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Self-assembly growth of high-quality SiC nanowires from Si thin films deposited on single-crystalline SiC wafers
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Self-assembly growth of high-quality SiC nanowires from Si thin films deposited on single-crystalline SiC wafers Byeong Geun Kim a,b, Byung-Sung Kim c, Soon-Mok Choi a,n, Ji Eun Lee b, Seong-Min Jeong b, Myung-Hyun Lee b, Won-Seon Seo b a
School of Energy, Materials and Chemical Engineering, Korea University of Technology and Education, Cheonan 330-708, Korea Energy & Environmental Division, Korea Institute of Ceramic Engineering and Technology, Jinju 660-031, Korea c Department of Engineering Science, University of Oxford, Oxford OX1 3PJ, UK b
art ic l e i nf o
a b s t r a c t
Article history: Received 31 May 2016 Received in revised form 5 August 2016 Accepted 6 September 2016
It was recently reported that well-ordered graphene layers were directly grown on the surface of SiC single crystals by only thermal annealing. Based on this phenomenon, we successfully demonstrate the self-assembly growth of SiC nanowires from Si-deposited SiC. After deposition of Si thin films on SiC single crystals by sputtering, they were annealed at 1200 °C and 1400 °C for 10 h under Ar gas atmosphere. High-quality SiC nanowires were grown on the surface of Si-deposited SiC annealed at 1400 °C, while only graphitic carbon spheres were formed at 1200 °C. Carbon atoms, which originated from the SiC single crystals, diffused into the films and functioned as carbon sources for the growth of SiC nanowires. The Si thin films were oxidized during thermal annealing and acted as both the Si sources for SiC nanowires and the diffusion path of carbon atoms. We believe that this study can help advance the crystal growth of nanostructures on SiC and the preparation of SiC-based nanoelectronic devices for various applications such as field emission and power devices. & 2016 Published by Elsevier Ltd and Techna Group S.r.l.
Keywords: Silicon carbide Graphene Nanowires Stacking faults Sputter Films
1. Introduction Silicon carbide (SiC) has been widely used in various fields due to its excellent physical and chemical properties [1]. In particular, SiC nanowires (NWs) have been developed to enhance the fieldemission [2–4] and mechanical properties of composites [5]. SiC NWs have been grown by many methods such as vapor-liquidsolid (VLS) [6–8], template-assisted [2,3] and vapor-solid (VS) processes [9–12]. Among them, the VS growth method has received attention because it is suited to the mass production of SiC NWs through a simple process, and it enable the self-assembly growth of high-quality SiC NWs [9]. In the VLS method [6–8], metal particles are used as catalysts, and they remain at the tips of the SiC NWs after completion of the growth process, but this does not occur in SiC NWs grown by the VS method [9–12]. Most researchers have reported that gaseous SiO play an important role in the VS method of growing SiC NWs [9–12]. The main growth reactions, which have been suggested by many research groups, are expressed as follows: SiO (g) þ2C (s)-SiC (s)þ CO (g) n
Corresponding author. E-mail address: [email protected] (S.-M. Choi).
(1)
SiO (g) þ3CO (g)-SiC (s)þ 2CO2 (g)
(2)
Reactions (1) and (2) are related to the formation of SiC nuclei and the growth of SiC NWs, respectively. Si or Si oxide powders have been used as Si sources to induce these reactions [9–12]. Gaseous SiO can be generated by the dry oxidation of Si at 1000 °C, as follows [13]: Si (s) þSiO (s)-2SiO (g)
(3)
They can also be formed by the reaction between silica powders and graphite, as follows [9,10]: SiO2 (s)þC (s)-SiO (g) þCO (g)
(4)
Graphite and carbon black powders are generally used as carbon sources [9–12]. Recently, studies on the catalyst-free growth of graphene layers have been reported [14,15]. When SiC single crystals are annealed at temperatures higher than 1200 °C under vacuum or Ar gas atmosphere, graphene layers are grown on the surface of SiC single crystal [14,15]. When the bonding between Si and C atoms is broken by high thermal energy, Si atoms are preferentially evaporated due to their low vapor pressure. As a result, graphene layers are formed by a rearrangement of excess carbon
http://dx.doi.org/10.1016/j.ceramint.2016.09.047 0272-8842/& 2016 Published by Elsevier Ltd and Techna Group S.r.l.
