  We have investigated ab initio the existence of localized states and
resonances in abrupt GaAs/Si/GaAs (110)- and (100)-oriented heterostructures
incorporating 1 or 2 monolayers (MLs) of Si, as well as in the fully developed
Si/GaAs (110) heterojunction. In (100)-oriented structures, we find both
valence- and conduction-band related near-band edge states localized at the
Si/GaAs interface. In the (110) systems, instead, interface states occur deeper
in the valence band; the highest valence-related resonances being about 1 eV
below the GaAs valence-band maximum. Using their characteristic bonding
properties and atomic character, we are able to follow the evolution of the
localized states and resonances from the fully developed Si/GaAs binary
junction to the ternary GaAs/Si/GaAs (110) systems incorporating 2 or 1 ML of
Si. This approach also allows us to show the link between the interface states
of the (110) and (100) systems. Finally, the conditions for the existence of
localized states at the Si/GaAs (110) interface are discussed based on a
Koster-Slater model developed for the interface-state problem.
