Chemical vapor deposition of graphene on refractory metals: The attempt of growth at much higher temperature

Synthetic Metals 247 (2019) 233–239

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Chemical vapor deposition of graphene on refractory metals: The attempt of growth at much higher temperature ⁎

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Xing Fana, Jie Sunb, , Weiling Guoa, , Xiaoxing Kec, Chunli Yand, Xuejian Lia, Yibo Donga, Fangzhu Xionga, Yafei Fua, Le Wanga, Jun Denga, Chen Xua a

Key Laboratory of Optoelectronics Technology, College of Microelectronics, Beijing University of Technology, Beijing, 100124, China National and Local United Engineering Laboratory of Flat Panel Display Technology, College of Physics and Information Engineering, Fuzhou University, Fuzhou, 350116, China c Institute of Microstructures and Properties of Advanced Materials, Beijing University of Technology, Beijing, 100124, China d Department of Information and Automation, Library of Fuzhou University, Fuzhou, 350116, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Graphene Chemical vapor deposition Refractory metal High temperature growth

Large area graphene is usually grown by chemical vapor deposition on Cu or Ni catalysts at ∼1000 °C. For most materials, high temperature leads to high quality. However, graphene growth at even higher temperatures is rarely reported. Therefore, here we systematically investigate the graphene deposition on refractory metals i.e. metals with extremely high melting points. The growth parameters and material characterizations are given in detail. On Ta which readily forms carbides during the carbon deposition, the growth mode is monolayer due to the chemical absorption of excess carbon in the bulk metal. On Re, there is no carbide formed (except in extreme conditions), which greatly simplifies the scenario. Because of the relatively high carbon solubility in Re, the growth temperature has to be limited in order not to drift into the dominantly multilayer graphene regime caused by the carbon segregation. Graphene with reasonable quality has been achieved, although not as good as expected. For example, on Ta, the residual bonds between the graphene and substrate deteriorate the graphene crystalline quality. Despite the difficulties in refractory metal etching, the transfer technique of the graphene is also explored. This research contributes to the fundamental understanding of the graphene growth theory and technology on refractory metals.

1. Introduction Graphene is a monolayer of sp2 hybridized carbon atoms forming two-dimensional (2D) honeycomb crystal lattice. Three of the four valence electrons form σ-bonds in the basal plane whereas the fourth πelectron orbital is oriented perpendicularly. It is the basis of other sp2 carbon allotropes e.g. 0D fullerenes and 1D carbon nanotubes. In 2004, graphene was first isolated from graphite by micromechanical exfoliation on ∼300 nm SiO2 on Si which offers the best optical contrast due to interference effect [1,2]. As the first truly 2D crystal known to mankind, graphene has excellent properties such as high optical transmittance, electrical and thermal conductivities [3–5], Young’s modulus and breaking strength [6], and surface to volume ration. It has high potentials in semiconductor devices, composite materials, as well as in environment and energy sectors [7–9]. However, the exfoliated graphene are small (typically in μm) and there is a need to develop scalable methods for large area graphene

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production. To date, there are mainly two techniques for this. One is the catalytic chemical vapor deposition (CVD) on metals from hydrocarbon precursors [10–13]. Compared with micromechanical exfoliation [1] and liquid phase exfoliation [14] which only produce flakes, it is much more compatible with semiconductor technologies and thus has received huge interest from industry. Another is the high temperature annealing of SiC in vacuum or Ar [15–17], which results in the desorption of Si from the surface leaving excess carbon behind forming graphene epi-layers. Unfortunately, it is a very expensive technology. Also, the epitaxial graphene is hardly transferrable, making it only feasible to use the graphene on SiC, which is costly and size limited (typically < 4 inch). CVD technique does not have these problems. Also, recent advances [18] have greatly improved the quality of CVD graphene. Hence, although we need to further consolidate the industrial compatibility and reduce the thermal and material costs, CVD is already considered to be the most logical choice for graphene application in post-Si electronics.

Corresponding authors. E-mail addresses: [email protected], albertjeff[email protected] (J. Sun), [email protected] (W. Guo).

https://doi.org/10.1016/j.synthmet.2018.12.016 Received 9 July 2018; Received in revised form 10 December 2018; Accepted 16 December 2018 0379-6779/ © 2018 Elsevier B.V. All rights reserved.

