An ultrathin, 1-6 monolayers (MLs) thick, Si delta-layer, is embedded in bu
lk GaAs. The normally observed self-assembling with resulting phase separat
ion can be avoided up to delta-layer thicknesses of 4MLs, which opens the p
ossibility to study two-dimensional (2D) properties of this III-V/IV hetero
structure. Optical, electrical, transport and structural characterization o
f the Si delta-layer has been carried out. In luminescence, two novel emiss
ion bands are observed, which are blue-shifted as the width of the Si delta
-layer is reduced, indicating pronounced 2D properties. The derived results
on transition energies and electronic structure are compared with theoreti
cal predictions obtained by a self-consistent approach.