Theoretical investigations of wide-bandgap semiconductor nanowires for optoelectronic applications

Abstract

Improving existing optoelectronic devices is a crucial step in satisfying humanity's increasing demand for electricity. This work explores different ways to achieve this goal. First density functional theory (DFT) calculations are performed on functionalized ZnO and GaN surface structures to investigate possible changes to their structural, electronic, and optical properties due to the attached functional groups. For both materials, attaching thiol groups leads to intra-gap states, which are found to be optically active for ZnO. Aiming at bigger GaN model sizes in future works compared to standard DFT approaches, a DFTB model was developed for GaN surface nanostructures. The interatomic interaction parameters were validated against standard DFT, achieving acceptable performances on bulk Ga, bulk GaN, and surface GaN systems. Another possible route to modify the electronic properties of semiconductor nanostructures is doping. ZnO bulk was doped with cobalt atoms to model different intrinsic defect complexes. Many-body GW calculations were employed to investigate their electronic structures. One defect complex is identified to be responsible for the experimentally observed photoluminescence. Due to the continuing decrease in size of electronic devices, the standard gate oxide SiO2 needs to be replaced, since today's required film thicknesses expose a crucial weakness of SiO2, a high tunneling leakage current. Possible candidates to be used as a replacement are hafnium silicate nanostructures, that avoid the described weakness. In a first step a density functional-based tight binding (DFTB) model for HfO2 was developed and validated against standard DFT calculations, achieving a very good performance for Hf bulk and HfO2 bulk. The obtained parameters were then used in a MD study on amorphous HfO2 systems to discuss their structural and electronic properties. In a second step this model was extended by silicon and applied to amorphous hafnium silicate structures to evaluate the influence of different Hf:Si ratios.