SiC(111) composite coatings by laser chemical vapor deposition

Accepted Manuscript Electrically conducting graphene/SiC(111) composite coatings by laser chemical vapor deposition

Qingfang XU, Zhao Deng, Qingyun SUN, Rong TU, Song ZHANG, Meijun Yang, Qizhong Li, Lianmeng Zhang, Takashi Goto, Hitoshi Ohmori PII:

S0008-6223(18)30600-6

DOI:

10.1016/j.carbon.2018.06.038

Reference:

CARBON 13243

To appear in:

Carbon

Received Date:

24 May 2018

Accepted Date:

14 June 2018

Please cite this article as: Qingfang XU, Zhao Deng, Qingyun SUN, Rong TU, Song ZHANG, Meijun Yang, Qizhong Li, Lianmeng Zhang, Takashi Goto, Hitoshi Ohmori, Electrically conducting graphene/SiC(111) composite coatings by laser chemical vapor deposition, Carbon (2018), doi: 10.1016/j.carbon.2018.06.038

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ACCEPTED MANUSCRIPT

Electrically conducting graphene/SiC(111) composite coatings by laser chemical vapor deposition Qingfang XU 1, Zhao Deng1, #, Qingyun SUN 1, Rong TU 1, Song ZHANG 1,*, Meijun Yang1, Qizhong Li2, Lianmeng Zhang1, Takashi Goto1, 3, Hitoshi Ohmori4 1State

Key Laboratory of Advanced Technology for Materials Synthesis and

Processing, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, People’s Republic of China 2Hubei

Key Laboratory of Advanced Technology for Automotive Components,

Wuhan University of Technology, Wuhan 430070, China 3Institute

for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku,

Sendai 980-8577, Japan 4

Institute of Physical and Chemical Research, 2-1 Hirosawa, Wako, Saitama 351-

0198, Japan *Corresponding author: Song Zhang; Tel/Fax: +86-27-87499449 E-mail: [email protected], [email protected] #This

author contributed equally to this work and should be considered co-first

anthors.

Abstract Graphene/SiC (G/SiC) composite coatings with <111>-orientation were in-situ deposited using hexamethyldisilane (HMDS) as precursor by laser chemical vapor deposition (LCVD). The concentration and distribution of graphene sheets in SiC

ACCEPTED MANUSCRIPT matrixes were controlled by changing total pressure (Ptot) in the reaction chamber. The investigation of growth mechanism showed that the photolytic of laser played an important role during the depositions. Observation by high-resolution transmission electron microscopy (HRTEM) revealed that graphene nucleated on SiC(111) lattice planes. As the increase of Ptot, the concentration of graphene decreased and the grain size of SiC grains and graphene sheets increased. The distribution of graphene in SiC matrix significantly affected the electrical conductivity () of the composite coating. The highest  is over 2 orders of magnitude larger than that of the graphene/semiconductor composites fabricated by other strategies. The high  makes the G/SiC(111) composite coating very promising for the applications in mechanical, energy, and sensor-related areas.

Key words: graphene/SiC coating; orientation; microstructure; laser CVD; electrical conductivity

Introduction Graphene with exceptional charge transport, thermal, optical, and mechanical properties, is considered one of the strongest materials ever produced[1][2][3][4][5]. Therefore, it becomes an ideal filler in the fabrication of conducting and robust polymer [6] and ceramic composites [3][7][8][9]. Among the advanced ceramic materials, SiC stands out due its superb high-temperature, high-power, highfrequency, and high-radiation resistance [1][2][3][10]. Besides, 3C-SiC can be deposited by chemical vapor deposition (CVD) below Si melting point (1683K),

ACCEPTED MANUSCRIPT which is compatible with the current Si-based semiconductor industry [11][12]. The addition of graphene as filler was envisaged to further enhance SiC ceramic properties, including better tribological and electrical performance [3]. At present, G/SiC (graphene/SiC) composites have been produced mainly by sintering technique [13][14][15][16][17][18][19]. In the sintering technique, nanoplatelets or graphene oxide were mixed with SiC powders in solvents, followed by a densification at high temperatures [3]. Unfortunately, these experiments may lead to both graphene agglomeration (forming aggregated flakes) and its structural degradation [3]. Compared to bulk G/SiC composite material, G/SiC composite coatings has a wider range of applications, such as distributed Bragg reflector [20], Micro-ElectroMechanical Systems (MEMS) [21], electromagnetic shielding coating [22], and solar cells [23]. However, a few studies were reported on the preparation of G/SiC composite coating by CVD technique. Zhuang et al [24] prepared G/SiC composite coatings by microwave plasma CVD. The room temperature electrical conductivity of the coatings was 9.6×103 S/m, which was at least 94 times higher than that of the materials prepared by sintering technique [3][25]. The SiC matrixes in these studies were randomly oriented. However, the orientation of SiC affects its properties and application. In the case of 3C-SiC, as the (111) plane of 3C-SiC is a polar surface, the chemical and electrical properties are dependent on the polarity [26]. On the other hand, the (110) plane of 3C-SiC is nonpolar, and thus it is an ideal surface to investigate the relaxation and electronic properties of 3C-SiC [26]. Furthermore, the concentration, layer number (N), and the distribution of graphene in SiC matrix play a

