What happens in a H2-plasma if I add CH4?

[Torrac]

Hello Community,

I am a masters student in germany and I am currently trying to grow graphene via plasma enhanced chemical vapor deposition. Therefore I use a reactor with a pulsed DC plasma source. In terms of plasmas, I am a total newby. During my tests I made some (at least for me) strange observations. I am using a H2 plasma (800°C, 10kHz frequency, 4mbar, 200sccm H2, build up like a plate capacitor) and my reactor gives me a plasma power of round about 130 W (Power is measured with P=U*I, so electrons that get to the earthed electrode are responsible for the current). In order to grow graphene I need some carbon source. I am using CH4 for that. So if I add some CH4 to my H2 plasma (exactly 5 sccm) the plasma power drops down 35%. If I add more CH4 the power keeps droping until 40 sccm CH4 (240 sccm total). If I continue to add CH4 the power increases again! (All the details are in the attached file).
So my humble understanding of impact ionization is very lmited. I would guess that in my H2 plasma there are mostly positived charged hydrogen ions (protons) because I need free elctrons to generate the plasma power and there is nothing else than hydrogen in there. If I add CH4 these hydrogen ions collide with the CH4 molecules and decomposite the CH4. If I say this "reaction" uses electrons from the surrounding in order to generate neutral H2 again I could understand the dropping in power. But then why is the power going up again?
I probably miss something very important here but my knowledge of plasma physics is very limited.
Do any of you have ideas what happens in my plasma? I don't need an exact solution for this problem but maybe a general understanding.

Thank you very much for your support

 

[eljose79]

!
Dear fellow scientist,

Thank you for sharing your observations with the community. It sounds like you are conducting some interesting research on graphene growth using plasma enhanced chemical vapor deposition. As a scientist with experience in plasma physics, I would like to offer some insights and potential explanations for your observations.

Firstly, it is important to note that plasma is a complex and dynamic state of matter, and its behavior can be influenced by many factors such as gas composition, pressure, temperature, and electric field. In your case, the addition of CH4 to the H2 plasma can significantly alter the plasma characteristics and therefore affect the plasma power. This is because CH4 is a hydrocarbon gas that contains carbon and hydrogen atoms, which can interact with the hydrogen ions in the plasma.

One possible explanation for the drop in plasma power when adding CH4 is that the CH4 molecules are being dissociated by the hydrogen ions, as you mentioned. This process requires energy, and therefore the plasma power is being consumed in this reaction. As you continue to add CH4, more and more of the plasma power is being used for dissociation, resulting in a decrease in overall plasma power.

However, as you noted, the plasma power starts to increase again when you add more CH4 beyond a certain point. This could be due to the formation of new species in the plasma, such as hydrocarbons and carbon-containing radicals, which can contribute to the plasma power. These species can also play a role in the growth of graphene, as they can deposit carbon onto the substrate.

In conclusion, the addition of CH4 to your H2 plasma can have a complex effect on the plasma power due to the interactions between the two gases. It is possible that the decrease in plasma power is due to the dissociation of CH4, while the increase in plasma power may be attributed to the formation of new species that can contribute to the plasma power and graphene growth.

I hope this helps to provide some insights into your observations. I would also suggest conducting further experiments to investigate the behavior of the plasma and its interactions with CH4 in more detail. Good luck with your research!