The present disclosure provides channel structures of a semiconductor device and fabricating methods thereof. The method can include forming a superlattice structure with first nanostructured layers and second nanostructured layers on a fin structure. The method can also include removing the second nanostructured layers to form multiple gate openings; forming a germanium epitaxial growth layer on the first nanostructured layers at a first temperature and a first pressure; and increasing the first temperature to a second temperature and increasing the first pressure to a second pressure over a first predetermined period of time. The method can further include annealing the germanium epitaxial growth layer at the second temperature and the second pressure in the chamber over a second predetermined period of time to form a cladding layer surrounding the first nanostructured layers.