Physica E 43 (2011) 1415–1418

Contents lists available at ScienceDirect

Physica E

1386-94

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/physe

Graphene films grown on Si substrate via direct depositionof solid-state carbon atoms

J. Tang, C.Y. Kang, L.M. Li, W.S. Yan, S.Q. Wei, P.S. Xu n

National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, PR China

a r t i c l e i n f o

Article history:

Received 17 January 2011

Received in revised form

15 March 2011

Accepted 16 March 2011Available online 6 April 2011

77/$ - see front matter Crown Copyright & 2

016/j.physe.2011.03.014

esponding author.

ail address: psxu@ustc.edu.cn (P.S. Xu).

a b s t r a c t

Graphene films were grown on Si(1 1 1) substrates at different temperatures (600, 700 and 800 1C) by

directly depositing solid-state carbon atoms through solid-state molecular beam epitaxy (SSMBE). The

structural properties were characterized by reflection high energy electron diffraction (RHEED), Raman

spectroscopy and near-edge X-ray absorption fine-structure (NEXAFS). The results showed that SiC

precipitates primarily formed at 700 and 800 1C, as a small quantity of carbon atoms were deposited.

When carbon atoms continued depositing, graphene films formed at 800 1C and amorphous or

polycrystalline carbon thin films formed at 700 and 600 1C. We think that graphene films could be

grown on Si(1 1 1) substrates because of the high activity of carbon atoms at 800 1C and weak

interaction between carbon atoms and silicon atoms of substrate due to a SiC buffer layer stabilizing

the surface of Si substrate.

Crown Copyright & 2011 Published by Elsevier B.V. All rights reserved.

1. Introduction

Due to its remarkable electronic properties, such as theexceedingly high charge carrier mobility and ballistic transportat room temperature, graphene has attracted much attention as apotential material for the next generation of electronic devices [1].Since the discovery of isolated graphene through mechanicalexfoliation of highly oriented pyrolytic graphite (HOPG) [2],various methods have been explored to produce this material.These include reduction of graphene oxide [3], chemical vapordeposition of hydrocarbons on transition metal surfaces [4,5],splitting carbon nanotubes to form graphene ribbons [6] andultrahigh vacuum (UHV) decomposition of a-SiC wafers [7,8].However, many of these techniques require unusual substratematerials. With its compatibility with Si technology, graphenegrown on Si substrate might be one of the most promisingcandidates as a material for beyond graphene-based CMOStechnology. Recently, Hackley et al. [9] have explored the growthof carbon films on Si substrates by firstly depositing amorphouscarbon layer and then annealing at different temperatures. Theyfound that graphite carbon could form on Si(1 1 1) wafers.However, it is not clear whether graphene can be grown on Sisubstrate at reasonable temperatures via directly depositingsolid-state carbon atoms or not.

011 Published by Elsevier B.V. All

In this paper, we explore graphene films grown on Si(1 1 1) atdifferent temperatures by directly depositing solid-state carbonatoms. The structural properties of the samples were character-ized by RHEED, Raman spectroscopy and NEXAFS.

2. Experimentals

The samples were prepared in a home-made MBE system withbase pressure of 6.0�10�8 Pa. Si and C sources with purity of99.999 % were both supplied by electron-beam evaporators [10].The evaporation rates of Si and C were measured by quartz crystaloscillators (MAXTEKTM-350) with the precision of 0.1 nm. The Siwafers were cleaned by chemical method, namely: (1) dipped incarbon tetrachloride, acetone and alcohol for 5 min in order toremove grease; (2) treated with H2SO4:H2O2 (1:1) for 3 min towipe off metal powder and organic contaminations; (3) etched in5% HF solution for 3 min to remove the surface native oxides and(4) rinsed several times with deionized water. After being dried ina flux of N2, the wafers were placed into the chamber immedi-ately. At first, a Si buffer layer with the thickness of around 5 nmwas grown on all Si(1 1 1) substrates at 800 1C with the deposi-tion rates around 0.3 nm min�1. Then, the samples were coveredby carbon atoms at the evaporation rates of 0.2 nm min�1 anddiffferent substrate temperatures of 600, 700 and 800 1C. Thethickness of carbon films was around 3 nm. After deposition,the samples were cooled down to room temperature in 60 min.

During the process of preparation, the sample surfaces werein situ monitored by RHEED, which was operated at accelerating

rights reserved.

J. Tang et al. / Physica E 43 (2011) 1415–14181416

voltage of 21 kV and ejecting current of 50 mA. Raman spectrawere measured using Ar ions laser with a wavelength of514.5 nm. The C K-edge NEXAFS measurement was performedin the total electron yield (TEY) detection mode on beamline U19at the National Synchrotron Radiation Laboratory, China.

3. Experimental results and discussion

As shown in Fig. 1, a series of RHEED patterns were acquiredalong the [1 1 0] azimuth in the preparation process of the samples.

