Ab initio treatment of silicon-hydrogen bond rupture at Si/SiO2 interfaces

Even after more than 50 years of development, a major issue in silicon-based technology is the understanding of the $\mathrm{Si}/{\mathrm{SiO}}_{2}$ interface and its defects, particularly the unsaturated silicon dangling bonds which have to be passivated by hydrogen during fabrication. Although it is well known that hydrogen dissociation from an initially passivated interfacial Si dangling bond results in an electrically active defect, there is still no consensus on the actual microscopic Si--H bond-breaking mechanism, despite a significant research effort. The most thorough theoretical study in the field was published by Tuttle and Van de Walle 20 years ago. Although it was then suggested that the hydrogen dissociates most likely into a bond-center site, no clean argument for bond rupture could be given at that time. In order to take a fresh look at this highly important problem, we employ the latest $ab$ initio methods available, including the method of well-tempered metadynamics and nudged-elastic-band calculations based on density functional theory (DFT). This allows us to study the interactions of a Si--H bond with its realistic environment in a three-dimensional Si/$a--{\mathrm{SiO}}_{2}$ interface in considerable detail. Using classical force fields and well-tempered metadynamics in conjunction with DFT, we provide new insights into the dissociation kinetics. We find that one of the previously suggested dissociation paths only leads into a neutral, metastable state which would not facilitate bond dissociation. By sampling the configuration space in greater detail than ever before, we propose a trajectory whereby the H first moves towards an adjacent Si and in a second step relaxes into a configuration between the next-nearest Si--Si bond. The final statistical analysis on a large number of defects on this amorphous interface yields potential energy surfaces and barriers which are in excellent agreement with experimental values.