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Wafer scale catalytic growth of graphene on nickel by solid carbon source
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Available at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/carbon
Wafer scale catalytic growth of graphene on nickel by solid carbon source A. Delamoreanu a b
a,b,* ,
C. Rabot a, C. Vallee b, A. Zenasni
a
CEA-LETI-Minatec Campus, 17 rue des martyrs, F-38054 Grenoble Cedex 9, France LTM, UJF-Grenoble1/CNRS/CEA, 17 rue des martyrs, F-38054 Grenoble Cedex 9, France
A R T I C L E I N F O
A B S T R A C T
Article history:
We report the growth of graphene by solid carbon source on full 8-inch wafers. The first
Received 18 March 2013
step is to deposit the fine-tuned carbon source and the nickel catalyst thin films on top.
Accepted 19 August 2013
The second step is to anneal the stack for driving carbon throughout the catalyst and grow
Available online 27 August 2013
graphene upon the surface. The graphene can be optimized by tuning the stack design and the annealing conditions. Via this method, graphene with a transmittance of 91.9% and a sheet resistance of 565 Ohm/sq has been obtained. Our technique provides a rapid route to grow large scale graphene using standard microelectronic facilities. 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
Graphene, first restricted to a monolayer of sp2 hybridized carbon atoms, and then investigated also as few layers, has focused significant attention thanks to its fascinating electronic, mechanical, optical and thermal properties [1]. Initially devoted to the table-lab experiments and prototyping in upstream devices, considerable efforts have been made to extend the coverage of high-quality graphene to larger surfaces and to tune in order to reach a fully manufacturing process flow of graphene [2–4]. Among the reported approaches, the mechanical exfoliation of highly oriented pyrolytic graphite is definitely not scalable even though it provides the bestquality graphene sheets [5]. Epitaxial growth from silicon carbide (SiC) are being extensively investigated [6], providing some breakthroughs in knowledge, but constraints to design the electronic devices directly on the expensive SiC substrates. Recently, measurable advances in chemical vapor deposition (CVD) of graphene on metal patterns have been achieved, which open a way to the development of graphene in real life applications [7–9]. The use of nickel [10,11] and copper [12,13] as catalysts for graphene synthesis has stimulated various
applications due to the scalability and transferability of graphene grown on these substrates. Most of these CVD processes on nickel and copper are based on the exposure of carbon species to hot metallic surface that catalyze the carbon to form, depending on its solubility, mono to few layers of graphene [14,15]. However, the process often requires several steps for substrate preparation prior to deposition thus becoming costly in time and consumption. Additionally the process usually needs high synthesis temperature, close to the melting point threshold of the metal catalyst. This can lead to metal dewetting or potential poisonings of the annealing furnaces used for the deposition. The present work emphasizes a simple and quick route to grow graphene on 8-inch wafers (Fig. 1a), in semiconductorcompatible environment. In this approach, we take advantage of the solubility of carbon in transition metals (here a thin film of nickel) at high temperature that segregates at the metal surface to form graphene (Fig. 1b). To do so, a solid carbonbased source (e.g. amorphous carbon or amorphous silicon carbide) is deposited as a thin film and embedded between the substrate and a thin metal film of (50–300 nm). The stack is subsequently heated to high temperature (600–800 C). The graphene quality and the number of layers are strongly
* Corresponding author: Fax: +33 (0)4 38 78 58 92. E-mail address: [email protected] (A. Delamoreanu). 0008-6223/$ - see front matter 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.08.037
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Fig. 1 – Graphene grown by solid carbon source on nickel. (a) An image of as-grown graphene film on 8 in wafer. (b) The wafer with Si/SiO2/a:SiCH/Ni stack used for graphene growth.
dependent on the stack features and annealing conditions. This method reminds the standard metallurgic process to purify single crystalline metals where thermal annealing was performed to drive carbon impurities out of the metal bulk [16]. First proofs of concept have demonstrated that the carbon impurities segregation upon appropriate metal can results in the formation of graphene [17–19]. In an original way, the present work is performed in a semiconductor clean room platform, dedicated to nano-electronics. In the absence of reliable method to deposit large surface of metal single crystals, we have successfully grown 8-inch graphene wafers on polycrystalline thin films of nickel via the so-called segregation process without any external gaseous carbon sources, thus avoiding the inherent problems related to the use of gas sources as described in the following. The resulted coverage of graphene upon the wafer is highly uniform with more than of 99% of coverage (fully interconnected) over the whole area of polycrystalline nickel. In principle, this method is controlled by the vertical processes (diffusion, segregation) and then is not limited by the horizontal extent of the wafer. The advantage of this method is that the graphene growth can be finely controlled by the carbon source thickness and annealing conditions whereas other methods can lead to substantial issues due to potential inhomogeneity in delivering gas source upon large wafer surface [20]. In addition, to increase the size of the grains of the metal catalyst with appropriate crystalline orientation (1 1 1), a first annealing step of the metal surface is often performed in CVD processes whereas in the current process, the carbon diffusion and the crystalline orientation of nickel grains are simultaneously occurring, gaining time process and cost.
2.
Experimental
2.1.
Graphene growth by solid carbon source
All depositions have been performed on 8-inch silicon wafers (1 0 0). The process developed involves the deposition of solid carbon sources on 300 nm silicon oxide. The silicon oxide is used as a barrier to the diffusion of atoms inside the silicon wafer. The solid carbon source contains silicon atoms to enhance the adhesion. Indeed, if the deposition is made with only amorphous carbon, the annealing would likely result in delamination of the carbon/SiO2 interface that will damage the whole wafer and induce a poisoning of the chamber due
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to carbon dust volatilization. 7–80 nm of an amorphous hydrogenated silicon carbide (a:SiCH) is deposited by plasma enhanced chemical vapour deposition with a minimum of 20% Si (in mass). A polycrystalline nickel thin film (10– 200 nm) is then deposited upon the carbon solid source by physical vapour deposition (PVD) at room temperature (Fig. 1b). The stack Si/SiO2/a:SiCH/Ni is then annealed in situ (to avoid any oxidation of the nickel film) inside a tube furnace (600–800 C) under argon flow and a total pressure of 50 Torr during (1–15 min). The sample is then kept in a cooling chamber with a high cooling rate (5–10 C/s).
2.2.
