In Situ Study on Oxygen Etching of Surface Buffer Layer on SiC ( 0001 ) Terraces

Thermal decomposition of SiC has been used for the fabrication of high quality monolayer graphene and graphene nanoribbons on semi-insulating substrates. In this work, we propose a selective oxygen etching method to remove buffer layers on SiC surfaces that are connected to monolayer graphene formed from step edges. A thermal treatment in an extreme low partial pressure oxygen diluted by argon atmosphere was found to be effective to etch only the buffer layers and remain monolayer graphene areas intact, which might be significant for the application of graphene to electric/spintronic devices. The etching processes of surface buffer layer investigated by in situ scanning electron microscopy and scanning tunneling microscopy revealed an etching rate dependence on a distance from a step edges, suggesting a distribution of crystallinity of surface buffer layer on a terrace. [DOI: 10.1380/ejssnt.2017.13]


I. INTRODUCTION
Graphene, known as the most ideal two-dimensional material, shows its unique electric properties [1][2][3][4] for next generation electronics [5][6][7][8].Since single layer graphene was firstly fabricated by exfoliation from graphite, several methods have been discovered to grow graphene layers effectively.Thermal decomposition of SiC is an approach to obtain epitaxially grown graphene directly on an insulating substrate.High quality monolayer graphene can be usually formed on SiC(0001) surfaces by annealing under an inert gas atmospheres or Si background [9,10].During the process of graphene growth on SiC(0001) the first carbon layer, so-called "buffer layer" covers the terraces before the formation of graphene.Carbon atoms in a buffer layer are partly bonded to Si atoms of SiC substrate, which makes the properties of the buffer layer different from those of graphene.Subsequently, SiC underneath decomposes with forming a new buffer layer, which decouples the first buffer layer and transforms it into a graphene layer.Therefore the surface buffer layer actually plays a significant role as a precursor for monolayer graphene growth.However, there is just few report focusing on the quality of surface buffer layer [11,12], and thus more investigation of the surface buffer layer is necessary.
In addition, Graphene nanoribbons are very important for graphene applications to electronic/spintronic devices.Nakada et al. reported [13] unique features of edge stats of monolayer graphene nanoribbons, which depends on edge structure.Figure 1(a) shows monolayer graphene ribbon formed at the edge of step bunching.As shown in the figure, left edge of the ribbon is seamlessly connected to the buffer layer, because the monolayer graphene was a buffer layer before decoupled by a new buffer layer underneath.In this situation, edge state explained above does not emerge.It has been revealed by Sclauzero et al. [14] that the presence of C atoms with sp 2 -to-sp 3 rehybridization enhances the chemical reactivity of buffer layer.Here, we suggest a selective oxygen etching method to remove the surface buffer layer on terraces and leave the monolayer graphene intact (Fig. 1(b)).
It has been already reported that oxygen treatment can be used for decoupling the buffer layer from the SiC(0001) substrate by forming an oxide layer under the buffer layer [15][16][17][18].A similar phenomenon using hydrogen intercalation has also been well studied, which can transform a buffer layer to a quasi-free-standing monolayer graphene [10,19,21].Oida et al. [15] have reported that a low-temperature annealing in an oxygen (O 2 ) atmosphere (1 atm, 250 • C, 5 s) turns the buffer layer to a monolayer graphene by forming a thin oxide layer (0.3 nm thickness) underneath the buffer layer.Following these studies, some other groups found water vapor or air can also be used as oxygen source for the intercalation.Ostler et al. [16] reported that annealing in a water vapor atmosphere (∼28 mbar) resulted in formation of monolayer graphene from buffer layer surfaces (500 • C, 30 min) and bilayer graphene from monolayer graphene surfaces (650 • C, 30 min).Oliveira et al. [17] investigated the oxygen intercalation to monolayer graphene surfaces by the annealing in air (600 • C, 40 min) and fabricated the decoupled bilayer graphene on SiC surfaces.Recently, they fabricated quasi-free-standing bilayer graphene nanoribbons by annealing a SiC substrate with monolayer graphene nanoribbons formed near step edge regions under the same condition (air, 600 • C, 40 min) [18].They also found that the surface buffer layers on terraces were etched after the annealing in air.
In this study, we tried etching connected buffer layers without decoupling buffer layer underneath monolayer graphene by using an extreme low partial pressure of oxygen to fabricate free edge monolayer graphene as shown in Fig. 1(b).In addition, we investigated the etching processes of surface buffer layer to clarify crystallinity of buffer layers because it directly influences the quality of formed monolayer graphene.

