The present invention relates generally to semiconductor devices and more particularly, to a structure and method of forming a fin using double trench epitaxy. The fin may be composed of a III-V semiconductor material and may be grown on a silicon, silicon germanium, or germanium substrate. A double trench aspect ratio trapping (ART) epitaxy method may trap crystalline defects within a first trench (i.e. a defective region) and may permit formation of a fin free of patterning defects in an upper trench (i.e. a fin mold). Crystalline defects within the defective region may be trapped via conventional aspect ratio trapping or three-sided aspect ratio trapping. Fin patterning defects may be avoided by utilizing a fin mold to grow an epitaxial fin and selectively removing dielectric material adjacent to a fin region.