Silicon is a known trace contaminant in diamond grown by chemical vapor deposition (CVD) methods. Deliberately Si-doped diamond is currently attracting great interest because of the attractive optical properties of the negatively charged silicon-vacancy (SiV-) defect. This work reports in-depth studies of microwave activated H2 plasmas containing trace (10-100 ppm) amounts of SiH4, with and without a few % of CH4, operating at pressures and powers relevant for contemporary diamond CVD, using a combination of experiment (spatially resolved optical emission (OE) imaging) and two-dimensional plasma chemical modelling. Key features identified from analysis and modelling of the OE from electronically excited H, H2, Si, and SiH species in the dilute Si/H plasmas include (i) fast H-shifting reactions ensure that Si atoms are the most abundant silicon-containing species throughout the entire reactor volume, (ii) the low ionization potentials of all SiHx (x ≤ 4) species and efficient ion conversion reactions ensure that even trace SiH4 additions cause a change in the dominant ions in the plasma volume (from H3+ to SiHx+), with consequences for electron-ion recombination rates and ambipolar diffusion coefficients and (iii) the total silicon content in the reactor volume can be substantially perturbed by silicon deposition and H atom etching reactions at the reactor walls. The effects of adding trace amounts of SiH4 to a pre-existing C/H plasma are shown to be much less dramatic, but include (i) a Si substrate or fused silica components within the reactor is a ready (unintended) source of gas phase Si-containing species, (ii) OE from electronically excited Si atoms should provide a reliable measure of the Si content in the hot plasma region and (iii) Si atoms and/or SiC2 species are the most abundant gas phase Si-containing species just above the growing diamond surface and thus the most likely carriers of the silicon incorporated into CVD diamond.
Mahoney, E. J. D., Ashfold, M. N. R., Mankelevich, Y. A., & Truscott, B. S. (2020). Optical Emission Imaging and Modelling Investigations of Microwave Activated SiH4/H2 and SiH4/CH4/H2 Plasmas. Journal of Physical Chemistry A, 124, 5109-5128. https://doi.org/10.1021/acs.jpca.0c03396