Carbon molecular beam epitaxy on various semiconductor substrates

Materials Research Bulletin 47 (2012) 2772–2775

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Carbon molecular beam epitaxy on various semiconductor substrates S.K. Jerng a, D.S. Yu a, J.H. Lee a, Y.S. Kim a, C. Kim b, S. Yoon b, S.H. Chun a,* a b

Department of Physics and Graphene Research Institute, Sejong University, Seoul 143-747, Republic of Korea Department of Physics, Ewha University, Seoul 151-747, Republic of Korea

A R T I C L E I N F O

A B S T R A C T

Article history: Available online 26 April 2012

Direct graphene growth on semiconductor substrates is an important goal for successful integration of graphene with the existing semiconductor technology. We test the feasibility of this goal by using molecular beam epitaxy on various semiconductor substrates: group IV (Si, SiC), group III–V (GaAs, GaN, InP), and group II–VI (ZnSe, ZnO). Graphitic carbon has been formed on most substrates except Si. In general, the crystallinities of carbon layers are better on substrates of hexagonal symmetry than those on cubic substrates. The flatness of graphitic carbon grown by molecular beam epitaxy is noticeable, which may help the integration with semiconductor structures. ß 2012 Elsevier Ltd. All rights reserved.

Keywords: A. Thin films B. Epitaxial growth C. Atomic force microscopy C. Raman spectroscopy D. Microstructure

1. Introduction Graphene growth on insulating substrates can accelerate the advent of graphene-based nanoelectronics. Especially the growth compatibility with semiconductor is an important issue. Molecular beam epitaxy (MBE) has been used for this purpose by several groups, and direct depositions of graphitic carbon by MBE on semiconductors and insulators, such as Si(1 1 1), SiC, and Al2O3 were reported recently [1–7]. Experiments have shown the existence of honeycomb ordering and Dirac cones [2,4], and even a Dirac-like peak has been observed in the transport measurements [6], showing the potential of graphene growth by MBE. Therefore, it is timely to study the growth behavior of carbon on other substrates. Here, we report the results of carbon MBE on widely used semiconductor substrates from group IV (Si, SiC), group III–V (GaAs, GaN, InP), and group II–VI (ZnSe, ZnO). When it is available, substrates of different cutting directions were used to study the dependence on symmetry. As far as we know, this is the first report of carbon MBE on group II–VI semiconductors in the pursuit of graphene growth. On most substrates except Si, we observed graphitic carbon growth, characterized by strong D and G peaks in the Raman spectra. Graphitic carbons on 6H-SiC(0 0 0 1) and GaAs(1 1 1)B, both having hexagonal symmetry, showed pronounced 2D peaks, also. There is a tendency that the graphitic carbons on hexagonal substrates have better crystallinities than those on cubic substrates. Atomic force microscopy (AFM) revealed that the surfaces of graphitic carbons on 6H-SiC(0 0 0 1) and GaN(0 0 0 1) were as flat as the substrates, implying the uniformity in the

coverage. Such flatness is an important virtue for the integration with semiconductor devices, as the quality of graphitic carbon approaches that of graphene in the future. 2. Experimental 2.1. Preparation Samples were prepared by a home-made ultra-high-vacuum MBE system. Carbon was supplied by heating a highly oriented pyrolytic graphite filament up to 1600 8C. During the growth, the pressure of the chamber was kept below 1.0  10 7 Torr with the help of liquid nitrogen flowing in the shroud. Epiready substrates were purchased from commercial vendors. The growth temperature, TG, was in the range of 900–1000 8C, except for substrates of low melting temperatures such as InP(1 0 0). Typical thickness of carbon film was 3–5 nm. Other details about the growth procedure can be found elsewhere [6]. 2.2. Characterization Raman scattering measurements were performed by using a McPherson model 207 monochromator with a 488 nm (2.54 eV) laser excitation source. The spectra recorded with a nitrogen-cooled charge-coupled device array detector. AFM images were taken by a commercial system (NanoFocus Inc.) in a non-contact mode. 3. Results and discussion 3.1. Group IV

