**4. Graphene preparation from the structure Co/SiC**

Graphene films were prepared by thermal processing of the Co/SiC structures, which differed in thickness of the cobalt layer (10, 50, 100, 200, and 300 nm) [31–33]. Co deposition was performed using a DC magnetron sputtering apparatus at room temperature. Alternatively, the Co layer with thickness 300 nm was deposited by the e-gun evaporation. For studying basic parameters of the prepared graphene films, Raman spectroscopy was chosen.

#### **4.1. Raman spectroscopy**

(SI-SiC) was used, and the graphene film on the interface between SiC and the metallization was tested (after the annealing, the silicide layer was etched off by HNO<sup>3</sup> acid, and the graphene/ SI-SiC structure was obtained). For the measuring of electrical parameters by van der Pauw method, the Au(30)/Cr(10) contacts prepared by evaporation were applied. Obtained results

**Figure 4.** Raman spectra of Ni/SiC sample after the annealing and after the exfoliation. (Reprinted with permission from

is surface resistivity, μH is Hall mobility, and c<sup>s</sup>

charge carriers). Hall mobility of the prepared graphene films is very low probably due to large

The synthesis of graphene on silicon carbide substrates by the Ni-silicidation reaction shows



The method is scalable—so it can be applied for the growth of large-area graphene films. However, the technique has a disadvantage—the graphene films are prepared on the conductive plate. For application in microelectronics (unipolar transistors and other devices), the

 **(Ω) μH (cm2**

1050 60 527 ± 2.5 300 ± 7.3 3.85 × 1013 ± 7.3 × 1011

/Si) is necessary. Alternatively, it is possible to apply

**/Vs) cs**

 **(cm−2)**

preparation of epitaxial graphene on SiC (minimally 1300°C).

method (from several minutes to several hours).

transfer on the dielectric substrate (SiO2

**Annealing conditions** ρ**<sup>s</sup>**

Reprinted with permission from J Electrical Engineering.

**Table 3.** Electrical parameters of graphene on SI SiC.

**T (°C) t (s)**

is the concentration of

are shown in **Table 3** (ρ<sup>s</sup>

J Electrical Engineering.).

two significant advantages:

**3.6. Conclusion**

concentration of defects in graphene layers.

212 Graphene Materials - Structure, Properties and Modifications

Raman spectra of several Co/SiC structures, which differ in thickness of the Co layer [31], are shown on **Figure 5**. The spectra were normalized to the same size of the G band, in order to compare D and 2D bands. All structures were formed at temperature of 1080°C, and the annealing period was 10 s. The graphene film was not prepared for the structure with 10 nm-thick cobalt layer. The resulting layer shows character of amorphous carbon with very low thickness (the signal is very weak—especially, the 2D band and the spectrum are noisy). With increasing thickness of the Co layer, the 2D band intensity grows up. This gives evidence of decreasing number of carbon monolayers in the formed graphene film. There is about five carbon monolayers for the structure Co(50 nm)/SiC. In the case of the structure with the thickest cobalt layer, a three-layered graphene film has been created. Similarly, crystallinity of the formed graphene films improves when thickness of the Co layer is growing. In the case of the structure Co(300 nm)/SiC, the crystal size in the graphene film is approximately 110 nm.

**Figure 5.** Raman spectra of several Co/SiC structures differing in Co layer thickness. (Reprinted with permission from J Mater Sci: Mater Electron, No. 3995250684677.).

An interesting effect was noticed at the structure Co(300 nm)/SiC. In the case of shorter annealing periods and in dependence on the graphene growth temperature, two different phases have emerged on the surface of the structures—a dark and a light. With the increasing of annealing period, the area of the dark-phase domain was increasing, and, finally, it was spreading over the whole area of the structure. Quality of the structure surface was nevertheless worse. **Figure 6** shows a photograph of the structure Co(300 nm)/SiC annealed at 990°C for 60 s. From the analysis in **Figure 7**, it is obvious that the dark phase contains the graphene film and cobalt silicides, while the light one does not include graphene and silicides. The XPS analysis has been used for verification of the phenomena [31]. The reaction of Co with SiC starts probably at defects and then gradually propagates over the surface of the structure. The effect does not occur at structures with a thinner cobalt layer, and their surface is smoother and homogenous [31].

