**2. Structure preparation**

graphite from the oriented pyrolytic graphite. The main disadvantage of the method is that it

Today, many methods are used for graphene growth. Graphene is very frequently prepared by a chemical vapor deposition (CVD) method [7–9]. Thin foils of Cu, Ni, or other metals are used as substrates. The method is scalable (graphene films with large dimension can be prepared), but the graphene film must be transferred onto a dielectric substrate for the next

The next method is a high-temperature decomposition of SiC, which is sometimes called as an epitaxial growth of graphene (EG) [10–12]. In the method silicone atoms sublimate from the surface of SiC substrate at high temperature—1100–1600°C in high vacuum and remaining carbon creates graphene. The method can be used for industrial growth of graphene due to full-wafer technology. Careful control of the sublimation process has recently led to the growth of very thin graphene film over the SiC surface, with only single graphene layer.

The vacuum sublimation of SiC usually produces graphene films with small crystallinity (30–200 nm) [13, 14] due to surface SiC roughening and creation of deep pits. The preparation of graphene by decomposition of SiC in an argon atmosphere of about 100 kPa gives better layers. This method gives SLG films with greater domain sizes. Graphene parameters can be also improved by increasing the growth temperature (up to 2000°C) since SiC decomposition

A special sort of method of graphene preparation is the so-called transfer-free method [15].

a metal-catalyzed crystallization of amorphous carbon (a-C) by thermal annealing. Polymer layer [16] and thin SiC layer [17] are used very frequently as the carbon source instead of a-C. Carbon atoms diffuse into a metal layer at elevated temperatures followed by their precipita-

The graphene growth on SiC substrates at relatively low temperature is very perspective [18]. The technique applies the Ni/SiC system as a basic structure. The method is very promising for the transfer of graphene layers from the SiC substrate to other substrates (mainly on dielectric one). By the annealing of the Ni/SiC system, carbon-rich products can be obtained at

This method has been developed by various groups of authors in many ways. The Ni(200 nm)/SiC structure was applied in the work [18]. Graphene was grown on the structure surface by annealing at 750°C (the speed of 25°C per second), with follow-up cooling with no specified velocity. The prepared graphene showed the FLG character. The results of the mentioned work are in discrepancy with the results of works [19, 20], where only thin Ni films (from single to tens nm) were applied. Heating speed was slightly lower (4°C per second), and cooling velocity was approximately 20°C per second. Prepared graphene had a character of FLG and SLG. Authors of work [21] elaborated with the Ni/SiC structure, where the thickness of nickel layer did not exceed the value of 100 nm. The annealing of the structure produced at 1100°C for 300 s (heating and cooling velocities were not specified; a rapid thermal process was used). After the annealing process, the created silicide layer was etched off, and a thin graphene film of FLG type remained on the silicon carbide substrate. Lastly, the modificated method described in the study [22] is also valuable. A

the Ni-SiC interface, and the graphene film is segregate on the top of the Ni layer.

/Si structure. The synthesis of graphene is based on

occurs at 1500°C under argon atmosphere rather than at 1150°C in vacuum.

/Si). Temperature or plasma CVD processes are used in practice.

produces only small pieces of graphene—graphene flakes.

206 Graphene Materials - Structure, Properties and Modifications

application (e.g., SiO2

The method comes from a metal/C/SiO2

tion as graphene during the cool-down step.

N-type 4H-SiC substrate wafers, Si-face polished, 4° off-axis, and doping level 4 × 10<sup>18</sup> cm−3 (supplied by SiCrystal A.G.) were used in our experiments. Majority of metallization were deposited using e-beam evaporator at 135°C in the vacuum of 2 × 10−4 Pa; alternatively, a DC magnetron sputtering apparatus with Ar plasma was used. Prior to the deposition of metals, the wafers were cleaned by the following wet chemical process: 5 min in acetone (ultrasound bath), 5 min in NH4 OH:H<sup>2</sup> O:H<sup>2</sup> O2 (5:1:1) (ultrasound bath), 5 min in 5% HF (ultrasound bath), 10 min in boiling water, and finally drying by nitrogen.

