Deposition of carbon nanolayers by PECVD on ceramic substrates

Graphene layers and nanostructures were deposited on silicon dioxide (SiO2/Si) and silicon carbide (SiC) substrates at low gas pressure (1 – 5 torr) by microwave discharge PECVD (Plasma Enhanced Chemical Vapor Deposition). The advantage of this method is the relatively low temperature (600-700°C) of the substrate in the deposition process. The diffusion processes of hydrocarbon radicals on the surface of the substrates have a significant effect on the homogeneity of deposited structures. The deposited graphene nanotubes on SiC were analyzed by scanning electron microscope (SEM) and Raman spectroscopy is applied for characterization of the graphene layers. The deposited carbon layers on SiO2 were analyzed by atomic force microscope and their thickness (12-20 nm) were determined.


Introduction
The unique mechanical and electromagnetic properties of graphene and graphene nanostructures [1], have made this material a subject of intense research because it finds many applications in new technologies.Of particular interest are the various methods for depositing graphene and carbon nanostructures on dielectric substrates.An important technological problem is to prepare a graphene layer on silicon carbide and silica substrates for future applications in microelectronics and in communications systems (absorbers, antennas) [2].
The deposition of carbon/graphene layers on silicon carbide dielectric substrates [3] involves surface processes, and due to the low solubility of carbon in silicon, high substrate temperatures are required to obtain the graphene layers.Simulation results show effective graphene deposition only at substrate temperature over 2000°C in the CVD and annealing process [4].Experimental data for minimal low temperature for deposition of graphene structures on silicon carbide [5,6,7] in CVD method, gas annealing process, and hot filament methods in the range of 950-1000°C.
Our investigations on the deposition of carbon nanostructures on silicon carbide substrates by PECVD in a planar microwave discharge in a gas mixture of H 2 and CH 4 , brings up-to-date information for the development of new method and model for the deposition of graphene on ceramic materials.Previous studies on the deposition of thin carbon and graphene layers on metal substrates at atmospheric pressure have demonstrated the effectiveness of microwave discharges for PECVD [8].
In this work, we propose an innovative method for deposition on the bottom surface of a porous silicon carbide substrate of carbon layers (graphene structures) by low-pressure PECVD.The advantages of this method are the relatively low temperature (600-700°C) of the substrate in the deposition process and the production of homogeneous thin carbon layers over a large area.The process is carried out parallel in time on SiC and on SiO 2 substrates.

Experimental setup
The PECVD method for the deposition of graphene nanostructures on SiC and SiO 2 /Si was developed, and the processes were carried out at low pressure (1 -5 torr) in a metal vacuum chamber (figure 1).The experimental setup includes SAIREM GMP20 2.45 GHz, 2kW microwave generator with amplifier and water cooling.The microwaves propagate along a waveguide to the plasma exciterpart of a rectangular waveguide with two inclined slots.The system has the possibility of load (exciter and plasma) matching with a triple stub and an additional sliding short.The exciter is cooled by an air flow system.The microwaves radiated by exciter pass into the chamber through a quartz window and cause a breakdown in the gas mixture.The ignited surface wave discharge sustains the plasma column in the chamber.The gas mixture is fed into the chamber by a feed system of argon, hydrogen, and methane gases.The amount of each gas is controlled by a Bronkhorst flow controller system, then the gases are mixed in a gas mixture box and fed into the chamber.The substrates for graphene layer deposition are placed on a heated substrate holder.A heater with an adjustable power supply is located under the substrates and the temperatures of the holder and, respectively, of the substrate are determined by thermocouples.
The light spectrum of the plasma column is recorded by an optical system including a collimator, an optical fiber, and HORRIBA 500i and HR Ocean Optics spectrometers.Data processing is performed by a computer system and LIFBASE software.Substrates of porous silicon carbide with dimensions of 20x20x1 mm and SiO 2 /Si with dimensions of 5x5x1 mm were used in the deposition process.The pressure of the gas mixture (hydrogen/methane) in the deposition chamber was between 1 -5 torr and the temperature of the substrates was controlled in the range of 600-700°C.The plasma processes cause the decomposition of the carbon-containing gas and as a result, the hydrocarbon radicals fall on the heated substrates.As a result of chemical processes between the radicals, a process of formation/deposition of graphene sheets and structures begins on the surface.The deposition time of the layers and structures is selected between 90 min and 270 min depending on the experimental conditions.

