  Chemical adsorption of the species at semiconductor surfaces is analyzed
showing the existence of the two contributions to adsorption energy: bond
creation and charge transfer. It is shown that the energy of quantum surface
states is affected by the electric field at the surface, nevertheless the
potential contribution of electron and nuclei cancels out. The charge transfer
contribution is Fermi level independent for pinned surfaces. Thus for Fermi
level pinned at the surface, the adsorption energy is independent on the Fermi
energy i.e. the doping in the bulk. The DFT simulations of adsorption of
hydrogen at clean GaN(0001) and silicon at SiC(0001) surfaces confirmed
independence of adsorption energy on the doping in the bulk. For the Fermi
level nonpinned surfaces the charge contribution depends on the position of
Fermi level in the bulk. Thus adsorption energy is sensitive to change of the
Fermi energy in the bulk, i.e. the doping. The DFT simulations of adsorption of
atomic hydrogen at 0.75 ML hydrogen covered GaN(0001) surface confirmed that
the adsorption energy may be changed by about 2 eV by the doping change from n-
to p-type.
