For many condensed matter problems, atomic type orbitals offer good computational efficiency. The clean surface of Si was studied by using the program DMOL3 and results were obtained in excellent agreement with well-converged plane-wave calculations. A study was then made of the effect of Ni on the diffusion of dimer vacancies. In the absence of certain impurities, dimer vacancies on the (001) surface of Si could be annealed away. Adding Ni, even in tiny amounts, could stabilize the vacancies. Both the local density approximation and generalized gradient corrections approximations were used. Use of generalized gradient approximations did not change any of the qualitative conclusions, although it did modify the details slightly (notably, bond lengths, surface dimer buckling, and the binding of the Ni atom to the dimer vacancies site were increased). The calculations showed that Ni binds very strongly to dimer vacancies, from which it was deduced that a dimer vacancies with a Ni attached could no longer diffuse. Thus, a pair of Ni atoms on a single dimer row could trap those dimer vacancies that lie between them, leading to the stabilization of the dimer vacancies.

Density-Functional Study of the Stabilization of the Si(001) Dimer Vacancy by Ni. A.Horsfield, E.Akhmatskaya, R.Nobes, J.Andzelm, G.Fitzgerald, N.Govind: Physical Review B, 2002, 66[8], 085309 (6pp)