Detailed balance between impact ionization and Auger recombination of trions in GaAs/AlAs quantum wells

We studied the photoluminescence of (e1:hh1)1S excitons (X) and negatively charged excitons (trions, X-) in quantum wells (QWs) having a low density (n(e) < 5 x 10(10) cm(-2)) two-dimensional electron gas (2DEG). Mixed type I-type II GaAs/AlAs quantum wells are studied in which the 2DEG is photogenerated in the type I QWs and n(e) is determined by the excitation intensity. We find that, while for T less than or equal to 12 K, the X and X- relative intensities can be described as a system of two levels at equilibrium, it cannot be so described for T greater than or equal to 16 K. The most pronounced difference between the two temperatures is that for n(e) < 5 x 10(9) cm(-2), the X- intensity decreases with increasing n(e) in favor of the X intensity. This and other observations are interpreted as due to deviations from the detailed balance between X- formation and annihilation processes required for thermodynamic equilibrium. (C) 1998 Academic Press.