The present disclosure describes a semiconductor structure and a method for forming the same. The method can include forming a fin structure over a substrate. The fin structure can include a channel layer and a sacrificial layer. The method can further include forming a first recess structure in a first portion of the fin structure, forming a second recess structure in the sacrificial layer of a second portion of the fin structure, forming a dielectric layer in the first and second recess structures, and performing an oxygen-free cyclic etching process to etch the dielectric layer to expose the channel layer of the second portion of the fin structure. The oxygen-free cyclic etching process can include two etching processes to selectively etch the dielectric layer over the channel layer.