Mechanisms of Hydrogen Passivation and Reaction Pathways for Neutral Oxygen Vacancies in Amorphous Silica

Amorphous silica (a-SiO₂), with excellent insulating properties, uniform disordered structure and good thermal stability, is the preferred material for field oxide layers, gate insulation layers and passivation layers in numerous semiconductor devices. However, in space environments, oxygen vacancies resulted from high-energy particle radiation and their interactions with hydrogen atoms in a-SiO₂ could lead to enhanced low-dose-rate sensitivity, potentially causing threshold voltage shifts and leakage current increases in semiconductor devices. These seriously threaten the operation safety of spacecraft, and the exploration of related reaction mechanisms is crucial. A first-principles calculation is employed to investigate the neutral oxygen vacancies in amorphous silica and their reaction mechanisms with hydrogen atoms. Five types of neutral oxygen vacancies are identified, namely VD, VB, VF, VBP₄ and VDSi configurations. A significant positive correlation is observed between the defect formation energy and the distance between two defect silicon atoms. The VD configuration may become the major defect type in irradiation or fabrication due to the lowest defect formation energy. VF and VB configurations display comparable Fermi contacts to those of Eγ′ centers. The presence of electron pairs leads to zero fermi contacts in VD, VBP₄ and VDSi configurations. To reactions between oxygen vacancies and hydrogen atoms, the previous investigations often pay more attention to the reactions with hydrogen atoms at the middle-sites of oxygen vacancies. And, a critical characteristic of the disordered a-SiO₂ structure is neglected by this approach: the reactions may extend into the neighboring network and occur at side-sites of oxygen defects. For a full understanding of actual reactions, both the middle-sites and side-sites are considered for hydrogen atoms in present investigations. It’s revealed that hydrogen atoms passivate neutral oxygen vacancies through two distinct mechanisms: Si-H bond formation or silanol group generation. These processes yield two classes of neutral hydrogenated oxygen vacancies, VH and VOH configurations, which can be further classified into seven distinct configurations based on the orientation of dangling bonds and Si-H bonds. By combining the analyses of ELF maps and EPR simulations, it is demonstrated that VBBH and VBMH configurations have comparable EPR parameters to those of Eγ′ center, implying that hydrogen passivation processes may interfere with the identification of E′ center. The formation of silanol group in VBB OH configuration provides theoretical bases for explaining water molecules formation within oxide layers and at interfaces. This study elucidates the hydrogen-induced crossnetwork migration and silanol group formation pathway, collectively revealing the dual role of hydrogen in passivating defects and inducing secondary defects. A microscopic explanation is derived from these findings for the enhanced low dose rate sensitivity in bipolar devices.