Graphene films were produced by thermal treatment of metal/SiC structures in a small vacuum chamber equipped with a resistively heated table (Boralectric heating element). First off, samples were degassed at 300°C for 5 min and then annealed at 600–1100°C in a pressure bellow 3 × 10−4 Pa. Temperature was measured with an optical pyrometer. Heating rate was approximately 17.5°C/s and cooling rate was 15°C/s.

Samples were analyzed by means of the Raman spectroscopy using a DXR Raman microscope spectrometer of the company Thermo Fisher Scientific equipped with confocal Olympus microscope. Solid-state Nd:YAG laser (wavelength 532 nm, maximum power 10 mW) was used as excitation source. Measurement conditions were 7 mW power, ten accumulations of 10 s. scans, grating with 900 lines/mm, and aperture 50 μm slit. A multichannel thermoelectrically cooled Charge Coupled Device (CCD) camera was used as detector. Magnification 50x provided measurement spot size ~1 μm<sup>2</sup> . For X-ray photoelectron spectroscopy (XPS), measurements were applied in two apparatus. Majority of the measurement were done in an ESCAProbe P apparatus from Omicron NanoTechnology Ltd. (vacuum 10−8 Pa, Al anode, energy of monochromated X-ray source 1486.7 eV, analyzed area with size of 1 mm<sup>2</sup> , depth profiling by Ar ions sputtering). Alternatively, for XPS measurement was apply a NanoESCA apparatus from Omicron NanoTechnology Ltd. equipped with a photoemission electron microscopy navigation technique. The apparatus obtained these radiation sources: Hg-lamp (5.2 eV), HeI (21.2 eV), and a monochromatic X-ray source Al K-alpha (1486.7 eV). The spectra processing and evaluation were produced by CasXPS software. Peak component fitting was done by symmetric Gaussian (70%)-Lorentzian (30%) peaks and Shirley background. Atomic force microscopy (AFM) analysis was conducted in a Veeco CP II apparatus in the tapping mode. Photos on structures were made on an optical microscope Jenavert G0685, magnification 2000. Electrical measurements were done on a computer-controlled workplace by a standard van der Pauw method.

#### **3. Graphene preparation from the structure Ni/SiC**

Growth of graphene films was done on a set of Ni/SiC structures with the different thickness of nickel layers (1, 5, 10, 50, 200, and 300 nm) and in the different type of thermal forming [25, 26]. Raman spectra were measured for all samples after annealing. The structure with the thinnest nickel layer showed the Raman spectrum with a very small G band, while the D and 2D bands were absent. Apparently, due to a shortage of Ni in this sample, the necessary quantity of carbon atoms for growth of graphene films by SiC decomposition was not released. So, this structure was omitted in the other text.

#### **3.1. Raman spectroscopy**

In **Figure 1**, an example of Raman spectra of structures differing in the thickness of the deposited nickel layer is shown. Annealing was carried out in the same way for all samples (heating temperature 1080°C, annealing period 10 s). All Raman spectra in **Figure 1** contain the main characteristic bands of carbon materials designated as D (1350 cm−1), G (1580 cm−1), and 2D (2700 cm−1). The integral intensity ratio of the D and G bands (ID/IG) can be applied for the quantification of defects in graphene. An equation for the calculation of the crystallite size L<sup>a</sup> in the graphene layer using any laser radiation can be written as [27]

$$L\_i(nm) = \frac{560}{E^\circ(eV)} \left(\frac{I\_D}{I\_c}\right)^{-1} \tag{1}$$

where E is the energy of laser which was applied in the Raman analysis. The integral intensity ratio of the 2D and G bands (I2D/IG) can be applied for the calculation of carbon layer number in the graphene layer. Alternatively, the value of the 2D band full width at half maximum (FWHM) can be used [28]. SLG shows the value of I2D/IG in the range 3–3.5 (ideally 4), and FWHM of the 2D band is then 25 cm−1.

**Figure 1.** Raman spectra of Ni/SiC samples with Ni thickness in the range of 5, 10, 50, 200, and 300 nm. (Reprinted with permission from Thin Solid Films No. 3995281012757.).

