NEGATIVELY CHARGED EXCITONS (X(-)) AND D- TRIPLET TRANSITIONS IN GAASAL0.3GA0.7AS MULTIPLE-QUANTUM WELLS/

From a combination of low-temperature photoluminescence (PL) and far-i nfrared magnetospectroscopy on several GaAs/AlGaAs multiple quantum we ll samples with different donor doping (well only, barrier only, both well and barrier), we have identified a recombination line due to nega tively charged excitons (X(-)). We have also studied the effects of ex cess free electrons in the wells on the X(-) line. Magneto-PL for low- density barrier-only doped samples shows both free exciton and X(-) re combination lines at all values of held studied. However, for more hea vily doped samples the behavior is very different. As magnetic field i s increased, three distinct features evolve from the broad free carrie r recombination lines. At low fields all three features are Landau lev el recombination lines. At a field corresponding to filling factor nu= 2 the lowest energy line undergoes a discontinuous change in slope, an d above this field it evolves into the X(-) line. In related far-infra red magnetospectroscopy studies we have made a clear identification of one of the predicted negative donor ion (D-) triplet transitions.