ACCEPTED MANUSCRIPT key role in determining the electrical conductivity ( of the composite. To our knowledge, there is hardly any report investigating the concentration, N and distribution of graphene in SiC matrix in the case of coatings. Our group developed a continuous laser CVD (LCVD) technic [27,28]. Because of pyrolytic and photolytic effect, the high-power continuous laser is able to control the decomposition of precursor to deposite SiC coatings with preferential orientation [10][29]. In this study, we demonstrated a one-step approach to prepare electrically conductive G/SiC composite coatings with <111> orientation by LCVD using hexamethyldisilane (HMDS) as precursor. The concentration, N and distribution of graphene, as well as  were investigated. The combination of the outstanding physical and chemical properties of graphene and <111> oriented 3C-SiC could open a window to many application, such as MEMS, sensors, and solar cells fields.

Experimental A cold-wall type LCVD was constructed to prepare the G/3C-SiC composite coatings on single crystalline Si(111) (Hefei Kejing Materials Technology Co. Ltd., Hefei, China, 1~10 Ω·m ) substrates. Si substrate, 10 × 15 mm2 in size, was cleaned in a peroxide solution (H2O: NH4OH: H2O2 = 5:1:1) to remove organic contamination and dipped in a hydrofluoric acid solution (H2O: HF = 50:1) to remove native oxide. Liquid hexametyldisilane (HMDS, Tokyo Chemical Industry Co., Ltd., Tokyo, Japan, 98%) was vaporized at 298 K and carried into the CVD chamber by 25 sccm Ar gas (Wuhan Xiangyun Chemical Co., Ltd., Wuhan, China, 99.999%) after deposition chamber was evacuated to 5 Pa. Meanwhile, 500 sccm H2 gas (Wuhan Xiangyun

ACCEPTED MANUSCRIPT Chemical Co., Ltd., Wuhan, China, 99.999%) was flowed into the chamber as dilution gas. The total pressure (Ptot) was 200~800 Pa. An enlarged diode continuous laser beam (InGaAlAs, wavelength = 1060 nm, 20 mm in diameter) was irradiated on the Si substrate. The deposition temperature was fixed at 1573 K. The temperature distribution on the substrate was measured by a thermographic camera (JENOPIK Inc., VarioCAM). The deposition time was 20 min. More details of the laser CVD apparatus are available in our previous works [10][30][31]. The crystal phase and orientation of the coatings were analyzed by X-ray diffraction (XRD, Rigaku Ultima III, 40 kV, 40 mA, Japan) with Cu-Kα radiation (wavelength λ = 0.15406 nm). The composition was detected by Raman spectrum (inVia. Renishaw, 633 nm He-Ne laser, UK) and X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi/ESCALAB 250Xi, USA). The microstructure of the coatings was observed by field emission scanning electron microscopy (FESEM; FEI Quanta-250, USA) and transmission electron microscopy (TEM, JEOL JEM-2100, 200 kV, Japan). The deposition rate (Rdep) was calculated from the thickness (T) and deposition time.  was derived from the sheet resistivity measured by four-probe method (HL5500PC, Accent Optical, UK) and T.