It is obvious that the ratio I2D/IG from the Raman spectra of all samples on **Figure 1** increases, and opposite to the fact, the ratio ID/IG decreases, while the thickness of Ni layer is growing. The resulting graphene film then has less defects or bigger crystals, respectively, and contains smaller number of carbon monolayers for samples with a thicker Ni layer. The graphene film on the Ni(200)/SiC structure shows ID/IG = 0.41, I2D/IG = 0.66 and FWHM = 59 cm−1. Using the data from Ref. [28] and the formula (1), the number of carbon monolayers was determined to be 3.8 and crystallite size L<sup>a</sup> = 45 nm. The graphene film prepared on the Ni(300)/SiC structure is similar the number of carbon monolayers is slightly lower (3.5), and crystallite size is lower too (40 nm).

**3. Graphene preparation from the structure Ni/SiC**

in the graphene layer using any laser radiation can be written as [27]

this structure was omitted in the other text.

208 Graphene Materials - Structure, Properties and Modifications

*La*

FWHM of the 2D band is then 25 cm−1.

permission from Thin Solid Films No. 3995281012757.).

**3.1. Raman spectroscopy**

Growth of graphene films was done on a set of Ni/SiC structures with the different thickness of nickel layers (1, 5, 10, 50, 200, and 300 nm) and in the different type of thermal forming [25, 26]. Raman spectra were measured for all samples after annealing. The structure with the thinnest nickel layer showed the Raman spectrum with a very small G band, while the D and 2D bands were absent. Apparently, due to a shortage of Ni in this sample, the necessary quantity of carbon atoms for growth of graphene films by SiC decomposition was not released. So,

In **Figure 1**, an example of Raman spectra of structures differing in the thickness of the deposited nickel layer is shown. Annealing was carried out in the same way for all samples (heating temperature 1080°C, annealing period 10 s). All Raman spectra in **Figure 1** contain the main characteristic bands of carbon materials designated as D (1350 cm−1), G (1580 cm−1), and 2D (2700 cm−1). The integral intensity ratio of the D and G bands (ID/IG) can be applied for the quantification of defects in graphene. An equation for the calculation of the crystallite size L<sup>a</sup>

> (*nm*) <sup>=</sup> \_\_\_\_\_ <sup>560</sup> *E*4 (*eV* ) ( *I* \_\_*<sup>D</sup> I G*) −1

where E is the energy of laser which was applied in the Raman analysis. The integral intensity ratio of the 2D and G bands (I2D/IG) can be applied for the calculation of carbon layer number in the graphene layer. Alternatively, the value of the 2D band full width at half maximum (FWHM) can be used [28]. SLG shows the value of I2D/IG in the range 3–3.5 (ideally 4), and

**Figure 1.** Raman spectra of Ni/SiC samples with Ni thickness in the range of 5, 10, 50, 200, and 300 nm. (Reprinted with

(1)

Further, an influence of temperature of the annealing process on parameters of the graphene films was tested. **Figure 2** illustrates dependence of the I2D/IG ratio on the annealing temperature, which was gained from Raman spectra of the graphene films on the Ni(200 nm)/SiC structures. The films were prepared by annealing of the structure in the temperature changing from 710 to 1080°C. The annealing time was in all cases 10 s. From the obtained dependence, it is evident that the graphene film with minimal number of carbon monolayers was obtained in the case of the highest annealing temperature (1080°C).

We studied also the quality of carbon films that were formed at the boundary between the metallization and silicon carbide substrate (lower graphene). In these experiments the annealed metallization was etched off firstly. The etching was produced in the mixture of acids HNO<sup>3</sup> :HF = 3:1 for 10 min. Analysis of the structures was carried out by the Raman measurement again. **Table 1** obtains the results—the values of ratios I2D/IG and ID/IG of the structures before (Ni surface) and after (SiC surface) the etching. It is evident from the results that the lower graphene film analyzed on the SiC surface showed for the most experiments higher I2D/IG ratio than in the graphene film on the metallization surface. Evidently, the graphene films on the SiC surface show a lower number of carbon monolayers. The great disadvantage of lower graphene is the much higher ID/IG ratio and so higher concentration of defects (smaller crystals). The crystallite size for the graphene film from the last line of **Table 1** is 67 nm before the etching and 16 nm after the etching of the nickel metallization.