Please cite this article as: B.G. Kim, et al., Self-assembly growth of high-quality SiC nanowires from Si thin films deposited on singlecrystalline SiC wafers, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.09.047i
B.G. Kim et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
3. Results and discussion
Fig. 1. A schematic diagram showing both research direction and goal in this study.
atoms. This indicates that SiC itself can be a candidate for a source of carbon atoms. Previously, we confirmed that sputter-deposited Si thin films are amorphous and can be easily transformed into Si oxide by thermal annealing at 600 °C [16]. If dry oxidation occurs in Si thin films, they may provide gaseous SiO at high temperature. If Si thin films deposited on SiC single crystals are annealed at high temperatures (above 1200 °C), SiC NWs may be grown by a supply of gaseous SiO and solid C from Si thin films and SiC single crystals, respectively, by the VS mechanism. This idea is illustrated in Fig. 1. To prove our hypothesis, Si thin films were deposited on SiC single crystals by sputtering. After they were annealed at 1200 °C and 1400 °C, we investigated their morphological and structural changes. The growth mechanism of SiC NWs will be discussed herein. Deng et al. grew SiC NWs directly on SiC ceramic substrates using a catalyst-assisted thermal heating process [4]. They confirmed that the heat of SiC NWs on SiC substrates can efficiently dissipate and the stability in field emitting SiC NWs can be improved. If our goal could be achieved, our findings may be applied in the development of new optoelectronic devices.
2. Experimental Single-crystalline 6H-SiC wafers (Tankeblue Semiconductor Co. Ltd.) were used as both substrates and carbon sources. They were cleaned with acetone and ethanol solutions for 10 min via an ultrasonic system in turn. Si thin films were deposited on them using radio frequency (RF) sputtering equipment with an Si (99.999%) target. The RF power applied to the Si targets was fixed at 100 W, and Ar (99.999%) gas was injected at a rate of 30 sccm (standard cubic centimeters per minute) to form an Ar ion plasma. The working pressure was 35 mTorr. Contaminants on the Si target were removed by pre-sputtering for more than 10 min. The deposition of each Si thin film was performed for 128 min at room temperature. After deposition was completed, all samples were annealed at 1200 °C or 1400 °C for 10 h in a horizontal tube furnace under Ar gas atmosphere. The morphologies of the samples were observed by a field-emission scanning electron microscopy (FESEM, JSM7001F, JEOL Ltd.). The structural changes of the samples were analyzed by Raman equipment (LabRam ARAMIS, Horiba JobinYvon) equipped with an Ar-ion laser (514.5 nm) at room temperature. In preparation for transmission electron microscopy (TEM, JEOL-2100F, JEOL Ltd.) observation of the SiC NWs, they were physically detached from the samples and dispersed onto a Cu grid.