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Fig. 1. (a) Optical image of a tantalum foil after graphene deposition at ∼2000 °C for 15 min with 5 sccm C2H2, 1000 sccm Ar and 20 sccm H2. The silver colored Ta turns into golden tantalum carbides. From the enlarged image, it is apparent that the transition occurs only on the surface. (b) XRD characterization of a 25-μm-thick Ta foil after growing graphene on top and quench cooling (1600 °C, 1000 sccm Ar, 20 sccm H2, 5 min annealing and 5 min growth with 10 sccm CH4). The foil has been partially turned into carbides, which are dominantly TaC and Ta2C. (c) A comparison of CVD graphene on Ta and Cu at 1000 °C with similar conditions. The substrates are 275 nm SiO2/Si. The Ta-grown graphene is uniform and monolayer. (d) and (e) Raman spectra of Ta-grown graphene deposited at 1000 °C (d) and ∼3000 °C (e) and transferred to SiO2/Si. In (d), the sample is grown with 5 sccm C2H2, 1000 sccm Ar and 20 sccm H2 for 5 min. In (e), the upper curve corresponds to graphene grown with 5 sccm C2H2, 1000 sccm Ar, and 20 sccm H2 for 17 min, and the lower is grown with 50 sccm CH4, 950 sccm Ar and 20 sccm H2 for 15 min.

2. Experimental method

Apart from Cu and Ni, many other catalysts have been used in the graphene CVD, such as Pt, Ru, Ir, Co, Pd. Nevertheless, we have noticed that nearly all of these growth temperatures are around 1000 °C. On the other hand, as a general principle, an elevated temperature is beneficial in terms of the material quality, because higher temperature offers higher energy, lower nucleation and longer migration length, helping the atoms to overcome the energy barrier and diffuse along the surface until they are incorporated in their proper positions in the lattice. To the best of our knowledge, graphene CVD at temperatures much higher than 1000 °C (i.e. ≥ 1200 °C) has never been explored systematically, which is because of, at least in part, the limitation in available graphene CVD equipment. Therefore, our fundamental research question is: Could higher temperature indeed improve the quality of CVD graphene? In this work, we carry out a systematic study of graphene growth on refractory metals, with a special focus on tantalum and rhenium. Ta and Re are typical refractory metals that usually forms and does not form carbides under usual graphene CVD conditions, respectively. The growth parameters, mechanisms, and transfer are described in detail. The produced graphene is characterized with microscopies, Raman spectroscopy and electrical measurements. It is concluded that an elevated temperature significantly higher than 1000 °C is beneficial to the graphene deposition. However, the effect is not as significant as expected, because there are other negative factors as well. We believe this work is unique and timely, which is valuable to give some insights for the very high temperature growth of graphene and other 2D materials.

Graphene thin films were grown in commercial cold wall nanocarbon CVD systems (Aixtron, Black Magic) on refractory metals. After organic cleaning (5 min in isopropanol and 5 min in acetone), the graphene deposition was performed from a hydrogen, argon, acetylene gas mixture. The growth time, exact gas composition, and other relevant parameters will be given in the subsequent sections. The substrate temperature was monitored by a thermal couple or an infrared pyrometer. The rates for heating and cooling are ∼300 °C/min and ∼1600 °C/min, respectively. The system is a cold wall CVD with a local heater, and therefore quench cooling is possible. Two graphene transfer processes, etching [19] and bubbling [20], were used (both using PMMA polymer mechanical support). For Ta, it was chemically etched in HF:HNO3:H2O = 1:1:6 solution and the graphene was wet transferred to SiO2/Si. For Re, during the electrochemical bubbling (500 mA) in a 0.25 M NaOH electrolyte solution, if the Re-graphene was used as the cathode, not every transfer was perfectly successful. Sometimes the graphene was not fully delaminated, possibly due to impurities in the Re resulting in the strong adhesion of graphene at those local points. If the Re-graphene was used as the anode instead, the Re would be etched, but not completely due to the stop of electrolysis caused by the loss of Re electrode integrity. Alternatively, we found that H2O2 (1:1 diluted) could etch cm-sized Re foils in 24 h. Re was oxidized into Re2O7 which was water soluble: 2Re + H2O2 + 6H2O = 2H2ReO4 + 5H2. 234

