Theory of hydrogen diffusion and reactions in crystalline silicon

The behavior of hydrogen in crystalline silicon is examined with state-of-the-art theoretical techniques, based on the pseudopotential-density-functional method in a supercell geometry. Stable sites, migration paths, and barriers for different charge states are explored and displayed in total-energy surfaces that provide immediate insight into these properties. The bond-center site is the global minimum for the neutral and positive charge states; in the negative charge state, the tetrahedral interstitial site is preferred. The positive charge state is energetically favorable in p-type material, providing a mechanism for passivation of shallow acceptors: electrons from the H atoms annihilate the free holes, and formation of H-acceptor pairs follows compensation. Also addressed are the issues of molecule formation and hydrogen-induced damage. A number of different mechanisms for defect formation are examined; hydrogen-assisted vacancy formation is found to be an exothermic process.