When the Si-deposited SiC was annealed at 1400 °C for 10 h, SiC NWs were grown on the surface of the Si thin films (Fig. 2(a)). Fig. 2(b) shows a low-resolution TEM image of an as-grown SiC NW. Fig. 2(c) and (d) present enlarged TEM images of the tip and side of an SiC NW in Fig. 2(b). No metal particle, which is a typical characteristic of samples grown by the VLS method, was found at the tip of the SiC NW (red arrow in Fig. 2(b) and (c)). The SiC NW was surrounded by a thin amorphous layer ( 5 nm). The spacing between the fringes was about 0.25 nm (inset of Fig. 2(c)), which is the d-spacing of the crystalline (111) plane of 3C-SiC phase. Inset (right bottom) of Fig. 2(e) is a low-resolution TEM image of another SiC NW, and Fig. 2(e) presents an enlarged TEM image of it. A high-resolution TEM (HRTEM) image (inset of Fig. 2(d)) as well as SEAD patterns of insets in Fig. 2(d) and (e) indicates that no stacking faults existed in the SiC NWs. To investigate the growth behaviors of SiC NWs, FESEM and Raman analyses were performed on Si-deposited SiC annealed at 1200 and 1400 °C (Fig. 3). When Si-deposited SiC was annealed at 1200 °C, small spheres with diameters of 680 nm were partially formed (Fig. 3(a)). When the annealing temperature was increased to 1400 °C, SiC NWs and large spheres (size of 8 mm) were formed together (Fig. 3(b)). Fig. 3(c) shows a high-magnification FESEM image of Si-deposited SiC annealed at 1200 °C. Small spheres had grown together and formed a round group, which is denoted in Fig. 3(c) as #1. The surface area without particles was flat, and it is denoted in Fig. 3(c) as #2. Large particles, etch pits and SiC NWs were observed in the FESEM image of Si-deposited SiC annealed at 1400 °C (Fig. 3(d)). The particles and surface without SiC NWs are denoted in Fig. 3(d) as #3 and #4, respectively. Raman analysis was carried out on #1–#4 (Fig. 3(e)–(f)). No SiCrelated peaks were observed in the Raman spectra of #1 and #3. However, peaks at 1356.5 cm 1, 1584.6 cm 1, and 2705.5 cm 1 were found, which are consistent with the D, G and 2D peaks of graphite including graphene [17]. Among them, the D peak is related with defects, and the D/G ratio shows the crystallinity of graphene [17]; when the D/G ratio is low, the crystallinity is better. In our case, the intensity ratios of D to G were 0.2 and 0.1 at #1 and #3, respectively. Fig. 3(f) shows the Raman spectra of #2 and #4. Three peaks with high intensity were commonly observed at 769.4 cm 1, 791.8 cm 1, and 970.4 cm 1 in both regions, which are consistent with transverse optical (TO) and longitudinal optical (LO) phonon peaks of 6H-SiC (inset of Fig. 3(f)) [18], indicating that they are the peaks of 6H-SiC substrates. Three small peaks were found near 1530.0 cm 1, 1583.5 cm 1, and 1717.3 cm 1, which were assumed to be the typical peaks of graphite [17]. Moreover, the intensity of the D peak was greater than or similar to that of the G peak. This indicates that graphitic carbon spheres, which were formed at 1200 °C and 1400 °C, consisted of well-organized graphene, while graphite with poor crystallinity existed on the surface of Si-deposited SiC. In particular, the appearance of graphitic carbon spheres as well as SiC NWs strongly indicates that the SiC substrates were thermally decomposed above temperature of 1200 °C because the possible carbon source only existed in SiC, not Si thin films. When the SiC surface was thermally decomposed, free carbon atoms could diffuse into the surface of Si-deposited SiC. Si atoms could combine with the Si thin films or evaporate due to their relatively low melting point. According to previous reports, the number of graphene layers increases in proportion to annealing temperature and duration [14,15]. The duration (10 h) in this study was much longer than that (30 min–1 h) of the previous reports [14,15]. This means that sufficient carbon sources are provided to allow the
Please cite this article as: B.G. Kim, et al., Self-assembly growth of high-quality SiC nanowires from Si thin films deposited on singlecrystalline SiC wafers, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.09.047i
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Fig. 2. (a) FESEM and (b)–(e) TEM images and SEAD patterns (insets) of SiC NWs grown on Si-deposited SiC annealed at 1400 °C for 10 h.
