Thermodynamics of Excitons and Biexcitons in a Gallium Arsenide Multiple Quantum Well.
Excitons inside a GaAs quantum well have been the subject of many theoretical and experimental investigations in the past. A number of recent experiments have shown that, at low temperature, excitons combine in pairs to form a significant number of biexcitons inside a GaAs quantum well. Theories on biexcitons have suggested that these experimental results are due to increased biexciton binding energy caused by confinement. These previous studies, thus, indicate that biexcitons must be included in any complete description of 2-dimensional, low-temperature exciton dynamics. A subject that, to my knowledge, has not been studied in great depth is the thermodynamic properties of the excitons and biexcitons inside a GaAs quantum well. In this thesis, I investigate the thermodynamics of a mixture of 2-dimensional excitonic and biexcitonic gases inside a 100-A GaAs multiple quantum. By analyzing time-resolved photoluminescence data to obtain instantaneous densities as functions of time, I show that the inter-converting excitons and biexcitons quickly establish a thermal and chemical equilibrium. The data demonstrate that, as the excitons and biexcitons decay, their relative densities are well predicted by a chemical-equilibrium theory. When the excitons and biexcitons obey Maxwell -Boltzmann statistics, i.e. when pair density is low, the chemical-equilibrium theory predicts the well known law of mass action which quadratically relates the exciton and biexciton densities. When the pair density is high and/or the equilibrium temperature is low, the law of mass action is no longer applicable, and the equilibrium density relationship deviates from the quadratic density relationship. I theoretically show that the deviation arises from the Bose -Einstein statistics of the excitons and biexcitons. By demonstrating the characteristic deviation from the law of mass action, I also present experimental evidence of Bose-Einstein statistics among the 2-dimensional excitons and, particularly, the biexcitons.
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- Physics: Condensed Matter; Physics: Optics