Dear Reham, many thanks for posting this very interesting technical question. To my knowledge (I may be wrong) only ammonia and hydrogen have thus far been employed as etching gases in the PECVD synthesis of vertically aligned and pure CNTs. In this context, please have a look at the following potentially useful article which might help you in your analysis:

Synthesis of Patterned Vertically Aligned Carbon Nanotubes by PECVD Using Different Growth Techniques: A Review

https://www.iith.ac.in/~ashok/Papers/Aparna_Pap.pdf

(see attached pdf file)

and

Vertically Aligned Carbon Nanotubes Production by PECVD

Chapter Vertically Aligned Carbon Nanotubes Production by PECVD

The second article is a book chapter which has been posted by the authors as public full text on RG. Thus you can freely download it as pdf file.

I hope this helps. Good luck with your research and best wishes, Frank Edelmann