**Figure 6.** Example of a structure surface with Co layer 300 nm thick annealed at 990°C for 60 s. (Reprinted with permission from J Mater Sci: Mater Electron, No. 3995250684677.).

**Figure 7.** Raman spectra of the Co(300 nm)/SiC structure annealed at 990°C for 60 s. (Reprinted with permission from J Mater Sci: Mater Electron, No. 3995250684677.).

An interesting effect was noticed at the structure Co(300 nm)/SiC. In the case of shorter annealing periods and in dependence on the graphene growth temperature, two different phases have emerged on the surface of the structures—a dark and a light. With the increasing of annealing period, the area of the dark-phase domain was increasing, and, finally, it was spreading over the whole area of the structure. Quality of the structure surface was nevertheless worse. **Figure 6** shows a photograph of the structure Co(300 nm)/SiC annealed at 990°C for 60 s. From the analysis in **Figure 7**, it is obvious that the dark phase contains the graphene film and cobalt silicides, while the light one does not include graphene and silicides. The XPS analysis has been used for verification of the phenomena [31]. The reaction of Co with SiC starts probably at defects and then gradually propagates over the surface of the structure. The effect does not occur at structures with a thinner cobalt layer, and their surface is smoother

**Figure 6.** Example of a structure surface with Co layer 300 nm thick annealed at 990°C for 60 s. (Reprinted with

permission from J Mater Sci: Mater Electron, No. 3995250684677.).

**Figure 5.** Raman spectra of several Co/SiC structures differing in Co layer thickness. (Reprinted with permission from J

and homogenous [31].

Mater Sci: Mater Electron, No. 3995250684677.).

214 Graphene Materials - Structure, Properties and Modifications

Slightly, other results were obtained in the case of cobalt layers prepared by evaporation [32]. The metallization was deposited onto SiC substrates at elevated temperature (135°C). The substrates were treated in situ by DC Ar plasma before the metal deposition. The layer thickness was 300 nm (the best thickness from previous research) [31]. **Figure 8** shows dependence of the I2D/IG ratio on annealing time of the structure annealed at 850°C. The results confirm that the sample annealed for 10 s is the best and shows character of nearly BLG. Contrary to the earlier work [31], there has not been observed formation of two phases on the sample surface during graphene growth. Samples' surface was homogenous after annealing, and the graphene phase spread all over the surface.

**Figure 8.** Ratio of 2D and G band intensities in Raman spectra of graphene prepared by annealing at 850°C and at various times. (Reprinted with permission from Applied Surface Science No. 3995260811830.).

#### **4.2. Graphene exfoliation**

Graphene films grown on the structure metal/SiC are not suitable for direct application in microelectronics. This is because of graphene having been produced by the reaction of a metal with silicon carbide leading to formation of silicides and carbon. The graphene film is created on a layer of silicides with large electrical conductivity, and thus it does not allow constructing electronic structures (such as unipolar transistor). A certain opportunity is to apply the lower graphene film which is created at the SiC-metal boundary. This graphene film shows however worse parameters than the surface graphene film (the graphene film on the silicide layer) [25]. Preparation of the lower graphene was tested with the Co/SiC structure as well [31]. **Table 4** shows results, which, apart from basic parameters of the technological process, contain also the values of I2D/IG and ID/IG before etching (the surface graphene) and after etching (the lower graphene on the surface of SiC substrate). It is evident from the table that the lower graphene shows in majority cases worse values of the I2D/IG and ID/IG ratios.

Another possibility to form a graphene film on a nonconductive substrate is to transfer the graphene film from the Co/SiC structure onto a dielectric substrate (SiO<sup>2</sup> /Si). **Table 4** shows an example of obtained results—the last two columns in the table. It is evident from the table that the transfer of graphene has smaller influence on its parameters than the preparation of the lower graphene. In addition, the silicon carbide surface after the etching-off of the metallic layer evinces greater surface roughness than with the transferred graphene.

Graphene films prepared on the metal/SiC structure are not appropriate for direct measurements of electrical parameters since they are situated on a conductive silicide layer. So it is necessary to transfer the graphene film on a dielectric substrate. **Table 5** gives the example of an obtained result [32]. The table includes parameters (sheet resistance, mobility, and hole concentration) of the graphene film prepared by the annealing of the Co(300 nm)/ SiC structure at 1050°C for 120 s. The graphene film has been transferred onto the SiO<sup>2</sup> /Si substrate.

