Hello Community,
I am currently trying to grow graphene via plasma enhanced chemical vapor deposition. Therefore I use a reactor with a pulsed DC plasma source. During my plasma tests I made some (at least for me) strange observations. I am using a H2 plasma (800°C, 10kHz frequency, 4mbar constant, 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 a carbon source, in my case CH4. 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 (200 sccm H2 + 40 sccm total gas flux). 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
Jan