  Vapor growth of semiconductors is analyzed using recently obtained dependence
of the adsorption energy on the electron charge transfer between the surface
adsorbed species and the bulk [Krukowski et al. J. Appl. Phys. 114 (2013)
063507, Kempisty et al. ArXiv 1307.5778 (2013)]. Ab initio calculations were
performed to study the physical properties of GaN(0001) surface in ammonia-rich
conditions, i.e. covered by mixture of NH3 molecules and NH2 radicals. The
Fermi level is pinned at valence band maximum (VBM) and conduction band minimum
(CBM) for full coverage by NH3 molecules and NH2 radicals, respectively. For
the crossover content of ammonia of about 25% monolayer (ML), the Fermi level
is unpinned. It was shown that hydrogen adsorption energy depends on the doping
in the bulk for the unpinned Fermi level, i.e. for this coverage. Surface
structure thermodynamic and mechanical stability criteria are defined and
compared. Mechanical stability of the coverage of such surfaces was checked by
determination of the desorption energy of hydrogen molecules. Thermodynamic
stability analysis indicates that initally equilibrium hydrogen vapor partial
pressure steeply increases with NH3 content to attain the crossover NH3/NH2
coverage, i.e. the unpinned Fermi level condition. For such condition the
entire range of experimentally accessible pressures belongs showing that vapor
growth of semiconductor crystals occurs predominantly for unpinned Fermi level
at the surface, i.e. for flat bands. Accordingly, adsorption energy of most
species depends on the doping in the bulk that is basis of the possible
molecular scenario explaining dependence of the growth and the doping of
semiconductor crystals on the doping in the bulk
