increase in the density of defects in the Raman spectra. Hence, it is plausible that it activated the defects previously formed before application of the current, resulting in the formation of atomic-size holes, i.e., vacancies, and penetration of oxygen through the graphene overlayer. For instance, one possibility is that the hole was initially filled with something such as hydrogen, which terminates a dangling bond of carbon at the defects, and that this filling desorbed or moved because of the local bonding deformation induced by the current application. This idea is consistent with vacancy formation in the graphene after hydrogenation, with resulting dangling bonds of carbon atoms terminated by hydrogen. These vacancies consist of hydrogen-terminated zigzag edges.26 Once hydrogen terminates the dangling bonds at the vacancies, most molecules cannot penetrate the interface from the air through the defects at room temperature because hydrogen atoms share the position of the hexagonal lattice instead of a carbon atom26 and disturb the penetration mechanically. In this model, it is theoretically predicted that carbon atoms at these vacancies shift out of the graphene surface by 0.4−0.7 Å. A similar salient structure, with carbon atoms shifted upward, was observed on hydrogen-intercalated graphene on SiC by scanning tunneling microscopy.27 To form this structure, carbon atoms must first be removed to form vacancies. This is unlikely to occur during the hydrogenation process because 1000 °C is not high enough to cause this reaction. However, it is conceivable that vacancies form during the graphene formation process and that hydrogen terminates the dangling bonds during the hydrogenation process. In fact, after the graphene formation by high temperature annealing on the surface of the SiC(0001), areas with the 6√3 × 6√3 R30° reconstruction exist (approximately 1% in area estimated using the phase image of an atomic force microscope). It is also conceivable that vacancies preferentially form in this area during hydrogenation. Thus, the process is not clear, but vacancies with the zigzag edges composed of hydrogen-terminated carbon are highly likely to exist on the real graphene surface after the hydrogenation process. Meanwhile, because the hydrogen-terminated zigzag edge is chemically active,28 chemical reactions with physisorbed gases in the air would occur at the defects because of the current flow, resulting in the removal of the filling (the hydrogen) and the 26394

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The Journal of Physical Chemistry C from the surface and would not be detected even if SiO were emitted from the interface between SiO2/Si. Furthermore, the characteristics would be closely related to the fact that divalent Si has not been observed on the oxidized surface of SiC(0001). In our case, thanks to the presence of the graphene overlayer, the SiO remains at the interface, and it was observed by XPS. When graphene covers a surface, it disturbs the desorption of volatile molecules. Then, they can be detected, and their chemical state can be analyzed by XPS because graphene is the atomically thin and photoelectrons are emitted from the surface through the graphene overlayer without large attenuation. Thus, graphene is greatly valuable for this type research. However, the Si2+ state was not found on the surface of oxygen-intercalated graphene/SiC(0001) formed by annealing in air or oxygen even though the structure in that case−that the oxidized Si at the interface under a graphene overlayer is formed at the interface−is similar to that of the present study. This difference is due to the reaction temperature, as mentioned in the first paragraph of this section. At high temperatures, the SiO at the interface could desorb through defects as in the experiment involving UHV annealing described in the previous section. The condition of this study, room temperature oxidation, is also an important factor allowing the observation of SiO.

anomalous phenomenon, which is triggered by the application of current.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b08631. Atomic force microscopic analyses of annealed sample surfaces under ultrahigh vacuum for interface-oxidized hydrogenated graphene/SiC (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(F.M.) E-mail: f-maeda@fit.ac.jp, Telephone: +81-92-6063564. ORCID

Fumihiko Maeda: 0000-0002-1349-1916 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are indebted to S. Isobe and S. Takagi of Nagaoka University of Technology for their help in the sample preparation and evaluation during their stay as trainees at the NTT Basic Research Laboratories. All experiments were conducted at the NTT Basic Research Laboratories when F.M. worked for Nippon Telegraph and Telephone Co. This work was partially supported by Grants-in-Aid for Scientific Research (22310077) from the Japan Society for the Promotion of Science (JSPS).

5. CONCLUSIONS The magnitude of the electrical current employed for mobility measurements is small, and it causes only minor stimulation. Hence, we share a common assumption that the structure of the measured object is preserved after the measurements. However, we question this assumption for the case of quasifree-standing graphene formed by hydrogen intercalation of epitaxial graphene on 4H-SiC(0001). We analyzed the surfaces of such samples by photoelectron spectroscopy and Raman spectroscopy after mobility measurements using the Van der Pauw method. We found that Si at the interface between graphene and bulk SiC becomes oxidized very gradually while graphene does not and that this oxidation does not occur without the application of current. Since it is clear that the current application triggered the microscopic structure change in the graphene overlayer and the oxygen penetration through the graphene overlayer to the interface resulting in interface oxidation, conventional oxidation mechanisms, which can appropriately explain the case of oxygen-intercalated graphene/SiC(0001), cannot apply for our case. Thus, we consider the scenario that the vacancies had already formed after the hydrogenation process and that they are activated by the application of current for the VDP measurements. However, this scenario is based on inferences, so concrete evidence is necessary to show its validity. Furthermore, we examined the chemical state of the oxidized Si and revealed that it is mainly divalent, which has not been observed either on the oxidized surface of the SiC(0001) or in the case of oxygen-intercalated graphene/SiC(0001). We revealed that the divalent Si is attributed to SiO, despite the fact that SiO is ordinarily difficult to observe by XPS because it is volatile and usually desorbs from the surface under the UHV. Thus, our experimental result is anomalous because the oxidation is different from conventional oxidation on SiC(0001)-related surfaces and SiO was observed on the surface. We conclude that the presence of the graphene overlayer and the oxidation at room temperature caused this very gradual and

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DOI: 10.1021/acs.jpcc.7b08631 J. Phys. Chem. C 2017, 121, 26389−26396