#### **4.3. Influence of cooling rate**

Authors of a number of studies, which deal with growing of graphene by synthesis on metal/ SiC structures [18, 20, 23, 34], postulated that parameters of graphene films are dependent on the cooling rate (CR) of the structures after the annealing is stopped. We have tried to verify the statement experimentally. The structures annealed at 1050°C for 120 s were chosen for


**Table 4.** Basic parameters of graphene films—surface graphene, lower graphene, and exfoliated graphene.


**Table 5.** Electrical parameters of the graphene layer after the exfoliation process.

the experiment. The tested structures were mounted on a special molybdenum tray prior to the annealing process, and the tray was resistively heated by large current. The tray enables us to control cooling temperature within the CR from 2 to 70°C/s. The highest CR is given by switching off a power supply, and the structure cools down independently. The CR was set by controlling the passing current. **Figure 9** shows the results. The graph represents dependence of the resulting I2D/IG and ID/IG ratios of prepared structures on the CR. It is obvious from the graph that the CR has nearly no influence onto the I2D/IG ratio (the number of carbon monolayers in the graphene film). However, the CR has a significant influence on the ID/IG ratio (the number of defects in the prepared graphene film). The faster cooling is the more defects in the graphene films are created.

#### **4.4. XPS analysis**

**4.2. Graphene exfoliation**

216 Graphene Materials - Structure, Properties and Modifications

substrate.

**Co layer thickness [nm]**

**4.3. Influence of cooling rate**

**Annealing temperature [°C]**

**Annealing time [s]**

Graphene films grown on the structure metal/SiC are not suitable for direct application in microelectronics. This is because of graphene having been produced by the reaction of a metal with silicon carbide leading to formation of silicides and carbon. The graphene film is created on a layer of silicides with large electrical conductivity, and thus it does not allow constructing electronic structures (such as unipolar transistor). A certain opportunity is to apply the lower graphene film which is created at the SiC-metal boundary. This graphene film shows however worse parameters than the surface graphene film (the graphene film on the silicide layer) [25]. Preparation of the lower graphene was tested with the Co/SiC structure as well [31]. **Table 4** shows results, which, apart from basic parameters of the technological process, contain also the values of I2D/IG and ID/IG before etching (the surface graphene) and after etching (the lower graphene on the surface of SiC substrate). It is evident from the table that the

lower graphene shows in majority cases worse values of the I2D/IG and ID/IG ratios.

graphene film from the Co/SiC structure onto a dielectric substrate (SiO<sup>2</sup>

layer evinces greater surface roughness than with the transferred graphene.

Another possibility to form a graphene film on a nonconductive substrate is to transfer the

an example of obtained results—the last two columns in the table. It is evident from the table that the transfer of graphene has smaller influence on its parameters than the preparation of the lower graphene. In addition, the silicon carbide surface after the etching-off of the metallic

Graphene films prepared on the metal/SiC structure are not appropriate for direct measurements of electrical parameters since they are situated on a conductive silicide layer. So it is necessary to transfer the graphene film on a dielectric substrate. **Table 5** gives the example of an obtained result [32]. The table includes parameters (sheet resistance, mobility, and hole concentration) of the graphene film prepared by the annealing of the Co(300 nm)/ SiC structure at 1050°C for 120 s. The graphene film has been transferred onto the SiO<sup>2</sup>

Authors of a number of studies, which deal with growing of graphene by synthesis on metal/ SiC structures [18, 20, 23, 34], postulated that parameters of graphene films are dependent on the cooling rate (CR) of the structures after the annealing is stopped. We have tried to verify the statement experimentally. The structures annealed at 1050°C for 120 s were chosen for

**Technological parameters Surface graphene Lower graphene Exfoliated graphene**

300 1080 120 0.76 0.34 0.75 0.66 0.63 0.06 100 1080 120 0.76 0.10 0.88 0.37 0.73 0.20

**Table 4.** Basic parameters of graphene films—surface graphene, lower graphene, and exfoliated graphene.

**I2D/IG ID/IG I2D/IG ID/IG I2D/IG ID/IG**

/Si). **Table 4** shows

/Si

The analysis was performed on the structure Co(300 nm)/SiC which was prepared by the evaporation process [32]. The structure for the XPS analysis is special, unlike regular structures. The cleaning, annealing, and analyzing of the sample were done in the NanoESCA apparatus. The sample was inserted into the apparatus, and the process of the experiment was done as follows:

**Figure 9.** I2D/IG and ID/IG ratios for different cooling rates. (Reprinted with permission from Thin Solid Films No. 3995281012757.).


