Gate-all-around integrated circuit structures having germanium nanowire channel structures, and methods of fabricating gate-all-around integrated circuit structures having germanium nanowire channel structures, are described. For example, an integrated circuit structure includes a vertical arrangement of horizontal nanowires above a fin, each of the nanowires including germanium, and the fin including a defect modification layer on a first semiconductor layer, a second semiconductor layer on the defect modification layer, and a third semiconductor layer on the second semiconductor layer. A gate stack is around the vertical arrangement of horizontal nanowires. A first epitaxial source or drain structure is at a first end of the vertical arrangement of horizontal nanowires, and a second epitaxial source or drain structure is at a second end of the vertical arrangement of horizontal nanowires.