Modeling of Intrinsic Defects in Amorphous SiO₂ Using Quantum Molecular Dynamics Methods

Point defects in amorphous and vitreous SiO₂ play an important role in various fields of science and technology, including microelectronics, multilayer optical coatings, and fiber optics. In microelectronics, SiO₂ is used as an insulating layer in semiconductor devices, and defect-related electronic states within the band gap of SiO₂ can act as traps for electrons and holes, leading to leakage currents and parasitic charge accumulation. In multilayer optical coatings composed of alternating oxide layers with low and high refractive indices, where amorphous SiO₂ films serve as low-index optical layers, defects in SiO₂ result in excess absorption and ultimately reduce the laser-induced damage threshold of the coatings. In silica-based optical fibers, point defects determine radiation-induced optical losses, governed by defect-related electronic states within the band gap of SiO₂. Impurity-related, and in some cases intrinsic, defect states in SiO₂ also govern the UV laser inscription of Bragg gratings in optical fibers, as well as the performance of fiber optical amplifiers and lasers.
To date, a large number of studies have investigated intrinsic and impurity defects in SiO₂. However, until recently, in modeling studies, “reasonable” hypothetical defects were typically constructed manually either in crystalline SiO₂ or in atomic clusters with structures resembling that of amorphous SiO₂. In the latter case, embedded cluster methods were employed, or dangling bonds at the cluster boundaries were passivated with hydrogen atoms. In both approaches, the energy of the initial system was locally optimized using quantum mechanical (quantum chemical) methods by varying the positions of defect atoms and, in some cases, atoms in their immediate vicinity. A consistent application of quantum mechanical methods to generate amorphous SiO₂ structures and their defects has been used only rarely and in a limited manner.
The present work is devoted to a new approach to modeling amorphous SiO₂ and its defects. In this approach, amorphous states were obtained by simulating the melt–quench process of an initial crystal using quantum molecular dynamics methods. This process involves heating the crystal above its melting temperature, equilibrating the melt, cooling it down to room temperature, and stabilizing the resulting structure. The licensed VASP 5.4.4 software package was used. Amorphous structures of both stoichiometric and oxygen-deficient SiO₂ were obtained at different melt equilibration temperatures. Oxygen-deficient states were modeled by performing melt–quench simulations starting from crystals containing one or two oxygen vacancies. The atomic network structure of the resulting amorphous states and the structures of the identified defects are described, including both known and newly identified point defects in SiO₂. In total, nine defects were identified in stoichiometric SiO₂ and seven defects in oxygen-deficient SiO₂. For comparison, results obtained using the same method for amorphous HfO₂ and ZrO₂, as well as the properties of an amorphous SiO₂–Ta₂O₅ bilayer structure generated by the same approach, are presented. Based on these results, general conclusions are drawn. This approach enables the natural formation of not only low-energy intrinsic defects but also impurity-related defects in amorphous structures.