Graphene transfer
In order to perform optical and electrical characterizations, the graphene films are transferred to a host substrate. After the synthesis, the stack is coated with 800 nm sacrificial poly(methyl methacrylate) (PMMA) layer [21] that was spincoated on the graphene film. The sample was, then, annealed at 180 C during 3 min. Afterwards, the substrate is immersed in an acid bath to etch the underlying nickel film. The later process was performed using iron(III) chloride (FeCl3 – 27%) which gave the most satisfactory result (compared with a substrate etched using HCl). The samples are immerged, from a couple of minutes to few hours depending on the sample size, in FeCl3 solution to etch the nickel. Contrarily to HCl that creates bubbling, mechanically damaging the graphene, iron chloride etches nickel very fast without any bubbles. Once the nickel is etched, the PMMA/graphene film was detached from the sample, and rinsed several times in deionised water. The PMMA/graphene film is then collected from beneath in water and stuck on (Si/300 nm SiO2) or quartz substrates. The sample was then dried carefully with nitrogen gas and heated up at 90 C during 20 min to improve the adherence of graphene to its substrate and to remove slowly the water. To dissolve the PMMA support layer, the sample was immersed in a hot acetone bath at 50 C during 10 min. After the transfer, a hot plate bake at 200 C is added to enhance the adhesion on the substrate and eliminate the polymer residues [22].
2.3.
Samples characterization
As-grown graphene films on nickel and transferred graphene on various substrates were characterized with X-ray diffraction (XRD, PANanalytical X’Pert PRO MRD XL), scanning electron microscope equipped with scanning transmission electron microscopy (SEM-STEM, Hitachi S-5500), transmission electron microscope (TEM, FEI Titan 80–300 Cs corrected at 80 kV). The Raman measurements were performed at room temperature using a 488 nm laser excitation (Raman spectrometer, JY LabRam). A 100x objective was used and the spot size of the laser was 1–2 lm with a laser power of 8 mW. The transmittance of graphene transferred on quartz substrate was measured from 280 to 1400 nm (Cary 500 UV–VIS–NIR spectrophotometer). Square pads (800 lm size, 200 lm interdistance) of nickel (20 nm) and gold (100 nm) were evaporated on top of graphene laying down on silicon dioxide to measure the sheet resistance.
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3.
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Results
The process used in the present work can be fine-tuned by changing the stack conditions. The thicknesses of both nickel and a:SiCH are important parameters for the graphene growth. The solid source can feed the process with carbon depending on the thickness of a:SiCH. Once the degradation occurs the carbon would diffuse through the whole nickel film to reach the surface and arrange itself to match the crystallographic orientation of nickel grains (in preferential way, hexagonal structures (1 1 1)) to make graphene. Besides, the carbon diffusion is based on the annealing conditions and so is the nickel structure. The influence of these parameters is investigated in order to have a better control of the graphene growth.
3.1.
Nickel structure
Scanning electron microscopy shows that the as-deposited nickel films reveal small grains with diameter size 20– 200 nm randomly distributed over the whole surface (Fig. 2a inset). When heated at high temperature, all grains grow to a large extent due to the thermal-induced movement of grains boundaries. Grains become larger with a diameter size of 1–2 lm (Fig. 2b inset, nickel grains can be seen underneath the graphene sheet). In order to appreciate the annealing effect on metal crystallinity, Fig. 2a and b shows X-ray Diffraction (XRD) spectra of a sample before and after annealing. The sample consists of a wafer template of silicon covered with 300 nm SiO2/80 nm a:SiCH/50 nm Ni, where the nickel is the top layer. XRD of as-deposited nickel thin film (Fig. 2a) shows predominately 1 1 1, 2 2 2 orientations. Residual nickel grains with different misorientations 1 0 0 and 3 1 1 can also be observed. After 15 min heating at 800 C, relative intensities of 1 1 1 and 2 2 2 peaks strongly increase and there are no residual orientations (Fig. 2b). This result shows that the as-deposited nickel film contains almost sufficient grains with the desired orientation (1 1 1), and the annealing is mostly used to grow nickel grains. Misoriented and small grains are known to create defects in the graphene layers [23]. Larger grains mean larger mono-domain of graphene. Annealed samples show also characteristic diffraction peaks of thick multilayer-graphene with 0 0 2 and 0 0 4 orientations at 26.4 and 54.5, respectively – i.e. with graphene planes parallel to the nickel surface. The sharp XRD 0 0 2 peak of graphene is similar to graphite XRD peak. This is due to the important thickness of graphene on this sample annealed at 800 C during 15 min. The coverage is estimated to 30 layers of graphene based on the Raman spectra.
3.2.
The influence of the amorphous silicon carbide
The effect of the carbon feedstock is investigated by controlling the amount of carbon via different a:SiCH thicknesses in the following stack (Si/300 nm SiO2/a:SiCH/100 nm Ni). The wafer is annealed at 800 C during 15 min and cooled down. Fig. 3a–c show SEM images of samples with 20, 40 and 80 nm a:SiCH, respectively. Graphene flakes can be easily distinguished in Fig. 3c inset by brightness contrast and thanks to wrinkles that are attributed to the difference in
the thermal expansion coefficients of graphene and nickel, which induces a thermal stress during the cooling. The resulting coverage of graphene is increasing with the amount of carbon in the stack. This means that the coalescence of graphene domains is enhanced by further carbon flow. Indeed, a balance between the carbon feedstock and the annealing condition to enhance the carbon segregation and the nickel thickness should be found to reach the full coverage. With a nickel thickness of 100 nm and 15 min of annealing at 800 C, the best thickness condition for the solid carbon source is 80 nm. The full coverage is almost reached with this value. The continuous coverage of graphene upon nickel grains means a diffusion effect of carbon throughout the nickel thickness. However, at the first stage of synthesis, graphene sheets cover nickel grains and also cross the grain boundaries without discontinuities, meaning no preferential segregation is occurring. No holes or dark spots are observed at the grain intersection meaning that diffusion and carbon segregation simultaneously occur.
3.3.
