A concise discussion concerning physical principles of the UHV ALE is
presented in this paper. Emphasis is put on the kinetic processes occu
rring in the near surface transition layer (NSTL) during the growth of
semiconductor compounds. The NSTL is considered here as a model for t
he growth region in which UHV ALE takes place. This model is based on
the following assumptions: (i) for anions and for cations, constituent
s of the grown semiconductor compound, transition layers, intermediate
between the well-ordered crystalline substrate (epilayer) and the gas
eous, disordered phase of the impinging beams are created in the ALE p
rocess, (ii) after desorption, during the dead times in the ALE deposi
tion cycle, of the loosely bound anions or cations, also a part of the
first atomic monolayer of these constituents (nearest to the substrat
e/epilayer surface), desorb into the UHV environment. With the NSTL mo
del of UHV ALE all most important features, characteristic to this gro
wth technique, can be explained in a satisfactory way.