Epitaxial structures of a fin-like field effect transistor (FinFET) device includes a substrate, a fin structure including two fins, inner and outer fin spacers formed along both sidewalls of the fins, and isolation regions formed around the fins. The FinFET device further includes a gate structure formed over the fin structure and an epitaxial structure formed over the fin structure in a source/drain region. The epitaxial structure is formed by merging the fins with at least one epitaxial semiconductor layer and includes an air gap having a volume determined by the height and separation distance of the inner fin spacers.