EFFECT OF DONOR IMPURITIES ON FAR-INFRARED MAGNETOSPECTROSCOPY OF ELECTRONS IN QUASI-2-DIMENSIONAL SYSTEMS

Results of recent far-infrared magnetospectroscopic studies of a serie s of GaAs/AlGaAs multiple-quantum-well samples with Si donors S-doped both in the 20 nm well centres (2 x 10(10) cm(-2)) and in the 60 nm ba rrier centres ((3.5-28) x 10(10) cm(-2)) are described. In the presenc e of excess electrons a strong singlet transition of the D- ions is ob served, and one of the two predicted triplet transitions has been iden tified. At higher electron densities these transitions evolve continuo usly into blue-shifted singlet-like and triplet-like bound magnetoplas mon transitions. For the two highest-density samples the singlet-like transition exhibits clear discontinuous changes in slope versus field at integral Landau level filling factors (FFs). Plots of the differenc e in energy between the many-electron singlet-like transition and the D- singlet transition normalized by the high-field Coulomb energy vers us FF, exhibit cusp-like behaviour at FFs 1 and 2 at 4.2 K for the hig hest density sample, and at 1 and 4/3 at 1.7 K for the lower density s ample. The results at integral FFs are compared with recent theoretica l calculations. These experiments indicate that controlled doping of a random array of donor ions can be used to probe the nature of many-el ectron states in confined systems.