Graphene growth
With respect to the standard microelectronics requirements, the annealing temperature and time of process have to be the lowest as possible. That must be the case for industrialcompatible synthesis of graphene by solid-source carbon diffusion. In the meantime, the carbon diffusion throughout the metal becomes lower when the time and temperature are reduced. Therefore, to match with those specifications, the nickel has to be thin in order to let sufficient amount of carbon reach the surface. That must be operated up to 20 nm of nickel to avoid any metal dewetting. Then, the targeted thickness is set at 50 nm considered as a potential optimum. Annealing temperature is set at 700 C. Fig. 4a shows scanning electron microscopy of a sample with 50 nm of nickel on top of 80 nm a:SiCH annealed at 700 C during 3 min. The coverage rate is estimated to be more than 99% on the whole wafer. Uncovered surface of nickel is spotted with white clusters that show an effective oxidation of nickel when exposed to air (Energy-dispersive X-ray spectroscopy (EDX) analysis not shown). It is worth noticing that no trace of nickel oxide is detected (using EDX) where the graphene grows, demonstrates that the graphene overlayer is a superior protective coating particularly for materials that easily oxidize in an uncontrolled way when exposed to air [24]. Fig. 4d displays the Raman spectrum of the corresponding sample transferred on 300 nm SiO2 (Fig. 4b). It shows clearly the presence of graphene thanks to the appearance of characteristic 2D peak at 2722 cm 1. This spectrum exhibits also a G peak at 1583 cm 1 and a small D peak at 1361 cm 1 with a ID/ IG ratio of 0.08 indicating a relatively low defect density probably originating from few sp3 carbon atoms or from defects from pentagon and heptagon rings. The nature of these defects is not yet fully understood, but they presumably come from multiple domain coalescence at the grain boundaries. The low D peak shows that the transfer has little impact on graphene quality and does not induce additional defects. I2D/IG ratio is equal to 0.52 and a 2D full width half maximum
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Fig. 2 – Grain growth during the annealing and graphene formation. The XRD spectrum (a) of as-deposited stack Si/SiO2/ 80 nm a:SiCH/50 nm Ni shows a random distribution of the orientation with predominance for the nickel (1 1 1). The XRD spectrum (b) after annealing at 800 C during 15 min shows only the (1 1 1) orientation and the thick graphene sharp peak. Insets show the corresponding SEM images. of 63 cm 1 allows estimating the growth of about 7–15 layers of graphene. Transmission electron microscopy measurements were carried out on the graphene transferred onto silicon–nitride grids. The sample shows 2–10 layers on different grains with an average of 6 layers (see Fig. 4c). Fig. 4e and f shows STEM images (transmission mode in a SEM at 30 kV) of graphene film transferred on a silicon–nitride TEM grid. The bright field (e) and the dark field (f) pictures (STEM 30 kV) show that the graphene domains are lm size and the difference in contrast between those domains can be explained by a different number of layers on each one. The graphene domains fit well with the nickel grain size, which comforts the assumption that the growth occurs mainly from the grain boundaries.
The nickel thickness can be adjusted in order to control the number of graphene layers. A thicker nickel layer would decrease the amount of carbon (thermally activated) reaching the surface and therefore a thinner graphene film should be obtained. Fig. 5a shows a SEM image of a graphene film transferred on SiO2 and obtained with 200 nm of nickel on the same stack and with the same annealing condition. After the transfer of graphene on SiO2, multi-layer graphene flakes are visible and cover 50% of the sample surface. Raman spectroscopy on this graphene film shows that between these flakes there is a monolayer of graphene interconnecting them together (Fig. 5b). Indeed, the I2D/IG ratio is equal to 2.5 indicating the presence of monolayer whereas on the flakes this ratio is equal to 0.48. This graphene obtained from 200 nm
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pads. The voltage between two Ni/Au pads is scanned from 5 to +5 volts and the resistance was checked on 20 different places over the sample. The average sheet resistance measured was 622 Ohm/sq for the 50 nm nickel stack and 565 Ohm/sq for the 200 nm nickel stack. The highest quality graphene in terms of a combination of both highest optical transmittance and lowest sheet resistance is obtained with a stack of 300 nm SiO2/80 nm a:SiCH/200 nm Ni that has been annealed to 700 C for 3 min. The small difference of resistance between the two samples could be explained by the doping effect of graphene by FeCl3 during the etching process. Indeed, only the first layer of graphene is exposed to the acid and therefore could be doped [25]. If the current is carried mainly by the first layer, this could explain the small difference in conductivity. Fig. 6b shows graphene transferred on flexible polyethylene terephthalate (PET) substrate. Graphene has a high bendability and could be used in flexible devices.
4.
Fig. 3 – 15 min process at 800 C with (Si/SiO2/a:SiCH/100 nm Ni) stack using different carbon source thickness. SEM images of (a) 20 nm a:SiCH, (b) 40 nm a:SiCH and (c) 80 nm a:SiCH samples. Inset shows high resolution (220 k) SEM images of graphene wrinkles. The resulted coverage of graphene on the nickel surface is increasing.
nickel stack is therefore thinner than the one obtained from 50 nm nickel one. It is also the thinnest obtained in this experiment. For the exactly same experimental conditions, Ni films with different thickness have been tested. They showed a drastic decrease of the graphene coverage for Ni films thicker than 200 nm. While for Ni films thinner than 200 nm, the average thickness of graphene increased, with particularly thick graphene at the nucleation points.
3.4.
Graphene as a transparent conductive material
The transmittance measurements on graphene transferred on quartz substrate (Fig. 6a) confirm that graphene from the 200 nm nickel stack is thinner. The spot diameter is 5 mm and the transmittance was measured in the range from 280 to 1400 nm laser light wavelength. The transmittance at 550 nm wavelength increases from 79.5% for the 50 nm nickel stack to 91.9% for the 200 nm nickel stack (Fig. 6c). The number of graphene layers is then adjusted by the metal thickness and the annealing conditions. The sheet resistance of graphene films on SiO2 was measured with the evaporated Ni/Au
Discussion
The use of solid carbon source to grow graphene leads to the reduction of the number of parameters involved during the process. Besides, all the steps are performed on standard semiconductor clean room equipments. That makes the process stable, reproducible and suitable for industrial production. The only important parameters are process-based (the thickness of the layers and the annealing conditions). A better understanding of the of each parameter effects is useful to have a fine control of the growth. The polycrystalline nickel layer used as a catalyst for the graphene growth has been characterized by XRD measurements and showed the importance of annealing on the metal grains. During this step, nickel grains are growing and the metal becomes more crystalline. Defects, evidenced by D peak in Raman spectra, are more likely located at the grains boundaries. It was evidenced that, when the epitaxial relationship between graphene and the metal grain is established, graphene domains are misoriented at the footprint of metal grain boundaries [26]. This sideline displays defects (pentagon and heptagon instead of hexagon graphene) which are scattering sites of carrier lowering the mobility and reducing the conductivity of graphene [27]. Large metal grains with less grain boundaries are then necessary for reducing the number of defects in the graphene layers. The solid source provides the amount of carbon that will diffuse through the catalyst. The graphene formation upon diffusion mechanisms of solid carbon source relies on the advances made in the metallurgic purification of metal foil heated at high temperature to induce carbon impurities diffusion to the surface. Typically, at high temperature, the intercalated carbon source becomes damaged and carbon atoms can either diffuse through the metallic films (carbon is highly soluble in nickel: 0.5% at 800 C [28]) or is catalyzed to form an intermediate carbon phase within the nickel film. Literature is already discussing the formation of nickel carbide Ni2C and the growth of graphene at the expense of this carbide phase [29]. This phenomenon is excluded in the temperature range used where the carbide phase is out of equilibrium. Besides, in the current experiment, X-ray Diffraction
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Fig. 4 – Graphene from (80 nm a:SiCH/50 nm Ni) stack. (a) SEM images of the stack annealed at 700 C during 3 min. (b) The graphene film transferred on 300 nm SiO2, 2 · 3 cm2. (c) TEM picture showing multilayer graphene with 6 layers. Scale bar: 2 nm. (d) Raman spectra (488 nm) of graphene films on SiO2. (e and f) Bright Field and Dark Field SEM-STEM picture (30 kV) of graphene transferred on a silicon–nitride grid. Hole diameter: 2.5 lm. Each lm size graphene domains has a different number of layers showing a different contrast in transmission mode. Red line shows the graphene domains. (A colour version of this figure can be viewed online.)
