Cálculos de primeiros princípios em nanofios semicondutores

• Autor(a) principal: Moreira, Marcos Dionizio
• Data de Publicação: 2009
• Tipo de documento: Tese
• Idioma: por
• Título da fonte: Repositório Institucional da Universidade Federal Fluminense (RIUFF)
• Texto Completo: https://app.uff.br/riuff/handle/1/19108
• topic: Oxidação de superfície; Nanofios semicondutores; Hetero-estruturas; Estrutura zinc-blend; Estrutura wurtzita; CNPQ::CIENCIAS EXATAS E DA TERRA::FISICA

In this work we study the electronic and structural properties of semiconductor nanowires InP and InAs and heterostructured nanowires InP/InAs. We also investigate the oxydation of the InP nanowires surface by means of hidroxyl radicals. All wires studied here were simulated in both experimentally found structures (Zinc-Blend and Wurtzite). We used first principles calculations based on the density functional theory. The GGA exchange-correlation potential was used. Electron-ions interactions are described by Troullier-Martins norm-conserving pseudopotentials. Total energies are obtained by self-consistently solving the Kohn-Sham (KS) equations and the KS orbitals are expanded using linear combinations of pseudo-atomic orbitals. To understand how the nanowires properties change with diameter, we studied five different diameters. In the structural analysis of these wires we estimate that there is a 7 Å wide region near the surface where relaxations are significant. We also propose analytical expressions to fit the variation of the wires gap with diameter relative to the bulk obtained in our work and also experimentally. For very small diameters some wires present indirect gaps. In the study of the oxydation of InP wires, we show that when all dangling-bonds are saturated by OH radicals the energy gaps diminish by means of the introduction of localized states near the top of the valence band and the bottom of the conduction band. We show that the energetically favorable saturation by OH radicals consist of bonding all surface P atoms with H atoms and all surface In atoms with OH radicals. In this case, the energy gap is larger and cleaner (with respect to localized states) than the fully OH radical saturated one. For the longitudinal heterostructures, we show that the interfaces formation energies for the InP/InAs case increase for increasing wire diameters. The swap between a P atom and a As atom in the interface is energetically favorable for the ZB wire indicating a alloyed interface. On the other hand, the same swap for the WZ structure is not energetically favorable indicating an abrupt interface. Interestingly, the introduction of a stacking fault defect in the WZ interface is energetically favorable. As far as the electronic properties are concerned, we show that in the boundaries of the bands several atomic orbitals mix, thus the energy gaps of the heterostructures depend on the number of layers of InP and InAs.