Optical properties of semiconductor nanostructures in the presence of disorder

摘要

The study of the optical properties of disordered heterostructures in the last years has been gradually moving from the stage of simple investigation of the electronic states to the exploration of possible "applications" in the broad sense. One very promising research line consists in the study of the fundamental physics of disorder using excitons in QWs. The behavior of a quantum-mechanical particle in a disorder potential is not completely understood. Anderson's scaling theory is a strong argument in favour of the idea that in less than three dimensions all one-particle eigen-states are localized, whereas in three or more dimensions the system displays a transition between localized and extended states. Localization induces universal behaviors such as, e.g., self-avoided level statistics (level repulsion), enhanced backscattering, or universal quantum fluctuations. Many of these phenomena have been observed in the case of disordered excitons in QWs. Furthermore, excitons display features complementary to other disordered systems. As an example, in contrast to conduction electrons in metals, excitons have a bosonic character at low density, their mutual interaction is weak and density-tunable, and they always relax to the bottom of the disorder band where they can be detected optically. This makes QW excitons a very promising candidate for a systematic exploration of the universal properties of disordered systems. In addition to more conventional applications of exciton localization to light-emitting devices, strongly localized excitons, otherwise called natural quantum dots, are advocated as one possible building block of the future technologies for quantum information processing. The strong localization enhances the exciton nonlinear properties, in particular the phase space filling due to the Pauli exclusion principle for electron and holes. In this regime, the present description in terms of center-of-mass degrees of freedom becomes incomplete and the separate influence of disorder on the electron and hole states must be taken into account. In particular, the transition between the composite boson character at moderate localization to the nonlinear quantum dot-like behavior at strong localization still needs a thorough investigation, and questions such as the number of excitons needed in order for significant phase space filling to take place, as a function of the localization length and energy, are still open. The theoretical description of localized Coulomb correlated electron-hole pairs and of their nonlinear properties beyond the center-of-mass approximation is thus the next step to take in this direction.