Results and discussion The θ-2θ diffraction patterns of the coatings prepared at Ptot = 200~800 Pa are shown in Fig. 1(a). Only 3C-SiC (111) peaks at 35.61o are identified, indicating <111> orientation of the SiC matrix. The grain size (S) of 3C-SiC was calculated from the full width at half-maximum (FWHM) of (111) peak via the Scherrer equation

ACCEPTED MANUSCRIPT [32]. As Fig. 1(b) shown, S was increased from 30 to 520 nm with increasing Ptot from 200 to 800 Pa. The surface and cross-section morphologies of the coatings are displayed in Fig. 2. As increase of Ptot, the deposits exhibit different topographies. At 200 Pa, SiC grains have irregular shape. As Ptot increased to 600-800 Pa, SiC grains exhibit pyramid-like structure with six-fold symmetry, and the “pyramids” grown larger with the increase of Ptot. The hexagonal pryamid is the typical morphology of <111>oriented 3C-SiC because of twins boundary and/or double position boundary [33]. The coatings display dense cross-section. T and Rdep increased from 0.05 to 6.5 m, and 0.15 to 18.5 m/h, respectively, with increasing Ptot from 200 to 800 Pa [30]. Fig. 3 shows the Raman spectra of the coatings prepared at various Ptot. The typical features of graphite including the G (~1588 cm-1) band and 2D (~2675 cm-1) band are observed. The intensity of 2D peak is higher than that of G peak, which indicates the presence of few layer graphene [34][35]. D (~1343 cm-1) band and D' (~1620 cm-1) band are also observable because of the existence of defects or disordering in graphene [24]. In addition to the features related to graphene, the peaks located at ~790 and ~973 cm-1 are aslo present. These peaks can be assigned to the transverse optical (TO) phonon and longitudinal optical (LO) phonon of SiC, respectively, confirming the formation of G/SiC composite [24][36]. The peak intensity ratios of I2D/IG and IG/ID are summarized in Table 1. The ratio of I2D/IG is usually used to determine the thickness of few layer graphene. In this study, the I2D/IG values for the spectra of the coatings were 1.24~2.20. These values

ACCEPTED MANUSCRIPT agree

with

those

generally

reported

for

graphene

with

N

=

1~2

[35][37][38][39][40][41]. The FWHM of 2D peak (F2D) is also sensitive to N [42][43]. As Table 1 shown, at 600~800 Pa，F2D is 50~55 cm-1, which is consistent with monolayer and bilayer graphene [44][43]. At 200 and 400 Pa, F2D is 158 and 114 cm-1, respectively, which are much larger than those at 600~800 Pa. As the previous studies reported, this discrepancy in F2D may be caused by two factors: the increase of defect density [45] and/or the increase of N (N > 3) [46][47]. The ratio of IG/ID is usually used to estimate the grain size (L) of graphene by Equ. 1 [48]. As Table 1 shown, as the increase of Ptot from 200 to 800 Pa, L increased from 5.01 to 58.19 nm. 𝐿 = (2.4 × 10

‒ 10

4 𝐼G

)𝜆 ( 𝐼D )

(1)

Where λ represents the laser wavelength of Raman spectra, and λ = 633 nm in this work. As Fig. 4(a) shown, the C1s XPS spectra of coatings deposited at 200~800 P were fitted by Gaussian-Lorentzian function after Shirley type background subtraction. Three XPS peaks were used to fit original spectra of C1s, corresponding to Si-C (~283.3 eV), sp2 C-C (~284.5 eV) and C-Ox (285.4~288.6 eV) bondings, which were attributed to the SiC, graphene, and oxycarbide, respectively [49][50][51][52]. With the increase of Ptot, the relative peak area of graphene decreased, indicating the concentration of graphene decreased. An amount of oxycarbide was also detected due to the O2 adsorbed during air exposure. The highresolution XPS spectra for Si2p are displayed in Fig. 4(b). The Si2p spectra were well fitted by a single Gaussian-Lorentzian line centered at 103.3 eV, which was assigned

ACCEPTED MANUSCRIPT to Si-C bonding [53]. The nearly perfect symmetry of the Si2p spectra excludes any Si-O-C or Si-O bondings. To further investigate the distribution, the quality and the layer number of graphene in the composite coatings, the cross sections of the coatings were observed by TEM. 3C-SiC (denoted as SiC) grains were observed with size of ~30 nm at Ptot = 200 Pa, which is actually in good accordance with the XRD result. Fig. 5(a) shows the cross-sectional TEM bright-field (BF) with the selected area electron diffraction (SAED) pattern from the square c. A strong (solid line) and weak (dash line) of diffraction spots of 3C-SiC along the <10-1> zone axis with mirror relation were obtained. The (111) planes of the two sets were both parallel to the surface of Si substrate, indicating the SiC matrix was <111> oriented. The weak set of diffraction spots can be assigned to the planar defects, such as microtwins with relatively lower density. A multilayer (N = 5) graphene (denoted as G) was spreaded on the surface and boundary of SiC grains as Fig. 5(b) and (c) shown, respectively. Besides, multilayer graphene in thickness of 5-6 nm was observed between 3C-SiC(111) lattice planes, as Fig. 5(d) shown. Nevertheless, the lattice image was not sharp but slightly spreaded out because of the disordering in the graphene. The graphene sheets were directly adjacented to the 3C-SiC crystallite with its basal plane parallel to the (111) planes of 3C-SiC, showing an intimate contact between graphene and 3C-SiC. Zhuang et al [24] supposed this phenomenon was contributed to SiC “split” in to two parts by “graphene wedges”. Fig. 6(a) shows the cross-sectional TEM BF image at Ptot = 800 Pa with the

