Improvement in Film Quality of Epitaxial Graphene on SiC(111)/Si(111)

by SiH4 Pretreatment

Shota Sanbonsuge1, Shunsuke Abe1, Hiroyuki Handa1, Ryota Takahashi1,

Kei Imaizumi1, Hirokazu Fukidome1�, and Maki Suemitsu1;2

1RIEC, Tohoku University, Sendai 980-8577, Japan2JST/CREST, Chiyoda, Tokyo 107-0075, Japan

Received November 30, 2011; revised February 18, 2012; accepted February 27, 2012; published online June 20, 2012

The epitaxy of graphene on 3C-SiC/Si (GOS) has attracted much attention owing to its viability to fuse graphene with Si-based technologies. It is

known that the surface condition of the 3C-SiC thin film before graphitization plays a decisive role in determining the quality of the GOS film. We

have investigated the effect of the pretreatment of the 3C-SiC thin film in vacuo at a SiH4 partial pressure of 6:7� 10�4 Pa on the subsequent

formation of graphene. As a result, it is revealed that the SiH4 pretreatment restores the defects on the SiC surface, such as the Si vacancy and

point defects formed by the presence of native oxides, and improves the quality of graphene. The effect is found to be highest when the substrate

temperature is 1173K. # 2012 The Japan Society of Applied Physics

1. Introduction

Graphene, a single layer of sp2-bonded carbon atoms, isbecoming a promising material for the next-generationdevices owing to its excellent properties, such as highcarrier mobility1) and chemomechanical stability.2) One ofthe issues of graphene toward device applications, however,is to grow large-area graphene films on solid substratesadequate for device applications. The epitaxial grapheneformed by the annealing of SiC bulk crystals to sublimatesurface Si atoms is beneficial in this respect.3–7) Twoadvantages of this epitaxial graphene can be cited: thesimplicity of the epitaxial method and the high quality of thegraphene.7)

One of the largest issues to be tackled for this epitaxialgraphene in considering its device applications is the lack ofcost effectiveness; the SiC bulk-crystal substrates are morecostly than Si ones by roughly two orders of magnitude. Tosolve this issue, we have developed a new epitaxial methodfor graphene using 3C-SiC thin films on Si substrates(graphene on silicon: GOS).8–17) GOS can be obtained bysimply annealing 3C-SiC thin films in ultrahigh vacuum(UHV).8–17) By using GOS as a channel, we succeeded inproducing field-effect transistors (GOS-FETs).18–21) Even theinverter action of the GOS-FETs, if not perfect, has beendemonstrated.22–24) Thus, the GOS technology has a highpotential for industrial applications.

The film quality of GOS, however, still remains as achallenge. To improve the film quality of graphene in GOS,the surface conditions of the base 3C-SiC thin film should beimproved.10) Seo et al.25) and Ferrer et al.26) improved theflatness of the surface of bulk SiC crystals by annealing inSiH4 atmosphere25) and under Si flux.26) The effectiveness ofthe pretreatments in the surface flattening is understood to bedue to the promotion of surface atom diffusion and stepbunching.

In this study, we have adopted this SiH4 pretreatment inthe GOS process. As a result, the pretreatment is found toimprove the quality of GOS. The improvement is not,however, due to the flattening of the surface, but to therestoration of defects within the surface domains of 3C-SiC(111) thin films.

2. Experimental Methods

Si(111) substrates, after being degreased by ultrasonicationwith acetone and ethanol, were introduced into an UHVchamber (base pressure: 6:7� 10�8 Pa). After the flashannealing of the substrates up to 1473K, 3C-SiC thin films(about 100 nm thick) were grown on the substrates at 1323Kby gas-source molecular beam epitaxy using monomethylsi-lane (CH3SiH3: MMS) as a single precursor. The MMSpressure was fixed at 4:0� 10�2 Pa.8–17) After the evacua-tion of MMS, the samples were annealed in a SiH4

atmosphere (6:7� 10�4 Pa) at 873–1423K for 30min. Afterthis SiH4 pretreatment, the samples were annealed in theUHV chamber at 1523K to grow epitaxial graphene. Atomicforce microscopy (AFM) measurements were conducted toevaluate the surface flatness of the sample. The measure-ments were carried out in atmospheric environments atroom temperature by using a noncontact mode AFM (Veecodi Innova). The quality of GOS was evaluated by Ramanscattering spectroscopy conducted using a Raman micro-scope (Renishaw inVia Microscope) operated at a laserenergy of 2.41 eV.