**Figure 2.** Ratio of 2D and G band intensities in Raman spectra of graphene on the Ni(200)/SiC structures annealed at various temperatures. (Reprinted with permission from Thin Solid Films No. 3995281012757.).


**Table 1.** Results of Raman analysis of the graphene structures (the ratios of band intensities) on the nickel surface and after the metallization was etched off (graphene on the SiC surface).

#### **3.2. Thickness of graphene layers**

The thickness of graphene films was calculated mainly from a dependency between the number of carbon monolayers and the I2D/IG ratio along with the FWHM obtained from Raman spectra [28]. An example is shown in **Table 2**. It shows values of the number of carbon monolayers (N) that were determined concurrently from the I2D/IG ratio and from the FWHM. There is a quite good conformity among the values. The values of N determined from the I2D/IG ratio are a bit higher than the values determined from the FWHM.

The XPS analysis was used for verification of the results in the case of the Ni(200 nm)/SiC structure (annealing temperature 1080°C, annealing time 10 s). The process is based on the attenuation of photoelectrons that are excited from the material under the graphene film [3]. Thickness of the graphene film can be determined from the formula

$$\mathbf{d} = -\lambda \ln \text{(I/I}\_o\text{)}.\tag{2}$$

where I and Io are intensities of XPS signal of a selected element that was measured on the surface of the graphene film and from the material under the graphene film, respectively. λ represents inelastic mean free path of electrons in the graphene film. Signal of Ni<sup>2</sup> p3/2 with the electron energy of 853 eV was selected in our experiments. The value of λ = 2.4 nm was gained from literature [29]. The surface of the graphene film was analyzed at first (I = 400 electron/s), and then the film was progressively sputtered off with Ar ions. Intensity of the Ni<sup>2</sup> p3/2 signal Io = 4500 electron/s was reached when the graphene film was completely removed. Thickness


**Table 2.** Number of carbon monolayers N for the chosen graphene structures.

of the graphene film 2.5 nm was calculated from the formula (2). The number of carbon monolayers in the prepared graphene film can be gained by dividing the obtained thickness of graphene film by the thickness of the monolayer of carbon (0.335 nm). In graphene films, however, the distance of carbon monolayers can change from 0.55 to 0.70 nm [30]. So we can conclude that the number of carbon monolayers in the graphene film on the Ni(200 nm)/SiC structure shows value from 3.6 to 4.5 and it corresponds to the data in **Table 2**.

#### **3.3. Graphene morphology**

The morphology of structure surfaces was studied by the AFM measurement. **Figure 3** shows the surface morphology of the Ni(300 nm)/SiC structure annealed at 1000°C for 120 s. Massive reaction of the nickel film with SiC substrate occurred in the annealing process, and the reaction was not homogeneous which is confirmed by large roughness R<sup>a</sup> = 31 nm. The prepared graphene film lays on the metallization surface, and consequently it shows large number of defects and thus has low crystallinity.

#### **3.4. Graphene transfer**

**3.2. Thickness of graphene layers**

**Annealing temperature [°C]**

210 Graphene Materials - Structure, Properties and Modifications

where I and Io

**Thickness of Ni** 

**Annealing temperature [°C]**

**[nm]**

**Thickness of Ni layer [nm]**

are a bit higher than the values determined from the FWHM.

Thickness of the graphene film can be determined from the formula

The thickness of graphene films was calculated mainly from a dependency between the number of carbon monolayers and the I2D/IG ratio along with the FWHM obtained from Raman spectra [28]. An example is shown in **Table 2**. It shows values of the number of carbon monolayers (N) that were determined concurrently from the I2D/IG ratio and from the FWHM. There is a quite good conformity among the values. The values of N determined from the I2D/IG ratio

**Table 1.** Results of Raman analysis of the graphene structures (the ratios of band intensities) on the nickel surface and

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

**Parameters of graphene growth Ni surface SiC surface**

200 1080 10 0.40 0.65 1.20 0.46 200 1080 60 0.43 0.55 1.24 0.65 200 1080 180 0.27 0.50 1.10 0.67

**Annealing time** 

**[s]**

Reprinted with permission from Thin Solid Films No. 3995281012757.

after the metallization was etched off (graphene on the SiC surface).

