Structure and mechanisms of formation of point defects in HfO₂, MgO and hexagonal boron nitride

• Main Author: Strand, Jack William
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2019
• Subjects: Point defects; Spectrum analysis

In this thesis, density functional theory (DFT) methods are used to model a range of defects and defect processes in three functional dielectric materials: MgO, HfO2 and hexagonal boron nitride (hBN). My results demonstrate that a novel implementation of time-dependent DFT in CP2K provides accurate optical properties of oxygen vacancies in MgO. In amorphous (a-) HfO2 the existence of self-trapped holes and electrons is predicted. These trapped states are found to be energetically deeper than their crystalline counterparts and are separated, on average, by 8 Ȃ . Calculated optical spectra of electron traps agree well with exhaustive photo-depopulation spectroscopy experiments. It is then shown that the average formation energies of oxygen vacancies and interstitials in a-HfO2 are close to those in monoclinic (m-) HfO2, however they follow a distribution. My calculations of optical spectra of oxygen vacancies in a-HfO2 demonstrate that the characteristic blue luminescence of HfO2 is likely due to the oxygen vacancy in its +2 state. It is also found that a 3.6 eV luminescence can be caused by a radiative tunnelling transition between a hole and a +1 charged oxygen vacancy. Next, oxygen Frenkel pairs (FPs) in HfO2 are studied. A barrier of 6.6 eV must be overcome to generate a FP in m-HfO2. Charging (by injection of electrons) decreases this barrier by over 4 eV. Similar barrier reduction, due to carrier localisation, is found for FP generation in a-HfO2, however with a broader energy distribution. It is demonstrated that both formation energies and barrier heights are reduced when FPs are created adjacent to vacancies or vacancy clusters. Finally, a range of intrinsic defects in hBN layers are modelled. In particular, it is predicted that divacancies stabilise N-N and B-B bridges between layers. These bridges sufficiently lower formation energies of Frenkel pairs.