growth of SiC NWs. After the thermal process, excess carbon atoms could condense and formed high-quality graphitic carbon spheres via a rearrangement by thermal annealing at high temperature. In addition, two peaks related to Si and Si oxide were observed in the Raman spectra of #2 and #4. The inset of Fig. 3(f) presents the enlarged Raman spectrum of #4 in a range of 400–600 cm 1 (red arrow in Fig. 2(f)), which was divided by Lorentz fitting. The peaks at 482.4 cm 1 and 509.5 cm 1 were distinguished, and they were found to be those of amorphous Si (480 cm 1) and Si oxide (505 cm 1), respectively [19]. This indicates that Si thin films were partially transformed into Si oxide films by thermal annealing. The thermal oxidation may be due to the residual O2 in the furnace or Ag gas [16]. When the annealing temperature was further increased, hence, the films induced the formation of gaseous SiO by reactions (3) and (4). Moreover, it is known that low oxygen partial pressure as well as high temperature facilitates the formation of gaseous SiO [13]. If oxygen partial pressure can be controlled, the higher yield of SiC NWs may be achieved. When Si-deposited SiC was annealed at 1400 °C for 10 h, the growth of SiC NWs was observed by FESEM, but this was not observed at 1200 °C. This demonstrates that SiC NWs may be grown by consecutive occurrence of reactions (1) and (2). Moreover, no defects (especially, stacking faults) were observed in the as-grown SiC nanowires in our study (Fig. 2). According to previous works, stacking faults are easily formed at in SiC NWs [2–12]. It has been reported that the formation of stacking faults in SiC NWs is induced by their fast growth rate [12]. SiC single crystal was covered with Si thin films in our case, and free carbon atoms had to pass through the films to contribute to the growth of SiC NWs. This may cause a low growth rate of SiC NWs. We demonstrated the defectfree growth of SiC NWs on Si thin films, which were deposited on
graphite, recently [20]. Although further study is needed, we surmise that Si thin films may play an important role in the growth of high-quality SiC NWs. Based on our experimental results, the self-assembly growth of SiC NWs is summarized as follows. When Si-deposited SiC is gradually annealed, Si thin films are transformed into Si oxide films by the diffusion of O2 gas. Carbon atoms are generated by high thermal energy at the interface between the oxidized Si thin films and SiC single crystals, and diffuse into Si oxide films. Gaseous SiO are formed by the reactions between solid SiO2 and solid C (reaction (4)) or between solid Si and solid SiO (reaction (3)). SiC nuclei are formed by a reaction between gaseous SiO and solid C (reaction (1)). Finally, SiC NWs are spontaneously grown by a reaction between SiO and gaseous CO (reaction (2)). The oxidized Si thin films plays two important roles on the growth of SiC NWs from Si-deposited SiC, namely, the diffusion path of carbon atoms and Si sources for growing SiC NWs [21,22]. The growth mechanism of SiC NWs is illustrated in Fig. 4.
4. Conclusion Single-crystalline SiC NWs were spontaneously grown on Sideposited SiC via simple thermal treatment. Si thin films were deposited on SiC single crystals by RF sputtering at room temperature. When they were thermally annealed at 1400 °C for 10 h under Ar gas atmosphere, SiC NWs were grown on the surface of Si-deposited SiC. No traces of catalyst or defects, such as metal particles and stacking faults, were found in the SiC NWs. Graphitic carbon spheres were observed for Si-deposited SiC annealed at temperatures above 1200 °C. When the annealing temperature
Please cite this article as: B.G. Kim, et al., Self-assembly growth of high-quality SiC nanowires from Si thin films deposited on singlecrystalline SiC wafers, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.09.047i
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Fig. 3. (a)–(d) FESEM images and (e)–(f) Raman spectra of Si-deposited SiC annealed at 1200 °C or 1400 °C for 10 h. The peak sum of the fitted curves is represented by black dotted lines, and ‘a-Si’ denotes amorphous Si.
thermal decomposition of SiC single crystals occurred at the interface between Si (or oxide) thin films and SiC single crystals, free carbon atoms were diffused into the films and used as carbon sources for SiC NWs. Si thin films were oxidized and acted as a channel of carbon diffusion and a Si sources for SiC NWs at high temperature. After the growth of SiC NWs, excess carbon atoms condensed and formed graphitic carbon spheres.
Acknowledgments This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2014R1A1A2057767). This work was supported by the Defense Acquisition Program Administration (DAPA) and the Agency for Defense Development (ADD), Republic of Korea.