The XPS analysis was used for verification of the results in the case of the Ni(200 nm)/SiC structure (annealing temperature 1080°C, annealing time 10 s). The process is based on the attenuation of photoelectrons that are excited from the material under the graphene film [3].

d = −λ ln(I/ Io), (2)

face of the graphene film and from the material under the graphene film, respectively. λ rep-

electron energy of 853 eV was selected in our experiments. The value of λ = 2.4 nm was gained from literature [29]. The surface of the graphene film was analyzed at first (I = 400 electron/s),

Io = 4500 electron/s was reached when the graphene film was completely removed. Thickness

resents inelastic mean free path of electrons in the graphene film. Signal of Ni<sup>2</sup>

and then the film was progressively sputtered off with Ar ions. Intensity of the Ni<sup>2</sup>

**Parameters of graphene growth I2D/IG N FWHM N**

200 1080 0 0.53 5 66 4.7 200 1080 10 0.65 3.8 59 3.4

**Annealing time** 

**[s]**

Reprinted with permission from Thin Solid Films No. 3995281012757.

**Table 2.** Number of carbon monolayers N for the chosen graphene structures.

are intensities of XPS signal of a selected element that was measured on the sur-

p3/2 with the

p3/2 signal

**Figure 4** shows an example of Raman spectra of a structure after annealing at 950°C, annealing period 30 s. The solid line represents spectrum after the annealing; the dashed line represents spectrum of the graphene film transferred onto the SiO<sup>2</sup> /Si substrate. The transfer was done by the etching of the silicide layer by the mixture of HF and HNO<sup>3</sup> acids (ratio 3:1) and with help of polymethyl methacrylate (PMMA). It is possible to estimate that graphene from **Figure 4** contains four carbon layers, its crystallite size is 43 nm, and the transfer increases its value on 82 nm. The difference is probably done by smoother surface of SiO<sup>2</sup> .

#### **3.5. Electrical parameters**

The basic electronic parameters of prepared graphene were measured. Experiments were done with the metallization prepared by the evaporation of nickel (thickness of 300 nm). For the measurements it is necessary to have a dielectric substrate; therefore, a semi-insulating SiC plate

**Figure 3.** AFM picture of the graphene film surface prepared on the Ni(300)/SiC structure annealed at 1000°C for 120 s. (Reprinted with permission from J Electrical Engineering.).

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

(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 are shown in **Table 3** (ρ<sup>s</sup> is surface resistivity, μH is Hall mobility, and c<sup>s</sup> is the concentration of charge carriers). Hall mobility of the prepared graphene films is very low probably due to large concentration of defects in graphene layers.

#### **3.6. Conclusion**

The synthesis of graphene on silicon carbide substrates by the Ni-silicidation reaction shows two significant advantages:


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 transfer on the dielectric substrate (SiO2 /Si) is necessary. Alternatively, it is possible to apply


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

semi-insulating SiC substrate together with the lower graphene. The formed silicide layer must be etched off. The lower graphene film can be applied directly for the preparation of electronic devices.

The following conclusion can be done from the results. In our experiments, the Ni films of assorted thicknesses ranging from 1 to 300 nm were applied. Graphene was grown on all samples except one with Ni 1 nm thick. The Ni(200 nm)/SiC structure produced the best results. The quality of lower graphene films growth on the SiC surface was studied after removing of the annealed metallization. In majority of cases, lower graphene films were thinner, but they showed lower crystallinity, which was caused by rough SiC surface after the reaction of Ni with silicon carbide. Thermal forming of the Ni/SiC structures was carried out at the temperature range from 710 to 1080°C and for diverse times. The temperature of 1080°C and time of 10 s showed the best process conditions. The graphene film with character of FLG (the number of carbon monolayers in the range from 3 to 4) was produced on the base of the Ni(200 nm)/SiC structure. Longer annealing period had no noticeable influence to the graphene film parameters. Thickness of the graphene films was checked up by the XPS analysis; the value of 2.5 nm was obtained for the mentioned sample, which is similar to the value gained from the results of Raman spectroscopy. Very good agreement in results was given by both independent methods.
