Atomic and electronic structures of the ridgeline and the apex of stacking-fault tetrahedron (SFT) defect in Si films are studied by using the first-principles calculations. It is shown that the electron and hole states of bulk Si are localized around the SFT faces, the SFT ridges, and the SET apexes. This feature occurs due to the unique atomic geometries in these SFT structures, such as stacking-fault layers, dimer bonds, and the Si-dome structures. © 2005 Elsevier B.V. All rights reserved.