Bound-state symmetries and optical transitions in GaAs/AlAs quantum wells and superlattices with impurities and defects

We consider (GaAs)(m)(AlAs)(n) superlattices as single crystals whose struc ture depends on the growth direction and numbers of monolayers within the s labs of constituent materials. We study point defects such as impurities (s ubstitutional and interstitial) or single vacancies and molecular defects s uch as paired impurities, double vacancies, or vacancy-impurity complexes i n (GaAs)(m)(AlAs)(n) superlattices grown along the [001], [110], and [111] directions. The possible site symmetries of the defects as well as the stat e symmetries for carriers bound to them have been determined. In contrast t o bulk GaAs or AIAs, no defect can present T-d symmetry. The atoms located in the center of the slabs occupy, in most of the [001]- and [110]-grown su perlattices, sites with higher symmetries (D-2d and C-2v, respectively). Th is results in different selection rules for optical transitions involving t he same impurity atom which substitutes the same host atom (Ga, Al, or As) at sites with different symmetries. The effect can be important in optical spectra of superlattices with very thin slabs. The modifications of the sel ection rules when including the spin-orbit interaction have been derived. Q uantum wells can be treated as a particular case of superlattices when barr iers become very thick. We present their three-dimensional diperiodic space groups. Quantum wells do not differ from superlattices from the points of view of possible site symmetries and selection rules for optical transition s involving defects. All the above results are also valid for any pseudomor phic superlattice or quantum well made of two binary compounds with zinc-bl ende structures and identical cations or anions, such as in the GaN/AIN sys tem.