Hydrogen passivation of titanium impurities in silicon: Effect of doping conditions

While the contamination of solar silicon by fast diffusing transition metals can be now limited through gettering, much attention has been drawn to the slow diffusing species, especially the early 3d and 4d elements. To some extent, hydrogen passivation has been successful in healing many deep centers, including transition metals in Si. Recent deep-level transient spectroscopy (DLTS) measurements concerning hydrogen passivation of Ti revealed the existence of at least four electrical levels related to TiiHn in the upper-half of the gap. These findings challenge the existing models regarding both the current level assignment as well as the structure/species involved in the defects. We revisit this problem by means of density functional calculations and find that progressive hydrogenation of interstitial Ti is thermodynamically stable in intrinsic and n-doped Si. Full passivation may not be possible to attain in p-type Si as TiiH3 and TiiH4 are metastable against dissociation and release of bond-centered protons. All DLTS electron traps are assigned, namely, E40′ to TiiH(-/0), E170′ to TiiH3(0/+), E(270) to TiiH2(0/+), and E170 to TiiH(0/+) transitions. TiiH4 is confirmed to be electrically inert.