Structural, electrical, and vibrational properties of Ti-H and Ni-H complexes in Si

Hydrogen passivates many electrically active impurities in Si. While most passivation reactions are understood experimentally and theoretically, the interactions between H and transition-metal (TM) impurities of the 3d series are poorly understood. H does trap at various TMs but new electrically active levels in the gap appear following hydrogenation. No specific complex has been assigned to any of these new lines and even the number of hydrogen atoms involved is only assumed. Electrical studies of Ti-H and Ni-H interactions have sometimes produced conflicting results. We report here the results of systematic first-principles studies of Ti-H and Ni-H interactions in Si. The stable structures, binding energies, electrical properties, and vibrational spectra are predicted. Several of the observed complexes are identified. We find one electrically inactive Ti-H complex, {Ti_i H₄}, but no inactive Ni-H complex. We show that, if enough H is available, substitutional Ti and Ni are expelled to interstitial sites, leaving a partially H-saturated vacancy.