Shallow impurities in quantum-well structures: A far infrared view

Recent high magnetic field, far infrared transmission and photoconductivity experiments on shallow donor impurities (Si) doped in the centers of the wells, the centers of the barriers and the edges of the wells in GaAs/AlGaAs multiple-quantum-well (MQW) structures are described. For donors doped in the centers of the wells, transitions to various high excited states have been observed. The assignment of these transitions and their relationship to the hydrogenic transitions in bulk (three dimensional) semiconductors is discussed. The edge-doped samples show dramatic differences in line shape for otherwise identical samples doped at the top and bottom of the GaAs wells. It is concluded from comparison with model calculations that the Si donors are redistributed along the growth direction, and such modeling should permit the determination of the impurity distribution non-destructively. Samples doped in the centers of the barriers show two lines at frequencies lower than that of the well-center impurities. The dominant line is due to electrons in the wells bound to their parent positive donor ions in the centers of the barriers. Possible origins of the weaker line are discussed.