Yannick Wimmer Dipl.-Ing. Dr.techn. Publications

Silicon dioxide (SiO₂) is a widely used material in many different technologies, such as light guides, thin-film coatings and dielectrics in semiconductor electronics. For many years, it has been suspected that hydrogen (H) plays a key role in many detrimental effects in SiO₂. The concentration of H in the oxide material is on the same order of magnitude as the experimentally observed defect density, which corresponds to only a handful of defects in modern devices. In order to understand recently discovered defect transformations, we used density functional theory (DFT) calculations to investigate the possible migration barriers for H in SiO₂.
An H migration mechanism, used to explain proton (H⁺) movement through amorphous SiO₂, has already been well established in the literature. However, the experimental results of the activation energy, as well as the theoretical calculation of the migration barriers, which have to be overcome during this process, are controversial. Furthermore, H⁺ migration is primarily considered in the literature, as well as its corresponding migration process, which in the following is referred to as "hopping" and is depicted in Fig. 1. For the neutral hydrogen (H⁰), such a hopping process has not been reported in the literature until now.
In order to determine the migration barriers, first configurations as in Fig.1 were created using DFT calculations. Then, the minimum energy path interlinking such configurations was calculated using nudged elastic band calculations. The migration barriers shown in brown in Fig. 2 are the migration barriers from a handful of starting points to their ten nearest neighbors. By far the most likely transition is always determined by the lowest barrier in each set, however, as presented in red in Fig 2. The hopping transition barriers for H⁺ and H⁰ turn out to be, on average, at a comparable height. This means that hopping in both charge states seems possible.

Fig. 1: Example of H migration in SiO₂. The H breaks off at one bridging O atom and rebinds to a neighboring one.
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Fig. 2: Statistically distributed barriers for the transitions similar to the one shown in Fig. 1. The complete statistics are shown in brown, but the determining transition is always the lowest one of each set (red). The barrier heights of the red distribution can be easily overcome at room temperature.