Fig. 5 – Graphene from (80 nm a:SiCH/200 nm Ni) stack. (a) SEM image of the stack annealed at 700 C during 3 min and transferred on 300 nm SiO2. (b) Raman spectrum (488 nm) of the transferred graphene on SiO2. The blue spectrum shows monolayer graphene and the red one multilayer graphene. The coverage rate of each is about 50% as shown by SEM images. (A colour version of this figure can be viewed online.)
measurements show no carbide formation during the annealing and only the presence of nickel phase. This means that
the carbon sources are mainly thermally degraded and that carbon atoms dissolves into the metal and diffuse through
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Fig. 6 – Graphene transferred on quartz substrate (a) and flexible PET (b). Transmittance of graphene films transferred on quartz substrate (c). The transmittance is 79.5% at 550 nm for the nickel 50 nm and 91.9% for the nickel 200 nm. Inset shows the transmittance and the sheet resistance for the nickel 50 nm and the nickel 200 nm samples.
it toward the metal surface. The bottom right domain on the dark field picture (Fig. 4f) shows some steps on the edge of the domain with different contrast. These steps could be attributed to the graphene layers meaning that the graphene is growing from the centre to the edge of the domain. This observation matches with the graphene growth mechanism that involves preferential nucleation of graphene at the metal grains boundaries [30]. During the annealing step, the carbon content in the nickel film increases while the carbon content in the a:SiCH film falls. When only the a:SiCH thickness is increased, the carbon content on the catalyst surface becomes more important. Since the Si/C ratio in the carbon source is constant throughout the thickness, the carbon source is gradually consumed in favor of carbon species that are driven from the bottom to the top of the stack to form the graphene sheets. The carbon amount reaching the surface can be set by the source thickness. Brightness differences show that some grains, instead others, favour a rapid diffusion of carbon. This is certainly at the origin of inhomogeneous segregation of graphene on top of nickel surface, already observed in previous works [31,32]. It is worth noticing that XRD analysis showed that the as-deposited nickel thin films support some residual nickel grains formation with different misorientations, namely 1 0 0 and 3 1 1. Those residual misorientations are transformed into 1 1 1 orientation during the annealing as observed from XRD spectra obtained after the annealing step. Since the carbon degradation and the simultaneous diffusion start at the first stage of annealing, those misoriented
grains could slowdown graphene formation. The use of asdeposited nickel film with only (1 1 1) phase orientation would probably reduce this inhomogeneity in graphene synthesis. When process conditions are tuned, sufficient carbon species are provided (in a:SiCH) and more extended graphene is obtained with high length wrinkles that cross nickel grains boundaries without any discontinuities (that has been evidenced in monolayer graphene) [33]. When only the nickel thickness is increased, less carbon can reach the surface and the graphene is, at the end, thinner. For instance, with 200 nm of nickel, the measured graphene transmittance is 91.9%, while with 50 nm of nickel, graphene transmittance is 79.5% (at 550 nm). The difference of carbon amount on the surface suggests that not all the carbon from the nickel is segregating on the nickel catalyst. In the current work, SEM images show that graphene segregates in a way that it is not always following the footprint. The perfect side of some graphene domains cannot be explained simply by inhomogeneous segregation of carbon upon individual grain. When sufficient carbon species are provided at the metal surface, the graphene growth by carbon segregation and the coalescence of the different graphene domains to form a complete layer covering the entire substrate surface can occur. Recently, Lahiri et al. [34] suggested that there is a large barrier for carbon attachment to the edge of a graphene island in graphene/Ni system. Density functional theory (DFT) calculation showed that the edge carbon atoms that have unsaturated dangling bonds form strong bonds with the nickel atoms in the first
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surface layer of the substrate. Therefore, the attachment of a single carbon atom to the graphene edge requires the breaking of these bonds, which is expected to produce a large activation barrier for such chemical reaction. This suggestion can be partially true as it was shown that the graphene islands are exclusively nucleated on the ascending side of atomic step edges [35–37]. This is presumably promoted by the fact that carbon atoms at graphene edges can accordingly form stabilizing-like bonds with the metal atoms at the step edge. Even though the coalescence of graphene islands is more likely to match with the experimental observations, at this stage, we cannot conclude on the exact mechanisms that occur.
5.
Conclusions
In summary, we have synthesized large area graphene on 8inch wafers. Using a solid carbon source, a reliable and facile route for growing graphene in an industrial fashion has been demonstrated. This technique is rapid, relatively simple and compatible with standard microelectronic facilities which make it low cost. The technique is not limited by the size and it can be easily scaled up to 12 in wafer or more. The number of layers can be controlled by the stack parameters and the annealing conditions. Graphene with a high transmittance (91.9%) and a low sheet resistance (565 Ohm/sq) has been obtained and successfully transferred to SiO2 substrates. It is also a good transparent conductive material that could be used as transparent conductive electrode in a large variety of applications in the near future.
Acknowledgements This work has been done in the frame of European funded FP7-NMP-GRENADA project and was partly supported by the French RENATECH network. We thank Dominique Lafond for TEM measurements and Patrice Gergaud for assistance during XRD measurements.
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[28] Natesan K, Kassner T. Thermodynamics of carbon in nickel, iron–nickel and iron–chromium–nickel alloys. Metall Trans B 1973;4:2557–66. [29] Lahiri J, Miller T, Adamska L, Oleynik II, Batzill M. Graphene growth on Ni(1 1 1) by transformation of a surface carbide. Nano Lett 2011;11(2):518–22. [30] Saadi S, Abild-Pedersen F, Helveg S, Sehested J, Hinnemann B, Appel CC, et al. On the role of metal step-edges in graphene growth. J Phys Chem C 2010;114(25):11221–7. [31] Weatherup RS, Bayer BC, Blume R, Ducati C, Baehtz C, Schlo¨gl R, et al. In situ characterization of alloy catalysts for lowtemperature graphene growth. Nano Lett 2011;11(10):4154–60. [32] Reina A, Jia X, Ho J, Nezich D, Son H, Bulovic V, et al. Large area, few-layer graphene films on arbitrary substrates by chemical vapor deposition. Nano Lett 2009;9(1):30–5.
