EXCITON SCATTERING IN QUANTUM STRUCTURES

Abstract

This thesis is devoted to the theoretical study of exciton scattering due to free carriers and excitons, and their corresponding contributions to the exciton linewidth broadening in semiconducting quantum well structures, which have not been adequately addressed as compared to other broadening mechanisms such as scattering by optical and accoustic phonons, defects, interface disorders, etc. In the present calculations, we employ a finite confining potential well model which, though requires more computation relative to an infinite one, reveals interesting features that are absent in the latter, In particular, we find that the exciton scattering cross sections initially decrease with decreasing well width until a critical narrow well limit where they reach a minimum and then increase with any further decrease in well width, This is contrary to the prediction of decreasing cross sections with decreasing well width using the infinite confining potential model. Interestingly, the enhanced cross sections in the narrow well limit also exhibit trends approaching those characteristic of bulk exciton scattering. Such quasi 3D excitons have been proposed by several researchers in their calculations of the exciton binding energy in quantum wells, Here we show that this quasi 3D behavior can also be deduced from the exciton cross sections which reflect the nature of the excitons. Comparisons are made with previous models to gain further insight to the scattering mechanisms of excitons due to free carriers and excitons in quantum wells. We also studied the effects of a transverse electric field on the exciton cross sections and linewidth broadening due to scattering by free carriers and excitons. It is found that the excitons in narrow wells are generally less sensitive to changes in electric field while in wider wells, the electric field could either increase or decrease the cross sections, and the corresponding linewidth broadening, depending on the type of scatterers and the nature of the scattering. Physically, the electric field not only polarizes the exciton, but also enhances (reduces) the interaction between particles of like (unlike) charges within the exciton and the scatterers. As the extent of these effects depend on both the strength of the field as well as the width of the confinement, the exciton cross sections can exhibit different behaviors within a range of well widths and electric fields. We have also discussed the relative importance of free carrier-exciton and exciton-exciton interactions in the presence of the electric field. Based on their relative lifetimes and magnitudes of cross sections, it is concluded that exciton-exciton interactions in quantum well structures could gain importance over free carrier-exciton interactions for increasing electric field strengths.