THERMODYNAMICS OF BIEXCITONS IN A GAAS QUANTUM-WELL

Photoluminescence from GaAs quantum wells displays sharp spectral features associated with the free exciton. At high excitation density and low temperature, an additional component appears just below the lowest (heavy-hole) excitonic emission. Previous studies have attributed this feature to the biexciton, consisting of two electrons and two holes, because it appears below the exciton line and grows superlinearly with respect to the exciton intensity. In this paper, we considerably strengthen this identification by analyzing time- and space-resolved photoluminescence data that imply a dynamic chemical equilibrium between the excitons and biexcitons, i.e., a law of mass action. This thermodynamic (complementary to spectroscopic) evidence for biexcitons is based on the interdependent temporal decay and spatial transport behavior of the two components on a picosecond time scale. As the excitons recombine, the law of mass action describes the equilibrium over more than two orders of magnitude in exciton density as determined from the measured photoluminescence intensity and volume of the exciton gas. © 1994 The American Physical Society.