* Corresponding author. Tel.: +82 2 3408 3398; fax: +82 2 3408 4316. E-mail address: [email protected] (S.H. Chun). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.04.123

Fig. 1 shows the Raman spectra from carbon films on Si(1 0 0) and Si(1 1 1). Characteristic Raman peaks of graphene or

S.K. Jerng et al. / Materials Research Bulletin 47 (2012) 2772–2775

Fig. 2. Raman spectra of carbon films grown at 1000 8C on 6H-SiC(0 0 0 1).

Fig. 1. Raman spectra of carbon films grown (a) at 1000 8C on Si(1 0 0) and (b) at 900 8C on Si(1 1 1).

graphitic carbon are not observed at all. This is in accord with the results of Ref. [1], where the authors noted that amorphous carbon layer grown at 560 8C was necessary for graphitic carbon growth at higher temperature. We did not try this method, because here we wanted to compare the carbon growth on different semiconductor substrates systematically. However, our results show that a suitable template on Si is critical for the growth of graphitic carbon. In contrast, carbon MBE on 6H-SiC(0 0 0 1) results in graphitic carbon formation as shown in Fig. 2. The D, G, and 2D peaks are clearly seen. Judging from the observed Raman spectra and theoretical models [8,9], we may call this carbon layer as nanocrystalline graphite (NCG). In fact the degree of graphitization is close to that of graphitic carbon on sapphire substrates [6]. Previous reports claimed the growth of epitaxial graphene from low energy electron diffraction and angle resolved photoelectron spectroscopy [2,4], and the only Raman spectra available include strong background peaks, making the comparison of peak intensities hard [5]. Here, we provide the fitting results of Raman peaks in Table 1 as a reference for further studies. The large intensity ratio of D peak to G peak (ID/ IG) and clearly observed 2D peak are strong signs of NCG formation [9]. 3.2. Group III–V There has been no report of carbon MBE on semiconductor substrates other than group IV, except an observation of

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graphitized carbon from thermally sublimated GaAs:C [10]. Here, we present the growth of graphitic carbon on group III–V by direct deposition. Fig. 3a shows that the carbon film grown on GaAs(1 0 0) does not bear honeycomb structures. On the contrary, the carbon deposited on GaAs(1 1 1)B by MBE is graphitic carbon of relatively high crystallinity as proved by the completely different Raman spectra shown in Fig. 3b. This is in sharp contrast to our previous study using sapphire substrates where the substrate symmetry hardly changed the crystallinity [6]. Further theoretical studies are necessary to resolve this discrepancy. Fig. 4 shows that the chemistry between the substrate and the carbon is also important. On InP(1 0 0) and GaN(0 0 0 1), we observe the D and G peaks of similar strength, but 2D peak is identified as a kink. Note that GaAs(1 0 0) and InP(1 0 0) have cubic symmetry, while GaAs(1 1 1)B and GaN(0 0 0 1) have hexagonal symmetry. We conclude here that both composition and symmetry affect the carbon growth mechanism on group III–V semiconductors. 3.3. Group II–VI We study the growth of carbon on group II–VI semiconductors of hexagonal symmetry: ZnO(0 0 0 1) and ZnSe(1 1 1). Both O-face and Zn-face ZnO substrates were tested, but the results were almost the same. As shown in Fig. 5a, it is hard to discern the D and G peaks due to the strong background signal from the ZnO substrate. However, we notice bumps

Table 1 Fitting results of the Raman spectra for various semiconductor substrates. Mixed Gaussian and Lorentzian functions are used to fit D, G, and 2D peaks. Substrate

Peak (D) (cm

Si(1 0 0) Si(1 1 1) 6H-SiC(0 0 0 1) GaAs(1 0 0) GaAs(1 1 1) InP(1 0 0) GaN(0 0 0 1) ZnO(0 0 0 1) ZnSe(1 1 1)