ACCEPTED MANUSCRIPT SAED pattern from the square c. Two sets of strong diffraction spots of 3C-SiC along the <10-1> zone axis with mirror relation were aslo obtained. The (111) plane of the two sets were both parallel to the surface of Si substrate, indicating the SiC matrix was <111> oriented. The elongated spots in SAED image imply a large amount of twins and stacking faults in the <111> direction [10]. The columar grains grew from the substrate with a thickness of approx ~4 μm. 3C-SiC grains larger than 3 μm are observed in the coating which is different from the XRD result in Fig. 1(b). Pujar and Cawly have investigated the effects of stacking faults on the XRD patterns of 3C-SiC experimentally and by simulation [54][55][53]. They supposed that presence of stacking faults and twins led to change in the diffraction peaks which could provide misleading results when these features were used for crystallite size determinations. Many dark striations corresponded to twins of stacking faults lied on (111) planes, and paralleled to the surface of substrate, were investigated in our previous works [10][30]. Their formation is a universal phenomenon during the growth of 3C-SiC because of their low formation energy. Only a small quantity of 1~2 layer graphene was observed on the surface of the coating, also exhibiting an intimate contact between graphene and 3C-SiC as Fig. 6(b) shown. Graphene was hardly observed at the root and the middle of the coating, as Fig. 6(c) and (d) shown, respectively. According to the XPS, Raman and TEM results, a schematic model of the distribution, the size, the layer number and the content of graphene in the composite coatings is given in Fig. 7. At Ptot = 200 Pa, multilayer graphene was distributed on SiC grain surface and inside of SiC grain. The large amount of graphene and the small

ACCEPTED MANUSCRIPT grain size of SiC enable graphene network constructed with high continuity At Ptot = 400-600 Pa, the graphene sheets with mono- or bi-layer were mainly distributed on SiC grain surface. At Ptot = 800 Pa, the concentration of graphene was further decreased. Mono- or bi-layer graphene was covered on the surface of the coating with larger size. The formation of graphene sheets is mainly depended on the decomposition of HMDS, which has been studied by quadrupole mass spectra (QMS)[57] and electron impact mass spectra (EIMS)[58]. They reported the decomposition steps as follows: Step 1

(CH3)3-Si-Si-(CH3)3 → (CH3)3-Si-Si-(CH3)2+ + CH3-

Step 2

(CH3)3-Si-Si-(CH3)2+ + CH3- → (CH3)n-Si-H4-n (n = 1~3) + CnHm

Step 3

(CH3)n-Si-H4-n (n = 1~3) → SiC (solid) + CnHm

The decomposition and deposition mechanism of HMDS at Ptot = 200 Pa and 800 Pa are depicted in Fig. 8, respectively. Because of lowest concentration of HMDS and carrier gas atoms at Ptot = 200 Pa, laser photons had superfluous energy to break the Si-C bonds in [CH3-Si] species over the step 3, which led to the generation of much more CHx [29]. Thus, the more graphene sheets with multilayer were formed together with SiC and prohibited the film growth (lower gowth rate) as shown as Fig. 8(a). At Ptot = 800 Pa (Fig. 8(b)), the concentration of precursors and carrier gas atoms increased. Laser beam was dispersed by these molecules in the path of traveliing to substrate, and the energy of laser photon may partly lose. Thus, laser photons had appropriate and enough energy to complete the 3-step-reaction [59]. HMDS was

ACCEPTED MANUSCRIPT generated to supply the formation of SiC with higher growth rate and few layer graphene just on the surface of the coating. As Fig. 9 shown,  and concentration of graphene (ratio of C-C/C-Si bonding, Rb) decreased, whereas S increaised with the increasing of Ptot. This electrical conductivity of the graphene/SiC coatings is more than 10-9 times larger than that of intrinsic SiC because of the excellent electrical conductivity of the continuous graphene network [60]. Table

2

compares

graphene/semiconductor

the

composites

electrical fabricated

by

conductivity

of

several

different

strategies.