Results
The dependence of parameters of the obtained carbon structures on the experimental conditions was investigated in terms of gas mixture pressure, microwave power, and deposition time.Simultaneous deposition was performed on silicon carbide and SiO 2 /Si at a substrate temperature of 650 degrees in a surface wave discharge in a hydrogen and methane gas mixture at an absorbed microwave power of 500W.
Optimal conditions for graphene deposition on SiC substrate are in the range of 2-3 Torr gas pressure and deposition time 90 min.
Analysis of the deposited structures on the SiC substrate by scanning electron microscope (SEM) shows that a thin graphitic layer is deposited under the graphene nanowires and nanotubes on the substrate surface.SEM images show very thin graphene nanowires with diameters smaller than 20 nm and graphene nanotubes with diameters of 60 to 140 nm (figure 2).A thin homogeneous carbon layer is observed also on the SiO 2 /Si substrates.The deposited structures are due to diffusion processes of carbon atoms and molecules on the inverted surface of the SiC-substrate as a result of chemical reactions of hydrocarbon radicals initiated by the high temperature of the substrate.Raman spectroscopy studies showed in the spectrum the typical D, G, and 2D peaks for graphene structures (figure 3).The narrow G peak was recorded at 1580 cm -1 and D peak at 1320 cm -1 is caused by disordered structures in graphene sheets.2D peak associated with the second order of the D peak is characteristic of multilayer graphene structures on the substrates.For the ratio between the intensities of the D and G peaks associated with the crystal structure of the graphene sheet, we obtain ratio I(D)/I(G) = 1.05, which is consistent with expectations for graphene multiwall structures.The deposition of graphene layers and structures on silicon carbide is related also to the synthesis of new composite metamaterials [11].An atomic force microscope was applied to determine the of the graphene layers on the SiO 2 /Si substrates.The porous silicon carbide substrate with deposited graphene nanostructures is characterized by a high vertical non-uniformity which makes difficult thickness measurements by AFM.An example of an AFM image of a silicon carbide surface with graphene layers is shown in figure 5. Results from AFM measurements for SiO 2 substrates show a thickness of the graphene layers between 12 and 20 nm (figure 6).A scratch on the deposited layer helps for the thickness measurements.The graphene layer depth was determined as 20 nm.In the case of a very thin homogeneous layer is difficult to obtain SEM images.Raman spectrum with D, G, and 2D peaks confirmed the presence of graphene-type layers on the SiO 2 /Si substrates (figure 4).

Conclusion
A novel method for depositing carbon nanostructures on the inverted surface of silicon carbide substrate by PECVD in a low-pressure microwave plasma has been developed.The deposition process is carried out at substrate temperature in the range of 600-700°C, whose temperature value is lower than in other methods for graphene deposition on silicon carbide.The parameters of deposited structures were determined by SEM analysis and the graphene layer thickness was measured by AFM on SiO 2 /Si.Results from Raman spectroscopy confirm the graphene type of deposited nanostructures.

Figure 1 .
Figure 1.Experimental setup for deposition of graphene layers and nanostructures.

Figure 2 .
Figure 2. SEM images of carbon nanowires (a) and carbon nanotubes (b) on SiC substrate.

Figure 3 .
Figure 3. Raman spectrum of carbon nanowires on SiC substrate.

Figure 5 .
Figure 5. AFM images of profile of graphene deposition on SiC substrate.