  We have studied the segregation of P and B impurities during oxidation of the
Si(100) surface by means of combined static and dynamical first-principles
simulations based on density functional theory. In the bare surface, dopants
segregate to chemically stable surface sites or to locally compressed
subsurface sites. Surface oxidation is accompanied by development of tensile
surface stress up to 2.9 N/m at a coverage of 1.5 monolayers of oxygen and by
formation of oxidised Si species with charges increasing approximately linearly
with the number of neighbouring oxygen atoms. Substitutional P and B defects
are energetically unstable within the native oxide layer, and are
preferentially located at or beneath the Si/SiOx interface. Consistently,
first-principles molecular dynamics simulations of native oxide formation on
doped surfaces reveal that dopants avoid the formation of P-O and B-O bonds,
suggesting a surface oxidation mechanism whereby impurities remain trapped at
the Si/SiOx interface. This seems to preclude a direct influence of impurities
on the surface electrostatics and, hence, on the interactions with an external
environment.
