Direct Synthesis of Graphene on Silicon at Low Temperature for Schottky Junction Solar Cells

Graphene thin films synthesized directly at low temperature (550 ̊C) on silicon substrate by microwave (MW) surface wave plasma (SWP) chemical vapor deposition (CVD) using the cover on substrates for avoiding plasma emission ultraviolet ray’s effect during film deposition. Analytical methods such as Raman spectroscopy, Transmission electron microscopy (TEM) and Scanning electron microscopy (SEM), four-point probe method, and JASCO V-570 UV/VIS/NIR spectrophotometer were employed to characterize the properties of the graphene films. Here, we report that it is possible to grow graphene directly on the silicon substrate (without using catalyst) due to the high radical density of MW SWP CVD. Furthermore, we fabricated graphene/ silicon Schottky junction solar cells with an efficiency of up to 6.39%. Compared to conventional silicon solar cells, the fabrication process is greatly simplified; just graphene is synthesized directly on n-type crystalline Si substrate at low temperate.

thod 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 highquality 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.

Experimental Details
Graphene film synthesized by MW-SWP CVD on silicon substrate.   shows schematic diagram of the MW-SWP CVD system. In this system, MW power, gas flow rate and deposition time-duration are controlled by touchscreen 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 10 11 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 (C 2 H 2 : 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. Journal of Materials Science and Chemical Engineering

Results and Discussion
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  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 sp 2 -hybridized carbon with an amount of sp 3 -hybrodized carbon, while the G-peaks represent graphite-like sp 2 -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   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

Conclusion
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 de-