Graphene is a single layer of carbon that has a hexagonal (honeycomb) lattice with a carbon-carbon distance of 0.142 nm. It is the first two-dimensional crystalline material and represents whole 2D material. Since the discovery of mono and few layers of graphene film (2004), it has become an attractive research topic in nanomaterial society due to its many unique properties such as physical, chemical, electrical, mechanical, thermal, conductive, and possible use of low-cost transparent electrodes, touch screen panel, photovoltaic solar cells and microelectronic devices [1] - [12].

However, obtaining the quality of graphene films by low-cost deposition method is a still significant challenge, a single process of graphene synthesis at low temperature is highly desirable. In fact, researchers are concentrated to do research on the different methods of graphene synthesis in recent years [8] [10] [11]. Several methods have been explored to prepare graphene on various substrates, like, thermal CVD, micromechanical cleavage or chemical exfoliation of graphite, thermal decomposition of SiC on copper and nickel substrates [12] [13] [14]. Among various methods, thermal CVD has been a main popular method to synthesize graphene everywhere, however, it requires high temperature (above ~1000˚C), catalyst films, post-transfer, and additional catalyst removal process are needed. For leaving this complicated graphene synthesis process, direct growth of graphene without using any other catalyst is very attractive.

In this work, we synthesized graphene directly (without using catalyst) on silicon substrate by MW-SWP CVD using the hydrocarbon as a source gas at low temperature (550˚C). We believed that the direct synthesis of graphene on silicon is possible due to the high radical density of MW-SWP CVD. MW-SWP CVD is a promising plasma source among the various types of existing plasma sources for the deposition of graphene thin film [15] [16]. For making the high-quality graphene, we used the cover on substrate for avoiding plasma emission ultraviolet ray’s effect; ultraviolet rays could increase defect density of graphene films. In addition, we fabricated graphene/silicon (Gi/Si) Schottky junction solar cells and characterized their photovoltaics properties. Figure 1 shows a schematic of the Gi/Si Schottky junction solar cells development process steps.

Graphene film synthesized by MW-SWP CVD on silicon substrate. Figure 2

shows schematic diagram of the MW-SWP CVD system. In this system, MW power, gas flow rate and deposition time-duration are controlled by touch-screen computer system. Due to the large stage diameter (20 cm) of the CVD, it is possible to deposit a relatively large area of graphene thin film or make simultaneous film deposition on various substrates in one experiment. The maximum MW power of the CVD system is 3 kW, whereas the stage temperature can be controlled up to 800˚C. The MW-SWP is produced in a 300 mm cylindrical vacuum chamber by introducing a 2.45 GHz MW through a quartz window via slot antennae. The MW introduced through the slot antennae drops exponentially below the quartz window where the electron density exceeds the cut-off density. A high plasma density with a uniform electron density of more than 10¹¹ cm^-3 is formed in the vacuum chamber and broadened in the downstream region due to particles diffusion [15]. Unlike other plasma sources, the SWP is a promising plasma source for large-area thin film deposition.

Argon (Ar: 100 sccm) and acetylene (C₂H₂: 10 sccm) were used as carrier and source gases for plasma formation. The detailed substrate cleaning process is described elsewhere [16]. The MW CVD chamber was evacuated to a base pressure at approximately 5 × 10⁻⁴ Pa using a turbo pump. The launched microwave power was typically 1000 W and a constant gas composition pressure is maintained at 15 Pa and substrate temperature was 550˚C during deposition. In this work, we used the cover (Figure 3) on the top of the substrate for the quality of graphene synthesis to avoid plasma ultraviolet ray’s effect during plasma irradiation. We synthesized graphene with different cover heights (open, 25 cm and 15 cm) and compared its properties.

The graphene films were characterized, by Raman spectroscopy, TEM JEOL JEM-2100F and SEM JEOL JSM-6510LA for structural properties. Four-point probes method for sheet resistance and JASCO V-570 UV/VIS/NIR spectrophotometer for transmittance of the films. The current-voltage characterization of graphene/silicon (G/Si) Schottky junction solar cell was tasted using JASCO SS-200 W solar simulator in the dark and under AM 1.5 simulated solar radiation.

We believed that the MW-SWP CVD has produced ultraviolet rays during plasma emission. Due to the ultraviolet rays, synthesized graphene films has increased defect density, for making quality of graphene we should have to avoid ultraviolet ray’s effect during plasma emission. We made the cover on the substrate so that ultraviolet ray’s could not hit directly on the surface substrate (Figure 3). Here we synthesized three types of graphene film with different cover heights (open, 25 cm and 15 cm) during deposition.