– – 1353 – 1356 1363 1358 – 1358

1

)

Peak (G) (cm – – 1596 – 1602 1610 1605 – 1606

1

)

Peak (2D) (cm – – 2704 – 2707 – – – 2697

1

)

ID/IG

I2D/IG

FWHM(G) (cm

– – 1.73 – 1.81 0.99 1.34 – 1.52

– – 0.35 – 0.26 – – – 0.31

– – 71 – 69 73 75 – 66

1

)

FWHM(2D) (cm – – 94 – 80 – – – 128

1

)

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Fig. 3. Raman spectra of carbon films grown (a) at 900 8C on GaAs(1 0 0) and (b) at 900 8C on GaAs(1 1 1)B.

Fig. 5. Raman spectra of carbon films grown (a) at 900 8C on ZnO[0 0 0 1] (Zn-face and O-face) and (b) at 900 8C on ZnSe(1 1 1).

near 1350 cm 1 and near 1600 cm 1, which may be related to graphitic carbon formation. On ZnSe(1 1 1), in contrast, we observe the D and G peaks clearly, implying the formation of graphitic carbon. Although the degree of graphitization is small at present (judging from the small 2D peak), it deserves further optimization. 3.4. Surface roughness

Fig. 4. Raman spectra of carbon films grown (a) at 700 8C on InP(1 0 0) and (b) at 900 8C on GaN(0 0 0 1).

For the integration of graphitic carbon with semiconductors, the surface properties are also important. We find that the carbon films are so flat that scanning electron microscopy does not provide useful information. Instead we have taken some AFM images of graphitic carbon layers on substrates of hexagonal symmetry to investigate the surface morphology. As shown in Fig. 6, there are differences depending on the symmetry of crystal structure. The graphitic carbons on 6H-SiC and GaN, both having wurtzite crystal structures, are surprisingly flat (Fig. 6a and b). The mean roughness parameters, Ra, from 1 mm  1 mm scans are 0.5  0.1 nm (6H-SiC) and 0.3  0.1 nm (GaN), respectively. These values are similar to those of substrates used in the experiments, suggesting a uniform coverage of carbon. On the other hand, the surfaces of graphitic carbon on GaAs(1 1 1)B and ZnSe(1 1 1), both having zinc blende crystal structures, are quite rough (Fig. 6c and d: note that the Ra of each substrate is about 0.1 nm). We find that the roughness mainly comes from the high growth temperature. Similar roughness is observed by simply annealing GaAs(1 1 1)B or ZnSe(1 1 1) at 900 8C, without the carbon supply (not shown). This may be due to the thermal decomposition of As or Se at the growth temperature [11].

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Fig. 6. 1 mm  1 mm AFM images of graphitic carbon (a) on 6H-SiC(0 0 0 1) (the mean roughness parameter Ra = 0.5  0.1 nm), (b) on GaN(0 0 0 1) (Ra = 0.3  0.1 nm), (c) on GaAs(1 1 1)B (Ra = 1.8  0.2 nm), (d) on ZnSe(1 1 1) (Ra = 10  1 nm).

4. Conclusions In summary, we tested the possibility of direct graphene growth by carbon MBE on group IV (Si, SiC), group III–V (GaAs, GaN, InP), and group II–VI (ZnSe, ZnO) semiconductor substrates. Raman spectra showed evidences of graphitic carbon formation on most substrates except Si. Especially we observed graphitic carbon growth on GaAs(1 1 1)B, GaN(0 0 0 1), InP(001), and ZnSe(1 1 1) for the first time. We found a tendency that the graphitic carbons on hexagonal substrates had better crystallinities than those on cubic substrates. A uniform coverage was observed on substrate of wurtzite crystal structure, increasing the hope of integration with semiconductor devices. Acknowledgments This research was supported by the Priority Research Centers Program (2011-0018395), and the Basic Science Research Programs (2011-0026292, KRF-2008-3130C00279), and the Center for

Topological Matter in POSTECH (2011-0030786) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST).

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