In one aspect thereof the invention provides a structure that includes a substrate having a surface and a plurality of fins supported by the surface of the substrate. The plurality of fins are formed of Group IVA-based crystalline semiconductor material and are spaced apart and generally parallel to one another. In the structure at least some of the plurality of fins comprise an amorphous region forming a nanowire precursor structure that is located along a length of the fin where a Group III-V transistor is to be located. A method to fabricate the structure and other structures is also disclosed.