Graphene Epitaxially Grown on Vicinal 4H-SiC(0001) Substrates∗

The growth mode of graphene epitaxially grown on vicinal SiC(0001) substrates as well as on-axis SiC(0001) surface has been investigated by scanning tunneling microscopy and low energy electron diffraction. The results show that the graphene favors to start to emerge from the edge of the surface steps, forming narrow graphene domains at the step edges on the vicinal surface with 4◦ off-axis angle, well separated by carbon rich (6 √ 3×6√3) domains, suggesting the possibility of obtaining graphene nano-ribbons. The graphene film grown on vicinal surface with larger off-axis angle (21◦) shows a wave-like morphology, exhibiting the continuity of graphene film across the step edges. [DOI: 10.1380/ejssnt.2009.29]


I. INTRODUCTION
Graphene has triggered intensive research activities in recent years both in theoretical and experimental fields [1] due to its attractive properties that make it an excellent candidate for the next-generation nano-devices [2][3][4].Especially, a great interest has been put on the unusual electronic structures induced by various geometries, e.g., graphene nano-dot [5], graphene bilayer [6], and graphene nano-ribbons [7,8].Among them, the graphene nanoribbons (GNRs) possess the most promising potential for the development of nano-technology.Previous theoretical study has predicted that GNRs may show unusual transport properties depending on the orientation of edges, which hold great potential for the spin based quantum computing and spintronics [10,11].GNRs have been divided into two distinct groups depending on the orientation of the edges, the armchair and zigzag ones [9].In the GNRs with zigzag edges, the transport properties depend typically on the existence of the edge states [12], i.e., the electronic states localized near the graphene domain edges.More importantly, due to their high degeneracy of these localized electrons, these states are predicted to be spin-polarized [13], showing localized ferromagnetic ordering.This unusual feature makes zigzag GNRs highly expected for the spin-polarized current device [14,15].Nevertheless, these attractive properties are still in lack of support in the experimental aspect.To promote the practical application of GNRs in the future, which will mainly depend on their unusual electronic structures, it is of great importance to synthesize suitable GNRs for the experimental study on the electronic properties, such as by means of angle-resolved photoemission spectroscopy etc.
Recent reports have shown that high quality graphene film low defect concentration in a large scale can be grown on the metal surfaces, such as Ni(111) [16], Ru(0001) [17], Ir(111) [18] etc., using low pressure chemical vapor deposition (CVD) methods at high substrate temperature.The graphene on the Ir(111) metal surface has been revealed to exhibit a continuity across the terraces and step edges [18], leading to the difficulties to obtain GNRs on its vicinal surfaces.
Thermal decomposition of SiC(0001) at high temperature in an ultra-high vacuum has been proved to be an alternative method to obtain high quality free-standing graphene film with large scale [4,19].Besides, the sub-ISSN 1348-0391 c 2009 The Surface Science Society of Japan (http://www.sssj.org/ejssnt)strates with vicinal surfaces are always of great technical importance to the nano-engineering with the selfassembling of one-dimensional and quasi one-dimensional systems.In our present work, vicinal surfaces of n-type 4H-SiC(0001) with off-axis angles of 4 • and 21 • as well as on-axis SiC(0001) surface have been used as the substrate for epitaxial growth of quasi one-dimensional graphene film, i.e. the GNRs.
Here we report the growth procedure of graphene on the stepped surfaces studied by scanning tunneling microscopy (STM), and show the growth pattern that graphene domain favors to emerge from the step edges of the vicinal surfaces, resulting in the formation of well separated narrow graphene domains on the step edges on the vicinal surface with small off-axis angle (4 • ) and wave-like graphene film on the vicinal surface with large off-axis angle (21 • ).These results hold a great importance not only for the scientific research to explore the unusual transport properties of graphene with various geometries, but also for the practical applications of the nano-scale engineering with carbon-based materials.

