Theory of Nickel and Nickel-Hydrogen Complexes in Silicon

Spin-polarized local density functional cluster calculations are carried out on substitutional Ni and Ni-H₂ complexes. We find that Ni⁻ undergoes a Jahn-Teller distortion along ⟨100⟩ with Ni moving slightly along the cube axis. The distorted state gives b₁, b₂, and a₁ levels in ascending order of energies within the gap in agreement with experiment. Several candidate NiH₂ defects are investigated: the lowest energy structure consists of a substitutional Ni atom together with two H atoms at antibonding sites to two Si neighbors of Ni. This gives H related vibrational modes and a spin-polarized charge density close to those reported for Pt-H₂.