A structure comprises a substrate, a mask, a buffer/nucleation layer, and a group III-V compound semiconductor material. The substrate has a top surface and has a recess from the top surface. The recess includes a sidewall. The first mask is the top surface of the substrate. The buffer/nucleation layer is along the sidewall, and has a different material composition than a material composition of the sidewall. The III-V compound semiconductor material continuously extends from inside the recess on the buffer/nucleation layer to over the first mask.