[33] Guermoune A, Chari T, Popescu F, Sabri SS, Guillemette J, Skulason HS, et al. Chemical vapor deposition synthesis of graphene on copper with methanol, ethanol, and propanol precursors. Carbon 2011;49(13):4204–10. [34] Lahiri J, Miller TS, Ross AJ, Adamska L, Oleynik II, Batzill M. Graphene growth and stability at nickel surfaces. New J Phys 2011;13(2):025001. [35] Coraux J, N’Diaye AT, Engler M, Busse C, Wall D, Buckanie N, et al. Growth of graphene on Ir(1 1 1). New J Phys 2009;11(2):023006. [36] Sutter PW, Flege JI, Sutter EA. Epitaxial graphene on ruthenium. Nat Mater 2008;7(5):406–11. [37] Sutter P, Sadowski JT, Sutter E. Graphene on Pt(1 1 1): growth and substrate interaction. Phys Rev B 2009;80(24):245411.
6 6 ( 2 0 1 4 ) 4 8 –5 6
Available at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/carbon
Wafer scale catalytic growth of graphene on nickel by solid carbon source A. Delamoreanu a b
a,b,* ,
C. Rabot a, C. Vallee b, A. Zenasni
a
CEA-LETI-Minatec Campus, 17 rue des martyrs, F-38054 Grenoble Cedex 9, France LTM, UJF-Grenoble1/CNRS/CEA, 17 rue des martyrs, F-38054 Grenoble Cedex 9, France
A R T I C L E I N F O
A B S T R A C T
Article history:
We report the growth of graphene by solid carbon source on full 8-inch wafers. The first
Received 18 March 2013
step is to deposit the fine-tuned carbon source and the nickel catalyst thin films on top.
Accepted 19 August 2013
The second step is to anneal the stack for driving carbon throughout the catalyst and grow
Available online 27 August 2013
graphene upon the surface. The graphene can be optimized by tuning the stack design and the annealing conditions. Via this method, graphene with a transmittance of 91.9% and a sheet resistance of 565 Ohm/sq has been obtained. Our technique provides a rapid route to grow large scale graphene using standard microelectronic facilities. 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
Graphene, first restricted to a monolayer of sp2 hybridized carbon atoms, and then investigated also as few layers, has focused significant attention thanks to its fascinating electronic, mechanical, optical and thermal properties [1]. Initially devoted to the table-lab experiments and prototyping in upstream devices, considerable efforts have been made to extend the coverage of high-quality graphene to larger surfaces and to tune in order to reach a fully manufacturing process flow of graphene [2–4]. Among the reported approaches, the mechanical exfoliation of highly oriented pyrolytic graphite is definitely not scalable even though it provides the bestquality graphene sheets [5]. Epitaxial growth from silicon carbide (SiC) are being extensively investigated [6], providing some breakthroughs in knowledge, but constraints to design the electronic devices directly on the expensive SiC substrates. Recently, measurable advances in chemical vapor deposition (CVD) of graphene on metal patterns have been achieved, which open a way to the development of graphene in real life applications [7–9]. The use of nickel [10,11] and copper [12,13] as catalysts for graphene synthesis has stimulated various
applications due to the scalability and transferability of graphene grown on these substrates. Most of these CVD processes on nickel and copper are based on the exposure of carbon species to hot metallic surface that catalyze the carbon to form, depending on its solubility, mono to few layers of graphene [14,15]. However, the process often requires several steps for substrate preparation prior to deposition thus becoming costly in time and consumption. Additionally the process usually needs high synthesis temperature, close to the melting point threshold of the metal catalyst. This can lead to metal dewetting or potential poisonings of the annealing furnaces used for the deposition. The present work emphasizes a simple and quick route to grow graphene on 8-inch wafers (Fig. 1a), in semiconductorcompatible environment. In this approach, we take advantage of the solubility of carbon in transition metals (here a thin film of nickel) at high temperature that segregates at the metal surface to form graphene (Fig. 1b). To do so, a solid carbonbased source (e.g. amorphous carbon or amorphous silicon carbide) is deposited as a thin film and embedded between the substrate and a thin metal film of (50–300 nm). The stack is subsequently heated to high temperature (600–800 C). The graphene quality and the number of layers are strongly
* Corresponding author: Fax: +33 (0)4 38 78 58 92. E-mail address: [email protected] (A. Delamoreanu). 0008-6223/$ - see front matter 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.08.037
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Fig. 1 – Graphene grown by solid carbon source on nickel. (a) An image of as-grown graphene film on 8 in wafer. (b) The wafer with Si/SiO2/a:SiCH/Ni stack used for graphene growth.
dependent on the stack features and annealing conditions. This method reminds the standard metallurgic process to purify single crystalline metals where thermal annealing was performed to drive carbon impurities out of the metal bulk [16]. First proofs of concept have demonstrated that the carbon impurities segregation upon appropriate metal can results in the formation of graphene [17–19]. In an original way, the present work is performed in a semiconductor clean room platform, dedicated to nano-electronics. In the absence of reliable method to deposit large surface of metal single crystals, we have successfully grown 8-inch graphene wafers on polycrystalline thin films of nickel via the so-called segregation process without any external gaseous carbon sources, thus avoiding the inherent problems related to the use of gas sources as described in the following. The resulted coverage of graphene upon the wafer is highly uniform with more than of 99% of coverage (fully interconnected) over the whole area of polycrystalline nickel. In principle, this method is controlled by the vertical processes (diffusion, segregation) and then is not limited by the horizontal extent of the wafer. The advantage of this method is that the graphene growth can be finely controlled by the carbon source thickness and annealing conditions whereas other methods can lead to substantial issues due to potential inhomogeneity in delivering gas source upon large wafer surface [20]. In addition, to increase the size of the grains of the metal catalyst with appropriate crystalline orientation (1 1 1), a first annealing step of the metal surface is often performed in CVD processes whereas in the current process, the carbon diffusion and the crystalline orientation of nickel grains are simultaneously occurring, gaining time process and cost.
2.
Experimental
2.1.
Graphene growth by solid carbon source
All depositions have been performed on 8-inch silicon wafers (1 0 0). The process developed involves the deposition of solid carbon sources on 300 nm silicon oxide. The silicon oxide is used as a barrier to the diffusion of atoms inside the silicon wafer. The solid carbon source contains silicon atoms to enhance the adhesion. Indeed, if the deposition is made with only amorphous carbon, the annealing would likely result in delamination of the carbon/SiO2 interface that will damage the whole wafer and induce a poisoning of the chamber due
49
to carbon dust volatilization. 7–80 nm of an amorphous hydrogenated silicon carbide (a:SiCH) is deposited by plasma enhanced chemical vapour deposition with a minimum of 20% Si (in mass). A polycrystalline nickel thin film (10– 200 nm) is then deposited upon the carbon solid source by physical vapour deposition (PVD) at room temperature (Fig. 1b). The stack Si/SiO2/a:SiCH/Ni is then annealed in situ (to avoid any oxidation of the nickel film) inside a tube furnace (600–800 C) under argon flow and a total pressure of 50 Torr during (1–15 min). The sample is then kept in a cooling chamber with a high cooling rate (5–10 C/s).
