  Low-energy electronic states in heterosrtuctures formed by ultranarrow layer
(single or several monolayers thickness) are studied theoretically. The host
material is described within the effective mass approximation and effect of
ultranarrow layers is taken into account within the framework of the transfer
matrix approach. Using the current conservation requirement and the inversion
symmetry of ultranarrow layer, the transfer matrix is written through two
phenomenological parameters. The binding energy of localized state, the
reflection (transmission) coefficient for the single ultranarrow layer case,
and the energy spectrum of superlattice are determined by these parameters.
Spectral dependency of absorption in superlattice due to photoexcitation of
electrons from localized states into minibands is strongly dependent on the
ultranarrow layers characteristics. Such a dependency can be used for
verification of the transfer matrix parameters.
