Charge states of the reactants in the hydrogen passivation of interstitial iron in P-type crystalline silicon

Significant reductions in interstitial iron (Fe_i) concentrations occur during annealing Fe-containing silicon wafers with silicon nitride films in the temperature range of 250°C-700°C. The silicon nitride films are known to release hydrogen during the annealing step. However, in co-annealed samples with silicon oxide films, which are hydrogen-lean, changes in the Fe_i concentrations were much less significant. The precipitation of Fe_i is ruled out as a possible explanation for the significant reductions. The hydrogen passivation of Fe_i, which is the complexing of monatomic H and isolated Fe_i forming a recombination-inactive hydride, is proposed as the most probable model to explain the reductions. Under the assumption that the reduction is caused by the hydrogenation of Fe_i, the reactants' charge states in the hydrogenation reaction are determined by two independent approaches. In the first approach, illumination is found to have a small but detectible impact on the reaction kinetics in the lower temperature range. The dominating reactants' charge states are concluded to be Fe⁰ + H⁺ as revealed by modelling the injection-dependent charge states of isolated Fe_i and monatomic H. In the second approach, the reaction kinetics are fitted with the Arrhenius equation over a large temperature range of 250°C-700°C. A reasonable fit is only obtained when assuming the reacting charge states are Fe⁰+H⁺. This supports the conclusion on the reacting charge states and also gives a value of the activation energy of hydrogenation in the 0.7-0.8eV range.