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The growth on Ta can be understood as follows. During the CVD, the tantalum carbide formation is explained by the C-Ta phase diagram [25]. The tantalum carbide readily forms in a large temperature range, from lower than 1000 °C to about 4200 °C [25]. In general, there are four kinds of carbides [23]: interstitial, covalent, intermediate and saltlike carbides. Carbides of Ta belong to the interstitial category, where the electronegativity between C and Ta is large, and the carbides are well conducting. Carbon atom has a much smaller dimension, allowing it to nest in the interstices of the Ta lattice. The composition is often indeterminate, making it hard to achieve stoichiometry. The graphene growth on Ta is similar to that on group VIII metals (like Ni) in terms of carbon absorption. At high temperatures, both can catalyze the graphene deposition from hydrocarbons and absorb some carbon into the bulk. For Ni, upon cooling down, the carbon solubility decreases, making the carbon dissolved in the metal segregate on the surface, resulting in growth of several graphene layers. However, in the case of Ta, the carbon is bound chemically (the bonds are partly covalent and partly ionic) and the carbon absorption is irreversible with respect to temperature changes. Therefore, the formation of carbides hinders the excess carbon precipitation during the cooling down, resulting in single layer graphene growth. Group IB metals like Cu can also grow monolayer graphene by virtue of the low carbon solubility, but their melting temperatures are low. In this respect, melting point of Ta is as high as 3017 °C, leaving much room for further increase of the growth temperature without bothering about multilayer formation. However, despite the uniformity in number of layers, the graphene grown on Ta at 1000 °C is of low quality, where the 2D peak in the Raman spectrum is hardly noticeable (see Fig. 1(d)). Hence, we have greatly elevated the deposition temperature to ∼3000 °C. As seen in the Raman spectra in Fig. 1(e), the graphene quality is much better than in Fig. 1(d), especially the CH4 grown sample. Therefore, the high temperature growth is indeed beneficial for the graphene quality. That if confirmed by Table 1, which is a summary of Raman peak ratios (I2D/IG, ID/IG) for all the Raman measurements in this paper, as well as a comparison with literature reported results [26]. The I2D/IG (and ID/IG) ratios for Fig. 1(e) are higher (and lower) than Fig. 1(d). Nevertheless, the Raman spectra indicate that the graphene quality is yet below our expectation, as evidenced by the D peak. Similar scenarios have been found in our growth experiments on Nb, W and Mo. Since the carbon source for the formation of metal carbides is actually the surface graphene, it is natural to believe that there are a number of chemical bonds between the graphene and the underlying carbides. Indeed, we have found it is not possible to delaminate the graphene from these metals by electrochemical bubbling [20] once the substrates are turned into carbides. This serves as indirect evidence that the graphene is partially chemically bonded to the foils. We therefore conclude that on these metals it is not easy to attain super high quality graphene due to the chemical bond distortion of the graphene lattice. Of course, the wet transfer process degrades the quality as well. Finally, we note that the I2D/IG ratios in Fig. 1(e) do not comply with that of standard monolayer graphene [19], and the reason may be because the monolayer graphene grown on Ta substrate is still quite disordered [27–29].

Table 1 Comparasion of the I2D/IG and ID/IG Raman peak ratios.

Fig. 1(d) Ta 1000 °C C2H2 Fig. 1(e) Ta 3000 °C C2H2 Fig. 1(e) Ta 3000 °C CH4 Fig. 3(a) Re 1500 °C Fig. 3(a) Re 1400 °C Fig. 3(a) Re 1300 °C Fig. 3(a) Re 1200 °C Fig. 3(b) Re 100 sccm CH4 Fig. 3(b) Re 50 sccm CH4 Fig. 3(c) Re 400 sccm Ar Fig. 3(c) Re 200 sccm Ar Fig. 3(c) Re 100 sccm Ar Standard growth on Cu [26] Standard growth on Ni [26]

I2D/IG

ID/IG

0.8 0.96 1.14 1.07 1.56 0.69 0.25 0.74 0.56 1.39 1.39 1.32 2 0.9

0.95 0.65 0.4 0.6 0.56 0.85 0.72 0.71 0.89 0.74 0.67 0.79 Almost no D peak Almost no D peak

The H2 also helped delaminate the graphene. Finally, using the bubbling technology to quickly etch most Re followed by the thorough etch in H2O2 was identified to be optimal in terms of yield and efficiency. HNO3 was not a choice in etching, since only concentrated HNO3, which led to severe graphene doping, could effectively etch Re. The Ta (99.9%, 20–50 μm thick) and Re (99.99%, 50 μm thick) foils were purchased from Goodfellow and Advent.

