Atomistic modelling of charge trapping defects in silicon dioxide

This thesis focuses on the atomistic modelling of electron trapping sites in silicon dioxide (SiO2) and interactions of hydrogen with the SiO2 network and the resulting defects they produce. They are discussed in the context of electronic device reliability issues. The results presented here were calculated in both crystalline (c-) and amorphous (a-) SiO2. Due to its disordered nature, modelling a-SiO2 is challenging and required the use of a melt-and-quench technique using a classical interatomic potential. All models were evaluated against experimental data to ensure that they are indeed representative of a-SiO2. Using density functional theory (DFT) and the models described above, extra electrons were shown to trap in pure c- and a- SiO2 in deep band gap states for the first time. They can trap spontaneously on pre-existing structural precursors in a-SiO2. The optical absorption spectrum of the intrinsic electron traps was calculated using time-dependent DFT and shows a peak at 3.7 eV, which is in good agreement with a previously unidentified experimental absorption peak measured at low temperatures. Electronic device fabrication processes widely employ hydrogen due to its perceived ability to improve their reliability. The results in this thesis show that both atomic and molecular hydrogen are involved in defect generation processes. Atomic hydrogen was found to interact strongly with strained Si-O bonds to form a stable defect. The barriers to create and annihilate this defect as well as the distribution of its properties were calculated. Hydrogen molecules were found to generate Si-H bonds which can trap holes to form a similar defect and release a proton which can modulate the defect's properties. These results provide insight into the atomistic nature of defects that can be involved in electronic device reliability issues and help guide the design of reliable fabrication processes.