A vacuum of 5 × 10−9 Pa was kept in the apparatus chamber during the process.

XPS analysis was performed in the sample's as-received state, after the sputtering and finally after the annealing. **Figure 10** shows all three survey spectra. The spectrum of as-received sample surface includes oxygen, carbon, and cobalt. Co and O probably create a native cobalt surface oxide, and carbon probably comes from an atmospheric contamination [35–37]. The sample surface after the sputtering is evidently free from C, and presumably the oxygen concentration likewise decreased. Silicon peaks appeared, and O is desirably absent at the sample after the annealing. The appeared Si is a product of the reaction between Co and SiC which yields Co silicides and free C [38, 39]. So Co and C are detected simultaneously. **Table 6** summarizes the atomic concentrations of all elements.

Detail spectra, which contain only narrow scans, are presented in the next paragraphs and figures with aim to give us a deeper chemical investigation. Firstly, it should be pointed out

**Figure 10.** Survey spectra in comparison. Main found peaks are denoted. (Reprinted with permission from Thin Solid Films No. 3995281012757.).


**Table 6.** Quantification results of the atomic concentration of elements from XPS analysis.


218 Graphene Materials - Structure, Properties and Modifications


marizes the atomic concentrations of all elements.

Films No. 3995281012757.).


A vacuum of 5 × 10−9 Pa was kept in the apparatus chamber during the process.

XPS analysis was performed in the sample's as-received state, after the sputtering and finally after the annealing. **Figure 10** shows all three survey spectra. The spectrum of as-received sample surface includes oxygen, carbon, and cobalt. Co and O probably create a native cobalt surface oxide, and carbon probably comes from an atmospheric contamination [35–37]. The sample surface after the sputtering is evidently free from C, and presumably the oxygen concentration likewise decreased. Silicon peaks appeared, and O is desirably absent at the sample after the annealing. The appeared Si is a product of the reaction between Co and SiC which yields Co silicides and free C [38, 39]. So Co and C are detected simultaneously. **Table 6** sum-

Detail spectra, which contain only narrow scans, are presented in the next paragraphs and figures with aim to give us a deeper chemical investigation. Firstly, it should be pointed out

**Figure 10.** Survey spectra in comparison. Main found peaks are denoted. (Reprinted with permission from Thin Solid

that all spectra probably shifted to higher binding energies roughly by 0.5 eV. No software spectrum correcting was produced.

**Figure 11** shows Co 2p peaks fitted by components on the basis of Ref. [37]. In the as-received state, a metallic Co component at around 779 eV, a CoII and CoIII components at around 781 eV, and satellites at slightly higher binding energies can be identified, all confirming the native Co oxide [35–37]. As it can be anticipated, the quantity of oxide-based components was decreased by the Ar sputtering. There seems to be only metallic Co detected at the annealed sample. Cobalt is represented by an asymmetric main peak and weak plasmons [37]. Identification of

**Figure 11.** Co 2p in comparison. (Reprinted with permission from Thin Solid Films No. 3995281012757.).

silicide bonds by XPS is complicated [40]. But, we can suppose that Co and Si in the annealed sample create silicides. The silicides are standard reaction products in the Co/SiC system under similar reactions, and they are detected by other analytic techniques, for example, XRD [38, 39].

Information obtained from detail O 1s spectra support so far drawn suppositions. **Figure 12** shows that there are at least two chemical states of O in the as-received sample. The component at around 530.5 eV comprises O bonds in the Co oxides [35, 37]. The second component at around 532.5 eV probably belongs to disrupt oxide structure, Co hydroxides, water, and possibly impurities [35, 37]. The sputtering of the sample decreases the quantity of impurities and the oxides/hydroxides. O disappeared completely at the annealed sample.

Detail C 1s spectra, first off in comparison, are shown on **Figure 13** and with fitted components on **Figures 14** and **15**. The C 1s peak at the as-received sample can be fitted by three components; the first one positioned at 285.5 eV belongs to adventitious carbon, the second one at 287 eV, and the third one at 289.5 eV both belong to bonds between carbon and oxygen [37]. The components are standard for contamination carbon. The components can be easily removed by the argon sputtering.

