The present disclosure provides for many different embodiments. An exemplary method includes forming a fin structure, forming a doped amorphous layer over a portion of the fin structure, and performing a knock-on implantation process to drive a dopant from the doped amorphous layer into the portion of the fin structure, thereby forming a doped feature. The doped amorphous layer includes a non-crystalline form of a material of the fin structure. In some implementations, the fin structure includes a crystalline material, and the knock-on implantation process converts at least a portion of the doped amorphous layer into the crystalline material (for example, by crystallizing the portion of the doped amorphous layer), such that the portion of the doped amorphous layer becomes a part of the fin structure. In some implementations, the fin structure includes silicon and the doped amorphous layer includes amorphous silicon, such that the knock-on implantation process crystallizes at least a portion of the amorphous silicon.

In some implementations, the method further includes performing a fin trimming process to reduce a dimension of the fin structure before forming the doped amorphous layer. In some implementations, a thickness of the doped amorphous layer is about equal to a thickness of the fin structure removed during the fin trimming process, and the knock-on implantation process causes the doped amorphous layer to become a part of the fin structure. In some implementations, the knock-on implantation process causes a portion of the doped amorphous layer to become a part of the fin structure. In such implementations, the method further includes oxidizing a remaining portion of the doped amorphous layer and removing the oxidized portion of the doped amorphous layer.