  In this contribution we present the results of Density-Functional Theory
(DFT) calculations of platelets as modelled by infinite planar arrangements of
hydrogen atoms and vacancies in (100) planes of silicon. From the observation
of the relaxed platelet structures and the comparison of their energy with the
one of hydrogen molecules dissolved in silicon we were able to evidence several
features. A planar arrangement of hydrogen atoms inserted in the middle of
Si-Si bonds proves unstable and Si bonds must be broken for the platelet to be
stable. In the (100) plane the most stable configuration is the one with two
Si-H bonds (a so-called SiH2 structure). It is possible to generate SiH3
structures which are more stable than hydrogen dissolved in Si bulk but less
than SiH2 structures but SiH1 or SiH4 sometimes observed in experiments prove
unstable.