3. Results and Discussion

Figure 1 shows Raman spectra of GOS with the SiH4

pretreatment at temperatures ranging from 873 to 1423K. Inall the spectra, well-defined G (�1600 cm�1), D (�1350

cm�1), and G0 (�2700 cm�1) bands appear. The G band is aband coming from a single-resonant Raman process and isassociated with a doubly degenerate in-plane transverseoptical (TO) mode and a longitudinal optical (LO) phononmode at the � point.27) The D (�1360 cm�1) band is a bandcoming from double-resonant Raman processes near the Kpoint and is related to microscopic defects because theenergy and the momentum are not conserved within thesimple electron–phonon scheme.27) The G0 band is theovertone of the D band. The G0 band, however, is not relatedto any defects, but to the intrinsic electronic structure ofgraphene28–30) and the two-dimensional crystallinity ofgraphene.29) Other features at 2970 and 3250 cm�1 areascribable to the combination mode of the D and G bands,and the overtone of the G band, respectively.31,32) Theappearance of these bands therefore indicates the formationof graphene on all the samples.�E-mail address: fukidome@riec.tohoku.ac.jp

Japanese Journal of Applied Physics 51 (2012) 06FD10

06FD10-1 # 2012 The Japan Society of Applied Physics

REGULAR PAPERDOI: 10.1143/JJAP.51.06FD10

The degree of the film quality is quantified by evaluatingthe intensity ratio of G and D bands (IG=ID), and that of G0

and G bands (IG0=IG), as shown in Fig. 2. The intensity ratioIG=ID indicates the density of defects at edges.27–30) On theother hand, the intensity ratio IG0=IG indicates the two-dimensional crystallinity of graphene.29) The intensity ratiosare calculated using the Raman spectra shown in Fig. 1. Thehorizontal dotted lines indicate the intensity ratios IG=ID andIG0=IG of GOS formed without the SiH4 pretreatment. Twofindings can be extracted from Fig. 2. First, the film qualityof GOS in terms of the defect density and the two-dimensional crystallinity is improved by the SiH4 pretreat-ment. This is because the intensity ratios of the pretreatedsamples are higher than those of the nonpretreated samples.Second, the SiH4 pretreatment is most effective when thepretreatment temperature is 1173K. This temperature can berelated to SiC surface reconstructions. In the high tempera-ture range from 1323 to 1423K, the 6H-SiC(0001) surface isknown to provide a disorder (1� 1) structure.33) In the lowtemperature range from 873 to 1073K, islands consisting ofexcess Si atoms are formed on the SiC surface.33) As

opposed to these temperature ranges, at 1173K, where GOSexhibits the best film quality, the SiC surface is uniformly(3� 3)-reconstructed.33) It is therefore suggested that thebest film quality is obtained at 1173K owing to the uniform(3� 3) reconstruction of SiC formed by the SiH4 pretreat-ment.

The effect of the SiH4 pretreatment on the surfacemorphology of 3C-SiC(111)/Si(111) was investigated byAFM, as shown in Figs. 3 and 4. Figure 3 shows AFMimages of SiC(111) on Si(111) obtained before (a) and after(b) graphitization, without the SiH4 pretreatment. The root-mean-square (RMS) value for the surface roughness (8.8 nm)does not change with graphitization. Figure 4 shows AFMimages of SiC(111) on Si(111) obtained before (a) and after(b) graphitization with the SiH4 pretreatment at 1173K. Thesurface of SiC after the SiH4 pretreatment is significantlyroughened (50 nm) compared with that before the SiH4

pretreatment (8.8 nm). Still, the local surface roughness inthe area shown by the red line shows similar value (8.7 nm)to that before the pretreatment. The marked increase inthe roughness of the pretreated surface is due to theformation of large islands, as shown in Fig. 4(a). The largeislands are ascribed to epitaxially grown Si(111) islands,34)

whose formation is understood to occur under the growthconditions in the vicinity of the boundary between the(3� 3)-reconstructed SiC and the formed of Si(111)islands.34) The Si(111) islands disappear in the AFM image

1000 1200 14000.5

0.6

0.7

0.8

0.9

SiH4 pretreatment temperature (K)

IG' / IGIG / ID

Peak

inte

nsity

, rat

io

without SiH4 pretreatment

Fig. 2. (Color online) SiH4 pretreatment temperature dependence of

IG=ID and IG=IG0 estimated from the spectra in Fig. 1. The horizontal dotted

lines indicate the intensity ratios IG=ID and IG0=IG of GOS formed without

the SiH4 pretreatment.

(a)

RMS:8.8 nm

(b)

RMS:8.8 nm

Fig. 3. (Color online) AFM images of (a) SiC(111)/Si(111) and (b) GOS

without SiH4 pretreatment. The dimensions of the scanned areas are

2� 2 �m2.

2000 3000Raman shift (cm-1)

Inte

nsity

(ar

b. u

nit) (a) 1423 K

(b) 1323 K

(c) 1173 K

(d) 1073 K

(f) 873 K

(e) 973 K

(g) without pretreatment

Fig. 1. (Color online) Raman spectra of GOS formed after annealing

SiC(111)/Si(111) in SiH4 atmosphere fixed at 6:7� 10�4 Pa at various

temperatures: (a) 1423, (b) 1323, (c) 1173, (d) 1073, (e) 973, and (f) 873K.

