The embodiments of mechanisms for forming source/drain (S/D) regions of field effect transistors (FETs) described enable forming an epitaxially grown silicon-containing material without using GeH4 in an etch gas mixture of an etch process for a cyclic deposition/etch (CDE) process. The etch process is performed at a temperature different form the deposition process to make the etch gas more efficient. As a result, the etch time is reduced and the throughput is increased.