References Fig. 4. Growth mechanism of SiC NWs from Si-deposited SiC with increase of annealing temperature.
increased from 1200 °C to 1400 °C, their size increased from 680 nm to 8 mm, respectively. The formation of graphitic carbon spheres as well as the growth of SiC NWs is the absolute evidence of the thermal decomposition of SiC single crystals, which were used as both substrates and carbon sources in this study. When the
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Please cite this article as: B.G. Kim, et al., Self-assembly growth of high-quality SiC nanowires from Si thin films deposited on singlecrystalline SiC wafers, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.09.047i
Contents lists available at ScienceDirect
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Self-assembly growth of high-quality SiC nanowires from Si thin films deposited on single-crystalline SiC wafers Byeong Geun Kim a,b, Byung-Sung Kim c, Soon-Mok Choi a,n, Ji Eun Lee b, Seong-Min Jeong b, Myung-Hyun Lee b, Won-Seon Seo b a
School of Energy, Materials and Chemical Engineering, Korea University of Technology and Education, Cheonan 330-708, Korea Energy & Environmental Division, Korea Institute of Ceramic Engineering and Technology, Jinju 660-031, Korea c Department of Engineering Science, University of Oxford, Oxford OX1 3PJ, UK b
art ic l e i nf o
a b s t r a c t
Article history: Received 31 May 2016 Received in revised form 5 August 2016 Accepted 6 September 2016
It was recently reported that well-ordered graphene layers were directly grown on the surface of SiC single crystals by only thermal annealing. Based on this phenomenon, we successfully demonstrate the self-assembly growth of SiC nanowires from Si-deposited SiC. After deposition of Si thin films on SiC single crystals by sputtering, they were annealed at 1200 °C and 1400 °C for 10 h under Ar gas atmosphere. High-quality SiC nanowires were grown on the surface of Si-deposited SiC annealed at 1400 °C, while only graphitic carbon spheres were formed at 1200 °C. Carbon atoms, which originated from the SiC single crystals, diffused into the films and functioned as carbon sources for the growth of SiC nanowires. The Si thin films were oxidized during thermal annealing and acted as both the Si sources for SiC nanowires and the diffusion path of carbon atoms. We believe that this study can help advance the crystal growth of nanostructures on SiC and the preparation of SiC-based nanoelectronic devices for various applications such as field emission and power devices. & 2016 Published by Elsevier Ltd and Techna Group S.r.l.
Keywords: Silicon carbide Graphene Nanowires Stacking faults Sputter Films
1. Introduction Silicon carbide (SiC) has been widely used in various fields due to its excellent physical and chemical properties [1]. In particular, SiC nanowires (NWs) have been developed to enhance the fieldemission [2–4] and mechanical properties of composites [5]. SiC NWs have been grown by many methods such as vapor-liquidsolid (VLS) [6–8], template-assisted [2,3] and vapor-solid (VS) processes [9–12]. Among them, the VS growth method has received attention because it is suited to the mass production of SiC NWs through a simple process, and it enable the self-assembly growth of high-quality SiC NWs [9]. In the VLS method [6–8], metal particles are used as catalysts, and they remain at the tips of the SiC NWs after completion of the growth process, but this does not occur in SiC NWs grown by the VS method [9–12]. Most researchers have reported that gaseous SiO play an important role in the VS method of growing SiC NWs [9–12]. The main growth reactions, which have been suggested by many research groups, are expressed as follows: SiO (g) þ2C (s)-SiC (s)þ CO (g) n
Corresponding author. E-mail address: [email protected] (S.-M. Choi).
(1)
SiO (g) þ3CO (g)-SiC (s)þ 2CO2 (g)
(2)
Reactions (1) and (2) are related to the formation of SiC nuclei and the growth of SiC NWs, respectively. Si or Si oxide powders have been used as Si sources to induce these reactions [9–12]. Gaseous SiO can be generated by the dry oxidation of Si at 1000 °C, as follows [13]: Si (s) þSiO (s)-2SiO (g)
(3)
They can also be formed by the reaction between silica powders and graphite, as follows [9,10]: SiO2 (s)þC (s)-SiO (g) þCO (g)
(4)
Graphite and carbon black powders are generally used as carbon sources [9–12]. Recently, studies on the catalyst-free growth of graphene layers have been reported [14,15]. When SiC single crystals are annealed at temperatures higher than 1200 °C under vacuum or Ar gas atmosphere, graphene layers are grown on the surface of SiC single crystal [14,15]. When the bonding between Si and C atoms is broken by high thermal energy, Si atoms are preferentially evaporated due to their low vapor pressure. As a result, graphene layers are formed by a rearrangement of excess carbon
http://dx.doi.org/10.1016/j.ceramint.2016.09.047 0272-8842/& 2016 Published by Elsevier Ltd and Techna Group S.r.l.
