Nanostructured materials of type IV and III-V doped with Mn.

Author(s)

Jeverson Teodoro Arantes Junior

• Document type: Doctoral Thesis
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Instituto de Física (IF/SBI)
• Defense date: 2007-12-04

Examining board members

Adalberto Fazzio; Marilia Junqueira Caldas; Euclydes Marega Junior; Roberto Hiroki Miwa; Raimundo Rocha dos Santos

• Advisor: Adalberto Fazzio

Abstract

In the present work, we investigate electronic, structural and transport properties of semiconductor nanostructures of type IV and III-V using first principles calculations. (I) As a starting point, we verify systematically the stability of substitutional Mn in Ge layers in Si/Ge heterostructures. We study the Mn-Mn magnetic interaction as a function of the lattice parameter of the substrate, and we find that the energy difference between the high and low spin configurations changes as the lattice parameter is modified. Using Si as a substrate, that energy difference favors the low spin configuration, whereas increasing the substrate lattice parameter the high spin configuration becomes more stable. (II) In the study of Ge nanowires, grown along the [110] and [111] directions, we investigate the variation of the energy gap as a function of the nanowire diameter. We study the (001) surface reconstruction for some nanowire diameters grown along the [110] direction. We did a systematic study of Mn doping in the Ge nanowires in order to verify which are the most stable substitutional sites. We also study the Mn-Mn magnetic coupling for their separation parallel to the growth direction as well as perpendicular to it. This study was performed for different distances between the impurities. (III) The gold particles observed in the top surface of the nanowires, a result of the Au droplet used as catalyst in the growth process, was the motivation of the study of the formation energy of Au isolated impurities in different positions and concentrations in the nanowires. These results make it possible to know if the Au atoms will move either along the surface or towards the bulk of the wire. (IV) We verify the behavior of the type-n and type-p doping in the electronic transmission properties for impurities positioned either in the central or in the (001) surface of Ge nanowires. Because of the importance of the surface in nanostructures, we calculate the changes in the electronic transmittance in the presence of a dangling bond and an OH molecule adsorbed in the surface. (V) We investigate how the quantum confinement modifies the behavior of the vacancy native defect in Si nanowires. From the formation energy difference for nonequivalent sites, we verify one possible pathway for the vacancy migration towards the (001) surface, and we calculate the migration barrier from the central region to the nanowire surface. We also calculate the effective-U, and find it to be negative in the bulk region. (VI) Finally, we also made a systematic study of nanowires of type III-V (InP and GaAs) as well as InAs nanoparticles doped with Mn. We study the equilibrium positions and the possibility of a magnetic order for the impurity in these nanostructures. For the nanoparticles, when the system is more confined the hole becomes more localized and, consequently, the energy difference between the high and low spin configuration still favors the high spin but becomes smaller. When we insert holes we can increase this energy difference. (AU)