Theoretical study of electronic structure and optical properties of semiconductor nanostructures

Abstract: In this thesis, the electronic structure and optical properties of semiconductor nanostructures are studied theoretically. Three types of nanostructures have been studied, silicon nanocrystals, free-standing III-V nanowires and free-standing GaAs/AlGaAs nanowire superlattices. The calculations have been carried out using an atomistic, empirical tight-binding approach. Silicon nanocrystals have attracted a great deal of attention after it was shown that they can emit visible light. In this thesis the highest occupied states and lowest unoccupied states of silicon nanocrystals are studied. The confinement energies of the states are calculated and the symmetries of the states are analyzed. To better understand the localization properties of the states, their wave functions are calculated and discussed. The major part of the work, contained within this thesis, regards calculations of the electronic structure and optical properties of free-standing nanowires. In all the cases studied the conduction bands of the nanowires show good parabolic dispersions. Also, the degeneracy of the light- and heavy-hole bands of the bulk material, is broken. The band structure for GaAs, InAs and InP nanowires, oriented along the [100] crystallographic direction, with square and rectangular cross-sections is studied. The effect of the cross-section aspect ratio is studied with regard to band structure and wave functions. For nanowires with a square cross-section the valence bands are found to show strongly non-parabolic dispersions. However, when the cross-section aspect ratio is increased, the dispersion of the bands become less non-parabolic. For large aspect ratios the topmost valence bands show good parabolic dispersion around the Brillouin zone center. In addition the optical properties of an InP [111]-oriented nanowire are studied, and the nanowire is found to have a strong optical polarization anisotropy. The band edge transition is found to be polarized parallel to the nanowire orientation. Lately, the model has been extended to handle the electronic structure of hetero-structured nanowires. In this thesis the band structure of [100]-oriented free-standing GaAs/AlGaAs nanowire superlattices is studied. The evolution of the band structure and the opening of band gaps in the valence and conduction bands are studied as a function of AlGaAs barrier thickness and nanowire lateral size.