Hello Community,

I am working with a CH4/H2 Plasma in a cold-wall reactor. If I use conducting substrates (like metalls) there is a homogeneous plasma visible above my substrate (roughly 1-2 cm aboce the surface of the substrate). Now if I use non-conducting or semi-conducting substrates (GaN on sapphire susbtrates) there is a clearly visible "hole" in the plasma just above the substrates surface. With my humble knowledge about plasma physics, I know that the non-emitting parts of the plasma mean that there are just positive ions and no more electrons (plasma sheath).

So my question is: what is the reason for "hole" above my substrate and what exactly happens there with the precursor molecules?

I would suggest that, because of the non existing potential drop between the substrate and the electrodes, there is no acceleration of positive ions torward the substrates surface there. But what kind of molecules/atoms define this region above the substrates surface?

Is there just a huge plasma sheath and thus I see no emission or is there simply no plasma at all and no decomposition takes place in this region?

Maybe some of you already stumbled over a similar problem.

With best regards