2.2.
Graphene transfer
In order to perform optical and electrical characterizations, the graphene films are transferred to a host substrate. After the synthesis, the stack is coated with 800 nm sacrificial poly(methyl methacrylate) (PMMA) layer [21] that was spincoated on the graphene film. The sample was, then, annealed at 180 C during 3 min. Afterwards, the substrate is immersed in an acid bath to etch the underlying nickel film. The later process was performed using iron(III) chloride (FeCl3 – 27%) which gave the most satisfactory result (compared with a substrate etched using HCl). The samples are immerged, from a couple of minutes to few hours depending on the sample size, in FeCl3 solution to etch the nickel. Contrarily to HCl that creates bubbling, mechanically damaging the graphene, iron chloride etches nickel very fast without any bubbles. Once the nickel is etched, the PMMA/graphene film was detached from the sample, and rinsed several times in deionised water. The PMMA/graphene film is then collected from beneath in water and stuck on (Si/300 nm SiO2) or quartz substrates. The sample was then dried carefully with nitrogen gas and heated up at 90 C during 20 min to improve the adherence of graphene to its substrate and to remove slowly the water. To dissolve the PMMA support layer, the sample was immersed in a hot acetone bath at 50 C during 10 min. After the transfer, a hot plate bake at 200 C is added to enhance the adhesion on the substrate and eliminate the polymer residues [22].
2.3.
Samples characterization
As-grown graphene films on nickel and transferred graphene on various substrates were characterized with X-ray diffraction (XRD, PANanalytical X’Pert PRO MRD XL), scanning electron microscope equipped with scanning transmission electron microscopy (SEM-STEM, Hitachi S-5500), transmission electron microscope (TEM, FEI Titan 80–300 Cs corrected at 80 kV). The Raman measurements were performed at room temperature using a 488 nm laser excitation (Raman spectrometer, JY LabRam). A 100x objective was used and the spot size of the laser was 1–2 lm with a laser power of 8 mW. The transmittance of graphene transferred on quartz substrate was measured from 280 to 1400 nm (Cary 500 UV–VIS–NIR spectrophotometer). Square pads (800 lm size, 200 lm interdistance) of nickel (20 nm) and gold (100 nm) were evaporated on top of graphene laying down on silicon dioxide to measure the sheet resistance.
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3.
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Results
The process used in the present work can be fine-tuned by changing the stack conditions. The thicknesses of both nickel and a:SiCH are important parameters for the graphene growth. The solid source can feed the process with carbon depending on the thickness of a:SiCH. Once the degradation occurs the carbon would diffuse through the whole nickel film to reach the surface and arrange itself to match the crystallographic orientation of nickel grains (in preferential way, hexagonal structures (1 1 1)) to make graphene. Besides, the carbon diffusion is based on the annealing conditions and so is the nickel structure. The influence of these parameters is investigated in order to have a better control of the graphene growth.
3.1.
Nickel structure
Scanning electron microscopy shows that the as-deposited nickel films reveal small grains with diameter size 20– 200 nm randomly distributed over the whole surface (Fig. 2a inset). When heated at high temperature, all grains grow to a large extent due to the thermal-induced movement of grains boundaries. Grains become larger with a diameter size of 1–2 lm (Fig. 2b inset, nickel grains can be seen underneath the graphene sheet). In order to appreciate the annealing effect on metal crystallinity, Fig. 2a and b shows X-ray Diffraction (XRD) spectra of a sample before and after annealing. The sample consists of a wafer template of silicon covered with 300 nm SiO2/80 nm a:SiCH/50 nm Ni, where the nickel is the top layer. XRD of as-deposited nickel thin film (Fig. 2a) shows predominately 1 1 1, 2 2 2 orientations. Residual nickel grains with different misorientations 1 0 0 and 3 1 1 can also be observed. After 15 min heating at 800 C, relative intensities of 1 1 1 and 2 2 2 peaks strongly increase and there are no residual orientations (Fig. 2b). This result shows that the as-deposited nickel film contains almost sufficient grains with the desired orientation (1 1 1), and the annealing is mostly used to grow nickel grains. Misoriented and small grains are known to create defects in the graphene layers [23]. Larger grains mean larger mono-domain of graphene. Annealed samples show also characteristic diffraction peaks of thick multilayer-graphene with 0 0 2 and 0 0 4 orientations at 26.4 and 54.5, respectively – i.e. with graphene planes parallel to the nickel surface. The sharp XRD 0 0 2 peak of graphene is similar to graphite XRD peak. This is due to the important thickness of graphene on this sample annealed at 800 C during 15 min. The coverage is estimated to 30 layers of graphene based on the Raman spectra.
3.2.
The influence of the amorphous silicon carbide
The effect of the carbon feedstock is investigated by controlling the amount of carbon via different a:SiCH thicknesses in the following stack (Si/300 nm SiO2/a:SiCH/100 nm Ni). The wafer is annealed at 800 C during 15 min and cooled down. Fig. 3a–c show SEM images of samples with 20, 40 and 80 nm a:SiCH, respectively. Graphene flakes can be easily distinguished in Fig. 3c inset by brightness contrast and thanks to wrinkles that are attributed to the difference in
the thermal expansion coefficients of graphene and nickel, which induces a thermal stress during the cooling. The resulting coverage of graphene is increasing with the amount of carbon in the stack. This means that the coalescence of graphene domains is enhanced by further carbon flow. Indeed, a balance between the carbon feedstock and the annealing condition to enhance the carbon segregation and the nickel thickness should be found to reach the full coverage. With a nickel thickness of 100 nm and 15 min of annealing at 800 C, the best thickness condition for the solid carbon source is 80 nm. The full coverage is almost reached with this value. The continuous coverage of graphene upon nickel grains means a diffusion effect of carbon throughout the nickel thickness. However, at the first stage of synthesis, graphene sheets cover nickel grains and also cross the grain boundaries without discontinuities, meaning no preferential segregation is occurring. No holes or dark spots are observed at the grain intersection meaning that diffusion and carbon segregation simultaneously occur.
3.3.