Fig. 1. RHEED patterns of Si substrate with Si buffer layer (S0) and the samples prepar

grown at 600, 700 and 800 1C, when 0.5 nm-thickness carbon films were deposited res

800 1C, when 3 nm-thickness carbon films were deposited, respectively.

Fig. 1 (S0) shows a clean Si(1 1 1) 7�7 reconstruction, which wasobserved after growth of Si buffer layer at 800 1C for all samples.Fig.1 (S1), (S2) and (S3) shows the RHEED patterns of the samplesgrown at 600, 700 and 800 1C, when around a carbon layer of around0.5 nm-thickness was covered on the substrates, respectively. InFig. 1 (S1), only the RHEED patterns of Si(1 1 1) 1�1 can beobserved, and the diffraction stripes of Si(1 1 1) 7�7 reconstructionsare washed out. It meant that the deposited carbon atoms justadhered to the surface of Si substrate, but did not react chemicallywith Si atoms of substrate to form SiC. As carbon atoms weredeposited at higher temperatures of 700 and 800 1C, a transformation

ed at different temperature. S1, S2 and S3 are the RHEED patterns of the samples

pectively; S10 , S20 and S30 are the RHEED patterns of the samples at 600, 700 and

J. Tang et al. / Physica E 43 (2011) 1415–1418 1417

from amorphous carbons to SiC precipitates occured. This progres-sion was marked in RHEED images, which gradually displayed faintSiC spots (Fig. 1 (S2)) and sharp SiC spots (Fig. 1 (S3)). The large andspotty diffraction patterns in Fig. 1 (S2) suggested three-dimensionalisland growth mode (Volmer–Weber) at 700 1C, while the sharpand streaky diffraction patterns in Fig. 1 (S3) suggested a mixedmode: layer by layer to a critical thickness and island formation(Stranski–Krastanov) at 800 1C [11]. For this reason, it could beinferred that the quality of SiC layer formed at 800 1C was better thanthat formed at 700 1C. Actually, this process of SiC precipitatesformed at 800 1C is similar to that of carbonization of Si surface inthe epitaxial growth of SiC films [12]. As carbon atoms continuedincreasing the diffraction spots of Si and SiC vanished, and then thediffraction rings of carbon films appeared. Fig.1 (S10), (S20) and (S30)shows the RHEED patterns of around 3 nm-thickness carbon films,which were deposited on Si substrates at 600, 700 and 800 1Crespectively. It shows that diffraction rings existed in all patterns,which indicates that the three-dimensional disorder films weregrown on Si substrates probably [13]. However, the ring at 800 1C(Fig. 1 (S30)) is tenuous, and the rings at 700 1C (Fig. 1 (S20)) and600 1C (Fig. 1 (S10)) are diffuse, which indicates that the degree oforder for the carbon thin films was improved as the growthtemperature increased from 600 to 800 1C.

Recently, Raman spectroscopy has been extensively utilizedas a convenient technique for identifying graphene layer. Fig. 2(a),(b), (c) and (d) shows the Raman spectra of the samples preparedat 600, 700 and 800 1C and HOPG, respectively. As shown inFig. 2(a) and (b), there were just weak D and G peaks inthe Raman spectra of the samples grown at 600 and 700 1C,respectively. Generally, the G vibration mode corresponds toE2g-symmertry phonons at the Brillouim zone (BZ) center ofgraphite, and the D mode corresponds to a breathing mode ofdisorder sixfold aromatic carbon ring [14]. Therefore, there wasevidence that amorphous or polycrystalline carbon thin filmsformed at 600 and 700 1C [15,16]. However, besides sharp D andG peaks located at 1348 and 1595 cm�1, respectively, 2D peaklocated at 2687 cm�1 was obviously observed in the Ramanspectrum of the sample at 800 1C (Fig. 2(c)). The presence of the2D band in Raman spectra of graphene had been explained on thebasis model of double phonons resonance in the highest opticalbranch near the K point of BZ [17]. Generally, the G and 2D peakswere regarded as the characteristic peaks in the Raman spectra ofgraphene [18]. Thus, it can be inferred that graphene films formed

Fig. 2. Raman spectra of the samples grown at 600 1C (a), 700 1C (b) and 800 1C (c),

and HOPG (d).