3. Results and discussion 3.1. CVD graphene on tantalum It is known that elevated temperature results in higher degrees of graphitization, and favors the formation of graphene lattice with less defects and impurities [21,22]. Refractory metals commonly refer to Ta, Nb, W, Mo, and Re. They are hard metals with melting points exceeding 2400 °C. We first test the graphene growth on commercial Ta foils and in-house evaporated 200 nm thick Ta films on Si substrates with 400 nm oxide. (When growing graphene on evaporated Ta films on SiO2, it is important to use relatively thick Ta i.e. ≥ 200 nm. Thinner Ta might be chemically “absorbed” by SiO2 at high temperature due to the reduction of SiO2 by Ta.) In our experiments, we find that Ta readily forms carbides already at 800 °C with C2H2 and 1000 °C with CH4, which can be easily recognizable simply by observing the color change. Fig. 1(a) shows the optical image of a typical Ta foil which has undergone graphene growth at ∼2000 °C. It is evident that the foil is turned into tantalum carbide which is golden yellow (monocarbide TaC and hemicarbide Ta2C [23]). However, as seen from the enlarged image in Fig. 1(a), at the cracked edge, the carbide peels off and the interior is still metallic Ta. Fig. 1(b) shows an X-ray diffraction (XRD) pattern of a Ta foil after 5 min of graphene growth at nominally 1600 °C. The curve is analyzed based on literature and Jade software for XRD. The strongest peak between 50° and 60° comes from Ta [24]. Characteristic peaks are detected as clear evidence of the formation of TaC [24]. According to Jade, ther unmarked peaks in Fig. 1(b) correspond to other carbide phases such as Ta2C. Despite the formation of μm-thick surface carbide, monolayer graphene is still grown catalytically. For example, at 1000 °C, the Ta foil is annealed with 20 sccm H2 and 1000 Ar for 5 min followed by adding 5 sccm C2H2 to initiate the 5 min graphene deposition. The pressure in the chamber is kept at 3.4 mbar yielding the acetylene partial pressure of 0.017 mbar. Fig. 1(c) shows optical images of the transferred graphene grown on Cu (left) and Ta (right) foils under similar conditions. The Ta-CVD graphene is uniform and continuous in large area, except for some cracks and wrinkles introduced during the transfer. It is known that the Cu-CVD graphene is predominantly single layer graphene [18,19]. In Fig. 1(c), the two graphene sheets are similar in color and contrast, suggesting that the Ta-grown graphene is most likely a single layer as well [2].

3.2. CVD graphene on rhenium Rhenium’s melting point is as high as 3186 °C, second only to tungsten. More importantly, among the five refractory metals, it is the only one that does not form carbides. This statement is true at least under common CVD conditions. Therefore, we have systematically studied the graphene growth on Re. The solubility of carbon in Re is estimated to be 0.35 at. % [30,31] at about 1000 °C. This value is somewhere between that for Cu (0.0027 at. % [32]) and those for Ni (1.26 at. % [33]) and Pt (1.14 at. % [34]) at ∼1000 °C. Therefore, in terms of the ability to produce uniformly monolayer graphene over large area, Re is in between Cu and Ni, Pt. That is, due to the differences in carbon solubilities, the CVD 235

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Fig. 2. (a) Optical image of a graphene sheet grown on Re foil at 1400 °C for 5 min with 20 sccm CH4, 1000 sccm Ar, 20 sccm H2 and transferred to 275 nm SiO2/Si. The graphene is largely uniform and continuous. The left part of the sample is uncovered. (b) Optical image of graphene and graphite grown on Re at > 2000 °C for 15 min with 50 sccm CH4, 950 sccm Ar and 20 sccm H2. Severe carbon segregation effect causes the deposition to be very nonuniform. The golden colored part is the thick CVD graphite film. (c) and (d) are low and high magnifications of SEM images of the Re-grown graphene (1400 °C for 5 min with 100 sccm CH4, 1000 sccm Ar and 20 sccm H2). In (d), some holes can be seen, which are ascribed to the transfer damages.

