First-principles study of defects and impurities in semiconductor nanowires and nitrides

Abstract

For device applications, it is necessary to generate charge carriers such as electron and hole, usually generated by impurity doping. However the doping efficiency is low in nanowires (NWs) and nitrides due to several reasons, thus a comprehensive theoretical study is required. In this thesis, the stability and the electronic properties of dopants and defects in NWs and nitrides are investigated by using the first-principles density functional total energy calculations. An introduction of the thesis and theoretical background of the calculation methods are presented in the first and the second chapter, respectively. In the third chapter, the origin of intrinsic hole carriers in Ge/Si core-shell NWs is investigated. As the diameter of NWs and/or the Ge core increase, the lowest unoccupied defect levels of the Si dangling bond and substitutional Au become closer to the valence band edge. The reason is attributed to the reduced quantum confinement effect and the valence band offset between Ge and Si. As a result, the defects located at the Si shell of the Ge/Si NWs induce the hole carriers which are confined to the Ge core. As the hole carriers are separated from the charge traps, the mobility is expected to be enhanced as compared to Si and Ge NWs. In the fourth chapter, the stability of the donor-pair defects in Si1-xGex alloy NWs are investigated. The stability is enhanced as the Ge concentration increases, resulting in the lower doping efficiency in Si1-xGex alloy NWs. The effects of reduced dimensionality, Ge chemical bonding, and strain on the stability of donor-pair defects in alloy NWs are discussed. In the fifth chapter, the stability of interstitial hydrogen and the energy barriers for H diffusion in Mg doped p-type GaN are investigated. Using the results for the energy barriers and diffusion paths for H diffusion as input, kinetic Monte Carlo simulations are performed to examine the thermal stability of hydrogen. The annealing temperature for complete...