II. EXPERIMENTAL
SiC substrates used in this study are commercial single-crystalline n-type 6H-SiC with a size of 12×3×0.33mm.The SiC(0001) faces are polished by a chemical-mechanical planarization method remaining a miscut angle of 0.14 • .Firstly, the SiC substrates were patterned with markers by laser lithography and reactive ion etching so that we can identify different positions on the surface of samples after various annealing stages.Before the annealing processes, SiC substrates were cleaned in acetone by an ultra-sonic cleaner for 30 min to remove surface impurity.
The samples supported on a molybdenum holder were annealed by direct current heating in an ultrahigh vacuum scanning electron microscopy/scanning tunneling microscopy (UHV-SEM/STM) apparatus with a customized preparation chamber, where a sample can be heated in various atmospheres (UHV to 1 atm).The sample surface structures can also be analyzed by reflection high-energy electron diffraction (RHEED) in the preparation chamber.The direct current heating always brings about a temperature distribution on the SiC substrates between two electrodes (higher temperature at the center and lower temperature near the electrodes).The annealing temperature was measured at the center of sample with an optical pyrometer.
After an introduction of samples to the preparation chamber, a hydrogen etching (0.1-atm-hydrogen at 1300 • C for 15 min) was first carried out for surface cleaning.Then, graphene was fabricated by annealing at 1550 • C for 1 hour in a 1-atm-Ar atmosphere, which was found to be an appropriate condition for the graphene growth from step bunches on wide terraces in our previous study [22].The selective oxygen etching was carried out in a mixed atmosphere (O 2 : 10 −3 Pa, Ar: 1 atm) at 1300 • C for 15 min, and changes of the surface structures were observed by the in situ SEM and STM.All the SEM images were obtained with a primary electron beam of 2 keV, because the formation of buffer layer and graphene on SiC substrates are easily distinguished by low-voltage SEM contrasts [11,23,24].Ex situ micro probe Raman spectroscopy was also carried out for graphene and buffer layer evaluation before and after the selective oxygen etching.To avoid adverse influences of the adsorbates such as oxygen and water after the exposure to air during Raman measurement, the graphene/SiC samples after the Raman measurement in air was not used for the subsequent etching processes.