growth condition. The effect of the carbon precursor partial pressure is also evaluated. The temperature is fixed at 1400 °C. Fig. 3(b) shows the Raman spectra for two growths at 50 sccm and 100 sccm CH4, respectively, while the argon and hydrogen are both 1000 sccm and 20 sccm. They correspond to CH4 percentages of 4.67% and 8.93% in the gas mixture. The total pressure in the chamber is approximately constant regardless of the flow rates, since it is mainly determined by the pumping capacity. Therefore, higher CH4 percentage is equivalent to higher CH4 partial pressure. Clearly, the graphene deposited with higher CH4 partial pressure has better quality, because of the higher G and 2D peaks in Fig. 3(b) (as compared with the D peak in the same sample). Therefore, we decide to continue to increase the CH4 partial pressure. We can not directly increase the CH4 flow further due to the limitation in our mass flow controller of the methane gas. Hence, we decrease the argon gas flow instead. Fig. 3(c) shows three graphene Raman spectra grown with 400 sccm, 200 sccm and 100 sccm Ar, respectively, while the CH4 and H2 flows are fixed at 100 sccm and 20 sccm. The methane percentages of the three depositions are dramatically increased to 19.23%, 31.25% and 45.45%. By comparing Fig. 3(c) with Fig. 3(b), we do see a further enhancement in the graphene quality, as evidenced by the even larger I2D/IG ratio and smaller ID/IG ratios (see Table 1). Nevertheless, comparing the three curves in Fig. 3(c), their Raman ratios in Table 1 are quite similar to each other, indicating that the graphene quality can hardly be increased even further by increasing the carbon precursor concentration from 19.23% to 45.45%. This phenomenon can be well understood by considering the carbon solubility. On Cu where the carbon solubility is negligible, the growth is based on surface catalysis, and therefore even tiny amount of CH4 is enough to accomplish the full coverage of graphene [19]. Excess carbon precursors generally will not improve the graphene quality further, but rather increase the nucleation density. On Re where the carbon solubility is much higher, or on Ta where the carbon can be chemically absorbed, too low a CH4 partial pressure leads to incomplete growth with microscopic or macroscopic holes and defects, resulting in low quality graphene. When the carbon concentration is increased, the quality is improved accordingly, until it saturates at the point when the precursor is enough for a complete growth. Not surprisingly, on Ni and Pt it requires even more CH4 to have a full coverage of graphene.

mechanism of graphene on Cu is almost purely surface catalysis [18,19], whereas the carbon segregation mechanism plays increasingly important roles in the growths on Re, Pt and Ni [35]. Indeed, it is found that the graphene grown on Re is largely monolayer (might have a limited amount of bilayer and multilayer patches) up to slightly more than 1400 °C, whereas on Ni and Pt the deposition mode is in multilayer regime already at ∼1000 °C [35]. Fig. 2(a) shows a typical graphene film grown at 1400 °C and transferred to 275 nm SiO2/Si substrate. The graphene is seen to be large-area continuous (in the order of 100 μm, where discontinuous regions are induced by the transfer) and most likely monolayer. At much more elevated temperatures e.g. > 2000 °C, however, the growth is never monolayer. An extreme example is shown in Fig. 2(b), where even graphite films can be grown by our CVD (see the golden film in the upper panel of the image). This is obviously due to the large amount of high temperature dissolved carbon segregated to the surface during cooling. The temperature effect on the graphene CVD is systematically studied. Fig. 3(a) summarizes the Raman spectra for four growths at 1200 °C–1500 °C with 100 °C intervals. The samples are annealed at the growth temperature for 5 min in 1000 sccm Ar and 20 sccm H2, followed by the introduction of 20 sccm CH4 to initiate the 5 min graphene growth. Then, the heater is quench cooled in the same atmosphere, and the gases are cut off at 300 °C. During the growth, the chamber pressure is around 12.1 mbar. The whole procedure including heating and cooling is approximately 25 min. Based on Fig. 3(a), one can conclude that the graphene crystalline quality is improved at increasing temperature from 1200 °C to 1400 °C, as evidenced by the decreased D peak at 1350 cm−1, the increased and sharpened 2D peak at 2680 cm−1, and the eventually merged double G peaks. This is also confirmed in Table 1. High temperatures are thus again confirmed to be beneficial to the graphene growth, since they lead to lower nucleation density, enhanced surface diffusion, and higher energy for the atoms to overcome energy barrier and be incorporated into the lattice. However, when the temperature reaches 1500 °C and above, the carbon segregation effect starts to be more and more significant. Upon cooling, carbon emits to the surface from defective points in Re, resulting in nonuniformity in the number of layers and defective lattice, which translates into slightly worse Raman signal at 1500 °C compared with 1400 °C (see Fig. 3(a) and Table 1). Therefore, 1400 °C is the optimal temperature in our 236