The

conductivity of the coatings prepared by CVD technique was higher than that prepared by sintering technique, which might due to the higher quality and better distribution of graphene in CVD. Because the process of mixing graphene and semiconductor powder may lead to both graphene agglomeration (forming aggregated flakes) and its structural degradation in sintering technique [3]. The graphene/SiC composite coating in this study processes the highest conductivity (7.61×105 S/m). To our best knowledge, it is 80 times higher than the highest conductivity ever reported. The ultra-high , intermediate band gap of SiC, feasible integration on different substrates as well as the good chemical and mechanical stability of SiC and graphene make this composite coating very promising for a wide range of applications.

Conclusion G/3C-SiC(111) composite coatings with high  were obtained by LCVD. Detailed HRTEM analysis revealed that graphene was distributed on the surface and

ACCEPTED MANUSCRIPT the grain boundary of SiC, as well as between SiC(111) lattice plane. Graphene constituted an electrical conductive network which enhanced the conductivity of the composite coating. As the decrease of Ptot, the concentration of graphene increased and S decreased, which enable the graphene network constructed with high continuity and compactness resulting in the increase of . The maxmium of  reached 7.61×105 S/m, which are over 2 orders of magnitude larger than that of graphene/semiconductor composites fabricated by sintering technique and other CVD method. Acknowledgement This work was supported by the National Natural Science Foundation of China (Nos. 51372188, and 51521001) and by the 111 Project (B13035). This research was also supported by the International Science & Technology Cooperation Program of China (2014DFA53090) the Natural Science Foundation of Hubei Province, China (2016CFA006), and the Fundamental Research Funds for the Central Universities (WUT: 2017II43GX, 2017III032, 2017YB004, 2018III016), and Science Challenge Project (No.TZ2016001). References [1]

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ACCEPTED MANUSCRIPT Figure Captions Fig. 1. XRD patterns (a) and the calculated S (b) of the coatings. Fig. 2. Surface and cross section SEM images of the composite coatings. Fig. 3. The Raman spectra of the coatings as a function of Ptot. Fig. 4. High-resolution C1s (a) and Si2p (b) spectra of the composite coatings as a function of Ptot. Fig. 5. Cross-sectional TEM image of G/3C-SiC composite coating deposited at Ptot = 200 Pa (a). HRTEM image of the coating surface in square b (b), HRTEM image of the grain boundary in square c (c), and HRTEM image of the square d (d). Fig. 6. Cross-sectional TEM image of G/3C-SiC composite coating deposited at Ptot = 800 Pa (a). HRTEM image of the coating surface in square b (b), HRTEM image of the grain boundary in square c (c), and HRTEM image of the SiC/Si interface in square d (d). Fig. 7. The schematic of graphene distribution in the composite coatings. Fig. 8 The schematic of the decomposition and deposition mechanism at Ptot = 200 Pa (a) and 800 Pa (b), respectively. Fig. 9 Effect of Ptot on s, Rb and S.

ACCEPTED MANUSCRIPT

Table 1. Effect of Ptot on the values of I2D/IG, IG/ID, F2D, the N and L. Ptot (Pa)

I2D/IG

IG/ID

F2D (cm-1)

N

L (nm)

200

1.24

0.13

158

N>3



400

1.67

0.15

114

N>3



600

1.51

1.20

50

1~2



800

2.10

1.51

55

1~2



Table 2 Electrical conductivity of the graphene/semiconductor composites fabricated using different methods. Matrix SiC(111) columar

Method

Conductivity (S/m)

Refs

Laser CVD

2.19×103~7.61×105

This work

SiC nanolaminate

Microwave plasma CVD

2.80×102~9.61×103

[22]

SiC microcrystals

Pressureless-sintered method

0.20~1.82

[17]

SiC microcrystals

Spark plasma sintering

8.30×10-5~1.02×102

[34]

Si nanoparticles

Physical mixing insolution

1.31×103~3.31×103

[58]

Si3N4 microcrystals

Spark plasma sintering

1.20×10~4.00×103

[59]

Al2O3 nanoparticles

Spark plasma sintering

1.00~4.38×103

[11]

ZnO/Zn(OH)2 nanoparticles

Wet chemistry

1.20×10-5~2.11

[60]

grains