The Effect of Increased Methane Flow Rate on Electronic Correlation of Amorphous Silicon Carbon (a-SiC: H)

In this study, we report for the first time that the addition of methane (CH 4 ) flow rate in the p-type a-SiC: H layer greatly affects the electronic correlation in increasing the conversion efficiency of solar cells. The a-SiC: H p-type layer was grown using Plasma Enhanced Chemical Vapor Deposition (PECVD) on Indium Tin Oxide (ITO) substrate with various methane flow rates. The a-SiC: H p-type layer was characterized including the complex dielectric properties and the complex refractive index using Ellipsometric Spectroscopy (ES), while the surface roughness morphology was used Atomic Force Microscopy (AFM). In sample P-2 there is a change in the form of a decrease in the value of the refractive index <n> and the E 0 energy in the lower energy compared to the P-1 sample with a change of 0.3 eV, an increase in the optical gap and a decrease in the value of the real and imaginary dielectric function. While the influence of an increase in the carbon composition of the amorphous network order shows the addition of amorphous tissue disorder. Our results, show that the optical magnitude of the p-type a-SiC: H layer is not only affected by the amount of carbon in the film but also the hydrogen which is thought to contribute.


Introduction
The p-type layer of a-SiC: H is applied to the solar cell on top lapiasan, the p-type layer in solar cells based on a-SiC: H p-i-n. As a light-receiving layer, p-type layer should be transparent so that the incoming photons more much, which means the carrier concentration increases [1][2][3]. In order for the p-type layer to be transparent and able to increase the concentration of the charge carrier, carbon is added when depositing the p-type layer which is made thinner. With the addition of carbon at the time deposed p-type layer, dangling bonds on the wane and become more transparent layers so that the percentage of photons that arrive at the intrinsic layer will increase [4][5]. Therefore, the composition greatly affect the optical gap p-type layer of a-SiC: H.
The optical gap width of the p-type a-SiC: H layer can also be obtained larger in the presence of carbon, besides that the presence of the amount of hydrogen also affects the optical gap increase, so it is often used as a transparent layer that is doped in solar cell applications, sensors, and electrolysis [6][7][8]. Increasing the width of the optical gap with the presence of carbon in the network layer of the p-type a-SiC: H followed by a worsening of its electrical properties and increased disorder [9][10][11]. The presence of carbon can increase the width of the state density in the tail region of the energy band, which can decrease the drift mobility thus worsening the electrical properties, whereas the reduction in disorder is thought to be due to the presence of trigonal C=C sp 2 in the sp 3 tetrahedral amorphous network [12][13]. The state density in the tail of the valence and conduction bands is reflected in the slope of the optical absorption curve so that it can be used to determine the electrical properties and amorphous semiconductor disorder. The unfavorable electrical properties are also thought to be related to structural characteristics such as a high number of voids or the formation of certain hydrogen bonds [14]. Therefore, it is necessary to know the structural characteristics and their relationship with the amorphous semiconductor material disorder which affects their electrical properties.
Information about the optical gap of the p-type a-SiC: H layer can be obtained from the optical absorption coefficient α in the visible region, while the optical absorption coefficient α can experimentally be found from the refractive index of the N = n-ik complex as a function of energy [15][16]. Therefore, both the optical magnitude α and n as a function of energy needs to be known in the study of the optical characteristics of the p-type layer of a-SiC: H. This research will study the two optical quantities for the p-type layer a-SiC: H deposition result by RF-PECVD method. The effect of variations in the flow rate of methane gas on these two optical quantities will be studied with data on the amplitude ratio <ψ> and the phase difference ratio <Δ> between the p and s polarization light waves from the ellipsometric spectroscopy measurements.

Experiment Method
In this study, the p-type layer of a-SiC: H grown on Corning glass 7059 substrate which has been prepared with Indium Tin Oxide (ITO). The PECVD technique used consists of two multi-chamber deposition chambers, each of which is specifically used to deposit p-type a-SiC: H layers as shown in Fig 1. In this condition, the filament temperature is only determined by the radiation heater temperature (substrate temperature). Silane gas (SiH4) with a concentration of 10% in hydrogen (H2) is used as the source gas. Used as a dopant gas diborane (B2H6) gas concentration of 10% in H2 for the p-type dopant layer and the addition of methane (CH4) as a p-type layer optimizations that are transparent and capable of increasing the concentration of charge carriers [17]. The growth parameters used are shown in Table 1.
Elipsometric spectroscopy (ES) parameters ψ and Δ (ie, the ratio of the amplitude and the phase difference between the reflected light polarized p and s, respectively) were collected on a 70° angle of incidence, the photon energy range between 0.6 and 6.5 eV using elipsometric

Result and Discussion
Measurement <ψ> and <Δ> of ellipsometric spectroscopy for samples of P-1 and P-2 show in

Complex dielectric function <ε1> and <ɛ2>
The dielectric function <ε> describes the optical and electrical properties of the material over

Refractive index <n> and Extinction coefficient <k>
The values of the complex refractive index <n> and <k> obtained through ellipsometric spectroscopic measurements represent the <n> value which represents the refractive index of the material and the <k> value which represents the extinction coefficient of material absorption and energy lost due to the scattering process [22]. Figure 5 shows the optical properties of the sample layer P-1 and P-2 for measurements at the same temperature on the substrate ITO. In the P-2 sample, the p-type a-SiC: H layer quality was quite good in terms of the lowest n value compared to the P-1 sample. At energy <3.0 eV in <n> sample P-2, there is a very sharp decrease compared to sample P-1, this shows that the maximum absorption rate obtained is greater for sample P-2 than for sample P-1 even though for each sample it is measured at the same substrate temperature. During deposition, the substrate temperature greatly affects the diffusion process of the deposited atoms on the substrate. In this case because the temperature of the substrate will cause the atoms of the surface of the substrate to vibrate and as a result, the distance between the planes stretches, making the insertion process easier.
In the extinction coefficient <k> in sample P-2, it is found that> 3.0 eV is a smaller value than in sample P-1, this indicates that the results of sample P-2 are denser than sample P-1.
This shows the extinction coefficient tends to decrease with increasing flow rate of methane gas. Extinction coefficient curves obtained are in limited energy so that it will have an energy price that is owned by a second layer of p-type a-SiC: H. However, at energies> 1.2 eV, a more transparent p-type a-SiC: H layer is produced, this is evidenced by the value of <k> which is towards the negative, this is likely to occur saturation of the insertion process of atoms and the result is a layer that is not dense. with an increase in carbon concentration, even though the deposition process uses methane gas as a carbon source.

Conclusion
The

Acknowledgment
The author thankfully acknowledges the Ministry of Finance of the Republic of Indonesian, through the Lembaga Pengelolah Dana Pendidikan (LPDP), which has provided financial support through the Scholarship Indonesian. The authors would also appreciate LPPM Institut Teknologi Sepuluh Nopember for the use of experimental facilities at Research Center ITS.
We also acknowledge the NUS Singapore Synchrotron Light Source (SSLS) SE measurements. We are thankful to Prof. Andrivo Rusydi National University of Singapore who provided a fruitful discussion for this paper.
Obtained by measurement by angle 70o.     Possible band-structure scheme in P-1 and P-2. EF denotes fermi level. Thickness of arrows qualitatively indicates strength of the transitions