Graphene growth
With respect to the standard microelectronics requirements, the annealing temperature and time of process have to be the lowest as possible. That must be the case for industrialcompatible synthesis of graphene by solid-source carbon diffusion. In the meantime, the carbon diffusion throughout the metal becomes lower when the time and temperature are reduced. Therefore, to match with those specifications, the nickel has to be thin in order to let sufficient amount of carbon reach the surface. That must be operated up to 20 nm of nickel to avoid any metal dewetting. Then, the targeted thickness is set at 50 nm considered as a potential optimum. Annealing temperature is set at 700 C. Fig. 4a shows scanning electron microscopy of a sample with 50 nm of nickel on top of 80 nm a:SiCH annealed at 700 C during 3 min. The coverage rate is estimated to be more than 99% on the whole wafer. Uncovered surface of nickel is spotted with white clusters that show an effective oxidation of nickel when exposed to air (Energy-dispersive X-ray spectroscopy (EDX) analysis not shown). It is worth noticing that no trace of nickel oxide is detected (using EDX) where the graphene grows, demonstrates that the graphene overlayer is a superior protective coating particularly for materials that easily oxidize in an uncontrolled way when exposed to air [24]. Fig. 4d displays the Raman spectrum of the corresponding sample transferred on 300 nm SiO2 (Fig. 4b). It shows clearly the presence of graphene thanks to the appearance of characteristic 2D peak at 2722 cm 1. This spectrum exhibits also a G peak at 1583 cm 1 and a small D peak at 1361 cm 1 with a ID/ IG ratio of 0.08 indicating a relatively low defect density probably originating from few sp3 carbon atoms or from defects from pentagon and heptagon rings. The nature of these defects is not yet fully understood, but they presumably come from multiple domain coalescence at the grain boundaries. The low D peak shows that the transfer has little impact on graphene quality and does not induce additional defects. I2D/IG ratio is equal to 0.52 and a 2D full width half maximum
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Fig. 2 – Grain growth during the annealing and graphene formation. The XRD spectrum (a) of as-deposited stack Si/SiO2/ 80 nm a:SiCH/50 nm Ni shows a random distribution of the orientation with predominance for the nickel (1 1 1). The XRD spectrum (b) after annealing at 800 C during 15 min shows only the (1 1 1) orientation and the thick graphene sharp peak. Insets show the corresponding SEM images. of 63 cm 1 allows estimating the growth of about 7–15 layers of graphene. Transmission electron microscopy measurements were carried out on the graphene transferred onto silicon–nitride grids. The sample shows 2–10 layers on different grains with an average of 6 layers (see Fig. 4c). Fig. 4e and f shows STEM images (transmission mode in a SEM at 30 kV) of graphene film transferred on a silicon–nitride TEM grid. The bright field (e) and the dark field (f) pictures (STEM 30 kV) show that the graphene domains are lm size and the difference in contrast between those domains can be explained by a different number of layers on each one. The graphene domains fit well with the nickel grain size, which comforts the assumption that the growth occurs mainly from the grain boundaries.
The nickel thickness can be adjusted in order to control the number of graphene layers. A thicker nickel layer would decrease the amount of carbon (thermally activated) reaching the surface and therefore a thinner graphene film should be obtained. Fig. 5a shows a SEM image of a graphene film transferred on SiO2 and obtained with 200 nm of nickel on the same stack and with the same annealing condition. After the transfer of graphene on SiO2, multi-layer graphene flakes are visible and cover 50% of the sample surface. Raman spectroscopy on this graphene film shows that between these flakes there is a monolayer of graphene interconnecting them together (Fig. 5b). Indeed, the I2D/IG ratio is equal to 2.5 indicating the presence of monolayer whereas on the flakes this ratio is equal to 0.48. This graphene obtained from 200 nm
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pads. The voltage between two Ni/Au pads is scanned from 5 to +5 volts and the resistance was checked on 20 different places over the sample. The average sheet resistance measured was 622 Ohm/sq for the 50 nm nickel stack and 565 Ohm/sq for the 200 nm nickel stack. The highest quality graphene in terms of a combination of both highest optical transmittance and lowest sheet resistance is obtained with a stack of 300 nm SiO2/80 nm a:SiCH/200 nm Ni that has been annealed to 700 C for 3 min. The small difference of resistance between the two samples could be explained by the doping effect of graphene by FeCl3 during the etching process. Indeed, only the first layer of graphene is exposed to the acid and therefore could be doped [25]. If the current is carried mainly by the first layer, this could explain the small difference in conductivity. Fig. 6b shows graphene transferred on flexible polyethylene terephthalate (PET) substrate. Graphene has a high bendability and could be used in flexible devices.
4.
Fig. 3 – 15 min process at 800 C with (Si/SiO2/a:SiCH/100 nm Ni) stack using different carbon source thickness. SEM images of (a) 20 nm a:SiCH, (b) 40 nm a:SiCH and (c) 80 nm a:SiCH samples. Inset shows high resolution (220 k) SEM images of graphene wrinkles. The resulted coverage of graphene on the nickel surface is increasing.
nickel stack is therefore thinner than the one obtained from 50 nm nickel one. It is also the thinnest obtained in this experiment. For the exactly same experimental conditions, Ni films with different thickness have been tested. They showed a drastic decrease of the graphene coverage for Ni films thicker than 200 nm. While for Ni films thinner than 200 nm, the average thickness of graphene increased, with particularly thick graphene at the nucleation points.
3.4.