III. RESULTS AND DISCUSSIONS
Figure 2(a) shows a SEM image of SiC(0001) surface after oxygen etching for 15 min, where monolayer graphene extends about 2 µm in width from the step bunches (white v-shaped arrows), and a triangle form of graphene in a pit is found on a terrace.Striped morphology indicated by black arrows is a typical feature of monolayer graphene formed on SiC substrates, and the formation mechanism was discussed in our previous report [22].A Raman mapping of graphene-2D peak intensity in the same area of Fig. 2 ter oxygen treatment are shown in Fig. 3.The spectra (a) and (b) were taken from the surface buffer layer regions on the terraces, while spectra (c) and (d) were taken at the monolayer graphene region near the step bunch.Raman signals from the bare SiC substrates have been subtracted from all 4 spectra.Broad peaks around 1350 cm −1 , 1490 cm −1 and 1580 cm −1 in spectrum (a) indicate the presence of a buffer layer [14].However, these signals are absent in the spectrum (b), which means that the surface buffer layer was removed and the bare SiC surface appeared after the oxygen etching.In the spectra (c) and (d), on the other hand, the typical graphene peaks (G: ∼1580 cm −1 ; 2D: ∼2700 cm −1 ) and buffer layer peaks are observed, indicating that monolayer graphene remained after the oxygen etching.A red shift of 2D peak (spectrum (d)) is due to the formation of oxide (Si + and Si 4+ ) between carbon layers and SiC substrate after oxygen treatment [17,19], which means that a small amount of oxygen probably still remains under graphene layer and oxidize the SiC substrates in our case.However width of the 2D did not change which denies formation of bilayer graphene.Thus, it was confirmed that the annealing in the extreme low partial pressure oxygen atmosphere also caused selective etching of the surface buffer layer on terraces, while, the buffer layer underneath monolayer graphene still exists as the buffer layer.
In situ RHEED and STM observations also support the presence of buffer layer underneath the monolayer graphene after the oxygen treatment.Figure 4(a) shows a RHEED pattern from the SiC with graphene in the azimuth direction of [1010] before oxygen etching, showing diffraction spots of SiC (1×1), SiC (6 3) and graphene, as has been observed previously [25,26].The SiC (6 √ 3×6 √ 3), which indicates the presence of the buffer layer are still observed in the RHEED pattern for the oxygen etched surface (Fig. 4(b)).As shown by Raman spectra, surface buffer layer was completely etched by oxygen treatment, so that the 6 √ 3×6 √ 3 spots in the RHEED pattern originated from the buffer layer underneath the monolayer graphene.
Figure 5(a) shows a SEM image of an oxygen etched surface with monolayer graphene formed from step bunches.A STM image taken from a region indicated by a black frame in Fig. 5(a) includes the bare SiC area and the graphene area with the striped morphology, as shown in Fig. 5(b).Due to the difference of conductivity between graphene and SiC, the bare SiC areas, being placed in a lower position, give distinct noise in STM. Figure 5(c) shows a magnified STM image of the graphene area scanned in the black dashed frame in Fig. 5(b), where a clear moiré pattern of SiC (6×6) is observed.Such moiré pattern also indicates the existence of the buffer layer under monolayer graphene [27,28], that is consistent with the Raman and RHEED observations.In addition, besides the moiré pattern, a random undulation is observed as well, which is probably also due to the formation of small amounts of oxide under carbon layers, corresponding to the red shift of graphene 2D peak discussed above.
Figures 6(a-h) show a series of SEM images of surface changing by selective oxygen etching at two different positions on the same sample.As mentioned above, there is a temperature distribution along the sample during the direct current heating.As a result, graphene is first formed at the center area of SiC substrate with a size of 3-  of the etching at various temperature regions on the same sample by SEM.
Figures 6(a-d) show a series of SEM image in the buffer layer region before (Fig. 6(a)) and after oxygen etching for 5 min (Fig. 6(b)), 10 min (Fig. 6(c)) and 15 min (Fig. 6(d)).In Fig. 6(a), the buffer layer covers terraces before the oxygen treatment, and a bare SiC terrace at the left corner with a brighter contrast where buffer layer does not form yet. Line-like contrasts indicated by arrows are step bunches between the buffer layer covered terraces.After oxygen etching for 5 min (Fig. 6(b)), the buffer layer is etched and bare SiC surfaces appear with expanding its width about 100 nm at step bunches (v-shaped arrows), indicating that the etching of buffer layers starts from its edges.In addition, some bright spots as shown by black triangles appear on terraces, suggesting that the etching occurs at defect sites of a buffer layer.It is common that the edges and defects with a lower chemical stability will be etched first.Moreover, as is observed at the lower left corner of Fig. 6(b), a ruggedly etched edge appears at the downside of a step bunch which is different from the case of the uniform etching at other edges.Actually, some etching sites are also observed at the downside of step bunches, as indicated by black arrows in Fig. 6 (a larger area view of Fig. 6(b)).By subsequent oxygen etching (Fig. 6(c)), buffer layer is further etched form step bunches (v-shaped arrows) and defects (black triangles), thus forming wider areas of bare SiC surfaces.Moreover, the etching began to occur on the entire buffer layer area with forming bright freckles in it.Interestingly, in this SEM image (Fig. 6(c)), it can be observed that the etching of surface buffer layer occurs non-uniformly on a terrace.Figure 6(j) shows a schematic section view and a graph of average contrast change (gray value) across the dashed frame region from A to B in Fig. 6(c).It reveals a distribution of gray value across the terrace in the graph, which indicates higher buffer layer coverage at A side than that at B side.Therefore, surface buffer layer more likely be etched at upper side of a terrace (B side).According to the report from Strupinski et al. [11], the surface buffer layer will form in higher quality with a more ordered structure and less sp 3 hybridization under a condition of faster Si sublimation (i.e., more supply of free C atoms).They also reported that buffer layer formed near an upper side of a step bunch has higher quality due to the abundant supply of C atoms form the step bunch.Thus, our observation can be explained as follow; the surface buffer layer formed at the upper side of a step bunch has a wellorganized structure, while the high density of defects and sp 2 -to-sp 3 rehybridization in the buffer layer at the lower side of a step bunch due to a deficit of carbon makes it much easier to be etched.Then the ruggedly etched edge in figure 6(b) and the etching sites at the downside of step bunches (Figures 6(i)) also agree with this explanation.Finally, in Fig. 6(d), after oxygen etching for 15 min, the entire buffer layer was etched off and the whole surface changed to a bare SiC surface.
A series of SEM images in Figures 6(e-h) show the surface changes in a higher annealing temperature region where monolayer graphene nanoribbons had grown at step bunches with width of tens nm to about 200 nm, as shown by arrows in Fig. 6(e).The first 5-min oxygen etching did not bring about noticeable change in morphology, as shown in Fig. 6(f).As described above, in the lower tem-perature area buffer layer edges were firstly etched after oxygen etching for 5 min (see Fig. 6(b)).At the higher temperature area, however, such etching of buffer layer edges did not occur (Fig. 6(f)).As reported by Norimatsu et al. [29], completely-grown buffer layer at lower side of the step bunches is connected to the nanoribbons at upper side of step bunches, which prevents the buffer layer from etching at the step bunch.Subsequently, the etching of entire buffer layer areas started and finally only graphene nanoribbons remained on the SiC surface, as shown in Figures 6(g) and (h).Notably, comparing with case in Fig. 6(c), the etching form defects and the non-uniform etching on terraces did not arise at the region in Fig. 6(g), which means a faster Si sublimation on the higher temperature surface supplies sufficient C atoms to facilitate the formation of the high quality surface buffer layer.Such investigation also agrees with the report in the previous study [11].In addition, under a higher etching temperature in Fig. 6(g), oxygen atoms can diffuse faster and further on the sample surfaces, leading to the decrease in the number of etching sites and thus enlarging the area of an etched region.