(g) GOS without SiH4 pre treatment.

S. Sanbonsuge et al.Jpn. J. Appl. Phys. 51 (2012) 06FD10

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after graphitization [Fig. 4(b)], and the surface roughnessbecomes even smoother (4.8 nm). We understand thissmoothening in terms of Si atom sublimation at the hightemperature (1523K) used for the graphitization. Figure 5shows the surface roughness (RMS value) of GOS pretreatedwith SiH4 at various temperatures. It can be seen that thesurface roughness is smallest after the treatment at 1170K.This suggests that the improvement in surface smoothnessby the SiH4 pretreatment plays the most dominant role inthe improvement in the quality of GOS. Thus, the AFMroughness of 3C-SiC before graphitization cannot becorrelated with the film quality of GOS on the basis of theeffect of the SiH4 pretreatment.

Figure 6 displays the polarization dependence of theRaman spectra of GOS with (red) and without (black) theSiH4 pretreatment. Polarized Raman spectroscopy is apowerful tool for evaluating GOS, as has been alreadydemonstrated in our previous paper.10) Several points can benoted. First, the IG=ID ratio is higher in the s-polarizedspectra than in the p-polarized spectra for both of thesamples. Since the p-polarized spectra convey informationon the vibration component normal to the surface, this resultindicates that the portion of graphene that gives rise tosurface normal components of G and D vibrations ismore defective than the portion that contributes to surfaceparallel components. Second, the IG=ID intensity ratio in the

s-polarized spectra is increased by 47% with the SiH4

pretreatment from 0.86 to 1.26. In sharp contrast, the IG=IDintensity ratio in the p-polarized spectrum is only slightlyincreased with the SiH4 pretreatment, from 0.63 to 0.73.These results of the polarized Raman spectroscopy indicatethat the effect of the SiH4 pretreatment on the quality ofgraphene is more remarkable in the portion parallel to thesubstrate surface.10) This is because the G and D bands arethe vibrational modes that are parallel to the plane ofgraphene. It can be therefore speculated that, although theSiH4 pretreatment cannot suppress the surface roughness,e.g., steps, facets and islands, the quality of the SiC portionwithin the grains parallel to the surface is improved. Thesurface of the SiC thin film has a Si vacancy, a clusteringdefect, and native oxide.25,35,36) We therefore tentativelyconsider that some of these defects are restored by the SiH4

pretreatment (Fig. 7). This restoration improves the filmquality of SiC on the terrace; thus, the gross quality of theGOS film is increased. In a previous report on the SiH4

pretreatment of 6H-SiC(0001), the surface can be atomicallyflattened. The pretreatment of 3C-SiC(111) thin films onSi(111) in this study cannot produce an atomically flattened

(a)

RMS:50 nm

(b)

RMS:4.8 nm

Fig. 4. (Color online) AFM images of (a) SiC(111)/Si(111) after SiH4

pretreatment at 1173K (RMS of the portion surrounded by the red line is

8.7 nm) and (b) GOS formed on (a). The dimensions of the scanned areas are

2� 2 �m2.

1000 1200 14004

6

8

10

12

SiH4 pretreatment temperature (K)

RM

S

without SiH4 pretreatment

Fig. 5. SiH4 pretreatment temperature dependence of RMS estimated

from AFM.

2000 3000

(a)

(b)

(c)

(d)

Raman shift (cm-1)

Inte

nsity

(ar

b. u

nit) p-pol

s-pol

Fig. 6. (Color online) Polarized Raman spectra of GOS with and without

the SiH4 pretreatment at 1173K. (a) p-polarized spectrum with the SiH4

pretreatment, (b) p-polarized spectrum without the SiH4 pretreatment,

(c) s-polarized spectrum with the SiH4 pretreatment, and (d) s-polarized

spectrum without the SiH4 pretreatment.

S. Sanbonsuge et al.Jpn. J. Appl. Phys. 51 (2012) 06FD10

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surface, though, to some extent, the surface roughness isimproved. One of the reasons for the difference may be thatthe partial pressure of SiH4 in our study is not yet optimized.

4. Conclusion

Effects of the SiH4 pretreatment of 3C-SiC(111) on thequality of GOS have been investigated. It is clarified that1173K is the optimal temperature for the pretreatment,which is ascribed to the formation of a uniformly(3� 3)-reconstructed 3C-SiC(111) surface at this tempera-ture. The effect of the SiH4 pretreatment on the film qualityis related to the reparation of the defects in the grains on theterraces of the 3C-SiC(111) films. For future work, thepartial pressure of SiH4 for the pretreatment should also beoptimized to further improve the film quality of GOS.

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before SiH4 pretreatment after SiH4 pretreatment

defects reparation of defects

Fig. 7. (Color online) Schematics on the effect of the reparation of

defects in grains on SiC surfaces.

S. Sanbonsuge et al.Jpn. J. Appl. Phys. 51 (2012) 06FD10

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