  A theoretical modeling of the oxygen diffusivity in silicon and germanium
crystals both at normal and high hydrostatic pressure has been carried out
using molecular mechanics, semiempirical and ab initio methods. It was
established that the diffusion process of an interstitial oxygen atom (Oi) is
controlled by the optimum configuration of three silicon (germanium) atoms
nearest to Oi. The calculated values of the activation energy Ea(Si)= 2.59 eV,
Ea(Ge)= 2.05 eV and pre-exponential factor Do (Si) = 0.28 sm2 s-1, Do (Ge) =
0.39 sm2 s-1 are in a good agreement with experimental ones and for the first
time describe perfectly an experimental temperature dependence of the Oi
diffusion constant in Si crystals (T=350 - 1200 C). Hydrostatic pressure (P<80
kbar) results in a linear decrease of the diffusion barrier (dEa/dP = -4.38
10(-3) eV kbar-1 for Si crystals). The calculated pressure dependence of Oi
diffusivity in silicon crystals agrees well with the pressure enhanced initial
growth of oxygen-related thermal donors.