Graphene as a transparent conductive material
The transmittance measurements on graphene transferred on quartz substrate (Fig. 6a) confirm that graphene from the 200 nm nickel stack is thinner. The spot diameter is 5 mm and the transmittance was measured in the range from 280 to 1400 nm laser light wavelength. The transmittance at 550 nm wavelength increases from 79.5% for the 50 nm nickel stack to 91.9% for the 200 nm nickel stack (Fig. 6c). The number of graphene layers is then adjusted by the metal thickness and the annealing conditions. The sheet resistance of graphene films on SiO2 was measured with the evaporated Ni/Au
Discussion
The use of solid carbon source to grow graphene leads to the reduction of the number of parameters involved during the process. Besides, all the steps are performed on standard semiconductor clean room equipments. That makes the process stable, reproducible and suitable for industrial production. The only important parameters are process-based (the thickness of the layers and the annealing conditions). A better understanding of the of each parameter effects is useful to have a fine control of the growth. The polycrystalline nickel layer used as a catalyst for the graphene growth has been characterized by XRD measurements and showed the importance of annealing on the metal grains. During this step, nickel grains are growing and the metal becomes more crystalline. Defects, evidenced by D peak in Raman spectra, are more likely located at the grains boundaries. It was evidenced that, when the epitaxial relationship between graphene and the metal grain is established, graphene domains are misoriented at the footprint of metal grain boundaries [26]. This sideline displays defects (pentagon and heptagon instead of hexagon graphene) which are scattering sites of carrier lowering the mobility and reducing the conductivity of graphene [27]. Large metal grains with less grain boundaries are then necessary for reducing the number of defects in the graphene layers. The solid source provides the amount of carbon that will diffuse through the catalyst. The graphene formation upon diffusion mechanisms of solid carbon source relies on the advances made in the metallurgic purification of metal foil heated at high temperature to induce carbon impurities diffusion to the surface. Typically, at high temperature, the intercalated carbon source becomes damaged and carbon atoms can either diffuse through the metallic films (carbon is highly soluble in nickel: 0.5% at 800 C [28]) or is catalyzed to form an intermediate carbon phase within the nickel film. Literature is already discussing the formation of nickel carbide Ni2C and the growth of graphene at the expense of this carbide phase [29]. This phenomenon is excluded in the temperature range used where the carbide phase is out of equilibrium. Besides, in the current experiment, X-ray Diffraction
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Fig. 4 – Graphene from (80 nm a:SiCH/50 nm Ni) stack. (a) SEM images of the stack annealed at 700 C during 3 min. (b) The graphene film transferred on 300 nm SiO2, 2 · 3 cm2. (c) TEM picture showing multilayer graphene with 6 layers. Scale bar: 2 nm. (d) Raman spectra (488 nm) of graphene films on SiO2. (e and f) Bright Field and Dark Field SEM-STEM picture (30 kV) of graphene transferred on a silicon–nitride grid. Hole diameter: 2.5 lm. Each lm size graphene domains has a different number of layers showing a different contrast in transmission mode. Red line shows the graphene domains. (A colour version of this figure can be viewed online.)
Fig. 5 – Graphene from (80 nm a:SiCH/200 nm Ni) stack. (a) SEM image of the stack annealed at 700 C during 3 min and transferred on 300 nm SiO2. (b) Raman spectrum (488 nm) of the transferred graphene on SiO2. The blue spectrum shows monolayer graphene and the red one multilayer graphene. The coverage rate of each is about 50% as shown by SEM images. (A colour version of this figure can be viewed online.)
measurements show no carbide formation during the annealing and only the presence of nickel phase. This means that
the carbon sources are mainly thermally degraded and that carbon atoms dissolves into the metal and diffuse through
54
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Fig. 6 – Graphene transferred on quartz substrate (a) and flexible PET (b). Transmittance of graphene films transferred on quartz substrate (c). The transmittance is 79.5% at 550 nm for the nickel 50 nm and 91.9% for the nickel 200 nm. Inset shows the transmittance and the sheet resistance for the nickel 50 nm and the nickel 200 nm samples.
it toward the metal surface. The bottom right domain on the dark field picture (Fig. 4f) shows some steps on the edge of the domain with different contrast. These steps could be attributed to the graphene layers meaning that the graphene is growing from the centre to the edge of the domain. This observation matches with the graphene growth mechanism that involves preferential nucleation of graphene at the metal grains boundaries [30]. During the annealing step, the carbon content in the nickel film increases while the carbon content in the a:SiCH film falls. When only the a:SiCH thickness is increased, the carbon content on the catalyst surface becomes more important. Since the Si/C ratio in the carbon source is constant throughout the thickness, the carbon source is gradually consumed in favor of carbon species that are driven from the bottom to the top of the stack to form the graphene sheets. The carbon amount reaching the surface can be set by the source thickness. Brightness differences show that some grains, instead others, favour a rapid diffusion of carbon. This is certainly at the origin of inhomogeneous segregation of graphene on top of nickel surface, already observed in previous works [31,32]. It is worth noticing that XRD analysis showed that the as-deposited nickel thin films support some residual nickel grains formation with different misorientations, namely 1 0 0 and 3 1 1. Those residual misorientations are transformed into 1 1 1 orientation during the annealing as observed from XRD spectra obtained after the annealing step. Since the carbon degradation and the simultaneous diffusion start at the first stage of annealing, those misoriented
grains could slowdown graphene formation. The use of asdeposited nickel film with only (1 1 1) phase orientation would probably reduce this inhomogeneity in graphene synthesis. When process conditions are tuned, sufficient carbon species are provided (in a:SiCH) and more extended graphene is obtained with high length wrinkles that cross nickel grains boundaries without any discontinuities (that has been evidenced in monolayer graphene) [33]. When only the nickel thickness is increased, less carbon can reach the surface and the graphene is, at the end, thinner. For instance, with 200 nm of nickel, the measured graphene transmittance is 91.9%, while with 50 nm of nickel, graphene transmittance is 79.5% (at 550 nm). The difference of carbon amount on the surface suggests that not all the carbon from the nickel is segregating on the nickel catalyst. In the current work, SEM images show that graphene segregates in a way that it is not always following the footprint. The perfect side of some graphene domains cannot be explained simply by inhomogeneous segregation of carbon upon individual grain. When sufficient carbon species are provided at the metal surface, the graphene growth by carbon segregation and the coalescence of the different graphene domains to form a complete layer covering the entire substrate surface can occur. Recently, Lahiri et al. [34] suggested that there is a large barrier for carbon attachment to the edge of a graphene island in graphene/Ni system. Density functional theory (DFT) calculation showed that the edge carbon atoms that have unsaturated dangling bonds form strong bonds with the nickel atoms in the first
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surface layer of the substrate. Therefore, the attachment of a single carbon atom to the graphene edge requires the breaking of these bonds, which is expected to produce a large activation barrier for such chemical reaction. This suggestion can be partially true as it was shown that the graphene islands are exclusively nucleated on the ascending side of atomic step edges [35–37]. This is presumably promoted by the fact that carbon atoms at graphene edges can accordingly form stabilizing-like bonds with the metal atoms at the step edge. Even though the coalescence of graphene islands is more likely to match with the experimental observations, at this stage, we cannot conclude on the exact mechanisms that occur.
5.
Conclusions
In summary, we have synthesized large area graphene on 8inch wafers. Using a solid carbon source, a reliable and facile route for growing graphene in an industrial fashion has been demonstrated. This technique is rapid, relatively simple and compatible with standard microelectronic facilities which make it low cost. The technique is not limited by the size and it can be easily scaled up to 12 in wafer or more. The number of layers can be controlled by the stack parameters and the annealing conditions. Graphene with a high transmittance (91.9%) and a low sheet resistance (565 Ohm/sq) has been obtained and successfully transferred to SiO2 substrates. It is also a good transparent conductive material that could be used as transparent conductive electrode in a large variety of applications in the near future.
Acknowledgements This work has been done in the frame of European funded FP7-NMP-GRENADA project and was partly supported by the French RENATECH network. We thank Dominique Lafond for TEM measurements and Patrice Gergaud for assistance during XRD measurements.
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6 6 ( 2 0 1 4 ) 4 8 –5 6
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