A method of forming a circuit structure includes providing a substrate; forming recesses in the substrate; forming a mask layer over the substrate, wherein the mask layer covers non-recessed portions of the substrate, with the recesses exposed through openings in the mask layer; forming a buffer/nucleation layer on exposed portions of the substrate in the recesses; and growing a group-III group-V (III-V) compound semiconductor material from the recesses until portions of the III-V compound semiconductor material grown from the recesses join each other to form a continuous III-V compound semiconductor layer.