CREATION AND ANNIHILATION OF POSITIVELY AND NEGATIVELY CHARGED EXCITONS IN GAAS QUANTUM-WELLS

The electro- and photoluminescence of p-i-n and n-i-n GaAs double-barr ier resonant tunnelling diodes have been investigated for various conc entrations of free carriers (either holes or electrons) in the quantum well (QW). For these structure it is possible to change the relative electron or hole density quasi-continuously with applied bias voltage. At electron densities well below that for exciton unbinding, the form ation of negatively charged excitons (X(-)) is demonstrated. In asymme tric QW structures the electron densities can be increased above the c ritical density and exciton unbinding is observed in the luminescence spectra. We also report the observation of the positively charged exci ton (X(2)(+)) in the luminescence spectra of a GaAs quantum well. The conditions for forming X(2)(+) are compared with those needed to see t he negatively charged exciton (X(-)), the heavy-hole free exciton (X) and free carrier recombination.