II. EXPERIMENTAL
The experiments were performed in an ultra-high vacuum scanning tunneling microscopy (UHV-STM) chamber, being attached with a home made preparation chamber that equipped with low energy electron diffraction (LEED) apparatus.All of the STM measurements were performed at room temperature.The electrochemically etched tungsten tip was used in the STM measurements, which exhibited an ideally hyperbolic shape under the SEM observation (Fig. 1(a)).The STM images were analyzed with the program developed by Horcas et al. [20].The commercially available n-type 4H-SiC(0001) wafers with various off-axis angles (0 • (on-axis), 4 • , 21 • ) were cut into roughly 3 mm×5 mm with a diamond cutter and mounted on the Mo sample holder by Ta wires after being cleaned ultrasonically in acetone and ethanol.Then the samples were induced into the UHV chamber, followed by in-situ degassing at 450 • C by applying direct current for several hours.Before annealing the SiC at high temperature for growing graphene, the SiC substrates were heated at 800 • C under Si flux in order to remove the native oxides.The Si flux was generated by heating a slice of Si wafer at about 1300 • C. Due to the large electric current (7.5 A) of the Si flux generator (Fig. 1(b)), two Mo rods (φ 1 mm) were used for support.Ta foil was used for good conduction to avoid the melting of the Si wafer following the design from Yates [21].All of the samples were examined by LEED and STM after being annealed at each different temperature.

III. RESULTS AND DISCUSSION
A. On-axis SiC(0001) substrate The graphene synthesizing experiment with on-axis SiC(0001) substrate was regarded as the preparation step for the study of the growth mode on the vicinal surfaces, in order to develop a proper condition for graphene fabrication in the following experimental stages due to its equipment sensitivity.The synthesizing of graphene with on-axis SiC(0001) was basically following the method described in ref. [4].Real space images of sample surface observed by STM in each stage of surface reconstructions in the growth development are illustrated in Fig. 2 with corresponding LEED patterns.bon rich phase with (6 √ 3×6 √ 3)R30 • super lattice [25] as illustrated in Fig. 2(e).The STM image also shows the (6 √ 3×6 √ 3) super lattice (Fig. 2(f)), corresponding to a partial graphitized surface which is commonly regarded as the initial stage of the graphene growth [22].When the annealing temperature reached 1180 • C, the surface started to experience a surface reconstruction process of forming graphene layer, through which a single layer of carbon atoms arranged in honeycomb structure was formed.However, this reconstruction was rather difficult to be identified from the changes of LEED patterns (Fig. 3(a)), since only slight change of some spot intensities could be found.The periodicity of (6 still dominated the LEED patterns, as well as SiC(1×1).Nevertheless, the LEED spots of graphene could be faintly observed.In the STM measurement on this sample, which has been annealed at 1180 • C, two kinds of phase regions, marked with 1 and 2 (and 2'), can be observed.Although the both regions 1 and 2 show the same lattice periodicity, the image shows different brightness to each other.The profile along the direction from A to B in Fig. 3(b) has been taken to analyze these two regions.It shows that the difference of height is only 0.67 Å (Fig. 3(c)), which suggests the difference of brightness between regions 1 and 2 is probably mainly due to the difference of density of states (DOS), since the STM image records the contour of the Fermi level local density of states (LDOS) in prin- ciple.In another words, the graphene phase that formed in region 1 possesses larger DOS around the Fermi level due to the appearance of π band, while the (6 phase in region 2 has much smaller DOS around Fermi level.It is easy to identify the tiny honeycomb structure of carbon atoms in graphene in region 1 with some longrange supper lattice periodicity that is corresponding to the (6 √ 3×6 √ 3)R30 • periodicity, which is resulted from the geometry relaxation between SiC and graphene.In the right panel of Fig. 3(d), STM image of graphene with an area of 20 Å×20 Å is illustrated.In each unit-cell of graphene lattice, there exist two carbon atoms, marked in red and blue circle, corresponding to the two sub-lattices of graphene.