**Figure 12.** Fitted O 1s peaks in comparison. (Reprinted with permission from Thin Solid Films No. 3995281012757.).

**Figure 13.** C 1s in comparison. (Reprinted with permission from Thin Solid Films No. 3995281012757.).

**Figure 15** shows the most important carbon peak for us which was produced by the annealing. The peak is fitted with a single distinctive component positioned at 285 eV which has a relatively low FWHM (below 1 eV). C–C or C=C bonds are characteristic for the component and represent the formation of graphitic carbon phase during the annealing process [41, 42]. The peak was fitted by one symmetric component. Nevertheless, the peak shows slight asymmetry which is a mark of the metallic character of graphene [43]. The observed asymmetry of the C 1s peak in addition supports the presence of graphitic carbon.

#### **4.5. Conclusion**

silicide bonds by XPS is complicated [40]. But, we can suppose that Co and Si in the annealed sample create silicides. The silicides are standard reaction products in the Co/SiC system under similar reactions, and they are detected by other analytic techniques, for example, XRD [38, 39]. Information obtained from detail O 1s spectra support so far drawn suppositions. **Figure 12** shows that there are at least two chemical states of O in the as-received sample. The component at around 530.5 eV comprises O bonds in the Co oxides [35, 37]. The second component at around 532.5 eV probably belongs to disrupt oxide structure, Co hydroxides, water, and possibly impurities [35, 37]. The sputtering of the sample decreases the quantity of impurities

Detail C 1s spectra, first off in comparison, are shown on **Figure 13** and with fitted components on **Figures 14** and **15**. The C 1s peak at the as-received sample can be fitted by three components; the first one positioned at 285.5 eV belongs to adventitious carbon, the second one at 287 eV, and the third one at 289.5 eV both belong to bonds between carbon and oxygen [37]. The components are standard for contamination carbon. The components can be easily

**Figure 12.** Fitted O 1s peaks in comparison. (Reprinted with permission from Thin Solid Films No. 3995281012757.).

and the oxides/hydroxides. O disappeared completely at the annealed sample.

removed by the argon sputtering.

220 Graphene Materials - Structure, Properties and Modifications

The process of graphene growth was ground on an optimization of the thermal forming of the Co/SiC structure. The process was produced within a temperature range from 750 to 1080°C, and annealing time was changed in a range from 0 to 120 s. We were testing qualities of the prepared graphene films on samples' surface by the Raman spectroscopy. Nevertheless, the graphene film is also created at the metallization-SiC boundary; parameters of this so-called lower graphene are always worse than in the case of the surface graphene. This is produced by the reaction of SiC with Co leading to a large roughness of the silicon carbide surface at the interface. From the results, it is evident that the applied method produces graphene film with parameters of a bilayer one.

**Figure 14.** Fitted C 1s peak of the as-received sample. For clarity, peak components and background are vertically slightly offset. (Reprinted with permission from Thin Solid Films No. 3995281012757.).

**Figure 15.** Fitted C 1s peak of the annealed sample. For clarity, peak components and background are vertically slightly offset. (Reprinted with permission from Thin Solid Films No. 3995281012757.).

At the Co(300 nm)/SiC structures prepared by the sputtering deposition process, the creation of two surface phases has been observed at shorter annealing period. In the case of the dark phase, the reaction between SiC and Co creates cobalt silicides and simultaneously the graphene film. Silicides and graphene are not created in the case of the light phase. By increasing the annealing time, the light-phase area shrinks, and finally it disappears. The XPS depth profiling detected a layer with increased amount of carbon atoms in the dark phase. The layer presumably represents carbon atoms that precipitate toward the surface of metallization. The precipitation is completed, and graphene is formed on the structure surface after the long experiment.

By applying the Co evaporation together with the plasma modification instead of the sputtering, the surface of the structure was homogenous and covered by the graphene film after annealing. We presuppose that the growing of the graphene film on the whole area can be attributed to a positive influence of the plasma treatment of the structure surface that was carried out just before the cobalt evaporation.