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edges of the holes, folded graphene can typically be seen, indicating the origin of the holes might very well be attributed to the graphene transfer process. Some TEM images are summarized in Fig. 4. In Fig. 4(a), on the graphene film, there are darker patches in some regions. In the zoomed-in images in Fig. 4(b) and (c), these patches are seen to be composed of many smaller patches self-assembled in regular patterns. They are Re nanoparticles resulted from the chemical etching residue when transferring the graphene to TEM grids. High resolution TEM (HRTEM) image of the nanoparticles is shown in Fig. 4(b), and further zoomedin Fig. 4(c). Fast Fourier Transform (FFT) pattern from the nanoparticles shows no distinguishable patterns due to their tiny size. However, lattice fringes can be clearly identified and the lattice spacing ∼2.4 Å agrees well with the crystal structure of Re, as marked by d(100) in Fig. 4(c). We note that, in principle, one could also associate the fringes with Re2C which has the structure of hexagonal space group P63/mmc with lattice constants a = 2.84 Å and c = 9.86 Å [36]. However, as carbides of Re are normally synthesized under ultrahigh pressure and temperature [36], it is commonly regarded as a non-carbide-forming metal [37]. Thus, the possibility for the nanoparticles to be Re2C is small. This notwithstanding, we could not entirely exclude the possibility for the existence of Re carbide nanoparticles, as it is known that Re is more prone to the formation of surface carbides than bulk carbides [38,39]. Fig. 4(d) is the electron diffraction pattern taken from a certain position in the graphene film. At this specific region, the graphene is seen to be a bilayer with a rotation angle of ∼8°. After the Re-grown graphene is transferred to silicon substrate with 275 nm thermal SiO2, field effect transistors are fabricated with back gate configuration (The doped silicon is used as the global back gate). The electrical properties of the graphene are measured at room temperature in ambient pressure, without any special treatments such as electric current annealing. Fig. 5 shows the measured typical transfer properties of two graphene sheets grown at 1000 °C and ∼1300 °C. The field-effect mobility is calculated to be about 1500 cm2/(Vs). The mobility value is reasonably good, but not very impressive. The high temperature growth effect on the graphene quality is not obvious in this specific measurement. We have not been able to find convincing evidences indicating that the graphene grown on Re is of better quality than on Cu (Room temperature mobility of 2000 cm2/(Vs) is obtained in Cu-grown graphene produced in the same CVD system at 1000 °C [20].), even at temperatures significantly higher than 1000 °C (i.e. ≥ 1200 °C). 3.3. Discussion In fact, pursuing high quality graphite formation on refractory metals and their carbides at extremely high temperatures can be dated very early [40–42]. Recently, there are also attempts of growing graphene by CVD on those substrates [38,43–47]. For example, Aizawa et al. [43] grew “monolayer graphite” on TaC (111) by exposing 200 L ethylene at 800–1200 °C, which supports our conclusion that tantalum carbide formation helps grow monolayer graphene. Some researchers exposed Re (0001) to carbon-containing molecules and annealed in vacuum to obtain graphene [38,44,45]. Miniussi et al. [38] found that when growing graphene on Re, a complex interplay between carbon segregation, dissolution and carburization took place, in agreement with our scenario. However, all of those works were carried out on single crystalline metals or metal carbides, and nearly none of them was at temperatures much higher than 1000 °C. Therefore, the information of graphene growth under temperatures greater than 1200 °C is still very limited. Our work certainly fills the gap, at least to some extent. We have confirmed that on carbide forming refractory metals, due to the chemical binding of the dissolved carbon in the metals, the carbon segregation upon cooling is greatly suppressed, resulting in the formation of monolayer graphene even at extremely high temperature. On Re which does not form any carbides, on the other hand, the growth temperature has to be limited to be away from the regime of

