Disclosed are semiconductor structures with monocrystalline semiconductor fins, which are above a trench isolation region in a semiconductor substrate and which can be incorporated into semiconductor device(s). Also disclosed are methods of forming such structures by forming sidewall spacers on opposing sides of mandrels on a dielectric cap layer. Between adjacent mandrels, an opening is formed that extends vertically through the dielectric cap layer and through multiple monocrystalline semiconductor layers into a semiconductor substrate. A portion of the opening within the substrate is expanded to form a trench. This trench undercuts the semiconductor layers and extends laterally below adjacent sidewall spacers on either side of the opening. The trench is then filled with an isolation layer, thereby forming a trench isolation region, and a sidewall image transfer process is performed using the sidewall spacers to form a pair of monocrystalline semiconductor fins above the trench isolation region.