Excitons in Low-Dimensional Semiconductor Structures

Abstract

Confinement of excitons in low-dimensional structures leads to a strong enhancement of excitonic effect. They have impact on optical properties of these structures up to room temperature even for materials with low excitonic binding in the bulk. We will start in this chapter with the properties of excitons in quasi-2D structures (quantum wells). In particular we will illustrate the dependence of this properties on the confining potential, e.g., its width. The influence of disorder due to well-width fluctuations will be addressed as well as polariton propagation in quantum wells. We will further elaborate the consequences of coupling between neighboring wells—up to the limit of superlattices—on excitonic properties. This includes superlattices with type-II band alignment separating electrons and holes, strained-layer superlattices with tilted bandstructures due to piezoelectric effects, and modulation-doped superlattices. Two-dimensional properties of excitons are best realized in monolayer semiconductors. The excitonic states are here strongly influenced by the unusual bandstructure leading to specific valley-dependent properties. We then proceed to excitons in quantum wires and rods and finally come to quantum dots. We discuss the various confinement regimes of excitons in quantum dots and their consequences on optical spectra. We start with quantum-dot ensembles but we will then describe in detail the fine structure of excitonic states in single InGaAs quantum dots. The later is accessible via micro-photoluminescence or near-field spectroscopy. Another experimental method illustrated in this chapter is photoluminescence excitation spectroscopy (PLE).