Please cite this article as: B.G. Kim, et al., Self-assembly growth of high-quality SiC nanowires from Si thin films deposited on singlecrystalline SiC wafers, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.09.047i
B.G. Kim et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
3. Results and discussion
Fig. 1. A schematic diagram showing both research direction and goal in this study.
atoms. This indicates that SiC itself can be a candidate for a source of carbon atoms. Previously, we confirmed that sputter-deposited Si thin films are amorphous and can be easily transformed into Si oxide by thermal annealing at 600 °C [16]. If dry oxidation occurs in Si thin films, they may provide gaseous SiO at high temperature. If Si thin films deposited on SiC single crystals are annealed at high temperatures (above 1200 °C), SiC NWs may be grown by a supply of gaseous SiO and solid C from Si thin films and SiC single crystals, respectively, by the VS mechanism. This idea is illustrated in Fig. 1. To prove our hypothesis, Si thin films were deposited on SiC single crystals by sputtering. After they were annealed at 1200 °C and 1400 °C, we investigated their morphological and structural changes. The growth mechanism of SiC NWs will be discussed herein. Deng et al. grew SiC NWs directly on SiC ceramic substrates using a catalyst-assisted thermal heating process [4]. They confirmed that the heat of SiC NWs on SiC substrates can efficiently dissipate and the stability in field emitting SiC NWs can be improved. If our goal could be achieved, our findings may be applied in the development of new optoelectronic devices.
2. Experimental Single-crystalline 6H-SiC wafers (Tankeblue Semiconductor Co. Ltd.) were used as both substrates and carbon sources. They were cleaned with acetone and ethanol solutions for 10 min via an ultrasonic system in turn. Si thin films were deposited on them using radio frequency (RF) sputtering equipment with an Si (99.999%) target. The RF power applied to the Si targets was fixed at 100 W, and Ar (99.999%) gas was injected at a rate of 30 sccm (standard cubic centimeters per minute) to form an Ar ion plasma. The working pressure was 35 mTorr. Contaminants on the Si target were removed by pre-sputtering for more than 10 min. The deposition of each Si thin film was performed for 128 min at room temperature. After deposition was completed, all samples were annealed at 1200 °C or 1400 °C for 10 h in a horizontal tube furnace under Ar gas atmosphere. The morphologies of the samples were observed by a field-emission scanning electron microscopy (FESEM, JSM7001F, JEOL Ltd.). The structural changes of the samples were analyzed by Raman equipment (LabRam ARAMIS, Horiba JobinYvon) equipped with an Ar-ion laser (514.5 nm) at room temperature. In preparation for transmission electron microscopy (TEM, JEOL-2100F, JEOL Ltd.) observation of the SiC NWs, they were physically detached from the samples and dispersed onto a Cu grid.