Fig. 3. (a) Raman spectra of the Re-grown graphene at different temperatures (5 min, 20 sccm CH4, 1000 sccm Ar and 20 sccm H2). It is seen that higher temperatures lead to enhanced graphene quality. (b) Raman spectra of the Regrown graphene with different CH4 flows (1400 °C, 5 min, 1000 sccm Ar and 20 sccm H2). (c) Raman spectra of the Re-grown graphene with different Ar flows (1400 °C, 5 min, 100 sccm CH4 and 20 sccm H2). The higher the Ar flow, the lower the CH4 concentration. Based on (b) and (c), higher concentration of CH4 results in better graphene quality, but the effect tends to saturate in (c). The laser is at 512 nm. The CH4 percentages in the gas mixture are indicated in (b) and (c).

The morphology of the graphene grown at 1400 °C with gases CH4:H2:Ar = 5:1:50 at a total flow rate of 1120sccm on Re is observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Fig. 2(c), the graphene is largely uniform except a few darker regions possibly due to bilayer and multilayers. When zoomed in, small holes can be identified here and there (Fig. 2(d)). However, these holes are most likely not intrinsic. At the 237

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Fig. 4. TEM images of typical Re-grown graphene films deposited at the same condition as Fig. 2(c) and (d). (a) is an overview, where small patches are found. This image is intentionally taken at a place with many patches. In the zoomed-in images in (b) and (c), these patches are seen to consist of many self-organized “dots”, which is known as the Re residue nanoparticles. In (c), the lattice interplanar spacing of ∼2.4 Å agrees well with d(100) of Re. (d) is the selected area electron diffraction (SAED) pattern at a certain position in the film, where the twist angle between the two superimposed graphene layers is ∼8°.

substrate. On Re, as the temperature can not be pushed up too much, the quality is also moderate. Finally, there is no evidence showing that the catalytic ability of refractory metals is larger than that of Cu and Ni at the same temperature. Considering all the positive and negative factors listed above, the quality of graphene grown on refractory substrates is comparable or even worse than standard graphene grown on Cu and Ni, as seen in Table 1 ([26]).

dominantly multilayer graphene formation. In general, an elevated temperature does improve the quality of graphene, and the optimal growth temperature for graphene on Re is 1400 °C in our case. However, there are also negative effects of using refractory metals. After the high temperature process, Ta foils absorb considerable amount of H2 and become brittle, making it hard to handle the samples. It happens also for other refractory metals e.g. Re, although they are known to absorb less H2. Ta-grown graphene can not be transferred by bubbling. Although on Re it can be delaminated by bubbling, the success rate is lower than on Cu and Ni. Thus, refractory metals can hardly be reused after the CVD, leading to a high cost. On Ta, the graphene quality is affected by the residual bonds between the graphene and

4. Conclusions We have carried out a systematic study of the graphene growth by CVD on refractory metals at temperatures much greater than 1000 °C

Fig. 5. Typical electric field effects measured in transistors made in the Re-grown graphene. The graphene in (a) and (b) are grown at 1000 °C and ∼1300 °C (5 min, 25 sccm CH4, 475 sccm Ar and 20 sccm H2), respectively. The graphene quality is decent, but still lower than expected. 238

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(i.e. ≥ 1200 °C). The growth and characterization of the graphene on tantalum and rhenium are illustrated in detail. We have also studied the graphene depositions on Nb, W and Mo. As they are similar to the situation on Ta, they are not described in this paper. It is confirmed that very high temperature is indeed beneficial for the graphene growth. The growth mechanisms on carbide forming metals and carbide free metal are thoroughly discussed. Negative effects of using refractory metals in the graphene CVD are also summarized. Although the original motivation of this research is to pursue high quality graphene, since the results indicate that there are no clear benefits compared with Cu and Ni from an application perspective, the paper should be rather regarded as a fundamental understanding of the graphene CVD at extreme conditions on refractory metals, where the knowledge is very limited in literature.

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