Electrical Levels and Diffusion Barriers of Early 3d and 4d Transition Metals in Silicon

Abstract

Early transition metals (TMs) of the 3d and 4d rows are undesired contaminants in solar- and electronic-grade Si. From the theoretical standpoint, understanding the properties of these TMs in silicon still remains a challenging problem owing to the strong correlations among the TM d-electrons. The present study proposes a first-principles Hubbard-corrected DFT+U approach, with on-site parameters accounting separately for electron Coulomb (U) and exchange (J) effects. We use this approach together with conventional DFT to determine electrical levels and migration barriers of early 3d (Ti, V and Cr) and 4d (Zr, Nb and Mo) TMs in Si. Comparisons with experimental data allowed us to uniquely assign the deep levels in the gap appraising also the effect of on-site correlation. Our results also resolve existing controversies in the literature concerning the type and origin of the donor levels of Cr and Mo. For all the metals, with the exception of Cr, high barriers of interstitial diffusion are obtained, thus confirming that most of these TMs are slow diffusers in silicon.