Mechanism for hydrogen-enhanced oxygen diffusion in silicon

Oxygen diffuses in silicon with an activation energy of 2.53–2.56 eV. In hydrogenated samples, this activation energy is found to decrease to 1.6–2.0 eV. In this paper, a microscopic mechanism for hydrogen-enhanced oxygen diffusion in p-doped silicon is proposed. A path for joint diffusion of O and H is obtained from an ab initio molecular-dynamics simulation in which the O atom is “kicked” away from its equilibrium position with a given initial kinetic energy. After reaching a maximum potential energy of 1.46 eV above the ground state, the system relaxes to a metastable state on which a Si-Si bond is broken and the H atom saturates one of the dangling bonds. With an extra 0.16 eV, the Si-H bond is broken and the system relaxes to an equivalent ground-state configuration. Therefore, the migration pathway is an intriguing two-step mechanism. This path represents a 0.54-eV reduction in the static barrier when compared with the diffusion of isolated O in Si, in excellent agreement with experiments. This mechanism elucidates the role played by the H atom in the process: it not only serves to “open up” a Si-Si bond to be attacked by the oxygen, but it also helps in reducing the energy of an important intermediate state in the diffusion pathway.