A semiconductor device and a method of fabricating the semiconductor device are disclosed. The method includes forming a fin base on a substrate, forming a superlattice structure including first and second nanostructured layers on the fin base, forming a polysilicon structure on the superlattice structure, epitaxially growing a S/D region on the fin base and adjacent to the first nanostructured layer, forming an oxygen-rich outer gate spacer including a first dielectric material with a first non-stoichiometric composition on a sidewall of the polysilicon structure, forming an oxygen-rich inner gate spacer including a second dielectric material with a second non-stoichiometric composition on a sidewall of the second nanostructured layer, and replacing the polysilicon structure with a gate structure.