Mechanism for hydrogen-enhanced oxygen diffusion in silicon

Oxygen diffuses in silicon with an activation energy of 2.53-2.56 eV. In hy drogenated samples, this activation energy is found to decrease to 1.6-2.0 eV. In this paper, a microscopic mechanism for hydrogen-enhanced oxygen dif fusion in p-doped silicon is proposed. A path for joint diffusion of O and H is obtained from an nb initio molecular-dynamics simulation in which the O atom is ''kicked'' away from its equilibrium position with a given initia l kinetic energy. After reaching a maximum potential energy of 1.46 eV abov e 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 equi valent ground-state configuration. Therefore, the migration pathway is an i ntriguing two-step mechanism. This path represents a 0.54-eV reduction in t he static barrier when compared with the diffusion of isolated O in Si, in excellent agreement with experiments. This mechanism elucidates the role pl ayed 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.