Non-silicon fin structures extend from a crystalline heteroepitaxial well material in a well recess of a substrate. III-V finFETs may be formed on the fin structures within the well recess while group IV finFETs are formed in a region of the substrate adjacent to the well recess. The well material may be electrically isolated from the substrate by an amorphous isolation material surrounding pillars passing through the isolation material that couple the well material to a seeding surface of the substrate and trap crystal growth defects. The pillars may be expanded over the well-isolation material by lateral epitaxial overgrowth, and the well recess filled with a single crystal of high quality. Well material may be planarized with adjacent substrate regions. N-type fin structures may be fabricated from the well material in succession with p-type fin structures fabricated from the substrate, or second epitaxial well.