Chemical activity of hydrogen in a Si lattice.

Abstract

A tight-binding total-energy calculation shows that interstitial atomic hydrogen cannot be stable in a Si lattice. Atomic hydrogen reacts with Si bonds, giving rise to a center consisting of a \ensuremath{\sigma}-type Si(p)-H(s) bond and one unsaturated p-like dangling hybrid. It is shown that hydrogen is bound outside or inside the bond region, depending on the occupancy of the adjacent dangling hybrid. The mechanism of diffusion of such H centers is presented and discussed. It is shown that interactions between H centers and D centers lead to annihilation of dangling hybrids in the lattice, in spite of the fact that the number of Si-H bonds remains unchanged. On the other hand, collisions between diffusing H centers may lead to two types of effects: (1) formation of a ${\mathrm{H}}_{2}$ molecule in the Si lattice, or (2) formation of a stable defect containing two Si(p)-H(s) bonds. The first of the predicted effects gives fresh microscopic insight into the mechanism of low-temperature evolution of hydrogen from \ensuremath{\alpha}-Si:H. The second effect is responsible for the damage induced by hydrogen in crystalline Si.