When the Si-deposited SiC was annealed at 1400 °C for 10 h, SiC NWs were grown on the surface of the Si thin films (Fig. 2(a)). Fig. 2(b) shows a low-resolution TEM image of an as-grown SiC NW. Fig. 2(c) and (d) present enlarged TEM images of the tip and side of an SiC NW in Fig. 2(b). No metal particle, which is a typical characteristic of samples grown by the VLS method, was found at the tip of the SiC NW (red arrow in Fig. 2(b) and (c)). The SiC NW was surrounded by a thin amorphous layer ( 5 nm). The spacing between the fringes was about 0.25 nm (inset of Fig. 2(c)), which is the d-spacing of the crystalline (111) plane of 3C-SiC phase. Inset (right bottom) of Fig. 2(e) is a low-resolution TEM image of another SiC NW, and Fig. 2(e) presents an enlarged TEM image of it. A high-resolution TEM (HRTEM) image (inset of Fig. 2(d)) as well as SEAD patterns of insets in Fig. 2(d) and (e) indicates that no stacking faults existed in the SiC NWs. To investigate the growth behaviors of SiC NWs, FESEM and Raman analyses were performed on Si-deposited SiC annealed at 1200 and 1400 °C (Fig. 3). When Si-deposited SiC was annealed at 1200 °C, small spheres with diameters of 680 nm were partially formed (Fig. 3(a)). When the annealing temperature was increased to 1400 °C, SiC NWs and large spheres (size of 8 mm) were formed together (Fig. 3(b)). Fig. 3(c) shows a high-magnification FESEM image of Si-deposited SiC annealed at 1200 °C. Small spheres had grown together and formed a round group, which is denoted in Fig. 3(c) as #1. The surface area without particles was flat, and it is denoted in Fig. 3(c) as #2. Large particles, etch pits and SiC NWs were observed in the FESEM image of Si-deposited SiC annealed at 1400 °C (Fig. 3(d)). The particles and surface without SiC NWs are denoted in Fig. 3(d) as #3 and #4, respectively. Raman analysis was carried out on #1–#4 (Fig. 3(e)–(f)). No SiCrelated peaks were observed in the Raman spectra of #1 and #3. However, peaks at 1356.5 cm 1, 1584.6 cm 1, and 2705.5 cm 1 were found, which are consistent with the D, G and 2D peaks of graphite including graphene [17]. Among them, the D peak is related with defects, and the D/G ratio shows the crystallinity of graphene [17]; when the D/G ratio is low, the crystallinity is better. In our case, the intensity ratios of D to G were 0.2 and 0.1 at #1 and #3, respectively. Fig. 3(f) shows the Raman spectra of #2 and #4. Three peaks with high intensity were commonly observed at 769.4 cm 1, 791.8 cm 1, and 970.4 cm 1 in both regions, which are consistent with transverse optical (TO) and longitudinal optical (LO) phonon peaks of 6H-SiC (inset of Fig. 3(f)) [18], indicating that they are the peaks of 6H-SiC substrates. Three small peaks were found near 1530.0 cm 1, 1583.5 cm 1, and 1717.3 cm 1, which were assumed to be the typical peaks of graphite [17]. Moreover, the intensity of the D peak was greater than or similar to that of the G peak. This indicates that graphitic carbon spheres, which were formed at 1200 °C and 1400 °C, consisted of well-organized graphene, while graphite with poor crystallinity existed on the surface of Si-deposited SiC. In particular, the appearance of graphitic carbon spheres as well as SiC NWs strongly indicates that the SiC substrates were thermally decomposed above temperature of 1200 °C because the possible carbon source only existed in SiC, not Si thin films. When the SiC surface was thermally decomposed, free carbon atoms could diffuse into the surface of Si-deposited SiC. Si atoms could combine with the Si thin films or evaporate due to their relatively low melting point. According to previous reports, the number of graphene layers increases in proportion to annealing temperature and duration [14,15]. The duration (10 h) in this study was much longer than that (30 min–1 h) of the previous reports [14,15]. This means that sufficient carbon sources are provided to allow the
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Fig. 2. (a) FESEM and (b)–(e) TEM images and SEAD patterns (insets) of SiC NWs grown on Si-deposited SiC annealed at 1400 °C for 10 h.