at 800 1C. Compared to the G and 2D peaks of HOPG (Fig. 2(d))located at 1580 and 2726 cm�1, respectively, there were signifi-cant shifts of G peak (15 cm�1) and 2D peak (�48 cm�1) of thesample grown at 800 1C. According to Fig. 1 (S3), a SiC layer hadbeen produced before graphene films formed. Therefore, the shiftof Raman peaks should have some kind of relationship with SiCbuffer layer. Generally, the G peaks of epitaxial graphene (EG)grown on Si-terminated SiC (Si–SiC) and C-terminated SiC (C–SiC)had similar frequency (�1597 cm�1), which upshifted 17 cm�1

compared to that of HOPG. The G peak shifts of EG on C–SiC andSi–SiC were attributed to similar stress caused by lattice mis-match between graphene and SiC [19]. However, the 2D peak ofEG grown on Si–SiC and C–SiC were different. Compared to the 2Dpeak of HOPG (2726 cm�1), EG on C–SiC had lower 2D peak(�2682 cm�1), while EG on Si–SiC had higher 2D peak(�2736 cm�1). The 2D peak shift of EG on Si–SiC was attributedto the interface interaction between graphene and Si–SiC, asabout 30% of C atoms from the first graphene layer werecovalently bonded to SiC [20]. Conversely, such covalent bondswere absent at the interface between graphene and C–SiC [20].Our Raman results showed that the G- and 2D-peaks of thesample grown at 800 1C had similar frequency to that of EG onC–SiC. Therefore, the reason of G and 2D peak shifts of the samplegrown at 800 1C should be similar to that of EG on C–SiC. Inaddition there was a sharp D peak observed in Fig. 2(c), whichimplied that the graphene films grown at 800 1C had a largeconcentration of defects.

Concurrently, the NEXAFS spectra were obtained and used toeffectively characterize sp2 hybridized C atoms of the samples.Fig. 3(a), (b), (c) and (d) shows the C K-edge NEXAFS spectra ofHOPG, and the samples grown at 800, 700 and 600 1C, respec-tively. As shown in Fig. 3(a) and (b), the NEXAFS spectra of HOPGand the sample at 800 1C are well characterized by a sharp 1s-p�peak clearly located at 285.5 eV and the 1s-sn absorption peak;evidently the sn peak contained two shoulder peaks s�1 at291.2 eV and s�2 at 292.1 eV. Following Fig. 3(c) and (d) therewere weak pn and sn resonance absorption peaks, but theshoulder peaks of sn resonance absorption could not be distin-guished. Generally, the two-dimensional nature of grapheneresults in a strong directionality of the orbitals: s orbitals liewithin the basal plane, while p orbitals are directed perpendicularto the basal plane. Compared with the samples at 700 and 600 1Cthe sn peak in the NEXAFS spectrum of the sample at 800 1C had

Fig. 3. C K-edge NEXAFS spectra of HOPG (a) and the samples grown at 800 1C (b),

700 1C (c) and 600 1C (d).

J. Tang et al. / Physica E 43 (2011) 1415–14181418

higher intensity and distinguishability, which indicated that theordered degree of s orbitals in the plane of carbon films, thatformed at 800 1C was larger than those at 700 and 600 1C [21,22].The results of NEXAFS are in agreement with the results of Ramanspectra above. In addition, for the absorption spectra of allsamples, a peak at 287.1 eV existed, which came from C¼O pn

state brought by surface oxidation [23]. As shown in Fig. 3(b)and (c) there is a peak located at 287.6 eV, which comes from C–Sisp3 hybridization state of SiC [24], which is consistent with theobservation of RHEED.

Combined with the results of Raman and NEXAFS, we coulddraw a conclusion that the carbon films grown at 800 1C indeedshowed graphene characteristics and the carbon films grown at600 and 700 1C just displayed the features of amorphous orpolycrystalline carbon. During the process of the sample growth,the substrate temperatures played a key role. Firstly, due to thelow activity of carbon atoms at 600 1C, the carbon atoms evapo-rated just heaped up on the Si surface. Then, the activity of carbonatoms raised as the substrate temperature increased to 700 1C. Inthis case, the carbon atoms started to react chemically with Siatoms of substrate, but the carbon atoms did not have enoughenergy to form ordered six-membered rings in-plane. Finally, onincreasing the temperature to 800 1C, the carbon atoms hadenough activity for bonding reciprocally and forming well-orga-nized six-membered rings in-plane [25,26]. Simultaneously, awell SiC buffer layer produced at 800 1C could stabilize thesubstrate surface and prevent the reaction of C atoms with Siatoms of substrate. However, it is undeniable that the quality ofgraphene grown at 800 1C is still not satisfactory. In order toimprove the quality of graphene film, the optimized processesincluding appropriate substrate temperature and deposition rateof C atoms need to be performed.

4. Conclusion

Graphene films have been grown on Si(1 1 1) substrates at800 1C by directly precipitating solid-state carbon atoms, whileamorphous or polycrystalline carbon thin films formed at 600 and700 1C. The results of RHEED showed that well-ordered SiC layerprimarily formed at 800 1C, when a small quantity of carbonatoms were deposited. We thought that the substrate tempera-ture played an important role in the formation of graphenefilms on Si(1 1 1) substrates. Meanwhile, a well SiC buffer layer

primarily formed was helpful to stabilize the surface of Sisubsrate and improve the formation of graphene films.

Acknowledgment

This work is supported by the National Science Foundation ofChina (Grant no. 50872128).

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