The embodiments of mechanisms for forming source/drain (S/D) regions of field effect transistors (FETs) described enable forming an epitaxially grown silicon-containing layer with reduced number of particles on surface of recesses. The described mechanisms also reduce the effect of the residual particles on the epitaxial growth. The mechanisms include controlled etch of a native oxide layer on the surfaces of recesses to reduce creation of particles, and pre-CDE etch to remove particles from surface. The mechanisms also include reduced etch/deposition ratio(s) of initial CDE unit cycle(s) of CDE process to reduce the effect of residual particles on the formation of the epitaxially grown silicon-containing layer. With the application of one or more of the mechanisms, the quality of the epitaxial layer is improved.