title: Crystal Phase Effects in Si Nanowire Polytypes and Their Homojunctions.
abstract: Recent experimental investigations have confirmed the possibility to synthesize and exploit polytypism in group IV nanowires. Driven by this promising evidence, we use first-principles methods based on density functional theory and many-body perturbation theory to investigate the electronic and optical properties of hexagonal-diamond and cubic-diamond Si NWs as well as their homojunctions. We show that hexagonal-diamond NWs are characterized by a more pronounced quantum confinement effect than cubic-diamond NWs. Furthermore, they absorb more light in the visible region with respect to cubic-diamond ones and, for most of the studied diameters, they are direct band gap materials. The study of the homojunctions reveals that the diameter has a crucial effect on the band alignment at the interface. In particular, at small diameters the band-offset is type-I whereas at experimentally relevant sizes the offset turns up to be of type-II. These findings highlight intriguing possibilities to modulate electron and hole separations as well as electronic and optical properties by simply modifying the crystal phase and the size of the junction.
authors.forename: Michele
authors.initials: M
authors.lastname: Amato
authors.forename: Thanayut
authors.initials: T
authors.lastname: Kaewmaraya
authors.forename: Alberto
authors.initials: A
authors.lastname: Zobelli
authors.affiliation: Dipartimento di Fisica, Università di Roma Tor Vergata , Via della Ricerca Scientifica 1, 00133 Roma, Italy.|INFN, Laboratori Nazionali di Frascati, Via E. Fermi 40, I-00044 Frascati, Italy.
authors.forename: Maurizia
authors.initials: M
authors.lastname: Palummo
authors.affiliation: Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus de Bellaterra , 08193 Bellaterra, Barcelona, Spain.
authors.forename: Riccardo
authors.initials: R
authors.lastname: Rurali
keywords: Silicon nanowires; band offset; density functional theory; electronic structure; silicon polytypes