Isolated H and D in crystalline material were studied by using the path-integral Monte Carlo method. The interactions between Si atoms were modelled by using the Stillinger-Weber potential, and the Si-H interaction was parametrized on the basis of previous pseudopotential density-functional calculations for this system. The finite-temperature properties of these point defects were analyzed for temperatures ranging from 50 to 600K. The H and D were found to be stable at the bond-center (B) site. The average values of the kinetic and potential energies of the defect were compared with those expected for the impurity within the harmonic approximation. At low temperatures, the kinetic energy was larger than the potential energy; due to a strong anharmonicity of the potential surface for the impurity around the B site. The density distribution of an impurity at the B site exhibited axial symmetry around the Si-B-Si axis. The width of the density distribution, along the bond direction, was about half of that which was found for directions that were perpendicular to the symmetry axis.

C.P.Herrero, R.Ramírez: Physical Review B, 1995, 51[23], 16761-71