Figure 4 shows the Raman spectra (left) of graphene films deposited with different cover heights. Raman spectroscopy is performed at room temperature. Three peaks centered at 1346, 1576, 2691 cm⁻¹ are assigned to the D (disorder mode), G (graphite mode) and 2D (D mode overtone) modes of graphene respectively. It is reported that the D-peak represents disordered sp²-hybridized carbon with an amount of sp³-hybrodized carbon, while the G-peaks represent graphite-like sp²-hybridized carbon in the deposited film [17]. The presence of 2D peak shows a good agreement of graphene structure formation into the film. The Raman spectra of carbon material are quite remarkable in order to study the quality of graphene structure [18]. It is believed that the defect of the film is decreased when the film’s I_d/I_g ratio is small. Ratio of the I_d/I_g is decreasing and FWHM of 2D peaks is also becoming narrower with decreasing height of the cover. It means that the defect density of graphene has decreased and crystalline structure of the films is improved. It is reported that the FWHM of 2D peak and

I_d/I_g of graphene film is decreased means the defect density decreased and quality of the graphene film is improved [19] [20]. It is agreed that the sheet resistance of the graphene films is decreased from 16.3 kΩ/sq to 1.3 kΩ/sq with decreasing cover height, shown in Figure 4 (right).

Figure 5 shows the SEM image of graphene film deposited on silicon wafer directly with cover height 15 cm only. It observed that the six-sided, five-sided and other shape polyhedra, which indicate the nucleation of hexagonal and pentagonal carbon rings into carbonaceous nuclei. Moreover, the shape polyhedra are joined each other, indicating the joining behavior of polyhedral shape nanoflakes to form monolayer graphene. We found the same film nature of graphene films with other cover heights.

TEM is used to examine the morphology of the graphene, shown in Figure 6. TEM image reveals the layers and crystalline information of the graphene film synthesized on silicon substrate directly with the cover height of 15 cm. Few layers of graphene (~3 layers) formation can be observed (b). The intensity pattern (c) of the graphene layers showed that an inter-planar spacing of about 0.38 nm. A fast fourier transform (FFT) image (d) showed a graphite crystalline structure.

The transmittance spectrum of graphene film deposited with cover height 15 mm is 94.31%@550 nm shown in Figure 7. It is reported that transmittance of one-layer graphene is 97.7%, [21] [22] [23]. Our results are qualitatively similar (~3 layers of graphene; Figure 7, with 94.31% transmittance) with the results found by other researchers. Comparatively, we got very good quality and high transmittance graphene with cover height 15 mm than with other height covers. Here we concluded that the ultraviolet rays should avoid during deposition by MW plasma CVD method for the quality of graphene films.

In addition, we fabricated G/Si schottky junction solar cells using graphene deposited with a cover height of 15 mm. Figure 8 shows the schematic diagram (a), band diagram (b), current-voltage (I-V) curve (c) and photovoltaic parameters

of G-Si Schottky junction solar cells (d). It showed that photovoltaic behavior with maximum open-circuit voltage (V_oc) 0.43 V, short circuit current density (J_sc) 24.01 mA/cm², fill factor (FF) 0.62 and conversion efficiency (η) 6.39% are obtained. Compare with conventional Si solar cells, the fabrication process is greatly simplified; just graphene is synthesized directly on n-type crystalline Si substrate at low temperature (550˚C). G-Si Schottky junction solar cells, the graphene film works on the silicon substrate as a carrier separator (photocarriers) and transport. Moreover, due to high transparency of graphene, most of the incident light is absorbed by silicon, the caused value of short circuit current density is high.

Graphene film is deposited directly on silicon substrate by MW-SWP CVD with three different cover heights at 550˚C for avoiding plasma emission ultraviolet ray’s effect, using the hydrocarbon as a source gas. The structural properties of the films were investigated by SEM and TEM measurements. It showed that the formation of a graphene layer on silicon having a crystalline structure with a hexagonal spot in FFT image of graphene was conformed. Raman’s result also confirmed that the graphene layers are grown on the silicon substrate. The ratio of I_d/I_g is decreasing and FWHM of 2D peaks is also becoming narrow with decreasing height of the cover. The sheet resistance decreased 16.3 kΩ/sq to 1.3 kΩ/sq with decreasing height of the cover. We fabricated graphene/silicon Schottky junction solar cells and characterized their photovoltaic behavior under dark and light illumination. The open voltage (V_oc) is 0.43 V, short circuit current (J_sc) is 24.01 mA/cm², fill factor (FF) is 0.62 and conversation efficiency (η) is 6.39% are obtained.

We would like to express sincere thanks to Mr. Kawamura for his help during TEM measurement.

The authors declare no conflicts of interest regarding the publication of this paper.

Adhikari, S., Zhu, R. and Umeno, M. (2021) Direct Synthesis of Graphene on Silicon at Low Temperature for Schottky Junction Solar Cells. Journal of Materials Science and Chemical Engineering, 9, 1-9. https://doi.org/10.4236/msce.2021.910001