First Principles Studies On Electronic Structures Of HfO₂ Systems And Tensile Properties Of Si Nanowires

First principles calculations based on density functional theory have been performed to investigate the electronic structures of HfO2 systems and the tensile properties of silicon nanowires. For HfO2, the Y defect in cubic structure, the low Miller index surfaces and the interface between silicon and amorphous HfO2 are studied. For silicon nanowire, the tensile behavior of Si[001] nanowire is simulated.This thesis consists of six chapters. ChapterⅠintroduces the experimental progress, the industrial usage of the HfO2 and Si nanowires. It is emphasized that theoretical simulations should be conducted to help understand and predict the physical phenomena. ChapterⅡgoes through the basic theories and the explicit methods adopted.ChapterⅢ, chapter IV and chapterⅤare all concerned with HfO2 systems. In chapterⅢ, we discuss the effect of Y doping on the structural stability and defect properties of cubic HfO2·It is found that Y doping would increase the stability of cubic phase relative to the monoclinic phase. The energetics indicates that the complex defect, formed by oxygen vacancy and Y substitute, prefers the single positively charged states. The corresponding defect level is located within the valence band. This explains the experimental observation that gap states related to oxygen vacancy defects become non-detectable in Y-doped HfO2 films.ChapterⅣis related to the structural, electronic and magnetic properties of low Miller index surfaces in monoclinic, tetragonal and cubic HfO2·We find the atomic relaxation lowers the surface energies of the systems; The ionic feature of Hf-O bond is important to understand the relaxation of the anions and cations; The most energetically favorable surfaces are stoichiometric, but the less stable non-stoichiometric ones could be observed in experiments under some chemical environments if the energy difference is not large; And d0 magnetism has been found in some oxygen rich non-stoichiometric surfaces, which results from the large spin exchange energy of the surface oxygen 2p orbital and the high density of states of the oxygen 2p electrons near the Fermi level. With the energet-ics, we propose a reasonable explanation for recent controversial observations of ferromagnetism in HfO2·ChapterⅤdeals with the atomistic and electronic properties of amorphous HfO2/Si(001) interface. The amorphous HfO2 model structure is generated by ab initio molecular dynamics simulations within the "melt-and-quench" scheme. Calculations indicate that the simulated amorphous HfO2 essentially shows the characteristics of the experimental amorphous HfO2 structure. The results sug-gest that atomic coordination of interface Si atoms would significantly affect the interface electronic properties. With the band lineup of the core level, the va-lence band offset is determined to be 2.62±0.35 eV, in good agreement with the experimental data.In chapterⅥ, quasi-static simulations are carried out to study the structural evolution and the breaking mechanism of the Si[001] nanowire with small radius.