An ultrathin, 1–6 monolayers (MLs) thick, Si δ layer, is embedded in bulk GaAs. The normally observed self-assembling with resulting phase separation can be avoided until δ-layer thickness of 4 MLs, which opens the possibility to study two-dimensional (2D) properties of this III–V/IV heterostructure. Optical, electrical, transport, and structural characterization of the Si δ layer has been carried out. In luminescence, two novel emission bands are observed, which are blueshifted as the width of the Si δ layer is reduced, indicating pronounced 2D properties. The derived results on transition energies and electronic structure are compared with theoretical predictions obtained by a self-consistent approach. © 1998 American Institute of Physics.