B. 4 • off-axis vicinal SiC(0001) substrate
Based on the growth conditions of graphene specified for our experiment system with on-axis SiC(0001) substrate in last section, vicinal surface of SiC(0001) with off-axis angle of 4 • was used for the epitaxial growth of quasi one-dimensional graphene film, i.e. the GNRs.
After being degassed in the preparation chamber at 450 • C by the same method described with on-axis substrate, the sample with vicinal surface of SiC(0001) was transferred into STM chamber to observe the as-received stepped on the sample surfaces.Though clear STM image could not be acquired at this stage due to the incomplete desorbing of surface contaminants, uniform straight step edges could be observed in large area, where the average width of the steps was calculated by Fast Fourier Transformation to be 3.89 nm.This width is corresponding to a step height of h = 3.89 nm × tan4 • = 0.27 nm, which could be regarded as the 1/4 of the unit-cell height of 4H-SiC crystal (1.005 nm) with acceptable error ranges.In another word, the steps on the as-received sample surface were of atomic-height, corresponding to the height of one Si-C bilayer.
After the procedure of removing the native oxide on the surface, the sample with vicinal surface was annealed at 1000 • C until the appearance of ( √ 3× √ 3)R30 • LEED pattern (Fig. 4(a)).Faint one-dimensional patterns could also be observed, indicating the stepped feature of the sample surface.Fig. 4(b) illustrates the STM image of this surface within an area of 300 nm×300 nm.The step width became dramatically wider than that of as-received sample.As shown in the inset, the step width became typically 16 nm.With this step width, we noticed that the height of the step became typically around 1 nm, which directly corresponds to the unit-cell height of 4H-SiC.This kind of evolvement of the nano-step on the vicinal surface suggests the instability of the atomic height step, and could be understood as the trend of minimizing the surface energy, which can be described by the following equation [23]: We noticed that similar phenomenon had been reported previously in the research work of etching SiC(0001) surface by H 2 at high temperature [24].
Since it was uncertain whether graphene had formed at this temperature from the LEED pattern, we examined the surface with STM.
As shown in Fig. 5(b), the STM image clearly exhibits two kinds of phases on the stepped surface, which could give us some insight into the growth mode of graphene on the stepped surfaces.One of them is found at the step edge, and the other kind of phase is located at the inner side of the steps.The atomically resolved STM image reveals that the phase located at the edge is graphene, where honeycomb lattice structure could be identified in the topography image, and the other one is the carbonrich (6 Though the narrow graphene domain in Fig. 5(b) ex- hibits an irregular shape, the atomic structure of carbon atoms at the step edge could be investigated by extrapolating the atomic lattice from the inner side of the graphene domain.Both the armchair-type and the zigzagtype graphene edges were observed due to the irregular shape.This kind of narrow domain could be treated as quasi one-dimensional with a lattice vector k parallel to the edge.Besides, along the direction of k, the narrow domain of graphene in Fig. 5(b) can be regarded as a zigzag GNRs.Moreover, through the morphology profile across the step edge, as shown in Fig. 5(c), the height of the step is kept at about 1nm, showing stable feature of the unit-cell height steps.This observation experimentally revealed the growth mode of graphene on the stepped surface that the graphene phase favors to grow from the step edges, where Si atoms were selectively defused with priority as shown in Fig. 5(c).Graphene domain is located at the edges of the step (in X range: from 3 nm to 6 nm), where graphene domain appears to be a little bit higher than the ( 6√ 3×6 √ 3)R30 • phase on the left hand side due to the appearance of π band as described in the previous section.It suggests a growth behavior that the graphene domains overspread from the edges to the inner side of the surface steps during the growth process as depicted in the scheme in Fig. 5(d) (only the few top most layers are shown here, which are supported by the bulk SiC substrate), which could be understood through the thermal behavior of Si atoms that at high temperature Si atoms are moving in plan with the possibility of reaching the step edges and subsequently diffusing into the vacuum.This provides the possibility of synthesizing well separated and well ordered GNRs in large scale, if proper substrate has been prepared, since the geometry of graphene can be partly restricted by the shape of step edges.http://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) e-Journal of Surface Science and Nanotechnology In the following experiment, we have tried the epitaxial method on vicinal SiC(0001) surface with larger off-axis angle of 21 • .After the same pre-cleaning procedure of removing native oxide, the Si-rich (3×3) pattern could not be observed on the LEED image.Instead, a clear one-dimensional feature showing the periodicity of surface steps appeared on the LEED pattern (Fig. 6(a)), indicating an average step width of 2.8 nm.The STM measurement revealed large amount of surface steps with uniformed direction, on which (3×3) Si-rich phase with large defect concentration were found (Fig. 6(b)) due to the dominant anisotropy of the substrate.
Then this sample was annealed at 1000 • C for desorbing excessive Si atoms until obvious changes could be observed in the LEED image, where bright lines appeared between SiC(1×1) spots (Fig. 6(c)), indicating the formation of extra one-dimensional structures.Fig. 6(d) shows the STM image of this surface.Atoms in one dimensional arrangement were found on the stepped surface.The distance between two atoms in the same atomic chain was as large as that of the lattice constant of SiC (∼0.6 nm), which kept consistency with the bright line that emerged in the LEED image.
When the annealing temperature reached 1180 • C, the bright lines in the LEED pattern representing the one dimensional atomic chains disappeared, suggesting that those atomic chains were composed of silicon atoms, which were desorbed with elevating annealing temperature.At the same time, the LEED spots representing the surface steps became rather weak and indistinct, suggesting that the periodicity of steps had been somehow destroyed.
STM image of this surface is illustrated in Fig. 7(a).The step edges become difficult to be identified.The surface showed wave-like terrace morphology.The morphol- Since the STM image could be acquired with low bias voltage (sample bias: 0.2 V), there should be finite density of states near the Fermi level in the electronic structures, probably originated from the π band of graphene.
A close observation on this surface would reveal the super structure with ( √ 3× √ 3)R30 • periodicity, which is usually found in the disordered graphene film [26].The I-V curve measured in this region (Fig. 7(d)) shows semiconducting behavior with the energy gap of ∼0.4eV located at the Fermi level, which is consistent with the previously reported ARPES results obtained on single layer graphene film grown on on-axis SiC(0001) surface [27].However, the location of the gap center shows discrepancy, which has not been well understood, probably due to the ( √ 3× √ 3)R30 • disordering effect or the difference of the SiC substrate such as the doping level.The wave-like morphology of graphene film grown on 21 • off-axis vicinal SiC(0001) surface suggests that the graphene films on different surface step are bonded together (Fig. 7(e)).This result can be understood from the growth mode that has been depicted in the last section.Due to the large offaxis angle, the narrow width of surface steps makes it easy to form graphene that cover from the step edges to the inner side of the steps.Moreover, the narrow width of surface steps also means the large amount of step edges, which can provide more channels for the diffusion of Si atoms that make it easier to form large area of graphene domain.