As mentioned above, it is generally postulated that parameters of the graphene films prepared by segregation depend very strictly on the CR of the structures after finishing the annealing process. Our results are different from this statement. The number of carbon monolayer in graphene has been practically independent on the CR; however, the graphene defectivity (the I D/IG ratio) is rapidly increasing. The possible explanation of the phenomena can be seen in the fact that the surface carbon layer (the graphene film) is formed not only by the segregation process after finishing the annealing as originally asserted but also during the annealing period. We assume that the CR modifies morphology and crystallic structure of the silicide layer and thus it increases the concentration of defects in the graphene film [44].

**Figure 14.** Fitted C 1s peak of the as-received sample. For clarity, peak components and background are vertically

**Figure 15.** Fitted C 1s peak of the annealed sample. For clarity, peak components and background are vertically slightly

offset. (Reprinted with permission from Thin Solid Films No. 3995281012757.).

slightly offset. (Reprinted with permission from Thin Solid Films No. 3995281012757.).

222 Graphene Materials - Structure, Properties and Modifications

For the measurement of the electrical parameters, we apply the graphene film transferred onto the SiO2 /Si structure. The transfer of graphene slightly increased the graphene defectivity. The phenomenon is caused by interruption of covalent bonds between Co and carbon atoms in graphene. Unsaturated bonds are formed within the graphene due to etching off of Co, and so the number of defects in graphene film is increased [45]. Obtained value of sheet resistance is in agreement with data published in literature [45–47]. The charge carrier mobility is very small taking into consideration the maximal published values, but the graphene films with the same number of carbon monolayers produced by the CVD method [46, 48] show the charge mobility with similar or only slightly higher value. It is probably caused by the presence of significant number of defects which are absent at the best graphene structures prepared by mechanical exfoliation [4]. Grain size and cracks in the metal play a great role at the graphene transfer from polycrystalline metal substrates [46, 49]. Due to positive sign of the Hall constant, our graphene films show hole conductivity which is attributed to residues of PMMA used for the graphene transport [50].

XPS analysis of the Co/SiC structure together with its annealing and cleaning was performed in situ in the Ultra High Vacuum (UHV) apparatus. Contrary to our standard preparation process [31], special cleaning of the tested structure was performed by degassing and by Ar ions bombardment. The process significantly reduced the surface contamination. The UHV in situ structure cleaning and subsequent analysis provided clean and very well-defined experimental conditions. In **Figure 15**, it is obvious that the Co/SiC reaction formed the graphene film on the surface of the structure as intended.

#### **5. Application of other metals**

The mentioned method is suitable for many other materials, not only for Ni and Co. Selected metals have to react with silicon carbide by the way silicides and carbon rich products are formed at the metal-SiC interface. The accumulation of graphite at the top of the metal is produced during the cooling period of the growth process. The stability or reactivity of SiC versus the number of metals was studied on the basis of phase diagrams [51]. Pd, Ge [33, 52], and Fe [53] were tested for graphene growth. In majority cases FLG was prepared.

#### **6. Influence of additive materials**

The above-described process of graphene growth is influenced by solubility of carbon within the used metal. In practice, nickel and cobalt are most frequently used; solubility of carbon in these metals is relatively high, and consequently carbon diffusion is easy. By modifying the carbon solubility in metal, it is possible to influence the quantity of carbon, which gets into the volume of metallization during the reaction of the metal with SiC, and in this way, the parameters of formed graphene are possible to influence.

Preparation of graphene was carried out on the Ni/SiC and Co/SiC structures, where the following materials were added into the metallization: germanium, copper, and gold [54]. Solubility of carbon in these materials is very low compared to Ni and Co. The main goal of the work was then to study the influence of admixtures onto parameters of prepared graphene films. Adding the admixture materials to nickel did not improve the prepared graphene films very much. Certain improvement was observed for the structures with copper, where the presence of copper in the metallization manifested itself by lowering the defect quantity of graphene. Another situation is for the metallization with cobalt. Germanium and gold did not improve the graphene parameters; however, copper proved its significant meaning. BLG (bilayer graphene) with very low defect quantity was detected on the Co structure with molar content of Cu 20% (annealing at 1000°C for 10 s).