IV. CONCLUSIONS
Annealing processes of thermally decomposed SiC(0001) in an O 2 -Ar atmosphere was investigated by in situ SEM/STM.We found that the annealing selectively etched the surface buffer layer on SiC terraces and left the monolayer graphene intact, showing that we have fabricated monolayer graphene without the connection to a buffer layer on SiC substrates.It was also found that, the non-uniform etching occurred on buffer layer region during the etching process, indicating distribution of the crystallinity of surface buffer layer on a terrace.This study must be significant for the fabrication of monolayer graphene and its applications in graphene-based devices.

FIG. 1 .
FIG. 1. Schematic cross sections of SiC substrates with graphene ribbon on their surfaces (a) before and (b) after the selective oxygen etching.

FIG. 2 .
FIG. 2.(a) SEM image of the thermally decomposed SiC(0001) surface after selective oxygen etching, (b) Raman mapping of Graphene 2D peak at the same area of (a).

FIG. 3 .
Figure2(a) shows a SEM image of SiC(0001) surface after oxygen etching for 15 min, where monolayer graphene extends about 2 µm in width from the step bunches (white v-shaped arrows), and a triangle form of graphene in a pit is found on a terrace.Striped morphology indicated by black arrows is a typical feature of monolayer graphene formed on SiC substrates, and the formation mechanism was discussed in our previous report[22].A Raman mapping of graphene-2D peak intensity in the same area of Fig.2(a) is shown in Fig.2(b).The mapping image well corresponds to the SEM image, indicating that graphene remains along the step bunches and in the pit after the oxygen etching.Raman spectra of the graphene samples before and af- 4 mm and surrounded by buffer layer and SiC areas.Utilizing this temperature distribution, we can track the behaviors http://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) Volume 15 (2017) Wang, et al.

FIG. 5 .
FIG. 5. (a) SEM image of a graphene formed surface after selective oxygen etching, (b) a STM image scanned in the black frame in (a), (c) STM image scanned in the black dashed frame in (b).All the STM images were taken at V b = −1.5 V, It = 0.1 nA.