IV. CONCLUSION
We have synthesized and characterized graphene nanofilm epitaxially grown on various SiC substrates, inhttp://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) cluding on-axis SiC(0001) surface and vicinal SiC(0001) surface with off-axis angles of 4 • and 21 • .The measurements performed by scanning tunneling microscopy on the stepped surfaces showed that graphene domain emerged from the step edges, being separated by carbonrich ( 6√ 3×6 √ 3)R30 • phase, with the advantage of overcoming the issue of the continuity of graphene across the terrace and step edges.It indicates a growth mode that Si atoms at the edge are firstly desorbed, leaving C atoms behind to form narrow graphene domains at the edges.Additionally, the phenomenon that the steps on 4 • offaxis vicinal SiC(0001) surface bunched from single atomic layer height to unit-cell height of 4H-SiC has also been observed in our work.Finally, the attempt of trying to fabricate graphene on the vicinal surface with large offaxis angle resulted in the wave-like graphene morphology, suggesting the importance of substrate that will affect the growth of graphene significantly.

Fig. 2 (
FIG. 2: LEED patterns and STM images obtained with elevation annealing temperature.(a) and (b) After being annealed at 800 • C under Si flux; (c) and (d) After being annealed at 1050 • C; (e) and (f) After being annealed at 1100 • C.

FIG. 5 :
FIG. 5: (a) and (b) LEED patterns and STM image obtained on 4 • off-axis SiC(0001) after being annealed at 1130 • C; (c) Morphology profile along "A" in (b); (d) Scheme of growth mode of graphene on stepped surface.