The embodiments of mechanisms for forming source/drain (S/D) regions of field effect transistors (FETs) described uses Cl2 as an etchant during the epitaxial formation of the S/D regions. The mechanisms involve using an asymmetric cyclic deposition and etch (ACDE) process that forms a preparation layer enable epitaxial growth of the following epitaxial layer with transistor dopants. The mechanisms also involve soaking the surface of substrate with dopant-containing precursors to enable sufficient incorporation of transistor dopants during the epitaxial growth of the S/D regions. By using Cl2 as etchants, the mechanisms also enables high throughput of the epitaxial growth of the S/D regions.