growth of SiC NWs. After the thermal process, excess carbon atoms could condense and formed high-quality graphitic carbon spheres via a rearrangement by thermal annealing at high temperature. In addition, two peaks related to Si and Si oxide were observed in the Raman spectra of #2 and #4. The inset of Fig. 3(f) presents the enlarged Raman spectrum of #4 in a range of 400–600 cm 1 (red arrow in Fig. 2(f)), which was divided by Lorentz fitting. The peaks at 482.4 cm 1 and 509.5 cm 1 were distinguished, and they were found to be those of amorphous Si (480 cm 1) and Si oxide (505 cm 1), respectively [19]. This indicates that Si thin films were partially transformed into Si oxide films by thermal annealing. The thermal oxidation may be due to the residual O2 in the furnace or Ag gas [16]. When the annealing temperature was further increased, hence, the films induced the formation of gaseous SiO by reactions (3) and (4). Moreover, it is known that low oxygen partial pressure as well as high temperature facilitates the formation of gaseous SiO [13]. If oxygen partial pressure can be controlled, the higher yield of SiC NWs may be achieved. When Si-deposited SiC was annealed at 1400 °C for 10 h, the growth of SiC NWs was observed by FESEM, but this was not observed at 1200 °C. This demonstrates that SiC NWs may be grown by consecutive occurrence of reactions (1) and (2). Moreover, no defects (especially, stacking faults) were observed in the as-grown SiC nanowires in our study (Fig. 2). According to previous works, stacking faults are easily formed at in SiC NWs [2–12]. It has been reported that the formation of stacking faults in SiC NWs is induced by their fast growth rate [12]. SiC single crystal was covered with Si thin films in our case, and free carbon atoms had to pass through the films to contribute to the growth of SiC NWs. This may cause a low growth rate of SiC NWs. We demonstrated the defectfree growth of SiC NWs on Si thin films, which were deposited on
graphite, recently [20]. Although further study is needed, we surmise that Si thin films may play an important role in the growth of high-quality SiC NWs. Based on our experimental results, the self-assembly growth of SiC NWs is summarized as follows. When Si-deposited SiC is gradually annealed, Si thin films are transformed into Si oxide films by the diffusion of O2 gas. Carbon atoms are generated by high thermal energy at the interface between the oxidized Si thin films and SiC single crystals, and diffuse into Si oxide films. Gaseous SiO are formed by the reactions between solid SiO2 and solid C (reaction (4)) or between solid Si and solid SiO (reaction (3)). SiC nuclei are formed by a reaction between gaseous SiO and solid C (reaction (1)). Finally, SiC NWs are spontaneously grown by a reaction between SiO and gaseous CO (reaction (2)). The oxidized Si thin films plays two important roles on the growth of SiC NWs from Si-deposited SiC, namely, the diffusion path of carbon atoms and Si sources for growing SiC NWs [21,22]. The growth mechanism of SiC NWs is illustrated in Fig. 4.
4. Conclusion Single-crystalline SiC NWs were spontaneously grown on Sideposited SiC via simple thermal treatment. Si thin films were deposited on SiC single crystals by RF sputtering at room temperature. When they were thermally annealed at 1400 °C for 10 h under Ar gas atmosphere, SiC NWs were grown on the surface of Si-deposited SiC. No traces of catalyst or defects, such as metal particles and stacking faults, were found in the SiC NWs. Graphitic carbon spheres were observed for Si-deposited SiC annealed at temperatures above 1200 °C. When the annealing temperature
Please cite this article as: B.G. Kim, et al., Self-assembly growth of high-quality SiC nanowires from Si thin films deposited on singlecrystalline SiC wafers, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.09.047i
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Fig. 3. (a)–(d) FESEM images and (e)–(f) Raman spectra of Si-deposited SiC annealed at 1200 °C or 1400 °C for 10 h. The peak sum of the fitted curves is represented by black dotted lines, and ‘a-Si’ denotes amorphous Si.
thermal decomposition of SiC single crystals occurred at the interface between Si (or oxide) thin films and SiC single crystals, free carbon atoms were diffused into the films and used as carbon sources for SiC NWs. Si thin films were oxidized and acted as a channel of carbon diffusion and a Si sources for SiC NWs at high temperature. After the growth of SiC NWs, excess carbon atoms condensed and formed graphitic carbon spheres.
Acknowledgments This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2014R1A1A2057767). This work was supported by the Defense Acquisition Program Administration (DAPA) and the Agency for Defense Development (ADD), Republic of Korea.
References Fig. 4. Growth mechanism of SiC NWs from Si-deposited SiC with increase of annealing temperature.
increased from 1200 °C to 1400 °C, their size increased from 680 nm to 8 mm, respectively. The formation of graphitic carbon spheres as well as the growth of SiC NWs is the absolute evidence of the thermal decomposition of SiC single crystals, which were used as both substrates and carbon sources in this study. When the
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Please cite this article as: B.G. Kim, et al., Self-assembly growth of high-quality SiC nanowires from Si thin films deposited on singlecrystalline SiC wafers, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.09.047i