The (GaAs)m(AlAs)n superlattices were treated as single crystals whose structure depended upon the growth directions and numbers of monolayers within the slabs of the constituent materials. A study was made of point defects such as impurities (substitutional or interstitial), single vacancies and molecular defects such as paired impurities, double vacancies, and vacancy-impurity complexes in superlattices which had been grown along the [001], [110] or [111] directions. In contrast to bulk GaAs or AlAs, no defect could exhibit Td symmetry. The atoms which were located in the center of the slabs occupied sites with higher (D2d, C2v) symmetries in most [001]- and [110]-grown superlattices, respectively. This resulted in different selection rules for optical transitions which involved the same impurity atom, when substituted for the same host atom (Ga, Al, As) at sites with different symmetries. This effect could be important in the optical spectra of superlattices with very thin slabs. Modification of the selection rules was required upon including spin-orbit interactions. Quantum wells could be treated as a particular case of a superlattice when the barriers became very thick. Their 3-dimensional biperiodic space groups were presented. Quantum wells did not differ from superlattices with regard to possible site symmetries and selection rules for optical transitions that involved defects. All of the results were valid for any pseudomorphic superlattice or quantum well which was made up of 2 binary compounds which had the zincblende structure and identical cations or anions.

Bound-State Symmetries and Optical Transitions in GaAs/AlAs Quantum Wells and Superlattices with Impurities and Defects P.Tronc, Y.E.Kitaev, A.G.Panfilov, M.F.Limonov, G.Wang, V.P.Smirnov: Physical Review B, 2000, 61[3], 1999-2007