#### **7. Conclusion**

The text of this chapter was taken from the publication of the author and his other colleagues [25, 26, 31–33, 54]. The chapter is focused on graphene preparation by silicidation of the metal/SiC structure, where nickel and cobalt were studied in the form of the metal. Graphene is formed due to SiC decomposition by annealing of the said structure at the temperature of 800–1000°C. SiC reacts with the metal to form silicides and free carbon, which diffuses into the layer of the metal and its silicides. While cooling down the structure, carbon gets segregated onto the surface of the metallization in the form of graphene. Graphene gets formed also at the metal/SiC boundary; this lower graphene has poorer features. When using SI-SiC, it can however be used, for example, for construction of graphene transistors directly, without the need to transfer it onto a dielectric substrate.

Generally, we can say that graphene prepared from the structure containing cobalt reaches better parameters than for the structure with nickel. For the best result, preparation of bilayer graphene with low failure rate can be considered. Raman spectroscopy was chosen for the basic diagnostic method, for it is an easily accessible, nondestructive, and fast analysis.

Within the text, optimizing of graphene preparation by annealing is gradually discussed, including the questions related to the influence of cooling rate onto graphene parameters. Contrary to the results published in literature, it was found that the influence of cooling rate is negative. Further, we followed the question of the thickness of the prepared graphene using the XPS analysis. This analysis was also used for verification of graphene present on the used substrates. Surface of graphene films was studied using AFM. Transfer of graphene was carried out by means of PMMA onto the SiO2 /Si substrates. Basic electrical parameters were measured at graphene on dielectric substrates. In the closing part of the chapter, the influence of additional materials, which decrease carbon solubility in the metallization and which thus influence features of the prepared graphene film, was discussed. It was proven that the most suitable material is copper.

Graphene preparation by synthesis from the metal/SiC structure is a promising and simple method, which does not require high temperatures, complicated technological apparatuses, or handling with dangerous gasses. Therefore, the concerned method is of good prospects at graphene preparation for a number of applications. The mentioned method is very perspective for future microelectronics, because it can be used for direct growth of graphene films on semiconductor substrates (especially SiC).

## **Acknowledgements**

The work was pursued under a financial support from the Czech Science Foundation, Project No. 17-00607S.

### **Author details**

Petr Machac

experimental conditions. In **Figure 15**, it is obvious that the Co/SiC reaction formed the gra-

The mentioned method is suitable for many other materials, not only for Ni and Co. Selected metals have to react with silicon carbide by the way silicides and carbon rich products are formed at the metal-SiC interface. The accumulation of graphite at the top of the metal is produced during the cooling period of the growth process. The stability or reactivity of SiC versus the number of metals was studied on the basis of phase diagrams [51]. Pd, Ge [33, 52],

The above-described process of graphene growth is influenced by solubility of carbon within the used metal. In practice, nickel and cobalt are most frequently used; solubility of carbon in these metals is relatively high, and consequently carbon diffusion is easy. By modifying the carbon solubility in metal, it is possible to influence the quantity of carbon, which gets into the volume of metallization during the reaction of the metal with SiC, and in this way, the

Preparation of graphene was carried out on the Ni/SiC and Co/SiC structures, where the following materials were added into the metallization: germanium, copper, and gold [54]. Solubility of carbon in these materials is very low compared to Ni and Co. The main goal of the work was then to study the influence of admixtures onto parameters of prepared graphene films. Adding the admixture materials to nickel did not improve the prepared graphene films very much. Certain improvement was observed for the structures with copper, where the presence of copper in the metallization manifested itself by lowering the defect quantity of graphene. Another situation is for the metallization with cobalt. Germanium and gold did not improve the graphene parameters; however, copper proved its significant meaning. BLG (bilayer graphene) with very low defect quantity was detected on the Co structure

The text of this chapter was taken from the publication of the author and his other colleagues [25, 26, 31–33, 54]. The chapter is focused on graphene preparation by silicidation of the metal/SiC structure, where nickel and cobalt were studied in the form of the metal. Graphene is formed due to SiC decomposition by annealing of the said structure at the temperature of 800–1000°C. SiC reacts with the metal to form silicides and free carbon, which

and Fe [53] were tested for graphene growth. In majority cases FLG was prepared.

phene film on the surface of the structure as intended.

**5. Application of other metals**

224 Graphene Materials - Structure, Properties and Modifications

**6. Influence of additive materials**

parameters of formed graphene are possible to influence.

with molar content of Cu 20% (annealing at 1000°C for 10 s).

**7. Conclusion**

Address all correspondence to: petr.machac@vscht.cz

Department of Solid State Engineering, University of Chemistry and Technology, Prague, Czech Republic

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